United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
EPA-600/8-80-028
August 1980
Research and Development
Design Handbook for
Automation of
Activated  Sludge
Wastewater Treatment
Plants

-------
                RESEARCH REPORTING SERIES

Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology.  Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:

      1.  Environmental  Health Effects Research
      2.  Environmental  Protection Technology
      3.  Ecological Research
      4.  Environmental  Monitoring
      5.  Socioeconomic Environmental Studies
      6.  Scientific and Technical Assessment Reports (STAR)
      7.  Interagency Energy-Environment Research and Development
      8.  "Special" Reports
      9.  Miscellaneous Reports

This report has been assigned to the "SPECIAL" REPORTS series. This series is
reserved for reports targeted to meet the technical information needs of specific
user groups. The series includes problem-oriented reports, research application
reports, and executive summary documents. Examples include state-of-the-art
analyses, technology assessments, design manuals, user manuals, and reports
on the results of major research and development efforts.
 This document is available to the public through the National Technical Informa-
 tion Service, Springfield, Virginia 22161.

-------
                                       EPA-600/8-80-028
                                       August 1980
      DESIGN HANDBOOK FOR AUTOMATION OF
ACTIVATED SLUDGE WASTEWATER TREATMENT PLANTS
                     by

               Alan W. Manning
                David M. Dobs

                  EMA, Inc.
         St. Paul, Minnesota  55101
           Contract No. 68-03-2573
               Project Officer

              Irwin J. Kugelman
        Wastewater Research Division
 Municipal Environmental Research Laboratory
           Cincinnati, Ohio  45268
 MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
     OFFICE OF RESEARCH AND DEVELOPMENT
    -U.S. ENVIRONMENTAL PROTECTION AGENCY
           CINCINNATI, OHIO  45268

-------
                                  DISCLAIMER
     This report has been reviewed by the Municipal Environmental Research
Laboratory, U. S. Environmental Protection Agency, and approved for
publication.  Approval does not signify that the contents necessarily
reflect the views and policies of the U. S. Environmental Protection Agency,
nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.

-------
                                   FOREWORD


     The Environmental Protection Agency was created because of increasing
public and government concern about the dangers of pollution to the health
and welfare of the American people.  Noxious air, foul water, and spoiled
land are tragic testimony to the deterioration of our natural environment.
The complexity of that environment and the interplay between its components
require a concentrated and integrated attack on the problem.

     Research and development is the necessary first step in problem
solution and it involves defining the problem, measuring its impact, and
searching for solutions.  The Municipal Environmental Research Laboratory
develops new and improved technology and systems for the prevention,
treatment, and management of wastewater and solid and hazardous waste
pollutant discharges from municipal and community sources,  for the
preservation and treatment of public drinking water supplies, and to
minimize the adverse economic, social, health, and aesthetic effects of
pollution.  This publication is one of the products of that research; a most
vital communications link between the researcher and the user community.

     This manual is intended to serve as a systems engineering handbook to
assist wastewater treatment plant design engineers in the application of
automation in activated sludge wastewater treatment processes.
                                      Francis T.  Mayo,  Director
                                      Municipal  Environmental  Research
                                      Laboratory
                                     iii

-------
                                   ABSTRACT


     This report is a systems engineering handbook for the automation of
activated sludge wastewater treatment processes.  Process control theory and
application are discussed to acquaint the reader with terminology and
fundamentals.  Successful unit process control strategies currently in use
are discussed.  Alternative methods of control and implementation are presented
where other considerations such as reliability or flexibility are
important.  A method for preparing a cost effective analysis is detailed
through the use of examples.  Currently available instrumentation is
reviewed to serve as a guide for the selection of instruments for specific
applications.  The design guide section reviews some of the aspects of
control system design and includes examples of documentation required to
convey the engineer's and user's requirements.  The concluding section
presents recommendations for further studies which will advance the
application of automation in wastewater treatment.

     This report was submitted in fulfillment of Contract No. 68-03-2573 by
EMA, Inc. under the sponsorship of the Environmental Protection Agency.
Work was completed as of January, 1979.
                                     iv

-------
                                   CONTENTS

Foreword	   "Mi
Abstract	    iv
Figures	    vi
Tables	    ix
Abbreviations and Symbols	     x
P&ID Legend of Symbols 	    xi
Instrument & Device Identification 	   xii
Acknowledgment	xiii

     1.   Introduction and Purpose 	     1
     2.   Process Control  	     6
     3.   Control Strategies 	    30
               Interceptor Storage 	    36
               Flow Equalization	    40
               Plant Lift Stations	    44
               Bar Screening	    47
               Grit Removal	    50
               Primary Clarification and Sludge Pumping	    54
               Hydraulic Flow Control	    59
               Dissolved Oxygen & Blower Control 	    64
               Cryogenic Oxygen Generation 	    68
               Return Activated Sludge 	    74
               Waste Activated Sludge	    83
               Chemical Feed	    89
               Post-Chlorination 	    94
               Ozonation	   103
               Gravity Thickening	   110
               Flotation Thickening  	   115
               Anaerobic Digestion 	   120
               Vacuum Filtration 	   128
               Centrifugation  	   134
               Roll Press Dewatering 	   139
               Plate Press Dewatering  	   145
               Incineration  	   151
               Return Liquors  	   161
     4.   Alternate Control Approaches 	   167
     5.   Cost Effective Analysis	   206
               Appendix-Cost Effective Analysis Tables 	   254
     6.   Available Instrumentation  	   292
     7.   Design Guide	   345
     8.   Recommended Future Activities  	   335

Glossary	.-	   403

-------
                                   FIGURES

Number                                                                  Page

 2-1     Open-loop control 	     7
 2-2     Closed-loop control 	     8
 2-3     Flow control loop	     9
 2-4     Typical control loop element response 	    10
 2-5     Pressure control loop . ,	    11
 2-6     On-off control	    12
 2-7     Differential gap on-off control 	    13
 2-8     Proportional control input-output relationship, gain = 1  .  .    14
 2-9     Effect of gain adjustment on input-output relationship  ...    15
 2-10    Proportional control offset 	    16
 2-11    Proportional mode control response	    17
 2-12    Elimination of offset by manual reset 	    18
 2-13    Integral mode controller action	    19
 2-14    Integral plus proportional mode controller action 	    20
 2-15    Proportional plus integral mode control  response  	    20
 2-16    Proportional plus derivative mode controller action 	    22
 2-17    Proportional plus integral plus derivative control response .    22
 2-18    Direct control of dissolved oxygen  	    25
 2-19    Cascade control of dissolved oxygen 	    26
 2-20    Feedforward control 	    27
 2-21    Feedforward (ratio) control of chlorination 	    28

 3-1     Lift station control for interceptor storage  	    37
 3-2     Flow equalization control	*	    42
 3-3     Lift station wet well  level control	    46
 3-4     Bar screen control	    49
 3-5     Aerated grit chamber control	    52
 3-6     Primary clarification and sludge pumping  . .	    55
 3-7     Flow control	    61
 3-8     Flow control, MOV method	    62
 3-9     Dissolved oxygen and blower control 	    66
 3-10    Cryogenic oxygen generation 	    70
 3-11    Activated sludge process flow diagrams  	    75
 3-12    RAS control	    77
 3-13    WAS control	    85
 3-14    Chemical feed control-flow pacing 	    91
 3-15    Chemical feed control-mass flow pacing  	    92
 3-16    Chlorine feed control-flow proportional	    96
 3-17    Chlorine feed control-compound loop	    98
 3-18    Chlorine feed control-double C9mpound loop  	    99
                                      VI

-------
                            FIGURES (Continued)


Number                                                                 Page

 3-19    Chlorine feed control-ratioed feedback  	    101
 3-20    Ozonation control 	    106
 3-21    Gravity thickener control 	    112
 3-22    Flotation thickening control   	    117
 3-23    Digester control  	    123
 3-24    Vacuum filter control 	    130
 3-25    Centrifuge control  	    136
 3-26    Roll press control	    141
 3-27    Plate press process diagram 	    147
 3-28    Incineration controls 	    153
 3-29    Incineration offgas handling   	    154

 4-1     Dewatering feed system	    172
 4-2     Manual control	    173
 4-3     Analog control characteristics  	    175
 4-4     Local analog control  	    176
 4-5     Distributed analog control  	    178
 4-6     Central analog control  	    179
 4-7     Central digital configurations  	    182
 4-8     Central digital control  	    183
 4-9     Distributed digital configurations  	    184
 4-10    Distributed digital control  	    185
 4-11    Hybrid control systems  	    187
 4-12    Analog control with digital  data logging  	    188
 4-13    Digitally directed analog control  (DDAC)  	    189

 5-1     Conventional activated sludge with drying beds  	    208
 5-2     Conventional activated sludge with incineration 	    209
 5-3     Liquid train P&ID - 5 mgd	    211
 5-4     Solids train P&ID - 5 mgd	    212
 5-5     Liquid train P&ID - 10 to 300 mgd	    213
 5-6     Solids train P&ID - 10 to 300 mgd	    214
 5-7     Control system panel comparison 	    217
 5-8     General central digital  system configuration  	    221
 5-9     Alternative control system costs  	    225
 5-10    Chemical  feed control methods 	    242
 5-11    Power demand control  	    245
 5-12    Material  and energy costs, June 1978	    247
                                    vii

-------
                              FIGURES (Continued)

Number                                                                  Page

 7-1     Typical constant speed pump interface (maintained contact
           control)	   353
 7-2     Typical constant speed pump interface (momentary contact
           control)	   354
 7-3     Secondary clarification and RAS piping schematic  	   356
 7-4     Control system configuration  	   357
 7-5     Secondary clarification and RAS P&ID	   360
 7-6     P&ID Symbol Legend	   361
 7-7     Variable speed pump interface 	   365
 7-8     Typical incremental controller interface  	   366
 7-9     Typical mounting detail-sludge density analyzer 	   372
 7-10    RAS local control panel	   373
                                    vm

-------
                                  TABLES


Number                                                                  Page

 2-1    Summary of Control Functions 	    24

 3-1    Unit Process or Control Strategy Utilization 	    33
 3-2    Steady State Modulation  	    71

 4-1    Control Comparison Summary 	   191

 5-1    Control Equipment and Wiring Capital Cost Summary  	   228
 5-2    Operations Annual Manhours - Conventional Control  	   230
 5-3    Process Operator Tasks 	   231
 5-4    Operating Attendant II Tasks 	   231
 5-5    Operating Attendant I Tasks  	   232
 5-6    Operational Manpower Assignments 	   233
 5-7    Operator Staffing Comparison - Conventional vs. Centralized
                                                  Control	   235
 5-8    Maintenance Requirements per Component 	   236
 5-9    Central Analog Maintenance Requirements  	   237
 5-10   Central Digital Control Maintenance Requirements 	   238
 5-11   Maintenance Manpower-Conventional Control  	   239
 5-12   Annual Operations & Maintenance Manpower Costs 	   240
 5-13   Annual Material and Energy Costs 	   248
 5-14   Present Worth Analysis 	   249
 5-15   Annual Cost Analysis	   251

 7-1    RAS Input/Output List	   363
 7-2    Instrumentation Summary List 	   368
 7-3    Magnetic Flowmeter and Transmitter 	   371
                                      IX

-------
                          ABBREVIATIONS AND SYMBOLS
PIPING AND INSTRUMENTATION SYMBOLS
     The legend of symbols on page xi was derived from Instrument Society of
America "Instrument Symbols and Identification," (ISA - S5.1, 1973).   These
symbols are used extensively in Section 3 - Control Strategies.
INSTRUMENT AND DEVICE IDENTIFICATION LETTERS
     A list of instrument letter codes is provided on page xii.   These
letter codes appear in the so-called "balloons" on the piping and
instrumentation drawings (P&ID's).  For example, the designation FIC  is
decoded as follows:  The first letter is identified as "flow" from the first
column; the succeeding letters represent "indicate" and "control" from the
second column.  Additional gramatical modifiers must be added to form a
complete designation, "flow indicating controller."
MISCELLANEOUS ABBREVIATIONS AND SYMBOLS
CL£    chlorine
CPU    central processor unit (computer)
CRT    cathode ray tube
C/S    constant speed
DO     dissolved oxygen
FB     feedback
I/F    interface
I/P    current to pneumatic converter
LEL    lower explosive limit
MUX
02
SEQ
SP
V/S
A
x
multiplexer
oxygen
sequence control
setpoint
variable speed
differential
multiply
divide
extract square root

-------
              P a ID  LEGEND OF SYMBOLS
                 o
                 LOCALLY
                 MOUNTED
               BACK OF PANEL    PANEL
                 MOUNTED     MOUNTED
           ORIFICE
         FLOWMETER
                        VENTURI
                     ROTAMETER
     MAG OR SONIC
     FLOWMETER
                   EQUIPMENT SYMBOLS
COMPRESSOR     PUMP      BLOWER      MOTOR

                    VALVE BODY SYMBOLS
                                             WEIR
      GLOBE
       BALL
                              PLUG
                                             FLUME
BUTTERFLY  KNIFE/GATE
      PINCH
      CHECK
                              SLUICE    BACKPRESSURE  PRESSURE
                              GATE     REGULATOR   REDUCING
                                                 REGULATOR
                     VALVE  ACTUATORS
 T        9
DIAPHRAM
ELECTRIC
ROTATING
                                                T
                       SOLENOID  PNEU CYLINDER  ELECTRO  PNEU. DPHRM
                                W/PILOT VALVE  HYDRAULIC  W/POSTIONER
                  SIGNAL LINE  SYMBOLS
                                            XXX
          ELECTRIC    PNEUMATIC    SONIC OR    CAPILLARY
                              ELECTROMAGNETIC   TUBE
                             XI

-------
              INSTRUMENT & DEVICE IDENTIFICATION
LETTER    FIRST LETTER
SECOND AND SUCCEEDING LETTERS
   A      Analysis
   B      Burner Flame
   C      Conductivity
   D      Density
   E      Voltage (EMF)
   F      Flow Rate
   G      User Choice
   H      Hand (Manual)
   I      Current Elec)
   J      Power
   K      Capicator
   L      Level
   M      Motor
   N      Moisture
   0      Torque
   P      Pressure or Vacuum
   Q      Quantity or Event
   R      Radioactivity
   S      Speed or Frequency
   T      Temperature
   U      Multi variable
   V      Valve or Damper
   W      Weight or Force
   X      Vibration, Motion
   Y      Computer
   Z      Position
Alarm
Close or Decrease
Control
Open or Increase
Primary Element
Failure

High
Indicate
Light
Control Station
Low
Operate or On/Off
Start/Stop or Open/Close
Overload

Totalize
Recorder
Switch
Transmitter
Multifunction
Valve  or  Damper

Excess
Relay or Compute
Drive Actuate or
Final Control Element
                            xii

-------
                               ACKNOWLEDGMENTS


     Sincere thanks is offered to the management and operating staffs of the
treatment plants visited for the purposes of gathering information
utilized in this report.  During the execution of this report, over seventy-
five facilities were visited.  It would be impossible to acknowledge each
group individually.  Certain people and organizations did contribute an
inordinately large amount of time and information.  The contributions of the
following individuals in particular are gratefully acknowledged.


Mr. Dale Bergsted and Ms. Joanne Hart of the Metropolitan Minneapolis-
     St. Paul Wastewater Treatment Plant.

Mr. Timothy McAloon, Mr. Richard Fries and Mr. Edward Pytel of the
     Metropolitan Chicago Sanitary District.

Mr. John Nelson of the Metropolitan Denver Sewage Disposal District.

Mr. Stanley LeSeur, Mr.  John Warren and Mr.  Michael  Mathews of the Hillsboro,
     Oregon Unified Sewerage Agency.

Mr. Gerald Seymour of the Metropolitan Cincinnati Sewer District.

Mr. Andy Letterman of the Springfield, Missouri Waste Treatment Plant.

Mr. Paul Papke of the Central Contra Costa Sanitary  District.

Mr. Edward Becker of the San Jose-Santa Clara Water  Pollution  Control Plant.

Mr. Phillip Habrukowich  of the Ocean County Utilities Authority.


     In addition to all  of those mentioned above, the authors  wish to
acknowledge the guidance and assistance provided by  Joseph Roesler, project
officer during the initial part of the activity.
                                    xiii

-------
                                  SECTION 1

                           INTRODUCTION AND PURPOSE
     A primary method for achieving cost effective abatement of water
pollution is optimization of process and system control in wastewater treat-
ment plants.  Process control on a real-time basis is of vital concern
because the quantity and quality characteristics of municipal wastewater
exhibit significant time variation.

     In the past, the overall economic framework of the water pollution
control industry has not favored the same methods of achieving process
control optimization as have been utilized in the chemical process indus-
tries.  Historically the water pollution control industry has favored high
capital expenditure for oversized or redundant units, and employment of
relatively large operational staffs of low to semi-skilied operators.
Recent tightening of effluent discharge standards, and the relative cost of
materials, construction labor, opertional labor, electronic equipment,
chemicals and power have forced a reassessment of the traditional tech-
niques.  Cost effective process control can now be best achieved by the
techniques of the chemical process control industry, i.e., design of process
unit capacity "close to the limit," and the use of small highly skilled
operational staffs, aided by the extensive use of instrumentation and
automation.

     The key factor in this new process control optimization philosophy is
the effective application of instrumentation and automation techniques.
Unfortunately, in many cases initial applications of instrumentation and
automation in the water pollution control field were less than roaring suc-
cesses.  Many reasons for significant failures in these initial  installa-
tions have been identified (1) and much activity is now underway to correct
these situations.  However, the damage has been done such that instrumenta-
tion and automation for process control suffers from a poor reputation in
the water pollution control industry.

     Among the major problems identified as the cause of failures in highly
instrumented and automated wastewater treatment plants are:

     1.   Lack of knowledge of and experience with the application of
          instrumentation equipment and automation in the sanitary
          engineering profession.

-------
     2.   Lack of understanding of the correct techniques of process control
          on the part of design engineers and users.

     3.   Misapplication of otherwise acceptable sensors and instruments.

     4.   Lack of acceptable instruments and sensors.

     5.   Improper maintenance of instrumentation and automation equipment.

     6.   Lack of identified proven control strategies for some processes.

     7.   Poor coordination between the control engineers and the other
          engineering disciplines involved in design of wastewater treatment
          systems.

     In fact, it is possible with the present level of technology and engi-
neering knowledge to successfully design, construct and operate a highly
instrumented and automated cost effective wastewater treatment plant, capa-
ble of meeting virtually all discharge standards.  Indeed there have been
several such plants put on stream in the last few years.  A basic problem is
putting the information about this technology and these engineering tech-
niques in the hands of the user community.

     The USEPA has actively pursued this educational goal by supporting the
development of three previous publications on instrumentation and automa-
tion.  In October of 1976, a report (2) was issued as a result of a compre-
hensive survey of wastewater treatment plants which documented the status of
treatment plant instrumentation and automation.  In December of 1976, a
second report (3) was issued which analyzed potential applications of
instrumentation and automation at wastewater treatment plants.  Various
proposed automated strategies were evaluated with respect to their technical
and economic validity.  In June of 1977, a third report (4) was issued which
completely examined dissolved oxygen control.  The last two reports are
written as technical documents for instrument and control system engineers.
In that these reports are the first of a kind, one can conclude that this
process industry is just beginning to realize the benefits of instrumenta-
tion and automation.

     This document is the fourth in this educational series.  Its specific
aim is to present to the sanitary design engineer the up-to-date information
required to adequately cope with the design of activated sludge plants which
will be equipped with cost effective instrumentation and automation.  The
objectives of this document are to provide a base of information, procedures
and reference material to designers, users and regulatory agencies in an
easily referenced handbook to aid them in making decisions regarding equip-
ment selection or specification for automation.

     The manual is divided into seven sections organized to guide the
consulting engineer in control "system" design for wastewater treatment
plants.  The objective for each section of this manual is as follows:

-------
PROCESS CONTROL

     This is a discussion of basic concepts of automatic process control.
The objective is to acquaint the reader with terminology and fundamentals,
and to provide examples of control relevant to wastewater treatment.

CONTROL STRATEGIES

     This section is a compilation of documented process control strategies
that are currently in use.  The objective is to summarize the generally
accepted, successful and field verified control strategies for specific unit
processes and varying plant capacities.

ALTERNATE CONTROL APPROACHES

     The objective of this section is to introduce the consulting engineer
to the alternate methods of control implementation.  Various manual and
automatic equipment configurations are described and important features such
as reliability, flexibility and expandability are discussed in each case.

COST EFFECTIVE ANALYSIS

     Cost is a consideration in every selection process.  The objective of
this section is to detail a method for preparing a cost effective analysis
by performing such an analysis on three alternate control systems for seven
plant sizes.

AVAILABLE INSTRUMENTATION

     The common element of each alternate control system is measurement of
the process variable.  Flow, pressure, temperature, etc. must be sensed and
converted to a signal representative of the measured variable for trans-
mission to the devices controlling the process.  The objective of this
section is to establish an appreciation for the capabilities and limitations
of instrumentation and to serve as a guide for the selection of instruments
for specific applications.  For each instrument, a minimum preventive main-
tenance program is suggested to ensure the instrument will remain
operational.

DESIGN GUIDE

     The objective of this section is to consolidate the information pre-
sented in the preceeding sections into a procedural guide demonstrating some
of the aspects of control system design for wastewater treatment processes.
The guidelines are intended to be used by design engineers, users and regu-
latory agencies in the proper approach for control system design.  Attention
to detail is stressed.  Examples are included with special emphasis on
explaining the documentation and drawings which are necessary to convey the
engineer's and the user's requirements and desires.  Checklists are provided
for guidance.

-------
RECOMMENDED FUTURE ACTIVITIES

     The objective of this section is to present recommendations for further
studies and research which will advance the application of instrumentation
and automation in the wastewater treatment industry.  The goal is to multi-
ply the benefits of previous research by performing additional work which
will broaden the use and value of this and other reports dealing with auto-
mation of wastewater treatment facilities.
                                      4

-------
                                  SECTION 1
                                  REFERENCES
1.   Research Needs for Automation  of Wastewater Treatment Systems,
     Proceedings of a workshop held at Clemson,  SC,  Sept.  23-25,  1974.

2.   Instrumentation and Automation Experiences  in Wastewater-Treatment
     Facilities, EPA-600/2-76-198.   October,  1976.

3.   Selected Applications of Instrumentation and Automation in
     Wastewater-Treatment Facilities, EPA-600/2-76-276.  December,  1976.

4.   Design Procedures for Dissolved Oxygen  Control  of Activated  Sludge
     Processes,  EPA-600/2-77-032.   June,  1977.

-------
                                  SECTION 2

                               PROCESS CONTROL
 INTRODUCTION
     This section will introduce basic concepts of automatic process control
including specific application examples relevant to wastewater treatment.
No control theory background is assumed in the presentation.  A formal
approach to the subject requires the use of differential equations and
Laplace transform techniques, but this is beyond the scope of this discus-
sion.  A major objective of this section will be to provide an understanding
of commonly used control terminology.  Formal definitions of the terms used
in this section are listed in the glossary at the end of this manual.

REASONS FOR PROCESS CONTROL

     Control can be considered as some action taken to maintain the objec-
tives of operation by balancing the supply and demand of a process over a
period of time.  Variations in operating conditions can take many different
forms and can occur at any point within a process.  A few examples of these
variations, more coirmonly referred to as "disturbances," include supply and
demand load changes, changes in flowstream quality, and changes in ambient
conditions.  Satisfactory operation can sometimes be achieved with only
minor and infrequent adjustments to correct or limit the deviation of
measured values from some selected reference.  In this situation, observa-
tion and manual corrective adjustment is often satisfactory.  In most cases,
however, it becomes impractical or impossible to manually control the
process within acceptable tolerances due to factors such as operator
fatigue, quantity and complexity of control decisions required, and the in-
ability of an operator to make adjustments quickly and in a consistent
manner.  An automatic controller is required to overcome these problems.
The controller is a device that accepts a signal representing the variable
to be regulated, compares it with a setpoint which is a reference source
representing the desired level of operation, and generates an output to a
control device that influences the variable to be regulated.

-------
CONTROL LOOPS

Open-Loop Control

     A generalized block diagram of an open-loop control system  is shown  in
Figure 2-1.  The final control element (modulating valve, variable speed
pump, etc.) is set at one point within its operating range or operated
according to a fixed program  (time clock operation of primary sludge  pump-
ing, for example).  The distinguishing feature of open-loop control is that
a continuous measurement of the variable to be controlled is not  available
or is not connected to the controls so that there is no assurance that the
control objective is actually being achieved.  Control will be satisfactory
only if the control element is properly set or programmed for a particular
set of process conditions and the conditions remain unchanged during  opera-
tion.  If the original conditions are disturbed, operator intervention will
be required to make the necessary adjustments to maintain balance within  the
process.  Open-loop control is satisfactory for noncritical applications
where conditions do not vary significantly and close regulation of the
process is not required.
                                  DISTURBANCES

CONTROL
ELEMENT

MANIPULATED
VARIABLE

/
1 i
PROCESS



      ACTUATOR
        I
      CONTROL
        T
                       Figure 2-1.  Open-loop control
     A manually adjusted variable speed pump control is an example of open-
loop control because typically the actual speed is not monitored to provide
correction to the control if a change in suction or discharge head should
cause the pump speed to drift from the speed setting selected.

-------
Closed-Loop Control

     Figure 2-2  depicts  a block diagram of a closed-loop control system.   In
this case, a  continous measurement of the controlled variable  is made  and
routed to the controller.   The controller compares this measurement or feed-
back signal with the  setpoint and any resulting deviation or error  is  used
by the controller to  generate an output to apply corrective action  to  the
final control element.   The term "closed-loop" describes the path that is
formed about  the process by the controlled variable measurement, the con-
troller, and  the output  to the control element.  The affects of any change
in the system propagate  around the loop.  A change in the measurement  causes
the controller output to change, the output change causes some condition of
the process to change, the change in the process affects the controlled
variable, and so on.
                                  DISTURBANCES
      CONTROL
      ELEMENT
MANIPULATED
 VARIABLE
PROCESS
               CONTROLLER
                 OUTPUT
                 SIGNAL
                                   CONTROLLER
CONTROLLED
 VARIABLE
                                  FEEDBACK
                                   SIGNAL
                                     SETPOINT
                       Figure 2-2.   Closed-loop control.
     The flow control  system shown schematically in Figure 2-3  is  an  example
of closed-loop control.  The objective is to maintain the flow  rate at  some
desired value by  adjustment  of  the modulating valve.  In this example,  flow
is the controlled variable used as feedback to the controller.  The valve
position is manipulated  by the  controller to reduce or eliminate any  devia-
tion of the flow from  the setpoint value.  The valve position is therefore
referred to as the manipulated  variable.
                                      8

-------
                                    SETPOINT
          FEEDBACK
          SIGNAL
CONTROLLER
 OUTPUT
 SIGNAL
                                   CONTROLLER
                                                       i

FLOW
XMTR
1^

ACTUATOR
C
L-1 f\

MANIPULATED VARIABLE
(VALVE POSITION)
                                                            CONTROLLED VARIABLE
                                                                (FLOW RATE)
                       Figure 2-3.  Flow control  loop.
     There is the potential for instability  (cycling or  oscillation of the
controlled variable with increasing amplitude)  in  closed-loop  control.  The
system will become unstable if the controller acts to  augment  an  imbalance
in the system rather than oppose it.  This condition can occur if the char-
acteristics of the controller are improperly matched with the  characteris-
tics of the control loop.

Dynamic Characteristics of Control Loops

     Each element of a control loop including the  sensor,  transmitter, con-
troller, actuator, control element and largely  the process  itself,  requires
time to react to a change in its input.  The response  of the complete
control loop will be the sum of the responses of all elements  in  the loop.
It is necessary to understand two basic types of time  elements  in order to
correctly match the controller characteristics  with those of the  process.

     Deadtime is that period of time which elapses between  the moment a
change is introduced into an element and the moment a  response begins to
occur.  Absolutely no response can be detected  during  this  time period.  An
example of deadtime in a process is that which  is  present in a chlorine con-
tact chamber.  If chlorine is injected into the influent at the chamber
entrance and the chlorine residual is measured  in the  chamber  effluent,
there will be a period during which a change in the chlorination  rate is un-
detectable.  This is due to the time it takes for the  affected  portion of
the flow to traverse the length of the chamber.  Control  of the chlorination

-------
rate based on the residual measurement will be poor at best because there is
always a lack of current sensor information to direct the control device
metering the chlorine.  This illustrates the control problems typically en-
countered in any process where significant deadtime exists in the control
loop.

     A more common type of time lag is that due to capacity involving the
storage of material or energy.  A wet well is a simple example of the occur-
rence of capacity within a process.  Unlike the previous example involving
deadtime, an increase in flow into or from a wet well results in an immedi-
ate though small change in the rate at which the level rises or falls.  This
system is considered to have a delay due to capacity because the level is
slow to respond as compared to the immediate change in flow rate that was
introduced.  Another example of the effect of capacity is evident in the
control of dissolved oxygen in an(aeration basin by regulating the aeration
air flow supply.  The DO in the large volume contained in the basin will
change very slowly even if a large change is made in the air flow rate.

     An index called a time constant is used to characterize the capacitive
response of a control loop element.  The time constant is defined as the
time it takes after a step change is made in an element's input for the
response of the element to reach 63.256 of its final equilibrium value.  The
response indicated in Figure 2-4 could represent the behavior of a process
variable, a sensor or a control device.
            INPUT

                                                         TIME CONSTANT
                                                         DEADTIME
             Figure 2-4.  Typical control loop element response.
                                     10

-------
AUTOMATIC CONTROLLERS

     The pressure control  system shown  in Figure 2-5 will be used as an
example to explain the operation of  an  automatic controller in more detail.
The objective of the control  system  is  to maintain a tank pressure of 60 psi
by adjusting the high pressure  air inlet  valve.   We will assume that strict
regulation of the tank air pressure  is  required  for proper operation of the
processes using the air supply.
                                     SETPOINT
 UNREGULATED
  AIR SUPPLY
   SOURCE
                                                PRESSURE  INDICATING CONTROLLER


                                                    	1
                                                                   PRESSURE
                                                                  TRANSMITTER
REGULATED
AIR SUPPLY
 SOURCE
                     Figure 2-5.  Pressure  control  loop.
On-Off Control

     The simplest controller  action that we might apply in this example
would be to fully open or close the air supply valve,  depending on whether
the tank pressure is below or  above 60 psi.   When the  final control element
is driven to one extreme or the other in response to a comparison of the
controlled variable with a setpoint, this  is  an example of two-position or
on-off control.  Using on-off  control, the pressure control system responds
as indicated in Figure 2-6.  When the pressure falls below the setpoint, the
controller opens the valve.  The tank pressure builds  up to 60 psi and
begins to excered the setpoint  before the control system can respond to close
the valve.  The pressure peaks and begins  to  fall due  to demand loading.
When the pressure falls below  60 psi, the  cycle repeats itself.
                                      11

-------
            CONTROLLED
             VARIABLE
            MEASUREMENT
              CONTROL
              ELEMENT
              ACTION
CONTROL
 POINT
                                               TIME
                         Figure  2-6.   On-off control.
     On-off control causes the controlled  variable to  oscillate about the
setpoint with an amplitude and frequency that  is  dependent on the capacity
and time response of the process.  The  amplitude  and frequency will  remain
the same as long as the load on the process  does  not change.   If the compo-
nents of the pressure control system were  idealized so that the valve could
be made to open or close instantaneously in  response to infinitesimal
errors, the tank pressure error would approach  zero but the frequency of
control valve cycling would approach infinity.

     In general, on-off control provides satisfactory  regulation only when
the process has a high capacity in relation  to  the load demand.  A large
process capacity also acts to buffer the input  variations  resulting  from a
control action that drives the final control element from  one extreme to the
other.

     A familiar example of an application  of on-off control is a thermostat-
ically controlled heater.  On-off control  is not  satisfactory in the.pres-
sure control example because the control action causes excessive valve wear
and the controlled variable (tank pressure)  may not be well regulated
depending on the amount of load in relation  to  the capacity of the tank.
                                      12

-------
     A variation of on-off control called differential-gap control is often
used to reduce the wear on the final control element.Asystem response
curve for this type of control is indicated in Figure 2-7.  A band is
defined about the setpoint value such that when the controlled variable
drops below the lower limit, the valve is opened.  The valve remains open
until the upper limit of the band is exceeded.  The valve then remains
closed until the controlled variable again drops below the lower limit.
Differential-gap control is commonly used in noncritical level control
applications where the only requirement is to prevent a tank from over-
flowing or running dry.
          CONTROLLED
           VARIABLE
          MEASUREMENT
          CONTROL
          ELEMENT
          ACTION
_r
                                    UPPER CONTROL
                                       LIMIT


                                    LOWER CONTROL
                                       LIMIT
                                                        OPEN
                                                        CLOSE
                                            TIME
                Figure 2-7.  Differential  gap on-off control.
     On-off control is the simplest and least expensive form of automatic
feedback control, but it is often unable to provide the regulation required
or is impractical due to the characteristics of the process or control
system.  Methods of control which direct a final  control  element to interme-
diate points within its range (referred to as throttling control) were
developed to resolve these problems.   The remainder of this section on  auto-
matic controllers will describe methods of generating throttling control
action.

Proportional Control

     In proportional control, the controller positions the final control
element based on the amount of deviation of the controlled variable from  the
setpoint.  A fixed linear relationship is then established between the  con-
trolled variable measurement and the  controller output as indicated by  the
                                     13

-------
input-output  line in Figure 2-8.  The  line represents all of the  possible
operating points of the system.  The slope of the line represents the amount
of proportional  action provided by the  controller, referred to  as the pro-
portional gain«   This gain is the ratio of the change in controller  output
to tne cnange in the input.
       VALVE
     POSITION
      % OPEN
  PROPORTIONAL
   CONTROLLER
    OUTPUT
100  -
                 80  -
                60  -
                                    30PSI MEASURMENT INPUT POSITIONS VALVE AT 70% OPEN
                20  -
                                                PS I INPUT POSITIONS VALVE AT AOI OPEN
                                                        SLOPEJ=1   (CONTROLLER GAIN)
                                          60
                                 80
100
PS I
                              CONTROLLED VARIABLE
                                 MEASUREMENT
    Figure 2-8.   Proportional control  input-output relationship,  gain = 1.
                  (Pressure control example)
     Proportional  Gain =
                                   A Output
                           A(setpoint  - measurement)
     Many controllers have this gain  adjustment expressed as proportional
band which  is  the  percent change of the  controlled variable measurement that
vRTT cause  the controller output to swing  from 0 to 100%.  Proportional band
is related  to  proportional gain as follows:
                                 100
     Proportional  Gain =
                          % Proportional  Band
                                       14

-------
      If the measurement  range  in  our example is 0 to 10U psi, a valve posi-
tion  correction  of  1%  for  a  deviation of 1  psi  represents a gain of 1 or a
proportional band of 100%.   A  gain  of 1  was arbitrarily chosen to be repre-
sented in Figure 2-8.  Figure  2-9 shows  the effect of varying the gain set-
ting  on the input-output relationship.   At  a gain of zero,  changes in the
controlled variable measurement do  not affect the valve position so there is
no control.  If  the controller could be  adjusted for infinite gain, on-off
control action would result  because infintesimally small deviations from the
setpoint would drive the valve fully open or closed.
                                      ON-OFF
                                     CONTROL
                 100
                  80  -
                  60  -
    % OF CONTROLLER
     OUTPUT RANGE
                  20  -
                  0
                                                   G=0
                                                     G-.5
NO CONTROL
                                                                      G= GAIN
                                         G= oo
                     0      20     40     60     80     100

                                 % OF MEASUREMENT RANGE

     Figure 2-9.  Effect of gain adjustment on  input-output relationship.


     For consistency, all of the proportional control  input-output charac-
teristics illustrated in this section are shown  as  having  negative slopes
indicating that the output decreases as the measurement  value  increases.
These controllers are sometimes classified  as "reverse acting."   Controller
characteristics having a positive slope are representative of  "direct acting
controllers."

     An important characteristic of  proportional control  is that  it can
never return the controlled variable back to the setpoint  after a load
change.  This is difficult to understand until  one  recalls that proportional
control produces corrections proportional to deviations.   No correction will
be produced unless there is some deviation.  It  is  obvious that any load
                                     15

-------
change wil.l  require  some control element movement to correct for  it.   There-
fore, after  any  load change there must always be some deviation remaining,
otherwise  there  would be no proportional action to provide correction.   The
residual steady  state error that is inherent in proportional control  is
called offset.   The  occurrence of offset can be demonstrated in our pressure
control system example by referring to Figure 2-10 in the following discus-
sion.  Assume that the process load conditions are such that the  controlled
variable is  at the setpoint and the valve is positioned at 40% open  (point  A
on the input-output  line).   Let us now disconnect the controller  output  from
the valve  so that  it remains at 40% open.  A load change occurs that  causes
the tank pressure to drift  to 20 psi and remain at this point.  If the con-
troller is now reconnected  to the valve, we see that the input-output
relationship of  the  controller dictates that the valve position will  be
driven to  80% open for a tank pressure measurement of 20 psi (point B).
When the valve position increases to the 80% open position, the tank  pres-
sure increases from  20 psi.  This reduces the tank pressure deviation so
that the controller  no longer calls for the valve to remain at 80% open.
The operating point-begins  to move down along the line toward point A.   As
the tank pressure continues to increase, the pressure deviation is reduced
further which in turn reduces the controller's corrective action.  Finally,
a new equilibrium point is  reached (point C) where there is just  enough  pro-
portional  action due to the remaining deviation to balance the effect of the
load disturbance.  The tank pressure and the offset will remain constant for
as long as the current load condition continues to exist.
       CONTROLLER
        OUTPUT
                100 %
                   80 -
                   60 -
              A= INITIAL OPERATING POINT


              B= CONTROLLER RE-CONNECTED
                AFTER A LOAD CHANGE OCCURS


              C= FINAL EQUILIBRIUM POINT
                    (SEE TEXT)
      BIAS
      POINT
                   20
                            20
40      60      80     100*
                                           t
                         CONTROLLED
                          VARIABLE
                         MEASUREMENT
                                        SETPOINT

    Figure 2-10.  Proportional  control  offset (pressure control  example).
                                      16

-------
     Where operating conditions  are  such that the maximum offset that occurs
is within the permissible  variations  of  the  controlled variable, a propor-
tional controller will provide satisfactory  control.   In an attempt to
reduce offset, it seems logical  that  all  we  need do is require the control-
ler to provide more correction for the same  amount of error.   This repre-
sents an increase in the proportional gain.   This will certainly be effec-
tive in reducing the offset, but it  has  the  effect of making the system far
too sensitive to disturbances.   At high  gain settings, the controller tends
to overcorrect severely when a disturbance occurs and results in a system
that oscillates in an unstable manner.   Figure 2-11 Indicates the response
of the pressure control system to a  large load disturbance when three
different gain settings are used.  Note  that the high gain setting results
in the least amount of offset but also wider and more prolonged cycling of
pressure before stable control is achieved.

                               PROPORTIONAL MODE SETTING

                                 — —- MODERATE
                                 	HIGH
         OUTLET
         PRESSURE
         (PS I)
                   60
                   50
                                         SETPOINT
OFFSET
                        STEP  INCREASE
                         IN  LOAD
                         OCCURS
                                                          TIME
              Figure 2-11.  Proportional mode  control  response.
                            (Pressure  control  example)
     Proportional controllers are usually  provided  with  an adjustment to
allow the output to be set at approximately  50% whenever the  controlled
variable equals the setpoint.  As the  controlled  variable measurement devi-
ates from the setpoint, the output will  vary from this reference level (or
bias) by a proportional amount.  The output  of the  controller is expressed
by the following equation:

     Output = Proportional Gain  (Setpoint  -  Measurement) + Bias

This adjustment can be used to eliminate offset.  An  operator could adjust
the controller output bias to move the final control  element  as much as
                                      17

-------
 needed  to return the controlled variable  to  the setpoint value.  A bias
 adjustment or manual reset is represented in Figure 2-12 by an upward shift
 of the  input-output line without changing its  slope.   In this way, the same
 value of the controlled variable measurement can be made to produce a
 different controller output than when the line was  fixed.
                                     SETPOINT
              100
               80 -
     CONTROLLER
      OUTPUT    6o _
               20 -
A=  INITIAL EQUILIBRIUM POINT

B=  OPERATING POINT MOVED TO SETPOINT
        (BY MANUAL RESET)
                                                          CONTROLLER OUTPUT BIAS
                                                          INCREASED (MANUAL RESET)
                                                     100
                              CONTROLLED VARIABLE
                                MEASUREMENT
              Figure 2-12.  Elimination of offset  by manual  reset.
Integral Control

      If offset  cannot be tolerated, the use of  a  high  proportional gain to
reduce offset may be unacceptable because it  can  cause the system to become
unstable.  The  other alternative, manual reset, can  be used but is almost
always impractical  because the operator must  readjust  the controller every
time  a significant  disturbance occurs.  An additional  control  mode referred
to as integral  mode or automatic reset is required to  automatically perform
the reset function.  The output of a controller due  to integral action is
the time integral  of the error so that the output becomes a function of both
the magnitude and duration of the error rather  than  a  function of the magni-
tude  only as in proportional  control.  Because  error is integrated, the
controller output will  continue to change as  long as a difference exists
between the controlled variable input and the setpoint.  The smallest error
will  cause the  controller output to ramp up or  down, driving the control
element as much as  is needed  to eliminate the error.
                                      18

-------
     Figure 2-13 demonstrates  how  an  integral mode  controller responds to a
step change in the controlled  variable  input.   It  is  important to note that
only the response of the controller itself  is indicated.   The controller
output has been disconnected from  the process (i.e. the control  loop has
been opened) so that the controller input remains  unaffected by the output.
               CONTROLLER
                 INPUT
               CONTROLLER
                 OUTPUT
                                              SETPOINT
ERROR
                                                       TIME
                Figure 2-13.  Integral mode controller  action.
     Integral action is very rarely used by itself because  it  is  slow to
act.  It is most often combined with proportional action  so that  the  two
modes act simultaneously.  Figure 2-14 indicates the output of  a  proportion-
al plus integral controller in an open-loop response to a step  change in the
input.  The integral time indicated in the Figure is the  amount of  time
required for integral action to duplicate the amount of output  change due to
proportional action alone.  The controller adjustment for integral  mode is
often calibrated in repeats per minute which is the number of times per
minute the amount of change caused by proportional action is repeated by
integral action.

     Let us now apply proportional plus integral mode control to  our  pres-
sure control example.  Assume that the proportional gain  has been adjusted
to a moderate value that regulates the pressure in a nonoscillatory manner
when a load disturbance occurs.  Figure 2-15 indicates the response of the
system associated with three different values of integral mode  actior, that
are added to the proportional controller.  Note that in each case,  offset is
completely eliminated but the higher values of integral action  cause  the
system to respond with less stability.  Too much integral action  causes the
controller's output to change faster than the rate at which the process can
respond and cycling results.
                                     19

-------
           CONTROLLER
             INPUT
           CONTROLLER
             OUTPUT
                                             SETPOINT
           PROPORTIONAL
             ACTION
                                                        TIME
                                    INTEGRAL
                                     TIME
           INTEGRAL
           "REPEAT"
            ACTION
Figure 2-14.   Integral plus proportional  mode  controller action.
                         PROPORTIONAL SETTING FIXED

                          INTEGRAL  MODE SETTING:
                         	 LOW
                         	— MODERATE
                         —--- HIGH
 OUTLET
PRESSURE
  (PSI)
60


50


1*0
                                                                   SETPOINT
                                                           TIME
                    STEP  INCREASE
                      IN  LOAD
                      OCCURS
Figure 2-15.   Proportional  plus  integral  mode  control  response.
                (Pressure control  example)
                                  20

-------
      Integral mode  action  has a disadvantage that becomes a  serious  problem
in some situations.  For example, during startup of a process, the con-
trolled variable  is often far from the setpoint.  The controller's integral
mode  function interprets this deviation as a large offset error and  rapidly
drives the final  control element to compensate.  By the time the controlled
variable reaches  the setpoint, the integral action may have  built up the
output to the limit of the controller so that  it continues to apply maximum
correction to the control device.  The result  is that the controlled vari-
able  drastically  overshoots the setpoint value and cycles about it for a
prolonged period  of time.  This problem is referred to as reset windup.  To
avoid this problem, it may be necessary to start the process with the inte-
gral mode turned  off until the proportional action brings the controlled
variable into equilibrium.  Automatic means of accomplishing this are some-
times available as an option on automatic controllers.

Derivative Control

     The addition of a third mode of control called derivative or rate is
sometimes desirable in the control of some processes to overcome system
inertia.  Basically, derivative action provides an immediate output when a
deviation begins  to occur, so that the correction supplied to the final
control element is initiallly greater than would normally be provided by
either proportional or integral mode action.  More exactly, the output of
the controller due to derivative mode action is proportional to the rate at
which the controlled variable deviates from the setpoint.

     Derivative mode action will  occur only when the controlled variable
changes so that under steady state conditions,  it makes no contribution to
the controller's  output.  For this reason, derivative mode is used only in
combination with proportional or proportional  plus integral control.   Figure
2-16  indicates the output response of a proportional  plus derivative mode
controller when a steadily increasing input is  applied.  Again, note  that
this is an open-loop response because the controller  output has been  dis-
connected from the control device.  The rate time indicated in the figure is
the time interval by which derivative acTTon 13vances the effect of propor-
tional action upon the final  control  element.   Derivative control  is  often
described as having an "anticipatory" effect because of the way it advances
the action of the controller.

     The example pressure control  system responds to  proportional  plus inte-
gral plus derivative control  as Indicated in Figure  2-17.   It is  assumed
that the proportional and integral mode adjustments  have  been set  at  levels
that are judged to be optimum for  this system.   With  these settings,  three
different values  of derivative mode action are  added  to the controller and
in each case, the response to a step load change is  indicated.   The  addition
of the proper amount of derivative action has  the effect  of stabilizing the
system in a shorter period of time.   As we have seen  in the cases  of  propor-
tional and integral mode control,  excessive derivative action also degrades
the stability of the system.
                                     21

-------
              CONTROLLER
                INPUT
              CONTROLLER
               OUTPUT
             DERIVATIVE-
               ACTION -
SETPOINT
                                           PROPORTIONAL
                                              ACTION
                                                        RATE
                                                        TIME
               TIME
    Figure 2-16.   Proportional plus derivative mode controller action.
                     t
                        PROPORTIONAL AND INTEGRAL SETTINGS FIXED

                             DERIVATIVE MODE SETTINGS:
                            	 LOW
                            —— MODERATE
                            — HIGH

	 V sf r —
1 vCs •^^//
i
i
i
i
i
i
i
t
STEP INCREASE
IN LOAD
OCCURS







TIME



                                                                       SETPOINT
Figure 2-17.   Proportional plus  integral  plus derivative  control  response.
                (Pressure  control  example)
                                       22

-------
 Controller Tuning

     Adjustment of the controller modes to obtain good system response  is
 referred  to as tuning the controller.  There  is a problem  in defining what
 constitutes "good" system response because the criteria will vary for dif-
 ferent types of processes.  For example, offset can be tolerated in  some
 processes but not in others.  It may be desirable in some  cases to have the
 system respond without overshoot and we are not too concerned with the
 length of time it may take for the controlled variable to  reach the  set-
 point.  In other cases, we may require strict regulation that quickly
 returns the system to the setpoint and we are willing to tolerate some  over-
 shoot of  the controlled variable in order to  achieve the faster response.
 The control modes are selected and adjusted to match the characteristics of
 the control loop.  Numerous procedures for controller tuning have been  pub-
 lished that are based on math matical and trial-and-error methods (1)(2).

 Automatic Controller Summary

     Table 2-1 summarizes the information presented on proportional, inte-
 gral and  derivative mode control.

 ADVANCED  CONTROL CONCEPTS

 Cascade Control

     For  some applications, improved control can be obtained through the use
 of two conventional feedback controllers connected in series.  In this  way,
 two complete feedback loops are formed, one within the other.

     To illustrate the need for and the application of cascade control, con-
 sider the dissolved oxygen (DO) control system shown in Figure 2-18 where
 the sensed variation in DO is used to manipulate the position of the air
 valve.  Due to the large process capacity, disturbances in the air flow or
 the influent's demand for oxygen are not immediately sensed by the DO ana-
 lyzer.  Once the DO measurement begins to change appreciably, the control
 system can begin to act to modify the air flow to the aeration basin.  There
 may be so much time lag in the process due to capacity, however, that the
 control is ineffective or unacceptable.  For example,  by the time the effect
 of a disturbance in the air supply pressure is sensed by a change in DO, the
 pressure disturbance may have subsided or the pressure may have changed to a
 third value.  At this time, the correction signal from the controller based
 on DO becomes inappropriate for compensation of the current air supply
 conditions.

     Two sources of disturbances that influence air supply pressure may be
 present.  In a typical multiple tank system where all  of the tanks  utilize a
 common air supply, the changing demands of each tank can disturb the air
 supply pressure (only one aeration  tank is shown in Figure 2-18 for simpli-
 city).  Also,  where multiple blowers are used to match the demands  of the
process, the starting and stopping  of blowers  introduces additional  pressure
 disturbances.


                                     23

-------
                                              TABLE 2-1.   SUMMARY OF CONTROL  FUNCTIONS
                        Mode
        Function
        Operation
       Notes
                   Proportional Action
Restores process equili-
brium by counteracting
disturbance effects.
Repositions the final
control element in
proportion to the magni-
tude of deviation from
setpoint.
Adjustment calibration
is % proportional band
or gain.
Action approaches on-
off control at highest
gain setting.
Action reduces, but
cannot eliminate,
deviations resulting
from load changes.
Used in noncritical
control applications
only.
                    Integral Action
                    (Automatic Reset)
Eliminates steady-state
deviation from setpoint.
Repositions the final
control element according
to magnitude and duration
of deviation from setpoint.
ro
Adjustment calibration
is repeats per minute.
Action stops only when
process is at setpoint.
Sustained deviation
from setpoint will
create a "wind-up" i.e.
control device reaches
an extreme.
Reset time should not
be set faster than the
process deadtime.
                   Derivative Action
                   (Rate)
Reduces time required
for controlled variable
to stabilize.
Repositions the final
control element according
to the rate of deviation
from setpoint.
Adjustment calibration
is rate time.
Acts only when the
control variable is
moving.
Aids in overcoming
system inertia due to
capacity.
Not used in control
loops with prominent
deadtimes.
Not used in control
loops that have a high
noise content.

-------
                          DO SETPOINT
                             I
                                00 CONTROLLER
       AIR
      FLOW
                                                       AT 1 DO TRANSMITTER
                                                       AE 100 SENSOR
v/VX VX VX '
                 VX
              Figure 2-18.  Direct control of  dissolved oxygen.
     Figure 2-19 illustrates the addition of a control  loop  for  air flow
with its setpoint derived from the DO controller.  The  DO controller is  said
to be "cascaded" into the air flow controller.  The DO  control  loop is
referred to as the primary, outer or master loop.  The  air flow  control  loop
is the secondary, inner or slave loop.  With this arrangement, disturbances
in the air supply pressure that would affect the flow are now corrected  by
the inner flow control loop.  Corrective action is now  initiated without
having to wait for the DO to change as was the case under single loop con-
trol.  If a disturbance in the influent demand for oxygen occurs,  it appears
that the response of the cascade control system will be much as before
except the corrective action must pass through two controllers.  It can  be
shown, however, that the addition of an inner feedback  loop  around  the
control valve has the effect of increasing the speed of response and thus
contributes some improvement in the speed of response of the overall system.

     In general, selection of the inner and outer cascade control  loops  for
proper operation and improved control requires that the inner loop  have  a
faster response time than the outer loop, and that the  inner loop  be chosen
as the one affected by the major disturbances in the system.  The  control-
lers are tuned in the same way as single loop controllers.  The  inner loop
should always be tuned first.
                                      25

-------
                          DO SETPOINT

                              I
        AIR
       FLOW
                                 00 CONTROLLER
                                 FLOW
                                SETPOINT


                                   FLOW
                                 CONTROLLER
\S \S
          A1A DO XMTR
                                                       AE 1 DO SENSOR
               Figure  2-19.   Cascade control  of dissolved oxygen.
Feedforward Control

     Nearly all of the  preceeding material  has been devoted to conventional
feedback control which  is  control  based  on  the comparison of the controlled
variable measurement with  its  desired  value and the use of any deviation to
direct the manipulation of an  input  to a process to reduce or eliminate the
deviation.  Feedback control may not be  satisfactory in some processes due
to two disadvantages inherent  in the concept.   The  most obvious disadvantage
is that the control system does  not  act  until  after a disturbance has caused
an error in the controlled variable  to exist.   Less obvious is the fact that
the effect of the corrective action  of the  control  system is not felt until
after the changing process conditions  have  propagated around the entire con-
trol loop.  These disadvantages  are  not  significant in most common process
control applications such  as flow control,  but they severely limit the ef-
fectiveness of feedback control  of processes with significant deadtime.

     A method known as feedforward control  can be used as an alternative to
feedback control.  Feedforward control involves the measurement of one ^r
more inputs to a process that  are prone  to  disturbances and any deviations
in their values are compensated  by. manipulation of  process inputs before the
disturbance affects the controlled variable (see Figure 2-20).  In theory,
if the exact amount of corrective action required can be predicted and cor-
rectly applied, no deviation of  the  controlled variable will ever occur.  In
practice, this is difficult to achieve because all  of the possible sources
of disturbances must be accounted for  and the  effect of the manipulated
variables on the controlled variable must be thoroughly understood.  Feed-
forward control is a form  of open-loop control because the controlled
                                      26

-------
variable measurement  is  not  used  by the automatic controls in the regulation
of the process.  In other words,  the "input"  to the process is measured  and
used for control rather  than the  "output"  product, and there is no propa-
gation of changing conditions  around a loop as in closed-loop control.
         PRIMARY
       DISTURBANCES
                   TRANSMITTER
                                                SECONDARY
                                                DISTURBANCES
t f
I PROCESS INPUT LOAD

SENSOR

i

i
*
                                                 PROCESS
                                                  A
                  V
                CONTROLLED
                 VARIABLE
                                                       MANIPULATED
                                                       '  PROCESS
                                                          INPUT
                    CONTROLLER
                       T
                     SETPOINT
                      Figure 2-20.   Feedforward control.
     As an example of  an  application  for  feedforward control, consider the
chlorination control problem discussed  earlier as an example of a process
having a large deadtime.  Recall that the measurement of chlorine residual
in the chamber effluent was not  an  adequate  basis for control of chlorine
feed due to the deadtime  or transport delay  between the process input and
the controlled variable measurement.  The primary disturbance to which this
system is subjected can be described  as a change in the demand for
chlorine.  Using a feedforward approach,  the rate of chlorine application
could be based on a proportional relationship with a measurement of the
chamber inflow as shown in Figure 2-21.   Provided that the characteristics
of the influent do not vary, and there  are no other disturbances that affect
the chlorine demand, this strategy  would  provide good control of the
chlorine residual.  The fact that the system has no way of correcting for
disturbances other than hydraulic load  changes could represent a serious
limitation.
                                      27

-------
CL DILUTION
2 WATER
FEEDFORWARD
SIGNAL
1 	 *
i
1
1
1
1
t
1
>S
CFEJ
PROCESS riu*i

LOAD •* 	 -J



J t

CHLORINATOR






1













CL2 RE
/'"N INT
CA|J






































1
1
1
1
JL
(AT)
\-~fS XMTR
^-k


CL2 CONTACT CHAMBER cl-2 RESIDUAL
(CONTROLLED
VARIABLE)
          Figure 2-21.  Feedforward (ratio) control of chlorination.
     This control strategy is actually a special case of feedforward control
referred to as ratio control and is very commonly used in processes involv-
ing chemical feecTRatio control is often used in these processes because
there is no practical way to measure the actual controlled variable in order
to apply feedback control.

     In general, pure feedforward control alone is not sufficient if strict
regulation is required.  In these cases it is necessary to add feedback to
obtain the advantages of both types of control; feedforward providing
advance compensation for major disturbances sensed in the process inputs,
and feedback providing a trimming effect to correct for minor disturbances
sensed as variations in the output of the process.
                                     28

-------
                                 SECTION 2
                                 REFERENCES
1.   Murrill, Paul W., Automatic Control of Processes.  International
     Textbook Co., Scranton, PA, 1967.   pp. 345-365.

2.   Soule, L. M., Tuning Process Controllers,  Chemical Engineering.
     December 1, 1969, pp. 101-104.
                                 SECTION 2
                                BIBLIOGRAPHY
Considine, Douglas M., Process Instruments and Controls Handbook, Second
Edition.  McGraw-Hill, New York,  1974,  pp. 18-6 - 18-50.

Johnson, Ernest F., Automatic Process Control.  McGraw-Hill  Book Co., New
York, 1967.

Lloyd, Sheldon G., and Gerald D.  Anderson, Industrial Process Control.
Fisher Controls Company, Marshalltown,  Iowa,  1971.

Principles of Process Control.  Study guide issued by Instrument Society
of America, Pittsburgh, Pennsylvania, ISBN 87664-108-7.

Shinsky, F. M., Process-Control Systems.   McGraw-Hill,  New York, 1967.
                                     29

-------
                                   SECTION 3

                               CONTROL  STRATEGIES
 TECHNICAL APPROACH

 Objective

     In keeping with the purpose of the Design Handbook, this section of the
 report deals with evaluation  and documentation of actual process control
 strategies which are being utilized in the field.  The purpose here is not
 to recommend control strategies, but rather to document strategies that have
 been found to be satisfactory.  All process control strategies presented are
 based on working systems and  document field visits and field observations.
 The descriptions given are purposely brief and to the point.  The control
 strategies are not described  in long complex technological terms, but rather
 in a manner that encourages the reader to examine the processes studied and
 to select the particular process areas of interest.

     It was found to be difficult to define if use of a control strategy
 insured good performance of the process because, in most instances, the
 objective of the "overall" control loop is to maximize the efficiency of a
 system of processes.  In addition, in many instances a measurement of the
 actual controlled variable is not available.  For these reasons, it is sel-
 dom possible to provide documentation proving that a process control
 strategy is successful.

     During each plant interview data was requested to illustrate the per-
formance of a control strategy.  Publication of such data is beyond the
 scope of this report.  It can be said, however, that data collected in a
 timely manner, enabling a meaningful  determination of performance, is only
rarely available.

     The strategies are not related to specific plants because the actual
physical  makeup of equipment  at the various plants differs.  The reader>s
cautioned to look at the technical approach documented and interpret this
 approach in light of the various configurations of equipment found in the
field.

Processes to be Studied

     The processes chosen to be evaluated and documented were indicated by
EPA to be of specific importance because of the present and/or future
expected frequency of their use.  There are some processes wffich were not


                                      30

-------
addressed because it was felt that the extent of their use is relatively
low.  The processes studied include all of those utilized in a typical
activated sludge treatment plant.  Therefore, both liquid treatment control
strategies and solids treatment control strategies are included.

     The specific processes studied and documented in this report are as
f o 11 ows:

                                               Liquid    Solids
                                               Train     Train      Page

    Interceptor Storage                          X                    36
    Flow Equalization                            X                    40
    Plant Lift Stations                          X                    44
    Bar Screening                                X                    47
    Grit Removal                                 X                    50
    Primary Clarification and Sludge Pumping               X          54
    Hydraulic Flow Control                       X                    59
    Dissolved Oxygen & Blower Control            X                    64
    Cryogenic Oxygen Generation                  X                    68
    Return Activated Sludge                      X                    74
    Waste Activated Sludge                       X                    83
    Chemical Feed                                X                    89
    Post-Chlorination                            X                    94
    Ozonation                                    X                   103
    Gravity Thickening                                     X         110
    Flotation Thickening                                   X         115
    Anaerobic Digestion                                    X         120
    Vacuum Filtration                                      X         128
    Centrifugation                                         X         134
    Roll Press Dewatering                                  X         139
    Plate Press Dewatering                                 X         145
    Incineration                                           X         151
    Return Liquors                                         X         161


     Again, these unit processes represent either frequently found unit
processes in the wastewater industry or processes which are believed to be
increasing in use.  There are processes that possibly should be added to
this list and the list of unit processes is growing every day.   The point is
that this report is not intended to be exhaustive.  The intent  is to
document workable control strategies that exist today.  Hopefully with
future research and future studies additional processes can be  documented
for other equipment which is not addressed here.

Method of Data Collection and Process Control Evaluation

     The many waste treatment organizations in the country were canvassed
for organizations that had multiple facilities and multiple sizes of plants
in addition to a full range of the unit processes under study.   These orga-
nizations were further evaluated in terms of their application  of process


                                      31

-------
control to their particular plants and field visits were arranged.  An
interview form was developed which ensured consistency in the plant inter-
views.  The purpose of the interview was to gather information relating to
how the control strategy was implemented and how it performed.  The inter-
views were in-depth analyses.  At least one day was spent at each facility.
The day was spent discussing control strategies with process control engi-
neers and operators.  The interviews concentrated on use of instrumentation,
use of observations, use of laboratory data and specifically what an opera-
tor or process control engineer does to keep the process under control.

     Table 3-1 is a matrix showing the wastewater treatment organizations
which were visited to gather data for this section and details the unit
processes utilized by these organizations.  Most of the organizations visi-
ted have multiple facilities.  These are listed with the wastewater treat-
ment organization.  The Chicago Sanitary District, as an example, has seven
treatment facilities listed, all of which contributed data to the evaluation.

     Plant visits began with a tour of the facility.  After the tour of the
physical facility, the interview took place using the interview form.  Typi-
cally a process control engineer and the shift operator were interviewed.
In-depth questions were asked regarding their technical approach to main-
taining process control on a unit process basis.  Performance data was
requested.  Remembering that process control strategies, especially in the
wastewater industry, are not just execution of a specific task, but include
many observations, the interview was directed at what observations the
process control engineer or operator associated with his evaluation of a
workable control strategy.

     In summary, the method of evaluation of a specific control strategy for
a unit process included in-depth discussion with technically competent
engineers and operators, a plant visit to verify and validate the equipment
utilized, and a technical discussion of operator observations which validate
and verify the viability of any control strategy.

Method of Documentation

     The method of documentation developed is intended to explain in a con-
cise fashion the technical approach to each process control strategy.  The
most important criterion established for the documentation of the strategies
was clarity.  Each strategy is described in a consistent manner with a level
of technical detail encouraging a consulting engineer or government official
to make reference to this section for guidance in selection of a control
strategy.  Because of the varying differences in physical configuration, it
was not possible to document the actual equipment at each plant but rather
the technical approach of the control strategy.  In this report, each
control  strategy is broken down into eight sections.  Those sections and a
brief explanation of the purpose of each section are as follows:
                                      32

-------
TABLE 3-1.  UNIT PROCESS OR CONTROL STRATEGY UTILIZATION
ORGANIZATION
Minneapol is
St. Paul
(MWCC)
Chicago, MSO
Metro Denver
Unified
Sewerage
Agency
C incinna t i
MSD
City of
County of
City of
Ocean
County
Sewerage
Authority
TREATMENT
FAC 1 L 1 TY
Metro
Seneca
Blue Lake
Hastings
Chaska
West-Southwest
Calumet
North Side
O'Hare
Eagan
Hanover
Streamwood
Lemon t
District No. 1
Durham
Rock Creek
Forest Grove
Mi 1 1 Creek
Muddy Creek
Little Miami
Springfield, MO
Contra Costa
San Jose
North
South
Central
UNIT PROCESS OR CONTROL STRATEGY
Interceptor Storage
X




X
X
X


















c
o
tJ
ID
N
3
o-
UJ
|
IT









X
X



X











V*
c
o
(D
(fc.
C
1)
a!





X
X
X
X
X




X
X

X
X

X
X
X
X
X
x
Bar Screening
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
5
cr
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Ol
c
a.
0)
Ol
•a
I/I
u
a.
X
X
X
X

X
X
X
X
X
X


X
X
X
X
X
X
X
X
X
X
X
X
X
8
I
8
u.
X








X



X
X



X
X
X
X
X
X


1
I
(J
O3
1
X

X
X

X
X
X
X
X
X
X
X

X


X
X
X

X
X
X
X
X
c
o
u
c
u
c
u
1
o
"




X








X
X
X

X


X





V
3
c
3
a
X
X
X
X
X
X
X
X
X
V
Ol
1
1
0
5
X
X
X
X
X
X
X
X
X
X X
X ! X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X


X
X
X
X
X
X
X
X
X
X
X
X
X
•o
u.
ID
u
'i
.c
X
X



X


X
X
X
X
X
X
X
X
X
X
X
X
X
X

X


c
IB
c
u
0
u
1
£
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Ozonation




















X





Ol
c
c
V
u
t-
u
X





X
X

X
X




X

X
X
X
X
X
X
X



Flotation Thickening
X
X







X



X
X
X



X
X

X



Anaerobic Digestion



X

X
X


X
X
X
X



X
X


X

X
X
X
X
c
o
ro
u.
|
u
ID
•>
X
X



X







X



X
X
X
X

X




c
\-
c
V
X








X




X


X



X
X

X
X
£
u
V
1
fl
I/)
V
O.
i
X

























c
0)
in
v\
V
a.
0)
a.
X














X











Incinerat Ion
X
X












X



X
X
X
X
X




c
4>
U
O
a,
in
V.
0
o-
-J
c
3
4>
oc
X




X











X





X


                          33

-------
1.    Introduction - In this section a brief description of the unit
     process being studied is included.   This is intended to give the
     reader a feeling for the general circumstances and variability of
     equipment configurations which are found in the field for this
     particular unit process.

2.    Objective - The purpose of this section is to identify the overall
     objective of a particular control strategy and to give some indi-
     cation of how this particular control strategy might be dependent
     on some other working process.  Also, some indication of the
     stability of the process (in terms of the objective of the control
     strategy) is included.

3.    Factors Affecting Process Performance - Any process is composed of
     three identifiable components.  The input or load on the process
     is the first component.  The process itself is the second, and the
     output, or result of the treatment process is the third compo-
     nent.  From the field observations this portion of control
     strategy documentation is intended to explain the dynamic nature
     of each control strategy to allow a better understanding of why
     process control of the particular unit process is necessary.

4.    Control Strategy - This is the section which documents the speci-
     fic workable control strategies.  The objective of this section is
     to clearly show how the control strategy is intended to work.  In
     addition, constraints of the control strategy in terms of its
     meeting the objective stated is also explained.  A process and
     instrumentation diagram (P&ID) is included to document each con-
     trol strategy as it would be executed.  The documentation is brief
     and is not in complete technical detail but rather outlines the
     basics of the control loop and the control strategy.

5.    Other Considerations - In the evaluation of any control strategy,
     there are observations, calculations and lab tests which operators
     and process control engineers utilize to verify whether a control
     strategy is performing.  In addition, they utilize this data to
     alter control strategies.  The intent of this section is to
     amplify the importance of operator observation, calculations and
     lab data and to point out how this information is utilized to
     either supplement or modify the control strategy being implemented.

6.   Instrumentation Utilized - The keys to any process control^
     strategy are the sensors, the controlling devices, and the annun-
     ciation of problems to an operator.  The objective of addressing
     instrumentation separately is to illustrate the importance of the
     sensors and to show the reader where instrumentation is used now
     in the field and where it can be expected to perform with
     reasonable realiability.
                                 34

-------
7.   Variation of Strategy With Plant Size - It was found that the
     process control strategy used varies with plant size.  Control
     strategies are not typically utilized in very small plants.
     Instead, methods where an operator simply fixes the operating
     point of a control device (e.g.  valve position, pump speed) are
     relied upon.  At the large plants, where unit processes are confi-
     gured in multiple banks of process equipment, the control strate-
     gies are dominated by operations that control the amount of
     equipment in use at a time.  It  was therefore important to discuss
     differences in control strategy with plant size.  For some unit
     processes, significant variation with plant size was found.  In
     other cases, there is no difference, because whenever the unit
     process is utilized, a control strategy similar to the one docu-
     mented is executed.  The objective of this section is to clearly
     indicate to the reader, with brief explanation, how the process
     control strategy might change as the plant size changes.

8.   Expected Performance - This section outlines the performance which
     can be expected from correct execution of similar control strate-
     gies.  In most cases, the expected performance is given in terms
     of conventional criteria of efficiency observed in the field.  In
     some cases, statistics and specific numbers are referenced which
     can be used as a practical guide.
                                 35

-------
                             INTERCEPTOR STORAGE
OBJECTIVE
     Interceptor storage is utilized to reduce plant flow variations within
the limits of water level imposed by the interceptor collection system and
the minimum velocity required to prevent sludge deposition within the inter-
ceptor.  The objective is the stabilization of the treatment process through
stabilization of the influent load.

FACTORS AFFECTING PROCESS PERFORMANCE

Load Characteristics

     The  load on the storage process is influenced greatly by storm water
infiltration, diurnal flow variations (especially rate of change of flow)
and ground water infiltration.

Process Characteristics

     The process of flow equalization is constrained by the usable storage
volume in the interceptor.  This usable portion is frequently less than the
volume available within the full cross section of the interceptor due to the
fact that some collection system laterals may enter the interceptor below
the top of the pipe.

     The  use of interceptor storage for flow equalization requires that the
plant raw sewage pumping equipment have very high redundancy and availa-
bility so that interceptor level is always controllable.  Field installa-
tions report that if the interceptor is being filled to even out a diurnal
peak, a storm can cause the system to be surcharged quickly.  High capacity
pumping equipment must be available to prevent surcharge.  Pump system con-
trol reliability and redundancy must be considered in the design of the
system.                                                             \

CONTROL STRATEGY

     Implementation of the control strategy requires level sensors, a vari-
able speed pumping system, and pump sequence program controls (see Figure
3-1).  An operator selects a plant flow rate setpoint to be maintained by
the pumping system based on interceptor level, observed level rate of
change, time of day, weather and any other information that may influence
                                      36

-------
CO
                           Figure 3-1.  Lift station control for interceptor storage.

-------
plant loading.  The operator's judgement in selecting and changing the set-
point is critical.  Field observations indicate that the selection of a cor-
rect setpoint (to balance the system) is an intuitive action.  Typically the
operator does not have a nomograph or formula to use.  Adequate experience
is the key to success.  The plant flow setpoint is changed frequently by
small amounts in order to keep the interceptor level within some operating
limits and yet dampen out daily flow variations.

     Pumped flow is both a controlled and manipulated variable.  Interceptor
and wet well levels are additional measured variables.

INSTRUMENTATION UTILIZED

     The instrumentation and control devices typically used for this process
are as follows:

     1.   Interceptor level - bubbler level sensor, can be located in a
          manhole a short distance from the plant.

     2.   Wet well level - bubbler level sensor, backed up by high and low
          level alarm float switches.

     3.   Variable speed pumping system.

     4.   Plant flow meter.

VARIATIONS IN STRATEGY WITH PLANT SIZE

     The use of interceptor storage for flow equalization cannot usually be
applied in the smaller plants because the interceptors tend to have limited
storage capacity and the diurnal flow variation is often too great.  If the
control strategy is implemented in a small  plant which is loaded by a dis-
proportionally large interceptor, good performance can be expected but
sludge deposition in the interceptor could become a problem.  This problem
may possibly be solved by nightly or weekly drawdown of the interceptor
level.

     Large plants usually employ more sophisticated pump control logic for
implementing this control strategy.  If the pump control logic is executed
by a control program in a digital control system, the flow control system
can be programmed to adapt itself based on excessive rate of change of" level
and anticipation of level operating limit violation.
                                      38

-------
EXPECTED PERFORMANCE

     The degree of modulation of flow variation which can be achieved is a
function of a number of parameters including:   the actual diurnal variation
of flow, the volume of interceptor storage available, the degree of operator
involvement and the use of adequate instrumentation.   At the West-Southwest
plant in Chicago, interceptor storage allows operation of the plant at a
nearly constant rate of 1,200 mgd (52,800 dm3/s).   without this technique,
the daily peak would be 1,440 mgd (63,360 dm3/s) and  the daily minimum
would be 800 mgd (35,200 dm3/s).   Similar results  were obtained at the
Calumet treatment plant, also in  Chicago.
                                      39

-------
                              FLOW EQUALIZATION
INTRODUCTION
     Flow to most municipal wastewater plants in operation today exhibits
significant diurnal variation.  One means of improving the process perform-
ance is the use of a specially designed equalization storage basin to dampen
out diurnal flow variations.  This discussion describes the implementation
of side-line equalization for a low capacity gravity flow plant.

OB JECTIVE

     The primary objective of flow equalization control is improvement of
the performance of the wastewater treatment process through the reduction of
diurnal flow variations.

     Almost all processes within a plant are, to some degree, affected by
changes in influent feed rate.  Achievement of a nearly constant flow rate
has its greatest benefit in the improvement of the performance of solids/
liquid separation processes.  Problems with operation due to short circuit-
ing are made worse by hydraulic flow variations.  Problems of this nature
are widespread in currently operating wastewater treatment plants.  Other
conditions caused by uncontrolled and large flow variations include
undesired overflows and biological washouts.

     When provisions for the reduction of flow variations are made in the
design of a plant, the quantity and size of the equipment may be reduced.
Clarifiers and other facilities can be sized to handle average rather than
peak loads.  Simpler arrangements of weirs and gates can be used to control
flow splitting with greater accuracy when the range of flow rates to be
controlled is reduced.

FACTORS AFFECTING PROCESS PERFORMANCE

Load Characteristics

     Diurnal flow .patterns, which are used to determine the equalization
basin  capacity required, vary from day to day (especially weekend as
compared to week day flow) as well as seasonally, and are affected by storm
inflow and infiltration.                                            J
                                      40

-------
Process Characteristics

     The process is characterized by its capacity to store wastewater.   If
the size of the equalization basin is large enough, the variation of  any dry
weather flow can be reduced to provide a constant plant flow rate.

     Flow equalization requires the capability to divert selected flows  to
the equalization basin when inflow is above average and to supplement normal
plant flow with storage bleedback in the proper amount when plant inflow is
below average.  Sufficient rangeability is therefore required in the weirs,
valves and pumps used in this process.

     The instrumentation required for level and flow measurements used in
conjunction with this process must be reliable and accurate in order for the
system to function because these parameters are the basis for the entire
control strategy.

CONTROL STRATEGY

     The process discussed here as an example is a "side-line" diversion
process as opposed to an "in-line" process where all wastewater flow  is
normally routed through the equalization basin.

     The system requires a flow splitter with an adjustable weir, variable
speed pumping system, flow metering for plant and return flow and a level
sensor for basin level (see Figure 3-2).  The adjustable weir is set to
allow overflow to occur at a desired plant flow rate so that all flow in
excess of selected rate will be diverted to the basin.  The flow metering
equipment and the variable speed pumps are used to return stored wastewater
at the proper rate to achieve a constant plant flow.

     Total plant flow is the controlled variable.  Weir position and pumped
return flow are the manipulated variables which are adjusted to achieve
constant plant flow.  Equalization basin level is used as an override to
indicate when the control strategy should be abandoned.

     The operator's positioning of the weir determines at what point flow is
diverted to equalization.  The decision to supplement plant flow with
storage can be made by a switchpoint associated with the plant flow trans-
mitter.  The pumping rate is determined by the difference between inter-
ceptor flow and the constant plant flow setpoint chosen.  The control
strategy must, of course, be overridden if a basin level limit is reached.
If a high level limit is reached,  the weir must be repositioned to prevent
continued diversion.  If a low limit is reached,  the pumps are turned off.
                                      41

-------
                                                                    I DESIRED PLANT FLOW
                SPLITTER BOX
r\5
       DE-GRITTED
>             INFLUENT

          '
                                           Figure  3-2.   Flow equalization  control.

-------
INSTRUMENTATION UTILIZED
     The following types of instrumentation and control devices have been
used to implement the control strategy.
     1.   Basin level - bubbler level sensor.
     2.   Plant flow - parshall flume or mag meter.
     3.   Returned flow - mag meter.
     4.   Weir position - manual loading station.
     5.   Variable speed pump controls - eddy current coupling or SCR drive
          with sequence program logic.
     6.   Return flow controller - PID mode type.
VARIATIONS IN STRATEGY WITH PLANT SIZE
     The example discussed applies to gravity flow plants of low capacity.
Larger plants requiring a raw sewage pump station may use a portion of the
pumped flow for diversion to an equalization basin, and later return flow to
the plant by gravity flow through modulating valves.
     The economics of the construction of an adequately sized equalization
basin generally limits the application of this strategy.  The largest plant
where side-line flow equalization was observed was 30 mgd (1320 dm3/s)
capacity.
EXPECTED PERFORMANCE
     Field experience has shown that the objective of dry weather flow
equalization can be achieved provided the basin has been adequately sized,
the,operator is able to make the required adjustments for varying diurnal
patterns, and the process instrumentation and control devices are reliable.
In general, plant flow and performance are influenced greatly by frequency
and amplitude of diurnal variations.
                                      43

-------
                             PLANT LIFT STATIONS
 INTRODUCTION
     Lift station level control is essential for plant operation, especially
in instances where there is limited capacity to store wastewater for flow
equalization.  In most cases the actual level is not critical as long  as it
is maintained within acceptable limits.  A lift station control system using
variable speed pumps is examined in this discussion.

OBJECTIVE

     Where limited storage capacity is available, a level control approach
must be taken to approximately match the pumping rate with the influent
flow.  Within the constraints of level control, it is desirable to minimize
abrupt changes in the pumped flow rate since the disturbances will propagate
through the plant treatment processes.  Frequent pump starts and stops also
represent wasteful energy usage.

FACTORS AFFECTING PERFORMANCE

Load Characteristics

     The system must respond to amplitude and rate of change of the diurnal
flow pattern.  Storm water inflow and ground water infiltration represent
large additional loads on the system.

Process Characteristics

     The dominant characteristic of the system is capacity.  There is a
trade-off between the use of a low capacity wet well  to reduce construction
costs, and the use of a large wet well for flow equalization.  Field obser-
vations indicate the wet well  storage capacity time should be a minimum of
ten minutes at maximum pumping rate.   This will  allow some response time
should problems develop with equipment in the field.

     The range of individual pump control available and the length of time
required to bring a pump on or off line may be constraints limiting the
achievement of the stated control  system objectives.

CONTROL STRATEGY

     Variable speed pump control  is most often based  on proportional  mode
control only.  Although rarely used,  a small  amount of  integral  mode action


                                      44

-------
may be added to drive the wet well level toward a setpoint.  This additional
use of integral mode would provide some energy savings at low flow if the
setpoint is in the middle to upper range of the wet well.  Derivative mode
action is not used because the capacity of the system and small fluctuations
in the level would lead to instability in the control system.

     The control system P&ID is shown in Figure 3-3.  Wet well level is the
controlled variable.  Pump speed is the manipulated variable.  Pumped flow
may be an additional measured variable.

     There are many possible variations observed in the field for the combi-
nation of speed control and pump sequencing used with multipump installa-
tions.  The design of the control equipment will depend on the hydraulics of
the system.

OTHER CONSIDERATIONS

     Variable speed drive reliability is an important consideration since
loss of a drive may require manual operation of pumps at constant speed.

INSTRUMENTATION UTILIZED

     The following equipment is commonly used in the systems:

     1.   Wet well level - bubbler sensor backed up by high and low level
          alarm float switches.

     2.   Variable speed pump controls - eddy current coupling, SCR drive,
          wound rotor motor drive, or variable frequency drive.  The latter
          must be carefully designed for the particular application.

VARIATIONS IN STRATEGY WITH PLANT SIZE

     The objective of maintaining wet well level within acceptable limits
remains the same regardless of plant size but the execution of control  will
vary.  Plants of less than 1 mgd (44 dm3/s) capacity typically use on/off
level switches and constant speed pumps.  A pump is started at a high level
and stopped at a low level switch.  Plants having capacity in the range of
10 to 50 mgd (440 to 2200 dm3/s) commonly use variable speed control of
two to four pumps of equal size.  Larger plants (100 to 300 mgd - 4400  to
13000 dm3/s) may have multiple pumps of varying size to handle wide load
variations smoothly.  The sequence control logic becomes far more compli-
cated when many pumps of different capacity are used.

EXPECTED PERFORMANCE

     Field observations show that most lift station level control systems
provide satisfactory operation since precise level  control is rarely
required.
                                      45

-------
Figure 3-3.   Lift station wet well  level  control.

-------
                               BAR SCREENING
OBJECTIVE
    Screening is performed to protect plant equipment from damage by
removing large floating objects and any other debris that may be carried
along by the influent stream.  Screening may be fine or coarse, depending
on the situation.
FACTORS AFFECTING PERFORMANCE
Load Characteristics
    The nature of the interceptor system, i.e. the amount of debris
present, often determines the success of any bar screen control strategy
applied.  If large amounts of sticks, rocks and trash are present,
frequent breakdowns due to overloading of the cleaning mechanisms may be
observed.  Large objects which block the flow path are the primary load on
the process.
Process Characteristics
    The screens have very little influence on downstream processes effi-
ciency but do prevent catastrophic mechanical failure.  The reliability of
the cleaning mechanism is the most important aspect of the screening
system.
CONTROL STRATEGY
    Field observations show that bar screen cleaning can be accomplished
by any of the following control methods:
    1.   Periodic manual operation (requires operator attention
         periodically during each shift).
    2.   Continuous operation (wastes energy and wears out equipment).
    3.   Individual time clock operation.
                                                          *    o
    4.   Operate when a high differential level occurs.
    5.   Combination of 3 and 4.
                                      47

-------
     Normally, strategy 4 is used.  The accumulation of materials on the
screens causes a differential level on the order of a few inches to occur
across the screen (see Figure 3-4).  Differential level is used because the
absolute level will vary widely.  Once a cleaning cycle is initiated by the
differential level switch, the control circuitry should be interlocked to
require the cleaning mechanism to make a complete cycle before turning off.
This prevents the mechanism from blocking the flow stream if the high dif-
ferential level condition is cleared midway through a cycle.  Multiple
screens should be cleaned consecutively when a high differential level is
sensed.

INSTRUMENTATION UTILIZED

     The following instrumentation and control devices are typically used:

     1.   Differential level - two bubbler tubes and a differential pressure
          cell.

     2.   Bar screen controls - timing and sequence logic.

     3.   Alarms - rake drive torque overload.

VARIATIONS  IN STRATEGY WITH PLANT SIZE

     Small  plants generally do not have mechanically cleaned bar screens.
Cleaning is manual followed by comminution of the collected solids.  Fre-
quent cleaning is often not required because the interceptors tend to be
relatively  free of harmful debris.  The moderate size plants typically
employ three or four locally automated screen units.  Frequent maintenance
is  again generally not required.  Large plants must often operate their
cleaning equipment continuously due to the great amount of material that is
often found in large interceptors.

EXPECTED PERFORMANCE

     Within the constraint of the nature of the materials present in the
interceptor, both differential level and time clock operation have been
proven as effective means of control.
                                      48

-------
                                       TIMERS AND SEQUENCE
                                           LOGIC
10
                       FLOW
                                              Figure 3-4.   Bar screen control

-------
                                 GRIT REMOVAL
INTRODUCTION
     There are three basic types of grit removal facilities, velocity con-
trolled, cyclone degritter, aerated grit chamber.  This discussion is appli-
cable to the monitoring and control of aerated grit chambers, because of
their advantages over the other two types, and their frequency of use in new
plants.

OBJECTIVE

     Grit, consisting of sand, gravel and other inert materials, is removed
to avoid physical damage to downstream plant equipment.  Grit damages pump
impellers, collection mechanisms, etc., and settles out in undesirable areas
such as settling tanks and digesters.  Equipment damage can lead to
equipment failure and subsequent loss of process control.

FACTORS AFFECTING PERFORMANCE

Load Characteristics

     Gritty substances are present in almost all wastewaters, especially
storm water.  Influent from combined sewer systems presents difficulties due
to the  large variations in the quantity of grit that must be removed.  The
grit load varies with the time of day, the flow rate to the plant and the
amount  of storm flow.

Process Characteristics

     In an aerated grit chamber, a spiral roll velocity is imparted to the
flow, the roll velocity being controlled by the chamber dimensions and the
air flow rate supplied to the chamber.  The roll velocity in an aerated grit
chamber can be held nearly constant regardless of the influent flow>ate by
adjusting the air flow.  The purpose of the roll is to resuspend organic
solids  which have settled with the grit.  With proper adjustment of air  !
flow, consistent removals can be achieved.

     Removal of grit from the chamber often poses more of a problem than the
original separation from the wastewater.  Mechanical equipment such as screw
or belt conveyors typically require much maintenance.  The odor and other
environmental conditions associated with the process often lead to lack of
maintenance.
                                      50

-------
CONTROL STRATEGY

     Since aerated grit chambers are designed to nearly eliminate  variations
in performance due to hydraulic load changes, the instrument and control
requirements are minimal (see Figure 3-5).  In the field, air flow is fixed
at a specific rate and is only adjusted seasonally to compensate for varia-
tions in air density with temperature.  Aeration blowers at the larger
plants often have inlet butterfly valves to adjust air flow.  Blower current
monitoring is sometimes used to indicate air flow output.  Although more
frequent adjustment of air flow is desired, in practice it is only rarely
observed.

     Mechanical grit removal equipment can be operated continuously, but
since it may take some time for grit to accumulate, time clock controls most
often are used to start the screw conveyor and, after a time delay, start  a
bucket and chain grit conveyor to load a grit storage hopper.  The  storage
hopper may be equipped with a high level alarm sensor which is interlocked
to shut off the collection system.

     The controlled variable, grit removal efficiency, is entirely qualita-
tive and thus cannot be directly measured.  Air flow and timer adjustments
are the manipulated variables.  Blower current (related to air flow) is a
measured variable.

OTHER CONSIDERATIONS

     The controlled variable being unmeasurable, the operator must rely on
observation of environmental conditions to judge the success of the grit
removal process.  For example, reports of grit volumes removed or  observa-
tion of grit in primary sludge will  influence the control  strategy.

INSTRUMENTATION UTILIZED

Sensors

    * Typically, no sensors are used other than perhaps a motor current
transmitter and a storage hopper level switch.  Occasionally air flow to
grit will be measured.

Controlling Devices

     1.   Manual loading stations - may be employed for setting inlet valve
          positions on blowers larger than 100 HP-

     2.   Timers and sequence controls - mechanical grit removal.
                                      51

-------
en
ro
                   HK\ [_f\

                  V7
Fl
1
1
— 1
i!
I
1
V '
                                                                                              PRIMARY TREATMENT
                                     Figure 3-5.  Aerated  grit chamber control.

-------
Annunciator Alarm Points

     1.   Grit conveyor drive overload.

     2.   Storage - high level.

     3.   Blower surge.

VARIATIONS IN STRATEGY WITH PLANT SIZE

     When aerated grit removal is used, the control strategy is similar
regardless of the size of the facility.  The small plants seldom use aerated
grit removal but apply velocity type.  Intermediate size plants may use
positive displacement aeration blowers if air flow requirements are low.
Large plants often have centrifugal blowers with individual inlet valves for
balancing air flow contributions from the multiple operating units.

EXPECTED PERFORMANCE

     Grit removal efficiency data is not readily available.  The operators
interviewed assumed that the process operated efficiently and commented
mostly about maintenance.  Most plants using aerated grit chambers report
satisfactory performance.  On the other hand, larger plants using horizontal
flow (velocity controlled) grit removal reported poor performance due to the
frequent need to bring chambers in and out of service to maintain proper
chamber velocities.  Velocity control with sutro weirs or parabolic cross
sections were observed only rarely and appeared to operate satisfactorily.
                                      53

-------
                   PRIMARY CLARIFICATION AND SLUDGE PUMPING
INTRODUCTION
     Primary clarifiers are tanks used to remove or reduce suspended solids
and organic loading from the wastewater before it goes to secondary treat-
ment units.

     Suspended solids removal is a function of flow and tank dimensions,
thus process control is limited to flow equalization, flow proportioning to
all units and avoidance of short circuiting.  The control strategy discussed
here concentrates on the removal of the sludge which accumulates in the tank.

     Clarifiers may be rectangular, square, or circular in shape.  In
rectangular tanks, the wastewater flows from one end to the other and the
settled sludge is moved to a hopper, usually at the inlet end, either by
flights set on parallel chains, or by a single bottom scraper set on a
traveling bridge.  Floating materials such as grease and oil are collected
by a surface skimmer and then removed from the tank.

     In circular tanks, the wastewater usually enters in the middle and
flows toward the outside edge.  Settled sludge is pushed to a hopper that is
in the middle of the tank bottom, and floating material is removed by a
surface skimmer connected to the sludge collector.

     The sludge pumping system typically includes some method of detecting
clarifier sludge level (optical probes, air lifts for operator determina-
tion, or light transmitting devices), clarifier sludge removal rakes,
isolation valves from the clarifiers, variable speed sludge pumping system,
sludge density sensors and sludge flow meters (see Figure 3-6).

OBJECTIVE

     The principal objectives of primary clarification are removal of
settleable solids and removal of floatable solids.

     Field observations indicate .that the primary clarification process is
controlled via the sludge pumping operation.  The objectives most often
found for primary sludge pumping are as follows:

     1.   Maintain as consistent a pumping rate as possible in order to
          minimize operational labor intensity.
                                      54

-------
                                                                         MANUAL
                                                                        SPEED ADJUST
                                                                         OVERRIDE
Ol
en
             AND
      J SEQUENCE LOGIC

'^T^
'   I
    |_   ACTIVATE
                                                                                      LOW DENSITY
                                                                                     "OVERRIDE
                                                                                DENSITY
                                                                                CONTROL
                                                                                LOGIC
                CLARIFIERS
                                    Figure 3-6.   Primary  clarification and  sludge pumping.

-------
     2.   Try to pump at a rate which will maintain a consistent primary
          sludge composition (density).

     3.   Observe, minimize and control any buildup of primary sludge in the
          clarifiers.

     4.   Be aware of downstream processes and minimize any disturbances on
          these.

FACTORS AFFECTING PROCESS PERFORMANCE

     A number of factors affect the performance of sedimentation tanks,
including the following:

     1.   The hydraulic overflow rate expressed in flow per unit time per
          unit top area of the tank.  This is a velocity term and is equal
          to the sedimentation velocity of particles which will be
          completely removed from the wastewater.

     2.   Detention time which provides the opportunity for coagulation of
          small particles to larger, faster settling ones.

     3.   Wastewater characteristic (wastewater strength,  freshness, and
          temperature; types and amount of industrial waste; and the
          density, shapes and sizes of particles).

     4.   Pretreatment operations (carryover of grit and screenings).

     5.   Nature and amount of any in-plant wastes recycled to the plant
          ahead of the primary clarifier.

Load Characteristics

     The load continuously varies hydraulically and in composition.
Changing waste characteristics and poor hydraulic distribution cause
problems.  Recycled in-plant waste also causes shock loads.

Process Characteristics

     Clarifiers are usually quite long or  large in diameter which "can cause
a problem in sludge collection and removal.  The sludge should be removed at
a rate comparable to the rate at which it  is deposited. The problem arises
in moving the sludge from where it is deposited to where it is removed.
This is done by use of a mechanical sludge rake which typically moves very
slowly, raking the sludge to a collection  well at the center or end of the
clarifier.

     Grease and oil  content make measurement "in situ"  extremely difficult.
If the sludge level  is too high,  the clarifier efficiency  drops off due to
smaller working volume.   If the sludge level  is too low, the clarifier
                                      56

-------
sludge removal system is operating at less than optimum conditions.  Proper
sludge pumping requires reliable sludge level metering and a reliable method
of metering sludge composition to maintain consistency for downstream
processes.

CONTROL STRATEGY

     The underlying control philosophy is to achieve a consistent total
solids concentration in the sludge (underflow) and to remove all the sludge
from the clarifiers.  The system consists of a bank of clarifiers from which
the sludge is sequentially pumped (usually on a timed basis).  When a clari-
fier's sludge is to be pumped either in its sequential turn or due to a high
sludge level, the clarifier's isolation valve is opened and the sludge is
pumped out.  The valve is closed at the end of the timed period, when the
sludge level falls to its low level or when the composition (solids content)
is below a preset cutoff concentration.

     The variable speed pumps are fed from a common line into which the
isolation valves empty.  The speed of the pumps is determined by the sludge
quality sensors or set after operator observation of lab data.  When the
pumps are speeded up, more water is drawn into the system, reducing the
sludge solids concentration.  Similarly,  when the solids concentration level
is too low, the pumps are slowed down so  less water and more sludge is
pumped.  Field observations indicate that because of sludge collection time
constants, response to changes in pump speed is sometimes slow.

     The controlled variable is usually the flow and the suspended solids
concentration or sludge blanket level in  the sludge.   The manipulated vari-
able is the sludge flow while the measured variables (lab or on-line)
include sludge level in the clarifiers, sludge flow,  sludge suspended solids
concentration and suspended solids concentration in the clarifier liquid
overflow.

     The constraints are that the instrumentation requires much maintenance
and, therefore, is typically not reliable enough to use for on-line measure-
ments.  For this reason, lab tests must be conducted.   Also,  the makeup of
the loads and the diurnal flow pattern greatly affect  the load.

OTHER CONSIDERATIONS

     At least once every shift the suspended solids in the liquid is
observed.  At the same time, any evidence of the sludge going septic should
be noted.  The pumped sludge itself should be occasionally observed to be
sure that it isn't becoming too weak or septic.  Lab  tests are periodically
conducted on both the suspended solids concentration  of the clarifier over-
flow and sludge in order to determine the clarifier efficiency.

INSTRUMENTATION UTILIZED

     The types of instrumentation and control devices  found to be used for
the process are as follows:


                                      57

-------
     1.   Sludge level - optical probes or ultrasonic detectors.

     2.   Sludge flow - usually heated tube mag meter with ultrasonic
          cleaning.

     3.   Sludge suspended solids concentration - optical or nuclear density.

     4.   Variable speed pumps - preferably not diaphragm pumps, but if
          used, stroke adjustable rather than speed controllable types are
          preferred.

     5.   Isolation valves - one per clarifier.

     6.   Variable speed pump control - eddy current coupling or SCR drive
          with sequence program logic.

     7.   Annunciators - for pump failure, clarifier level low and incorrect
          density.

VARIATIONS IN STRATEGY WITH PLANT SIZE

     The configuration discussed here is typical for medium sized plants.
For plants that are on the large end of the scale, there is more likelihood
of individual pumping from clarifiers.  For small packaged plants, typically
less than 1 mgd (44 dm3/s), a primary clarifier for removal of suspended
solids typically is not provided.

EXPECTED PERFORMANCE

     Field experience has shown that the objective of minimizing the inven-
tory buildup in the clarifiers while controlling pumping rate and sludge
composition (density) is difficult to accomplish.  Automation of the sludge
pumping process helps achieve the objectives but lack of maintenance on
sensors limits success.  Most operators usually revert to flow control to
minimize the inventory buildup and let composition vary as it will.
                                      58

-------
                            HYDRAULIC FLOW CONTROL
INTRODUCTION
     Control of volumetric flows to multiple units within a particular unit
process was the most requested improvement from plant operators during our
field visits.  Most plants visited used splitter boxes or weirs.  The prob-
lem observed in the field is that splitter boxes and weirs are fixed and the
flow splitting process is dynamic.  Splitter boxes work acceptably between
two tanks with small design flows.  As the design flows increase or the
number of tanks increase, these methods lead to widely varying hydraulic
distribution and very poor solids distribution.

     Controlled flow distribution allows the flow and solids to be dispersed
equally among all  on-line units and allows the operator the latitude to
adjust the distribution based on feedback from plant performance.

OBJECTIVE

     The primary objective of flow control is to split process influent flow
among multiple trains in a controlled manner.  Field observations indicate
that this can be achieved by two different methods.  The first is used when
the total incoming flow is measurable.  The desired flow per train is then
simply total flow divided by number of trains in service.  The second
strategy, referred to hereafter as "most open valve" (MOV), is used when the
total incoming flow is unknown.  This is a more complex method which is not
as desirable a$ the first because the inherent instability of the control
system leads to periodic oscillations in the flows controlled.

     When provisions are made for flow splitting among multiple process
trains, the chances of undesired washouts and overflows are greatly reduced
so that more consistent results are achieved.

FACTORS AFFECTING PROCESS PERFORMANCE

Load Characteristics

     The process load is characterized by the plant influent diurnal flow
pattern, which will vary from day to day (especially week day flow as
compared to weekend flow) as well as due to storm inputs.  Peak diurnal
variations were observed as great as 3 to 1.
                                      59

-------
Process Characteristic^

     The hydraulic control application is characterized by the need to
adjust resistance to flow to equally split the total flow into more than one
train.  This ability is dependent on the hydraulics of the piping configura-
tion or splitter box and the load changes which take place.

     The instrumentation used for flow measurement and control in this
process must be highly reliable because feedback control is applied which
depends on a continuous signal of feedback flow.

CONTROL STRATEGY

     Two strategies are discussed here.  The first one is used when the
total flow to be split is measurable (see Figure 3-7).  Total flow is
metered and divided by the total number of trains in service to determine
the  desired flow per train.  A multiplier/divider sends a common setpoint
signal to the train flow controllers.  Each individual train flow meter
compares the actual flow with the desired flow for adjustment of the valve.
Total flow and individual flow per train are the measured variables, valve
positions are the manipulated variables, and the individual train flows are
the  controlled variables.

     The second flow control strategy, MOV, can be utilized when a measure-
ment of the influent flow is not available (see Figure 3-8).  This strategy
requires the initial valve positions to be manually set for equal flow when
the  system is initially made operable.  This also establishes an initial
value of the "most open valve" for the master valve controller.  Once this
is accomplished, the control system is activated with the flow setpoints at
the  set conditions.  The individual train flow meters supply feedback to the
flow controllers to keep the flow rates constant.  Variations in total flow
cause the valve controllers to move all valves to maintain flow, producing a
position other than the operator entered MOV position.  Through the MOV high
level select module, the "most open valve" is selected.  This valve's posi-
tion  is passed on to the MVC which compares it to the operator entered MOV
setting initially entered.  A corrected setpoint (percent change) is then
passed to the valve controllers.  This feedback loop is repeated until the
MOV  position agrees with the operator entered MOV setting.  The strategy
accomplishes flow control with the valves as wide open as possible to assure
that the total load is handled and energy loss through the valves in
minimized.

OTHER CONSIDERATIONS

     For proper flow control the operator should, at least once per shift,
observe the individual train to check that flow is being split equally.
When the MOV strategy is used, the operator must also enter the desired MOV
setting and the initial valve positions.  This need only be done on ini-
tialization of the control loop and after large flow changes (storm flows).
                                      60

-------
en
                       I"

                     rUY\ TOTAL FLOW
                           NO. TRAINS  IN SERVICE
                                                                      TYPICAL OF 3
 I

 I
 I
J.

FT

T

j-
FE
                                                    I



                                                    FE
                                                                   i
                                                   Figure  3-7.   Flow  control

-------
                                                                                                      VALVE POSITION
ro
                                       Figure 3-8.   Flow control, MOV method.

-------
     In some instances it may be desirable to have unequal flows  going
through the different trains.  This can be accomplished by the use of ratio
stations having weighting ratios determined by the operator to correspond
with the desired loading.

INSTRUMENTATION UTILIZED

     The following types of instrumentation and control devices are
typically used for the first method:

     1.   Flow meters - mag meters.

     2.   Flow controller - PID type.

     3.   Multiplier/divider logic module.

     Instrumentation and control devices typically used to implement MOV
control are as follows:

     1.   Flow meters - mag meters.

     2.   Flow controller - PID type.

     3.   MOV high level select module.

     4.   MVC (master valve controller) module.

VARIATIONS IN STRATEGY WITH PLANT SIZE

     Use of the first strategy is preferrable but is limited by the need to
measure total flow.  Flow splitting for processes within the larger capacity
plants must often rely on an alternate strategy such as the MOV method
because it is not economically feasible to meter large flows.

EXPECTED PERFORMANCE

     When using the first method, there should be little deviation from
equal flow splitting.  The accuracy of control of the individual  flows can
be expected to be within the accuracy of the flow meters.

     The MOV method has a flow control cycle period (natural period of
oscillation) of about two hours.  The diurnal flow pattern changes cause an
inherent amplitude inaccuracy.  Large load changes like those  caused by
storm flows can cause the MOV control system to go unstable.
                                      63

-------
                      DISSOLVED OXYGEN & BLOWER CONTROL


INTRODUCTION

     The subject of aerator DO control has received extensive coverage in
EPA publication "Design Procedures for DO Control of Activated Sludge
Processes," EPA-600/2-77-032, June 1977.  This discussion will highlight
briefly the control strategy used in a typical process configuration
including centrifugal blowers with diffused aeration.

OBJECTIVE

     The activated sludge process, where used, is a main component in a
secondary wastewater treatment plant.  Control of oxygen level in the acti-
vated sludge reactor is an important factor in maintaining the stability of
the process.  If poor control results in DO levels that are too low, aerobic
bacterial activity may be reduced and/or poorly flocculating organisms can
predominate.  Both will result in poor effluent quality.  Excessive DO
levels  are  produced by excess levels of air flow.  Not only is this a waste
of energy,  but  it may cause the mixed liquor to become dispersed or frag-
mented, thereby reducing solids capture in the final clarifier.

FACTORS AFFECTING  PERFORMANCE

Load Characteristics

     Aeration requirements are affected by the mass  of organic material
resident  in the aeration chamber.  This in turn  is the result of the volume
and  composition of returned sludge,  and the volume and composition of the
raw  wastewater  influent.  Industrial waste contributions may affect the bio-
degradeability  of  the wastewater.  If stormwater  influent is a large compo-
nent of the influent  load, the objectives of DO control may be almost
impossible  to achieve due to  the  accompanying wide variations in hydraulic
and  organic loading.

Process Characteristics

     Field  observations  indicate  that control of the DO  level in aeration
tanks  is  difficult because the dominant characteristic of the process  is the
large  volumetric capacity of  the  aeration tank.   Direct blower air flow
manipulation cannot be effectively used to control the level of DO  in  aera-
tion because the time lag between a  change in  air flow and the resultant
change  in the DO level often  leads to instability.   Manipulation of blower
air  flow  is also constrained  by the  need to sustain  adequate mixing in the


                                      64

-------
 aeration tank  and the turn-down  limits of the blowers  (these  are  frequently
 oversized).

     Measurement of DO presents  difficulties in mounting  instruments  to
 obtain representative readings,  and in the accuracy  and reliability of the
 instrument.  Responsive instruments providing reasonable  accuracy with
 acceptable maintenance requirements are currently available.

 CONTROL STRATEGY

     Automatic closed-loop control of aeration air flow as discussed  here
 has been demonstrated, but is not typically implemented.  The control philo-
 sophy typically involves control of blower discharge pressure and oxygen
 dissolution as separate loops to decouple the demands of  each process, thus
 reducing the degree of interaction between the two (see Figure 3-9).

     The object of the blower control subsystem is to maintain constant
 discharge header air pressure to ensure stable operation  of the individual
 DO control loops.  A cascade control system, having pressure as the primary
 controlled variable and air flow as the secondary, is used to provide pres-
 sure regulation.  Blower speed,  inlet guide vanes, or suction throttling
 valve positions are manipulated to control the output of  the individual
 blowers.  Sequence control logic is required for starting and stopping
 blowers to meet varying system demand and to alternate the operation of
 blowers for equalization of wear.

     A separate cascade control, having aeration basin DO as the  primary
 controlled variable and air flow as the secondary, is used to obtain good
 system response and stability in maintaining the desired  DO level.  A slow
 acting controller uses measured dissolved oxygen as the feedback.  This
 controller uses the comparison of measured dissolved oxygen versus the
 desired dissolved oxygen to call for more air flow or less air flow.  The
 air flow is usually maintained by a conventional flow controller  whose set-
point is adjusted periodically by the slow acting dissolved oxygen
 controller.

OTHER CONSIDERATIONS

     The operator must periodically observe the color and odor of the mixed
 liquor as a check on the DO setpoint employed.   The results of half-hour
 settling tests and lab DO measurements may indicate that modification of
system setpoints is required.  Other lab tests  to be performed include
measurements of MLVSS and SVI.

 INSTRUMENTATION UTILIZED

     Instrumentation and control devices found in use are as follows:

 Sensors

     1.   DO probes - galvanic or polarographic.


                                      65

-------
1 DESIRED HEADER
I PRESSURE
' DESIRED DO LEVEL
                                                 L~  DO CONTROLLER
                                                 rD
TYPICAL OF TWO
            CENTRIFUGAL
              BLOWERS
                                                                    v	'DO

                                                          HHtH
                                                         llttiit
           AERATION TANKS
                                                                                      EFFLUENT
         Figure  3-9.   Dissolved oxygen and  blower  control.

-------
      2.    Aeration  air flow - orifice plates,  venturi  tubes or annubars (the
           latter  provides  a measurement that is  not as stable as the others).

      3.    Header  pressure  - diaphragm force  balance.

      4.    Blower  flow  - venturi.

      5.    Header  air temperature  -  sometimes used  to compute volumetric air
           flow  at standard conditions.

Modulating Control  Devices

      1.    Butterfly valves - these  typically have  good response and
           rangeability.  Air flow rangeability,  however,  is limited by the
           working pressure not the  valve.

Controlling Devices

      1.    PID mode  controllers for  DO,  air flow  and pressure.

      2.    Sequence  control  logic  is  sometimes used for automatic  control of
           blower  startup and shutdown.

Annunciator Alarms

      1.    Blower  alarms -  bearing and winding temperature,  high vibration,
           current sensing  for impending surge condition.

      2.    DO level  alarms  -  useful for  sensor failure  as well  as  process
           degradation  alarm.

VARIATION  IN STRATEGY  WITH  PLANT  SIZE

      Small capacity plants  (less  than 1 mgd  - 44 dm3/s) may have  a  single
oxidation  tank equipped with a positive displacement blower  and manual  DO
controls.  A control strategy similar to that discussed is  typically used  in
plants ranging from 5  to 50  mgd (220 to 2200 dm3/s).  The larger  plants
(100  to 300 mgd - 4400 to  13000 dm3/s) may follow a similar  control strat-
egy but startup and shutdown of blowers is not often under  automatic control
due to the increased complexity of the procedures and the consequences  of
possible misoperation of devices associated with the large blowers.

EXPECTED PERFORMANCE

     Automatic closed  loop control of dissolved oxygen can offer  significant
advantages in adaptation to  variations in loading and  in energy savings.  At
the present time, however, most DO control is manually implemented.  Auto-
matic DO control has been successfully implemented at the John Eagan plant
in the Chicago Sanitary District, consistently maintaining the DO level at  a
desired setpoint on a  long-term basis.
                                      67

-------
                         CRYOGENIC OXYGEN GENERATION
INTRODUCTION

     Cryogenic generation of high purity oxygen (95 to 98%) from air has
recently begun to be used in wastewater treatment.  This process provides
oxygen which is used as an alternative to air in biological treatment pro-
cesses and as a feed stream to ozone production.  It produces oxygen by
distillation of partially liquified air.  Ambient air is first compressed by
a multistage compressor.  The air is then cooled and a portion condensed by
a combination of heat exchanges with product and waste streams and expansion
in a turbine.  The expansion turbine provides the majority of refrigeration
in a standard plant while miniplants obtain refrigeration from purchased
liquid oxygen.  Two distillation columns finally separate the air into high
purity oxygen and nitrogen.  Several suppliers furnish complete cryogenic
plants which use compressors, expansion turbines, heat exchangers and
distillation columns to separate oxygen from air.  Cryogenic plants are
complex  but  steady state operation  is often fully automatic.

OBJECTIVE

     The objective of  cryogenic plant operation is to produce the pure
oxygen required for use in  treatment plant operation.  A guiding principle
of cryogenic plant operation is minimization of energy use.

FACTORS  AFFECTING PROCESS PERFORMANCE

Load Characteristics

      The oxygen  demand is determined by the  oxygenation  and  ozonation
processes.   These processes are  affected  by  diurnal  and  seasonal fluctua-
tions in hydraulic  and organic  loadings.

      The oxygen  supply is air which is  of constant quality (about  21%
 oxygen).  Changes  in  ambient temperature  have  little effect  on  distillation
 column  operation since the  compressed  air is cooled  to  a constant  tempera-
 ture with a cold water heat exchanger  and the  cryogenic  plant components are
 well insulated.

 Process Characteristics

      The response  of a cryogenic plant to load changes  must be  slow in order
 to avoid severe  column upsets.   Upsets  could result  in  column flooding or
 physical damage to  the columns.


                                       68

-------
     Product purity  is limited by column  design  and  is  variable  only over a
small range.  Usually setpoints for product and  waste nitrogen purities  are
not changed once the plant is on line.

     Energy is conserved in many areas, but those of particular  importance
are:

     1.   Maintaining a low cold end temperature differential to  limit heat
          losses via the waste nitrogen stream.

     2.   Inhibiting vaporization of stored liquid product since  making
          liquid requires four times as much energy as  producing  gas.

     3.   Eliminating overproduction of oxygen.

     This last consideration is constrained by the turndown capacity of  the
cryogenic plant which is usually 6056 of full capacity.  Slow control
responses inevitably result in venting to the atmosphere of some  product.
An important factor improving the process energy balance is that  the cryo-
genic plant is heavily instrumented to achieve complete automatic control.

CONTROL STRATEGY

     The cryogenic process is very complex as can be appreciated  from the
simplified P&ID provided (Figure 3-10).   In order to minimize column  upsets,
several parameters are controlled such that the ratio of their value to  the
cold end flow is kept constant.  The compressor capacity which determines
plant production is controlled by a mass flow controller using product out-
put as feedback.  This arrangement speeds up responses over configurations
with feedback closer to the compressor discharge.
     The primary controlled variable is usually product oxygen flow.  In
situations where the cryogenic plant is too slow in response or is shut
down, oxygen is obtained from vaporization of liquid oxygen stored during
times of overproduction.  Product pressure becomes the rate controlling
variable regulating vaporization.  The manipulated variable is ultimately
compressor discharge flow.  Measured variables are:  cold end flow, product
purity, product pressure and product flow.

     The cryogenic plant is limited in achieving the objective by the
constraints of slow response and limited turndown capability.

     During startups and shutdowns plant operation is manual until automatic
control loops are activated.  Table 3-2 describes the relationships of the
major control loops during steady state operation.
                                      69

-------
 OPERATOR
I ENTRY
 OPERATOR
(ENTRY RATIO
 OPERATOR
(ENTRY RATIO
 OPERATOR
(ENTRY RATIO
                              Figure  3-10.   Cryogenic  oxygen generation.

-------
                     TABLE 3-2.  STEADY STATE MODULATION
     Device Name
                     Device
                     N umber*
   Feedback
Controlling Device
Compressor Guidevanes


Cold End Turbine Feed
Valve

LPC Reflux Valve

HPC Level Control Valve


LOX Transfer Valve


GOX Vent Valve
GOX Make Valve
LOX .Vaporization Valve
                       1


                       2


                       3

                       4


                       5


                       6
                       8
GOX flow


Unbalance air
flow

Reflux flow

MAC liquid level
High and low
levels

GOX flow
                              GOX flow
Pressure
QIC, mass flow
controller

RIC, ratioed to CE
flow

RIC, ratioed to CE flow

LIC, level indicating
controller

On/off controller
RIC, GOX make valve
(7) controller only
when make valve is
full open

RIC, ratioed to CE
flow based on product
purity

PIC, pressure
indicating controller
*Device number indicated on Figure 3-10

Abbreviations:
CE
GOX
HPC
LOX
LPC
Cold end
Gaseous oxygen
High pressure column
Liquid oxygen
Low pressure column
                                      71

-------
OTHER CONSIDERATIONS
     A problem inherent with high purity oxygen  is the potential for explo-
sion and combustion.  Many materials that  are  normally difficult to oxidize,
burn or explode in  an oxygen rich environment.   To reduce the hazard two  gel
traps adsorb hydrocarbon  impurities  in the cold  end air stream  and the
liquid oxygen.  A hydrocarbon  analyzer will  automatically alarm at 25% LEL.
In addition, the operator must periodically  check the  liquid produced for
clarity (cloudiness is caused  by carbon dioxide  particles,  hence other
hydrocarbons may be present) and test for  the  presence of acetylene.
INSTRUMENTATION UTILIZED
     The sensors and control devices listed  below are typically used in the
process and  are normally  reliable and accurate.
Sensors
     1.    Gas flow  - orifice plate often with  pressure and  temperature
           correction.
     2.    Temperature.
     3.    Pressure.
     4.    Oxygen purity,  Nitrogen purity - membrane covered polarographic
           electrode.
     5.    Level -  differential pressure.
Modulating Control  Devices
     1.    Centrifugal compressor - inlet guide vanes or butterfly throttling
           valve with butterfly valve on recycle  for surge protection.
     2.    Gas flow control  valves -  butterfly  valves.
     3.    Liquid flow control  valves - gate  valves.
On/Off  Control Devices
      1.    Compressor.
     2.    Turbine  expander.
     Startup and shutdown of these are usually manual operations.
                                       72

-------
Controlling Devices

     1.   PIC - pressure indicating controller.

     2.   LIC - level indicating controller.

     3.   RIC's - controlled flows are ratioed to cold end air flow.

     4.   QIC - mass flow indicating controller.

     5.   Level switches - float pots.

Alarms

     1.   Hydrocarbon - LEL.

     2.   Liquid oxygen level in LPC, high and low.

     3.   Turbine discharge low temperature.

     4.   Compressor and turbine operating alarms, such as high oil tempera-
          ture, low oil pressure, vibration and coolant high temperature.

     5.   LPC and HPC high pressure.

VARIATION IN STRATEGY WITH PLANT SIZE

     Cryogenic plants larger than about 180 TPD have an energy recovery
system linked to the expander turbine which can reduce energy requirements
by about 2 to 3%.  Plants less than about 40 TPD usually have positive dis-
placement type, rather than centrifugal, compressors.  Compressor discharge
is then varied by step unloading or by recycle, which leads to inefficient
energy utilization.  Oxygen production rates are controlled in the same way
as in larger plants.  Mini-cryo plants (6 to 16 TPD) obtain required refri-
geration from purchased liquid oxygen rather than from a turbine expander.

EXPECTED PERFORMANCE

     Data showing the performance of cryogenic plants in regard to the
stated objective are lacking due to limited operating experience.  Operators
have indicated that the plants operate very well with a minimum of attention
or shutdowns.  However, system constraints have caused oxygen losses.  For
example, the plant at Springfield, Missouri, is designed for 50 TPD but is
only operating at about 30 TPD which is the minimum that can be produced.
System requirements are often less than 30 TPD so excess production must be
vented to the atmosphere.  The other major constraint is sluggish response
to load changes.  On at least one occasion at the Denver plant, load changes
were occurring too fast for the cryogenic plant to keep up or to stabilize.
To maintain adequate oxygen, the cryogenic plant capacity was manually set
(by controlling compressor discharge) at a fixed capacity.  This open-loop
control operation resulted in venting or underproduction of oxygen.


                                      73

-------
                           RETURN ACTIVATED SLUDGE
INTRODUCTION
     The activated sludge process is utilized extensively in its original
form as well as in the modified forms.  There are five main versions of the
process in common usage (Figure 3-11).  Conventional plug flow and complete
mix are the most common forms found in larger plants.  Step aeration is
gaining in popularity for use in large plants because of its greater flexi-
bility.  Contact stabilization is utilized in plants with flows of less than
10 mgd (440 dm3/s).  Extended aeration is usually used at flows of less
than 0.5 mgd (22 dm3/s).  For purposes of this discussion, only control of
sludge return and withdrawal from the clarifiers is considered.  Control of
wasting and of dissolved oxygen are discussed independently.  The need for
an integrated control strategy for the entire activated sludge process is
discussed in the Reconmended Future Activities, Section 8.

OBJECTIVE

     The primary objective of return activated sludge (RAS) control is to
maintain the stability of the activated sludge process.  This is accom-
plished by returning an active mass of microorganisms in a sufficient
quantity to remove the biodegradable organics from the influent wastewater.

     The stability of the process is of key importance.  Any perturbation of
the process may affect the ability to separate solids in the clarifier and
will degrade effluent quality.  Changes in the clarifier removal efficiency
affect the makeup and density of the returned sludge.  This may result in a
spiral decay of process performance.  Any fluctuations in the process will
affect bacterial growth rates which will affect the quantity of solids
wasted to the thickener.

     The microbial culture which dominates this process is subject to popu-
lation shifts; because growth and species predominance are controlled by a
variety of environmental parameters which are constantly changing.  In
addition, the actual effect of variations in environmental parameters on
microorganism predomination is only poorly understood.  Due to this very
dynamic environment, the process is difficult to control.  In order to
consistently control the process, the strategy must compensate for process
variations.  This can be (partially) accomplished by careful control of
return solids flow.
                                      74

-------
 INFLUENT   I
  T
  PLUG FLOW
AERATION  TANK
   INFLUENT
cn
  SLUDGE  RETURN
                  CONVENTIONAL
                PLUG  FLOW
               AERATION  TANK
                SLUDGE  RETURN
                 STEP AERATION
                                          WASTE  SLUDGE
INFLUENT
i
I
ff
II
i i
SLUDGE
f 4
U
1 1

RETURN
^f SETT
V TA»
                                                                                                         EFFLUENT
                                                    EFFLUENT     INFLUENT
                            WASTE  SLUDGE
                                                            ALTERNATE
                                                            WASTE SLUDGE
                                                            DRAWOFF POINT
                                                                                      COMPLETE  MIX
                                                                                                               WASTE  SLUDGE
                                                                     CONTACT
                                                                       TANK
                                                                                                                       EFFLUENT
                                                                                   STABILIZATION
                                                                                     TANK
                                                                          CONTACT STABILIZATION
                                                                                                            WASTE  SLUDGE
                             INFLUENT
                                       T_TT"
                                         GRIT     I
                                        REMOVAL   I
                                                        AERATION
                                                                                      EFFLUENT
                                                      24 HR. DETENTION

                                                              RETURN SLUDGE
                                                      EXTENDED  AERATION
                                Figure  3-11.   Activated sludge process flow diagrams.

-------
FACTORS AFFECTING PROCESS PERFORMANCE

Load Characteristics

     The activated sludge process is subjected to a variety of loads.  Diur-
nal fluctuations in both hydraulic and organic loading are often of large
magnitude and high frequency.  The process is affected by changes in water
temperature.  Industrial discharges of concentrated organics or 'toxins may
have dramatic effects on the process.  The method of control used may even
introduce additional process loads.  The dynamic conditions of the process
result in variations in the concentration and viability of the return
sludge.  Any load change on the process may have delayed repercussions due
to resultant changes in return sludge characteristics.

Process Characteristics

     Environmental conditions such as temperature and physical characteris-
tics such as pH  all affect the growth rates and kinetics of the micro-
organisms in the process.  Physical conditions such as short circuiting,
uneven mixing or problems due to  design affect process performance.  Distri-
bution of both liquid and solids  to multiple units is often out of control
and  results in large differences  between supposed parallel units.  The
process is subject to constraints on the ability to transfer oxygen to the
medium.

     Two factors have a major affect on the process.  One is the flow rate
which  is a short lag parameter.   Excessive flow which can occur during the
daily  peak can result in a poor effluent irrespective of the condition of
the  biological culture.  Here the effect is due to an excessive overflow
rate in the secondary clarifier.  The second factor is the effect of process
or load changes  on the biological culture.  This has a long lag period.  It
may  take several days to several  weeks for the effect of a process condition
change to be manifest in predomination in the biological culture.  Control
is further hindered by the difficulty in measuring process parameters.
Reliable on-line instruments for  some important parameters such as TOC, ATP
and  BOD are not  yet available.

     The most significant fact regarding the process is that the process
 itself generates the  active biomass to sustain the process.  If the process
degrades,  a degraded  biomass is formed and when returned causes further
process degradation.  The bacteria returned to the process are very sensi-
tive to process  fluctuations.

CONTROL STRATEGY

     Two types of control strategies are extensively utilized  in the field
 to control RAS.  One  returns sludge at either a fixed flow rate or a rate
paced  to the flow rate of primary effluent.  This can be considered a volu-
metric control strategy.  The second is based on clarifier sludge  inventory
control.  Typical  instrumentation is illustrated in Figure 3-12.
                                       76

-------
                        I FLOW RATIO
                                                                  TO
                                                                AERATION
                          RAS PUMPS
Figure 3-12.   RAS control.

-------
Volumetric RAS Control

     Determination of the volumetric return rate is generally by operator
experience based on the anticipated organic loading and present sedimenta-
tion characteristics of the sludge.  One widely used method of determining
the average volumetric rate of sludge return is based on a short time sedi-
mentation test conducted in the field.  A sample of mixed liquor is put in a
one or two liter graduate and allowed to settle for one-half hour.  The
initial and final sludge volume is recorded.  The ratio of final sludge
volume to initial sludge volume is multiplied by the primary effluent flow
to determine the rate of sludge return.  When the rate of return is fixed
(ratioed to average primary effluent flow), the MLSS in the reactor will
vary inversely with the primary effluent flow.  If the return rate is
ratioed to instantaneous primary effluent flow, the MLSS remains constant
but more severe flow transients are produced in the reactor and final
clarifier.

     Although volumetric control of return rate is widely practiced and
generally seems effective, it suffers from significant drawbacks.  The
required sludge return should be a mass rate of flow rather than a volume
rate of flow.  As long as sludge concentration after clarification remains
relatively constant, control on a volumetric basis is satisfactory.  How-
ever,  during upsets produced by microorganism shifts and during peak flow
periods, the sludge concentration will change significantly.  In addition,
when high rates of return are used, the return sludge concentration can then
vary considerably.  For these situations, volumetric control of return rate
is  not satisfactory.  Another constraint is that return rates are limited by
the capacity of the sludge return pumps available in the plant and the
availability of a reservoir of return sludge in the secondary clarifier.

     Volumetric return sludge control is relatively easy to implement and is
reasonably successful under conditions where sewage flow and characteristics
do  not change significantly.  RAS rates are generally 20 to 50% of the
primary effluent flow rate.

Clarifier Sludge Inventory Control

     The objective of the withdrawal strategy is to maintain a desired
sludge inventory (in the clarifier) within the constraints of RAS volumetric
demand.  Control is implemented by measuring or observing sludge blanket
levels and adjusting return to maintain an an adequate sludge blanket.

     Checking the depth of the sludge blanket in the clarifier is the most
direct method for determining the inventory.  The location of the sludge
blanket may be found by several types of devices.  Some are commercially
available while others must be made by the operator.  The following are some
of  the different types of detectors observed:

     1.   A series of air lift pumps mounted within the clarifier at various
          depths.
                                      78

-------
     2.   Gravity flow tubes located at various depths.

     3.   Electronic sludge level detector - a light source and photo-
          electric cell attached to a graduated handle or drop cord.  The
          photoelectric cell actuates a meter, buzzer, light, etc.

     4.   Sight glass finder - a graduated pipe with a sight glass and light
          source attached at the lower end.

     5.   Some type of portable pumping unit with a graduated suction pipe
          or hose.

     The blanket depth should be kept to less than one-fourth of the clari-
fier sidewall water depth.  The operator checks the blanket depth on a
routine basis, making adjustments in the RAS to control the blanket depth.
If the depth of the sludge blanket is increasing, the increase may result
from having too much activated sludge in the treatment system, and/or,
because of a poorly settling sludge, or plugging of the sludge removal
system.  Long-term corrections must be made that will improve the settling
characteristics of the sludge or remove the excess solids from the treatment
system.

     Measurements of the sludge blanket depth in the clarifier should be
made at the same time each day, or continuously.  The best time to make
these measurements is during the period of maximum daily flow, because the
clarifier is operating under the highest solids loading rate.  The sludge
blanket should be measured daily, and adjustments to the RAS rate could then
be made.  Adjustments in the RAS flow rate should only be needed occasion-
ally if the activated sludge process is operating properly.

     Availability of sludge is dependent on the performance of the clari-
fiers and the activated sludge process.  Due to the instability of the
aeration process loading to the clarifier, both flow and solids often
fluctuate.  Hydraulic splitting problems often lead to unbalanced clarifier
loadings.  One or two clarifiers may receive a disproportionately high load
of solids even when flow is evenly split.   Clarifier solids separation
performance is dependent on solids loading and hydraulic loading as well  as
the settling characteristics of the sludge floes.

     The inability to accurately measure the sludge level and concentration
hinders implementation of a control strategy.  Sludge collection by vacuum
or scrapers has historically been less than optimum.  Since the quantity of
sludge required to treat the wastes is the first priority, the consistency
must suffer at high volumetric demands.

INSTRUMENTATION UTILIZED

Sensors

     Sensors were observed to be in use in the field for measurement of the
following parameters:


                                      79

-------
     1.    MLSS - suspended solids analyzer.
     2.    RAS flow - magmeter or ultrasonic.
     3.    RAS suspended solids - optical or nuclear.
     4.    Clarifier underflow - magmeter or ultrasonic.
     5.    Sludge level - optical, ultrasonic or airlift.
     6.    TOC - both influent and effluent (may be performed in  lab).
     7.    RAS wet well level switch.
Modulating Control Devices
     1.    Variable speed pump drives.
     2.    Valves (plug).
Controlling Devices
     1.    Switching logic for pumps.
     2.   Flow  controllers  (PID).
     3.    Level controllers.
Annunciator Alarm Points
     Annunciators should be provided for:
     1.   Low  and high sludge, levels in clarifier.
     2.   Wet  well  level.
     3.   Pump failure.
     4.   Sensor failures.
VARIATION IN  STRATEGY WITH  PLANT SIZE
     Extended  aeration is often  used in small plants with flows  of  less  than
0.5  mgd  (22  dm3/$).  Flow arrangement  is similar to conventional treatment
with the exception  that primary  treatment  is omitted as shown  in Figure
3-12.  The goal  in  extended aeration is to completely  oxidize  the organic
material  in  the waste.  Aeration times  are usually about 24 hours and
require  greater oxygen inputs.  With this process configuration, all acti-
vated sludge generated is returned.  No wasting is typically required.
                                      80

-------
     Plants treating flows of less than 5 mgd  (220 dm3/s) may  utilize
contact stabilization.  Flow arrangement is as shown  in Figure 3-12.
Adjustment of return rate is controlled by adjusting  air flow  rates to  air
lift pumps.  A longer lag time in response to return  demand  is  present  in
the sludge aeration basin.  Again with contact stabilization,  all the sludge
generated is returned to stabilization (and ultimately contact)  and wasting
is only necessary very infrequently (monthly).

     In our plant surveys conventional, complete mix  and step  aeration  were
observed.  Flow diagrams for these processes are shown in Figure 3-11.  The
strategy of control does not vary significantly among these forms of the
activated sludge process, but implementation of the strategy will vary  due
to variations in process arrangement.

OTHER CONSIDERATIONS

     As shown in the Minneapolis-St. Paul Metro and Denver #1 plants, labo-
ratory tests and physical observations such as color  and smell of return
sludge all weigh heavily in an operator's decision to override the control
strategy.  Chemical, organic and settling tests are made each shift or more
frequently and may indicate that significant changes  in strategy are
required.  Other lab tests include MLVSS, influent BOD and effluent BOD.
Each shift a F/M ratio and mass balances are calculated as a check on cali-
bration of sensors and control devices.  Any lab test or observation that
indicates a need for an immediate change in control strategy is typically
repeated before action is taken.

     In the previous discussion no indication was given of the need to
periodically waste excess activated sludge from this process.  In fact, as
detailed in the next section, controlled wasting of sludge must be con-
ducted.  Over the long run, sludge not wasted must be returned to the
aerator and vice versa.  Thus, on the average, sludge return and sludge
wasting are linked, not independent parameters.  In this section and the
next one these parameters have, however,  been treated as independently vari-
able.  This was done because accepted control strategies in the field treat
them as independent.  In addition, temporary storage of sludge will  permit
some degree of independent action.  The degree of independence  is a function
of sewage and recycle flows, tankage configuration and mode of activated
sludge operation.  Conventional activated sludge provides little storage as
only a portion of the clarifier capacity is available.  Step aeration
provides more as the front portion of the aerator is available when all
sewage flows are routed to the downstream compartments.  Contact stabiliza-
tion systems provide the most storage since the whole of the sludge
reaeration volume is available for short  term storage.  Although in the
short term independent action is possible,  it must be remembered that on a
daily average basis return sludge and sludge wasting are linked and  will
limit the degree to which the other can be varied.

     This will  be taken up again in Section 8 under research needs for
activated sludge control.
                                      81

-------
EXPECTED PERFORMANCE

     A good deal of work has been done at the Denver Metro plant regarding
instantaneous control of return sludge via TOC and other advanced control
strategies.  To date, this work has shown little advantage.  The volumetric
or sludge inventory methods utilized in the field have provided adequate
process stability under most circumstances.  Further work on control  of this
process is indicated.
                                      82

-------
                            WASTE ACTIVATED SLUDGE
INTRODUCTION
     The activated sludge process (including all variations) will generate
solids which inevitably must be wasted.  Some process configurations produce
more sludge to be wasted than others.  Sludge can be wasted from the under-
flow of a single chosen clarifier, from the RAS pumping well, from a sepa-
rate WAS pumping well or directly from one aeration reactor or all the
reactors.  Some process configurations allow both wasting from the clarifier
underflow and from the aeration tank.

     Field observations indicate that wasting of the activated sludge is
normally done by removing a portion of the RAS flow.  The waste activated
sludge is either pumped to thickening facilities and then to a digester, or
to the primary clarifiers where it is pumped to a digester with the raw
sludge.

     The alternate method for wasting sludge from the mixed liquor in the
aeration tank was not frequently observed.  There is much higher concentra-
tion of suspended matter in the RAS than there is in the mixed liquor.  When
wasting is done from the mixed liquor, larger sludge handling facilities are
required.  Many plants do not have the flexibility to waste from the mixed
liquor nor is there sufficient sludge handling facilities to handle the more
dilute sludge.  For these reasons, this discussion will concentrate on
control strategies for wasting sludge from the RAS pumping well.  This
configuration was most often observed in the field.

     WAS pumping influences mainly the solids train processes in the short
term.  Rapid changes in wasting rates affect solids train performance but
contribute only small hydraulic effects on the liquid train.  Improper
wasting rates maintained for prolonged periods will seriously affect the
performance of the activated sludge process.

OBJECTIVE

     The objective of sludge waste control is to control the level of solids
in the activated sludge system.  If the quantity of solids becomes too high
or too low, the performance of the process will degrade.  Excess solids in
the system produce too high a solids loading on the clarifier.   If the level
of solids in the system is too low, sufficient organisms will not be present
to remove the required organics from the wastewater.  Predomination of
poorly settling organisms has often been traced to too high or  too low a
solids level  with respect to the organic loading.


                                      83

-------
FACTORS AFFECTING PROCESS PERFORMANCE

Load Characteristics

     All load characteristics discussed in RAS also influence the control of
WAS pumping rates.  If the cell synthesis rate is disturbed by physical or
environmental changes, the wasting rate must be altered if the system  is to
stay in balance.

     The effect of the loads influencing control of WAS flow are not as
dynamic as those affecting RAS flow control since solids residence time is
of the order of days as compared to a  liquid train residence time of hours.
Overflow suspended solids should be considered as unintentional sludge
wasting and should be accounted for in determining WAS requirements.

Process Characteristics

     WAS flow  is generally expressed in terms of mass of sludge to be  wasted
per  day.  The  availability of a measure of mass flow is dependent on the
ability to measure both sludge flow and density accurately.  Measurement of
these  variables  and qualities such as  cell synthesis rate  are difficult and
in some cases, not possible.  The typical rangeability designed in the WAS
pumps may also create problems at peak demands if mass flow is to be
maintained.

CONTROL STRATEGY

     Wasting of  the activated sludge can be done on a intermittent or  con-
tinuous basis.   The intermittent wasting of sludge means that wasting  is
conducted on a batch basis from day to day.  Intermittent wasting of sludge
has  the advantage that less variation  in the suspended matter concentration
will occur during the wasting period,  and the amount of sludge wasted  will
be more accurately known.  The disadvantages of intermittent wasting are
that the sludge  handling facilities in the treatment plant may be loaded at
a higher hydraulic loading rate and that the activated sludge process  is out
of balance for a period of time until  the microorganisms regrow to replace
those  wasted over the shorter period of time.

     The simplest and most commonly used approach in controlling the amount
of sludge wasted  is to waste enough to maintain a nearly constant MLVSS.
This technique usually produces good quality effluent as long as the
incoming wastewater characteristics are fairly constant with minimal varia-
tions  in  influent flow rates.  The operator tries to maintain a constant
MLVSS  concentration in the aeration tank to treat the incoming wastewater
organic load.

     Field observations produced four  control strategies commonly used to
control the  activated sludge wasting rate.  Typical control and instrumenta-
tion is illustrated in Figure 3-13.
                                      84

-------
                                                     OPERATOR
                                                    CALCULATION
                                       LSLI	
00
CD
          FROM CLARIFIERS
                                   RAS
                                 WET WELL
                                                           MASS FLOW
                                                           CONTROLLER
                                                           WAS PUMPS

                                                                                                        MASS FLOW
                                                                                                       I CALCULATOR
                                                                                                	I   I	
FT
r
i
i.
FE
               I
               i.

              DT
                                                                                                                       TO  THICKENING
                                                      Figure  3-13.   WAS  control

-------
     1.    Controlled Solids Retention Time (SRT) - A quantity of WAS is
          calculated daily based on the system so.lids and the selected SRT
          value.   The SRT value used is determined by experience at each
          plant.   The optimum value shifts seasonally.  The wasting rate is
          then set to maintain a desired SRT.

                     Pounds of System Solids (aeration and clarification)
               SRT =	
                       Pounds Wasted Daily   (intentional and unintentional)

     2.    Biosynthesis - A wasting rate in pounds per hour is calculated
          based on the rate of new cell synthesis.  This system has been
          implemented at the Denver plant with very good results.

     3.    Setpoint based on mass flow - A setpoint is determined and varied
          on a daily basis.  The setpoint is based on maintaining a desired
          range of F/M ratio in the aeration basin.

     4.    Setpoint by flow - A WAS flow setpoint is determined based on the
          target level of MLSS in the aerator.

     In the WAS process, the volumetric flow is typically both the
controlled and manipulated variable.  Measured variables include MLSS,
effluent suspended solids, primary effluent suspended solids and RAS solids
concentration.

     WAS flow control is limited by the rangeability of the WAS pumps.  The
use of WAS control calculations based on solids balance is limited by the
ability to accurately measure all of the necessary parameters.  The frequent
unavailability of certain measurements due to the high maintenance require-
ments of the instruments also limits the use of WAS calculations in a
control  strategy.

OTHER CONSIDERATIONS

     All operator and laboratory observations of the process pertaining to
RAS also apply to the WAS flow.  MLSS is observed and used as an override on
the WAS calculation if the solids are too low or too high.  Most plants
surveyed used a daily solids balance to check the calibration of measurement
devices.  If the process has poorly settling sludge as indicated by a high
SVI reading, the WAS flow may be either stopped or run at maximum depending
on the assumed cause of the poor sludge compaction.

INSTRUMENTATION UTILIZED

Sensors

     The following sensors were observed to be in use in the field:

     1.    WAS flow rate - magmeter or ultrasonic.
                                      86

-------
     2.    WAS suspended solids - optical or nuclear.
     3.    Blanket level - optical, ultrasonic or airlift.
     4.    MLSS - suspended solids analysis.
     5.    Effluent suspended solids - usually a lab test can be done with
          analyzer.
     6.    TOC - influent and effluent, can be a lab test.
Modulating Control Devices
     1.    Pump drives.
     2.    Valves, if applicable.
Controlling Devices
     1.    Pump speed controllers.
     2.    WAS flow calculator.
Annunciator Alarms
     1.    Pump failures.
     2.    Low flow in pump.
     3.    Low return suspended solids.
     4.    Low wet well level.
VARIATION IN STRATEGY WITH PLANT SIZE
    "The wasting control strategy for the activated sludge processes
utilized in the larger plants—conventional, complete mix and step aera-
tion—do not significantly change with plant size.   Process design is
similar except in size of units and method of carrying out the strategy.  In
small plants the use of contact stabilization or extended aeration becomes
advantageous.  In a contact stabilization process,  wasting is typically
intermittent and infrequent.  Wasting is usually to the aerobic digesters.
     In a plant using the extended aeration process (usually less than 0.5
rogd, 22 dm3/s), wasting is done very infrequently and only when needed.
The goal in extended aeration is complete oxidation of all organic
material.  This strategy yields very little growth  in cell mass.   The non-
degradable solids eventually build up and must be cleaned out, usually once
or twice a year.
                                      87

-------
EXPECTED PERFORMANCE

     Denver has shown improvement in process consistency since implementing
a strategy of mass flow control based on a calculated biosynthesis or sludge
growth.  The key is not to maintain a flow, but a mass flow as solids con-
centration from the clarifiers does change radically.  Most other plants
visited waste based on MLSS set by a volumetric flow rate.  Process consist-
ency suffers but the monthly average performance is generally adequate with
laboratory diligence.

-------
                                CHEMICAL FEED
INTRODUCTION

     Chemical addition has many applications in wastewater treatment,
including polymer addition to aid suspended solids removal, sludge condi-
tioning prior to the various dewatering techniques, lime addition for
adjusting pH, and chlorine addition for disinfection.  Chlorination  is
covered in a separate section.  Three methods of chemical addition are
discussed here;  fixed rate, flow proportional and mass flow proportional.
These approaches are discussed because they were observed in the field.

OBJECTIVE

     Objectives  in chemical feed control include the improvement in  the
operation of the unit process and minimization of chemical usage.  In
solids/liquid separation, chemical addition promotes removal of suspended
solids by adding a flocculant.  In a sludge dewatering process, chemicals
are added to aid in the formation of sludge cake from which water can be
readily removed.  The particular chemicals that are used in dewatering will
depend on the method of removal used.  For example, lime may be added to
control the pH after use of a coagulant aid in dewatering.  Coagulation is
pH sensitive.

     The interactions of chemical feed are extreme.  If the unit processes
upstream are unstable, the chemical dose must be highly variable in  order to
compensate for the changes in feed character.  If the unit processes down-
stream are sensitive (dewatering), the chemical feed control must be precise
in order to maintain performance.

FACTORS AFFECTING PROCESS PERFORMANCE

Load Characteristics

     The required chemical dosage is difficult to determine because of the
number of variables which affect the required dose.  Also, the relationships
between the process goal and the chemical dose is often not well understood
(e.g. how much polymer is needed to produce a solids capture of 90%  in a
flotation thickener).  For this reason, empirical modeling is required to
determine dosages and optimization is not attempted.  If necessary, the
strength of the  stock chemical solution should be frequently monitored.
                                      89

-------
Process Characteristics

     The metering pumps, typically positive displacement with SCR drive
speed controls, provide good rangeability to control chemical addition.
Flow feedback measurement is not generally required since the pump speed
provides a fair estimate of feed flow.

CONTROL STRATEGY

     Three strategies are describee below; operator set speed, flow pacing,
and mass flow pacing (see Figures 3-14 and 3-15).  In all cases the con-
trolled variable is chemical feed rate, pump speed is the manipulated
variable, and measured  variables are those that  are associated with the  load
including process feed  flow, influent suspended  solids, and occasionally
effluent suspended solids.

     Two important constraints of the control system are the large process
time constant  and the lack of  an on-line feedback signal.

     Chemical feed rate control  is typically based on process hydraulic flow
or mass flow and the degree to which the desired results are achieved  is
used to further modify  the chemical feed rate.   Chemical feed pumping  is
typically alternated between primary and standby pumps.  Pump control  can be
interlocked to shut off on day tank low level.

Operator Set Speed

     Chemical feed control could consist of simply having the operator set
the feed rate as determined by the process flow  rate.  However, this method
requires very frequent  operator  attention, otherwise much chemical will be
wasted.  Adequate load  following cannot be achieved with this method unless
there  is frequent operator attention.

Flow Pacing

     This method requires an operator entered ratio of chemical feed flow to
process flow.  The process stream flow rate is used as input to the ratio
controller to control the pump speed rate based  on an operator ratio.

Mass Flow Pacing

     This method is similar to above except a calculation of process feed
mass flow rate is utilized as the input to the ratio controller.  The
controller then drives  the chemical feeder based on an operator entered
ratio  setpoint around the load input (mass flow).

INSTRUMENTATION UTILIZED

     The following instrumentation and control devices are typically used:

     1.   Flow meters - mag meters.


                                      90

-------
      DESIRED FLOW
                                                                    SUSPENDED  SOLIDS
                                                                          OR
                                                                    MOISTURE CONTENT
                                                                         OR ON—LINE'S
                                                                      MEASUREMENT  )
                                                     SOLIDS/LIQUID
                                                      SEPARATION
                                                        OR
                                                     DEWATERING
                                                       PROCESS
Figure 3-14.   Chemical  feed  control  -  flow pacing.

-------
                                            DESIRED MASS FLOW
UD
ro
                                                                                        SOLIDS/LIQUID
                                                                                           SEPARATION
                                                                                              OR
                                                                                          DEWATERING
                                                                                            PROCESS
                                                                                                           SUSPENDED SOLIDS
                                                                                                                   OR
                                                                                                           MOISTURE CONTENT
                                                                                                          /'LAB OR ON  LINEN
                                                                                                          V.   MEASUREMENT /
T
                                    Figure  3-15.   Chemical  feed control  - mass  flow  pacing.

-------
     2.   Suspended solids analyzer (optional) - this is used to establish
          process mass flow.

     3.   Level switches in feed tank.

     4.   SCR metering pump drive.

     5.   FIC or MFIC flow or mass flow control - PID type.

     6.   Mass flow calculator.

VARIATIONS IN STRATEGY WITH PLANT SIZE

     Small plants typically use fixed rate or intermittent addition of
chemicals.  The intermediate sized plants usually use the flow pacing
method.  Large plants will typically use flow pacing and start/stop sequence
controls for a number of metering pumps.  The larger plants sometimes incor-
porate suspended solids measurements in the control strategy.  Mass flow
ratioing was observed, but infrequently.

EXPECTED PERFORMANCE

     The large time constants make measurements difficult and therefore the
improvements gained through the use of chemical feed control are difficult
to evaluate.  Field results have shown that chemical feed control allows
dewatering and separation processes to function properly, but the nature of
the loads involved make it nearly impossible to predict the improvement that
may be achieved by utilizing one mode versus another.
                                      93

-------
                              POST-CHLORINATION
INTRODUCTION
     Chlorine has historically been used in this country for wastewater
treatment plant effluent disinfection.  Chlorine gas is the form in which
this substance is most frequently used.  The dangers of this gas are widely
known.  At smaller facilities a hypochlorite solution is preferred.  It is
more expensive, but safer.  At very large facilities, hypochlorite is
becoming prevalent again because of the safety aspect.  Safety is also one
of the reasons for the adoption of ozone for effluent disinfection at new
plants.

     Because post-chlorin tion overwhelmingly includes use of chlorine gas
and because hypochlorite and ozone are discussed with other strategies,
post-chlorination with chlorine gas will be discussed here.

OBJECTIVE

     The specific goal of chlorination control is to maintain adequate
chlorine residual to insure that disease producing organisms (pathogens) are
destroyed.  The more complex process control systems are better able to
follow the load  and maintain the desired chlorine residual, saving excess
chlorination costs.  However, the uncontrolled hydraulics and difficulty of
measuring bacteria level affect the ability of the control system to
consistently meet the process objectives.

FACTORS AFFECTING PROCESS PERFORMANCE

Load Characteristics

     The load on the process is not the quantity of microorganisms, but
rather substances which exert a chlorine demand.  The load is organic,
inorganic, hydraulic and solids and follows diurnal flow patterns.  Also,
there are industrial dumps which can produce peaks of highly oxidizable
wastes in the plant influent.  With the exception of flow, on-line real time
measurement of other load parameters  (TOC, SOC, ammonia, nitrate, sulfides)
is difficult to  achieve.  Empirical relationships between these load para-
meters and chlorine demand are under  development.

Process Characteristics

     The most important process characteristic is that the final chlorine
residual is not  determined until long after (e.g. 15 minutes) chlorine has


                                      94

-------
been added.  This time is required to provide the contact time necessary to
effect kill of the organisms.  The final chlorine residual analysis  is some-
times not even done on site, further delaying the measurement.  Another
typical process characteristic is the crude mixing achieved in flow  over
weirs in the chlorine contact basin.

     Other difficulties arise in measuring chlorine gas flow rate.   Control
of the gas flow rate requires sensitive equipment to regulate a low  flow
under vacuum.  Instrumentation to measure suspended solids level for feed-
forward chlorine control  is increasing in reliability as is instrumentation
for TOC.

CONTROL STRATEGY

     A chlorinator is a device which takes gaseous chlorine and prepares a
stock solution of the chlorine in water and meters the stock solution into
the flow to be treated.  The vacuum regulating valve sets the flow of
chlorine/gas and thus controls the preparation of the stock solution.  The
orifice positioner meters the stock solution into the flow.

     There are four different control strategies commonly used for chlorina-
tion control.  They are,  in order of usage; flow proportional, compound
loop, double compound loop, and ratioed feedback.

     For all methods described below, the controlled variable is chlorine
residual (indirectly, the bacteria level).  The manipulated variable is gas
flow.  Measured variables include plant flow, and chlorine residual.  Sus-
pended solids and TOC would be included as measured variables if they are
used in conjunction with  plant flow as feedforward control, although this is
rarely observed in practice.

Flow Proportional

     This method requires a process influent flow meter, chlorinator and a
ratio station control.  The vacuum regulator is manually set to meter
chlorine for maximum plant flow.  The operator enters the ratio setting, as
determined from the lab analysis of the parameters of the load, to control
the orifice positioner in relation to the process stream's measured flow
rate.  Only manual monitoring of chlorine residual level is done,  so optimum
residual level cannot be  maintained.

     Instrumentation and  control devices typically used in conjunction with
the chlorinator are as follows (see Figure 3-16):

     1.   Flow meter - mag meter (process flow).
       i
     2.   Ratio station - orifice control.

     3.   Manual loading  station - vacuum regulation.
                                      95

-------
                              GAS
PROCESS WATER
                    I
                   O,
                   FE
                                                       RATIO
                                                       STATION
                                                  I
                                                VACUUM
                                                REGULATING
                                                VALVE
CHLORINATOR
                                                                      DILUTION
                                                                      WATER
                                                                 EJECTOR
                                                                                       Cl_2 CONTACT BASIN
                                                                                                                    CL2
                                                                                                                    RESIDUAL
                             Figure 3-16.   Chlorine  feed  control  -  flow  proportional.

-------
Compound Loop

     The compound loop process is set up so as to modulate both vacuum regu-
lation and orifice position.  In this control strategy feedforward control
based on process flow and feedback control based on chlorine residual are
employed.  The feedforward control is on the vacuum regulator while the
feedback control is effected on the orifice positioner.  This split must be
used because of the inability to utilize analog control devices with simul-
taneous feedforward and feedback elements.  A chlorine residual controller
(CRC) is used in addition to the devices described under flow proportional
control.  Compound loop control uses a ratio station with process flow input
and an operator entered ratio setting for vacuum regulation.  Orifice posi-
tion is set by the output of the CRC which uses an operator entered setpoint
and feedback from the chlorine residual analyzer.  This control strategy has
the advantage of closely matching the chlorine dosage to the requirements of
the process by controlling orifice position as well as vacuum regulation.

     Instrumentation and control devices typically used in conjunction with
the chlorinator are as follows (see Figure 3-17):

     1.   Flow meter - mag meter (process flow).

     2.   Chlorine residual analyzer.

     3.   Ratio station - vacuum regulation.

     4.   Chlorine residual controller - PID type.

Double Compound Loop

     Double compound loop uses a cascade control system to control orifice
position (gas flow).  An additional chlorine residual analyzer is used in
the contact basin to provide more rapid feedback control.   The process flow
through the chlorine contact basin takes approximately fifteen minutes in
this example.  To overcome the long delay in obtaining residual chlorine
measurement when the analyzer is at the end of the basin,  a second residual
chlorine analyzer is placed to obtain a sample at a position five (flow)
minutes into the basin.  The latter provides feedback to the CRC in the
secondary loop controlling orifice position.  The secondary loop CRC set-
point is provided by the output of the primary loop CRC using an operator
entered setpoint and the residual chlorine level at the exit of the chlorine
contact basin as feedback.  This method provides superior  load following and
can save a substantial amount of money through reduction in unnecessary
chlorine use.

     Instrumentation and control devices typically used in conjunction with
the chlorinator are as follows (see Figure 3-18):

     1.   Flow meter - mag meter.

     2.   Ratio station - vacuum regulation.


                                      97

-------
00
                                                                                     DESIRED RESIDUAL

                                                                                      SP
                                                                         CHLORINE
                                                                         RESIDUAL
                                                                         CONTROLLER
         PROCESS WATER
                                                                                                                        CL2
                                                                                                                       RESIDUAL
                                                                                           CL2 CONTACT BASIN
                                    Figure  3-17.   Chlorine feed control  - compound loop.

-------
vo
        PROCESS  WATER
                                 CLZ OAS
                                                        TRATIO
                                                          STATION
                                                      VACUUM
                                                     REGULATING
                                                       VALVE
                                                 [ORIFICE
                                              I	(POSITIONER
                                               CHLORINATOR
                                                                     CHLORINE
                                                                     RESIDUAL
                                                                     CONTROLLERS
EJECTOR
                                                                                                                           CL2
                                                                                                                           RESIDUAL
                                                                                             CL2 CONTACT BASIN
                                Figure 3-18.   Chlorine  feed  control  - double  compound  loop.

-------
     3.   Two chlorine residual analyzers.

     4.   Two CRC's - PID type.

Ratioed Feedback

     This method requires a flow meter, control valves, residual chlorine
analyzer, chlorine residual controller (CRC), and ratio controller.  The
vacuum adjust is manually set to meter chlorine for maximum plant flow.  The
CRC compares the chlorine residual with an operator entered setpoint and the
output varies the ratio controller's ratio setting.  The process stream's
flow measurement is wired to the ratio station as the driving signal.  The
feedforward flow times the variable ratio determines the output to the gas
flow control orifice.

     Instrumentation and control devices typically used in conjunction with
the chlorinator are as .follows (see Figure 3-19):

     1.   Flow meter - mag meter.

     2.   Chlorine residual analyzer - requires daily cleaning, but the
          primary benefit returned is good effluent residual control.

     3.   Ratio station - orifice control.

     4.   Chlorine residual controller.

     5.   Manual loading station - vacuum regulation.

     This configuration was shown in the field to produce highly variable
and unpredictable results because it attempts to combine feedforward and
feedback control with a single control device.

VARIATIONS  IN STRATEGY WITH PLANT SIZE

     Different size plants typically use different control strategies.   In
the small plants (less than 5 mgd - 220 dm3/s) flow proportional control
 is usually  adequate or the chlorine feed rate  is sometimes fixed.  Midsized
plants  (5 to 50 mgd, 220 to 2200 dm3/s) typically use compound  loop con-
trol.   Chlorine residual is often measured to determine the ratio settings
for control.  Larger plants (100 to 300 mgd, 4400 to 13000 dm3/s) usually
use flow proportional control, often sequencing the operation of multiple
chlorinators (although problems arise due to system complexity  and corrosion
of relays by chlorine gas).  Plants of this size may also automatically
control  the liquid chlorine evaporator delivering chlorine gas  to the
 chlorinator.
                                      100

-------
PROCESS WATER
                                              i
                                              VACUUM
                                            REGULATING
                                              VALVE
                                                               CHLORINE
                                                               CONTACT
                                                               CONTROLLER
                                                                              DESIRED RESIDUAL
                                                                 DILUTJON
                                                                   WATER
                                                                          \J RATIO
                                                                                STATION
                                                                                                                 CL2
                                                                                                                 RESIDUAL
                                                                                     CL2  CONTACT BASIN
                          Figure 3-19.   Chlorine  feed control  - ratioed feedback.

-------
EXPECTED PERFORMANCE  '

     Experience has shown that chlorine residual is difficult to control.
There are often significant deviations from the setpoint, usually excess
chlorination.  Control system performance achieving a chlorine residual
within 10/6 of setpoint will be a fortunate occurrence due to the dominance
of the process time lag and the problems associated with residual ana-
lyzers.  With use of flow proportional control only, it  is possible to stay
above the minimum desired level, but difficult to accurately adjust dosage
for load variations.
                                      102

-------
                                  OZONATION
INTRODUCTION

     The use of ozone in wastewater treatment for disinfection and odor
control has only recently begun to be implemented.  Ozone is generated by
high voltage, electric discharges in air or oxygen.  In general, the amount
of ozone generated is increased by increases in the oxygen bearing gas flow
rate, oxygen concentration, applied voltage level and electrical discharge
frequency.

     Ozone generation can be an independent process utilizing air as the
oxygen source.  However, ozone generation becomes cost effective if high
purity oxygen is available at the plant for other purposes since the energy
required per unit mass of ozone is about half of that which is required if
air is the feed gas.   Ozone is considered to be a more potent disinfectant
and oxidant than chlorine, and ozonation byproducts may not be harmful, in
fact, one byproduct is oxygen which is beneficial to water quality.

     For these reasons the major use of ozone in wastewater treatment in the
future will be as a disinfectant in conjunction with a high purity oxygen
source.  The oxygen is usually generated for use in pure oxygen activated
sludge systems.  The oxygen source will generally be a cryogenic plant for
flows of about 15 mgd (660 dm3/s) and greater, or a pressure swing adsorp-
tion (PSA) unit for lower flows.  Liquid oxygen can be purchased for reva-
porization for extremely small flows.

    ,In this report the model studied uses ozone for disinfection generated
from high purity oxygen supplied by a cryogenic plant, with ozonation off-
gases reused in an activated sludge process.

OBJECTIVE

     The objective of ozonation is disinfection of wastewater to meet
federal or state standards.  Disinfection is assumed to be accomplished with
the destruction of all disease producing organisms (pathogens).

     Disinfection is  a high priority goal in wastewater treatment because of
the potential health hazard of improperly treated sewage.  Ozone demand is
dependent on the quality and quantity of the liquid being treated and is not
synchronous with oxygen demand or supply.  This requires ozonation to be
widely rangeable in order to provide adequate disinfection at a reasonable
cost.
                                     103

-------
FACTORS AFFECTING PROCESS PERFORMANCE

Load Characteristics

     The required ozone dose is affected by changes in wastewater flow and
quality.  Diurnal and seasonal fluctuations in hydraulic loading and refrac-
tory organic loading are often large in magnitude.  Instabilities in plant
operation lead to varying loadings of dissolved organics, dissolved inorgan-
ics and suspended solids.  Industrial discharges may cause  large changes  in
pH which can alter ozonation efficiency.

Process Characteristics

     The ozonation process is controlled by mass transfer of  ozone  into the
wastewater.  Since ozone rapidly  decomposes and has slow diffusion  rates,  it
is  important to  intimately mix the ozone bearing gas with the wastewater.
The contact basin should be  designed for an average contact time of about
fifteen minutes  and  should be compartmentalized to prevent  short circuiting.

     The controlled  variable (disease producing organisms)  is not a measured
parameter in the automatic control system  but tests are performed to
indicate the possible presence of pathogens.  Ozone residual, which would
indicate ozone  in excess of  that  required  for disinfection, is  difficult  to
measure because  ozone rapidly decomposes.  Furthermore, ozone analyzers are
not ozone specific and require a  great deal of maintenance.  Thus,  ozone
production requirements  (in  an hour-to-hour sense) are  largely  determined
from feedforward parameters.

CONTROL  STRATEGY

     The high purity oxygen  feed  gas  is first pressurized by  a  boost  com-
pressor  to overcome  the  liquid depth  in the contact tank.   The  compressed
gas is sent  through  the  generators where ozone  is  produced  by a corona
discharge.   The  ozone bearing gas is  next  diffused  into the wastewater in
the contact  basin.   The  off-gases are removed from the  contact  basin  by a
blower which sends the gas  to the oxygen dissolution process.  These  gases
 are typically not recycled  to the ozonator.

     As  stated  earlier,  the  controlled  variable (fecal  coliform count)  is
 not available as an  automatic control parameter,  nor  is ozone residual.
Thus ozonation  is usually operated  in  an open-loop control  mode which  can
 lead to  over or  under disinfection.

     Manipulated variables  are corona discharge voltage or  frequency  (but
 not usually  both)  and oxygen flow.   These  variables  are manipulated to
 increase or  decrease the amount of ozone produced.

      Measured  variables  can include the following:

      1.    Flow,  pressure and temperature  of the boost compressor discharge.
           These are  used to determine mass flow rate.


                                      104

-------
     2.   Flow of boost compressor inlet.

     3.   Flow of off-gas blower inlet.

     4.   Flow of effluent to be disinfected.

     5.   Contact basin pressure.

     6.   Generator discharge ozone concentration.

     7.   Ozone residual (dissolved in water).

     8.   Ozone purity (gas phase).

     There are several factors limiting the ozonation control system from
meeting the disinfection objective.  As indicated previously, the primary
constraint is that the controlled variable is not readily measured thus
requiring open-loop operation.  Another constraint is that the oxygen supply
varies with the oxygenation requirements of other sections of the plant.  If
these coincide with oxygen demand for the ozonation, system capacity could
be overcome.

     An ozone-oxygen system has been operating continuously for about five
months at the Southwest Treatment Plant in Springfield, Missouri.  The auto-
matic controls used are indicated in Figure 3-20 and discussed below.  The
P&ID is a simplified representation; for example, there are actually thir-
teen generators although only two are shown.

     Startup and shutdown of generators is performed manually; the number of
generators operating at one time is based on the operator's experience in
obtaining the required ozone for the least energy.

     During steady-state operation the following automatic controls are
active:

     1.   Plant flow controls oxygen feed flow.

     2.   Contact basin pressure controls off-gas flow rate.

     3.   Operator controls generator output based on feedforward from plant
          flow and feedback from generator discharge ozone generation.

OTHER CONSIDERATIONS

     Ozone required for disinfection increases with increasing suspended
solids (SS) levels, nitrite levels and total organic carbon (TOC or COD)
levels of the effluent to be treated.   The operator should adjust the ozone
production to compensate for fluctuations in these.  Filtration prior to
ozonation reduces the variation in SS.
                                     105

-------
                                          DOSAGE
OPERATOR   DESIRED
 ADJUST    PRESSURE
   I        i
VACUUM
  l
HIGH PURITY 0)
FROM CRYO
 PROCESS INFLUENT
                                                                                                                            TO
                                                                                                                       OXYGENATION
                                                                                                                DISINFECTED EFFLUENT
                                           Figure 3-20.   Ozonation control.

-------
     Fecal coliform counts and laboratory ozone residual tests show if over
or under disinfection is occurring.  The operator uses this information and
experience to adjust ozone dosage accordingly, usually once per shift.
     The operator must also periodically inspect the equipment performance.
INSTRUMENTATION UTILIZED
     The following lists contain typical instrumentation for an ozonation
system.  Unless otherwise stated, the sensors and devices are capable of a
high degree of accuracy and reliability.
Sensors
     1.   Gas flow meters - orifice plates with correction for temperature
          and pressure.
     2.   Plant flow - flume.
     3.   Temperature (generator gas discharge).
     4.   Pressure (generator gas discharge).
     5.   Ozone concentration - infra-red.
     6.   Ozone residual - electrode with ozone gas permeable membrane or
          other amperometric method.  These instruments usually have high
          maintenance and poor accuracy, however improvements in both areas
          are being made.
Modulating Control Devices
     1.   Centrifugal compressors - inlet guide vanes or butterfly
          throttling valve control flow until surge point is reached.
          Bypass lines with butterfly valves control recycle to give net gas
          flows below surge conditions.
     2.   Generators - variable voltage transformer or discharge frequency
          oscillator.
On/Off Control Devices
     1.   Compressors.
     2.   Generators.
     3.   Purge blower.
                                     107

-------
Controlling Devices

     1.   PIC's.

     2.   FIC's.

     3.   Manual loading stations.

Annunciator Alarms

     1.   Compressor surge.

     2.   Compressor aftercooler gas temperature.

     3.   Compressor aftercooler coolant temperature and pressure.

     4.   Generator coolant temperature and pressure.

     5.   Ozone purity high and low.

     6.   Contact basin pressure.

     7.   50% Lower explosive limit (LEL).

     8.   Supply gas dew point.

VARIATION OF STRATEGY WITH PLANT SIZE

     For small plants (less than about 15 mgd - 660 dm3/s), the oxygen
source would probably not be a cryogenic plant, but rather pressure swing
adsorption (PSA) or vaporization of purchased liquid oxygen.  This may
change the economic picture and was not observed in the field.  The size
cutoff is not exact because a high BOD influent would require more oxygen,
hence a cryogenic plant might be justified at flows less than 15 mgd (660
dm3/s).

     In large plants the number of generators required can become very high
because they are not currently manufactured with large capacities.  For
example, the Springfield, Missouri, plant has thirteen generators at a
design flow of 35 mgd (1540 dm3/s).

EXPECTED PERFORMANCE

     Good control of the ozone generation process can be expected.  Opera-
tors at the Springfield plant have not experienced any control problems
related to generation.   They report good disinfection reliability; however,
there were periods when the disinfection objective was not met.  This
occurred during a period of system adjustment.  Initially six generators
were put on line; each  one set to produce about 300 pounds of ozone per
day.  Total coliform inactivation  was obtained so the number of operating
generators was reduced  until  the ozonation objective was no longer achieved


                                     108

-------
to determine the minimum required.  For current plant flows, three or four
generators are operated, depending on the effluent flow and suspended
solids.  In this operating mode, excess disinfection most often occurs.

     Although this plant's ozonation system is operating well now, part of
this success is due to operation at partial capacity.  The c.'yo plant is
designed to produce 50 tons per day of oxygen but is currently being oper-
ated at about 30 tons per day, which is the maximum turndown achievable so
that excess oxygen must be vented.  The excess production does, however,
minimize cryo-ozone-oxygenation system interaction because the required
ozone supply of oxygen is always available.  Loads where oxygen generation
energy conservation controls can be implemented will provide a more strin-
gent test condition for the ozonation control system.

     No operating data is available at this time to quantify the reliability
in meeting the objective.
                                     109

-------
                              GRAVITY THICKENING
INTRODUCTION
     Gravity thickening is generally utilized to concentrate primary or a
mixture of primary and waste activated sludge.  Thickening tanks are typi-
cally the center feed, vertical picket type.  Polymer is sometimes used to
improve thickener performance.  Gravity thickening is usually followed by
additional dewatering, or biological stabilization.

OBJECTIVE

     The primary objective of gravity thickening is to concentrate the
sludge as much as possible within the constraints of the process to maintain
as consistent an underflow quantity and composition as can be achieved.  Of
secondary importance  is operation of the process at maximum solids capture
efficiency and minimization of chemical usage.  Optimization of hydraulic
loading with dilution water is also desired for improved performance.

     The solids processes downstream of the thickeners are directly affected
by the performance of the thickening process.  Poor performance will greatly
affect the volume of  sludge withdrawn from the tank.  The thickener overflow
liquor is usually not treated separately but  is returned to the head of the
plant.  Thus, overflow solids may represent a significant solids and organic
load on the  liquid train processes.
                        *
     The process operates by gravity separation and is by nature stable.
Stability is often affected by hydraulic control of multiple units.  Typi-
cally the degree of flow control employed is  less than adequate, often
resulting in a flow or solids imbalance between units.  Stability may also
be affected  by floating solids (and scum) from septic sludge if sludge  is
maintained in the system too long or dilution water flow is either insuffi-
cient or excessive.

FACTORS AFFECTING PROCESS PERFORMANCE

Load Characteristics

     The  loads on a gravity thickener are difficult to predict since quan-
tity and composition  of the feed is determined by how well the primary  and
secondary clarifiers  perform as well as the wasting schedule adopted.   If a
combination  of primary and waste activated sludge is fed, the hydraulic and
solids loads are more unpredictable.  Dilution water added as a ratio of
                                      110

-------
primary flow represents a significant hydraulic load on the process.  Dilu-
tion appears, however, to favor thickening perhaps by washing out fines
which disrupt thickening.

Process Characteristics

     Field observations indicate that the process is susceptible to short
circuiting problems.  Solids removal is also troublesome, resulting in the
development of septic conditions and associated odors.

     The measurement of underflow concentration and blanket level is diffi-
cult.  Sensing equipment for these applications has improved so that capa-
bility exists for obtaining acceptable measurements.  Often, however, the
sludge blanket interface is very diffuse which leads to insensitive
measurements.

CONTROL STRATEGY

     The goal of the strategy is to maintain a consistent and maximum
concentration of sludge in the underflow (see Figure 3-21).  Thickeners are
usually fed continuously, with flow split to all units in operation.  Dilu-
tion water flow is controlled as a ratio of sludge flow.  Sludge withdrawal
is typically a batch operation, but if continuous pumping is employed, each
unit is usually equipped with individual pumps.

     Underflow concentration is the controlled variable in the strategy.
The manipulated variables are the underflow pumping rate, influent rate,
dilution water flow and, if applicable, polymer addition.  Typical measured
variables include total flow, individual unit flow, density of influent,
sludge blanket level and underflow concentration.

OTHER CONSIDERATIONS

     Lab analysis typically supplements the control system.  Underflow and
overflow solids should be analyzed each shift to check instrumentation and
establish efficiency of solids capture.  Sludge level and sludge color are
also periodically checked.  Process modifications  may be made based on these
observations.  Sludge volume ratio (SVR, volume of thickener sludge blanket
divided by the sludge volume pumped per day) is a good control calculation
occasionally used to modify strategy seasonally.  This parameter is analo-
gous to SRT and gives a measure of sludge age in the thickener.

System Constraints

     Field observations indicate that the following process characteristics
can constrain the ability of a control  system to meet stated objectives:

     1.   The ability to consistently control feed and equally split the
          flow is usually not provided.

     2.   Concentration of feed sludge is variable.


                                     Ill

-------
           DESIRED
          | RATIO
DESIRED
FLOW
ro
                                                                                     OVERFLOW RETURN
                                                                                     TO HEADWORKS
                                          Figure 3-21.   Gravity  thickener control.

-------
     3.   Collection and removal of concentrated sludge  is difficult.
     4.   Response time of sensors is slow.
     5.   The use of positive displacement pumps results in flow surges that
          affect flow measurement.
INSTRUMENTATION UTILIZED
     The following instruments and control devices are applicable and were
observed in the field:
Sensors
     1.   Flow - typically magnetic flow meter (others observed include
          ultrasonic).
     2.   Density - a difficult measurement but can be accomplished with
          optical or nuclear devices.
     3.   Level - optical, ultrasonic or air lift with visual observation.
Modulating Devices
     1.   Plug valves on thickener feed and dilution water.
     2.   Polymer metering pumps.
     3.   Variable speed or stroke sludge pumps.
On/Off Device
     1.   Underflow isolation valves for common pumped systems.
Controlling Devices
     1.   Flow indicating controller - typically PI mode.
     2.   Sequence control logic for sludge withdrawal valves.
     3    Ratio controllers (for dilution water and polymer).
     4.   Multipliers.
VARIATION IN STRATEGY WITH PLANT SIZE
     In small  plants  (less than 1 mgd,  44 dm3/s),  use of gravity thick-
eners is usually not  practical.  Moderate size  plants (5 to 50 mgd,  220 to
2200 dm3/s)  utilize the strategy previously described.  Large plants (100
to 300 mgd,  4400 to' 13000 dm3/s)  employ a similar  strategy but each
                                     113

-------
thickener is usually equipped with separate pumps to allow independent
control of underflow pumping.  The control equipment, and hence control
strategy, becomes more complex.

EXPECTED PERFORMANCE

     This strategy should be capable of producing a consistent underflow
concentration, which is a function of sludge characteristics.  Overflow
solids should be checked as use of the control strategy does not guarantee
improvement in capture efficiency.  At the Ocean County, New Jersey plant,
measurement of overflow solids is used to aid the operator in adjusting
pumping rate.
                                      114

-------
                             FLOTATION THICKENING
INTRODUCTION
     Flotation thickening is generally used for the concentration of waste
activated sludges.  Most older thickeners may be rectangular, but most new
installations observed are circular.  The flotation process feeds air into
the sludge under pressure so that a large amount of air can be dissolved.
When the air comes out of solution in the thickener, small bubbles attach
themselves to sludge particles and float them.  Chemical flotation aids
(polymer) are used to help improve sludge concentration and capture
efficiency.

     The thickeners are in the middle of the solids process train and are
highly interactive with both the activated sludge process and the solids
disposal train.  Thickeners are usually followed by either a holding or
blending tank.  Performance of the thickening process will affect the sludge
processing train.

     Field observations indicate that the process operates with inconsistent
results because of varying loading and uncontrollable process characteris-
tics.  WAS loading, recycle requirements, bubble size, etc. are not directly
controllable and make process control complex.

OBJECTIVE

     The objective of flotation thickening control is to concentrate the
feed sludge to as high a concentration as practical with the minimum of
chemtcal feeds.  In addition, maximum capture of solids is desirable to
minimize the effect of returning solids to the head of the plant.

FACTORS AFFECTING PROCESS PERFORMANCE

Load Characteristics

     The material being delivered to the flotation thickener units varies in
composition as well as quantity.  Hence the major process loads are hydrau-
lic and solids application rates.  The process is very flow sensitive and
rapid changes in loadings cause inconsistencies in performance.  The
characteristics of the WAS and the associated need for polymer is usually
based on manually implemented rise tests and the requirements may change
more rapidly than can be detected by these tests.
                                     115

-------
Process Characteristics

     The primary operating variables for flotation thickening are:  recycle
pressure, recycle ratio (flow), feed solids concentration, detention time,
air/solids ratio and use of chemical aids,  the process typically has three
control degrees of freedom; polymer dosage, recycle rate  and thickening
time.  The relationship between solids concentration and  control settings
are difficult to determine and cause the process control  system to require
labor  intensive observation.

     The long time lags of the process makes understanding of interactions
difficult.  Cause and effect relationships  are not typically known in the
field.  Measurement of overflow and thickened sludge solids may be done with
optical instruments.  These instruments are, however, not often applied due
to the frequent maintenance required.

CONTROL STRATEGY

     Field observations indicate that the typical control strategy is fre-
quently implemented manually.  Refer to Figure 3-22 in the following discus-
sion.  The flow is distributed to  on-line operating units (equally or
unequally, depending on the thickener performance) by the operator.  Overall
(system) hydraulic changes are made slowly  and infrequently.  Solids load-
ings are calculated frequently and rise tests performed.  These results lead
to adjustment in polymer dose and/or recycle ratio.  Also observations of
the blanket thickness lead to changes in frequency and duration of collector
operation.  Smaller, rectangular flotation  thickeners typically collect
continuously (bottom and top).

     Typically:

     1.   Feed flow is distributed (controlled to operating  units).

     2.   Polymer dose is controlled by a ratio to mass flow or a ratio to
          flow.

     3.   Recycle flow is ratioed  to feed flow.

     4.   Air flow  is ratioed to recycle flow.

     5.   Blanket formation observations  (rise tests) control need for
          adjusting above ratios.

     The controlled variable for the process is thickened sludge  concentra-
tion and secondarily, capture efficiency.   Manipulated variables  are polymer
dosage, WAS flow to each unit, recycle flow and thickening time.  Measured
variables can include influent suspended solids and air flow.  Suspended
solids measurements on the floated sludge and the subnatant  are becoming
more prevalent with improvements in instrument reliability and maintenance
                                      116

-------
 FEED
RATIO
STATIONS
FROM
WAS
PUMPS
      POLYMER FEED
I                                                        RETENTION
                                                        TANK         PRESSURIZING
                                                                      PUMP
                                                                                                                     JFFLUENT
                                                                                                                       TO
                                                                                                                      STORAGE
                                   Figure  3-22.   Flotation thickening  control.

-------
requirements.  Implementation of a consistent control strategy is con-
strained by limited understanding of the process dynamics and the multitude
of variables affecting it.
     Thickener startup sequencing is usually carried out manually from
gallery panels.  The requirement for unit startup or shutdown is usually
based on sustained changes in solids loading.  Sludge is often allowed to
remain in idle thickener for up to 48 hours if restart  is anticipated within
this time.
OTHER CONSIDERATIONS
     Laboratory  analysis for capture efficiency on each thickener is often
used to make adjustments in control strategy and as a check on sensors.
During sludge blanket rise tests, the operator observes clarity of subnatant
and thickness of blanket which will also be factored into the strategy.
Modified jar tests are also performed occasionally to determine optimum
polymer dose.  Odors and color of floated sludge are also observed.
INSTRUMENTATION  UTILIZED
     Instrumentation and control devices observed are as follows:
Sensors
     1.   Suspended solids - optical sensor located on  influent and
          occasionally, on subnatant and thickened sludge.
     2.   Flow measurements - magmeters on influent, recycle and thickened
          sludge pump discharge  lines; orifice for air  flow and occasionally
          recycle flow.
     3.   Level  transmitter - bubbler type located in sludge box.
On/Off Control Devices
     1.   Floated sludge collector mechanism (typically on timer).
     2.   Sludge pumps.
     3.   Air compressor.
     4.   Polymer metering pumps.
Controlling Devices
     1.   Ratio  station for polymer feed.
     2.   Flow indicating controller for influent flow.
                                      118

-------
     3.   Ratio station - may be used for influent flow control, recycle
          control and polymer.
     4.   Timers and sequence controls.
Typical Annunciator Alarms
     1.   "No flow" output from magmeter transmitter.
     2.   High torque on collector mechanism.
     3.   Low recycle pressure.
     4.   High/low level in floated sludge sump.
VARIATION IN STRATEGY WITH PLANT SIZE
     Control strategy remains consistent, regardless of plant size.  Smaller
plants have one or two units and do not require flow distribution controls.
In larger plants, the number of process units dictates more process control
either automatically or manually implemented.
EXPECTED PERFORMANCE
     The Denver Metro plant and others have shown that desired thickened
sludge concentrations can be met 96% of the time through close control of
hydraulics, mass flow polymer pacing and control of thickening time based on
sludge blanket rise tests.
                                     119

-------
                             ANAEROBIC DIGESTION
INTRODUCTION
     In this process, the organic matter in the sludge is biologically
stabilized in the absence of oxygen.  Anaerobic digesters may be "low rate"
or "high rate."  For the purposes of this report, high rate will be
addressed.  Both single and two stage systems are utilized, depending on the
nature of the material to be digested.

     Modern systems are "high rate" systems utilizing one or two stages.
The sludge stabilizes in the first stage, while the second stage provides
settling and thickening.  In a single-stage system, the secondary digester
is replaced by some other thickening process.  The digester is heated to 85
to 95op and usually provides 10 to 20 days detention of the sludge.

     The process has been successful when fed primary sludge or combinations
of primary sludge and secondary sludge.  The larger the fraction of second-
ary sludge, the lower the inherent dewaterability of the digested product.

     The process converts about 40 to 70 percent of the organic solids to
gas (depending on the biodegradability of the sludge) reducing the amount of
solids to be disposed.  About two-thirds of the gas produced in the process
is methane.  Anaerobic digester gas is frequently used as a fuel for heating
digesters and buildings and for engines that drive pumps, air blowers and
electrical generators.

OBJECTIVE

     The primary goal in anaerobic sludge digestion is to stabilize the
sludge so that it is easy to dewater  and can be economically and aestheti-
cally disposed of with minimal public health hazards.  Secondary objectives
include recovering the fuel generated and reducing the weight and/or volume
of sludge to be dewatered.

FACTORS AFFECTING PERFORMANCE

Load Characteristics

     The anaerobic process is mostly controlled by the substrate removal
kinetics of the methane-forming bacteria.  These bacteria grow slowly and
have long generation times compared to those of aerobic or facultative
bacteria.  Thus, long retention times (SRT) are required in the reactor.
Methane formers are very sensitive to the presence of oxidizing agents^ pH,


                                      120

-------
sludge composition and temperature.  If the pH drops below 6.5, methane
formation is severely retarded and the organics in the sludge are not stabi-
lized.  Temperature changes of as little as +5op will produce process
upsets.  Although detention times are long, the digester charging procedure
should be carefully designed so large batches are not imposed each time a
digester is charged.  Composition changes, both in terms of organic makeup
and density, lead to problems with operation.  Realizing that these loads
are generated from liquid train unit processes, the loads are unpredictable
and often highly varying.  Where industrial wastes are a substantial contri-
butor, toxic components (i.e. heavy metals) impose an additional disturbance
on the process.

Process Characteristics

     Anaerobic sludge digestion progresses in two stages.  First facultative
anaerobic bacteria, called acid-forming bacteria, convert the complex
organic material in the sludge to simple organic material, primarily organic
acids.  Some carbon dioxide is formed and some stabilization occurs during
the first step.  Second, the organic acids are converted to carbon dioxide
and methane by anaerobic bacteria called methane-forming bacteria.  Most of
the sludge stabilization occurs in this step as the organics are converted
into gas, water and a limited quantity of biological mass.  A small amount
of hydrogen sulfide may also be produced.  The acid formers respond much
more quickly to environmental and process changes than the methane bacte-
ria.  This often leads to pH upsets due to an imbalance between acid
production and acid conversion to methane.

     The process is primarily characterized, controlled and sensitive to the
following variables:

     1.   Food supply (organic content, physical composition, density).

     2.   Hydraulic feed rate.

     3.   Time.

     4.   Temperature.

     5.   pH.

     6.   Mixing (homogenous environment).

     Because of the slow growth of the methane bacteria, the process is
dominated by a very long lag time.  This necessitates careful control  and
monitoring of intermediate variables in order to recognize and react to
upset conditions.

     The process environment is such that measurement of any process para-
meter is difficult.  Sampling for laboratory analysis of process variables
is also difficult because of the large volumes involved in the unit process
equipment.


                                     121

-------
CONTROL STRATEGY

     In order to achieve the objectives of the process, the control strategy
must address each of the following variables from a monitoring  and control
standpoint (refer to the process P&ID in Figure 3-23).

Food Supply

     Organisms in the primary digesters are most efficient when sludge  is
furnished in small quantities at frequent intervals.   If too much sludge  is
added to the digesters  in a short time, the acid-forming step may be pre-
dominant.  This will cause the volatile acids to increase and the pH to
decrease, which is an unfavorable environment for the  gas-forming step.
Foaming may also occur.  Incomplete digestion with accompanying odors will
result from the lack of balance between the two steps.  Thus, primary
digester influent sludge should be pumped as continuously and consistently
as possible over 24 hours; however, care must be exercised not to pump
sludge which is too thin, to make sure that detention  time guidelines
(below) are not violated.  Successful control systems  observed  in the field
are designed to split the mass of incoming sludge as equally as possible
between on-line digesters.

Detention Time

     The design digester detention time typically is 15 to 20 days.  The
actual detention time can be calculated by knowing the quantity of sludge
pumped to the digesters and the digester volume.  A chart should be prepared
to show the detention time with various influent sludge flow rates.  Diges-
ters should not be operated with a detention time of less than 10 days  over
an extended period, as  the system tends to become biologically unstable
under these conditions.  Detention times longer than 25 days insure
stability but may not be economically justifiable.

Temperature

     Effective mixing of incoming sludge with the contents of the digester
is necessary to provide the organisms with their essential food supply  and
to maintain a uniform temperature throughout the primary digesters.  The
primary digester 'temperature should be maintained within +_ 5op of the
selected temperature.   The temperature setpoint should be between 90 and
lOOop for optimum operation.  The control strategies observed showed that
digester temperatures could be maintained within 5op of setpoint.
     Anaerobic digestion  can proceed  efficiently with  pH  levels from  about
6.6  to 7.4;  however,  the  optimum  range  is  from 6.8  to  7.2.  Outside these
limits, digestion efficiency decreases  rapidly.  The pH level  in  all  systems
 is set by the bicarbonate buffer  system.   Since volatile  acids titrate  as
alkalinity,  volatile  acid determinations are required  to  determine the
actual bicarbonate  alkalinity  level.  The  bicarbonate  alkalinity  should.,


                                      122

-------
                      TIMERS AND
                     .SEQUENCE LOGIC
I\J
CO
                                                                                                                         TYPICAL
X
""""I
-f \J
                                                                                                                        TO
                                                                                                                       DEWATERING
                                                                                                                     SECONDARY
                                                                                                                     DIGESTERS
                                                            PRIMARY
                                                            DIGESTERS
                                                     Figure 3-23.   Digester control.

-------
always exceed the volatile acids by at least 1,000 to 2,000 mg/1.  Control
strategies typically do not include pH adjustment although most all plants
required adjustment periodically.  If pH adjustment is employed, sodium
bicarbonate or sodium carbonate should be used in preference to lime.  The
sodium salts are easily soluble so distribution in the digester is not a
problem.  Lime is difficult to dissolve and distribute in the digestion
liquor.

Mixing

     Proper mixing of the digesters is essential to ensure the raw sludge
and heat  is evenly distributed throughout the digester.  Mixing also
decreases the chances of a scum layer buildup.  Most control strategies
observed  included provision for automatic gas mixing and some also included
provision for concentrating the gas mixing in certain areas within the
digester  via rotary valves.  Mechanical mixing is also successfully used.

Gas Production

     The  efficiency of the primary digesters is indicated by the volume and
content of gas produced.  Gas production from the primary digesters when 50
percent volatile solids destruction is being achieved has been found to vary
between 12 and 18 sfm/lb volatiles destroyed.  This usually produces a gas
which  is  65 to 50 percent methane.  The control strategy should include
daily  calculations of gas yield and monitoring of gas content.  The fraction
of the gas which is methane is a function of the type of organic compounds
fed to the digester.  Carbohydrates yield a 50-50 Clfy - CO? mixture.
Fats always yield higher fractions of methane than carbon dioxide with the
methane increasing as the length of fully acid chain increases.  Proteins
also yield higher fractions of methane than carbon dioxide with methane
yield  determined by the number of amino groups and the length of the carbon
chain.  An increase in carbon dioxide content of the gas can mean an upset
is in  progress or it can indicate a change in sludge characteristics.  When
the carbon dioxide fraction is greater than 50%, it is a sure sign of
trouble.  Generally, all of the gas which will be produced from a charge of
feed will occur within 24 hours of the charge.  Thus a significant decrease
in gas production can  indicate trouble unless the feed volatile solids have
also decreased.

Supernatant Flow

     Supernatant withdrawal and recycle must be considered in the control
strategy, as the characteristics of the supernatant can overload the liquid
treatment processes.  It is best to return supernatant to other plant units
at the time when it will have the least effect.  Usually, it is best to do
this when the raw wastewater flow to the plant is at its daily minimum; it
is not good practice to add the supernatant load during peak flows.  Inade-
quate  digestion can result in poor quality supernatant which can lower
overall plant performance when recycled.  In systems where the digester is
vigorously mixed at all times, anaerobic digestion yields no supernatant,
rather the supernatant results from subsequent dewatering steps.


                                     124

-------
OTHER CONSIDERATIONS
     In the digestion process, operator observations and laboratory data are
extremely important.  Volatile acids and alkalinity analysis should be per-
formed every shift.  Loads are calculated, cover and liquid  level are
observed to program digester feeding.  The process is very  .abor intensive
with heavy operator involvement.  Calculation of the methane production per
unit of volatile solids charged is very indicative of system performance.
Laboratory monitoring of pH, volatile acids, bicarbonate alkalinity and
total alkalinity are neccessary to truly know the health of the digester.
Volatile acid/total alkalinity ratios of less than 0.5 yield good operating
results based on our field observations, provided alkalinity is in the range
1,500 to 5,000 mg/1.
INSTRUMENTATION UTILIZED
Sensors
     1.   Sludge flow - heated tube and ultrasonically cleaned mag meters.
     2.   Density - typically nuclear, optical is becoming more usable.
     3.   Temperature - resistance bulb in a thermal  well.
     4.   Cover level - electromechanical devices.
     5.   Liquid level - flange mounted diaphragm sensor.
     6.   Gas flow - turbine or displacement meter.
     7.   Gas composition - (optional).
     8.   Lower explosive limit detectors.
Modulating Control  Devices
     1.   Variable speed pumps - occasionally used.
     2.   Two speed or variable speed recirculation  pumps.
On/Off Control Devices
     1.   All pump motors.
     2.   Valves.
     3.   Gas compressors.
                                     125

-------
Controlling Devices
     1.   Temperature controllers.
     2.   Sequence programmers.
Annunciator Alarms
      1.  Blending tank level.
      2.  Cover level - high and low.
      3.  Liquid level - high  and low.
      4.  Temperature - high and  low.
      5.  Recirculation compressor failure.
      6.  Heat exchanger recirculator pump failure.
      7.  All pump failures.
      8.  Valve failures.
      9.  Transfer pump high pressure.
     10.  Digester mixer (compressor) alarms.
     11.  Digester gas pressure.
     12.  Explosive  limit  alarms.
 VARIATION IN  STRATEGY WITH  PLANT  SIZE
     Field  observations indicate  that small plants  (less than  5 mgd,  220
 dm3/s)  typically utilize single-stage digestion with drying beds.  Use of
 instrumentation  is minimal  and operation  of the digesters  is sporadic.  Per-
 formance  is predictably inconsistent.  Operation  could  be  significantly
 improved with  some manual  measurements of volatile  acids two or three times
 per  week.   Sludge pumping  should  not be conducted continuously at  these
 plants  unless  it  is  monitored  in  some way.
     For the  intermediate  sized plants (5 to  50 mgd, 220 to 2200 dm3/s)
 two-stage or  single-stage  operation  is likely to  be found  with a relatively
 high use  of instrumentation.   The documented  control strategy  was  observed
 (fully  automated) in several plants.  Use of  on-line gas composition  ana-
 lyzers  was  not prevalent because  normal laboratory  techniques  could be used
 for  the moderate number of digesters involved.
     For  the  larger  plants  (100 mgd, 4400 dm3/s,  and larger) the number of
 digesters increased  rapidly.   Individual  digesters  are  controlled  as  .above,
                                     126

-------
but the overall set of digesters needs inventory control.  The sequence
logic becomes complex and the inventory control strategy is more frequently
manually implemented.  More on-line use of gas composition is observed
because of the number of digesters to monitor.  The City of Philadelphia
uses a strategy of inventory management.

EXPECTED PERFORMANCE

     Anaerobic sludge digestion normally proceeds with minimum trouble if
instrumented and controlled properly.  Field observations indicate, however,
that performance is highly variable in terms of frequency of digester
failures, yield of gas (quantity), yield of gas quality and percent vola-
tiles destroyed.  A well operated digester can convert 50 percent of the
organic solids to water and gas, can yield a gas mixture which is up to 65
percent methane having a heat value of up to 600 BTU/scf and can produce
approximately 18 scf of this gas for every pound of volatile matter de-
stroyed.  Field results, however, show the following range of performance:

     30 to 50 percent organics converted

     45 to 65 percent methane in gas

     400 to 600 BTU/scf

     12 to 18 scf/lb volatiles destroyed

     These variations could be due to improper operation or due to charac-
teristics of the feed.  A digester may produce a digested sludge (low in
volatile acids, having a tarry odor, a humis-like consistency, and draining
readily) while operating consistently on the low side of these ranges.  This
may indicate room for improvement or that the sludge being treated is high
in nonbiodegradable organics.
                                     127

-------
                              VACUUM FILTRATION
INTRODUCTION
     Vacuum filtration is the predominant method of mechanical sludge
dewatering.  Vacuum filters are made by several manufacturers with a variety
of filter media available.  All filters operate on basically the same prin-
ciple and are similar, therefore they will not be differentiated in this
section.

     Filtrate from the process, which is very high in organics, is returned
to the head of the plant.  If vacuum filtration is followed by incineration,
variability of cake moisture content can seriously affect the performance
and fuel efficiency of the incineration units.

     The operation of a filter is relatively unstable due to the large
number of process variables.  Typically holding tanks are sized such that
there is little buffering capacity to handle the wide variability of feed.
In addition, field observations indicate the process is very labor intensive
and requires constant operator adjustment.

OBJECTIVE

     The primary objective of vacuum filtration is to produce a consistently
dry sludge cake while maintaining high production (yield) and economical use
of chemicals.  Field observations indicate that most filters are controlled
based on yield with only secondary attention paid to solids content and
solids capture.

FACTORS AFFECTING PROCESS PERFORMANCE

Load Characteristics

     Vacuum filters are one of the last elements in the treatment process
and as a result are subject to the loads imposed by all other elements of
the process.  Field observations indicate that the material normally dewa-
tered by a vacuum filter is a mixed sludge.  The composition of that sludge
is highly variable between primary and waste activated.  In addition, vary-
ing quantities of septic sludge and scum make the composition that much more
unpredictable.  The sludge age also changes the dewaterability characteris-
tics.  Chemical dose must be varied to compensate for variations in sludge
filterability characteristics.  The relationship between required chemical
                                     128

-------
dose and characteristics of the feed is variable and not well understood.
Poor understanding of the chemical interactions often leads to improper
doses of chemicals and a reluctance to attempt optimization.

Process Characteristics

     The vacuum filtration process is very complex due to the number of
process variables (see Figure 3-24).  The variables involved are:

     1.   Vat level.

     2.   Chemical dose (lime, ferric, polymer).

     3.   Feed rate.

     4.   Chemical - sludge mixer speed.

     5.   Drum speed.

     6.   Vacuum applied.

     7.   Vat agitator speed.

          Vat level is very important in process performance and is a
difficult parameter to control.  The time lag between pump response and
liquid delivery coupled with erratic waves caused by the vat agitator makes
maintenance of a constant level difficult.  The other process variables are
very interactive and difficult to relate.  The vat level is generally
related to feed rate, vacuum applied and drum speed although the chemical
conditioning must be correct so that the mat is formed in the cake.  Effec-
tiveness of chemical conditioning is dependent on pH, sludge age, mixing and
filterability of raw mixture.  Thickness of the cake varies with several of
the parameters and yield changes dramatically with several parameters also.
Field observations show that solids capture, yield and cake solids content
all can and usually do vary dramatically during the day.  Measurement of
sludge parameters are difficult due to the nature of the material.   Control
devices are typically adequate to properly control the process.

CONTROL STRATEGY

     Vat level is maintained by controlling sludge feed flow in a cascaded
manner.  Ferric dose is typically ratioed either to mass flow or volume
flow.  Lime is proportioned to ferric.  Mixer and drum speed manual adjust-
ments are infrequently performed.  The mixer speed is typically not adjusted
because the control relationship is not clear.  Drum speed is not frequently
adjusted because it affects the operation dramatically.
                                     129

-------
OJ
o
                                            MASS
                                         'UYY*LCULATOR
                                                  -1
                                                    [FERRIC
                                                     ATIO
 DESIRED
I LEVEL
                                                                                                    TANGENTIAL
                                                                                                    RECEIVER
                                                                                                                   FILTRATE PUMP
                                             Figure  3-24.   Vacuum  filter control.

-------
     The primary controlled variables are total yield and cake moisture
content. A secondary controlled variable is solids capture, although  in the
field only little concern is paid to this.  The manipulated variables are
feed solids flow, chemical feed, mixer speed, drum speed and vat level.
Measured variables are the cake moisture content (lab), production rate,
chemicals added, vacuum pressure, drum speed and feed flow.

     Field observations show that consistent performance of a vacuum  filter
is constrained by the number of variables and their interactions.  Yield can
be measured but not the true yield—dry solids.  Because of variable  loads
even the level is difficult to control in the field.  Overflows are common.
A filterability index which is easy to measure and use is needed for  chemi-
cal feed" control.  This process is run in a crisis mode in the field.

     Control of process startup and shutdown is typically manual.  Drum
drive speed is manually set based on desired yield of cake.  Chemical doses
are ratioed to flow based on Buchner funnel or filterability measurements
performed in the laboratory.

OTHER CONSIDERATIONS

     Operator input is crucial to performance due to the complexity of the
process.  Without the existence of a measurable filterability index, opera-
tors must be able to recognize changes in sludge and cake composition and
tune the process accordingly.   Chemical addition rates and vacuum level must
be observed frequently to insure process stability.  Lab tests and loading
calculations must be performed every shift.  Yield, cake solids content and
solids capture must be the main concerns of the operator.   In the real
world, however, the operator's main concern is to maintain a uniform sludge
cake discharge from the drum.

INSTRUMENTATION UTILIZED

Sensors

     1.   Level - bubbler or flange mounted diaphragm, used in holding tank
          and vat.

     2.   Feed flow - ultrasonically cleaned, heated tube magnetic flow
          meter.

     3.   Sludge density - optical or nuclear devices.

     4.   Weight - strain guage type on cake conveyor belt.

     5.   Tachometer for drum speed and metering pump speeds.

Modulating Control Devices

     1.   Feed pumps.
                                     131

-------
     2.   Chemical metering pumps.
     3.   Drum drive.
     4.   Mixer drive.
On/Off Control Devices
     1.   Vacuum valve.
     2.   Filtrate pump.
     3.   Vacuum pump.
     4.   Vat drain  valve.
Controlling Devices
     1.   Ratio station for chemical feeds.
     2.   Level controller.
     3.   Flow controller.
     4.   Manual  loading  stations for  drum speed, mixer,  agitator.
Annunciator Alarms
     1.   Excess weight.
     2.   Low  and high  levels  in  holding  tank,  mixing  tank  and  vat.
     3.   No flow in feed pumps.
     4.   Vacuum  failure.
     5.   Zero speed on drum  and  pumps.
VARIATION IN STRATEGY WITH PLANT  SIZE
     Plants smaller  than  1 mgd (44  dm3/s) seldom  use vacuum filters.   In
moderate sized plants (5  to 50 mgd,  220 to 2200 dmS/s),  the control scheme
shown  in Figure 3-24 can  be considered typical  and  successful.   Level
control is occasionally manually  implemented.   Large plants employing
numerous units are often  reluctant  to  apply  instrumentation due to process
complexity.  Inventory control and  hydraulic distribution of sludge to
on-line units become more important  in the overall  strategy.
                                      132

-------
EXPECTED PERFORMANCE

     Consistency and repeatability are typically not observed in the field
due to the difficulties in process control implementation.  Product diurnal
moisture content variations of over 2Q% were observed in a typical day at
several treatment plants.  The large constraints on controlling variables
should be improved with better process understanding and maintenance
schedules.  The process is very labor intensive and expensive to operate.
                                     133

-------
                                CENTRIFUGATION
INTRODUCTION
     The theory of the centrifugation process is well understood and has
been applied very successfully in industry and wastewater treatment plants.
Lack of a good fiber content or mat in wastewater sludges has discouraged
centrifuge use in the past.  With improved equipment and polymer addition,
the use of centrifugation as a dewatering tool has been increasing in recent
years.  Disk and bowl type centrifuges are utilized with the bowl type used
most often for sludge dewatering.  Control of the units is usually provided
by the manufacturer with provisions for manual startup and shutdown
sequences.  The necessity of costly polymer addition at most installations
provides incentive for the use of a more careful control strategy.

     The centrifugation process is typically used in the middle of the
solids treatment train and typically is followed by incineration.  If cen-
trifugation is preceded by digestion, the capacity of the digester enables
inventory to be programmed.  Centrate from the process may present a signi-
ficant load on the head of the plant as the centrate solids observed in the
field are often over 1000 mg/1.

     Centrifugation is a physical process with only two important process
variables and should be stable if the load is stable.  The process is
controlled by two modulating devices which require adjustment to produce a
stable product.

OBJECTIVE

     The goal of centrifugation is to produce as dry a cake as possible and
maximize the efficiency of solids capture.  These two goals are in opposi-
tion to some extent as, field observations show, an increase in efficiency
will decrease cake solids content.  In most instances the costs associated
with improved cake dryness outweigh the benefits gained by greater capture
efficiency, although an acceptable tradeoff between the two must sometimes
be made.

FACTORS AFFECTING PROCESS PERFORMANCE

Load Characteristics

     Load to the centrifuge is determined by the upstream processes.  In the
field, pumping rates to the centrifuges are set daily based on the inventory
to be processed.  Dewatering characteristics of the sludge tend to vary -


                                     134

-------
depending on its origin and makeup.  If the centrifuge follows heat treat-
ment, the sludge load and consistency can vary considerably; but  if diges-
tion precedes, the sludge is more stable but much more difficult  to dewater.

     Field observations show that even if the feed rate is not changed very
often, the sludge composition changes and this can impose a large distur-
bance on the centrifuge process performance.  If the feed rate is highly
variable, the load disturbances are more dramatic.

Process Characteristics

     Field observations show that centrifugation is a predictable and
re liable.process when the sludge feed concentration is stable.  The varia-
bility of the dewatered cake is reduced and the cake failures are less
frequent than with vacuum filtration.  The bowl speed is typically set high
enough to handle composition load disturbances.  Occasionally the conveyor
speed is inadequate and pool depth gets excessive.  The process is dependent
on the following variables:

     1.   Bowl speed.

     2.   Conveyor (collector speed).

     3.   Pool depth (residence time).

     4.   Polymer dose (mat formation within cake).

     Centrifuges require a large amount of maintenance due to the high rota-
tional speeds and the mechanical equipment used.  Collection of concentrated
sludge is efficient.  Centrifuges usually require the use of polymer addi-
tion.  Process measurements are difficult to obtain and may interfere with
process operation.  Balance between capture efficiency and cake dryness is
sensitive and the poor resolution in the control devices make optimization
difficult.

CONTROL STRATEGY

     Field observations indicate that typically feed rates to the centri-
fuges are set based on the sludge volume produced in the upstream process.
Bowl speed and conveyor speed are manually set based on the desired solids
content of the cake (see Figure 3-25).  Capture efficiency is typically not
considered initially.  Polymer dose is ratioed to the volumetric feed flow
rate and adjusted as the sludge characteristics change.  Polymer  is used to
increase capture efficiency and hence, as the centrate solids increase the
polymer ratio is increased by the operator.

     The primary controlled variables are production rate and cake dryness.
Both variables usually are determined in the laboratory tests.  The second-
ary controlled variable is capture efficiency which is usually calculated on
a daily basis.  On-line calculation of capture efficiency would require
measurement of flow rate and solids levels at three locations on each


                                     135

-------
00
               FEED PUMP
                                                     POLYMER
                                                     RATIO
                                                        SOLIDS
                                                                         I	
                                                                   CENTRIFUGE
                                                                                    BOWL DRIVE
                                                                                    SPEED REDUCER
                                                                             V/S
                                                                                 	1 ST
                                                                                           	1
                                                                                 SCROLL CONVEYOR DRIVE
                                                                             fV SPEED REDUCER
                                                                                                                        CENTRATE
                                                Figure  3-25.   Centrifuge  control.

-------
centrifuge and hence typically is not done.  Manipulated variables  are the
feed rate, polymer ratio, bowl speed and scroll speed.  Measured  variables
are the flow and occasionally centrate suspended solids.  Sludge  solids
content is usually determined in the lab.
     Manual control adjustments typically utilize the following
relationships:
     1.   Product (yield) is influenced by feed rate.
     2.   Solids content is influenced by bowl speed.
     3.   Capture efficiency is influenced by polymer dosage.
     4.   Pool depth (residence time) is influenced by conveyor speed.
OTHER CONSIDERATIONS
     Centrifugation becomes very labor intensive when the consistency of
feed sludge varies frequently.  Operators will observe the centrate and
perform  solids analysis to aid in adjusting polymer controls.  Pool depth
or residence time is calculated from flow curves and used to adjust conveyor
speeds.  In some plants observed, the operator adjusts the polymer dose
hourly based on a viscosity measurement of the feed sludge.  Cake dryness,
centrate suspended solids and solids capture are analyzed daily to check on
instruments and process performance.
INSTRUMENTATION-UTILIZED
     The following instrumentation was observed to be in general use.
Sensors
     1.   Flow - mag meters, heated tube and ultrasonically cleaned.
     2.   Density - optical devices on feed.
     3    Speeds - tachometer or SCR drive indicators (these were not always
          utilized).
     4.   Viscometer (not typical) on feed sludge.
Modulating Control Devices
     1.   Sludge feed pumps - variable speed, SCR drive.
     2.   Bowl speed - mechanically variable speed.
     3.   Scroll speed - mechanically variable speed.
     4.   Polymer metering pump - variable speed, SCR drive.
                                     137

-------
On/Off Control Devices

     1.   All motors.

Controlling Devices

     1.   Manual loading stations (on feed flow, bowl speed, conveyor speed).

     2.   Polymer pump ratio controller.

Annunciator Alarms

     1.   Drive "required/not running."

     2.   Bowl vibration.

     3.   Motor temperature.

VARIATIONS IN STRATEGY WITH PLANT SIZE

     Plants with flows less than 1 mgd  (44 dm3/s) usually do not utilize
centrifuges.  Use of the control strategy discussed is typical in moderate
size plants and the centrifuges are usually operated seven or eight hours
per day.  Many of the newer plants have been equipped with improved instru-
mentation for centrifuge control.  Large plants may utilize many units and
control of feed to each unit becomes more difficult.  In the larger plants
centrifuge operation is generally continuous on all shifts.  As in other
sludge handling processes, inventory control becomes important as both plant
size and number of units on line increase.

EXPECTED PERFORMANCE

     Field observations indicate that depending on the sludge to be
dewatered, centrifuges can yield a 10 to 25 percent solids content cake.
Solids recovery can be in the range of 80 to 95 percent with polymer.
Diligent operation is needed, however, for consistent operation at the upper
end of these ranges.  This process typically requires a full time operator
station if consistent performance is desired.
                                     138

-------
                            ROLL PRESS DEWATERING
INTRODUCTION
     Roll presses have been used for years in the paper industry to dewater
bleached pulp.  The process is a combination of centrifugation and pressure
filtration where two drums are rotated in a pressurized vat and a cake is
formed between the two drums.  One drum is movable and is positioned pneu-
matically to control the "press" function.  There are a large number of
control devices and controlled variables and the process and process inter-
actions are complex.  A good deal of the process tuning takes place in the
field.

     The roll press has the capability to form a very dry cake (30 to 40%
solids) but must have a raw material with a high fiber content which is
capable of forming a strong mat.  Polymer can help to form this mat but
field experience shows that if the fiber content of the raw sludge
decreases, the polymer alone will not do the job.

     This characteristic causes this process to have the potential of
causing large disturbances in both the liquid and solids train.  When the
sludge cake fails, the moisture content of the product increases immedi-
ately.  Sometimes the production stops completely.  During this period the
sludge is being forced into the rotating drum and returned to the head of
the plant as pressate.  The characteristics of this material approach the
same characteristics as the raw material delivered to the unit during these
times.  Even under normal conditions, the pressate is a large load on the
liquid train.

OBJECTIVE

     The primary objective of this process is to produce the driest possible
sludge cake in a consistent manner.  Solids capture efficiency is not usu-
ally defined as an objective because typically the pressate contains an
extremely high solids content.  Typically the objective can only be met on a
primary sludge which has substantial industrial inputs.   Roll presses are
not utilized on mixed sludges or waste activated sludge because the weakness
of the floe making up these sludges makes it virtually impossible to retain
a mat.
                                     139

-------
FACTORS AFFECTING PROCESS PERFORMANCE

Load Characteristics

     Because the roll press is usually used to dewater primary sludge, the
most dynamic load is volumetric sludge flow.  Variations in sludge composi-
tion are less significant.  The quantity of primary sludge generated during
the day varies hourly.  In addition, control strategies in primary treatment
typically produce varying sludge concentrations which quickly destroy the
fiber mat leading to process  instability.  The quality and quantity of the
material fed to the roll press continuously imposes disturbances on the
process  and necessitates either intensive operational labor or automation.

Process Characteristics

     The process  itself has many process variables, many controlling devices
 and  in  addition,  has a significant  volumetric capacity (vat) which compli-
cates the timing.  The process requires a specific filterability character-
 istic  (fiber content) which the polymer then magnifies.  Without this
characteristic, the process is difficult to operate.  Each of the process
 variables  is controlled  by  a  different process measurement which causes  any
 control  strategy  to be highly interactive, causing  it to  lean toward
 instability.

      The use  of on-line  instrumentation  is difficult  due to the characteris-
 tics of the raw material.   The required measurements  of flow  and solids  must
 be maintained  daily to  assure operability.  Key variables  such  as fiber
 content on  flow material  and  solids content on  end  product  cannot be
 measured on line  but are determined in the  laboratory.

 CONTROL STRATEGY

      A roll press is  a highly instrumented  process;  however,  even with the
 instrumentation,  the process  is  labor intensive and requires  a  full time
 operator.   The control  strategy is illustrated in Figure  3-26.

      Controlled variables are as follows:

      1.   Sludge feed flow.

      2.   Polymer feed.

      3.   Chemical mix tank mixer speed.

      4.   Vat pressure.

      5.   Roller speed.

      6.   Roller spacing (nip gap).
                                      140

-------
                                                                              SLUDGE CAKE
                                                                              • WEIGHT
GAP SPACING
SLUDGE
 SUMP
                                   Figure  3-26.   Roll press control.

-------
     7.    Vat agitator speed.

     8.    Dilution water flow.

     Additional variables which are typically measured include:

     1.    Pressate suspended solids.

     2.    Discharge cake weight.

     The control strategy which is implemented in the field requires manual
equipment startup and cake development.  The formation of the initial mat is
very important and must be done manually.  Once a reasonable cake is
produced, the control system is activated and the strategy is implemented
automatically as follows:

     1.    A vat level.controller setpoints a feed flow controller.

     2.    Roller speed is maintained at  an operator setpoint based on cake
          production desired (open loop).

     3.    Polymer is ratioed to mass flow of the feed sludge.

     4.    Nip or roller spacing is controlled at an operator setpoint which
          is adjusted based on observed  cake solids estimate.  Operator
          observes pressate suspended solids and adjusts ratio up or down
          accordingly.

     5.    Dilution water controller is adjusted (setpoint determined) in a
          cascade fashion by an electrical current measurement on the vat
          mixer drive.

     6.    Speed of both the chemical conditioning mixer and the vat agitator
          mixer are controlled  and the setpoint determined by the mass flow
          of feed solids.

OTHER CONSIDERATIONS

     The system is labor intensive and full-time operator presence is
required for multiple units.  Care should be taken in setting roller speed
and gap spacing to control sludge cake moisture content.  Periodic observa-
tions should be made as well as lab tests on the sludge cake, filtrate, and
raw sludge characteristics.  Tests of filterability and jar tests for opti-
mum polymer dose are typically performed weekly.  Daily calculations of
actual polymer dosage, suspended solids  capture, and a solids material
balance are performed.

INSTRUMENTATION UTILIZED

     The types of instrumentation and control devices utilized in this
application are as follows:


                                     142

-------
Sensors
     1.   Flow meters - mag meters with ultrasonic cleaning.
     2.   Density (suspended solids) analyzers - optical (feed sludge  and
          pressate).
     3.   Pressure measurements - diaphragm force balance element  and
          transmitter.
     4.   Gap measurement - position transducer.
     5.   Weight - strain gauge and transmitter.
Modulating Control Devices
     1.   Variable speed drives for pumps, tank mixers  and press rollers.
     2.   Dilution water control  valve.
     3.   Air control valve.
     4.   Polymer metering pump.
On/Off Control Devices
     1.   All drive motors.
Controlling  Devices
     1.   Ratio  stations (polymer, mixer  speeds).
     2.   Multiplier/divider.
     ^3.   Controllers for flow, pressure,  current, gap  spacing and weight.
Alarms
     1.   Feed pump  or  flow.
     2.   Vat pressure.
     3.   Nip gap  limit.
     4.   Roll press  drive  and vat  agitator  electrical  current  limits.
     5.   Weight limits exceeded.
     6.    Sludge sump level  low.
                                      143

-------
VARIATION IN STRATEGY WITH PLANT SIZE

     The strategy remains basically the same whenever this process is
applied.  Field observations indicate that the unit process has only been
applied at larger treatment plants.  Since the startup must be manual,  there
is no automatic sequencing between multiple units, even in the larger
plants.  Operator involvement during normal daily performance would be sub-
stantially increased if cascade and ratio control were changed to manual
adjustment.  This would be the case in application of the roll press in
small treatment plants.

EXPECTED PERFORMANCE

     The Metropolitan Waste Control Commission of Minneapolis-St. Paul  has
demonstrated that a roll press can operate with reasonable polymer dosages
yielding a sludge cake of 30 to 35% solids as long as the raw sludge fiber
content is high enough.  The operation requires heavy labor intensity for
monitoring with frequent controller adjustment.  If the raw sludge to be
dewatered varies greatly in consistency, the roll press performance relative
to cake solids content and solids capture will be erratic.
                                     144

-------
                            PLATE PRESS DEWATERING
INTRODUCTION
     Plate and frame press dewatering is a batch process which  is utilized
to dewater sludges to the driest state practically achievable.  The process
is utilized on difficult to dewater sludges or to provide a very dry sludge
which can then be burned with little auxiliary fuel, or land filled economi-
cally.  Polymer or other dewatering aids are commonly used.

     Field observations show two types of plate and frame presses
available.  Both types are batch operations but one can be fully automated
and one requires the presence of an operator for the plate separation and
sludge removal steps.

     Interviews with operators indicated that the labor intensity required
with plate and frame presses is their biggest drawback.  In addition, in the
plate separation and sludge removal steps injury potential to the operator
is high.  For this reason, the fully automatic plate press will be described
here although in the field the manual sludge separation method is preva-
lent.  It is felt that fully automatic presses will dominate in the future.

OBJECTIVE

     The specific goal of this process is to dewater sludge to produce a
very dry sludge cake (as much as 40 to 50% solids).  This sludge cake then
is typically incinerated or disposed of on land (for comparison, vacuum
filtration,  a popular dewatering technique, produces a cake which is
approximately 20% solids).  Also, filtrate is much lower in suspended solids
than from any other method.

     Plate press operation interacts with both the solids and liquid
trains.  The degree of separation and concentration previously achieved in
the solids train will affect the performance of the plate press.  Incinera-
tion typically follows the plate press in the solids train and its perform-
ance is completely dependent on the dewatering process.  The liquid train is
usually less affected because of the pressate suspended solids content and
volumetric amounts of the filtrate, since the filtrate is typically less
than 100 mg/1 suspended solids and the volumetric flows are not continuous.
                                     145

-------
FACTORS AFFECTING PROCESS PERFORMANCE

Load Characteristics

     The load volume may change from day to day influencing the size and
number of batch operations performed per day.  The solids content of the
load will affect the capture rate.  The stability of the solids (in terms of
ability to form a pressable cake) will affect the dewaterability.

Process Characteristics

     The plate press process requires a sludge which forms a stable mat or
else an auxiliary matting base is needed.  In wastewater treatment
processes, the sludge is typically treated either by heating or adding
polymer to make the sludge more dewaterable.

     The plate press is a physical process and, therefore, is inherently
stable.  The process itself is a batch operation.  Each batch goes through a
specific sequence—-i.e. fill, press, remove pressed cake (repeat)- with sig-
nificant dead time between filling cycles.  Most sequence steps are timed.

CONTROL STRATEGY

     The system consists of the plate press itself, sludge feed pump,
pressurized water pump for dewatering (pressing) the sludge, cloth washing
pump for cleaning plate cloths and air compressor and vacuum pump for
clearing the lines (see Figure 3-27).

     Due to the plate press batch having a sequential mode of operation,  it
is very compatible with automation.  Operator assistance is requested when
needed and the automated sequence involved in running presses is as
follows.  The initial step assumes the plates are open and clean.

     1.   Filter press feed pump sequence
          The filter plates are closed and the feed pump turned on with feed
          valve open.  The pump is run until a timer times out, a high
          pressure is reached or the feed sump level is low.

     2.   Pressurizing pump sequence
          Once the press is filled with sludge, it is pressurized and
          squeezed through the filter cloth at greater than 200 psi
          pressures.  This is based on time.

     3.   Core blow with water sequence
          The pressurization cycle is followed by cleaning out the core of
          pressate liquor with water.  This usually lasts 15 seconds.

     4.   Core blow with air sequence
          Once the pressate is forced out, air is blown in to devoid the
          chamber of all liquid.
                                     146

-------
SCO A,
FILTER PRESS
FEED PUMP
SEOB,
PRESSURIZE
PUMP
SEQ C,
CORE BLOW
WITH WATER
SEQ D,
CORE BLOW
WITH AIR
SEQ E,
DRAIN
DIAPHRAGMS
SEQ F,
EXHAUST
DIAPHRAGMS
AND CAKE
CHAMBERS
SEQ G,
OPEN
FILTER PRESS
SEQ H,
CLOTH
MOTOR
SEQ I,
CUDTH
WASH
                                                                                              AIR  TO DIAPHRAGMS
                                                                                              VALVE
 EFFLUENT WATER
                                                        CLOTH
                                                        WASH
                                                        MOTOR
                                                               AIR  TO CORE
                                                                      VALVE
                                      SJCORE
                                        DRAIN
                                        VALVE
                  THERMALLY
                  CONDITIONED
                  SLUDGE
                  SUMP
                                                                                                         CLOTH WASH PUMP
                             SLUDGE
                           FEED PUMP
                                                                                             PRESS WATER
                                                                                             DRAIN VALVE
                  FILTRATE
                  SUMP
                                                                                                           SERVICE WATER
                                                                                                           RECEIVER  TANK
                                                                                     PRESS WATER
                                                                                     FEED VALVE
                                                    EXHAUST VALVE
                                                    VACUUM PUMf-
                                                                                                 PRESSURIZED
                                                                                                 WATER PUMP
                                   Figure 3-27.   Plate  press  process diagram.

-------
     5.    Draining diaphragms  sequence
          With air in core,  the diaphragms are drained by blowing air in
          sequence.  This takes usually a few minutes.

     6.    Exhausting diaphragms and cake chambers sequence
          A vacuum pump is started to draw out all exhausted pressate to
          sump.  This takes a variable time depending on the sludge
          dewaterability.

     7.    Open filter press  sequence
          Once the diaphragms are evacuated, the plates are opened by
          hydraulic pump and valves.

     8.    Cloth motor sequence
          The cloth motor is started and moves the filter cloth down and
          releases cake.  After a time, the cloth is moved back up.

     9.    Cloth wash sequence
          The cloth is now spray cleaned.  The wash pump is started and the
          cloth is moved down again through the spray.  Only 20/6 of the
          cloths are washed per cycle.  A 50 gpm supply (per press) of
          service water  is required to sustain a wash cycle.

     This completes one plate press cycle.  The plates are open and the unit
is clean.  A total cycle for a completely automatic press has a substan-
tially reduced total cycle time compared to the standard two to four hours
for conventional plate and frame presses.

     For plate presses the primary controlled variable is the daily through-
put and the secondary controlled variable is the percent solids in the
sludge cake (moisture content).  This is difficult to control since the
process is a batch operation.  The manipulated variables are batch time and
pressure for operation.  Additional measured variables are pressure, time
(for sequence  steps) and inventory conditions (e.g. sump level).

     Constraints on the system include the requirement that the sludge be
dewaterable (i.e. capable of forming a cake).  Also for any sequence to
operate requires that the previous process must be operating.

     The required volume of sludge to be treated per  day determines the
number of batches.  This is actually controlled by the time of a batch.  The
solids content of the discharge cake is controlled by the water operating
pressure.  The startup of a sequence can either be controlled by the ending
of the previous sequence or manually started.

     The operator must calculate the number of batches required each day and
then set the batch time.  Also, at least daily, laboratory observations of
filtrate suspended solids and cake moisture/suspended solids content should
                                     148

-------
be made to determine press efficiency or need for polymer addition to
promote better filterability.  A daily solids balance is usually performed
and the dewatered cake percent solids is determined so that performance can
be assessed daily.
INSTRUMENTATION UTILIZED
Sensors
     1.   Level transmitters for feed and filtrate sumps.
     2.   Pressure transmitters and/or switches for pressurized water, cloth
          wash and sludge feed.
Modulating Control Devices
     1.   Typically if polymer is added, a control valve or variable speed
          drive is utilized and a set dose rate utilized.
On/Off Control Devices
     1.   Feed pumps.
     2.   All valves.
     3.   Air compressor.
     4.   Hydraulic pump motor for opening/closing filter press.
Controlling Devices
     1.   Sequence programmers.
     2.   Sequence interlocking logic.
A1arms
     1.   Equipment not in status required (e.g.  valve closed,  but required
          to be open).
     2.   Sump level either high or low.
     3.   Pressure feed either high or low.
     4.   Electrical lockouts if plates out of place.
                                     149

-------
VARIATION IN STRATEGY WITH PLANT SIZE

     There is little variation in strategy with plant size, since each press
operates independently.  On small plants the sequence interlocks may be
missing so an operator is needed.  On  larger plants with duplicate trains
the degree of complexity of the control strategy  is increased due to common
service water, compressor and sumps, but typically each press has an indi-
vidual feed pump (common backup pumps).  The strategy is usually totally
automatic up to the plate separation step.

EXPECTED PERFORMANCE

      If the sludge  is stable  (filterable) excellent results (approximately
40% solids) can be expected with moderate polymer addition.  However,  if the
sludge  is varying  in dewaterability, large polymer additions are required.
Performance of a plate press with highly varying  sludge feed (volume and
quality) will be poor based on field observations.
                                      150

-------
                                 INCINERATION
INTRODUCTION

     This control strategy describes a general approach to incineration
control. . There are several manufacturers of incineration processes, each
with a method of control unique to their respective equipment and philosophy
of operation.  However, the intent of this document is not to describe
specific equipment but simply to point out a usable strategy.

     Other methods of control are feasible and frequently used, depending on
the manufacturer and the type of equipment supplied.  However, for the
purpose of this strategy, the following method was chosen as a representa-
tive and successful form of control.

OBJECT! VE

     The incineration process is a means of final disposal of sludge and
scum.  Combustion reduces the material to a sterile ash with approximately
\Q% of its original volume.  Since a fuel supply in addition to the dewa-
tered sludge (such as fuel oil or natural gas) is normally required, stable
operation with minimal use of auxiliary fuel is the prime goal.  Another
benefit resulting from stable incinerator operation is lower maintenance
costs, as thermal cycling speeds refractory breakdown.

FACTORS AFFECTING PROCESS PERFORMANCE

     Process operation is affected by certain other process load changes and
to a certain extent causes load changes in other processes.

     Interaction of the incinerator with other processes centers mainly on
its dependency upon upstream dewatering processes.  Wet cake incineration
dramatically changes the temperature of the upper hearths due to the addi-
tional water which must be driven off before combustion can occur.  This not
only places undo stress on the refractory of the affected hearths, but also
increases the auxiliary fuel consumption and as a result, operating costs.

     A byproduct of the incineration process is the offgas generated from
the sludge combustion.  To meet atmospheric emission requirements, the off-
gas is scrubbed by an impingement scrubber.  This device uses effluent water
to entrap the suspended particulates.  The effluent water then flows to the
head of the waste treatment plant, imposing a hydraulic load on the plant.
                                     151

-------
     A control system designed to stabilize the incineration process  is
confronted by two problem areas:  1) The variation  in sludge cake quality,
as mentioned earlier, and 2) The large capacity of  the incinerator itself,
resulting in long delay times.  The first will require an  immediate response
to maintain control.  However, because of the capacity of  the incinerator,
the effect will be delayed.

Load Characteristics

     Dewatered sludge quality consists of two dominant variables, percent
moisture of the cake and fuel content of the solids.  Each, if rapidly
changed, will tend to disrupt process stability and performance.  Wet cake
will result in greater consumption of auxiliary fuel and lowering the
burning zone which in turn will create a product which is  not completely
combusted and could lead to burning sludge in the incinerator cooling zone.

     The fuel content of the sludge cake solids will also  significantly
affect the location of the burning zone.  An increase in fuel content will
raise the burning zone due to a lowering of the flash point resulting in
combustion at a lower temperature.  Low fuel content will  require additional
auxiliary fuel in order to maintain the location of the desired burning
zone.  Industrial waste dumps may change the fuel content.  Industry shut-
downs for weekends, holidays, etc. will contribute  to complexity of this
control strategy by causing significant changes in the sludge volatility.

     Delivery of the sludge cake material is typically rather sporadic due
to the manner in which cake is discharged from the  vacuum  filter.  This will
also affect process performance to a degree and make control more difficult.

Process Characteristics

     The heart of this process is combustion which  in itself is a complex
phenomenon.  In the case of waste sludge incineration, it  is further compli-
cated by a lack of knowledge of the fuel content or moisture content of the
incoming sludge on a real-time basis.  These measurements  are not available
to the control system.  Therefore, feedback control is generally the method
used.  Due to the large capacity of incinerators, long time constants are
the norm where temperature control is involved, making proper tuning of
feedback control loops difficult to achieve.  Figures 3-28 and 3-29 illus-
trate the control and instrumentation applied.

     Not all aspects of the incineration process are sufficiently understood
to allow completely automatic control.  As an example, the incinerator
temperature profile is typically controlled by modulation of the rabble arm
drive speed, the idea being that the speed with which the  sludge moves
through the furnace will dictate in which hearth combustion will occur.  The
middle hearth is generally the most desirable location.

     Should the upper hearth temperature^be high enough to indicate that
combustion is taking place, the rabble arm drive speed is  increased in order
to move the burning sludge to the middle hearths.  However, increasing the


                                     152

-------
en
CO
                                                                                                            BURNER COMBUSTION
                                                                                                               AIR FANS
SLUDGE COMBUSTION
AIR FANS
                                                                                                       PUMPS
                                            Figure  3-28.   Incineration controls.

-------
en
              EMERGENCY
              STACK
                             FROM PRESSURE
                             TRANSMITTER
                                                            vv


NORMA

~
L
r






j





r


                   FROM
                   FURNACE
                                                                                       L.
                                                                                               ~i
                                                                                                       ZT

V _i_
1 " li /I
V^
L/PI
{ ^~^ SCRUBBER WATER
]r
OFF GAS

\

V /
A
                                                                                                                     IMPINGMENT
                                                                                                                     SCRUBBER
                                                                                                                    PLANT EFFLUENT
                                                                                                                           SUPPLY
                                    TO  ASH SLURRY TANK
                                           Figure  3-29.   Incineration offgas handling.

-------
speed of the rabble arm tends to break up the combustion material and expose
more volatile material to temperatures greater than their flash point.  The
short term effect will be to increase the upper hearth temperatures, exactly
opposite of that desired.  In the long term, the burning zone will move to
the lower hearths, but the rate at which this will occur is unpredictable.

     As one might suspect in a process of this nature, process variable
measurement difficulties hinder implementation of control.  The instruments
employed tend to be high maintenance items due to the extreme heat and the
corrosive and abrasive ash particles to which they are subjected.  Thermo-
couple burnout is not an uncommon occurrence.  Offgas oxygen and flow
measurement equipment will require regular cleaning due to dust.

     Some measurement error may occur as a result of measurement of nonrep-
resentative material.  For example, if burning sludge accumulates around a
thermocouple, it will result in a measurement significantly higher than the
actual hearth temperature (and shorten the life of the couple).

     Most of the control devices used for the incineration process are air
dampers which modify the combustion air and off gas flow through the fur-
nace.  Unlike most other control loops in the waste treatment plant, here
consideration must be given to the compressability of the medium being con-
trolled.  This factor adds still another dimension of difficulty to the
control.

     The interrelation of the process loop is another important factor.   For
example, a venturi scrubber typically requires a pressure drop across the
system in order to maintain proper operation.  Modulation of a venturi
throat provides the required differential pressure.  This loop is tied quite
closely to the incinerator pressure loop which modulates the induced draft
fan inlet damper.  Manipulation of the furnace pressure is performed by
pulling more air out of the furnace and through the scrubber system .  This
directly affects the differential pressure across the scrubber.

CONTROL STRATEGY

     The following description outlines a strategy employing excess oxygen
in the offgas to adjust the amount of sludge combustion air entering the
furnace.  Burner control will be based on respective hearth temperatures.
Temperature profile will be maintained by operator modulation of the rabble
arm drive speed.

Hearth Temperature Control

     A three-mode temperature control loop using the error between setpoint
and the respective hearth temperature to control the throttling of burners
should be provided for each hearth.   The feedback (typically thermocouples)
should be linearized according to the ISA standard.  The output is direct
acting and will control the burner combustion air valve.   The fuel con-
sumption rate is generally a mechanically fixed ratio to the combustion  air
flow and not automatically adjusted.


                                     155

-------
Sludge Combustion Air Control

     A cascade control loop using an excess oxygen control loop to setppint
a flow control loop on the sludge combustion air supply fan will maintain a
sufficient supply of oxygen for use in the combustion process.  The percent
of oxygen in the incinerator offgases is compared with the setpoint in the
two-mode oxygen control loop.  The direct acting output of the oxygen loop
is used to setpoint a two-mode flow control loop on the sludge combustion
air.  Error between the combustion air flow and setpoint generates a direct
acting output which operates the sludge combustion air damper in a manner to
provide the required oxygen level in the offgases.

Incinerator Pressure Control

     A two-mode negative pressure (vacuum) control loop using the error
between setpoint and furnace top hearth pressure to control the position of
the induced draft fan. inlet damper will maintain a negative pressure in the
furnace and keep the offgases within the incineration system.  In most
cases, upon failure of the induced draft fan or the scrubber system, the
output of this control loop would control the position of a damper in the
emergency bypass stack.

Scrubber Venturi Control

     Differential pressure developed across the scrubber venturi throat is
compared with the setpoint in a three-mode control loop.  The direct acting
output of the control loop modulates a positioner in the throat to control
the differential pressure and maximize particulate removal.

Scrubber Water Flow Control

     The scrubber system typically requires a controlled supply of effluent
water to insure proper equipment operation.  A two mode control loop
compares the flow in the scrubber water header with setpoint and generates
an output to position the scrubber flow control valve.

Ash Tank Water Level

     A portion of the spent scrubber effluent water is typically used to
produce an ash slurry to facilitate pumping to an ash lagoon.  Control of
the ash tank water level is via a two probe system with water being added in
a batch mode of operation.

Shaft Drive Speed Control

     This is a manual control loop with the closed-loop control within the
variable speed drive unit.  The operator selects the desired speed based
upon the temperature profile.
                                     156

-------
OTHER CONSIDERATIONS

     The operator may improve control by introducing his own feedforward
control based on various observations.  General characteristics of the
sludge cake such as moisture content could allow the operator to anticipate
the effects this will have on the furnace.  A moisture content lab test
should be performed each shift.  In addition, ash organic content and sludge
fuel content lab tests should be performed regularly.

     Observation of the color and location of the burner flame could alert
the operator to potential slag buildup on the burner.  Maintenance of that
burner could be scheduled in a timely manner and avoid a potential hazard.

     Examining the ash for completeness of combustion gives the operator an
indication of the detention time requirements.  He may increase or decrease
the detention time of the sludge by changing the rabble arm drive speed.
Lowering of the speed results in the sludge remaining in the furnace longer,
increasing the speed has the opposite effect.

     The operator should confirm the excess oxygen analyzer operation on a
regular basis using the ORSAT procedure.  Regular cleaning will  also be in
order for this particular analyzer.

     Observation of the stack gas color excursions may also provide insight
into general incinerator performance, particularly scrubber performance.
Measurement of opacity on line or simple observation using a visual
(Ringleman chart) approach will serve to be a warning of poor combustion and
particulate formation and/or removal.

INSTRUMENTATION UTILIZED

Sensors

     1.   Strain gauge weight conveyor - this instrument weighs  the sludge
          on the feed conveyor and,  in conjunction with the conveyor speed,
          provides a signal representative of the sludge feed rate.

     2.   Temperature - Type K thermocouples  are generally used  to  measure
          the following:  each hearth temperature,  cooling air exit
          temperature,  venturi scrubber entrance and exit temperatures,  ID
          fan inlet temperature.

     3.   Pressure - system pressures are required of the following:   top
          hearth furnace, differential across venturi  filter.

     4.   Excess oxygen - offgas immediately exiting furnace.  Analyzer
          indicates percent oxygen remaining  over and  above that  used in
          combustion.

     5.   Opacity- monitors stack gas after  ID  fan.   Indicates
          effectiveness of scrubber  system.


                                     157

-------
     6.   Gas flow - sludge combustion air flow is typically measured using
          an annubar with an appropriate differential pressure transmitter.
     7.   Damper position - The following damper positions are typically
          required for operator use in the manual mode:  sludge combustion
          air damper, burner throttling valve, bypass stack damper, venturi
          throat damper, ID fan inlet damper.
Modulating Control Devices
     The following valves, dampers and motors will require control via a PID
type controller:
     1.   Sludge combustion air damper (two controllers for cascade
          operation).
     2.   Burner throttling valves.
     3.   Bypass stack damper.
     4.   Venturi throat damper.
     5.   ID fan inlet damper.
     6.   Rabble arm drive (manual loading station only).
     7.   Scrubber effluent feed water valve.
     8.   ID fan motor, if variable speed (manual loading station only)
On/Off Devices
     The following devices will require start/stop or open/close control:
     1.   Sludge combustion air fans.
     2.   Burner air fan.
     3.   Burner oil pumps.
     4.   Feed conveyor.
     5.   Shaft cooling air fan.
     6.   Shaft drive.
     7.   Ash slurry pumps.
     8.   ID fan.
     9.   Each burner.
                                     158

-------
Alarm Points
     1.   High temperature in furnace.
     2.   High temperature at scrubber inlet.
     3.   High temperature at ID fan  inlet.
     4.   Low water flow to scrubber.
     5.   Low speed switch on rabble  arm.
     6.   Low air flow on cooling air.
     7.   High temperature on cooling air.
     8.   High level in ash tank.
     9.   Zero furnace pressure.
Controlling Devices
     In addition to the PID controllers described previously, sequence
control of the following items will be necessary:
     1.   Switching network for multiple blowers and pumps.
     2.   Burner control logic.
     3.   Emergency bypass logic to control furnace pressure in shutdown.
     4.   Startup sequence logic.
     5.   Ash system control logic.
VARIATIONS IN STRATEGY WITH PLANT SIZE
     There is no variation in actual  control strategy with plant size.  The
above described strategy is representative of what is required for plants  in
the 5 to 50 mgd (220 to 2200 dm3/s) range.  Plants less than 5 mgd (220
dm3/s) typically do not use the incineration process for sludge disposal
as it is not economically feasible.   Plants greater than 50 mgd (2200
dm3/s) must in addition cope with the problem of routing sludge to
multiple incinerators and incinerator maintenance.
                                     159

-------
EXPECTED PERFORMANCE

     In most incinerator installations observed, the goal of final disposal
of dewatered sludge is attained, however, not in the most economical
fashion.  Due to the instability of the process (for the reasons outlined
above), controls are typically in manual mode with auxiliary fuel flow at
maximum and sludge combustion air at maximum.  This meets the process objec-
tive, but at the expense of extreme and frequent temperature excursions
(causing refractory damage) and much higher operating costs.
                                      160

-------
                                RETURN LIQUORS
INTRODUCTION
     Field observations indicate that treatment of the return liquor from
the solids treatment unit processes is becoming a necessity.  All plants
visited acknowledged that return liquors from the various solids train
processes imposed large hydraulic, organic and solids loads on the liquid
train.  These loads comprise up to 3 to 5% of the plant flow, 10 to 20% of
the solids load and 10 to 20% of the organic load.  Not many operating
plants exist where the problem is properly handled.  Two methods were
observed (in various stages of implementation) which can be considered typi-
cal for the immediate future.  They are:  1) treatment of liquors separately
from the main plant flow, and 2) storage of liquors with programmed release
during off-peak hours.

     These two methods will be discussed in general terms with no detailed
proposed strategy because the strategy is so dependent on the approach used
to handle the return liquors.

OBJECTIVE

     The objective of the return liquors treatment is to treat or control
the material to be returned so that it does not disrupt the liquid treatment
process.  Return liquors represents a major disturbance on the process and
this disturbance, if coinciding with a diurnal load, will yield a process
upset which field observations show can last for weeks.   The liquid train is
susceptible to short term and/or long term disruptions caused by hydraulics,
organics and solids.  The goal of a return liquors treatment is to minimize
the affect of this known disturbance through control.

FACTORS AFFECTING PROCESS PERFORMANCE

Load Characteristics

     Loads may be from the gravity thickeners, vacuum filters, sludge
concentration tanks, flotation thickener, digesters, centrifuges or heat
treatment.  The waste liquid portion of these processes determines the
hydraulic load from liquors and the frequency of the loading.  These
processes are usually dynamic, and may be operated on a batch basis,  which
results in shock loads.  In addition, the shock loads and continuous loads
somewhat follow the plant diurnal pattern.
                                     161

-------
Process Characteristics

     When return liquors are stored prior to discharge into the liquids,
the storage capacity must be large enough to dampen the return liquor flow
peaks.  Based upon field observations, the detention times provided are too
small, so that the method is usually not successful.  At least three hours
detention time of return liquors storage (at peak plant flows) should be
provided.

     Treatment of the return liquor is usually divided into two trains.
Heavily solid-laden wastes are treated by physical/chemical means while
liquids containing high concentrations of organics such as heat treatment
return liquors are treated by biological means.  High costs are involved  in
physical/chemical treatment.  Measurements are difficult and sensors other
than flow measurement are seldom used.  The process must be able to handle
widely varying loads which may at times be difficult to treat.  Sufficient
buffering of return .liquors is the objective and with consistent control, a
reasonable return liquors performance has been demonstrated in the field.

CONTROL STRATEGY

     If the storage method is used, return liquor is collected in a
separate return liquor wet well.  A level control system, with a deadband
in the controller, is used to manipulate the flow from the wet well into
the liquids train.

     Treatment of return flow acts as a buffer and reduces the impact of
the return liquors on the plant.  If chemical feed is used, it is ratioed
to the return flow.  If biological treatment via rotating biological sur-
faces is used, the number of shafts on line is varied.  If activated sludge
is used, the unit process is always kept on line and the return sludge rate
is varied.

     In both control strategies, the solid and organic contribution of the
return liquor is the manipulated variable.  The release of return liquor
flows when the diurnal flows are low has been attempted, but is difficult
to successfully achieve in practice.  The manipulated variable in the stor-
age strategy is the flow valve that regulates the return liquor released
into the headworks of the plant.  In the treatment strategy, the chemical
dose or number of biological units running, is the manipulated variable.
Return liquor flow is measured continuously with suspended solids or
organics usually measured in the laboratory.

     Extreme variation in both the characteristics and quantity of return
liquors is a severe constraint on the process.  In the storage system, the
wet well volume is generally not sufficient to effectively buffer loads.

INSTRUMENTATION UTILIZED

     These instruments have been observed in general use:
                                   162

-------
Sensors
     1.   Flow - mag meter.
     2.   Density - optical or nuclear instruments are occasionally  used
          after treatment.
Modulating Control Devices
     1.   Variable speed chemical feed pumps.
On/Off Control Devices
     1.   Chemical pump drives.
     2.   Biodisks drives.
Controlling Devices
     1.   Flow indicating controllers.
     2.   Chemical ratio controllers.
     3.   Level indicating controller for wet well level with wide
          deadband.  This may include pumping logic.
Annunciator Alarms
     1.   Failed devices.
     2.   High wet well level.
VARIATION OF STRATEGY WITH PLANT SIZE
     Handling of return liquors is not an important problem in small treat-
ment plants because they generally do not have complex solids treatment
trains.  Storage strategies were observed in predominant use in moderate
size plants having return liquor handling facilities.  Large plants tend to
use separate treatment facilities to process return liquors.  At the
Minneapolis-St. Paul Metro Plant, biological and physical/chemical processes
are utilized to treat a 10 mgd (440 dm3/s) return liquor flow.
EXPECTED PERFORMANCE
     Although not many return liquors installations were observed, the
results are promising.  With adequate return liquors storage and good plant
management, timed release of return liquors can be successful.  At least
three hours of return liquors storage is necessary based on field inputs.
     Treatment of"return liquors is expected to reduce the solids load and
organic load on the liquid train by well over 50% at Minneapolis-St. Paul.
                                     163

-------
                                 SECTION 3
                                BIBLIOGRAPHY


Allen, R. K. and C. 0. Victor, Plant to Disinfect Wastewater with Ozone,
Water and Sewage Works. July, 1978.

Andrews, «J. F., Control Strategies for the Anaerobic Digestor Process,
Water and Sewage Works. March, 1975.

Belich, M. and F. N. VanKirk, California Plant Gets Straight A's in
Computer Control, Water and Wastes Engineering. March, 1975.

Bishop, D. F. and W. W. Schuk, Control Strategies for Automation of
Advanced Waste Treatment Systems, AICHE Symposium Series 145, Vol. 71,
1975.

Bollyky, L. J. and B. Siegal, Ozone Disinfection of Secondary Effluent,
Water and Sewage Works. April, 1977.

Brachen, B. D., What's Up With Oxygen? Parts I and II, Water and Wastes
Engineering. March  and April, 1977.

Ching, D. L., 0. M. Heimsted  and P. W. Gilligan, Computerized Control of
the Primary Sedimentation and Air Activated Sludge Process, Proceedings of
the Instrument Society of America conference Advances in Instrumentation,
March, 1977.

Environmental Protection Agency, Field Manual for Performance Evaluation
and Troubleshooting at Municipal Wastewater Treatment Facilities,
EPA-430/9-78-001. January, 1978.

Environmental Protection Agency, Flow Equalization, EPA Technology
Transfer Seminar Publication  May, 1974.

Environmental Protection Agency, Operations Manual-Sludge Handling and
Conditioning, EPA-430/9-78-002. February, 1978.

Environmental Protection Agency, Process Design Manual for Aerobic
Biological Treatment Facilities, EPA-430/9-77-006

Environmental Protection Agency, Process Design Manual, Wastewater
Treatment Facilities for Sewered Small Communities, October, 1977.

Etchart, D. Y. and B. Mishra, Survey on an On-Line Computer Control System
at Metro Denver, Proceedings of the Instrument Society of America
conference Advances in Instrumentation. March, 1977.
                                      164

-------
Fair, G. M., J. C. Geyer and D. A. Okun, Elements of Water Supply and
Wastewater Disposal, Second Edition, 1970.

Fertek, H. A., A Computer Control Strategy for Breakpoint Ch1 orination,
Proceedings of the Instrument Society of America conference Advances in
Instrumentation, March, 1977.

Flanagan, M. J., Control Strategies for the Activated Sludge Process,
Instrumentation Technology. June, 1977.

Flanagan, M. J. et al, Design Procedures for Dissolved Oxygen Control of
Activated Sludge Processes, Environmental Protection Technology Series,
EPA-600/2-77-032. June, 1977.

Genthe, W. K., J. F. Roesler and B. D. Bracken, The Case for Automatic
Control of Dissolved Oxygen, presented at 49th Annual Conference of Water
Pollution Control Federation, Minneapolis, Minnesota, October 3-8, 1976

Lesperance, T. W., A Generalized Approach to Activated Sludge,  Water &
Wastes Engineering. April-December, 1965.

Lieberman, N., Instrumenting a Plant to Run Smoothly, Chemical
Engineering. 1977.

Molvar, A. E., J. F. Roesler and R. H. Babcock, Instrumentation and
Automation Experiences in Wastewater Treatment Facilities, Environmental
Protection Technology Series, EPA-600/2-77-032. June, 1977.

Molvar, A. E., Selected Applications of Instrumentation  and  Automation in
Wastewater Treatment Facilities, Environmental Protection Technology
Series, EPA-600/2-76-276. December, 1976.

Metropolitan Waste Control Commission, St. Paul,  Minnesota,  Evaluation of
Wastewater Treatment Process Control Systems,  Case Study:  Metropolitan
Plant. June, 1976.

Nagel, C. A., State of the Technology Semi-Automatic Control of Activated
Sludge Treatment Plants, Environmental Protection Technology Series,
EPA-600/2-75-058. December, 1975.

Ortman, C., TOC, ATP and Respiration Rate as Control Parameters for  the
Activated Sludge Process, Environmental Protection Technology Series,
EPA-600/2-77-142. 1977.
                                     165

-------
Petersack, J. F. and R. G. Smith, Advanced Automatic Control  Strategies
for the Activated Sludge Treatment Process, Environmental Protection
Series, EPA-670/2-75-039. May, 1975.

Rice, R. E. and G. A. Mathes, A Demonstration, Direct Digital Control of a
Vacuum Filter, Proceedings of the Instrument Society of American
conference Advances in Instrumentation. March, 1977.

Rice, R. E. and G. A. Mathes, Direct Digital Control of a Vacuum Filter
Part II, presented at the Instrument Society of America conference on
Advances in Instrumentation, Philadelphia, Pennsylvania, October 16, 1978

Roesler, J. F., The State-of-the-Art for Automation of Sludge Handling
Processes, Proceedings of Third National Conference on Sludge Management,
Disposal and Utilization, Miami, Florida, December 14-16, 1976.

Roesler, J. F., Status of Instrumentation and Automation for Control of
Wastewater Treatment Plants, Proceedings of the Fourth United States/Japan
conference on Sewage Treatment Technology, October 28-29, 1975,
Cincinnati, Ohio.

Roop, R. N.  Evaluation of Residual Chlorine Control Systems, Journal
Water Pollution Control Federation. 1977.

Woodruff, P- H., Dissolved Oxygen Control of the Activated Sludge Process,
Progress in Water Technology, Vol. 6, Pergamon Press 1974.

Zickefoose, C. and R. B. Hayes, Anaerobic Sludge Digestion, Operations
Manual, EPA-430/9-76-001. February, 1976.
                                     166

-------
                                 SECTION 4

                        ALTERNATE CONTROL APPROACHES
PURPOSE

     A multitude of alternatives exist from which a selection can be made
when designing the control system of a wastewater treatment plant.  This
section will address those considerations which are vital to this selection,
and the general attributes of each alternative, relative to the selection
criteria.  In general, the following will be accomplished:

     1.   Development of a basic familiarity with the various control
          alternatives.

     2.   Identification of specific considerations relative to the parti-
          cular requirements of a given process or control problem.

     3.   Identification of the alternate(s) which should be considered for
          a particular plant application.

     This section is intended to allow the reader to subjectively reduce the
entire set of alternatives to a relative few which will  meet a given set of
requirements.  The reader may then perform a complete analysis of this sub-
set and do it within a practical time frame.  More extensive information to
assist in this formal evaluation is contained in Sections 5 and 7 of this
publication.
     The common types of control systems in use today are detailed here as
well as the more subjective or "gray-area" qualities of each.   The latter,
while not always quantifiable, must certainly be considered when evaluating
the alternatives.  Any evaluation which ignores these considerations,
selecting based solely on costs and ignoring these other aspects of
cost/benefit, may lead to the wrong choice.

     This section is not intended to pinpoint one control philosophy which
will be best for a given application, nor is it intended to be a "cookbook"
or "formula" which will automatically determine the solution to a control
problem by merely plugging in certain information.  Furthermore, it will not
attempt to apply any specific dollar values to costs or savings; these
                                    167

-------
depend far too much on specific configurations and process variables.  The
reader is directed to Section 5 for a more objective discussion of costs and
savings.

Assumptions

     The discussions presented in the section make two very important
assumptions.  First of all, the control of a plant is viewed as an inte-
grated system.  Whenever an alternative is discussed, it will be discussed
on a plant wide basis; application of the alternative to an individual loop
will be addressed by way of explanation but all discussion with respect to
the advantages/disadvantages will imply a systems approach.

     Secondly, each alternative will be addressed as if the treatment plant
is entirely controlled by a single method.  In practice, this is seldom the
case.  Control systems in most plants are, in actuality, combinations of
several of the cases presented.  The rationale for the procedure used is
that at each plant one method is dominant, even if several are in use.

SELECTION CRITERIA/CONSIDERATIONS

     Many factors go into the selection of the "best" means of meeting the
control objectives of a process.  The following topics should be considered
in any evaluation.  They are not presented in any specific order, i.e. there
is no order of importance attached to their sequence.  They are all very
important considerations.

Costs

     The costs associated with any control alternative include:

     Capital Equipment - the cost of any hardware necessary, such as panels,
     computers, peripherals, etc.

     Instrumentation - the cost of any sensors or control elements within
     the process.

     Installation - the cost of installing the capital equipment, the
     instrumentation and the wiring between them.

     Software - the cost of the computer programs which might be necessary.

     Operational Costs - the costs associated with the day-to-day operation
     of the control system, including operators, supplies, etc.

     Maintenance - the costs associated with keeping the control system
     fully operational, including maintenance of the capital equipment, the
     instrumentation and the software.

     Expansion - the costs required to expand the control system beyond the
     initial configuration.


                                    168

-------
Savings

     There are many areas of potential savings related to any selected
control alternative:

     Labor - Staff reductions may be considered with certain alternatives.
     This is particularly possible with respect to maintenance and opera-
     tional personnel.  Clerical labor is another potential area for savings.

     Chemical - Chemical usage may be reduced by some control alternatives.
     This is particularly important in larger plants and in heavily treated
     processes but is also an important factor in smaller plants.

     Energy - Selection of an alternative which is capable of controlling
     energy demand, both total and peak demand, will produce savings.

     Equipment Life - Equipment life may be extended by preventing such
     things as short-cycling or by alternating usage with backup devices or
     by more rigorous performance of preventive maintenance.

Intangibles

Reliability-
     The reliability of a control system must be evaluated with respect to
the overall process objectives.  This requires that the following topics be
addressed:

     Risks - What will happen if the control system fails and is not availa-
     ble?  Can the process survive?  How long?  What will it cost?

     Backup - What type of backup control is available?  What is required in
     order to switch to the backup mode?

Process Variability-
     Different types of processes require different degrees of control.   In
general, the following components determine the variability:

     Strength - is the influent strength generally consistent or does it
     vary from hour-to-hour or day-to-day?  Is the influent primarily one
     type (e.g. domestic, industrial) or does it change?

     Flow - does the flow change dramatically over time?  Is there a signi-
     ficant storm flow?

Expandability--
     Very few processes are static in terms of control  requirements or capa-
cities.  Future growth may not be fully identifiable but must almost always
be considered.  Growth may include additional process units, additional
process trains, additional sensors or, at least additional  information
requirements.
                                    169

-------
Flexibility of Control--
     Past experience dictates that flexibility in control strategy is an
absolute must for most processes.  Except for very small and static plants,
it is generally not practical to offer the operator only one control alter-
native.  He must be able to apply a variety of strategies, depending upon
the particular combinations of conditions with which he is faced at any
point in time.

Optimization of Results —
     The typical wastewater treatment process is being pressed by two almost
opposite influences, one requiring more tightly controlled effluent charac-
teristics while the other demands a reduction in operating expenses.  These
two influences are not mutually exclusive.  The control system must be
evaluated in terms of its ability to assist in the identification of these
complex relationships and then be "tuned" to make optimal use of the results.

Physical Space Requirements —
     Any control system will require some physical space.  How much is
necessary versus how much is available needs to be considered.  Addition-
ally, environmental conditions required in the control room should be
reviewed and considered.

Operator Qualifications —
     Critical to any control system is the operating personnel.  This staff
will be vital to the system's operation, and the specific requirements for
competent personnel must be considered.  How much training must they have?
How much education?

Operator Acceptance—
     The acceptance of the control system by the operators must also be
considered.  A very sophisticated system may be installed, but if the
operator won't use it (or cannot understand it), it will be of no practical
value and will certainly not solve the control problems it was intended to
solve.

Management Information—
     Every plant requires some degree of management information.  This may
include reports generated for various government agencies, operation summa-
ries for plant management review/monitoring, cost data for financial
reporting and the like.  Much of the data included in these reports must be
obtained from the control system.  Report requirements of the plant must be
considered and matched to the various alternative's ability to provide the
necessary data.

Methods of Control--
     The operator of a plant or process must make a variety of decisions
every day.  These decisions range from a simple selection of what device to
start, to decisions related to sudden device failures or significant load
changes.  The manner of response and the amount of information presented to
the operator by the control system is an important consideration.
                                    170

-------
     Essentially, there are two control extremes which may be  considered.
If the primary mode of control  is to wait for something to go  wrong  and  then
react, it is termed "crisis control."  The opposite of crisis  control
requires that the operator be able to plan for change or be able to  foresee
a problem before it is serious.  This is termed "anticipatory  control" and
is the more desirable.

     Certain control alternatives provide a much better potential for
reducing or eliminating the occurrence of crisis control situations.  When
evaluating control alternatives, consider the potential for crisis control
and the costs of such situations.

CONTROL SYSTEM ALTERNATES

     Various methods of control are available and in use today.  The basic
field equipment generally used  in today's systems is usually the same for
each system type, and is considered the same in this report.  The equipment
is usually electrical in the more sophisticated systems and includes the
instrumentation, the final operators, motors and motor controls, and other
electrical devices.  All nonelectrical equipment will be controlled by
transducing the electrical signals to the required mode and vice versa.
Pneumatic instrumentation is still common in small or less sophisticated
control systems.

     The field instruments, which include many different types of devices,
are mounted near the variables  to be measured.   The primary elements, such
as flow tubes, are mounted for  proper measurement of the desired variables.
The accompanying transmitter is mounted as near as possible to the primary
device to send the desired signals back to the control location by direct
hard wiring and/or multiplexer  telemetry equipment.   The field instrumenta-
tion measures such common things as flow, temperature, level and pressure,
and includes analyzers for different process variables,  including dissolved
oxygen, total organic carbon, chlorine residual and others.

     The final control of variables such as flow is  normally done by motor
control with electric operator  (as described below),  pneumatic operator or
hydraulic operator.  The pneumatic and hydraulic operators require local
transducing of the control output signal to these devices.

     The motors are controlled through motor control  centers,  which include
starters and related control circuits.  The motor control  center is in a
location central to the unit process being controlled.  The control circuits
are hard wired to the local maintenance type motor control stand,  and they
are hard wired and/or multiplexed back to the various  control  locations.
Control circuits include variable speed controls,  as well  as the normal
start/stop functions, motor overload and motor  running type signals.

     Several types of devices such as pressure switches,  control switches
and limit switches are used to  sense the current state of various processes
and equipment.  These discrete  signals are worked into the logic and alarm
circuits of almost all types of control systems.


                                    171

-------
     The following paragraphs describe the basic parts of several control
systems now in use in various wastewater treatment plants:

          Manual Control

          Analog Control
               Local Analog
               Central Analog
               Distributed Analog

          Digital Control
               Central Digital
               Distributed Digital

          Hybrid Control
               Loggers
               Digitally Directed Analog (DDAC)

     The field instruments and control devices  are considered  to be  common
to each system unless otherwise  noted.

     Each type of control will be discussed  relative  to  a single process
situation (Figure 4-1).  This example will show the application of each
control alternative to a sludge  pumping process containing  two pumps feeding
a common header.  One of two pumps will be required to deliver the necessary
sludge flow.  The flow is controlled by a motor operated valve and measured
by a flow meter.
                                       DEWATERING
                                       EQUIPMENT
                               TO NEXT
                               PROCESS
                     Figure 4-1.  Dewatering  feed  system.
Manual Control
          DECISION MAKER:'
          CONTROL LOCATION:
Human Being
The Physical Equipment
     Manual control  is  a method of  control where  all  process  adjustments  are
made by an operator.  The control points  are  concentrated  on  panels  near  the
                                     172

-------
process  or subprocess  controlled.   The panels are arranged with groupings of
controls as related  to specific functions or processes.  This type  of con-
trol  is  considered open loop and requires manual interpretation of  meter  and
chart readings with  subsequent manual adjustment of knobs or valves.   Opera-
tor communication between  panels is necessary to allow this type of control.

     Figure 4-2  shows  the  application of manual control to the sample sludge
flow control  loop being considered.  The operator starts the correct  pump(s)
via the  panel pushbuttons.  He observes the flow on the flow meter  and modu-
lates the control valve until  the desired flow is attained.  If a single
pump does not produce  the  required flow, the operator must start the  second
pump  and vice versa.   Similar  panels exist for each small unit process  to be
controlled.
               FLOW CONTROL
               PANEL NEAR
               EQUIPMENT
       MOTOR  .
     START/STOP
                                                FLOW INDICATOR
                                                VALVE CONTROL
                                                  DEWATERING
                                                   PANELS
                                                    l
                                                 DEWATERING
                                                 EQUIPMENT
TO NEXT
PROCESS
Analog Control
                          Figure  4-2.   Manual  control.
     "Analog" control  systems,  as  discussed in this  section,  refer to any
alternative which combines  various discrete hardware components into a
control system.  These discrete components  include a variety of devices
ranging from simple  analog  logic to sophisticated  sequence control logic.
This includes three-mode  controllers,  relays,  adders,  subtracters, composi
tors and the like.   This  definition,  while  not completely correct from a
pure control theory  point of view, is  practical when categorizing actual
control systems  in use today.
                                     173

-------
     The analog logic component of "analog control" refers to the use of
hardware which monitors a process variable and performs control adjustments
continuously.

     For example, a desired process condition such as flow, level or
temperature (called the setpoint) is compared to the actual value of that
condition (called the feedback or process variable).  A mathematical calcu-
lation between the two, executed by the analog hardware, determines if any
correction must be applied to the control element (pump speed, valve
position, etc.).

     The sequential logic component of analog control refers to the use of
hardware which performs a series of discrete control adjustments.  For
example, pumps are started or stopped based on a set of wet well level set-
points or an air blower is placed on line by performing a series of discrete
startup steps.

     Other than simple manual systems, process control utilizing analog
instrumentation is the most common form of control in use today.  All con-
trol (manual and automatic) eminates from panels located within the plant.
The number of panels depends on the type of analog control implemented and
the size of the plant.  Figure 4-3 summarizes the functions of the various
panels.  The major types of analog systems are Local Analog, Distributed
Analog and Central Analog.

Local Analog—

          DECISION MAKER:     Analog Hardware and Human Being
          CONTROL LOCATION:   The Physical Equipment

     The local analog or localized control systems are common at small
wastewater treatment plants.  In these systems, panels are located in the
vicinity of the equipment to be controlled.  The panels are arranged with
groupings of instruments, pushbuttons, switches and controls as related to
specific functions or processes.  The analog control portion of the panel is
usually a closed-loop type of control.  The measured process change is used
to control a valve or other device through the action of individual control-
lers.  These controllers may be a pneumatic type where all signals are
pneumatic in and out of the controller.  Predominantly, however, the
controllers and support instrumentation are electronic.

     The analog equipment mounted on the panel includes indicators of the
process variable and of the final control element.  The indication of these
variables is a controller in a compact arrangement.  The controller also has
switches for selecting auto or manual mode, adjustments for a setpoint to
the controller, adjustments for output in a manual mode, and other various
functions.  Signal conditioning sometimes applied includes square root
extraction for differential pressure type of flow devices, thermocouple
junction compensation, multiplication and division.  Integration of flow
signals is performed for a totalization of a given flow stream, or for out-
putting a signal to activate a sampler at a given quantity of flow rather


                                    174

-------
    OPTIONAL  —
    CENTRAL
    INDICATOR
    PANEL
ONE PANEL
PER ASSOCIATED
SET OF DEVICES/
LOOP
       INPUTS AND OUTPUTS
        . TO ASSOCIATED
        CONTROL DEVICES
Local Analog:   Control Is spread
throughout  the plant  in small,  local
panels.   Each  panel  typically controls
one or more  similar  devices.  Manual
sequencing and operator setpointing
are typical.   Interpanel  communication
is extensive.   Decisions are made  at
the equipment  level.  The central
panel provides indication only.
     CENTRAL
    INDICATOR
      PANEL
— OPTIONAL
                  ONE PANEL PER
                  SUBPROCESS OR
                    TRAIN
     INPUTS AND OUTPUTS
     TO ALL DEVICES OF
     A SUBPROCESS OR TRAIN
Distributed Analog:   Control of unit
processes  is  concentrated into panels
located throughout the plant.  All
devices associated with a subprocess
are controlled  from one panel.
Automatic  sequencing and cascade control
is typical.   Interpanel communication
requirements  are  reduced.  Decisions
are made at the subprocess level.  The
central panel provides indication only.
          CENTRAL
           PANEL
      INPUTS AND OUTPUTS
      TO ALL DEVICES IN
         THE PLANT
                      Central Analog:   Control of the entire
                      plant  is consolidated into a central
                      panel.  Automatic sequencing and cascade
                      are typical.   Decisions are made in
                      the control  room  so that communication
                      requirements at the local  level are
                      m i n i ma 1.
              Figure 4-3.   Analog Control Characteristics.
                                   175

-------
than time.  Automatic sequencing  of various pieces of equipment or the
starting and stopping of  control  functions  within a process are typically
not included.  These generally  remain  as  manual  (operator) functions.

     The motor control  on these panels is usually a set of start and stop
pushbuttons for starting  and  shutdown  operations.  The hardware typically
provides the required equipment safety interlocks necessary to ensure that
related conditions  are  in the correct  state prior to permitting action.  For
example, the operation  of a secondary  motor is physically prohibited if the
seal water valve  is  not open.  Additionally, the motor is monitored for such
things as whether or not  it is  running, motor overloads,  motor speeds, etc.
and the results are  indicated on  a small  field panel by lights or panel
meters.

     The location for local analog panels is usually at or near the devices
being controlled.   Communication  between  the panels is required to assure
material transfer between subprocesses and to allow proper operator
monitoring.

     Figure 4-4 shows the application  of local analog control to the sample
sludge flow loop being  considered.  The operator manually starts the desired
pump(s) just as was  done  in the manual control example.  However, instead of
manual modulation of the  control  valve,  the operator sets the desired flow
into the analog controller.  The  controller will automatically adjust the
valve and continue  to adjust  it such that the actual flow matches the set-
point.  Again, similar  panels would exist for all unit processes.
       MOTOR  .
     START/STOP
               FLOW CONTROL
               PANEL (NEAR
                EQUIPMENT)
 ANALOG
  FLOW
CONTROLLER
                                                  DEWATERING
                                                   PANELS
1 iA

t tt
DEWATERING
EQU 1 PMENT
                                                                    TO NEXT
                                                                    PROCESS
                       Figure 4-4.   Local analog control,
                                     176

-------
Distributed Analog—

          DECISION MAKER:     Analog Hardware (some human intervention)
          CONTROL LOCATION:   Subprocess

     Distributed analog control involves the use of larger panels designed
to allow complete control of a complete unit process from a single field
location.  The panels include controllers, square root extractors, indica-
tors, recorders, switches and all other equipment necessary to allow the
subsystem to be controlled.  These individual control panels typically
retransmit a small number of critical variables (alarm, status, analog) to a
centralized panel for monitoring.  The plant is divided into various major
processes to minimize the effects of operation from many locations, and to
maximize the efficiency of quality control within the individual unit
processes.  A process should have all the necessary controls and operating
information available on a panel so that an operator can control the
process.  The signals wired to a central location communicate key informa-
tion to the plant engineer to monitor the performance of the unit process
operators and to evaluate their performance and that of the entire process.

     Instrumentation used in the field is brought back to the unit process
or building control panels.  This instrumentation is electronic, except in
hazardous and explosive areas where pneumatic instrumentation should be
used.  The controls and electronic devices make cascade control possible and
grouping of various control functions to minimize the required operator
interface.  The discrete information such as whether or not a pump is run-
ning, overloads or critical alarms are shown on the unit process panel with
lights and annunciators.  The start-stop on the manual and other control
functions necessary on a control panel are pushbuttons and selector
switches.  Automatic sequencing may or may not be included as each distri-
buted panel is usually manned 24 hours per day.

     Required interprocess operator communication and control,  such as
secondary treatment, primary treatment, chlorination and the solids train,
is usually difficult.  Communication between these panels is usually verbal
(via telephone), and requires strict regulation and enforcement of rules.
In some cases, key process variables could be wired to multiple panels in
order to reduce the severity of the problem.  A central control panel would
optionally be included for monitoring important flows, alarms and status of
devices.  This would give the plant engineer the opportunity to monitor the
operation of various subprocess panels, analyze overall system needs,  and to
call for the required process control changes.

     Figure 4-5 shows the application of distributed analog control  to the
sludge flow example.  In this case, all elements of the dewatering process
are consolidated on a single panel near the process.  A single operator
controls all devices from this location.  Automatic sequencing of the pumps
and control of the flow is the same as for central  analog.   A similar panel
exists for each subprocess or train.
                                    177

-------
        DEWATERING PROCESS
      CONTROL PANEL LOCATED
           NEAR PROCESS
      MOTOR SEQUENCE
         CONTROL
 \
                                                 ANALOG
                                                  FLOW
                                               CONTROLLER
\

o o
O 0



B*
•
9
» i
/

B9
•
•

T '» 1
0
0 O
o o

M
                          J_
                                                        CONTROL PANELS
                                                          FOR OTHER
                                                          PROCESSES
                                                                     TO NEXT
                                                                     PROCESS
                    Figure 4-5.  Distributed analog control.
Central Analog—
          DECISION  MAKER:
          CONTROL LOCATION:
Analog Hardware  (some  human  interaction)
Central Control  Room
     Central  analog control  is  defined as the system where all control
operations of importance are controlled from a central location.  Typically,
central analog control  systems  have manual backup by the use of small main-
tenance panels in  the field. The analog controllers can be in the field
with remote setpoint stations in the central control room or the controllers
can be in the control room.   Central analog control systems generally
include full  sequence control.

     The central control panel  is located in a room with environmental
control.  The motor control  or  discrete type of signal handling for  a
central control system  is available from the panel as an override of the
automatic sequencing.   Alarm annunciators are located on the panel and can
be acknowledged by the  operator in the room.

     The analog portion of the  control system allows for more complex con-
trol schemes  when  located in a  central location.  Cascaded control from a
primary variable to several  controllers, possibly in a separate unit process
controlling a secondary variable, is possible.   This improves communications
between process panels.
                                      178

-------
     Figure 4-6  shows  the application of central analog control  to  the
sanple sludge flow  loop being considered.  The control is moved  from local
panels located  near the particular equipment to a central control  room.   The
start-stop pushbuttons for the pumps have been replaced by  sequencing logic
which will start and stop the devices automatically as flow requires.  The
control instrumentation is located on a much larger panel with similar
devices for all  of  the plant.
     The  operator  activates the sequence control logic and enters  a  setpoint
into the  controller.   Modulation and sequencing is automatic from  that  point
on.  Alarms  (e.g.  device failures) are annunciated on the panel.
           CENTRALLY LOCATED
            CONTROL PANEL
        MOTOR SEQUENCE
          CONTROL
                                                 ANALOG FLOW
                                                   CONTROL
 OTHER PROCESS
INSTRUMENTATION
                                                                I/O TO
                                                                OTHER
                                                                UNIT
                                                                PROCESSES
                                                                 TO NEXT
                                                                 PROCESS
                      Figure 4-6.   Central  analog control.
Digital Control Systems

     Use of the digital  computer to control industrial processes has
increased rapidly over the  past 15 to 25 years.   Certain industries, notably
petrochemical and pharmaceutical,  have been in the forefront of development
and application.  These  industries have made very complete and sophisticated
use of the digital computer in  order to achieve complete control of entire
process trains, plants and  even distribution networks (in the case of pipe-
line systems).  Analog signals  representing flows, pressures and so forth,
are converted to a form  which  can  be read and understood by a digital compu-
ter.  The computer makes control decisions  and issues field compatible
control outputs.
                                     179

-------
     In early application of the digital computer to process control, the
thrust was usually to replace analog control with the digital counterpart.
While this was usually of great economic benefit, it soon became apparent
that this was not to be the most far-reaching advantage of the digital
computer.  The fact that the digital computer could be programmed to perform
massive calculations and to make complex decisions, and to do them very
rapidly and consistently, proved to be the most important feature.  Because
of this ability, it is almost impossible to find certain unit processes,
such as a distillation column, which are not controlled by digital computers.

     Utilization of computers in many industries has greatly increased since
the advent of the minicomputer in the 1960's.  This technology offered an
economical alternative to manual and analog control systems even for small
installations and thus has proven to be the catalyst necessary for wide-
spread application of the digital computer to smaller, lower margin
processes.  This phenomenon is manifest in the wastewater treatment indus-
try, which has essentially experienced the birth and maturation of computer
control since about 1970.

     Another revolution began in the late 1970's with the advent of the
microcomputer.  This technology has greatly increased in recent years and
has made the "computer on a chip" an economically feasible alternative.  The
term "micro" refers to the physical size of the processor and has little to
do with describing the capabilities, since many microprocessors have more
instruction capacity than did early minicomputers.  However, microcomputers
do tend to have less capacity overall because of limited memory capacity.
Also, the number of circuits which will fit on one chip requires the use of
slower but more compact logic alternatives.

     Initial application of microcomputers has been to bury the processor in
a device, such as a terminal or a multiplexer, and to use it as an
"engine."  The microcomputer controls the device, while usually enhancing
its capabilities, and would communicate the appropriate information to a
higher form of computer.  As capabilities increased, more and more logic was
included and today the microcomputer is being utilized as an individual
process controller within a network and, in some cases, as a stand-alone
computer.

     The application of the digital computer within treatment plants has
matured very rapidly in the past 10 years.  The synergistic interaction of
computer economics and control/reporting requirements has forced this evolu-
tion.  New developments will undoubtedly continue to expand the computer's
application over the coming years.

     While there are nearly as many different hardware configurations in use
today as there are installations, there are essentially two different types
of systems:  1)  central digital, and 2)  distributed digital.  Each type is
discussed below in greater detail.
                                    180

-------
Central Digital —

          DECISION MAKER:     Computer
          CONTROL LOCATION:   Central Control Room

     In the case of  a central digital control system  (Figure 4-7),  all  of
the control with the main plant  is brought back to a  single computer  loca-
tion.  This single computer  location, with redundancy, performs all computa-
tion" and control within a single central processing unit (CPU).  Field
signals are brought  into the central location on either individual wiring or
telemetered in on a multiplexing scheme.  A multiplexer, or MUX for short,
is a hardware device which receives many field signals and routes them,  one
at a time, to the computer in a manner similar to the way in which the  phone
company transmits many calls over a single cable.  Control computations
and logic are performed upon the data and the control output is sent  to
final control elements.  Figures 4-7 (a) and (b) show some of the "flavors"
of this configuration.

     When the number of points being monitored and controlled in any  single
plant exceeds 1,500  to 2,000, a single computer system becomes overburdened
and unresponsive.  When this occurs, multiple computers may be used,  thereby
"sharing" the load.  Usually, each computer will be assigned to monitor  and
control one or more  logical portions of the process, e.g. primary treatment,
secondary treatment, sludge handling, etc.  Figure 4-7 (c) shows this confi-
guration.  For example, the  "A" sensors may be connected to the primary
treatment, while the "B" sensors are connected to the secondary section of
the plant.

     The computers may operate autonomously.  That is, they may each monitor
and control their own portion of the process and have no knowledge of,  or
interaction with, the remainder of the plant.  However, this will  cause
communication and coordination problems similar to those discussed with
respect to analog control.

     In most applications, the computers are linked together via communica-
tion lines and share various pieces of information.  Data obtained from one
subsystem and required by another is transmitted as needed.   Typically, all
reports are printed by one computer and contain data from all  subsystems.
That data is also transmitted via the communications network from all  sub-
system computers to the computer designated as the report printer.

     The computers also monitor the status of each other and serve as backup
for one another, should either fail.  When one computer takes  over complete
control, it typically operates in a degraded mode; that is,  essential
control actions are continued while nonessential or less critical  control is
suspended in order to maintain responsiveness and data integrity.

     In a central digital control system, microcomputers may be included as
part of the multiplexer.  Typically, they would perform all  process input
and output, error checking, conversion to or from engineering  units, etc.
Some sequencing logic and other analog replacement logic might be included.


                                    181

-------
 (a) Single  CPU
     (b)  Single CPU, On-Line Backup
      CPU
     MUX
  SENSORS AND
CONTROL  DEVICES
   ON-LINE
     CPU
BACKUP
 CPU
                                                     MUX
              SENSORS AND
            CONTROL DEVICES
            (c) JJual CPU, Each Backs  up the Other
                                          CPU
                                          "B"
                      .		I
                                          MUX
                                          "B"
          "A" SENSORS AND
          CONTROL  DEVICES
"B" SENSORS AND
CONTROL  DEVICES
   Figure 4-7.   Central  Digital Configurations
                                 182

-------
     Figure 4-8 shows  the  application  of central  digital  control to the
sanple sludge flow  loop being  considered.   The entire dewatering process
train  is wired to a  central  computer  system through  a common multiplexer.
All other process elements  are  similarly connected to the same computer.
This computer system performs  flow  control  and pump  sequencing based on
operator entered values or  other  processing information.   For example, when
the flow increases  from the  sludge  thickener,  the computer will automati-
cally  adjust the setpoint  of the  flow  control  loop.
               CRT'S
             PRINTERS
             RECORDERS
 CENTRAL
COMPUTER
                              TT
x'
' /
MUX

MUX
fyr*<
' '. N v
                                                MULTIPLEXERS FOR
                                                OTHER PROCESS AREAS
                                                             TO NEXT
                                                             PROCESS
                    Figure  4-8.   Central digital control.
Distributed Digital —

          DECISION  MAKER:      Computer
          CONTROL LOCATION:    Subprocess

     Distributed digital  control refers to  a hierarchial system which
includes multiple levels  of control  and control  decisions.  The distributed
system is composed  of  a number of  individual computers which control unit
processes or process trains.   These  process control computers are connected
(hardwired or telemetry)  to a  central computer system (see Figure 4-9).  The
local or process control  computers may be mini or micro computers.  The
central or master computer is  a mini.

     Different types of control  and  process decisions are made at each
level.  The objective  is  to distribute the  execution and processing among
the various computers  in  order to  increase  control responses and to increase
                                     183

-------
        (a)  Non-Redundant Local CPU
        MINI  OR MICRO
          COMPUTER
        CPU 6 MUX MAY
        BE AN INTEGRAL
        UNIT WHEN A   .
        MICRO COMPUTER
        IS UTILIZED
1 — ^
1 LOCAL
* CPU
"A"
1
"A"
1 MUX


"A" SENSORS
AND CONTROLS
I
CENTRAL
CPU
[ t
LOCAL
CPU
MB-
J
"B"
MUX


>
\





BACK-UP
CPU
(OPT 10

INPUTS AND
<* ~~- OUTPUTS
\


"B" SENSORS
AND CONTROLS
LOCAL
CPU
I I/MI
!
"C"
MUX


"C" SENSORS
AND CONTROLS
                                                                       QUANTITY OF
                                                                   EQUIPMENT AS REQUIRED
                                                                    FOR ALL SUBPROCESS
       (b)  Fully Redundant  Local  CPU
ON-LINE
CENTRAL
CPU


BACK-UP
CENTRAL
CPU "
                                                                     COMMUNICATIONS
                                                                        CHANNELS
/\
ON-LINE
LOCAL
CPU

BACK-UP
LOCAL
CPU
\
V
/\
ON-LINE
LOCAL
CPU

BACK-UP
LOCAL
CPU
                                                                    \z


MUX
"A"


MUX
"B"


MUX
"C"


OOO O O o OOO
L"A" SENSORS "B" SENSORS "C" SENSORS
AND CONTROLS. AND CONTROLS AND CONTROLS
V
                                      QUANTITY OF .
                                  EQUIPMENT AS REQUIRED
                                   FOR ALL SUBPROCESS
                Figure 4-9.  Distributed Digital Configurations

                                       184

-------
reliability and availability.  Each processing subsystem shares  the  pro-
cessing load and, because  it stands alone, is unaffected by failures of the
other systems.

     The subsystem computer.'s primary purpose is the control of  the  proces-
ses for which it  is responsible.  The subprocess computer will exchange data
with the central machine as required to  learn of related conditions  in  other
subprocess sections of the plant.  All of the data gathered will be  sent  to
the central machine for the data collection and recording responsibilities.
The central machine will transmit control functions to the individual sub-
process machines, allowing direction of  the total plant operation.

     The central computer  is used for linking the entire control system.
This includes the basic optimization changes necessary to achieve high  per-
formance in the total plant.  Power demand monitoring and control are estab-
lished from the central machine, giving  direction to the subprocess  centers
for various actions in starting  and stopping of motors.  The reporting  of
all data for operations or regulatory agency reports is performed at this
level.

     Figure 4-10  shows the application of distributed digital control to  the
sample sludge flow loop being considered.  Again, the entire dewatering
process is wired to a computer system.   However, in this case, only  the
sequencing logic  and flow control loop logic is contained in the local
computer.  Other  local computers contain similar logic related to the other
subprocesses within the plant.
                   OTHER LOCAL
                    COMPUTERS
LOCATED IN FIELD
NEAR EQUIPMENT
             Hi
             OTHER
            PROCESSES
                                                                     TO NEXT
                                                                     PROCESS
                  Figure  4-10.   Distributed digital control.
                                     185

-------
     Each local computer is connected to a centrally located master compu-
ter.  Current flows, device status, etc. are transmitted by the local
computers to the central computer which will be responsible for such things
as data collection and optimization.  If, for example, an increased thick-
ener flow was detected from one local computer, central would direct the
dewatering local computer to increase its present setpoint for flow.

Hybrid Control

     A combination of analog and digital control is sometimes accomplished
in the hybrid control systems.  In these cases, it is common to have a com-
puter that will receive data parallel to an analog control panel.  The
hybrid control situation can take two forms: 1)  Data Logging and, 2)
Digitally Directed Analog Control.

 Data Loggers —

          DECISION MAKER:     Analog Hardware and/or Human Being
          CONTROL LOCATION:   Unit/Subprocess/Central

     The use of the digital computer for data acquisition is very corrmon in
the United States.  These systems are quite often used in out-of-plant type
problems such as monitoring the flows within a waste control district.  All
of this monitored data can then be processed for such purposes as billing
the contributing communities and industries.

     This system of data acquisition has also become quite common in the
operation of wastewater treatment plants (see Figure 4-11).  Here, the prin-
cipal advantage is the recording capabilities of the computer.  The data,
which can be gathered on as little as one second intervals, can be stored
for later reporting and calculation.  For a medium to large wastewater
treatment plant where there are many flows, the volume of paperwork required
to keep good operating data becomes difficult and expensive.  With a data
acquisition system, it is possible to do this work on a continuous and rapid
basis.

     The computer may also be used to log process occurrences and to annun-
ciate alarm conditions.  Audible alarms and printed messages can alert the
operator to current or potential problems

     The data acquisition system is usually a single computer which monitors
the various analog and discrete inputs.  No control is issued by the comput-
er.  The data may be hardwired to the computer interface modules or it may
be telemetered, in the case of a large plant or out-plant situation, to the
computer center.

     The control of the process is accomplished by either manual control or
by one of the previously discussed analog systems.
                                    186

-------
 (a) Data Logging  System
                                                  REPORTS
                                                  ALARMS
                                                   CRT'S
                     PROCESS 1
  PROCESS "N1
(b)  Digital  Directed Analog Control  (DDAC)
                                                   REPORTS
                                                   ALARMS
                                                   CRT'S
                                                OPTIMIZATION
                    PROCESS  1
PROCESS  "N"
             Figure 4-11.   Hybrid  Control Systems,
                               187

-------
     Figure 4-12 shows the  application  of a logger system to the sample
sludge flow loop being considered.   In  the example, control is shown through
a distributed analog panel,  but  this  could be via a manual analog panel just
as well.  The panel provides control  in the same manner as discussed previ-
ously under distributed  analog control.  Flows,  device status, etc. are
transmitted from the panel  to a  central computer.  All other panels do the
same such that all data  capture  and reporting functions, as well as alarming
functions, occur at the  computer.
                                                 ANALOG FLOW
                                                 CONTROLLER
         DEWATERING PROCESS
        CONTROL PANEL LOCATED
           NEAR PROCESS
        MOTOR SEQUENCE
            CONTROL
                                                                     TO NEXT
                                                                     PROCESS
            Figure 4-12.   Analog control with digital data  logging.
Digitally Directed  Analog Control (DDAC)--
          DECISION  MAKER:
          CONTROL LOCATION:
Computer and Analog Hardware
Central Computer and Analog Panel
     This method  of control combines local analog control with  the  data
 reporting  and  calculating capabilities of the computer.  In DDAC  systems
 (see Figure  4-11),  all  control of the unit processes is accomplished  by
 local  analog control,  including conventional controllers and  sequencing
 logic,  but control  decisions can be initiated by the digital  system.
                                     188

-------
     Process inputs  are  transmitted  in  parallel to the panels  and  to the
computer.  The computer  will  log and retain the information, as  described
under data logger above.   It  will  prepare all reports and annunciate alarm
conditions as they appear.

     The computer will also contain  programs which will ar.alyze  the  process
inputs and will decide when certain  process changes or adjustments are
necessary.  Typically, only loops  which can be benefited by optimization
will be included.  Computer control  outputs will be to the analog panels and
will be in the form  of analog controller setpoint changes and  sequence logic
initiation.  Direct  outputs to field devices would be included only  for
complex sequences where  the computer could improve performance.

     Figure 4-13 shows the application  of DDAC to the sample sludge  flow
loop being considered.   It is identical to the discussion above  under  data
loggers except that  the  computer may also make control decisions and output
them to the local panel.  For example,  if an increased thickener flow  is
detected by the computer,  it  will  output a setpoint change to the analog
controller at the dewatering  panel.   The controller will then control  to the
new setpoint until the operator or the  computer directs otherwise.
                                     CENTRAL
                                     COMPUTER
        DEWATERING PROCESS
       CONTROL PANEL LOCATED
          NEAR PROCESS
       MOTOR SEQUENCE
         CONTROL
                                                    ANALOG FLOW
                                                     CONTROLLER
                                                                    TO NEXT
                                                                    PROCESS
           Figure 4-13.  Digitally directed analog  control  (DDAC)
                                    189

-------
COMPARATIVE EVALUATION

     This section presents a combination of the previous two discussions and
will put the benefits and drawbacks of each alternative into perspective.
Each important consideration outlined earlier will be addressed for each
general control alternative.  A summary of the discussion is found in Table
4-1.  The reader is cautioned that this summary contains certain terms which
are used for overall comparison and should not be construed to imply any
relative magnitudes.  For example, installation costs for a manual system
are shown as LOW.  This does not necessarily mean that it is inexpensive to
install such a system, but rather that the costs are low compared to other
alternatives.

Manual Control

Costs —
     In general, manual control is the least expensive alternative in terms
of the seven cost factors discussed previously:
     Capital Equipment - Local p
     ators I audible and visual).
panels with pushbuttons, gauges and annunci-
     Instrumentation - Flow meters, chlorine analyzers, thermocouples, etc.

     Installation - The lack of equipment would, of course, reduce the
     installation costs.  Since all panels are near the unit processes,
     there would be "minimal" wiring costs.

     Software - None required.

     Operating Costs - This will depend  largely upon the variability of the
     process and the size of the plant.  In a small, stable plant, one or
     two operators per shift could get to all panels within a reasonable
     time.  In a large, variable process, it might require one operator per
     panel per shift which could necessitate 50 or more personnel.  A 10 mgd
     (440 dm3/s) secondary plant, for example, might require five or more
     persons to operate it per shift.

     Maintenance - Sensor and panel maintenance.  Typically, two or more
     mechanically-oriented persons would be required.

     Expansion - Simple expansion can cause problems and increase costs,
     depending upon the amount of modification required to the various
     panels.  Addition of another pump is inexpensive  if there is space for
     it on the panel and relatively expensive if a new panel must be built.
     Wiring is direct between process and panel, thereby necessitating new
     wiring.

Savings--
     There are no savings associated with manual control as it is the basic
alternative and is the standard against which all others will be measured.


                                    190

-------
TABLE 4-1.  CONTROL COMPARISON SUMMARY
Consideration
Costs:
- Equipment
- Instruments
- Installation
- Operating
- Maintenance
- Software
- Expansion
Savings:
- Labor
- Chemical
- Energy
- Equip Life
Rel iability
Flexibil ity
Optimization
Space Requirements
Operator Qualifications
Operator Acceptance
Management Information
Methods of
Control
Automatic Control
Ava i lable
Manual
Low
Low
Low
High
I
No.i'-
I"W
None
None
None
None
High
Complete
None
Low
Low
Good
None
Pane 1 s .
Crisis Control
None.
Human Intuition
Analog
Loca,l
Moderate
Moderate
Moderate
High
High
None
High
Some
Some
None
None
Moderate
Low
None
Moderate
Moderate
Fair/Good
None
Panels.
Crisis Control
Feedforward,
Feedback
Central
Moderate
Moderate
Moderate
Low
High
None
High
Some
Some
None
None
Moderate
Low
None
Moderate
Moderate
Fa 1 r/Good
None
Panels.
Crisis Control
Feedforward,
Feedback,
Cascade
Distributed
Moderate
Moderate
Mod/High
Moderate
High
None
High
Some
Some
None
None
Moderate
Low
None
Moderate
Mod/High
Fa ir/Good
None
Panels.
Crisis Control
Feedforward,
Feedback,
Cascade
Digital
Central
High
High
Mod/High
Low
Moderate
Moderate
Moderate
High
Moderate
Moderate
Some
Low/Moderate
High
High
High
Mod/High
Good
Complete
CRTs.
Anticipatory Ctl
Feedfwd, Feedback,
Cascade, Adaptive
Distributed
High
High
High
Low/Mod
Moderate
High
Moderate
High
Moderate
Moderate
Some
Moderate
High
High
High
Mod/High
Good
Complete
CRTs.
Anticipatory Cti
Feedfwd, Feedback
Cascade, Adaptive
Hybrid
High
Mod/High
Hiqh
Hiqh
High
Moderate
High
Some
Some
Some
Some
Moderate
Low
Some
High
Mod/High
Good
Moderat.e
Panels/CRTs.
Crisis Control
Feedfwd, Feedback,
Cascade, Adaptive

-------
     Labor - In moderate to large plants,  manual  control  requires  far more
     operators than other alternatives.   In very  small  plants  (less  than  1
     mgd - 44 dm3/s)}  however,  labor costs would  be comparable to  other
     alternatives.

     Chemical - Chemical addition is largely uncontrolled,  unless  an opera-
     tor(s) continually adjusts dosage rates.  Excess chemical usage
     generally results.

     Energy - No practical consideration may be made.  Energy  consumption
     will be essentially uncontrolled.

     Equipment Life - No improvement.  Device alternation will be  left up to
     the operator and preventive maintenance will be manually  controlled  and
     scheduled.

Reliability—
     The control of the plant will be as reliable as the operator(s).
Operator intuition and experience will be the order of the day.  Sensor
failure will result in operator "guesstimates" of the current  process vari-
able.  Control hardware failure is generally unlikely.

Process Variability-
     Manual control is practical in small  volume, consistent processes.
When the flow fluctuates a great deal or when the influent characteristics
change, an operator cannot react quickly enough or often enough to ensure
consistent effluent.  Manual control generally cannot be expected  to produce
stable or consistent effluent in treatment plants unless the influent
characteristics are very constant and the plant very small.

Control Flexibility—
     In one sense, manual control is very flexible; the operator may do
whatever he wishes.  While this does provide flexibility, it also  leads to
inconsistent control.  Each operator will  have his own mode of control and
will essentially operate the plant differently.

Optimization of Results-
     Little, if any, optimization is possible.  Each unit generally operates
independently of every other unit.  There are no  data capture  capabilities,
short of manual recording, and any attempts at optimization must be manually
developed and controlled.

Physical Space Requirements-
     Space for the control panels is required. This requirement will vary
depending upon the number of control elements within each panel.  A large,
central panel may be included at a central location for process monitoring.

Operator Qualifications —
     The operators will need to be experienced in the particular plant being
controlled.  They must be well schooled in the treatment process and well
versed in the plant dynamics.  Operators at manual plants must be


                                    192

-------
disciplined to be on constant vigil for changes.  The training period  is
likely to be a relatively long apprenticeship.  Therefore, cross-training of
operators and the use of one or two "floaters"  is prudent  in  order  to  cover
illness, resignations, etc.  Mechanical aptitude is a necessary  criterion
for operator selection.

Operator Acceptance--
     Manual control systems are usually quickly accepted because  of their
simplicity.

Management Information —
     Very little assistance is offered by this concept.  Data may be
recorded on strip or circular recorders and manually transcribed.   All
calculations must be manual as is all report preparation.

Manner of Control —
     Crisis control is generally the norm.  The operator cannot readily
discern any trends in the process but rather must react to a situation after
it occurs.  More likely, peaks and valleys will come and go and no  modifica-
tion of control strategy will occur at all because the operator has  no
vehicle available which will alert him unless a crisis condition occurs.

General Usage--
     Manual control is generally only practical in small plants (0  to 5 mgd
- 0 to 220 dm3/s) or for unit processes.  Packaged plants are typically
manually controlled.  It is often found in physically small plants  and in
primary treatment plants with relatively constant flow and weak influent
characteristics.  It is generally not practical in larger plants or  in unit
processes which are 1) critical,  2) time dependent or, 3) multivariant.
There is a limit to the number of variables a person can comprehend,
evaluate and respond to over a period of time.

Analog Control

     Since the general considerations of local, central and distributed
control are very similar, they will be addressed in the same section.  These
comments apply to all  three unless specifically noted.

Costs —
     Full  analog control generally increases the overall cost of the control
system.  Typically,  a central  system costs more than a distributed configu-
ration, while a local  analog system is the least costly.

     Capital  Equipment - Includes local  panels, central  panel (if needed),
     analog controllers,  control  elements, remote setpoint stations, cascade
     controllers, sequence logic,  etc.  Interpanel  communication is also
     necessary via telephone,  radio or the like.

     Instrumentation - Flow meters,  chlorine analyzers,  thermocouples,  level
     sensors,  .etc.
                                    193

-------
     Installation - Installation costs are higher than manual control  since
     control lers must be wired and installed.   Centralized systems include
     more hardware and require more installation.

     Software - None.

     Operating Costs - Generally speaking, one operator per shift is
     required per analog panel or panel grouping except in small plants
     where an operator can handle several panels due to low flow or inter-
     mittent operation.  A central analog configuration in a large plant
     would require more than one person in the control room.  When two or
     three panels are in close proximity of each other, one operator can
     effectively manage all of them as long as the dynamics are reasonable.

     Maintenance - Maintenance of the control  system elements is extremely
     critical to analog systems.  A good preventive maintenance program is
     necessary.  A large number of maintenance personnel are required  to
     maintain the sensors and controllers.  This staff should be mechanical
     in orientation; some electronics experience would be necessary for the
     maintenance and repair of analog controllers, sequencing devices, etc.

     Expansion - Simple expansion can cause problems and increase costs.
     Large analog panels are very rigid and difficult to change.  Signal
     wiring is typically direct between process and panel and changes  will
     necessitate additional wiring.

Savings —

     Labor - Fewer operators are needed but additional maintenance personnel
     will probably be required.  However, the net should be an overall
     reduction over manual control.

     Chemical - Chemical feed can be paced by flow, thereby reducing total
     usage.  Dosage rates generally remain operator entered setpoints.

     Energy - Little energy savings may be realized.  Consumption may  be
     monitored and manually curtailed during peak times.

Reliability-
     Because of the number of devices, reliability can be a significant
problem.  Backup, in the case of a controller failure, will be manual  con-
trol from the appropriate panel.  Sensor failure would require an operator
to "guess" at current process conditions and make manual corrections
accordingly.

     Failure of the feedback signal in an analog control loop can cause
process upsets.  The controller cannot detect such occurrences and will
either drive the control output closed or will "wind up" and go to full
signal output.  The problem with sensors "holding" a value as they become
coated is severe with analog controllers.
                                    194

-------
Process Variability—
     Since closed-loop control is included with this alternative, process
variations may be sensed via feedback variables and appropriate adjustments
made automatically.  In general,  this will affect variations in flow.  It  is
still impossible to compensate for variations in the influent characteris-
tics unless cascaded loops are included.  Feedforward control is not
practical.

     Under local analog control,  certain variations, even in flow, cannot  be
compensated for without operator intervention.  For example, if a single
variable speed influent pump is being controlled by a wet well  level signal
and the influent flow increases above that which a single pump can handle,
the operator must manually bring a second pump on line.

     Process stability and the attainment of treatment standards is not
easily assured with this control  system.  Experience has shown that process
stability is not a strong point.   Each shift usually applies different
theories of operation and adjustments lead to process instabilities and
inconsistent results.

Control Flexibility--
     The flexibility of the control system is somewhat questionable.  After
the panels and controllers are mounted,  even a change from a flow control
loop to a mass control loop is not easy.  This kind of change would require
the purchase of a new hardware device and reconfiguration or redesign of a
panel.  Hence, it is generally difficult to attain a great deal of flexi-
bility in the control system.  However,  much like manual control, the
operator has a great deal of flexibility in determining control strategy,
device configuration and setpoint changes.

Optimization of Results—
     A certain degree of optimization is possible, particularly if cascaded
control is utilized.  However, any attempts at optimization will be a result
of manual data capture and analysis.   Because of the great amount of effort
required to implement any type of optimization, and the necessity of
continuing it over a long period  of time, it is doubtful if any great amount
will, in reality, be accomplished.

Physical Space Requirements —
     Substantially more space is  necessary than under manual control because
of the space required by the controllers.  Local control panels, each with a
similar requirement, and a central panel will require space.

     Utility services required at each panel would typically be power air,
instrument air and electric service (120 VAC).  Larger panels may require
environmentally controlled rooms.  Location of the panels becomes a critical
factor in any plant.  The Occupational Safety and Health regulations will
heavily affect the location because an operator will work in the vicinity of
the panel for 24 hours per day.  This will affect other designs such as
lighting, ventilation,  noise, etc. in addition to obvious problems when
located in potentially wet, hot or hazardous locations.


                                    195

-------
Operator Qualifications —
     The operator will need to be knowledgeable in the particular plant
dynamics and process control system.  Training will need to be "system"
oriented and thorough.  Operators must feel comfortable with the system.
The operator need not be technically oriented beyond basic mechanical skills
and process knowledge.

Operator Acceptance--
     Operator acceptance is high.  They usually feel part of the control
system.

     Operator acceptance quickly diminishes, however, when the number of
panels and/or the complexities of each reaches a point where the operator
cannot cope with the magnitude of the task.  This is particularly likely in
a centralized plant which grows to the point that the central control room
contains a large maze of panels with many operators trying to comprehend and
manage the process.

Management Information-
     Data is manually gathered, calculated and reported.

Methods of Control —
     The operator interfaces with the process via the various panels, and
typically does not react to trends but rather to alarm events.  Crisis
control is very much evident.  Interactions of process units are impossible
to detect, much less control.  Deviation alarms, indicated by the analog
controllers, assist the operator in reacting to upsets.  Discrete alarms
(high temperature, vibration, etc.) can be audibly annunciated.

     Because this system has no higher level control, off-line analytical
measurements cannot generally be used to automatically correct the process.
They can only be used to correct for the future and they must be analyzed
and interpreted by the local panel operator.

General Usage/Local Analog--
     Local analog control should be considered a viable alternative for
small plants (0 to 5 mgd -  0 to 220 dm3/s) and for specific unit processes
in other plants.  (For example, a plant may be all manually controlled
except for chlorination which may be put under analog control because of its
critical nature.)

     Small plants with variable  influent may utilize this alternative.  Unit
processes with a moderate number of control points are prime candidates.

     Local analog control is generally impractical in medium or  large scale
plants.  Communication, coordination and downtime risks are often serious
drawbacks of this type of control.  Labor costs can become a factor  in
larger plants.
                                     196

-------
General Usage/Central Analog-
     Centralized analog control is applied to medium size plants  (5  to 50
mgd - 220 to 2200 dm3/s).  It provides coordinated control with minimal
communication problems.  It may become too expensive for larger processes  or
in geographically spread out plants where wiring costs would surpass savings.

     It generally is not applied to larger plants.  As the plant  size
increases, so do the number of control loops, the variability of  the
process, the reporting requirements and the process costs.  Central analog
control in a large plant often results in a massive control room  with many
long, dense and cumbersome panels which are difficult to comprehend and
nearly impossible to use.  Data collection alone becomes a major  considera-
tion.

General Usage/Distributed Analog-
     Distributed analog control is generally applicable to medium to large
scale plants (10 to 100 mgd - 440 to 4400 dm3/s).  it offers centralized
coordination and data collection with less wiring expense and generally more
reliable operations.  Maintenance can be centrally controlled and overall
plant operation observed and managed.

     Distributed analog is typically not applicable for smaller plants;
local analog or central analog are probably better alternatives.  Very large
plants (100 mgd - 4400 dm3/s and over) are probably operating under such
severe cost, quality and reporting constraints that a more sophisticated
alternative is warranted, although in practice distributed analog is used.

Digital Systems

     Digital systems, both centralized and distributed, are discussed
below.  All comments are relative to both unless otherwise indicated.

Costs —
     In general, digital systems cost more than do analog systems.  However,
this is not true in a large plant where,  for example, the wiring costs of a
central analog system are much greater than the cost of a digital system
with remote multiplexers.  Similarly, a single CPU digital  system would
probably cost less than an extensive distributed analog system with many
different control rooms and comprehensive sequencing logic.  However, the
costs of developing computer software usually result in a higher cost for
the digital system.   Furthermore,  distributed configurations typically cost
more than centralized systems except when comparing a fully redundant,
multiprocessor centralized system to a simple distributed application.

     Capital Equipment - Includes all  computers, multiplexers,  maintenance
     panels (if needed), CRT's and other  peripherals.

     Instrumentation - Generally includes more on-line analyzers since the
     computer can make more extensive use of the information than can other
     alternatives.  Other instrumentation (flows, levels, etc.)  are the same
     except that all nonelectrical  signals must be transduced.


                                    197

-------
     Installation- - Must include installation of the computer hardware,  any
     panels and all wiring.   If remote multiplexing (i.e.  placing multi-
     plexers near the process unit)  is utilized, wiring costs can be reduced.

     Software - Will be a significant portion of the costs.   Magnitude
     depends upon the amount of control required.

     Operating Costs - Operating personnel will  depend upon  process size and
     control system type.  A single  processor centralized  configuration, for
     example, can be run by a single operator per shift.  A  dual processor
     configuration requires two.  A  large distributed system would probably
     be staffed by one operator for  each subsystem and a process engineer at
     the central computer.  One or more laborers per shift would be
     necessary in all instances.  In general, a reduction  in operating
     personnel can be planned for because of the centralized capability.

     Maintenance - Maintenance of all sensors is critical, just as for
     analog and manual systems.  In  addition, the computer hardware requires
     maintenance, as does the software.  A hardware maintenance contract
     from the computer vendor is often utilized, although  larger configura-
     tions will probably warrant hiring an in-house staff.  Similarly,
     changes to the programs (or new programming) may be contracted for  with
     a control system supplier or an in-house capability may be developed.

     Expansion - Modification of the control system is easier than with
     other alternatives.  Control points can typically be  added to the
     computer software in a matter of minutes; modification  of control
     programs requires one to five days time.  Wiring costs  are generally
     the most significant cost of expansion, unless major  additions to the
     process necessitate additional  control system hardware  or completely
     new programs.

Savings~
     Significant savings may be realized over all other control alternatives.

     Labor - Fewer persons are required to run the plant than with other
     alternatives.  In addition, reports and data analysis can be handled by
     the computer, thereby reducing  the clerical staff required.

     Chemical - Chemical optimization  is practical since application may be
     paced as a function of not only flow but of a complex set of influent
     characteristics as well.

     Energy - Total energy costs can be reduced using a digital system.
     This  is accomplished by minimizing consumption (reducing device starts
     and load balancing) and by load shedding when current consumption
     approaches the economical peak  demand level.
                                    198

-------
     Equipent Life - Process equipment life may be extended by utilizing a
     digital control system.   The computer can minimize equipment damage due
     to frequent cycling and/or long periods of continuous operation.  It
     can automatically alternate usage of redundant devices in order to even
     wear, and it can record run times and automatically schedule preventive
     maintenance tasks based on calendar time or device usage.

Reliability--
     Digital control systems are generally as reliable as other alterna-
tives.  Backup can be accomplished by redundant computers or from mainte-
nance panels.  Sensor failures can be sensed and either 1) the last control
output is held or, 2) an estimated value is calculated (e.g. flow calculated
from a level signal).  Nearly fail-safe reliability (99.9% uptime) can be
achieved 'if necessary.

Process Variability--
     A well defined and tuned digital control system can generally handle
most ranges of process variability.  Changes in flow, and in influent char-
acteristics (assuming proper sensors), can be compensated for utilizing a
wide variety of control techniques (feedforward, feedback, adaptive,
cascade).  Very tight, stable and consistent control can be achieved.  Rela-
tionships between processes,  process variables and laboratory data can be
programmed such that effluent quality standards can be consistently achieved.

Control Flexibility-
     Flexibility is provided because the control is implemented in soft-
ware.  Points and control algorithms may be specified on line without
disturbing the process.  If design errors are encountered, the previous
control mode can be used without hardware changes.  Typically, reports and
graphic CRT displays can be altered without any disruption of the process
and without significant costs.

Optimization of Results—
     Because of the data capture and analysis capability of the computer,
optimization is feasible and practical.  Complex relationships may be
determined and utilized in complicated control strategies because of the
flexibility provided.  Distributed systems provide the ultimate possibili-
ties because the central computer has plant-wide data available for optimal
load balancing, sludge routing and other critical and complicated subsystems.

Physical Space Requirements —
     Environmentally controlled space is typically required for the computer
hardware.  Multiplexers require less stringent conditions.  A room of
approximately 10,000 square feet per central computer subsystem is generally
required.  Isolated power (120 VAC) is normally needed as well as a separate
electrical ground.  Temperature and humidity controls are necessary.
                                    199

-------
Operator Qualifications--
     Operating staff, while fewer in number, must receive a significantly
different type of training than with analog systems and manual systems.
This is particularly true in a centralized system and at the central
computer of a distributed configuration where process interactions and
trends must be determined and handled.

Operator Acceptance—
     Because of the ease of data display and control operations, operator
acceptance comes rapidly.  This is particularly true in new plants with new
personnel.  "System" training and knowledge is the key.  Acceptance is
slower when upgrading an old system (e.g. analog control to digital
control), largely because of the unfamiliarity with the new methods.

Management Information —
     The ability to automatically capture and report data is one of the
greatest benefits of digital systems.  They offer plant management the
opportunity to easily and quickly collect, display and analyze current and
past process data in order to determine control problems, interactions and
solutions.  Various governmental reports can be generated automatically.

Method of Control--
     The operator of a digital control system views the process and
interacts with it via a CRT display (TV tube type display and typewriter-
like keyboard).  Existing systems use black and white displays while most
new installations utilize color displays.

     Older systems use all alphanumeric displays while recent installations
are depending heavily on graphic capabilities where color denotes certain
conditions such as running, not running, etc., and flashing characteristics
show unacknowledged  alarm conditions.  All conditions and readings appear in
real time.

     Crisis control  is minimizecj with digital systems.  The computer can
sense  trends, rate of change, operating limit violations, etc. and notify
the operator or take independent action before an alarm condition appears.
Graphic displays allow the operator to view overall trends and conditions.

General Usage/Central Digital~
     Centralized digital control is most common in medium range plants (10
to 100 mgd - 440 to  4400 dm3/s).  Some application in larger plants is
possible when multiple computers share the load or when applying control to
only certain process units.  The controlling factor appears to be the number
of process data points, with 2,000 points per computer system being a
practical design standard.  On-line backup capabilities further increase the
reliability of the system. .

     Today even smaller plants can justify a digital control system, as
indicated in the cost effective section.  Larger plants may find that the
coordination necessary between multiple computer systems is too difficult,
or costly, to attain and other alternates should be considered.


                                    200

-------
     This control alternative should also be considered when process vari-
ability is great, regardless of plant size, since it offers more complex
control ability.  Data reporting requirements warrant consideration and
plants which can anticipate future changes or expansion should consider this
option.

General Usage/Distributed Digital--
     Distributed digital appears to be the most applicable for large to very
large processes (100 mgd - 4400 dm3/s and over) and for processes with
very critical uptime requirements.  Applicability to smaller plants will
become more practical as the use of microcomputers in "process controllers"
becomes better defined and developed.

     The use of this alternative should be considered when cost and
reporting constraints are great, as well as when plant size and variability
increase.  Optimization possibilities make this type of system very
attractive.

Hybrid Control Systems

     Since data loggers and digitally directed analog control (DDAC) are
very similar, they will be addressed in the same comparative analysis.
Additionally, since hybrid control is a combination of analog and digital,
references will be made to the corresponding discussions whenever possible.

Costs —
     Hybrid control is generally more expensive than other alternatives
since it requires the components of analog control plus a digital computer.

     Capital Equipment - Includes local panels, analog controllers, control
     elements, pushbuttons, sequence logic, recorders, readouts,  interpanel
     communication, computer, multiplexers, peripherals.

     Instrumentation - Flow meters, analyzers, thermocouples, level sensors,
     etc.

     Installation - Includes not only the cost of panel installation,  but
     also the costs of installing a computer.  Additionally, wiring from all
     field inputs must be run from the panels to the multiplexer.

     Software - Software for input scanning, conversion, logging  and
     alarming is necessary.  Additionally, DDAC systems require control
     programs capable of analyzing process inputs  and outputting  control
     modifications.

     Operation - Operating staff would be comparable to the staff require-
     ments of the analog component of the system.   The computer system  will
     typically be a focal point for the plant or process engineer to monitor
     the process and will not add to the staff.
                                    201

-------
     Maintenance - Maintenance can be considered as the arithmetic addition
     or the analog control maintenance and the costs associated with the
     computer system hardware and software, the latter being slightly higher
     for DDAC systems than for straight data loggers.

     Expansion - Expansion will require that both the analog system and the
     computer system be expanded.  The already high costs of analog
     expansion are increased by the digital component.

Savings —
     Labor - Hybrid systems typically do not reduce labor costs beyond the
     savings already discussed under analog control.  The possible exception
     to this might be in larger plants where clerical staff reductions might
     result from the data collection and reporting components of the
     computer.

     Chemical - Data loggers will provide little in the way of savings
     beyond that offered by analog control.  DDAC systems, on the other
     hand, will be able to make use of complex control algorithms which
     include capabilities to adjust dosage rates based on process analyzers.

     Energy - Like chemical savings, DDAC systems offer the opportunity for
     load balancing and energy monitoring and control over those abilities
     of the analog component.  Data loggers do not.

Reliability—•
     In general, the hybrid control system is as reliable as the analog
system doing the control.  Since the only interface with the process is
through the analog panel, and since it is doing most of the process control,
it can be considered to be the critical link.  Failure of the computer
system will result in a loss of data and, in the case of DDAC, a loss of the
higher level control.

Process Variability—
     Comments relative to analog control are appropriate to data loggers,
since they provide no additional control capabilities.  DDAC systems,
however, do increase the capacity of the system to handle variations.  These
capabilities, which are the same as for digital systems, relate to the opti-
mization and sophistication possible because of the programming aspects of
the computer.

Control Flexibility-
     Data loggers offer no additional flexibility over straight analog
control.  Even DDAC is limited, since the final output to the control
elements is no more flexible than the analog panels permit.  However, the
fact that the computer can he programmed to provide a variety of, say, set-
point calculation alternatives, will increase the flexibility.
                                    202

-------
Optimization of Results —
     Data loggers will provide some assistance in plant optimization since
they offer data capture, analysis and reporting abilities.  Implementation
would still be manual as would followup.

     DDAC systems usually offer a reduced scope of capability in relation to
the digital systems.

Physical Space Requirements —
     The same space would be required for hybrid systems as for analog
systems plus a central computer room.

Operator Qualifications —
     Same as analog.

Operator Acceptance-
     Same as analog, since the process operator is still located at the
local panels.

Management Information-
     Hybrid systems will generate the same information as the digital system
will.

Methods of Control —
     In general, the operator still interfaces with the process through the
local panels.  The process engineer will be able to oversee the entire plant
via CRT displays on the computer and will be alerted by the computer to
potential problems before they occur.  Crisis control will be greatly
reduced.

General Usage—
     Hybrid systems can be generally found in plants ranging from 50 to 300
mgd (2200 to 13000 dm3/s) and are used with manual and all forms of analog
control.  It offers decentralized control with full reporting abilities.
They are generally too expensive for very small plants.   Future growth of
the microcomputer may make these systems more practical  for the smaller size
processes.
                                    203

-------
                                  SECTION 4
                                 BIBLIOGRAPHY


Amrehn, Dr. H., Report on Two Computer Installations With DDC Application.
Proceedings of 22nd Annual Instrument Society of America Conference, 1967.

Amrehn, Hermann and Otto Winkler, Backup for Process-Control Computers,
Chemical Engineering. March, 1970.

Bailey, S. J., Direct Digital Control-A Status Report,  Control Engineering.
November,  1972.

Bakke, Roger M., Theoretical Aspects of Direct Digital Control. IBM Control
System Development Center.

Davis, Guy E., Jr., Hybrid Digital-Analog Power Plant Control,  ISA
Transactions. Vol. 9, No. 4, 1970.

Etchart, David Y. and Bipin Mishra, Survey of an On-Line Computer Control
System at  Metro Denver,  Advances in Instrumentation, Vol. 32, Part 3.  ISA,
1977.

Fraade, David, 10 Years After-A Word to the Wise,  Instrument Practice.
July,  1969.

Gutshall,  T. L., C. F. Koch and G. Nines, DDC Initial Cost Justified,
Control Engineering. January, 1969.

Harrison,  T. J., Micros, Minis and Multiprocessing,  Instrumentation
Technology. February, 1978.

Lloyd  and  Anderson, Industrial Process Control. Fisher Controls Company,
1971.

Lombardo,  J. M., The Place of Digital Backup in the Direct Digital Control
System. Spring Joint Computer Conference, 1967.

Marks. Charles H.. Digital Process Control System Development,  Pipe Line
Industry.  June, 1967.

Minimizing Process Computer Maintenance,  Instrumentation Technology, Vol.
15 No. 1.  January, 1978.

Miller, James G., Introduction to Digital Control,  Instruments and Control
Systems. March, 1971.
                                     204

-------
Nagel, Carl A., State of the Technology Semi-Automatic Control of Activated
Sludge Treatment Plants. National Technical Information Service, U.S. Dept.
of Commerce, 1975.

Parsons, J. R., M. W. Oglesby and D. L. Smith,  Performance of a Direct
Digital Control System. Applied Automation, Inc.

Poisson, Normand A., Interfaces for Process Control,   Textile Industries.
March, 1970.

Rice, Raymond E. and George A. Mathes, A Demonstration Direct Digital
Control of a Vacuum Filter,  Advances in Instrumentation, Vol. 32, Part 3.
ISA, 1977.

Roesler, J. F., Evaluation of the Effectiveness of Automation,  Research
Needs for Automation of Wastewater Treatment  Systems.  USEPA,  1975.

Senstrom, Michael Knudson, A Dynamic Model  and  Computer Compatible Control
Strategies for Wastewater Treatment Plants. Clemson University Doctoral
Dissertation, 1975.

Shinskey, F. G., Process Control Systems.  McGraw Hill.

Skaggs, W. L. and R. M. Johnson, Economic  Application  of Direct Digital
Control. Workshop on the Use of Digital Computers in  Process  Control,
Louisiana State University, March 10, 1967, The Foxboro Company.

Smith, Cecil L., Digital Computer Process  Control.  Intext Educational
Publishers.

Systems Control, Inc.,  Demonstration of Digital Computer Application  in  a
Wastewater Treatment Plant, City of Palo Alto,  California,  1974.

Williams, T. J., Computers, Automation and  Process  Control  Annual  Review,
Ind. Eng. Chem.

Zikas, Algirdas J., Hardware Requirements  for a Typical DDC System. The
Foxboro Company.
                                    205

-------
                                  SECTION 5

                           COST EFFECTIVE ANALYSIS
INTRODUCTION
     When automating a wastewater treatment plant, several alternate control
approaches, described in Section 4, are available to the design engineer.
Field observations during the plant visits revealed that the use of multiple
conventional control areas often leads to poor operation and an unmanageable
situation.  Centralization can alleviate these problems and may be used if
the additional capital costs are offset by savings in operating costs.

     Although centralization of control has not been widely used to date in
wastewater treatment facilities, there are a few installations in the field
which have utilized the approach and have documented the savings. (1)(2)(3)
The installations have shown that with centralization there is a substantial
decrease  in the number of operating positions, some increase in maintenance
positions resulting in a net decrease in manpower costs.  For example, the
new Cedar Rapids, Iowa 39 mgd treatment plant with centralized digital
control is a more complex plant and treats twice the flow of the two
conventionally controlled treatment plants it replaced.  However, the new
plant operational staffing requires only one more person than staffed at the
two old plants.  The centrally controlled plant staff consists of 20
operators, a reduction of two operators and 20 maintenance personnel, and an
increase  in three maintenance people.

     A cost effective analysis should be performed to determine which
control approach would be the most economical solution for the application.
Once this economic information is obtained, the design engineer can then
analyze other control design features and capabilities that may be less
tangible  but no less important in selecting the control design approach.
These areas of consideration were discussed in Section 4.

     This cost effective analysis should be prepared on a "systems" basis
reflecting the estimated capital investment, operation, maintenance, materi-
als (chemicals) and energy costs for the alternate control systems accom-
plishing  the same control functions.  The potential savings in operation,
maintenance, chemicals and .energy costs obtainable by the application of
varying levels of automation and centralization should be examined in rela-
tion to the associated capital investments required for the control systems.
                                     206

-------
     This section documents a technique for performing  a cost effective
analysis of alternative control systems.  The procedure  is  presented by
comparing the costs of three types of commercially  available  control systems
(distributed analog control, centralized analog control  and centralized
digital control) for five plant sizes (5, 10, 50, 100 and 300 mgd)  with the
same general unit process flow sheets.  The flow sheets  were  modified only
to reflect the appropriate unit processes for each  plant size.

     The cost information utilized in this section  is for these  generalized
treatment schemes and should not be used for application to any  other
specific analysis.  The numbers are presented as a  method of  demonstrating
the cost analysis technique.  A control system cost effective analysis  must
be customized to an actual process flow sheet and an actual staffing plan
with all local conditions considered.

TREATMENT PROCESSES SELECTED

     Two types of wastewater treatment processes were selected by the EPA
for use as example applications for automation in this cost effective analy-
sis (Figures 5-1, and 5-2).  The processes selected for  the various
treatment plant sizes include:  1) conventional activated sludge, anaerobic
sludge digestion (sludge stabilization), sludge drying beds;  and 2)  conven-
tional activated sludge, anaerobic sludge digestion, sludge incineration
(sludge disposal).  Both include a main lift, diffused aeration, chlorine
disinfection,  plant water pumping, return liquors pumping and  scum
handling.  The conventional activated sludge process includes bar screens,
aerated grit chambers and primary treatment.

     The treatment process configurations developed for  the five plant  sizes
are hypothetical, but are in accordance with current design practices and
standards (4)(5).  The processes are configured as nearly alike as possible
so that general comparisons between control systems and  trends can be
clearly indicated.

     Piping and instrumentation drawings (P&ID's) are included in this
section to illustrate the treatment process and the application of automa-
tion for the five plant sizes,   A symbol sheet and device lettering  table
(pages xi,  xii) describe the equipment shown on the P&ID's.  For
simpilicity, analog control instrumentation symbology is used in the
schematics.   For the digital systems however, much of the control hardware
shown would be replaced by software control programming  in  the central
computer.
                                     207

-------
           PLANT
          INFLUENT'
ro
o
CO
                      MAIN   BAR     GRIT
                      LIFT  SCREENS CHAMBERS
 PRIMARY
CLARIFIERS
                                                              EFFLUENT
AERATION
  TANKS
SECONDARY
CLARIFERS
CHLORINE
CONTACT
CHAMBER
                                                          TO DRYING
                                                            BEDS
                             Figure  5-1.   Conventional  activated sludge with drying beds.

-------
ro
o
          PLANT
         INFLUENT
                     MAIN    BAR     GRIT
                     LIFT  SCREENS CHAMBERS
                             CHLORINE
                             CONTACT
                             CHAMBER
                                                                                                                  EFFLUENT
                                                    INCINERATION
VACUUM
FILTERS
HOLDING
 TANKS
                             Figure 5-2.  Conventional activated sludge with incineration.

-------
Conventional Activated Sludge With Drying Beds

     The 5 mgd (220 dm3/s) plant is detailed in Figures 5-3 and 5-4.  Raw
sewage and return liquor  is lifted from respective wet wells by dual vari-
able speed pumps.  Preliminary treatment consists of mechanically cleaned
bar screens and aerated grit chambers.  Variable volume centrifugal com-
pressors supply air for the grit chambers.  The flow is split between two
circular primary clarifiers.  Clarifier overflow is mixed with return
activated sludge and passes to plug flow diffused air aeration chambers.
The mixed liquor flow is  split between two rectangular secondary clari-
fiers.  The plant effluent is disinfected with chlorine before discharge.
Secondary clarifier underflow is discharged into a sludge wet well.  Return
activated sludge is boosted by dual variable speed pumps.  Plant water is
supplied by two constant  speed pumps.  Solids handling consists of gravity
thickening, anaerobic digestion and sludge disposal on drying beds.

Conventional Activated Sludge With Incineration

     Figures 5-5 and 5-6  detail the liquid and solids train piping and
instrumentation for the 10, 50, 100 and 300 mgd plants (440 through 13000
dm3/s).  The liquid train processes are the same as the 5 mgd (44 dm3/s)
plant with changes only in equipment sizes and quantities to match the
flow.  Sludge is stabilized via anaerobic digestion, vacuum filtered and
incinerated.

PROCESS CONTROL STRATEGIES

     The following brief  process control descriptions establish the level of
control for each unit process in the various treatment plant sizes.  Not all
process control strategies described apply to all treatment plant sizes.
Refer to the process piping and instrumentation drawings, Figures 5-3
through 5-6, for the unit processes applicable to each treatment plant size.

Influent Pumping

     Multiple, variable speed lift pumps are controlled by the raw sewage
wet well level.  Pump speed ramps up or down and pumps are sequenced on or
off in response to wetwell level.

Preliminary Treatment

Bar Screens--
     Bar screen cleaning  is controlled by a time clock or high level
differential across the screen on the larger plant sizes.

Grit Chambers—
     Degritting equipment is controlled by a time clock.  Air flow is
controlled in relation to hydraulic flow in the larger plants.
                                     210

-------
PLANT INFLUENT
 RETURN LIQUORS
                               Figure 5-3.   Liquid train P&ID -  5  mgd.

-------
ro
                     WAS
                  PRIMARY CLARIFIER SCUM
                                                    SECONDARY CLARIFIER SCUM
                                GRAVITY THICKENER SCUM
                                SCUM COLLECTION a PUMPING
                                                 Figure  5-4.   Solids train  P&ID  - 5  mgd,

-------
IN)
h-»
CO
           PLANT INFLUENT
             WET WELL
            RETURN LIQUORS
                                      Figure  5-5.   Liquid train P&ID -  10 to  300 mgd.

-------
ro
                                                                                                             "TYPICAL FOR"
                                                                                                              0 DIGESTERS
DTj M-


I
TJ



                 PRIMARY CLARIFIER
                                GRAVITY THICKENER SCUM


                                SCUM COLLECTION a PUMPING
PLANT
10
SO
too
ino
PHOCFSS ujni
2222?
? 2 5 2 It
'i 4 1. 2 0
'i ". H 4 l(.
Mi HI
2 2
1 1
'i '<
'i 8
IIIAN1 ITY
1 1 I 1
2*21
1 12 I'.
                                              LQJ

                                               HOLDING TANKS
ASH
        INCINERATORS
                                            Figure 5-6.   Solids  train P&ID -  10  to 300  mgd.

-------
Primary Clarifier

     Flow to multiple clarifiers  is dynamically  divided  by  a flow splitting
control scheme (see Hydraulic Flow Control, Section 3).   Unequal  flow
balance is available to compensate for variations  in performance  among the
clarifiers.  Sludge withdrawal  is based on blanket level  and sludge density
with timer overrides.

Aeration

     Multiple, variable flow compressors maintain  a constant pressure  on  a
common discharge header.  Flow  control valves regulate the  air flow to each
tank based on dissolved oxygen.  Density compensated RAS  flow is  ratioed  to
influent flow.

Secondary Clarifier

     Flow to multiple clarifiers is dynamically  divided by  a flow splitting
control scheme.  Unequal flow balance is available to compensate  for
hydraulic inequalities.  Sludge withdrawal is based on sludge blanket  level
and sludge density or timer overrides.

Chlorination

     Chlorine application is ratio controlled based on the  treatment plant
influent flow to the contact chamber influent with chlorine  residual feed-
back control.

RAS Pumping

     RAS pumping is ratio controlled based on the influent  flow to  the
treatment plants.  Multiple, variable speed return activated  sludge pumps
are controlled based on the flow demand in a common discharge header.
Sludge wet well level controls  are provided to override on  high and low
level.

WAS Pumping

     Multiple, variable speed waste activated sludge pumps  are flow ratio
controlled based on a manual flow control setpoint.  Sludge  wet well level
control overrides on high and low level.

Return Liquors Pumping

     Multiple, variable speed lift pumps are controlled by  the return
liquors wet well  level.  Pump speed ramps up or  down and  pumps are  sequenced
on or off in response to the variable load.
                                     215

-------
Gravity Thickening

     Flow to multiple gravity thickeners  is dynamically  divided  by  a  flow
splitting control scheme.  Unequal flow balance  is  available  to  compensate
for performance inequalities.  Underflow  withdrawal  is based  on  blanket
level and sludge density with timer overrides.   Dilution water is flow
ratioed to gravity thickener influent flow.

Anaerobic Digesters

     Primary and secondary digester feed  and withdrawal  are controlled by
timers and density sensors.  The digester temperature is controlled.
Digester gas flow and composition are related to the process  performance,
but are not incorporated into the process control.

Drying Bed Pumping

     Drying bed feed pumps transfer digested sludge  on a timed basis.

Sludge Holding

     Fill and withdrawal to holding tanks is controlled  by sequence logic
and high/low limit relays.

Vacuum Filtration

     Sludge is withdrawn from the holding tanks by variable speed pumps dis-
charging to a common header.  Pump speed  is a function of header pressure.
Sludge is transferred to the individual vacuum filters based  on  vat level.
Chemical addition (lime and ferric) is ratioed to the density compensated
sludge flow to each filter.

Incineration and Flue Gas Scrubbing

     Fuel and combustion air flows are based on  load and temperature.  The
secondary combustion air flow into the incinerator  is controlled by an
oxygen measurement in the flue gas.  Induced draft is controlled to maintain
a slight negative pressure in the incinerator.  Scrubber water,  chemical
addition and blow down is controlled by differential pressure and sequence
timers.

CONTROL SYSTEM CONFIGURATIONS

     For the purpose of a cost effective  analysis demonstration, three
commonly used control system configurations are applied  to five  activated
sludge process wastewater treatment plant sizes ranging  from 5 to 300 mgd
(220 to 13000 dm3/s).  Using terminology  defined in Section 4, the
configurations are:  1) conventional (distributed analog) control; 2)
central analog control; and 3) central digital control.  Figure  5-7
illustrates an example of the control panel configurations used  in the three
types of systems analyzed and defines three levels of equipment  locations
(field, local and central).

                                     216

-------
               CONVENTIONAL
              (DISTRIBUTED ANALOG)
CENTRAL ANALOG
CENTRAL DIGITAL
ro
                                              REMOTE SETPOINT
                                                STATIONS
                                                                                                         CENTRAL
                                                            LOCAL
                                                                                                          FIELD
                                    Figure 5-7.   Control system panel comparison.

-------
Field Mounted Equipment

     Field mounted equipment includes instrumentation, final control ele-
ments such as valves and pumps, motors, motor controls,  field  control
(maintenance) panels, and the associated wiring between  this equipment  and
the local control panel or multiplexer.  Instrumentation includes  items such
as flow meters, flow transmitters, pressure  transmitters,  level  transmitters
and analytical sensors (e.g. DO probes, etc.).  The  same field mounted
equipment is utilized regardless of the alternate  control  system employed.
Pneumatic instrumentation is not considered  in this  comparative  analysis
since it is not normally used with digital control.

     Primary elements such as flow tubes are installed for measurement  of
the desired variables.  The accompanying transmitters are  mounted  as near as
possible to the primary devices and maintenance panel.   The transmitter
signals are wired to the local control panel  or multiplexer.   Signal condi-
tioning devices, primarily square root extractors  for flow signal  lineariza-
tion, are mounted in the local panel.

     Final element positioners such as motors, pneumatic or hydraulic
systems are used to control the variable final control element.  The pneu-
matic and hydraulic positioners require field conversion of the  modulating
control signals transmitted to these  devices.  A means of  manual operation
for each final element is provided at that element for maintenance and
emergency control purposes.

     Motors are controlled through motor control centers (MCC) which include
disconnects, starters and related circuits.   Manual  controls for the motors
are installed at the motor control center, at the  motor,  or at both loca-
tions to aid maintenance and startup  operations.   Related  control  circuits
consist of variable speed drives, start/stop functions and motor protection
equipment including vibration monitors, interlocks and sequence  logic.

     Field maintenance panels are utilized in all  three  alternate  control
systems.  For the smaller plants, the maintenance  equipment will be located
at the device.  For the larger plants, the maintenance controls  will be
located at panels having control for  a number of related pieces  of equipment.

Conventional Control System  (Distributed Analog)

     The conventional control system  is a  localized  manual/analog  system
common at small wastewater treatment  plants  or at  individual unit  processes
in large treatment plants.  The basis of this control  is to divide the
treatment plant  into major subsystems, generally along unit process bounda-
ries.  Divided into multiple operating centers in  this way, the  plant can be
staffed by operators experienced in selected areas.  Each  unit process  has
one or more control panels with all the necessary  controllers, recorders,
information and  status indicators, switches, etc.  required to  operate the
process.  The panel  is usually  located in  the vicinity of the  equipment to
be controlled at  smaller plants and in area  control  rooms  at larger plants.
Verbal  and written  communication between operators is  required for process


                                      218

-------
coordination  and maintenance.  The  communication  is  often inadequate which
causes diversified  and,  at  times, conflicting  process  decisions.

     The analog control  equipment includes  indicators  for process variables
and final control elements.  The controllers have  switches for selecting
automatic or manual mode, setpoint  adjustments, manual  final  element adjust-
ments plus bias, gain and rate adjustments.  Internally mounted components
include signal conditioners, arithmetic modules,  integrators,  timers,
sequencers and relays.

     Motor control  on these panels  is usually  limited  to  start/stop push-
buttons.  Most of the interlocking  relays and  the  magnetic starters are
housed i.n a separate motor control  center.  Rotating and  mechanical
equipment is monitored for operational status, alarm and  shutdown condi-
tions.  These conditions are indicated on the  panel by lights,  meters  and
annunciator panels.

     A minimal number of process parameters, status and alarm  signals  from
each control panel  are wired to a central monitor  panel.   The  information
displayed is limited to  a few critical process parameters,  an  alarm indica-
tor for each process, and alarm and status  indications  for selected equip-
ment.  The central monitor panel does not have any control  provisions.

Central Analog Control System

     A central analog system controls all operations from  one  central  loca-
tion.  In practice, this is only practical  at  small plants because much of
the local control panel  equipment consists  of  internally mounted  components
(relays, timers, etc.) which would  require  excessive wiring to centralize.
This is a popular approach in other industries where separation distances
are smaller but is a rare occurrence at wastewater treatment plants larger
than 50 mgd.  In general practice, most of  the information  and  control  capa-
bilities available from the panel mounted devices  on the  local  panel are
made available at the central location;  the local panel remains essentially
the%same as in a conventional control system.  The analog  controllers  are
located in the local panels and connections are made to remote  setpoint
stations in the central control panel.  (As an alternative, the controlling
equipment may be located in the central  control room and the local  panels
used for backup operation only.)  Additional controllers  and panel  instru-
mentation for cascade, feedforward and closed-loop interprocess control  can
be incorporated.  Some switches, alarm annunciators, panel  lights  and  meters
are duplicated on both local and central panels.   Most  of  the  process
recorders remain in the field since much of the recorded  data  is  necessary
only for upset and process failure analysis.  The  local mounting  of
recorders reduces the complexity of the central analog  panel.

     A subtle yet important difference exists between the  conventional  and
central analog control approaches.  The large  increase  in  duplicate instru-
mentation and control is evident in central  analog control, but the advan-
tages of centralizing plant control must be considered.  Under  a
conventional control approach,  process decisions  are made  by local  operators


                                     219

-------
distributed throughout the plant.  The decisions made by these operators  are
based on the information available to each individual operator.  The
operators have limited communication with each other which may lead to
situations where conflicting process control decisions may be made.  The
centralized control approach improves information availability so that  a
operations supervisor can coordinate actions that may affect several
processes.  Process data from the entire plant is available to the plant
engineer at a central location and allows all control decisions  to be made
and implemented from the plant central control room.

Central Digital Control System

     Central digital process control can be implemented using centralized
hardware or distributed hardware, depending upon the location and number  of
computers.  In either configuration, operators are  stationed at  a central
location and their interface with the control system is basically the same.
Digital control using distributed hardware will not be considered in this
analysis, however.  The computer control system configuration chosen for  the
various plant sizes are shown in Figure 5-8.

     The digital control systems in this analysis make use of multiplexers
to reduce the wiring required to the central location.  The cost of a multi-
plexer can be offset and possible additional savings realized due to the
elimination of the voluminous amount of interconnecting wiring that would
otherwise be required.  Multiplexers are equipped with circuitry for inter-
facing the central computer to the field equipment.  Analog and  discrete
inputs from field equipment are converted to digital signals and are trans-
mitted to the computer.  Similarly, digital signals from the computer
representing modulating outputs or discrete control outputs are  received  by
the multiplexer and are converted from digital form to a form acceptable  to
the field devices.  The four input/output classifications  are as follows:

     Analog Input (AI)—Analog inputs consist of signals wired to the multi-
plexer representing process variables such as level, flow  or pressure.  The
input signals could be in the form of variable current, voltage, resistance,
pulse rate or pulse width.

     Modulating Output (MO)—Modulating outputs  include positional type
analog outputs  and  incremental (velocity type) outputs.  In either case they
are used  to control the operation of devices such as control valves or
variable  speed  pumps.

     Digital Input  (PI)—Digital inputs are in the  form of an open or closed
electrical contact wired to the multiplexer.  The inputs represent status,
device position or  alarm conditions.

     Control Outputs  (CO)—Control outputs are used for operating two-state
control  devices such  as motors (start/stop control) or valves (full open/
close control).  These outputs are in the form of relay contact  outputs from
the multiplexer.
                                      220

-------
a)   Hardware configuration  - 5 mgd
                              CONSOLE
                       BLACK AND
                       WHITE CRT
                        KEYBOARD
                                MUX
MINI COMPUTER
16 BIT WORD
(CORE ONLY)
   MUX
 INTERFACE
b)   Hardware configuration  - 10 to  300 mgd
                                       MINICOMPUTER
                                        16 BIT WORD
                                MUX


/ COLOR
I CRT J

KEYBOARD





















' COLOR
i CRT ,

KEYBOARD









/
/


                                           MUX
                                        INTERFACE

/


PRINTER
120 CHARS/SEC
^- 	 -
  QTY.  N
                                                                    PLANT
                                                                    SIZE
                                                                    (MGD)
                                                     MUX
 10
 50
100
300
                                    QTY. OF
                                    MUX' S
                                      (N)
 7
 8
11
14
          Figure'5-8.   General central digital system configuration,
                                           221

-------
     Locally mounted multiplexers can accommodate  large quantities  of  the
above type signals.  Communication with the central  computer  is  via a  high
speed data channel which requires only a few wires.

     Analog controllers and other instrumentation  found in  analog control
panels are replaced by computer control programming  (software).  All modula-
ting outputs from the computer are interfaced to field equipment through
manual loading stations.

     In place of the large central panels used  in  conventional and  central
analog control, one or more cathode ray tube (CRT) monitors and  keyboards
are used.  The CRT displays are generated by the computer and can include
dynamic process graphic diagrams as well as simulations of  controllers,
indicators, annunciators, recorders or any other panel mounted device.
Information pertinent to any subprocess, process or  group of  processes  can
be displayed on demand.  The operator can manipulate the process via the
keyboard as if a portion of a central analog control panel  were  before  him.
The digital control system retains all the advantages of central analog
process automation without incurring the costs  of  a  large hardware  installa-
tion.  A digital control system also offers additional features  and advan-
tages which were discussed in Section 4.  It should  be emphasized that  for
the purpose of an equal comparison, only those  control strategies performed
by the conventional and central analog alternate control- systems for each of
the individual processes are implemented by the digital control  system  to
closely model the analog control approach.

     The 5 mgd (220 dm3/s) plant configuration  includes a small  mini-
computer with core resident software.  Report generation includes event and
alarm logging, daily reports and monthly summaries.   Process  optimization
and Management Information System (MIS) functions  are not included.

     The 10, 50, 100 and 300 (440, 2200, 4400 and  13000 dm3/s) plants
incorporate disk memory based operating systems.   The disk  system allows
additional reporting functions including maintenance scheduling, trending,
greater historical data storage and recovery.   Dynamic process graphic
diagrams are available for display on a color CRT  operator's  console.

     A low speed logger is provided for the 5 mgd  plant.  These  loggers
typically print at a rate of 30 to 130 characters  per second.  A high  speed
printer is provided for the larger plants to accommodate the  larger report
volume requirements.  These printers typically  print at rates of 400 to 600
characters per second.
                                      222

-------
ASSUMPTIONS USED  IN EQUIPMENT & WIRING COST COMPARISON

     To compare the three control alternatives  on  an  equal  basis,  the
following assumptions have been made:

Field Mounted Sensors and Control Elements

     These equipment items are utilized regardless of the control  system
chosen because the same variables must be sensed and  controlled.
Consequently capital and recurring cost estimates  for these devices  are  not
included in this  analysis.

Maintenance Panels

     Periodic inspection, calibration and maintenance will be  necessary  for
all components of the process control system  including sensors  and control
devices.  Operators and technicians should have a  means of manual  control
from a local maintenance panel.  For the smaller plants, these  panels  are
adjacent to each device.  For the larger plants, these panels  are  in conve-
nient areas grouping a number of devices on a single maintenance panel.
These maintenance panels also serve as a backup control panel  in the event
that a failure disables any portion of the control system.  Cost and reli-
ability considerations dictate that these maintenance/backup panels be as
simple and straightforward as possible.  The panel should, however, contain
sufficient equipment to perform selected critical  tasks.  Since these panels
are common to all three alternative control systems, they are  not  included
in the cost estimates.

Level of Control

     The three alternate control systems are configured as nearly  identical
in control executions as possible.  Conventional control requires  operators
to implement the strategies.   Analog and digital automation have the poten-
tial of reducing operational  labor,  but require higher capital  investment
and cost more to maintain.  The computer control system detailed in this
cost comparison is not truly representative of a typical installation
because it simply emulates an analog control system.  Because  it is diffi-
cult to quantify the capabilities and flexibilities of a computer  based
system, in most cases they are ignored.  In cases where they could be quan-
tified, the savings are shown in the material and energy savings analysis.

EQUIPMENT COSTS

     Table A-l in the Appendix of this section lists the material  cost,
installation cost and total unit cost for the analog instrumentation and
equipment used in the conventional and centralized analog control
approaches.   Cost estimates represent second quarter 1978 averages.  It  has
been assumed that the control panels are preassembled, wired and tested  at
the factory, and that external wiring connections are made on screw type
terminal strips.
                                     223

-------
     The control, equipment quantities  and costs for  conventional  analog
control in 5 mgd to 300 mgd plants are summarized  in Appendix Tables A-2
through A-6.  These schedules indicate the  instrumentation  and  control
requirements, excluding field mounted equipment, for each unit  process
within the example treatment plants.

     Similarly, quantities and costs for central analog  control are
summarized in the Appendix in Tables A-7 through A-ll.

     Each unit process to be controlled by  a central digital control system
is evaluated in terms of the input/ output  signals required for monitoring
and control.  The computer input/ output (I/O) for each  plant size is
summarized in the Appendix in Tables A-12 through A-16.  Equipment costs
listed in the Appendix in Tables A-17 through A-21 for the  five digital
computer control systems are based on past  experience, manufacturing costs
and the I/O point counts.  As stated earlier, the costs  are based on a
digital computer system emulation of an analog control system.  For this
reason, the total cost estimates, particularly the memory size  and software
man-hour estimates, are not as high as would be expected if complex advanced
algorithms and management information processing were involved.  This is
particularly true for plant sizes 50, 100 and 300 mgd (220, 440 and 13000
dm3/s).

     The control equipment costs for each alternative control system are
plotted as a function of plant size in Figure 5-9.   The  control costs do not
include the field mounted sensors, final control elements and the
maintenance panels because they are common  to each alternative  control
system.  As a check on the reasonability of these control costs, the total
estimated capital costs for the five plant  sizes were calculated using the
EPA executive costing program.(6)  Three percentages (.1%,  1%,  and 10%) of
the total plant costs were calculated  and plotted  in Figure 5-9 for
comparison.

     Control wiring, which may add a significant amount  to  the  installed
cost of the control equipment, is treated separately in  the next section and
is not included in the above comparison.

CONTROL WIRING COSTS

     The cost of control wiring is individually analyzed for the three
alternative control systems and the five plant sizes.  The  analysis starts
with calculation of the cost per pair foot  of discrete,  analog  and digital
wiring.

     Wiring cost calculations are based on  Building  Cost Data 1977 (7).  The
costs per pair foot are derived by summing  material, installation, and over-
head and profit (O&P) costs.  Termination costs are  $1.76 per pair.  Three
types of wiring are considered:
                                      224

-------
o
Q

12
CO
o
o
a.
4
O
    I08
    I07
I05
   I04  —
      8
      6

      4
   I03
      2 —
      2 —
                               1.0% OF PLANT
                               CAPITAL COSTS
                                                  10% OF  PLANT
                                                  CAPITAL  COSTS
                                              0.1% OF PLANT
                                              CAPITAL COSTS
                             CONTROL COSTS IN THIS ANALYSIS
                             ------   CONVENTIONAL
                             —— — —   CENTRAL ANALOG
                             ————   CENTRAL DIGITAL
                             FIELD MOUNTED EQUIPMENT,
                             MAINTENANCE PANELS 8 WIRING
                             NOT  INCLUDED.

                             I   I  I  I  Mill      I   I   I  I I  I
                          10                 100
                           PLANT CAPACITY (MOD)
                                                            1000
           Figure 5-9.   Alternative control system costs.
                                 225

-------
Discrete Signal Wiring

     This is typically wiring from output relays to field  control  relays,
magnetic starters and solenoids  or wiring from  status  and  alarm  contacts
from field equipment.  Two #12 AWG stranded  conductors  constitute  one  pair.
Seven pair are installed per 1%" rigid conduit.
Discrete

(2) #12
7 Pr./l% Conduit
 Material   Installation

 2(.039)   +  2(.063)
1/7(1.35)  + 1/7(2.00)
                             0 & P           Total

                         +  2L0285)  =  .2610 $/Pr. Ft.
                         +  l/7(.95)  =  .6143 $/Pr. Ft.

                                         .8753 $/Pr. Ft.
Analog Signal Wiring
     This wiring  is typically for 4-20 ma  or  1-5  volt  signals.   Two  #14  AWG
stranded conductors with  an overall  plastic jacket  constitutes  one pair.
Seven pair are  installed  per 1%" rigid conduit.
Analog

#14 Pr
7 Pr./lh Conduit
 Material   Installation     0 & P

  .2100    +    .09      +    .05
1/7(1.35)  + 1/7(2.00)
                                             Total

                                         .3500 $/Pr. Ft.
                         +  l/7(.95)  =  .6143 $/Pr. Ft.

                                         .9643 $/Pr. Ft.
     A  single  composite  cost  for  the  discrete  and  analog  signal  wiring  was
developed to simplify  the  analysis.   A  distribution  of M%  discrete  wiring
and 53%  analog wiring  was  chosen.  This yields a composite  pair  cost of
$0.92/Pr. Ft.

Digital  Signal Wiring

     High speed,  low level  data link  is used to connect the multiplexers  and
the computer.  Two #14 AWG stranded conductors with  an overall plastic
jacket  constitutes one pair.  Two pair  are  installed per  3/4" rigid  conduit.
Digital
 Material   Installation
#14  Pr.               .2100
2 Pr./3/4  Conduit   1/2(.62)
           +    .09
           + 1/2(1.58)
                             0 & P           Total

                         +    .05     =  .3500 $/Pr. Ft.
                         +  l/2(.65)  = 1.4250 $/Pr. Ft.

                                        i 7?c;n $/pr. Ft.
                                      226

-------
     Wiring requirements  are  developed  from  the  schedules  of equipment
mounted in the centralized monitor panel or  the  central  control  room for the
conventional and central  analog control systems  respectively.  The wiring
for field mounted equipment that  is  independent  of the choice  of the control
configuration (level transmitters, flow transmitters, etc.)  is not included
in the wire count.  The wiring analysis for  a digital control  system
requires a schedule of multiplexer locations to  determine  the  pair
requirements.

     The wiring distances are developed from a model of  each treatment
plant.  The process centers maintain the same relative position  on a square
plant site.  The area of  the  plant site varies in a simple relationship to
capacity.  The land area  (exclusive  of  expansion space)  per  mgd  capcity was
estimated from several actual plants.   An average value  of 50,000  square
feet for each mgd was derived.

     Comparative wiring costs for the three  alternate control  systems  in
each plant size are shown in the Appendix in Tables A-22 through A-26  for
conventional control.  Tables A-27 through A-31  are for  central  analog and
Tables A-32 through A-36  are for digital control.  The digital multiplexer
assignments are shown in Table A-37.

     Control equipment costs developed previously and wiring costs  for the
alternate control systems are summarized in Table 5-1 for  comparison
purposes.  It should be noted that these are initial or  nonrecurrent costs.
Wiring costs for conventional control are minimal because  very little
information is brought back to a central location as discussed earlier.
Wiring costs for central  analog control are high due to  the  fact that  more
information and control signal wires are routed to the central location.
Substantial savings in wiring costs  are achieved in digital  systems  because
wiring is reduced by the use of multiplexers.  These savings are partially
reduced by the cost of the multiplexers.

MANPOWER ANALYSIS

     The primary objective of process centralization and automation  is  to
reduce the dependency on operational  labor.   The decreased operational
dependency is attained by adopting an "operation by exception" philosophy
where events out of the ordinary are detected by the control system  and
annunciated to an operator for action,  thus reducing or eliminating  labor
required for routine monitoring of equipment and manual  data logging.

     The potential annual cost savings  due to centralization are signifi-
cant.  The cost of labor for operations and maintenance of a typical waste-
water treatment facility normally approaches one-half of the total operating
budget.   If a capital investment for centralized control is  annualized over
twenty years at 7% interest and compared to  the annual  salary of an  operator
($15,000 per year), it can be shown that over $150,000 in capital cost can
be amortized and still  not equal  the  annual  cost of one operator.
                                     227

-------
                       TABLE 5-1    CONTROL EQUIPMENT AND WIRING CAPITAL COST SUMMARY
Plant Capacity - MGD (dm3/s)
5 10 50 100 300
(220) (440) (2200) (4400) (13000)
CONVENTIONAL
*
Equipment
Wiring*
Total
CENTRAL ANALOG
ro *
ro Equipment
oo

$85,612
10,021
$95,633

$113,585

$130,583
24,320
$154,903

$170,602

$189,100
69,606
$258,706

$244,116

$297,942
164,058
$462,000

$ 389,269

$ 532,266
574,749
$1,107,015

$ 688,470
*  Does not include field mounted sensors, final  control  elements and maintenance panels
+  Does not include wiring between field equipment and local  control  panels or multiplexers

-------
     The  analysis  compares  the  operational  labor requirements of conven-
tional  (distributed  analog)  control  systems  with two  centralized
systems—analog  and  digital.  Maintenance requirements  of the two
centralized  alternatives  are  then  analyzed  to  determine the  impact of added
instrumentation  associated  with centralization.   In this way,  a comparison
can be  made  in terms  of total labor  cost for operation  and maintenance.

     Data for estimating  manpower  requirements at  conventional  treatment
plants  has been  collected by  Patterson and Banker  (8) and  Burke (9).   The
data provides a  means for estimating staffing  requirements for  a variety of
conventional wastewater treatment  plants on  an average  basis.   Since  the
data is not  adjusted for  local  conditions or changing design  requirements,
it cannot be considered applicable for specific  treatment  plant estimates.
The United States  Environmental  Protection Agency  (USEPA)  has  analyzed this
data and has developed a  staffing  model.  The  model is  incorporated in a
computer program which accepts  definitions of  unit processes  and plant sizes
and calculates man-hours  of operational labor  and  maintenance  labor for  each
associated unit  process.  The purpose is to  provide average estimates  for
proper  plant staffing for planning studies assuming a conventional control
system  approach.   Manpower  in an actual conventionally  controlled  plant  may
be more or less  than these estimates.  However, more staff may  mean
overstaffing and a higher potential  for manpower savings and  less  staff  may
be an indication of improper operations.  The  EPA  program  forms  the basis
for conventional control  operation and maintenance staffing estimates  in
this analysis.

Operational Manpower Requirements

     The estimated operational manpower requirements for the five  plant
sizes are shown  in Table  5-2.  This  data is  based  on the conventional
control systems  analyzed  by Patterson and Banker and from  the USEPA pro-
gram.  These estimated man-hours are for operations only and do  not include
administrative time, laboratory time or manual labor time.  The  operations
man-hour estimates are then divided  by 1,800 man-hours  per year  per person
to account for weekends,  holidays  and sick leave.  At 1,800 man-hours  per
year, 4.9 people are required (8760/1800) for  each position that must  be
manned on a 24-hour basis seven days a week.  Although  there were many
staffing options observed in the field,  this analysis assumes that five
operational groups are required to operate a facility.

     To demonstrate potential operational labor reductions that  may be
achieved by centralization,  the tasks associated with three operator posi-
tions will be evaluated.  The task list examples provided  in Tables 5-3, 5-4
and 5-5 are for  secondary treatment  and were obtained during the field
visits.  The approximate  completion  times for  each task  are estimated  for
centralized control based on the fact that the control  system frees the
operator from much of the routine  equipment monitoring  and manual  data
logging that would otherwise be required.  The tasks and times were provided
by the Metropolitan Waste Control Commission during field visits.
                                     229

-------
                           TABLE 5-2.  OPERATIONS ANNUAL MANHOURS-CONVENTIONAL CONTROL
C*>
O


Influent
Pumping
Preliminary
Treatment
Primary Treat-
ment & Sludge
Aeration
IS
Secondary
Clarification
Chlorination
RAS
WAS
Gravity
Thickening
Digestion
Sludge Holding
Vacuum Filters
Drying Beds
Incineration
Reaeration
Miscellaneous
Total Manhours
Per Year
People Required
(1800 MH » 1 Han)
5 MGD
(220 dm3/s)
500
1,400
1,600
2,000
850
1,000
300
300
350
1,500
0
0
750
0
0
400
10,950
6
10 MGD
(440 dm3/s)
700
2,100
2,450
3,000
1,300
1,500
500
500
500
2,000
700
3,000
0
2,500
0
800
21,550
12
50 MGD
(2200 dm3/s)
2,000
5,000
5,000
6,500
3,300
3,500
1,800
850
1,200
10,000
1,700
10,000
0
7,500
0
800
59,150
33
100 MGD
(4400 dm3/s)
3,000
15,000
8,000
10,000
5,200
5,000
2,500
1,000
1,600
14,000
2,100
14 ,000
0
11,000
0
800
93,200
52
300 MGD
(1300 dm3/s)
5,000
30,000
16,000
20,000
10,000
10,000
5,000
2,000
3,200
28,600
4,200
28,000
0
22,000
0
900
184,900
102

-------
TABLE 5-3.  PROCESS OPERATOR TASKS

Task
Check compressor equipment
(motor, bearing, vibration)
Foam & solids carryover check
Clarifier drives and other
equipment checks
Pump and motor check (bearing,
temperature, vibration)
Valve, alarm & status change logging
RAS withdrawal control
(4 times per shift)
RAS distribution control
Air flow control
WAS control
Miscellaneous
Total Time
TABLE 5-4. OPERATING
Estimated Time to Complete
Conventional
Control
30
30
30
60
60
60
60
60
60
30
480
ATTENDANT II TASKS
(min/shift)
Centralized
Control
0
30
0
0
30
60
60
30
30
30
300


Task
Tank stickings collector operation
Outside cleaning (shovel)
Device checks
D.O. logging
Air supply logging
Mixed liquor flow logging
Centrifuge tests, settling tests,
logging
Waste & return flow checking & logging
Miscellaneous (device checks, alarms,
operator communications)
Total Time
Estimated Time to Complete
Conventional
Control
120
30
60
30
30
45
60
15
45
480
(min/shift)
Centralized
Control
120
30
0
0
0
0
60
0
45
255
               231

-------
                  TABLE  5-5.   OPERATING ATTENDANT I TASKS
                                       Estimated Time to Complete (min/shift)
Task
Settling tests (30 min.)
Centrifuge tests
Sampler line flushing
Sink Cleaning
D.O. probe flushing
Composite sample pickup and
distribution
Pump cleaning
Re! amp ing (average)
Floor and drain cleaning
Total Time
Conventional
Control
120
60
60
30
60
30
60
30
60
480
Centralized
Control
120
60
60
30
60
30
60
30
60
480
     Table 5-3 indicates that  centralization  can  reduce  the  time required
for typical operational functions performed by process operators  by  about
37%.  Table 5-4 indicates that typical operating  attendant II control  func-
tions and operating time are reduced by approximately 46%.   Table 5-5  shows
that the operational tasks for a typical operating  attendant I will  not  be
reduced or affected by centralization.  Checking  of devices  (preventive
maintenance), sampling, testing and cleaning  all  continue to be  necessary.
Centralization saves operational man-hours predominantly in  the  process
operator and operating attendant II categories.   Operation by exception,
made possible by the use of centralized control,  reduces the total number of
people needed to make control  decisions and log data.

     The average man-hour savings are greater than  25% in terms  of actual
operational functions and time in the examples cited.  A simple  average
reduction  in operational labor cannot be assigned on an  arbitrary basis
across the board.  The actual  number of operators assigned to a  facility
must be based on an analysis of the unit processes, equipment, monitoring
and control point quantities and other complexities.   In an  attempt  to esti-
mate potential operator staff  reductions for  centralized control, conven-
tional control staffing for the example treatment plants will first  be
reviewed.  The analysis will then briefly  describe  how the same  facilities
could be staffed with centralized,  automated  systems.  All of the data is
presented  in Table 5-6.  The process assignments  are  in  general  accordance
with actual assignments observed  at the plants  visited.
                                      232

-------
                                 TABLE 5-6.  OPERATIONAL MANPOWER ASSIGNMENTS

PLANT SIZE
•i
HGD - (dm7s)

5
(220)



10
(440)

50
(2200)



100
(4400)





300
(13000)







CONVENTIONAL CONTROL

OPERATOR
STATION
PLANT OPERATOR
OPERATING ATTENDANT



OPERATIONS SUPVR./
LIQUID TRAIN
SOLIDS TRAIN
DAY SHIFT ATTENDANTS
OPERATIONS SUPVR.
PRELIM/PRIM/SEC
DIGESTION/DEWATERING
INCINERATION
DAY SHIFT ATTENDANTS
OPERATIONS SUPVR.
PRELIMINARY/PRIMARY
BLOWERS
SECONDARY
DIGESTION/DEWATERING
INCINERATION
DAY SHIFT ATTENDANTS
OPERATIONS SUPVR.
PRELIMINARY
PRIMARY
BLOWERS
SECONDARY
DIGESTION
VACUUM FILTRATION
INCINERATION
DAY SHIFT ATTENDANTS
TOTAL TOTAL
MANPOWER MANPOWER OPERATIONAL
ASSIGNMENTS (DAY SHIFT) STAFF COMMENTS
1 - days "\ 6 Day shift only.
5 groups of 1 / 2 An shifts.



5 groups of 1 ~| 12 Secondary operator
1 serves as group
5 groups of 1 " supervisor.
2 J
S groups of 1
5 groups of 1
5 groups of 2
5 groups of 2
3
33 Three field
operator stations.
9


5 groups of 1 ^1 52 Five field
5 groups of 1
5 groups of 1
5 groups of 1
5 groups of 3
5 groups of 2
7
5 groups of 1
5 groups of 2
5 groups of 2
5 groups of 2
5 groups of 2
5 groups of 3
5 groups of 3
5 groups of 4
7 j
operator stations.

• 16



102 Sevan field
operator stations.


• 26




CENTRAL/AUTOMATED CONTROL

OPERATOR
STATION
PLANT OPERATOR
DAY SHIFT ATTENDANTS



OPERATIONS SUPVR.
OPERATING ATTENDANTS

OPERATIONS SUPVR.
LIQUID TRAIN
SOLIDS TRAIN
DAY SHIFT ATTENDANTS

OPERATIONS SUPVR.
LIQUID TRAIN
SOLIDS TRAIN
DAY SHIFT ATTENDANTS



OPERATIONS SURVR.
LIQUID TRAIN
SOLIDS TRAIN
BLOWERS
RAS/WAS
DIGESTION
VACUUM FILTRATION
INCINERATION
DAY SHIFT ATTENDANTS
TOTAL TOTAL
MANPOWER MANPOWER OPERATIONAL
ASSIGNMENTS (DAY SHIFT) STAFF COMMENTS
1 - days \ , 4 Only day shift
f * attended. Telemetry
3 - d»ys J provides alarms.
control system
records events on
off-hours.
5 groups of 1 \ 2 10 Control room
5 groups of 1 J Field
V

5 groups of 1 ^i 24 Control room
5 groups of 1 I 8 Field
5 groups of 2 Field
4 J Field

5 groups of 1
5 groups of 2
5 groups of 3
10
40 Control room
., Control room/field
ID
Control room/field




5 groups of 1 '
5 groups of 1
5 groups of 1
5 groups of 1
5 groups of 1
5 groups of 2
5 groups of 3
5 groups of 3
10 j
75 Control room
Control room/field
Control room/field
Field
91
23 Field
Field
Field
Field

CO
CO

-------
5.0 MGD (220 dm3/s) Plant—
     A conventionally controlled plant  is typically  supported  by either  one
full time operator attendant for each shift  and  a  plant  operator for  days
only or two shifts of three operators with one of  the  operating  people on
the day shift acting as plant supervisor.  The total operational  staffing
requirement is six.  If this plant were centralized, the plant would  be
manned only on the day shift (week days) by  a plant  operator and three
attendants.  A staff of four would be required and two operator  positions
could be saved.

10.0 MGD (440 dm3/s) Plant—
     With conventional control, this plant requires  full  time  operation  of
the liquid train by the operations supervisor and  one  full  time  operator in
the solids train.  Two additional day shift  support  attendants are  typically
provided for daylight operational support.   Total  operational  staffing
requirements are twelve men.  With a centralized system,  an operations
supervi- sor and one operating  attendant would operate the  plant.   This
would require a total staff of  ten, thus saving  two  operator positions.

50.0 MGD (2200 dm3/s) Plant—
     A conventional control system for  this  size plant requires  a full time
operations supervisor located in the control room.   There are  typically
three area control centers in the plant:  preliminary/primary/secondary,
digest!on/dewatering and  incineration.  One  operator per shift controls
preliminary, primary and  secondary.  There are typically two operators in
the digestion/dewatering  area and two operators  in the incineration area.
Three day shift operating attendants are required  to support the operational
staff.

     A centralized system would require a shift  supervisor  and three  opera-
tors per shift.  In addition, four day  shift support attendants  would be
required.  Nine operating positions would be saved.

100 MGD (4400 dm3/s) Plant—
     A conventional control system for  this  plant  requires  an  operations
supervisor in the control room  and full time operators at the  five  control
centers.  The control centers are preliminary/primary  treatment,  blowers,
secondary, digesters/dewatering and incineration.  Each  area is  manned by
one operator on each shift with the exception of digestion/dewatering which
requires three operators  per shift and  incineration, requiring two  operators
per shift.  Seven day shift operating attendants are needed to support the
operational staff.

     Centralization of this plant would require  an operations  supervisor and
two assistant operators (liquid and solids train supervisors)  in the  control
room.  The two assistant  operators would work in the field  part  of  the
time.  The liquid train operator supervises  one  attendant.  The  solids train
operator supervises two operating attendants.  Ten additional  operating
attendants support the operating staff  on the day  shift.  Centralization
would reduce the operational staff requirements  by twelve positions.
                                      234

-------
300 MGD (13000 dm3/s) Plant-
     A conventional control system for this size plant requires  an  opera-
tions supervisor on each shift.  There are seven operator  stations,  each
manned twenty-four hours per day.  Each of these stations  requires  more  than
one person on each shift because of the quantities of equipment.  The  sec-
ondary process requires two people per shift as would the  blowers,  prelimi-
nary, and primary treatment areas.  Dewatering and digestion each require
three operators per shift, and incineration requires four  operators  per
shift.  In addition the day shift would be staffed with at least seven
support operating attendants.

     Centralization of this size plant would require a shift supervisor  and
two operators (liquid and solids train supervisors) in the main control
room.  The two operators would also work in the field part of the time.   In
addition, there are operators at each of five stations in the field.
Blowers are manned by a single operator per shift.   The RAS/WAS system would
be manned by a single operator per shift.  The digesters would typically
require two operators per shift.   The vacuum filters and incinerators would
each require three operating attendants per shift.   In addition, the day
shift would have a support staff of at least ten operating attendants for
cleaning and preventive maintenance.  Centralization would reduce the opera-
tional staffing requirements by twenty-seven positions.

Summary of Operational Staffing—
   	Table 5-7 below illustrates  that on the average,  centralization can
reduce the total operational manpower requirements  by about 25%.



  TABLE 5-7.  OPERATOR STAFFING COMPARISON-CONVENTIONAL vs CENTRALIZED CONTROL

Plant Size
mgd (dm3/s)
5
10
50
100
300
(220)
(440)
(2200)
(4400)
(13000)
Conventional
System Staffing
6
12
33
52
102
Centralized
System Staffing
4
10
24
40
75
                                     235

-------
Control System Maintenance Manpower

     This analysis will determine the amount of additional maintenance  that
will be required for the centralized control systems in comparison  to
conventional control.  It will be shown that for the example configurations
chosen, the additional maintenance requirements for the central  analog
system are minimal and that there is no change in the total number  of main-
tenance people required for the central digital system.

     The analysis is based on the maintenance requirements associated with
the components used in the centralized control systems.  Table 5-8  shows  the
maintenance requirements for various control components as tabulated in a
recent EPA publication (10).
               TABLE 5-8.   MAINTENANCE REQUIREMENTS PER COMPONENT


          Component                                   Maintenance  (hours/year)

Annunciators                                                     2
PID Controllers, Ratio and Setpoint Stations                     8

Signal Converters                                                10

Recorders                                                        8

Control Switches                                                 2
     The central analog control system examined in this analysis requires  a
large number of additional items such as setpoint stations, switches  and
alarm annunciators that are not utilized in a conventional control system
(see Table 5-9).  Based on this increase in components, the additional man-
power required to maintain a central analog control system can be derived.
The component quantities are translated to maintenance manhours using Table
5-8 and the number of additional maintenance people required  is determined
by dividing total man-hours by 1800.  The results of this analysis show that
the increase in maintenance manpower is, at most, one additional person over
the requirements of conventional control.
                                     236

-------
      TABLE 5-9.   CENTRAL ANALOG MAINTENANCE REQUIREMENTS-COMPARISON WITH
                  CONVENTIONAL CONTROL

Componpnt

Setpoint
Station
Alarm
Annunciator
Control
Swi tch

Maint.
MH per
Mnif

8
2
2
Plant Capacity
5 10 50
'•>?n\ (44<^ (2200)
Qty. Mrs. Qty. Hrs. Qty. Mrs.
+21 +168 +32 +256 +47 +376
+10 « 20 +18 + 36 +29 *• 58
+20 + 40 +24 + 48 +27 + 54
MGD (dm3/«
100
(4400)
Qty. Hrs.
+77 +616
+49 + 98
+39 + 78

300
(13000)
Qty. Hrs.
+143 +1144
+ 96 +192
+ 46+92

 Increased
 Maintenance
 Manhours per
 Year
+228
+340
+488
+792
                                         +1428
 Additional
 People Required
 (1800 MH = 1  Man)
     The central digital  control  system examined in this analysis  requires
fewer components compared to  the  equivalent conventional control system (see
Table 5-10).  The major components  that are eliminated include recorders  and
PID controllers.  Other minor components such as signal conditioners,  total-
izers and timers are  also eliminated but these are not significant  and will
not be considered in  this analysis.   The decrease in components  is  used to
derive the reduction  in manpower  requirements in a manner similar  to  that
discussed under central analog control.

     A digital control system has additional maintenance requirements  asso-
ciated with the computer,  peripheral equipment and the multiplexers.   It  is
difficult to accurately estimate  the maintenance requirement for computer
equipment because an  exact component makeup cannot be determined.   Further-
more, generally applied and accepted maintenance values for the  various
components are not  available  at this time.   Experience has shown that  the
most significant maintenance  items  are the  multiplexer components  used for
input/output signals  at the local level. An estimate of one hour  per
input/output point  per year will  be  used to derive the maintenance
requirements for the  multiplexers and the computer system equipment.
                                      237

-------
     As indicated in Table  5-10,  the reduction in maintenance due  to  compo-
nent decrease is almost entirely  offset by the increased maintenance
requirements of the computer  system equipment; there is no significant
change in the maintenance manpower  requirements for the digital system  as
compared to conventional control.
       TABLE 5-10.  CENTRAL  DIGITAL  CONTROL MAINTENANCE REQUIREMENTS -
                    COMPARISON  WITH  CONVENTIONAL CONTROL

Component

Recorder
PIO
Controller
Multiplexer
I/O

Maint.
MH per
Unit

8
8
1/PT

5
(220)
Qty. Hrs.
-16 -128
-20 -160
+173
Plant Capacity
10 50
(440) (2200)
Qty. Hrs. Qty. Hrs.
-21 -168 -33 -264
-31 -248 -46 -368
+276 +361

M6D (dm3/s)
100
(4400)
Qty. Hrs.
-47 -376
-76 -608
+599


300
(13000)
Qty. Hrs.
-85 -680
-142 -1136
+1036

 Maintenance
 Manhours per
 Year Saved
115
140
271
385
780
 People Saved
 (1800 MH = 1 Man)
Summary of Operation  and  Maintenance Manpower Costs

     To estimate the  total  maintenance costs for the alternate control
systems, a base line  cost must  be established for conventional control.
Patterson and Banker's man-hour estimates for a conventionally controlled
treatment plant are listed  in Table 5-11 and are used to determine  the  base
line maintenance estimates. These numbers are translated to the  number of
people required through division by 1800 as explained earlier.  These
staffing requirements are then  adjusted for central analog  and central  digi-
tal control  according to  the results of the maintenance manpower  analysis.
                                      238

-------
                  TABLE  5-11.   MAINTENANCE MANPOWER-CONVENTIONAL  CONTROL


Influent
. Pumping
Preliminary
Treatment
Primary Treat-
ment & Sludge
Aeration/
Reaeration
Secondary
Clarification
Chlorinatlon
RAS
WAS
Gravity
Thickening
Digestion
Sludge Holding
Vacuum Filters
Drying Beds
Incineration
Total Manhours
Per Year
5 MGD
(220 dm3/s)
600
500
600
900
350
150
420
420
175
750
230
0
298
0
5,393
10 MGD
(440 dm3/s)
621
947
1,212
1,718
684
288
537
300
248
1,150
342
494
0
1,112
9,653
50 MGD
(2200 dm3/s)
1,500
3,000
2,800
3,300
1,500
1,500
800
500
450
4,500
700
1,300
0
2.800
24,650
100 MGD
(4400 dm3/s)
2,046
5,433
4,062
6,755
2,981
2,971
1,765
1,000
892
8,827
1,261
2,590
0
5.472
46,055
300 MGD
(13000 dm3/s)
2,046
10 ,000
8,000
14,000
6,000
6,000
3,500
2,000
1,700
17,000
2,500
5,000
0
10,000
87,746
People Required
(1800 MH = 1  Man)
14
26
49

-------
     An average labor cost of $15,000  has  been selected for both operations
and maintenance personnel based  on  a direct labor cost of $6.00 per hour and
an additional indirect  labor cost of 20%.   Since a portion of the staff at a
digitally controlled plant must  have specialized knowledge relating to the
control system and will typically require  higher than average pay, a labor
cost estimate of $15,500 was selected  for  the digital control systems.
Table 5-12 summarizes the operation and  maintenance requirements (staffing
and associated labor costs) for  the five plant sizes.
         TABLE 5-12.  ANNUAL OPERATIONS &  MAINTENANCE MANPOWER COSTS

5
(220)
Plant Capacity
10
(440)
MGD (*i3/s)
50
(2200)

100
(4400)

300
(13000)
                 Men  Cost
Men  Cost
Men  Cost
Men  Cost
                                                                   Men
                                           Cost
CONVENTIONAL
Operations
Maintenance
Total
CENTRAL ANALOG
Operations
Maintenance
Total
CENTRAL DIGITAL
Operations
Maintenance
Total

6 $ 90,000
3 45.000
$135,000

4 $ 60,000
4 60,000
$120,000

4 $ 62,000
3 46,500
$108,000

12 $180,000
5 75,000
$255 ,000

10 $150,000
6 90,000
$240,000

10 $155,000
5 77,500
$232,500

33 $495,000
14 210,000
$705,000

24 $360,000
15 225,000
$585,000

24 $372,000
14 217,000
$589,000

52 $780,000
26 390,000
$1,170,000

40 $600,000
27 405,000
$1,005,000

40 $620,000
26 403,000
$1,023,000

102 $1,530,000
49 735 ,000
$2,265,000

75 $1,125,000
50 750,000
$1 ,875 ,000

75 $1,162,500
49 759,000
$1,922,000
                                     240

-------
MATERIAL AND ENERGY COST

     Energy and chemical costs represent  approximately thirty percent of
wastewater treatment plants'  annual operating  cost  (8).   The  consumables of
most concern in a cost analysis are fuel, electrical  energy and  chemicals.
Most of the potential cost savings result from the  use of automated load
following control techniques.  Power demand control has  not been demon-
strated in this industry, but has shown large  savings in other industries.
Power factor correction can save on cost  of power,  but is not often utilized.

Load Following Savings

     Load following involves the definition of a process  model in  terms  of  a
mathematical formula.  This formula can be as  simple  as  a proportional
adjustment relative to the load (e.g. hydraulic load  following)  or  can be as
complicated as a logarithmic relationship describing  a pH titration curve.
Using the model, the required chemical feed is  calculated for actual  process
needs.  These feedforward control models  should be  accompanied by  feedback
to compensate for errors between the actual process load  and  the load
predicted from the mathematical model.  This combination  of feedforward  and
feedback control minimizes chemical costs and  optimizes  the process
operation.

     In Figure 5-10 (adapted from reference 10) the use  of two types  of  load
following is compared with constant rate  chemical feed to show how load
following can reduce the materials used for chemical  feed.

     The concept of load following, when  applied to dissolved oxygen  con-
trol, saves energy by minimizing aeration in a manner  similar to that shown
for chemical feed.  Feedforward control is based on organics  (mass  loading)
to be metabolized.  The mass of organics  is translated to a required  air
flow, and the measured dissolved oxygen provides feedback trim.  Dissolved
oxygen control  has been shown to yield a significant  reduction in energy use
when advanced control strategies are applied.

     Conventional control systems typically control chemical  feed on  an
open-loop basis.  This means that an operator  periodically sets  a  chemical
feed dose rate controller and that setting is  maintained.  Dissolved  oxygen
control is also conventionally implemented on  an open-loop basis.   The
operator increases or decreases air flow rates based  on  visual observations,
dissolved oxygen readings or laboratory data.  Accurate  load  following
cannot be achieved in these instances because  changes  in  loads are  usually
unpredictable.   Frequent manual adjustment would be required  to  compensate
for these changes.  The implementation of load following  generally  requires
feedforward and feedback controls that are typically  included in centralized
control systems.

     In estimating the average savings obtainable through the use of  load
following control, demonstrated experiences can be the only guide.   In a
study on dissolved- oxygen control of activated sludge  processes  (11),
involving twelve treatment plants, an analysis of the  percent improvement in
air supplied per unit quantity of BOD removed  indicates an average  air and

                                     241

-------
DOSE
        -CONSTANT SPEED
EXCESS  AMOUNT
         REQUIRED AMOUNT  4
DOSE
                    CONSTANT  FEED  RATE
        FLOW  PROPORTIONAL     ,— EXCESS AMOUNT
                    HYDRAULIC LOAD  FOLLOWING
DOSE
         REQUIRED AMOUNT
                     TOTAL LOAD  FOLLOWING
             Figure 5-10.  Chemical  feed control  methods,
                                                              TIME
         REQUIRED AMOUNT
                                                              TIME
                                                              TIME
                               242

-------
power savings of twenty percent.  This savings can be  realized  using
centrifugal blowers with adjustable inlet vane or suction throttling
constant pressure control.  The study analyzed plant sizes from 1 mgd  to  100
mgd.  Since blower power consumption in a treatment plant is at least  fifty
percent of the total plant power consumption, load following savings of ten
percent of total plant power consumption will be used for power cost
reduction due to dissolved oxygen control.

     An automated chlorine disinfection system can significantly reduce
chemical needs (12).  Load following chemical feed has been shown to reduce
both chemical usage and fuel consumption (13).  Chemical savings (chlorine)
greater than six percent have been demonstrated with use of simple  load
following via compound loop chlorination (14)(15).

     Cost for alum addition were calculated for a periodically  adjusted
constant feed rate, hydraulic load following and total (mass) load  following
at at a 10 mgd plant.  The hydraulic load following provided an alum savings
of 5% and the total load following provided an alum savings of 15%  over the
constant feed rate.  Similar cost comparisons have been made for methanol
addition for denitrification and for breakpoint chlorination.(16)   Since
smaller plants have a more widely varying diurnal load variation, the
anticipated savings might be greater (15% for alum for a 10 mgd plant) for  a
small plant then for a large plant.  However, equipment to follow these
diurnal variation (turndown ratios, multiple feeders) is more likely to be
found in larger plants then in small plants.  Consequently, for the  purposes
of this analysis, a 5% savings in chemical costs using centralized  analog
control will be applied to all plant sizes.  Since more complex process
models may be programmed in the digital computer to implement total  mass
load following, a 8% saving in chemical costs using centralized digital
control will be applied to all plant sizes.

Power Demand Control Savings

     Power company tariffs usually consider two factors in billing  rates:
amount of energy consumed and maximum power demand.  The basic charge  is for
energy used, whereas, the power demand charges can be viewed as penalties or
surcharges on the basic rate.  The power companies monitor power demand
during intervals of fifteen or thirty minutes.  The Cincinnati Gas  and
Electric Company (CG&E) defines power demand as "...the kilowatts derived
from the company's demand meter for the fifteen minute period of customer's
greatest use during the month" (17).  Power demand billing techniques  vary
throughout the country.  CG&E bill on a specified percentage (50%)  of  the
highest reading during a six month period.  Some other power companies bill
the power demand charges on the highest demand reading during each  month.
They may also be a "summer ratchet," where the demand reading during the
winter months cannot be less than a specified percentage of the peak summer
demand.  Therefore, there are significant economic justifications for  power
demand control.
                                     243

-------
     The total power load can be divided into two classes, base  loads  and
selected loads.  The base load is lighting, small appliances and major  loads
which are extremely critical to system operation.  Selected loads can  either
be reduced in power level or shed entirely for a short time as required  to
control demand peaks.

     The basic concept of power demand control involves shedding a block of
loads or preventing startups from occurring to prevent demand from exceeding
a setpoint; deferring consumption of that block of energy to a period when
the remainder of the electrical load is lower.  In this way, peaks are  low-
ered, but energy consumption is not reduced.  Figure 5-11 illustrates  this
concept.  A 500 KW reduction in the power demand peak is obtained by defer-
ring heavy energy use to a period of lower demand.  This amount of power
would otherwise be used to calculate a demand charge determined by the
monthly demand peak multiplied by a rate which can be as high as $1.50/KW/
month.  Therefore, keeping the power demand below the assigned peak demand
can result in significant savings.

     Power demand control cannot be implemented in a conventional or central
analog control system because the complexity of the necessary control  logic
requires the capabilities of a digital computer.  Industrial groups have
implemented power demand control techniques using computers.  Applications
have been reported (18, 19) presenting successful installations where power
costs were reduced by more than ten percent by reducing demand charges.
Numerous industrial installations can also be cited where savings like these
have been demonstrated.  As a result of these findings, a reduction of five
percent will be used as an estimate of the average power cost savings that
can be achieved through the implementation of power demand control with
digital systems.  The potential savings in a particular wastewater treatment
plant will depend upon the plant's operational flexibility (ability to defer
or shed electrical loads such as blowers without significantly affecting
overall plant performance).

Determining Plant Energy and Material Costs for Conventionally Controlled
Plants

     The material and energy cost estimates in 1971 for conventionally
controlled plants are based on data provided by the EPA.(20)  Material costs
were updated to June 1978 using a factor calculated from the wholesale price
index for industrial commodities.

          WPI(1978) _ 208.5
          WPI(1971
     Electrical energy costs were updated in a similar way to June 1978
based on the consumer price index for electricity.

          CPIE(1978)   209.6
          CPIE 1971
                                     244

-------
CJ1
         5000 ••
                 POWER
                 DEMAND
                   (KW)
ENERGY USAGE  WITHOUT
DEMAND  CONTROL
                                                             ENERGY USAGE DEFERRED
                                                             BY DEMAND CONTROL
                                             	/	DESIRED
                                                                                      TIME (HRS.)
                                    Figure 5-11.  Power demand control.

-------
     The base data and transitions are shown as follows:

                              Material
                                Cost         Correction          Material
                                1971           Factor            Cost 1978

  1 MGD (44 dm3/s)           $ 32,442           1.86            $   60,000
 10 MGD (440 dm3/s)           129,700           1.86               241,000
100 MGD (4400 dm3/s)          720,260           1.86             1,340,000


                               Energy
                                Cost         Correction           Energy
                                1971           Factor             Cost 1978

  1 MGD (44 dmS/s)           $ 10,530           1.91            $   20,000
 10 MGD (440 dm3/s)            62,300           1.91               119,000
100 MGD (4400 dm3/s)          498,260           1.91               952,000

     The updated data is plotted in Figure 5-12.  Cost data for 5, 50 and
300 mgd plant sizes are interpolated and extrapolated from a straight line
approximation and shown in Table 5-13.

Summary of Material and Energy Costs

     An energy and material cost analysis is important because these costs
represent almost one-third of the cost of operating a treatment plant.  With
consideration of the limited supply of conventional energy sources, conser-
vation becomes an environmental as well as economic concern.  Using the
percentage savings in material and power costs previously discussed, Table
5-13 indicates that significant savings can be gained from automation.
Conservative estimates of the savings to be expected have been indicated.
Greater savings than those indicated have been demonstrated in the field for
power and chemical feed at wastewater treatment plants.  Greater savings
than those estimated for power demand control have also been demonstrated in
the field for industrial installations.  The case histories referenced
illustrate that currently available technology can achieve these savings.
While the percentage savings selected for this demonstration cost analysis
are consistent with wastewater and industrial experience, a specific
analysis should be carried out for an actual process flow sheet taking into
account equipment design and operational flexibility.
                                     246

-------
CO
o:
o
Q


CO
I-
co
o
o
             I    I    I   III
I  I     III
III
                           10                  100


                          PLANT CAPACITY (MGD)



           Figure'5-12.  Material and energy costs,  June 1978,

                       for conventional  control.
                            1000
                                 247

-------
                 TABLE 5-13.   ANNUAL MATERIAL AND  ENERGY COSTS
                               Plant Capacity MGD  (dnr/s)
5 10 50 100 300
(220) (440) (2200) (4400) (13000)
CONVENTIONAL
Materials
Energy
CENTRAL ANALOG
Materials1
2
Energy
CENTRAL DIGITAL
Materials3
Energy2*4

$160,000
70,000

$152,000
63,000

$147,000
59,500

$260.000
130,000

$247,000
117,000
»
$239,000
110.500

$780,000
500.000

$741 ,000
450,000

$718.000
425.000

$1,300.000
900,000

$1,235,000
810,000

$1.196.000
765,000

$2,800,000
2,300,000

$2,660,000
2,070.000

$2,576,000
1,955,000
   1.  5% savings due to hydraulic load following control
   2.  10% savings due to DO load following control

   3.  B% savings due to mass load following control
   4.  5% savings due to power demand control
COST EFFECTIVE ANALYSIS SUMMARY

     The  economics of wastewater  treatment plant  automation have been
analyzed  in  terms of capital  investment in control  instrumentation,
operation and maintenance labor costs,  and material  and energy costs.   The
present worth of the equipment, labor,  materials  and energy costs  is
presented in Table 5-14.  The present worth method  conforms to the cost
effective analysis guidelines in  the Sewage Treatment Construction Grants
Manual (7-23-79),  using a 20 year planning period at seven percent annual
interest  with an useful life of 15 years for the  control equipment.  The
uniform series present worth factor is  10.954 for the annual operations and
maintenance  costs in Table 5-12 and the annual material and energy costs in
Table 5-13.   Only the control equipment, not the wiring, is assumed to  be
replaced  at  the original cost after the 15 years  of useful life.
                                       248

-------
                  TABLE  5-14.   PRESENT  WORTH ANALYSIS
                         (All  costs are in units of $1000)
                                       Plant Capacity MGD  (dm /s)
. „ 5 10 50 100 300
Present Worth1'2 (220) (440) (2200) (4400) (13000)
CONVENTIONAL
Capital3
Replacement
Operations
Maintenance
Material
Energy
Total
CENTRAL ANALOG
Capital3
Replacement '
Operations
Maintenance
Material
Energy
Total
CENTRAL DIGITAL
Capital3
4 5
Replacement '
Operations
Maintenance
Materials
Energy
Total

96
31
954
477
1,695
742
3,995
166
41
636
636
1,610
667
3,756
200
69
657
493
1,557
630
3,606
155
47
1,907
795
2,754
1.377
7,035
267
62
1,589
954
2,617
1.240
6,729
367
128
1,642
821
2,532
1,171
6,661
259
69
5,244
2,225
8,263
5.297
21,357
561
89
3,814
2,384
7,850
4.767
19,465
512
175
3,941
2,299
7,606
4,503
19,036
462
108
8,263
4,132
13,772
9.535
36,272
1,129
141
6,356
4,291
13,084
8.581
33,582
670
217
6,568
4,269
12,670
8,104
32,498
1,107
193
16,209
7,787
29,663
24.366
79,325
2,891
250
11,918
7,946
28,180
21 .930
73.115
914
277
12,316
8,047
27,290
20,711
69,555
1.  Present Worth  is  based upon a 20-year period at a 7% annual  rate of  return.
2.  Estimated cost June  1978.  In accordance with EPA methodology,  no escalation is assumed
    for annual  operating costs or replacement costs.
3.  Field mounted  sensors., final control elements, maintenance panels, and wiring between
    field equipment and  local control panels(or multiplexers) not included, because they are
    common to the  alternatives.
4.  Fifteen years  useful life for control equipment (excluding wiring)
5.  No salvage value  assumed for replacement control system after 20 years, because of high
    technical obsolescence and customized design.
6.  Does not include  administrative, laboratory or manual  labor  time.
                                       249

-------
     According to the guideline, the component costs should  be  calculted  on
the basis of the market prices prevailing at the. time of the cost-effective
analysis.  A single payment present worth factor of .3624  is used  to  obtain
the present worth of the replacement equipment.  The actual  future
replacement decision will depend upon many factors, such as  failure rates,
availability of spare parts and maintenance costs.

     The present worth analysis shows that central analog  control  is  cost
effective compared to conventional control and that centralized digital
control is cost effective compared to centralized analog control and
conventional (distributed analog) control for the five plant sizes.   While
the economic benefits of automation for these generalized  treatment schemes
have been illustrated through this cost-effective analysis,  the cost
information in this section should not be used for application  to  other
specific analysis.  Actual processes, process equipment, plant  influent,
staffing, electrical contracts and other local conditions  must  be  fully
considered in the cost-effective analysis.

     Table 5-15 is included to show the relative cost differentials on an
annual cost basis to provide a different viewpoint on the  economics of
centralization.  A capital recovery factor of 0.944 (7%, 20  years) was
applied to the present worth figures in Table 5-14 to obtain the annual
costs.  The annual costs for centralized analog control and  centralized
digital control were then subtracted from the annual costs for conventional
control to obtain Table 5-15.

     Automation has the potential of achieving many desirable objectives;
the most important of these is a reduction in operational  labor, chemicals
and power costs.  As operational labor is reduced, more effort can be
concentrated on maintenance.  The establishment of comprehensive preventive
maintenance programs will greatly aid in achieving continuous effective use
of instrumentation and control equipment and assure more consistent effluent
quality from wastewater treatment plants.
                                     250

-------
                 TABLE 5-15.  DIFFERENCE BETWEEN CENTRALIZED CONTROL AND CONVENTIONAL CONTROL

                                                   ANNUAL COST ANALYSIS

5
(220)
Plant Capacity
10
(440)
- MGD (dm3/s)
50
(2200)

100
(4400)

300
(13000)
CENTRAL ANALOG

Capital
Replacement
Operations
Maintenance
Material
Energy
Total
CENTRAL DIGITAL
Capital
Replacement
Operations
Maintenance
Material
Energy
Total


$+ 6,600
+ 1 ,000
- 30,000
+ 15,000
- 8,000
- 7,000
$- 22,400

$+ 9,800
+ 3,600
- 28,000
+ 1 ,000
- 13,000
- 10,500
$- 37,100

t
$+ 11,500
+ 1 ,300
- 30,000
+ 15,000
- 13,000
- 13,000
$- 28,200

$+ 20,000
+ 7,600
- 25,000
+ 2,500
- 21,000
- 20,000
$- 35,900


$+ 28,600
+ 1,900
-135,000
+ 15,000
- 39,000
- 50,000
$-178,500

$+ 23,900
+ 10,000
-123,000
+ 7 ,000
- 62,000
- 75,000
$-219,000


$+ 62,900
+ 2,900
-180,000
+ 15,000
- 65,000
- 90,000
$-254,200

$+ 19,000
+ 10,200
-160,000
+ 13,000
-104,000
-135,000
$-356,800


$+168,000
+ 5,400
-405,000
+ 15,000
-140,000
-230,000
$-586,200

$- 18,000
+ 7,900
-368,000
+ 24,500
-224,000
-345,000
$-922 ,800
 .
Ul

-------
                                  SECTION 5
                                  REFERENCES


 1.   Nelson, J. K. and B. B. Mishra, Management-The Key to Success With
     Computers.  Presented at the ISA Philadelphia, PA, October 15-19, 1978
     Conference, October 18, 1978.

 2.   Flanagan, M. J., Full Automation Goal of Water Reclamation Plant, Water
     and Wastewater Engineering.  December, 1975.

 3.   Roesler, J. F.,  A. Manning and R. Timmons, A Cost/Benefit Analysis for
     Automation of Wastewater Treatment Plants.  Presented at the IAWPR
     Workshop on Instrumentation and Control, May, 1977.

 4.   Standards for Waste Treatment Works.  New York State Dept. of
     Environmental Conservation, 1970.

 5.   Metcalf & Eddy,  Inc., Wastewater Engineering.  McGraw-Hill Book
     Company, 1972.

 6.   Male, J. W. and  S. P. Graef, editors of Applications of Computer
     Programs in the  Preliminary Design of Wastewater Treatment Facilities.
     Short course proceedings, August 15-19, 1977.

 7.   Building Construction Cost Data 1977, 35th Annual Edition of Means Cost
     Index.

 8.   Patterson, W. L. and R. F. Banker, Estimating Costs and Manpower
     Requirements for Conventional Wastewater Treatment Facilities.
     Environmental Protection Technology Series, 17090 DAN 09/71, October,


 9.   Burke, G. W., Jr., Estimating Personnel Needs for Wastewater Treatment
     Plants.  Journal Water Pollution Control Federation 48, 241, 1976.

10.   Molvar, A. E., Selected Applications of Instrumentation and Automation
     in Wastewater Treatment Facilities.  Environmental Protection
     Technology Series, EPA 600/2-76-276, December, 1976.

11.   Flanagan, M.J. and B.D. Bracker, Design Procedures for Dissolved Oxygen
     Control of Activated Sludge Processes.  Environmental Protection
     Technology Series, EPA-600/2-77-032, June 1977.
                                     252

-------
12.  Molvar, A. E., J. F. Roesler, R. H. Wise and R. H. Babcock,
     Instrumentation and Automation Experiences in Wastewater Treatment
     Facilities.  Environmental Protection Technology Series,
     EPA-600/2-76-198, October, 1976.

13.  Covery, John J.,  Joseph F. Roesler and Robert H. Wise, Automation and
     Control of Physical-Chemical Treatment for Municipal Wastewater.
     Applications of New Concepts of Physical-Chemical Wastewater Treatment
     Symposium, September 13-22, 1972.

14.  Manning, A. W., Direct Digital Control at the Iowa City Water Treatment
     Plant, Journal of American Water Works Association.  Volume 69, Number
     6, June, 1977.

15.  Kron, C., Direct Digital Control of the Iowa City Water Treatment
     Plant.  Presented at the National AWWA Conference, June, 1975.

16.  Roesler, J.F., Evaluation of the Effectiveness of Automation, Research
     Needs for Automation of Wastewater Treatment Systems, Clemson
     University, Clemson, SC, September 23-25, 1974.

17.  Cincinnati Gas and Electric Company, Rate General Service, PUCO No.  16,
     September, 1976.

18.  Thibodeau, G. J., Peak Power Demand Savings Through Computer Control of
     Production Electric Furnaces.  Digital Equipment Corporation, April,
     1977.

19.  Crowell, W. H., Power Demand Control for Lowering Operating Costs.
     Proceedings of the ISA Annual Conference, October, 1974.

20.  Smith, R., A Comparison Between Recently Computed Cost for Conventional
     Plants and Previous Estimates.  Memo to John J. Convery, July 6, 1972.
                                     253

-------
          SECTION 5
           APPENDIX
COST EFFECTIVE ANALYSIS TABLES
             254

-------
TABLE A-l.  UNIT PRICING

Instrument or Device
Recorder
Ratio Station
PID Controller
Setpoint Station
Panel Indicator
Totalizer
Signal Conditioner
Annunciator
Control Switch
Tinier
Panel Section
Material
$ 700
585
1,150
750
240
800
555
25/ PT
130
100
800
Installation
$ 40/PEN
50
50
40
40
40
30
30/PT
100
30
200
Installed
Unit Cost
$ 740
635
1,200
790
280
840
585
55/PT
230
130
1,000
           255

-------
          TABLE  A-2
                                                             [ udlPKCM  A.MALYSIS

                                                             CLNVtllT 1LNAL ANALJG
          INFLUENT  PUMPING
          PRELIM   TREATMENT
                      	__ ..  ._  .  «.t,rt
                                                                                                      _ 26	?Q

                                                                                                      55        zaa
                                                                                       _^t. 6 hCL
                  	xa	

                   -   130-   . .-1,000-

                         l	10*0.00	
                                                                                                                                                 790
          EL.ANJL-C.OJ5J
             INEERING  (.15)
         -_SJfSTE.H. tDJAL	 	

         _£P_A CCNSTRUCT1QN COST  ESTIMATE
          I/_t COST  AS  A PERCENT  OF COMAUtUCTION
                                                                                                                             	u,

                                                                                                                                    85,612

                                                                                                                               ..  3,930,619

                                                                                                                                    .._ 2.2

-------
                                                                     EQUIPMENT  ANALYSIS
cn
TAR 1 F A 7
IMDLC M _, CONVENTIONAL ANALOG
R4T10 PID PANEL TOTAL- SIGNAL ALARM CUNTRtll
RF.CQR05PS STATI3NS CONTROL INDICATORS IZERS CONDITION ANNUN SWITCHES
INFLUENT PUMPING 1 121323
PRELIM TREATMENT 3 32
PRIMARY TREATMENT 48 8
AERATION .'• i ;f" 2 57 3
SF.CONCARY CLARIFIER I 591 8

TIMERS/ TNTI PM
CLOCKS ETC SFCTICN
1
2 2
2
2
2
10 MGD
REMDTF
S.P. STA


CHLOftlNATION > 4 2
RAS PUMPING 2 322^22
MAS pwwiNfl;;it,-:!'«::;--:&a- ••.:• i 1112 2
If >%.'•'• ._ . . .
GRAVITY THICKENING 242 4
DICES TJON .',: :\ ,6 4 6 2 4 4
J ":!.-'.,
SLUDGE HOLDING 1 51 42
VACUUM faren •,'";•» 3 33 2 2
INCINERATION 10 4
MISCELLANEOUS 2 122425
INSTRUMENT TOTAL 21 1 31 60 10 13 44 24
UNIT COST 740 635 1,2JO 280 840 585 55 230
TOTAL COST 15,540 035 37,200 1 6 , 8 OU 8,400 7,605 2,420 5,520
PLANT COST
ENGINEERING (.15»
SYSTEM TOTAL
EPA CCNSTRUCTION COST CSTIMATE


2
4 2
2 I
2
1
3 1
11 18
1_30 UOOO
1,430 18,000


R







-
. 7^0
113,550
17, 033
130.5B3
U7fi 997
1 . 5
          I/C COST AS_A_PERCENT f^ ^CONSTRUCT ICH

-------

TABLE A-4

RECORDERS
INFLUENT PUMPING 1
PRELIM TREATMENT
PRIMARY TREATMENT
AERATION . : • : '* i,
SECONDARY CLARIFIER 1
CHLORINATION . .; ft-:' • ' '^'4
RAS PUMPING 4
MAS PUHPING , •;-;." 1
GRAVITY THICKENING 3
-,•• Dictsttdft ;•.£: ;-:•« 11
rv> ; ; V:?fe; •-' : .<:;
<£ SLUDGE HOLDING 1
CO
:", VACUUM FitTe*!,-'-, "o '-••' 5
.•''•'•'.'j; -.,•:: <: '
INCINERATION
MISCELLANEOUS'' .|£. '' ' 2
INSTRUMENT TOTAL 33
UNIT COST ' 740
TOTAL COST 24.420
PLANT CQSt
ENGINEERING 1.151
SYSTEM TOTAL . , 1
EPA CONSTRUCTION COST ESTIMATE
EUUlP_MfNT ANALVS.IS
so MGD
CUNVEJ4T1CNAL_ANALQG
RATI3 PIIJ PANEL TOTAL- SIGNAL ALARM CnNTRfll TlMFlf^/ fNTI PNL REMOTE
STATIONS CONTROL INDICATORS IZERS CONDITION ANNUN SWITCHES CLOCKS FTC SFCTION S.P. STA
1 2 1 <. i <• 1
3 3222
4 a a t
- 7 11 3 2
7 13 1 12 3
!lr? *;; /
544323 1
1112 2 1
63 6 3
9 11 Z 9556
71 6221
55 2 2 3
20 8 2
1 2 24 2 5 3 2
1 46 90 17 15 61 ?1 1? ?<3
635 1.70(1 7BO H«.f)
635 55.200 25.700 10rnHn 8,775 ^,555 fc,?10 1 f 5&f) 7qtnnn

^4 W-S
AOQ
_ _ _ . 28,319,122 _
0_. 7_

-------

;•' TABLE A-5
RATIO
'RECORDERS STATIONS
INFLUENT PUMPING 1
PRELIM TREATMENT
PRIMARY TREATMENT
AERATIONS r .•-.";>•'•}*. - « ;; "•' ; •>,
SECONDARY CLARIFIER 1
CHLORINAflON ' ' *$% _ .f'V^'IflffSK'
RAS PUMPING 8
HAS PUNPtNO &:'.£.-. ^: •• I
i • »K-4'' i , ' J^K '' '•'
. • . ikf* - i: ./F. »•'- -«'A .»!,»•
GRAVITY THICKENING 4
'• DIGESTION :*j$£Sfa £$», 1* '
SLUDGE HOLDING 1
VACUITN Fkf«fi?v :£$&&•.•. -.1 : 7 • . '-.
'-• • • ' ^:ffj'«HJl^' '
INCINERATION
HISCELLANEOUS. T "3$ 2
INSTRUMENT TOTAL 47 1
UNIT COST ;'' : '•[,[ 7*0 635
TOTAL COST 34, 780 635
PLANT COST ' ? ' ' ' •'
ENGINEERING (.15)
1 SYSTEM TOTAL '' ' ^•'•'•;
-------
                                                                     hUULPMEM ANALYSIS.
ro
CTl
O
TABLE A-6
RATU PtD
/.RECORDERS STATIONS CONTROL
INFLUENT HUMPING 1 1
PRELIM TREATMENT
PRIMARY TREATMENT Lb
AERATION It 20
SECONDARY CLARIFIER 1 33
CHLORINATION ; !. ,','.'• '1
RAS PUMPING 16 17
MAS PUMPING •('- V 1
GRAVITY THICKENING 8 16
DIGESTION '- "•<'> V) 2* ' 24
.; .-..•••••• 'v'
SLUDGE HOLDING 1
VACUUM FIlTtft. i 13 13
INCINERATION
MISCELLANEOUS' ,". ' • 2 1
INSTRUMENT TOTAL 85 1 142
UNIT CO$T 74p 635 L,20O
TUTAL COST 62.900 635 170.400
J PLANT CPST
ENGINEERING 1.151
! SYSTEM TOTAL ''. " . V^"
EpA CCNSTRUCTION COST ESTIMATE
f '• . •'""• ':::'- :•••
1 .. .I/C. COST.. AS. _A. PERCENT. OtlCflMSMJJCTlBM 	
CONVENTIONAL. ANALOG
PANEL IUTAL- SIGNAL 	 ALAHM .-CONTROL. T.IHER5
INDICATORS IZfiPS CCNMTION &NNUN SWITCHES CLOCKS
3 366
32 J2
36 4
65 I 04
6
16 16 4 2 4
114 4
8 16
26 2 24 8 8
13 1 12 3 3
13 3 3
60 24
225284
277 24 20 , 1H5 	 46 21
2aO . 	 840 .- _ - 5U5 55 230 130
77,560 20,160 11,100 )0,JL75 11), 58O 2,740





i/... CNTL PNL
FTC SFCTION
i
3
H
a
16
6
2
8
24
2
5
6
6
V6
1.000
96.000
133,

300 N
REMUT
S.P. S
790
462,840
69.426
532.266
,767,672
--O.U

-------
TABLE A-7


INFLUENT PUMPING
PRELIM TREATMENT ,-
*' ?.
PRIMARY TREATMENT
AERATION ' Ki
SECONDARY CLARIFIFR
CHLOR1NATIDN ' •< •
RAS PUMPING
HAS fimflN^,.|v!,^
GRAVITY THICKENING
DIGESTION :•&•"•*'• ?•"#•;•
• . . - : i i *• --
DRYING BEOS
MISCfLLANEOUi C'.iv.
X '» .' /
• • i . . . . i .•* .
INSTRUMENT TOTAL
UNIT tOST* '
TOTAL COST
PLANT COST \ •
ENGINEERING 1.15)
SYSTEM TOTAL
EPA CONSTRUCTION COST
I/C COST AS A PERCENT
CENTRAL ANALOO
PATH Pin PANEL TOTAL- SldNAL
RECORDERS STATIONS CONTROL INDICATORS I/ERS CONDITION
1 1112
*
2 4
1' 3 6
1 341
:.4::$T£? l "" l
1 1 L 2
1 . 1 12
2 42
6 462
1 1 1
2 1222
16 1 20 30 9 9
740 635 1,200 280 840 565
11,840 635 24,000 8,400 7 t^D 5,  1



ESTIMATE
OF CONSTRUCTION

. ALAKM
ANNUN
2
2
4
2
4

2

4
4

12
36
55
,9bO





5 MUD
CONTROL IlMhRS/ CNTL PNL KEMOTt
SHITCHFS CLUCKS ETC SFCTITN S. P. STA
4 1 1
4 2
1 I
1 3
1 3
•W V. l
4 1
4 1
1 4
6424
4 2 1
8231
40 10 12 21
230 130 liOOO rvo
4,200 1.300 12,000 l&.iYO
&£ 770
1 b , fi 1 *>
113,585
^ , 9 ^ n , f, 1 9
2.9

-------
TABLE  A-8
LUUH'MI.Mf ANALY'jIi




  Cl NTKAL ANALOG
	 _ .... RAT 13 IMD I'ANLL TUTAL- ilbNAL ALARM CUN1MJL T1MIHS/
RFCORD6RS STATIONS CDNThUL INIilCATIIRS I/FRS CUNIM1IHN ANNUN SWlTCMrS CLOCKS r
INFLUENT PUMPING 1 1 1 1 J 2 6
PRELIM TREATMENT 6 3 4 2
PRIMARY TREATMENT 4 u 8
AERATION I- 2 5 10 3
SfCUNCARY CLARIFIER 1 581 8
CHLOR1NATION \ 1 4
RAS PUMPING 2 3 2224
WAS PUMPING •'•''$*•*. l 1 11 «
GRAVITY THICKENING- 242 4
DIGESTION , •"••• 6 462 484
0\ SLUDGE HOLDING 1 51 442
ro
VACUUM FILTBH 3 3 2 4
INCINERATION 10 4
MISCELLANEOUS 2 1 2 2 4 20 10 )
.. INSTRUMENl-IfllAL Zl.- 1 Jl ia .10 _ IV . .... 62 48 11
UNIT COST . . 740 635 1,200 280 8*40". 585 55 230 130
T01AL CUST. 	 	 15.J40 	 635 . _ . J/,200 16,240 U.4UO ... _. diiyo J.410 11,040 1,430
: PLANT COST. 	 	 .
ENGINEERING (.191
SYSTEM TOTAL 	 	 	 . .
, . fcPA CCNSTKUCT10N COS.I E SHMA1E. 	 . . 	 	
I.NTL PNL RtMCH
Tr. SFCTICN S.P. STA
1 1
2
2 4
2 5
2 5
1
3
1
2 4
2 4
1
2 3
1
4 1
21 J2
1,000 790
21,000 25,280 -
148,365
22,255
170,620
8,'t7G,227
 1/C COST AS  A PERCENT OF CONSTRUCTION

-------
TABLE A-9
twUll'KLM ANALYilS



  LEN1KAL ANALUG
INFLUENT PUMPING
PRELIM TREATMENT
PRIMARY TRtATMTNT"
AERATION
SECONDARY CLAR IF IER
CM 10* I NAT 101*4.
MAS PUMPING
MAS nmm1-.*,, • •*••-..
GRAVITY THICKENING
OfOGJTTIGfil '.'•-"
r\3 - •
CT> SLUDGE HOLDING
CO
VACUUM PrtTfH
INCINERATION
MISCELLANEOUS
INSTRUMENT TOTAL
UN|T COST 	
TOTAL COST
PLANT COST 	
ENGINEERING (.151
SYSTEM TOTAL
RAT | )
HECOROFPS STATIONS
1
	 _. ._ 	
4
1
•• "'''jjRlS^*1
*
7/ i
U'fi
j
•.r'? 1 1
T '
1
•Iff 5

2
33 1
	 MO _ 635
24,420 6J5
.... ... .

FPA CCNSTRUCTIHN COST ESTIMATE 	
I/C COST AS A PERCENT OF CONSTRUCTION
PJI) p/VNfLL TOTAL- ilt'-IAL ALAHM
CONTROL INOILATUHS i/FfS CUNIITKIN ANNUN
1114^
6 3
4 a b
7 14 3
7 12 1 12
1
5 4 J <•
1 1 2
6 J 6
9 11 2 9
7 I 6
5 2
20 H
1 2 2 4 31
46 b4 12 16 VO
1,200 280 840 505 55
55,200 iJ,!>20 10, QUO V.3CO 4,V!>0
• - -.----.


CUNIhUL T1ME.HS/ (.NIL PNL HCMl.
SWITCHES r.uicKS FTC SFc^I(:^ s.c.
t i 1
422
2 7
3 '
•<-• ..; i
6 1 b
4 1 I
3 6
10 5 6 9
4 2 1
4 35
10 3 t 1
54 12 33 47
2JO 1JO UOCO 7VO
12t4«;Q lii>60 JJiOQQ 37,130
212,275
.._.._. J 1,841
244 ,116
._. 	 _2S,319,122
_0.9

-------
ro   _
TABLE A- 10
RECORDERS
INFLUENT PUMPING 1
PRELIM TREATMENT
PRIMARY TREATMENT
AERATION •
'.ii*-
SECONDARY CLARIFIER 1
CHLORINAtlOli; : '.'
RAS PUMPING 8
MAS PUHPJN6 ;'•:••;$, • '*"•
GRAVITY THICKENING 4
•6
1 • oieestw:i^.;fvj|^i^
SLUDGE HOLDING 1
VACUUM •ftLt£ti'^i'-"Rf >•:.
INCINERATION
MISCELLANEOUS • -^ '-•.", 2
•' I* "
INSTRUMENT TOTAL 47
UNIT COST 740
TOTAL COST 34. 780
, PLANT COST
FNr.lNFFRINr. 1.1 SI
SYSTEM TOTAL • :-'il
EPA CONSTRUCTION COST ESTIMATE
. .. _ . . 	 CENTRAL ANALOG. . .._....
RATIA P1D _PANtl TI1TAL- Slf-NAI ALARM tDNTROl TIMFBS/ tNTl PKL
STATIONS CONTROL INDICATORS IZERS CONPITION ANNUN SWITCHES CLOCKS ETC SECTION
111428 2
6 3632
8 16 16 4
12 24 4 4
17 32 1 32 8
r"'^M ••' B '• .
,.,:i^. r • *•
9 8428 2
1 14 8 1
84 8 4
12 14 2 12 12 6 12
91 8631
7 363
30 12 3
1 2 25 51 16 4 13
1 76 1 iH 1 h jn 1 in 7fl \f, 4n i^T4<>n 1 1 , ;nn flf^sn i7,Q4n 7,nHii >!9tOCC
3


.. -sn.fi
100 M
REMOT
S.P. S
1
8
12
17
1
9
1
8
12

7
1
.. .....7.7.
.-. 790
38,495
50*774
89-, .269
     	L/C_C.OSI_AS -A.PERCEJ1T OF.COJ4STRUCTLON	_.

-------
TABLE A- 11


INFLUENT PUMPING
PRELIM TREATMENT
PRIMARY TREATMENT
AERATION
SECONDARY CLARIFIES
CHLORINATION 5-':
kAS PUMPING
HAS PUMPW* ; .;;;
GRAVITY THICKENING
DieeSTtoM f .. . -fy,^
i>o ..''."/. 'i;:'i-- a.'.
CT> SLUDGE HOLDING
cn
VACUUM flLTW- •'-'';,"

INCINERATION
MISCELLANEOUS ;''.;
INSTRUMENT TOTAL
UNIT COST
TOTAL COST
PLANT COST
ENGINEERING (.15)
SYSTEM TOTAL
fPA CCNSTRUCTION COST


RECORDERS
1


16
1
• ;
16
' 1
8
: 26
1
13

2
85
740
62,900


•?
ESTIMATE
CENTRAL ANALOG
RATIO PIO PANEL TOTAL- SIGNAL
STATIONS CONTROL INDICATORS IZHRS CONDITION
1 1 1 <.
6
16 32
20 40
33 64 1
;,t£l
17 16 4
» 14'
t
16 8
*4 2* 2
13 1
13 .3, 	
60
1 22 S
1 142 252 24 20
635 1,200 280 840 565
635 170,400 70,560 20,160 11,700 15





_AJ^ARH_
ANNUN
2
3
32
4
64

2

16
24
12

24
9«
,81
55_
.455





CONTROL TIM E.R.S.
SWITCHES CLOCKS
8
12 6



12
8
8

16 8
6 3
6

16 4
92 21
2-JO i39
21,160 2,730





Z 	 CNl L JPNt-
ETC SFCT1QN
2
3
8
8
16

6
2
a
24
2
5
6
20
110
-.. l-< OCU ._..
110,000

1 53,
30d MGO
_A£MOTE
S.P. STA
1
16
20
33
1
1 7
1
16
24

13

1
143
790
JUZ^70
598, 670
_J9,, 80J)_
6 8 8^ ^ 7 0
767,672
I/C COST AS A PERCENT OF CONSTRUCTION
                                                                                                                                 0. 7

-------
                       INPUT/OUTPUT ANALYSIS
TABLE A-12
                                    5 MGD
                      CENTRAL DIGITAL CONTROL
                          ANlALDC-
         MODULATIN&  DISCRETE    CCN^RCL
    UNIT PROCESS
INPUT
OUTPUT
INPUT
OUTPUT
INFLUENT PUMPING
PRELIMINARY TRCAT<1ENT
PRIMARY TREATMENT
AERATION
SECONDARY CLARIFIER
CHLORINATION
RAS PUMPING
WAS PUMPING
GRAVITY THICKENING
DIGEST ION
DRYING BEDS
MISCELLANEOUS
4
2
6
7
8
1
5
4
10
10
1
6
1
2
3
3
1
2
1
4
4
1
2
2
8
1
9
5
2
10
6
8
2
I
2
2
4
2
2
10
8
COLUMN TOTALS
                          53
                          34
ANALOG /  DISCRETE
                                     87
PLANT TOTAL
                                    173
                               266

-------

TABLE A-13

UNIT PROCESS
INFLJENT PUMPING
PRELIMIMARY TREATS
PRIMARY TREATMENT
AERATION
SECONDARY CLARIFIE
CHLORINATION
RAS PJMPING
WAS PUMPING
GRAVITY THICKENING
DIGESTION
SLUDGE HOLDING
VACUUM FILTER
INCINERATION
MISCELLANEOUS
COLUMN TOTALS
ANALOG / DISCRETE
INPUT/OJTPl
JT ANALYSIS

CENTRAL DIGITAL CONTRni
ANALDG
INPUT
5
ENT 3
12
12
R 14
1
7
4
10
10
3
8
§
8
102

MODULATING
OUTPUT
1
4
5
5
1
3
1
4
4
3
1
32
134
DIStPETE
INPUT
3
P
16
2
17
6
2
10
6
4
4
16
86


10 MGO
CTNTRHL
OUTPUT
3
4
8
2
2
10
^

54
A42
PLANT TQTAt ?7A
267

-------

TABLE A-14

UNIT PROCESS
INFLUENT PUMPING
PRELIMINARY TRFAT1
PRIMARY TREATMENT
AERATION
SECONDARY CLARIFIF
CHLORINATION
RAS PJMPING
WAS PUMPING
GRAVITY THICKENING
DIGEST ION
SLUDGE HOLDING
VACUUM FILTER
INCINERATION
MISCELLANEOUS
COLUMN TOTALS
ANALOG / DISCRETE
PLANT TOTAL
INPUT/OUT
CENTRAL DI
ANALOG
INPUT
6
ENT 3
12
18
R 20
1
12
4
15
20
4
12
10
8
145


PUT ANALYSIS
GITAL CONTROL
MODULATING
OUTPUT
i
4
7
7
1
5
1
6
9
5
1
47
192



DISCRETE
INPUT
4
2
16
4
19
9
2
14
7
6
8
16
107



50 MGC
CCNTROL
OUTPUT
4
2
4
3
6
3
2
16
6
16
62
169
361
268

-------
INPUT/OUTPUT ANALYSIS
TABLE A-15
CENTRAL DIGI

UNIT PROCESS
INFLUENT PUMPING
PRELIMINARY TREATMENT
PRIMARY TREATMENT
AERATION
SECONDARY CLARIFIER
CHLORINATION
RAS PUMPING
WAS PUMPING
GRAVITY THICKENING
DIGESTION
SLUDGE HOLDING
VACUUM FILTER
INCINERATION
MISCELLANEOUS
COLUMN TOTALS
ANALOG / DISCRETE
ANALOG
INPUT
6
3
24
32
50
1
21
6
20
26
5
17
15
9
235

TAL CONTROL
MODULATING
OUTPUT
1
8
12
17
1
9
1
8
12
7
1
77
312

DISCRETE
INPUT
4
3
32
fi
49
14
4
20
9
9
12
25
1*9

1JO MGC
rrNTRr.
OUTPUT
4
8
4
16
4
4
21
9
25
9fl
287
PLANT TOTAL 599
269

-------

TABLE A-16

UNIT PROCESS
INFLUENT PUMPING
PRELIMINARY TREATMENT
PRIMARY TREATMENT
AERATION
SECONDARY CLARIFIER
CHLORINATION
RAS PJMPING
WAS PUMPING
GRAVITY THICKENING
DIGEST ION
SLUDGE HOLDING
VACUUM FILTER
INCINERATION
MISCELLANEOUS
COLUMN TOTALS
ANALOG / DISCRETE
PLANT TOTAL
INPUT/OUT
CENTRAL DI
ANALOG
INPUT
6
3
48
56
98
1
37
6
40
50
7
29
30
9
420


PUT ANALYSIS
GITAL CONTROL
MODULATING
OUTPUT
1
16
20
33
1
17
1
16
24
13
1
143
563



DISCRETE
INPUT
4
6
64
16
97
22
4
40
11
15
24
25
328



300 MGD
CCNTRCL
OUTPUT
4
6
16
4
32
4
4
35
15
25
145
473
1.036
270

-------
TABLE A-17  5 MGD (220 dm3/s) EQUIPMENT COST, DIGITAL
                            Quantity
Unit Cost
Total Cost
HARDWARE
Central Processor
Memory
Program Console
Program Load Device
Printer
Operator Console
Multiplexer Interface
Multiplexer
AI
MO
DI
CO
HARDWARE SUBTOTAL
SOFTWARE
Development/ Integration
Process Control
ENGINEERING
SYSTEM TOTAL

1
32K 16-Bit Words
1
1
1
1
1
6
64
22
53
34


550 MH
800 MH
640 MH


$ 14,000
2,500/4K
5,500
4,000
3,000
2,500
1,000
3,100
200
450
80
175


$45/MH
$45/MH
$45/MH


$











$

$
$
$

$

14,000
20,000
5,500
4,000
3,000
2,500
1,000
18,600
12,800
9,900
4,240
5,950
101,490

24,750
36,000
28,800

191,040
                         271

-------
          TABLE A-18  10 MGD (440 dnT/s)  EQUIPMENT COST, DIGITAL
                                      Quantity     Unit Cost
                                    Total  Cost
HARDWARE
     Central Processor
     Memory
     Program Console
     Disk (Fixed Head)
     Program Load Device
     Printer
     Operator Console (Dual)
     Multiplexer Interface
     Multiplexer
        AI
        MO
        DI
        CO

HARDWARE SUBTOTAL
            1
32K 16-Bit Words
            1
            1
            1
            1
            1
            1
            7
          102
           32
           88
           54
14,000
 2,500/4K
 5,500
20,000
14,000
 6,000
32,000
  ,000
  ,100
   200
   450
    80
   175
1
$  14,000
   20,000
    5,500
   20,000
   14,000
    6,000
   32,000
    1,000
   21,700
   20,400
   14,400
    7,040
    9,450

$ 185,490
SOFTWARE
     Development/1ntegrati on
     Process Control
ENGINEERING
        1,040 MH
        1,600 MH

          960 MH
$50/MH
$45/MH

$45/MH
          $  52,000
          $  72,000

          $  43,200
SYSTEM TOTAL
                                  $ 352,690
                                    272

-------
TABLE A-19  50 MGD (2200 dnT/s) EQUIPMENT COST, DIGITAL
                            Quantity     Unit Cost     Total  Cost
HARDWARE
Central Processor
Memory
Program Console
Disk (Fixed Head)
Program Load Device
Printer
Operator Console (Dual)
Multiplexer Interface
Multiplexer
AI
MO
DI
CO
HARDWARE SUBTOTAL
SOFTWARE
Development/ Integration
Process Control
ENGINEERING
SYSTEM TOTAL

1
48K 16-Bit Words
1
1
1
1
1
1
8
145
47
107
62


1 ,600 MH
2,800 MH
1 ,200 MH


$ 14,000
2.500/4K
5,500
20,000
14,000
6,000
32,000
1,000
3,100
200
450
80
175


$50/MH
$45/ MH
$50/MH


$ 14,000
30,000
5,500
20,000
14,000
6,000
32,000
1,000
24,800
29,000
21,150
8,560
10,850
$ 216,860

$ 80,000
$ 126,000
$ 60,000

$ 482,860
                          273

-------
TABLE A-20  100 MGD (4400 dnT/s) EQUIPMENT COST, DIGITAL
                              Quantity    Unit Cost    Total  Cost
HARDWARE
Central Processor
Memory
Program Console
Disk (Fixed Head)
Program Load Device
Printer
Operator Console (Dual)
Multiplexer Interface
Multiplexer
AI
MO
DI
CO
HARDWARE SUBTOTAL
SOFTWARE
Devel opment/ 1 ntegra ti on
Process Control
ENGINEERING
SYSTEM TOTAL

1
60K 16-Bit Words
1
1
1
1
1
1
11
235
77
189
98


2,000 MH
3,100 MH
1,600 MH


$ 14,000
2.500/4K
5,500
20,000
14,000
6,000
32,000
1,000
3,100
200
450
80
175


$50/MH
$45/MH
$50/MH


$ 14,000
37 ,500
5,500
20,000
14,000
6,000
32,000
1 ,000
34,100
47,000
34,650
15,120
17,150
$ 278,020

$ 100,000
$ 139,500
$ 80,000

$ 597,520
                           274

-------
         TABLE A-21  300 MGD (13000 dnT/s) EQUIPMENT COST, DIGITAL
                                       Quantity    Unit Cost    Total  Cost
HARDWARE
     Central Processor
     Memory
     Program Console
     Disk (Fixed Head)
     Program Load Device
     Pri nter
     Operator Console (Dual)
     Multiplexer Interface
     Multiplexer
        AI
        MO
        DI
        CO

HARDWARE SUBTOTAL

SOFTWARE
     Development/Integration
     Process Control

ENGINEERING
            1
72K 16-Bit Words
            1
            1
            1
            1
            1
            1
           14
          420
          143
          328
          145
        2,400 MH
        3,600 MH

        2,000 MH
14,000
 2,500/4K
 5,500
20,000
14,000
 6,000
32,000
 1
,000
,100
200
450
 80
175
$50/MH
$45/MH

$50/MH
$  14,000
   45,000
    5,500
   20,000
   14,000
    6,000
   32,000
    1,000
   43,400
   84,000
   64,350
   26,240
   25,375

$ 380,865
        $ 120,000
        $ 162,000

        $ 100,000
SYSTEM TOTAL
                                 $  762,865
                                    275

-------
                       HIRF  AMAI YST
TABLE A-22
CONVENT
UNIT PRDCESS
INFLUENT PUMPING
PiUM^KY TREATMENT
A CR AT I ON
SECONDARY CLAR1FIER
RAS PUMPING
*AS PUMPING.
GRAVITY THICKENING
DIGESTION
DRYING BEOS
MISCELLANEOUS
TOTAL
W IRE .92/ PR
T ERwilNATION 1. 76 / PR

IONAL ANALOG
FErT PAI
100
200
350
650
650
650
600
500
500
650
p
-FT



R
2
2
1
3
2
1
2
6
1
2




5 MGD
PAI R-FT
POO
400
1 ,950
650
1 .200
3 ,000
500
1 ,3JO
10,850
Q,982
39

TOTAL COST
10,021
                            276

-------
                          WIRE ANALYSIS
TABLE A-23                                                 10 MGD
                      CONVENTIONAL ANALOG
UNIT PRDCESS
INFLUENT PUMPING
PRIMARY TREATMENT
AERATION
SECONDARY CL4RIFIF.P
RAS PUMPING
WAS PUMPING
GRAVITY THICKENING
DIGEST ION
SLUDGE HOLDING
VACUUM FILTER
INCINERAT ION
MISCELLANEOUS
TCTAL
WIRE -92
TERMINATION 1.76
FEET
150
300
530
950
950
950
800
600
600
670
670
950

/ PR-FT
/ PR
PAIR
2
4
2
5
4
1
2
6
3
3
5
2
39


PAIP-FT
300
1 ,200
1,100
4»750
3,800
950
1,600
3,600
1 ,800
2,01o
3 ,330
1,900
26,360
2^,251
69

    TOTAL COST
                                                    2^,320
                              277

-------
                            WIPE  ANALYSIS
TABLE A-24
                        CONVENTIONAL  ANALOG
                                      50
          UNIT PROCESS
         FEFT
                               PATP-FT
INFLUENT PUMP ING
PRIMARY TREATMENT
AERATION
SECONDARY CLARIFIER
RAS PUMPING
WAS PUMPING
GRAVITY THICKENING
DIGEST ION
SLUDGE HOLDING
VACUUM FILTER
INCINERAT ICNJ
MISCELLANEOUS
TOTAL
*on
700
lf 200
2,100
2. 100
2, 100
1.850
1,510
1. 510
650
650
2, 100


4
7
4
1
3
11
4
5
10
2
57
Ann
2 ,800
4'rftOO
14,700
8 .400
2,100
5.550
16 ,610
6.O40
3,250
6 . 5 10
4,200
75,550
      w IPF
.92 /  P3-FT
      TERMINATION
1.76 / PR
                                  ion
      TCTAL COST
                              69,606
                                278

-------
                            WIRE ANALYSIS
 TABLE  A-25                                                 100 MGO
	CONVENTIONAL ANALOG        	

         UNIT  PROCESS           FEET      PAIR	PAIR-FT
INFLUENT PUMPING
PRIMARY TREATMENT
AERATION
SECCNCARY CLARIFIER
RAS PUMPING
WAS PUMPING
GRAVITY THICKENING
DIGESTION
SLUDGE HOLDING
VACUUM FILTER
INCINERATION
MISCELLANEOUS
TOTAL
WIRE _Q2/
TERMINATION 1.7t> /
400
1,000
1,650
3,000
3,000
3,000
2,600
2,150
2,150
9i>0
950
3,000

PR-FT
PR
2
6
8
17
6
1
4
14
5
7
15
2
91


800
8,000
13", 200
51,000
24,000
3 ,000
10,400
30,100
10,750
6,650
14,250
6,000
17d,150
163, flQS
160
     TOTAL COST                                      I6*f, 058
                                279

-------
                           WIRE ANALYSTS
TABLE A-26
                       CONVENTIONAL ANALOG
     WIRE
                                      300  MGD
UNIT PRDCESS
INFLUENT PUMPING
PRIMARY TREATMENT
AERATION
SECONDARY CLAfUFIER
RAS PUMPING
WAS PUMPING
GRAVITY THICKENING
DIGESTION
SLUDGE HOLDING
VACUUM FILTER
INCINERAT ION
MISCELLANEOUS
TOTAL
FEET
700
1,700
2.900
5,200
5.200
5,200
4.530
3,700
;• 3.700
1,650
1. 650
5*200

PAIR

16
16
33
1
8
26
7
13
30
2
170
PAIP-FT
I T4nn
27,200
46 .400
171 ,600
83.200
5,200
36.000
96,200
25f900
21,450
10,400
574,450
1.03 / PR-FT
                               57^,450
     TERMINATION
1.76 / PR
     TOTAL COST
                               280

-------
                           WISE ANALYSIS
TABLE  A-27                                                   5  MGD
	CENTRAL ANALOG       	
	DMT ?RJ:ESS	FEET	PAIR	PAIP-FT	
     INFLUENT PUMPING	100	7	700
PRELIMINARY TREATMENT
PRIMARY T3EATMEMT
AERATION
SECCNCARY CLARIFIEP
CHLO* I NAT ION
PAS PUMPING
WAS PUMPING
GRAVITY THICKENING
DIGESTION
DRYING BEOS
100
2JO
350
650
650
650
650
600
500
500
6
6
9
8
6
6
6
10
2<:
5
600
1,200
3,150
5.200
3,900
3 ,900
3,900
6.000
11,000
2,500
     MISCELLANEOUS               650       22       14,300
     TOTALTI356,350

     w IRE	.92 / PR-FT	51,8*12
     TERMINATION     1.76 / PR	199
     TCTAL COST                                     52,0*H
                               281

-------
                            WIRF  ANALYSTS
TABLE A-28
                          CENTRAL ANALOG
        10 MGD
UNIT PROCESS
INFLUENT PUMP ING
PRELIMINARY TREATMENT
PRIMARY TREATMENT
AERATION
SECONDARY CLARIFIER
CHLORINAT IQNJ
KAS PUMPING
rfAS PUMPING
GRAVITY THICon
6,700
3 1 3 5 O
3C,400
114,300
105.156
P90

     TOTAL COST
105,446
                                282

-------
                           WIPE ANALYSIS
TABLE A-29                                                  50  MGD
                         .CENTRAL ANALOG
UNIT PROCESS
INFLUENT PUMPING
PRELIMINARY TREATMENT
PRIMARY TREATMENT
AERATION
SECONDARY CLARIFIER
CHLORINATION
RAS PUMPING
WAS PUMPING
GRAVITY THICKENING
DIGESTION
SLUDGE HOLDING
VACUUM FILTER
INCINERATION
MISCELLANEOUS
TOTAL
WIRE .92/
TERMINATION 1.76 /
FEET
300
300
700
1,200
2,100
2, 100
2,100
2, 100
1,850
1,510
It 510
650
650
2, 100

PR-FT
PR
PAIR
11
7
12
21
20
6
16
6
15
39
6
14
10
43
228


PAIR-FT
3.300
2,100
8,400
25,200
42,000
12 ,600
33,600
12,600
27,750
53,890
12,080
9,100
6,500
90,300
344*420
316,866
401
     TOTAL COST                                    317,267
                               283

-------
                            WIRE-  ANALYSI S
TABLE A-30                                                  100 MGD
                          CENTRAL AMAtQG
UNIT PRDCESS
INFLUENT PUMPING
PRELIMINARY TREATMENT
PRIMARY TREATMENT
AERATION
SECONCARY CLARIFIES
CHLORINATIOM
RAS PUMPING
MAS PUMPING
GRAVITY THICKENING
DIGESTION
SLUDGE HOLDIMG
VACUUM FILTER
INCINERATION
MISCELLANEOUS
TOTAL
FEET
400
400
1.000
1,650
3*000
3,000
3.000
3,000
2. 600
2,150
2. 150
950
950
3,000

PAIR
11
9
24
36
50
10
26
10
20
50
11
20
1 5
69
361
PAIP-FT
4 ,400
3,600
24.000
59,400
150.030
30,000
7d.OOO
30,000
52 ,000
107,500
24 ,650
19,000
14 rP50
2C7,000
tt02 ,800
     w IRE	-92 /  PR-FT	738. 576

     TERMINATION	1.76  /  PR	
     TOTAL COST                                     739,211
                                284

-------
                            WIRF ANALYSIS
TABLE  A-31                                                  300 MGD
	CENTRAL ANALOG	_____

          UNIT PROCESS          FEET       PAIR       PAIP-FT
INFLUENT PUMP ING
PRELIMINARY TREATMENT
PRIMARY TREATMENT
AERATION
SECONDARY CLARIFIER
ChLOR INAT ION
RAS PUMPING
WAS PUMPING
GRAVITY THICKENING
DIGESTION
SLUDGE HOLDING
VACUUM FILTER
INCINERATION
MISCELLANEOUS
TOTAL
WIRE -92 /
700
700
It 700
2,900
5,200
5,200
5,200
5,200
4,500
3, 700
3,700
1,650
1,650
5,200

PR-FT
11
15
48
60
98
14
42
10
40
90
13
32
30
116
619

7.700
10,500
81 .600
174,000
509,600
72 ,8DO
21ti ,400
52,000
180*000
333 ,000
48,100
52,800
49,500
603,200
2,393,200
2,201,7^4
      TERMINATION	1.76 / PR	1.089
      TOTAL  COST
                                285

-------
                            HIPT  ANALYSTS
TABLE A-32
                      CENTRAL  DIGITAL  CONTROL
                                        5 MGD
          UNIT  PROCESS
     TOTAL
          FEET
PAIR
                       12
PA1R-FT
INFLUFNT PUMPING
PRIMARY TREATMENT
AERATION
SECCNCARY CLARIFIEP
GRAVITY THICKENING
DIGESTION
100
200
350
650
600
500
?
2
2
2
2
2
200
400
703
1 t300
1 ?200
1 »000
            4,800
       IhF
j.78 /
             8.
     T ERMINATIDN
1.76 / PR
                x 1
      TGTAL  COST
                                 8,565
                                 286

-------
                            hIRE ANALYSIS
 TABLE  A-33                                                   10 MGO
	CENTRAL DIGITAL CONTROL  	

          UNIT  PROCESS	FEET      PAIR	PMP-FT
INFLUENT PUMP ING
PRIMARY TREATMENT
AERATION
SECONCARY CLARIFIER
GRAVITY THICKENING
DIGESTION
VACUUM FILTER
TOTAL
150
300
550
S50
SCO
600
670
'
2
2
2
2.
2
2
2
14
300
630
1 ,100
1,900
1 ,600
1 ,200
1,340
8.040
     WIRE            1.78  / PR-FT
     TERMINATION      1.76 / PR
     TOTAL  COST
                                287

-------
                            WIRF ANALYSIS
TABLE A-34
                      CENTRAL DIGITAL  CONTROL
                                       50 MGD
         UNIT  PROCESS
     TOTAL
          FEET
PAIR
                       16
PAIR-FT
INFLUENT PUMPING
PRIM4PY TREATMENT
AERAT ION
GRAV ITY THICKENING
DIGESTION
VACUUM FILTER

1.
1,
L.

300
7UJ
200
850
510
650
2
4

-------
                            WIPE ANALYSIS
 TABLE  A-35                                                 IJO MGD
	CENTRAL DIGITAL CONTROL	


          UNIT PRXESS	FEET	PAIR	PAIR-FT
INFLUENT PUMPING
PRIMARY TREATMENT
AERATION
SECONDARY CLARIFIER
CHLORINATION
GRAVITY THICKENING
DIGESTION
VACUUM FILTER
400
1,000
1,650
3,000
3,000
2,600
2,150
950
2
4
4
4
2
2
2
2
800
4, ,000
6,600
12,000
6,000
5,203
4,30U
1,900
      TOTAL                                   22        40,600



      WIRF	1,78/ PR-FT	72,62*+


      TERMINATION	1.76 / PR	3d



      TOTAL  COST72,662
                                289

-------
                            MIRE  ANALYSIS
TABLE A-36
                     CENTRAL  DIGITAL CONTROL
                                      300 MGD
         UNIT PR3CESS
          FEET
PAIR
PAIP-FI
     INFLUENT PUMPING
     INCINERATION
            700
          1,650
PRIMARY TREATMENT
AERATION
SECONCARY CLARIFIER
CHLORINATIOM
GRAV ITY THICKENING
DIGESTION
SLUDGE HOLDING
VACUUM FILTER
1,700
2.900
5,2UO
5.200
4,500
3. 700
3,700
1.650
4
4
4
2
2
2
2
2
6,800
11 .600
20,800
10.400
9,000
7,400
7,400
3.300
            6.6JO
     TOTAL
                      28
           64,700
      IPE
 1.78/ PR-FT
          150,766,
     TERMINATION
1.76 / PR
               49
     TOTAL COST
                              150,815
                                290

-------
             TABLE A-37.   MULTIPLEXER ASSIGNMENTS
     Plant
     Size                       Quantity              Multiplexer Location
     5 MGD                         6                Influent Pumping

  (220 dotf/s)
                                                    Secondary Clarifier
                                                    Gravity Thickening
                                                    Digestion

    10 MGD                         7                Influent Pumping

       dm3/s)
                                                    Secondary Clarifier
                                                    Gravity Thickening
                                                    Digestion
                                                    Vacuum Filter

    50 MGD                         8                Influent Pumping

 (2200 dn,3/s)
                                                    Gravity Thickening
                                                    Digestion
                                                    Vacuum Filter

   100 MGD                         11                 Influent Pumping

 (4400 dm3/s)                                        f ; J™^^

                                                    2 - Secondary Clarifier
                                                    Chi on" nation
                                                    Gravity Thickening
                                                    Digestion
                                                    Vacuum Filter

   300 MGD                         14                 Influent Pumping
(13000 dm3/s)
                                                    2 - Secondary Clarifier
                                                    Chlori nation
                                                    Gravity Thickening
                                                    Digestion
                                                    Sludge Holding
                                                    Vacuum Filter
                                                    2 - Incineration
                                  291

-------
                                  SECTION 6

                          AVAILABLE INSTRUMENTATION
INTRODUCTION
     A wastewater treatment plant is faced with a difficult task of treating
an influent stream of continually changing quality and quantity while
required to consistently maintain effluent standards.   Also, ever increasing
costs of energy, labor and materials make the problem  more challenging.
However, the judicious application of instrumentation  to monitor key process
parameters and present the information for operational decision making can
aid in achieving the desired goals.

     This section provides a procedure which can be followed for the selec-
tion of an instrument, a discussion of maintenance requirements for long-
term operation and a summary of commonly used instrumentation in wastewater
facilities with recommendations for application.  By following the proce-
dures in this section, data can be compiled to aid in  making a final
decision to use an instrument.

GUIDE FOR INSTRUMENT SELECTION

     This discussion offers a guideline for users of instrumentation to  aid
in the selection and application of an instrument.  By examining the need
for measurement of a process parameter, determining the various methods  for
accomplishing the desired measurement and establishing a procedure for
making>a final selection, the probability of achieving a successful instal-
lation  is greatly increased.

     The general guide consists of a series of questions which, if individu-
ally addressed and answered or determined inappropriate for the case under
review, can lead to best conclusions.  Also, there is  a discussion of fre-
quently used terminology which will aid in interpreting instrument specifi-
cation sheets published by manufacturers.  By using the information
provided, there is an orderly procedure which can be followed for the evalu-
ation and selection of equipment to provide reliable measurement of a
parameter.
                                    292

-------
MEASUREMENT CONSIDERATIONS

Objectives

     The first consideration in the design of any measurement system should
include a statement of the objective.  Typical examples of wastewater
analysis and control are:

          Establishing a material balance.
          Control sludge wasting rate.
          Detecting LEL (lower explosive gas level) in closed tanks or wet
          wells.
          Control wet well level.
          Control of chemical feed addition.
          Monitoring final effluent quality.

     After the general objective is stated, the next step will be to analyze
measurement systems which will accomplish the stated objective.  Parameters
required for the analysis depend on the actual process information desired.
The following major points or questions should be considered in determining
the type of measurement system required.

Measurement For Control & Optimizing of the Resources

     What instrument accuracy is needed?  Accuracy will depend on the
process requirement.  For example, precise level measurement for interceptor
storage control is more critical than an application for wet well level
measurement where the primary concern is to prevent flooding the wet well or
running the pumps dry.

     Is the measurement required for relieving operating personnel of mono-
tonous and nonchallenging tasks?

     Can the process control tolerate momentary or extended periods of
interruption?  If momentary interruptions can create process upsets, signal
locking should be specified with the instrument.  A signal locking arrange-
ment will  store the last measurement whenever the signal is disturbed or
stopped for a period of time.  Will the process be interrupted often?  If
so, will  fouling of the sensor occur?  Provide necessary cleaning devices if
such interruptions are expected to affect performance.

Measurement For Safety

     The reliability of the instrument should be the major consideration
here.  Fail-safe arrangement should be considered where the process is
critical or involves personnel safety.

Measurement For Process Monitoring

     In this case, accuracy, repeatability and reliability requirements may
not be as stringent as they are in the first two considerations.  The reason


                                    293

-------
for monitoring the parameter should provide background on the accuracy
requirements.  Measurement used for billing purposes or NPDES compliance
reporting requires better accuracy and reliability than the measurement used
for maintenance or trending purposes.

Instrument Location

     The next step in the measurement system design is to determine a point
of measurement in the process.  Process characteristics largely determine
the optimum location of the point of measurement, however, the feasibility
of making the measurement at a specific point and accessibility may dictate
the location.

     There are three general areas in which instruments can be located for
process monitoring and control.  These include on the input to the process,
on the output of the process or in the process.  Influent and effluent
measurements of a unit process normally require sensor mounting on closed
conduits, wet wells or an open channel.  Obtaining a measurement in a
process involves installation of a sensor in an open or closed tank.

Limitations

     Once a  location for the measurement element is determined, the next
step will be to evaluate constraints which may exist where the measurement
is to be made.  This section lists a number of limitations under which a
selected instrument may have to work.  Each limitation should be carefully
reviewed considering the characteristics and performance data of the avail-
able instruments.

Physical--
     Evaluate the point of measurement to determine if physical restrictions
exist.  Is the location accessible for maintenance?  Is pipe size adequate?
Are up and down stream pipe characteristics conducive to representative
measurements?  Is there adequate space for installation?  These are only a
few of the physical characteristics that have to be examined before making a
final instrument selection.

Environmental—•

     Temperature - If the instrument has to work under extreme temperature
     conditions, means should be provided to protect the instrument against
     freezing, condensation and high temperatures.

     Vibration - If an instrument'is located on or near rotating or reci-
     procating equipment, accuracy and/or long term reliability may be
     affected by the vibration.

     Humidity - This should be checked against the instrument specification
     and specifically where sample gas or air lines are required, proper
     protection should be provided against collection of condensibles in the
     lines.


                                     294

-------
     Corrosive Gases - If corrosive gases are expected in the instrument
     environment, proper protection should be provided for the instrument in
     its enclosure.

     Cleaning - If steam or chemical cleaning is expected, verify that the
     sensor material would withstand the cleaning;  or if not, that proper
     protection is provided.

Process Material Characteristics —
     The process material characteristics should be reviewed to check their
compatibility with the performance and the life  of  the instrument.

     Foreign Materials - Undesirable materials like grease,  scum, entrained
     solids and chemicals can have detrimental effects on instruments
     exposed to them.  Each type of available instrument's performance
     should be checked in similar environment.

     Suspended Solids, pH - Value of suspended solids and/or pH of the
     process material will affect the performance of certain instruments.

     Temperature and Pressure - The sensors'  performance will be affected
     with respect to accuracy and the life of the sensor by extreme tempera-
     ture and pressure.  These should be considered separately from the
     outside environment consideration.

Rangeability—
     This characteristic of the instrument will  indicate its suitability for
the present requirement as well as future expansions.  The selected instru-
ment should be able to measure the process parameter over the range antici-
pated and if required, should be able to expand  or  suppress  the range if the
parameter's range is expected to change in the future.  The performance of
the instrument should remain within its  specified limits in  the total range.

Technology—
     A large variety of sensors and transducers  are available for measuring
each of the most commonly encountered parameters in the wastewater treatment
field.  These sensors are designed based either  on  different scientific
principles or different techniques using the  same principle.

     Each instrument considered should be evaluated against the requirement
for the measurement.  A less sophisticated, field proven instrument may be
more desirable than a new instrument claiming improved performance.

Available Measurement Methods

     After a measurement parameter and the location for the measurement are
determined, the next step is to locate an instrument to measure the parame-
ter.  Out of the multitude of instruments available, one needs to select the
most suitable instrument for the application.  This section  provides an
organized approach to obtain the information  about  available measurement
methods and potential suppliers of the instruments.


                                    295

-------
Literature Search-
     Information related to instrumentation applications,  theory  and  suppli-
ers is published in several different forms.  The applicable  publications
are discussed here.

     Trade Journals —

     1.   Instruments & Control Systems, a monthly publication, published  by
          Chilton Co., Randor, PA.

     2.   Control Engineering, a monthly publication of Technical Publishing
          Co., Chicago, IL.

     3.   Instrumentation Technology, a monthly publication of Instrument
          Society of America, Pittsburgh, PA.

     4.   Waste & Sewage Works, a monthly publication of Scranton Gillette
          Co., Chicago, IL.

     Equipment Guides—

     1.   Visual Search Micro Films (VSMF) a service provided by  Information
          Handling Services of Englewood, Colorado

     2.   Chemical Engineering Equipment Buyers' Guide, McGraw Hill
          Publication, New York, NY.

     3.   Chilton's Control Equipment Master, Chilton Book Co., Randor,  PA.

     4.   Environmental Yearbook and Products Reference Guide, Technical
          Publishing Co., Greenwich, CT.

     5.   Pollution Equipment News Buyers' Guide, a Rimbach Publication,
          Pittsburgh, PA.

     Reference Books—These reference books will provide detailed informa-
tion about different kinds of instruments available in the market.  These
references sometimes provide comparison of different types of instruments
available to measure a parameter.  Examples of these references are:

     1.   Chemical Engineer's Handbook, John H. Perry, McGraw Hill Book  Co.,
          New York, NY.

     2.   Liptak's Environmental Engineers' Handbook, B. G. Liptak,  Chilton
          Book Co., Randor, PA.

     3.   Process  Instruments & Controls Handbook, D. M. Considine,  McGraw
          Hill Book Co., New York, NY.
                                     296

-------
Manufacturer's Representative-
     Representatives of the instrument manufacturer can also provide the
information needed about the instrument evaluation for the application.  He
may be able to help you evaluate the application with respect to the instru-
ment he is representing.  If an applicable instrument is not made by the
firm he/she represents, he/she may guide you to another manufacturer who can
supply the required instrument.  Both factory and local sales representa-
tives can provide useful technical information and literature.

UNDERSTANDING INSTRUMENT SPECIFICATIONS

     It is essential that the individual evaluating instrumentation alterna-
tives understand instrument specifications so that comparisons  can be done
on a common base.  The following discussion explains commonly used terms
found in manufacturers instrument specifications.

     The instrument specifications published by suppliers are organized in
numerous ways.  For the sake of clarity, the following description is
organized in major sections and subsections similar to that used in many
specifications.

General Features

     This part of a specification describes the general use and capabilities
of the instrument.  These highlights can provide information relative to the
kind of environment in which the instrument can be used, the intended
service, rangeability, maintenance requirements and the kind of flexibility
the instrument offers.  Many times it is possible to determine  from this
information whether the instrument is worth considering for the requirements
of the measurement system.

Detailed Design Specifications

     If the general specification meets the general requirements of the
intended application, it is then necessary to study the detailed specifica-
tions of the instrument.

Material —
     The material of construction for each part of the instrument in contact
with the process should be described.  If not, the supplier should be con-
sulted to check the material compatibility with the process media and
environment.  Ascertain the instrument enclosure material also  meets the
requirements of the environment in which it will be working.

Connection Size—
     If the sensor or sensor enclosure is required to be physically con-
nected with the other process equipment or parts, size (including thread
specification if threaded connections are used) and the type of connector
should be specified.  Always try to specify and obtain standard size con-
nectors.
                                    297

-------
Mounting—
     The kind of mounting available for sensor assembly and the signal
converter should be specified.  The sensor could be in-line, pipe, duct,
tank mounted, or may require specially designed mounting.  The signal
converter enclosure could be portable, surface, pedestal or panel  mounted
type.

Utilities--
     This part of the specification should indicate necessary utilities like
electric power, air, steam or chemicals that may be required for either
instrument operation or calibration.  Rate of consumption, pressure, vol-
tage, temperature or concentration should be specified.

Performance Specifications

     Performance evaluation rates high on the list of items considered in
selecting an instrument.  Each item affecting the performance should be
considered against the requirements of the total system.  Major items that
affect the accuracy are repeatability, linearity, static and dynamic errors
and dead band.  These are figures published by the instrument manufacturers
and will hold only in ideal operating conditions and if the system is main-
tained according to the manufacturer's recommendations.

Accuracy—
     It is defined as the limit, usually expressed as a percentage of full
scale range, span or measured value, not exceeded by errors when the instru-
ment is used under certain reference conditions.  Verify the actual absolute
error across the range of measurement and the conditions under which the
accuracy would hold.  Caution:  Specific component accuracy is not to be
confused with loop accuracy where component accuracy may be compounded.

Repeatability-
     Repeatability is defined as the ability of an instrument to come back
to the same value of measurement at different times.  It is normally speci-
fied in terms of maximum error between two values of the same measurement
made at different times, but under the same operating conditions.   It is
expressed in terms of percent of the full scale value of the measurement.

Linearity-
     It is the closeness to which a curve formed of measurement points
approximates a straight line.  It is specified in terms of maximum deviation
between an average curve and a straight line.

Drift-
     Drift is defined as a change in the output for the same input and
operating conditions over a specified period of time.  It is normally
expressed in terms of maximum change in output across the measured range in
percent of full scale value.  The drift could be caused by sensor deteriora-
tion or signal converter output variation.
                                     298

-------
Sensitivity—
     The sensitivity of a measuring instrument is the minimum change in
value of the measured variable the instrument can sense and provide a useful
output.  Also, the minimum value which instrument can provide a useful sig-
nal within accuracy of specs.

Dead Band—
     The largest range through which the variable being measured can change
without the change being sensed by the instrument.

Limitations

     All instruments have restrictions which must be considered in order to
assure reliable performance within the specified accuracy of the instru-
ment.  These must be examined closely to determine if they may affect an
instrument's ability to perform as required.

Rangeability—
     This indicates the practical measured range over which an instrument
will perform within the specified accuracy and other performance criteria.
It is normally expressed as a ratio of maximum to minimum measured value.
At times different sizes of the same instrument may be necessary to cover
the range over which an instrument is expected to operate.  Other instru-
ments are designed to operate over multiple ranges which are switch select-
able.  Also, options are available with some instruments for expanding and
suppressing the range.

Environment—
     This specifies the range of parameters like temperature, pressure,
humidity or any other special environment parameters within which the
instrument can operate and provide results within the specified perform-
ance.  Verify these limits for both the sensor assembly and the signal
converter.

Physical —
     This is concerned with restrictions which should be observed for proper
installation.  Restrictions on direction, location or orientation of the
sensor in the process, maximum distance allowed between the sensor and the
signal converter and need for special accessories for mounting are discussed
here.

Options

     Instruments are designed to be suitable for as many applications as is
practical.  However, options are frequently offered to enable customization
of an instrument for a user's specific purpose.  It may enhance the reli-
ability, provide more flexibility, wider range, better control or may make
it suitable for harsher environment.
                                    299

-------
Maintainability--
     Options like ultrasonic cleaning, a corporation stop for in-line
installation, plug-in modules and local on/off control are offered for the
purpose of improving the maintainability and ease of calibration.

Output Signal--
     Special types and  levels of signals are offered to meet different
interface requirements.  If the line has intermittent flow or is often shut
down, the zero signal option should be purchased.

Controls —
     Local controls and indication are provided for ease of calibration.

Future Expansion-
     Span elevation or  suppression provisions or a range selector are pro-
vided to accommodate future expansion of the capability of the instrument.

Instrument Protection—
     Diaphragm seal for pressure transmitter, intrinsically safe equipment,
or  enclosures for hazardous locations provides safety of the equipment.
Heat trace or steam trace lines are offered for sample lines for protecting
the lines in colder and/or humid environments.

Instrument Design For Safety—
     If the  instrument  is expected to work in a hazardous area, provide
explosion proof  or intrinsically safe equipment.  Check if there is any
other optional equipment available for safety reasons.

Material-
     Special material of construction for the sensor and other wetted parts
or  special protection well are offered for harsher (or corrosive) process
media applications.  Check if enclosure material also is compatible with the
environment.

Installation-
     Special mounting assemblies or fixtures to facilitate the ease of
installation.

FINAL INSTRUMENT SELECTION

     In evaluating different instruments, in addition to their design and
performance  specifications there are other factors that should be consid-
ered.  This  section identifies many of these factors and discusses the
importance of each for  consideration in  arriving at the most suitable
instrument for the desired measurement system.

Performance  of the Instrument

     Performance parameters desired of the measurement system usually are
not subject  to compromise.  Determine the performance needs of the process
                                     300

-------
and select the instrument which will deliver the required accuracy, main-
tainability and reliability.  Selection of instruments with a greater
performance than is necessary will make the instrument costs excessive.

Accuracy-
     Accuracy should be considered with respect to the prc :ess requirement.
Inaccuracy because of only static error is not critical to automatic con-
trol.  In order to enhance protection or the reliability of the sensor, a
lower dtjji ee of accuracy may be acceptable.  Accuracy figure^ In terms of a
percent of measurement rather than a percent of maximum span will provide a
better picture of the measurement system.

Sensitivity-
     Sensitivity is not very critical in most of the applications in the
wastewater treatment process, except in the laboratory analysis.  Sacri-
ficing sensitivity of the sensor in favor of better protection and reli-
ability is usually valid.

Repeatability—
     This is important in automatic control of a process and the designer
should strive to obtain the best available.

Drift-
     This is important as it affects repeatability for automatic control
systems.  If a high rate of drift is unavoidable, automatic  recalibration of
the system at regular intervals should be specified.  Signal locking should
be specified for continuous control if automatic recalibration is specified.

Warranty—
     The engineer must consider the length of warranty period and items and
work covered in the contract.  It should provide for replacement of all worn
and defective parts detected during the warranty period.  If it  is  a new
equipment item, check the availability of the instrument for trial  period
use on the actual process.

Acceptance Testing—
     The basis for evaluation of an instrument has to be stated  in order for
a supplier to determine if his equipment can  achieve the desired perform-
ance.  Who will conduct the test, where the test will be conducted, the
duration over which performance must be maintained and the action to be
taken in a pass or fail situation must all be clearly stated.

Maintainability

     This is the weakest link in the total measurement system; but if
properly considered and prepared for, it can  insure the long-term successful
operation of the system.

Uniformity and Consistency--
     Specifying -instruments that are of the same make or model as that of
the existing instruments offers several  advantages.  The inventory of spare


                                    301

-------
parts and spare units is greatly reduced.  It reduces the need for training
maintenance persons and operators for a variety of brands of instruments.
There is less possibility of error in calibration or operation.

Maintenance Record--
     Determine the need for maintenance for the instrument.  It may not be
available in the catalogs or the instruction manual.  Insist on obtaining
the information on the performance record with respect to maintenance
requirement.  This should include custodial and preventive maintenance
requirements, and frequency of failure.

Required Skill Level for Maintenance-
     Make sure properly trained personnel will be available to maintain the
instrument.

Easy Access for Maintenance-
     Discuss the accessibility of the instrument parts that need mainte-
nance.  Can it be serviced in its installed position?  Does it provide any
built-in aids for maintaining the instrument?

On-Line Servicing—
     If it can be serviced on line, is it equipped with signal locking pro-
vision for continuous operation of the control loop?  Is the signal
converter equipped with an indicator and necessary controls for on-line
calibration?

Spare Parts and Serviceability-
     Are the spare parts and service people available in the area where the
instrument will be used?  Check the time within which a service person can
be available for service.  Will the spare parts be available for the instru-
ment for its expected life span?

Safety - Instrument and Personnel

     Does the instrument need venting or purging for safe operation?  Check
the classification of the area of the instrument location and ascertain the
available instrument is designed for use in the area.  Does the instrument
design meet all the OSHA requirements and any other prevailing local and
insurance company safety codes?  Does the instrument require protection
against  lightning?  If the instrument system  is used for measuring a safety
(of personnel and equipment) level of the environment in a room or an area,
a  redundant sensor or measurement system should be provided.

Installation Consideration—
     The following points should be considered with respect to installation
of the instrument on the process.

Location-
     Compare the space available and location of the measurement point with
the  size of sensor assembly available.
                                    302

-------
Alignment Restriction-
     Check if there is any alignment requirement for the instrument and see
if that would create any problem.

Distance Between the Sensor and the Signal Transmitter-
     Check that the distance permissible between the sensor and the trans-
mitter meets the installation requirements.

Utility Requirement —
     Check requirements for electrical power, air, steam and water for the
instrument.  Are they available in the area?  Do they meet the voltage,
pressure or temperature requirements?  Is there any special grounding
requirement?

Weather Protection—
     If the instrument is located outdoors, check availability of weather-
proof  enclosure for the instrument and junction box for the electrical
connections.  Proper protection and/or auxiliary heating system should be
provided for cold weather protection.

Costs

     Cost of the instrument is one of the major criteria in selecting the
instrument.  One should, however, consider the total cost of the instrument
over its life span rather than just initial cost of the instrument.  For
comparison, the total annual cost of each instrument should be used.  The
total  cost of the instrument should include consideration of the following:

          Purchase price
          Installation (labor, parts and materials)
          Spare/Replacement parts
          Operation  (include utilities and expendable materials)
          Maintenance labor

Evaluation of Supplier

     Obtain first hand, written statements from the supplier about the
operation, maintenance and reliability of the instrument.

Qualifications  of the Supplier

     This can be obtained by surveying the users of the supplier's instru-
ment  and/or visiting  and observing different installations.  A user survey
would  reveal both good and bad points of the instrument and supplier's  oper-
 ation  and  reliability.

 IMPORTANCE OF  INSTRUMENTATION MAINTENANCE

      Instrumentation forms the foundation or base of  any control system.   If
                                     303

-------
automation is to succeed, the sensors must be kept in working condition.
When properly maintained, instrumentation can provide information to cen-
tralize operation decision making in a remote facility.  To accomplish this
requires implementation of an effective maintenance program to insure the
availability of reliable process information.

The Maintenance Commitment

     Wastewater streams characteristically contain high concentrations of
grease, scum and suspended solids; all of which can interfere with the per-
formance of an instrument.  Since wastewater in contact with precise equip-
ment can cause fouling in a short period, instrumentation has often been
turned off and labeled unworkable when it failed to operate without atten-
tion for extended periods of time.  For this reason, instruments have
developed a poor reputation in  the wastewater industry.  To overcome these
problems requires a  conscious commitment on the part of the instrument
specifier, manufacturer  and user.

     An  instrument specifier has a responsibility to select equipment which
 is  satisfactory for  the  application  and provides information which is neces-
sary for safety or process operation.  The specifier and user must maintain
communications relative  to the  frequency of maintenance required for each
 instrument and the availability of personnel to perform the maintenance.
Also,  the specifier  must provide provisions for maintenance in the installa-
tion details.

      Instrument manufacturers  are responsible for maintaining an available
 supply of expendable and replacement  spare parts and for furnishing useful
 instruction materials so a user can maintain the equipment.   In addition, it
 is  also a manufacturer's responsibility not to exaggerate claims of perform-
 ance and to  be  realistic in  frequency and time to perform periodic
maintenance.

      The user,  having agreed  to use  the  instrumentation, is  responsible for
making available  a  trained  staff  capable of performing  all required mainte-
 nance  and with  sufficient manpower so the tasks can  be  performed when
 required.  This  is  the key  to  successful instrument  application.  Therefore,
 the user must  commit to clean,  calibrate and/or repair  the instrumentation
 as  required.   The importance  of this  commitment  increases with increasing
 levels of automation.  Although automation can minimize the  labor required
 for operations,  there is a  corresponding shift  to maintenance to insure the
 instruments  are reporting reliable  information  for making  operation
 decisions.

 Levels of Maintenance

      Instrumentation failures can be obvious as when the signal  suddenly
 drives to its  maximum or minimum value due  to  a component  breakdown.   Other
 failures cause gradual degradation of the instrument's performance  either
 resulting from fouling of the sensor or changes in  calibration  because of
 age or environmental conditions.   The second class  of failures  is  difficult


                                      304

-------
to detect since the loss of accuracy and sensitivity is too subtle to be
picked up in normal operations.  An effective maintenance program will be
capable of detecting and responding to both classes of failures.  To
accomplish this, maintenance can be classified as follows:

          Custodial
          Periodic Calibration
          Repair

Each is independently definable, typically performed by different personnel,
and each  is equally important to overall system performance.

     Custodial maintenance can be defined as the routine cleaning and/or
observation of an instrument to assure validity of the information.  An
instrument which is in contact with the material being measured will tend to
become coated.  Therefore, routine cleaning must be performed.  Optical
instruments are a good example of this type of maintenance.  One can wait
for an optical instrument to degrade enough to show an obvious failure, but
long before, the instrument has been providing an incorrect reading.

     Keeping instruments clean and checking for reasonableness and variabil-
ity solves about half the problems.  In wastewater treatment plants, the
ambient temperatures constantly change on a seasonal and daily basis.  The
sensor will start to be coated and the transmitter will drift with time.
Periodic calibration must be performed to ensure successful operation.

     The third category of maintenance is identified as repair/replace.  It
is not uncommon for a sensor and/or a transmitter "or some supporting
component" to fail.  The instrument is there for a reason.  Therefore, when
the instrument fails, its repair and/or replacement is the highest prior-
ity.  This requires spare parts, spare instruments and the technical
expertise to perform the work.  Again, repair maintenance is crucial to the
system communication and security.

Maintenance Organization

     Organizing an instrument maintenance program is important and a team
responsible for maintenance must be formed.  This team should be comprised
of personnel from operations, laboratory and electrical/mechanical trades.
The staff operators should perform the cleaning and reasonability checks.
They should be assigned specific and detailed tasks to be performed regu-
larly on every instrument.

     The periodic calibration should be performed under this organization by
a chemist or analytical technician.  In most instances, periodic calibration
can be accomplished by a chemist without the assistance of an electrical/
mechanical maintenance person.  This should be one of the objectives in the
specification of the instrument.  The person responsible to calibrate a
sensor and possibly a transmitter should not be required to rewire or have
                                    305

-------
to disassemble a device.  There are definitely times when this is not
possible,  htowever, for the most part, a chemist should be able to handle
the calibration.

     Repair maintenance would normally be accomplished by the electrical/
mechanical maintenance technician.  Spare parts is the key here.  Failure of
a transmitter or sensor can normally be diagnosed at a workbench while the
failed unit has been replaced by a spare.

     It  is very important that the management of this team develop the
cooperation and constructive attitude which is necessary.  Feelings of not
knowing  one's responsibilities or work tasks or feelings of not being
comfortable in performing certain maintenance functions must be avoided.
Training  and hands-on supervised instruction is important.  This must be an
on-going  task.

     Once the team  is organized, the next crucial item is establishing a
routine;  a discipline.  Each member of the team should be given specific
tasks to  be accomplished on a daily, weekly or monthly basis.  A schedule
should be developed for each instrument type.  As a start, custodial or
reasonability type maintenance should be performed every day until operating
experience determines what frequency  is actually required.  Periodic cali-
bration  should be specified for each type of instrument.  As examples, a
chlorine  residual analyzer should be calibrated daily or a magnetic flow
meter should be calibrated monthly.  The same approach is used for other
types of  instruments.  Care must be taken to consider the application when
establishing a maintenance schedule.  As an example, in an intermittently
pumped primary sludge line the magnetic flow meter electrodes become coated
very rapidly.  Continuous or portable ultrasonic cleaning along with
frequent  routine calibration is necessary here.

     Documentation  of the three levels of preventive maintenance must be
provided.  Records must be kept and verified.  This will allow development
of a more directly  applicable schedule because history will show what the
actual requirements are.  The documentation should include operator initials
along with comments and observations.  A form should be developed which is
practical and useable.

     Communication  with operations during the performance of the routine
maintenance is crucial.  When an instrument is specified, the rule was that
it was necessary for reporting, process control or safety.  Therefore, that
sensor is important and cannot be arbitrarily inactivated.  The shift opera-
tors who  are performing operator maintenance and the chemists and techni-
cians who are performing repair/replace maintenance must first receive
authorization from  central operations.  The management organization of an
instrument maintenance program requires daily communication with opera-
tions.   They must be in the loop and they must authorize actions or tasks
which are scheduled.
                                     306

-------
     The job of management and of the preventive maintenance team  is to
establish and maintain an "attitude" of thinking preventive instrument main-
tenance as opposed to crisis maintenance.  The fact is that the performance
of their duties is crucial to the central operation of the plant.  A desire
to keep instruments and sensors operational must be nurtured.  A pride in
their work must be developed.  The team should be aware of the challenge to
have all instrumentation functioning.

Implementation of an Instrument Preventive Maintenance Program

     If the organization of the maintenance team is structured properly and
an air of cooperation established, then implementation of the program should
be fairly smooth.  Again, the key is direction and management.  One person
must be in charge and given full responsibility for all three types of
instrument maintenance.  Proper operation of the sensors and transmitters is
that person's responsibility.

     The goals and objectives of the preventive maintenance program should
be made crystal clear to all the team members.  They are:

          Maintain and verify the validity of sensor measurements.
          Minimize sensor downtime.
          Have 100% of system instrumentation operational 95% of the time.
          Establish close working relationships and cooperation between team
          disciplines.
          Establish a team spirit in carrying out the goals and objectives
          set forth above.

     Maintaining the discipline required for an instrument preventive main-
tenance program requires that routine tasks be performed, that they be veri-
fied by management and that they be discussed periodically.  Communication
must be clear.  The tasks must be specific and well documented on a "work
order" form.  The daily work orders  should call out tasks for each shift
operator, each analytical technician and each electrical/mechanical techni-
cians  The work order answers questions like what, who, where, when and how
to perform the tasks which are his responsibility.  Successful implementa-
tion is dependent on communication.   In order to develop a pride and desire
to perform the repetitive tasks outlined, management should illustrate the
importance of these tasks by reviewing them regularly.  The team must be
aware that someone is reviewing their work and taking action as a result of
their findings.
                                     307

-------
                               AMMONIA ANALYZER
OPERATING PRINCIPLE:  Electrochemical transducers (ion selective electrodes)
generate a millivolt potential when immersed in a conducting solution
containing free ions to which the electrodes are responsive.

     Primary Element:  Gas sensing electrodes which sense ammonia/aimionium
     present in the sample solution.
     Transmitter:  Measures the potential generated between the electrodes.

GENERAL SPECIFICATIONS:  The system includes a sample filtering system,
sample conditioning (pH adjustment) equipment, a temperature controller,
reagent storage and supply equipment, electrodes and the analyzer.

     Analyzer:  This will measure the millivolt potential between the
     electrodes, control the pH of the sample, control reagent flow and
     automatically recalibrate the instrument at regular intervals and dis-
     play the amount of drift.  The analyzer also controls the sample
     temperature at which the system is calibrated.
     Accuracy:  _+ 10% of actual concentration of ammonia.
     Repeatability:  + 5% of span.
     Response:Maximum 10 minutes to a step change in ammonia concentration.
     Rangeability:  0.1 to 3 ppm to 1 to 50 ppm.

INSTALLATION:  The analyzer is normally located indoors.

     Electrical:  115 VAC, 150 VA for the analyzer and heater.
     Mechanical:  If the process stream being monitored is not pressurized,
     a pumping system will be required to deliver a sample flow of a
     constant pressure and rate.  If the process stream is adequately
     pressurized, pressure and flow regulation equipment will be required in
     the sample line.

APPLICATION:  May be used on raw sewage or primary effluent to provide early
warning of high ammonia concentration in plant influent; however, raw sewage
application  is not recommended because of the need for a clean sample.  Also
used on final effluent for monitoring NPDES compliance.

     Environment:  Sample temperature operating range between 32 and 120°F
     (0 and  50°C) and ambient temperature 32 to 105°F (0 to 40°C).
     Humidity - 10 to 100% relative humidity.
                                     308

-------
MAINTENANCE:  Analyzers in use today have not demonstrated a high
reliability.

     Custodial:  Daily check sample system to insure proper operation.
     Weekly check calibration and operation of the analyzer and clean the
     filter and electrodes.
     Preventive:  Monthly check filter, check sample tubing, reagent
     solutions and standardization solution.  Every other month, replenish
     reagent and standardization solution, replace electrodes and service
     the temperature control system.
     Frequency of Failure:  Estimated 3 to 4 times per month.

ACCESSORIES/OPTIONS:  Output— ma, volts or output relay contacts.
Recorder.  Auxiliary strainer.
                                     309

-------
                          CHLORINE RESIDUAL ANALYZER
OPERATING PRINCIPLE:  Chlorine being a strong oxidizing agent,  depolarizes
one of the two electrodes in an amperometric cell.   This results in an
electric current in proportion to the concentration of oxidizing agent.

     Primary Element:  Two dissimilar metal electrodes that form the ampero-
     metric cell act as a primary element.  Located in the analyzer chamber.
     Analyzer:  A filtered and pretreated sample water (pH is adjusted)
     enters the cell area where it reacts with a reagent like potassium
     iodine.  Oxidation of the reagents results in a separation of iodine.
     This quantity  is measured by measuring the current through electrodes.

GENERAL SPECIFICATIONS:  Electrodes used could be gold, copper or other
noble metals.  A continuous cleaning system should be provided for the
electrodes.

     Rangeability:  0 to 1 through 0 to 20 mg/1 with several intermediate
     ranges.
     Accuracy:  +_ 2% of actual value.
     Sensitivity:   0.01 mg/1.
     Response Time:  10 seconds from the time the sample enters the analyzer.

     All chemicals  for pretreatment and electrolysis should be provided with
     the equipment.  Quantities provided should last at least 60 days.  The
     chemicals supplied should have at least 60 days shelf life or the plant
     chemist can order them as required per manufacturer's specifications.

INSTALLATION:  The  analyzer is enclosed in a free-standing enclosure.  A
water sample  is drawn from the main stream and delivered to the analyzer at
rate and pressure recommended by the manufacturer.  A self-cleaning system
is used to filter the sample before it reaches the analyzer.  All wetted
material should be  corrosion resistant.

     Electrical:  115 VAC for instruments and pumps, 150 VA approximately.

APPLICATION:  Applicable in chlorination process and measuring chlorine
level in the final  effluent.  Fouling of the copper electrode affects the
performance of the  electrodes.  Electrodes seem to lose sensitivity when
operating near residual chlorine concentration of close to zero.

     Environment:   32 to 120°F (0 to 50°C) when furnished with automatic
     temperature compensation.
                                     310

-------
MAINTENANCE:

     CustodialI:   Daily visual checks for leakage,  clogged lines and
     calibration.  Refill  the chemicals required every 60 days.
     Preventive:   Servicing pumps.   Replace filter and tubing every 30
     days.  Clean electrodes every 30 days.
     Frequency of Failure:  Estimated 6 to 10 times a year.

ACCESSORIES/OPTIONS:   Titration equipment for calibration.   Sample lines.
Heat trace lines.  Sample pumps.  Continuous motor driven filter.   Recor-
der.  Output— ma or  volts or output relay contacts.
                                    311

-------
                          DENSITY ANALYZER - NUCLEAR
OPERATING PRINCIPLE:  Gamma radiation is absorbed as it passes through
material.  The absorption increases proportionately with the increase in
density of the material.

     Primary Element:  It consists of a source of Ganma radiation and a
     radiation detector mounted on a process pipe or vessel in diametrically
     opposite location.  The radiations received by the radiation detector
     are measured.
     Transmitter:  The remotely located transmitter measures the changes in
     radiations received and converts it to appropriate output units.

GENERAL SPECIFICATIONS:  Two types of primary elements are available, one
for radial mounting for pipes up to 36" in diameter and for pipe with "Z"
configuration with distance between the source and detector up to 36".

     Rangeability:  0 to 15% density.
     Accuracy:  + 1% (average) of span in 5 to 15% range.
     Linearity:  _+ 1% of span.

INSTALLATION:  A technician licensed by the Nuclear Regulatory Commission
(NRC)  is required for the installation.  The instrument is available with
special mounting for pipe or tank mounting.  A glass spool is recommended
with pipe installation.  Installation on a vertical run is preferred, but
the sensors should be mounted in horizontal plane.  The location of the
measuring point should be selected to minimize entrained air bubbles.
Provide isolation valves, water taps and drain connection on each side of
the meter for calibration.

     Electrical;  Need 115 VAC for detector and the transmitter.  Signal
     cable between the transmitter and the primary element should be
     installed in a conduit.  Provide an output contact to indicate zero
     flow when there is no flow.

APPLICATION:  Best suited where density range is 5 to 15% total solids and
fouling of the sensor  is a problem.  Entrained air bubbles in the liquid
will significantly affect the performance.  A license from AEC is required
to acquire the sensor  and for installation and maintenance of the Gamma
source.

     Environment;  Gauge heads and instruments— -20 to 140°F (-30 to
     60°C) and 0 tcr 90% RH.  Gauge heads available in NEMA 4 enclosures.

MAINTENANCE:  Requires a licensed technician.  Check calibration every 30
days.  If feasible, zero every 30 days.  The Garnna source will need replace-
ment every two years.  Should perform a "wipe" test every six months on the
source enclosure.
                                    312

-------
     Frequency of Failure:   Estimated 2 to 3 times a year.

ACCESSORIES/OPTIONS:   Output - ma and volt output.  Automatic source decay
compensation.  Automatic temperature compensation.  Glass pipe spool  for
protection against fouling.

PRECAUTIONS:   The sensor should be handled only by a qualified NRC  licensed
technician.  In case  of an  accident, fire or explosion,  or  lost or  stolen
sensor, the user shall notify the regional NRC office.
                                     313

-------
                          DISSOLVED OXYGEN ANALYZER
OPERATING PRINCIPLE:  Instruments for measuring dissolved oxygen (DO) are
designed on different principles.  Galvonic Cell - Molecular oxygen diffuses
through a membrane and reacts with the lead/silver electrode system to
produce a current proportional to the DO concentration.  Polarographic Cell
- Oxygen diffuses through a membrane; after which, the oxygen is reduced by
a small reference voltage applied across two metal electrodes.   This cell
produces a current proportional to the DO concentration.

     Primary Element:  Galvonic cell uses lead/silver electrodes.  Polara-
     graphic cell uses noble metal electrodes with polarized voltage across
     them.
     Transmitter:  This amplifies the sensor signal and converts it to the
     desired output.  Most units are equipped with switch selectable ranges.

GENERAL SPECIFICATIONS:

     Size:  Probes are available for tank or large vessel mounting and for
     pipe mounting.
     Rangeability:  Available in different ranges from 0 to 1 through 0 to
     30 fng/1.
     Accuracy:  Specified accuracy varies from + 0.5% to + 2% of span.
     Drift:0.05 to 0.5% per degree C in range 0 to 50°C.
     Response Time:  Average 30 seconds for 99% of the actual value.  This
     is typical, however, it may vary depending on manufacturer.

INSTALLATION:  Sensor needs to be immersed at the point of measurement.
Special length probe for deep water mounting is available.  With in-line
applications, install the probe with a corporation stop for ease of removal
for maintenance.  Take care not to introduce air in transport.   Maximum
distance between the sensor and the transmitter varies from 250' to 1,000'.

     Mechanical:  A sensor is installed in a stilling well.
     Electrical;  Either dry battery cells or 115 VAC.

APPLICATION:  Fouling because of grease, biomass or suspended solids build
up is one of the biggest problems with this instrument.  Provision for
cleaning with each probe is frequently required.

     Environment:  Most of the sensors are designed for submersible
     operation.  Ambient temperature 32 to 120°F (0 to 50°C).
     Temperature compensation is required.
                                     314

-------
MAINTENANCE:

     Custodial;   As experience deems necessary,  clean  the probe membrane.
     This can be anywhere from once per day to  once per month.
     Preventive:  Monthly check calibration.
     Frequency~of Failure:  Estimated 6 to 10 times a  year.

ACCESSORIES/OPTIONS:  Output— ma or volt output.   Multiple  range  with
selector switch.  Temperature compensation system  with temperature indicator
on signal converter.  Remote calibration unit.   Ultrasonic probe cleaning.
Agitator type cleaners are available but not  recommended since  they often
tend to accumulate hair and string and can become  a maintenance problem in
themselves.
                                    315

-------
                               FLOW - MAGNETIC


OPERATING PRINCIPLE:  Faraday's Law of Induction.  An electromotive force
(EMF) is generated when a conductive material passes through a magnetic
field.  The magnitude of the EMF is proportional to the speed of the
conductor through the field.

     Primary Element:  A conductive material tube with a magnetic coil
     embedded in it.  Has two electrodes mounted 180° apart.  The tube is
     normally supplied with a liner of different material to suit the
     process liquid.
     Transmitter/Signal Converter:  Measures the EMF generated in the
     primary element and converts to an output signal compatible with other
     instruments.

GENERAL SPECIFICATIONS:

     Primary Element Size:  Less than 1" through 96" diameter and larger
     sizes on custom order.
     Rangeability:  Velocity 3 through 30 feet/second.
     Accuracy:+ 1% of rate within 10:1 flow range.

INSTALLATION:

     Orientation:  Orientation does not affect the performance as long as
     the tube is full of  liquid.  The most  desirable is to mount it in a
     vertical section of pipe with upflow.  A system for flow calibration of
     the meter should be  incorporated in the equipment and piping design.
     Mechanical;  Upstream  and downstream straight pipe runs are not as
     critical as in case of other types of meters, but consult the
     manufacturer about the affects of pipe configurations on meter
     performance.
     ElectricalI:  Wiring  required between \he primary element and the trans-
     mitter.  A~lso needs power wiring for the transmitter.  Power required
     is approximately 5 watts per inch of meter diameter.  The magnetic coil
     driver circuit assembly should be located within 30 feet of the primary
     element.
     Grounding:  Provide  grounding rings between the metering tube and the
     fluid  if the liner is  nonconductive or when an insulated pipeline in
     used.  Also ground it  to the plant grounding system such as a cold
     water pipe.  The grounding of the fluid and meter must be done regard-
     less of kind.of tube used.

APPLICATION:  Frequently  used in applications containing high solids
concentrations.

     Environment;  Primary  element - pressure 0 to 250 psig, temperature 0
     to 3000p (-20 to 150QC) with teflon lining.
                                     316

-------
MAINTENANCE:

     Custodial:  Recorrmend weekly check of transmitter calibration.  Inspect
     inner wall and electrodes every two months if the fluid measured
     contains high entrained solids or grease.
     Preventive:  Calibration every other week.  Clean the electrodes every
     two to three months.  Check and service wiring, including grounding,
     every two months.  Flush the metering tube whenever the flow is stopped.
     Frequency of Failure:  Estimated 2 to 3 times a year.

ACCESSORIES/OPTIONS:  ma, voltage or pulse output.  Electrode ultrasonic
cleaning.  Low flow cutoff.  Field range adjustability.  Reduced flange
meter.  Liner material and electrode material.  Removable electrodes.
                                    317

-------
                             FLOW - ORIFICE PLATE
OPERATING PRINCIPLE:  A fluid flowing through a known size and shape of
restriction creates a pressure drop across the restriction.   The pressure
drop is proportional to the square of the flow.

     Primary Element:  This is a flat metal plate with a circular hole bored
     through it.The thickness varies from 1/16" through 1/4" depending
     upon the pipe size.  The hole can be concentric, eccentric or
     segmented.  Standards for the fabrication of orifice plates have been
     established by ASME.
     Signal Converter:  It is a differential pressure transmitter that
     measures the pressure drop across the orifice.  Location of the
     pressure taps for the transmitter can be in the flanges or the pipe at
     distances up and downstream from plate as described by ASME standard.

GENERAL SPECIFICATIONS:

     Primary Element:  Need to specify maximum flow available in different
     types of material.
     Rangeability:  Can be used with 1/2" through 72" diameter pipe sizes.
     Velocity measured 0 to 0.2 feet per second through 0 to 18 feet per
     second.
     Accuracy:  For 3:1 flow range + 2% of full scale.  Output should be
     obtained in a straight line reTation.
                                                              ""-V..
INSTALLATION:  For concentric orifice plate it is important that the hole of
the orifice be concentric with the inside diameter of the flow pipe.
Flanged type orifice assembly will make this installation easier.  Upstream
and downstream straight pipe lengths are more critical with the orifice
plate than the venturi.  If adequate straight length is not available,
straightening vanes shall be provided.  Arrangement should be provided to
clean taps either manually or automatically.  Make sure cleaning does not
affect accuracy of measurement.  The taps should be installed with shutoff
valves for easy removal of the transmitter.  Use diaphragm seals with
corrosive fluids.

     Electrical:  Power required only for the pressure transmitter.

APPLICATION:  In wastewater processes it is suitable for measuring gas
flows.  Use concentric orifice for flows with low concentrations of
entrained solids.  With high concentrations of entrained solids, use
eccentric orifice with bottom of the hole flush with bottom of the pipe
inside diameter.  With gas flow measurement, temperature compensation should
be provided.
                                     318

-------
MAINTENANCE:  Checking of the orifice,  the pressure taps and the calibration
every 60 days.  If used with fluids with entrained solids,  cleaning the flow
tube and orifice every 60 days.

     Frequency of Failure:   Estimated 2 to 3  times a year.

ACCESSORIES/OPTIONS:   Straightening vanes.  Pressure tap cleaning  system.
                                    319

-------
                            FLOW - PARSHALL FLUMES
OPERATING PRINCIPLE:  Open channel flow, when passing through a known
convergence and constriction in the channel, will produce a hydraulic head
at a specific point, upstream of the constriction.  The flow is
approximately proportional to the three-halves power of the head.

     Primary Element:  It consists of a converging upstream section, a down-
     ward sloping throat and diverging downstream section.
     Transmitter:  This can be any of several liquid level measuring devices
     with its output characterized to flow.

GENERAL SPECIFICATIONS:

     Primary Element Size:  Available with throat width from 3" to 40'  and
     can measure flow up to 1500 mgd.
     Accuracy:  + 5% of span.

INSTALLATION:

     Mechanical:  The crest of the flume must have a smooth surface.  No
     protrusion in the flume should be permitted.  If a stilling well is
     required for level measuring, it should be connected at a point 2/3 of
     the converging wall length upstream from the crest.  The connecting
     pipe between the flume and still well should be at least 2" in diameter
     and be horizontal.  If a still well is used, provide flushing drains
     and connections.  If used with sludge or raw sewage, provide for
     continuous flushing of the well with clean water.

APPLICATION:  Suitable for open channel flows and measuring raw sewage
flow.  Parshall flume is suitable where the head loss permitted is limited.

MAINTENANCE:  Periodic checking and cleaning of the stilling well and
connecting pipes.

OPTIONS/ACCESSORIES:  Options are in the type of sensor used to develop the
flow signal.
                                     320

-------
                                 FLOW - SONIC
OPERATING PRINCIPLE:  The meter measures the speed of sound in the upstream
and downstream direction simultaneously.  The resulting difference is
proportional to the flow rate.

     Primary Element:  Two electroacoustic transducers are mounted diago-
     nally  (for pipe sizes 3" and larger) or axially (for pipe sizes 2" and
     smaller) in a flow tube.  The transducer faces must be parallel to each
     other  and wetted by the liquid being measured.  The sensors do not
     protrude in the flow.
     Transmitter:  Measures the difference between the speed of sound waves
     from the sensors in the upstream and downstream flow direction and
     translates it to an output proportional to flow.

GENERAL SPECIFICATIONS:

     Size:  Available for pipe sizes from 2" to 120".  Specify same pipe
     size for the transducer assembly as main flow pipe.
     Rangeability:  This depends on the pipe and type of fluid flow.  Works
     up to Reynolds number of 100,000.
     Accuracy:  Accuracy depends upon the range of operating Reynolds
     number.  At higher Reynolds number, +_ 1% of reading is possible.
     Repeatability:  As high as 0.25% rate is possible.

INSTALLATION:  Should be located a minimum of 5 pipe diameters downstream
and 2 pipe diameters upstream from any deviation in straight pipe.   Certain
pipe configurations may require greater straight pipe runs.  The transducers
should be mounted in horizontal plane.

     Electrical:  10 watts at 120 VAC for both the sensor and meter.

APPLICATION:  For monitoring of liquid flows where air bubbles are  not
present and solids are not expected to exceed 4%.

     Environment:  Sensor— pressure limit up to 1000 psig.  Transducer—
     ambient temperature -100 to 300°F (-70 to 150°C).   Transmittei—
     ambient temperature 20 to 140°F (-10 to 60°C).

MAINTENANCE:

     Preventive:  Cleaning transmitter of dust and moisture and cleaning of
     electrodes every 60 days by flushing with high pressure steam  or water.
     Custodial:  Checking of calibration every 60  days.
     Frequency of Failure:   Estimated two to three times a year.

ACCESSORIES/OPTIONS:  Output—  ma and volt signal  and relay contacts.
Removable sensor without stopping the process.   Sensor  flushing ports.
Heater in the transmitter for cold weather operations.   Totalizer system.
                                    321

-------
                                FLOW - TURBINE
OPERATING PRINCIPLE:  A small turbine wheel located in the fluid flow spins
on its axes at a speed proportional to the velocity of the fluid.

     Primary Element:  A small turbine mounted on frictionless bearing is
     inserted in the pipe.
     Signal Converter:  Measures the turbine speed by using magnetic pickup
     which generates magnetic pulses proportional to the turbine speed.

GENERAL SPECIFICATIONS:  Each meter head and transmitter shall be in NEMA 4
enclosure.  The turbine shall be all 316SS with completely sealed bearings.
It shall be a completely balanced turbine.

     Rangeability:  1 to 45 feet/second.
     Accuracy:  + 2% of the span.
     Repeatability:  + 1% of the span.

INSTALLATION:  Shall be installed with means for easy removal and without
shutting the process off.  Shall provide a NEMA 4 junction box for electri-
cal connections.  Point of Ixation of the turbine shall be such to sense
the average velocity.  Install in a straight run or with straightening vanes,

     Electrical:  115 VAC, or 12 or 24 VDC.

APPLICATION:  This type of instrument is normally suitable for only clean
fluids flow streams.  Typical application is measuring effluent flow.
Turbine offers some pressure drop.  Entrained solids foul the element and
may deteriorate bearing's performance.

     Environment:  -20 to +150°F (-30 to 65°C).  Pressure up to 250 psig.

MAINTENANCE:

     Preventive:  Quarterly check the calibration.
     Frequency~of Failure;  Estimated once per year.

ACCESSORIES/OPTIONS:  Output— ma and volts or relay contacts.  High tem-
perature bearings.  Amplifier mounted in the turbine pipe.  Purged bearings.
                                     322

-------
                             FLOW - VENTURI TUBE
OPERATING PRINCIPLE:  A fluid flowing through a tube section containing a
convergence and constriction of known shape and area causes a pressure drop
at the constriction area.  The pressure drop is proportional to the square
of the flow.

     Primary Element:  It is a flow tube designed in four sections; main
     barrel section, entrance cone, throat section and discharge cone.  The
     main barrel section is of the same diameter as the main pipe inside
     diameter.  The entrance section converges from the barrel diameter to
     the throat section.  The throat section connects to the discharge
     section that diverges to the inside diameter of the main flow pipe.
     Taps are provided at the barrel section and at the throat to measure
     the pressure drop.
     Signal Transmitter:  It is a differential pressure transmitter that
     measures the pressure drop in the venturi.  The output is a squared
     function of the flow in the venturi.

GENERAL SPECIFICATIONS:

     Rangeability:  Available in 0 to 1 feet/second through 0 to 30 feet/
     second and can be used with pipe sizes from 3" through 48".
     Accuracy:  + 1.0% of span within 3:1  range.
     Repeatability:  + 1.0% of span.
     Output:Should ¥e obtained in a straight line relation.  It shall be
     designed to permit field calibration  by a gauge or a manometer.

INSTALLATION:   The venturi tube is available in different mounting designs;
flanged mounting type,  welded ends type or insert type.

     Mechanical:  Can be mounted in any orientation as long as the pipe
     remains full of liquid.  An arrangement should be provided for flushing
     the pressure taps either continuously or intermittently to prevent them
     from clogging.  Pressure leads to the transmitter should be installed
     with shutoff valves to facilitate removal  of the transmitter.  Accuracy
     of the meter can be greatly affected  by the up and downstream piping.
     A rule of thumb is to provide a minimum straight pipe from the orifice
     of 5 pipe diameters upstream and 2 pipe diameters downstream.

APPLICATION:  Used for  measurement of liquid flows.   The ratio of maximum to
minimum flow anticipated for an application should not exceed 3.  This is
necessary because the accuracy at low flow rates is  very poor.

     Environment:  Environmental constraints are limited to those of the
     differential pressure transmitter connected to  the pressure taps.
                                   323

-------
MAINTENANCE:

     Custodial:  At a frequency determined by the application and
     experience, rod out or flush the pressure sensing taps.
     Preventive:  Quarterly calibrate the differential pressure transmitter.
     Frequency~bf Failure:  Estimated 2 to 3 times a year.

ACCESSORIES/OPTIONS:  Special material of construction or linings.  Pressure
tap cleaning system manual or automatic.  Straightening vanes.
                                     324

-------
                             LEVEL - BUBBLER TYPE
OPERATING PRINCIPLE:  The hydrostatic pressure exerted by any fluid in an
open tank is directly proportional to the height and density of the fluid
above the point of measurement.

     Primary Element:  A constant flow of air is passed through a bubble
     tube inserted in the liquid.
     Signal Transmitter:  The change in pressure to maintain flow in the
     line that supplies air to the bubbler tube is measured.  The change in
     pressure is proportional to the liquid level above the bottom (zero
     level) of the bubble tube.

GENERAL SPECIFICATIONS:

     Primary Element:  Approximately 1/2" diameter tubing mounted in a rigid
     stand pipe.  Material of construction shall be corrosion resistant and
     suitable for the fluid.
     Rangeability:  The maximum level measurement depends on the pressure of
     the supply air.  Up to 185'  (with 80 psi  air) is available.
     Accuracy:  + 1% of actual head.
     Repeatability, Drift and Linearity:   Depend on the differential
     pressure transmitter used.
                                             t
INSTALLATION:

     Mechanical;  The bubble tube should be mounted in a rigid pipe a few
     inches longer than the maximum level measurement desired,  and at least
     3" above the bottom of the tank.  Provide either automatic or manual
     purging system for cleaning  the bubbler tube.  The air supply should be
     equipped with a filter, purge rotameter and a differential  pressure
     transmitter installed following the rotameter to sense a change  in
     level.  The purge lines to the bubbler should not be so small or long
     so that pressure losses in the line create errors in the measured level.

APPLICATION:  Suitable for corrosive liquids,  viscous liquids or liquids
with entrained solids.

     Environment:  Same as for the differential pressure transmitter.

MAINTENANCE:

     Custodial:  Calibration check and check for proper operation of air
     supply every 30 days.
     Preventive:   Check air  filter and calibrate every 60 days.
     Frequency~of Failure:  Estimated 2 to 3 times a year.

ACCESSORIES/OPTIONS:  Special  corrosion resistant material  for  the stand
pipe and bubbler tube.
                                    325

-------
                           LEVEL - CAPACITANCE TYPE
OPERATING PRINCIPLE:  The capacitance of a suitable electrical  capacitance
sensing element varies with the depth of the material in which  it is
immersed.  The electrical measurement of the capacitance will provide a
direct reading of level once it is calibrated and correlation of the depth
and the liquid surface level is determined.

     Primary Element:  It is composed of the probe head (outer  conductor),
     the electrode  (inner conductor) and insulation to electrically insulate
     the head from the electrode.
     Signal Transmitter/Converter:  The transmitter, when supplied with
     constant voltage power, will provide an output proportional to level.

GENERAL SPECIFICATIONS:

     Rangeability:  0 through 60 feet depending on probe length.  The kind
     of fluid should be specified for getting correct probe.  Probes are
     available in rod or cable form.
     Accuracy:  +; 1% of span.
     Repeatability:  +_ 2% of span with constant fluid properties.

INSTALLATION:

     Mechanical:  Both rod and cable type probes are available  with standard
     NPT threads and can be directly mounted in the tank top or with a
     bracket with a mating plug.  Vertical mounting is recommended.  With
     cable probe, minimum distance between the tank wall and the probe must
     be about 10% of the probe length.
     Electrical:  An electronic assembly is located in the probe head.
     Power to the head is supplied through the transmitter.  The probe and
     the transmitter can be separated up to 1,000 feet distance.

APPLICATION:  Suitable for different kinds of fluids like acids, slurries,
lime when a probe of proper material is available.  Fluids must be
electrically conductive.  Foam on the top of the liquid or bubbles in the
liquid will also affect accuracy.

MAINTENANCE:

     Preventive:  Monthly inspect, clean and calibrate.
     Frequency~of Failure:  Estimated 2 to 3 times a year.

ACCESSORIES/OPTIONS:  Output— ma or voltage or relay output contacts.  Time
delay output contacts.  Deadband function over measuring range.
                                     326

-------
                            LEVEL - DIAPHRAGM TYPE
OPERATING PRINCIPLE:  Static pressure exerted by the liquid is directly pro-
portional to the height of the liquid above the point of measurement.  A
diaphragm in contact with the fluid is flexed by the pressure.  The amount
of movement is sensed and calibrated to level.

     Primary Element/Transmitter:  An integrated pressure sensing assembly
     with the high pressure side of the diaphragm in direct contact with the
     process fluid.

GENERAL SPECIFICATIONS:

     Primary Element:  Available with different mounting flanges with
     different pressure rating and flange size.
     Rangeability;  0 through 750" of ^0 and -225 to +225" of HoO.
     Accuracy:  + 0.5% of span for ranges 20" through 750"  of ^0.
     Repeatability:  0.1% of span.

INSTALLATION:   Available with different length of extension lengths to fit
different wall sizes.  The diaphargm shall be mounted flush with the tank
inside wall.  Provide proper size vent and drain valves in  the low pressure
side of the primary element.  If the tank is covered and not vented to
atmosphere, the low pressure side has to be connected to the tank top.

     Electrical:  24-48 VDC.

APPLICATION:  For measurement of level in tanks constructed above ground.

     Environment:  Process— -40 to 350°F (-40 to 180°C).   Ambient— 0
     to 130°F (-20 to 50°C).

MAINTENANCE:

     Custodial;  Check the calibration, piping connection for  leak and the
     diaphragm for fouling every 60 days.
     Preventive:  Calibrate and clean  the diaphragm every 60 days.   Check
     electrical connections.
     Frequency of Failure:  Estimated 3 to 4 times a year.

ACCESSORIES/OPTIONS:  Integral  range elevation and suppression adjustment.
Flush or extended diaphragm.  Different diaphragm material  to  suit the fluid
characteristics.  Available as  a pneumatic instrument.
                                    327

-------
                           LEVEL - SONIC/ULTRASONIC


OPERATING PRINCIPLE:  Electrical impulses emitted from a sonic transmitter
are reflected back from the liquid air interface.  They travel at a fixed
velocity and the time required to sense the reflected signal is proportional
to the surface distance from the transmitter.

     Primary Element:  This includes a transducer that sends and receives
     the sound waves.  The enclosure is fitted with appropriate reflector to
     direct the sound waves in desired direction.
     Signal Transmitter:  The transmitter sends sonic signals to the primary
     element and detects the echo received back from the liquid surface
     through the primary element.  Sonic transmits at frequencies between
     16,000 and 20,000 Hz and ultrasonic operates above 20,000 Hz.  The
     transmitter times the period between sending and receiving the signal
     and outputs a signal proportional to level.

GENERAL SPECIFICATIONS:

     Primary Element:  Available for different environments and to be used
lary
iffe
     in different physical constraints.  Shall be supplied with integrally
     mounted temperature sensor for temperature compensation.
     Rangeability:  Available from 0 through 160 feet in different
     intermediate ranges.
     Accuracy:  + 1% of the span.  Better accuracies possible with
     controlled Temperature.
     Repeatability:  + 0.1% of span.
     Drift:  Maximum 0~.1% for a period of six months with temperature
     compensation.

 INSTALLATION:

     Mechanical:  Follow manufacturer's recommendation for the minimum sepa-
     r at ion of the transducer and the maximum expected liquid level.  The
     transducer can be mounted close to tank wall except where the tank side
     is tapered.  If mounted outdoors, the path of signal travel should be
     protected from wind.  Transducers are available with different mounting
     fixtures.
     Electrical:  Two cables required; one for the temperature probe and the
     other for the transducer.  Power for the transmitter is 115 VAC.

APPLICATION:  A noncontacting sensor for measuring liquid level.  Specify
 special material and type of enclosure for corrosive and outdoor
environment.  Ultrasonic's signal strength is substantially lower than the
 sonic's signal; therefore, ultrasonic is used for limited range of level.
Foam accumulation on a liquid surface may cause erroneous level readings.
                                     328

-------
     Environment:  Temperature 30 to 110°F (0 to 40°C)  for transmitter,
     -20 to 180°F (-30 to 80°C) for sensor with temperature compensa-
     tion.  Pressure— sensor + 0.5 psi above/below atmospheric pressure.
     Ultrasonic is more prone to dust,  humidity and wind interference than
     sonic units.

MAINTENANCE:

     Preventive:  Every other month check calibration and temperature
     compensation.
     Frequency of Failure:  Estimated 2 to 3 times  a year.

ACCESSORIES/OPTIONS:   Output— ma,  volts or contact closures.   Signal
locking.  Explosion proof enclosure.
                                     329

-------
                LOWER EXPLOSIVE LIMIT (LEL) DETECTOR/ANALYZER


OPERATING PRINCIPLE:  Combustible gas/air mixture, diffusing through a flame
arrestor, oxidizes on a catalytically treated hot sensing element.  Combus-
tible gas in the mixture burns at the element, thus raising its temperature
and electrical resistance.

     Primary Element:  The sensor is an active low temperature catalytic
     element arranged in one leg of the wheatstone bridge circuit.
     Analyzer;  It measures the change in resistance of the primary element
     by balancing the wheatstone bridge circuit.  The measure of the change
     in the resistance is converted to appropriate units for explosive gas
     concentration.

GENERAL SPECIFICATIONS:

     Rangeability:  0 to 100% LEL.  Adjustable relay output contacts
     normally set to operate at 25% LEL and 50% LEL.
     Accuracy;  + 2%.
     Minimum Detection:  5% LEL.
     Drift;Less than 0.5% over a period of 30 days.

INSTALLATION:  Locate the sensor in a location where combustible gas or
vapor is likely to concentrate.  If the sensor is located remote from the
point where it is desired to check for an explosive condition, a sample
system will be required.  Use a pump for drawing sample if the stream
pressure is not high enough to provide proper flow.  If required, install
heat trace line to protect the sample line from freezing and condensibles.
Use explosion proof conduits and junction boxes for wiring.  A gas drying
system may be required if the sample moisture content is high.

     Electrical:  115 VAC for the analyzer.

APPLICATION:  Used to detect an accumulation of combustibles and provide a
warning of an impending explosive condition.

     Environment:  0 to 200°F (-20 to 100°C) sensor.  Moisture in the
     sample deteriorates the sensor performance and can cause damage
     necessitating replacement of the sensing element.
                                     330

-------
MAINTENANCE:

     Custodial:  If a sample system is used, weekly check and clean as
     required.
     Preventive:  Monthly inspect sensor and calibrate analyzer using a
     cylinder of known gas sample.  Service sample pumps and moisture
     removal system if used.
     Frequency of Failure:  Estimated 3 to 4 times a year.

ACCESSORIES/OPTIONS:  Sample withdrawal system normally is  custom made.
Explosion proof enclosures.   Cylinders of gas samples containing a known
type and concentration of combustible material.
                                     331

-------
                               OXYGEN ANALYZER


OPERATING PRINCIPLE:  Different oxygen concentration in a strong nonuniform
magnetic environment will exert variable forces on a nitrogen filled dumb-
bell suspended in the environment.

     Transmitter:  The transmitter converts the forces exerted on the dumb-
     bell by measuring the resultant movement.  It uses optical and
     electronic equipment to convert the motion to electrical signal.
     Sample:  Normally a sampling system is required including a sampling
     probe, piping, flow controller and moisture removing equipment to
     provide dry sample of gas to the heated analyzer.  Analyzer temperature
     is controlled.

GENERAL SPECIFICATIONS:

     Rangeability;  Available in selectable ranges from 0 to 100% of
     oxygen.Individual range is switch selectable.
     Accuracy:  +_ 1% of full scale.
     Repeatability:  + 0.05% oxygen of full scale.
     Response Time:  TO seconds after the sample reaches analyzer.

INSTALLATION:  Locate the sampling probe at the measuring point.  Piping
between the probe and the analyzer should be protected against cold weather
and also from condensibles in the pipe.  Sample line length should be kept
to a minimum and should be made of 316SS.  A pump may be required in the
sampling system if the process stream is not at an elevated pressure.
Provide zero flow indication when there is no sample supplied to the
analyzer.

     Electrical:  Need 115 VAC connection to the analyzer.  Consumes about
     200 watts.
     Sample:  The sample should be delivered at specified pressure and flow
     rate.  Temperature of the sample flow can vary between 0 and 120°F
     (-20 and 50°C).

APPLICATION:  Can be used in measuring oxygen in vent gases from covered
oxygen aeration tanks.

     Environment:  Ambient temperature 32 to 120°F  (0 to 50°C).
     Humidity 0 to 90% RH.
                                    332

-------
MAINTENANCE:

     Custodial:  Daily check sample flow to analyzer.
     Preventivie:  Monthly check calibration with pure nitrogen and clean  dry
     instrument air.  Every other month check and clean sample system and
     check analyzer temperature control system.
     Frequency of Failure:  Estimated at twice per year.

ACCESSORIES/OPTIONS:  Output - ma or voltage or output relay contacts.  Heat
tracing for sample system.
                                    333

-------
                                 pH ANALYZER


OPERATING PRINCIPLE:  A potential is developed by an electro-chemical cell
when placed in a solution.  The potential varies with the hydrogen ion
concentration in the solution.

     Primary Element;  A glass pH electrode and a standard pH electrode,
     both filled with electrolyte form a pH probe.  These two electrodes
     along with a temperature sensor are assembled in a molded ceramic or
     plastic body.  Some probes use two glass electrodes and a reference
     electrode.  Some probes include a preamplifier to amplify the low level
     signal for noise immunity and for transmitting the signal for longer
     distance.
     Transmitter:  Measures the input signal from the probe and converts it
     to a desired output.

GENERAL SPECIFICATIONS:

     Primary Element:  This is available for both flow-through and immersion
     type configurations.
     Rangeability:  0 to 14 pH.
     Sensitivity?  0.001 pH.
     Repeatability:  + 0.02 pH.
     Nonlinearity;  0.1% of full scale.
     Response Time:  1 second.

INSTALLATION:

     Orientation:  Should be mounted vertically with the electrodes at the
     bottom.Flow-through sensors are available with standard threaded fit-
     tings.  A valve should be used with the sensor in flow-through
     application to facilitate the sensor removal for cleaning and repair.
     Immersion type sensor will need special brackets for mounting.  Care
     should be taken to protect the glass electrodes.
     Electrical;  Need 115 VAC for the transmitter.
     MechanicaT:  Isolate it from vibration environment whenever possible.

APPLICATION:  Used for pH control of plant influents and used extensively in
physical/chemical plants.

     Environment:  Probe-- 20 to 150°F (-10 to 65°C), 0 to 100 psi.
     Transmitter— automatic temperature compensation to 120°F (50°C).
                                     334

-------
MAINTENANCE:

     Custodial:  Weekly check calibration using a laboratory calibrated
     portable probe.  Visually inspect probe signal  cable for signs of
     damage or wear.  Every other week check and clean the electrodes.
     Preventive:  Once per month calibrate unit with known buffer solution.
     Quarterly it is recommended to clean the electrodes with 100% HCL.
     Depending on manufacturer,  recharge electrodes  every six months to two
     years.
     Frequency of Failure:  Estimated 3 to 4 times  per year.

ACCESSORIES/OPTIONS:  Output— ma and volt signals,  output relay contacts.
Digital indicator.  Range expander up to 10% of the  scale for better
accuracy.  Electrode ultrasonic  cleaner.
                                    335

-------
                  PRESSURE TRANSMITTER - ELECTRO-MECHANICAL
OPERATING PRINCIPLE:  Application of pressure on a diaphragm causes a
deflection which creates a change in the values of electrical components
(such as a capacitor, strain gauge, or inductor) attached to the diaphragm.

     Primary Element:  The diaphragms are mechanically connected with one of
     the electrical components.  The process pressure is transmitted to
     either one or both sides of the diaphragm through a fluid media.
     Transmitter:  Measures the changes in electrical component and converts
     into desired pressure units.

GENERAL SPECIFICATIONS:

     Rangeability:  Available in several ranges from 0 to 0.5 psi through 0
     to 1000 psig.  Also has range down to -200" of
     Accuracy:  Typical +-0.5% of span (includes linearity, hysterisis and
     repeatability).
     Dead band:  0.05% of span.
     Materials:  Wetted parts typically 316SS.  Other materials available as
     an option.

INSTALLATION:

     Mechani cal :  The transmitter should be located as close as possible to
     the measurement point.  Slope of the pressure tap lines should be
     suitable for the application.  Install shutoff valve and drains in all
     pressure leads.  Applications subject to clogging should have
     provisions for line flushing.  For vacuum applications, use a minimum
     number  of joints or valves to reduce the possibility of leaks.  Provide
     heat trace line if the transmitter location is exposed to freezing
     temperatures.
     Orientation:  Dependent on whether the monitored fluid is a gas, liquid
     or condensing vapor.
     Electrical :  Uses standard voltage DC power.  It uses two-wire signal
     transmission, and voltage depends on the load resistance.  Typical
     voltage used is 24 VDC.

APPLICATION:  Used for pressure, level and differential pressure flow
monitoring.

     Environment:  Most will operate normally in 0 to 150°F (-20 to
     65°C) and 0 to 95% relative humidity.  Check temperature effect on
     zero and span shift.
                                     336

-------
MAINTENANCE:  Reconmend calibration check  every 3 months.

     Frequency of Failure:   Estimated 3 to 4 times a year.

ACCESSORIES/OPTIONS:  Special materials for wetted parts.   Valve  manifold
for differential pressure application.   Kits for elevated  and  suppressed
zero ranges.  Diaphragm seals for  applications  involving corrosive,  viscous
or solids bearing fluids.  Absolute pressure calibration.   Locally mounted
indicator.
                                    337

-------
                           PRESSURE - FORCE BALANCE
OPERATING PRINCIPLE:  Applied pressures on two sides of the diaphragm is
measured by applying a balancing force through mechanical linkages.

     Primary Element:  The diaphragm to which the monitored pressure is
     applied is connected to the balancing device through mechanical
     linkages.  The magnitude of the balancing force represents the pressure
     value.
     Transmitter/Signal Converter:  Measures the balancing force through a
     transducer and converts it to desired output units.  Converter and
     primary element are an integral assembly.

GENERAL SPECIFICATIONS:

     Rangeability:  Available in several ranges from 0 to 0.5 psi through 0
     to 1000 psi and -200" of H^ to 0" of 1^0.
     Accuracy:  + 1% of span (includes linearity, hysterisis and repeat-
     ability).
     Dead Band:  + Q.1% of span.
     Materials':  Wetted parts typically 316SS.  Other materials available as
     an option.

INSTALLATION:  Same as pressure transmitter-electro-mechanical.

APPLICATION:  This type of transmitter is more suitable where the transmit-
ter is exposed to electrical noise or extreme temperature and humidity.

     Environment:  Ambient temperature -20 to 180°F (-30 to 80°C).

MAINTENANCE:  Check for leakage in the lines every month.  Recommend cali-
bration check every 3 months.  Check damping fluid and sealing fluid for
loss every 3 months.  Every 6 months check the linkages, the force motor and
balancing force assembly for wear and proper operation.

     Frequency of Failure:  Estimated 3 to 4 times a year.

ACCESSORIES/OPTIONS:  Same as pressure transmitter-electro-mechanical.  Also
available with pneumatic output.
                                     338

-------
                     SUSPENDED SOLIDS ANALYZER - OPTICAL
OPERATING PRINCIPLE:  Scattering of a beam of light by suspended particles.
The scattered light is proportional to the suspended solids.

     Primary Element:  It consists of a light source and photocells mounted
     in transparent wall sample chamber.
     Transmitter/Signal Converter:  This measures the amount of light
     received by the photocells when the sample fluid is in the chamber and
     compares it with the light received by the photocell through a sample
     of known suspended solids concentration.

GENERAL SPECIFICATIONS:

     Primary Sensor:  This is available in both pipe and tank mounted
     versions.
     Rangeability:  Available in different ranges up to 0-30,000 mg/1.
     Accuracy:  j[ 2% of full scale.

     The output will have to be averaged over a period of time.

INSTALLATION:  For tank mounting, different lengths of probes are available
with necessary mounting brackets.  The sensor should be submerged at least
12".  Sensor should be mounted where the fluid does not contain air
bubbles.  It is recomnended that the sensor be mounted at 15° incline from
the vertical axis.  Pipe mounted sensors should be installed with a
corporation stop for easy removal of sensor without shutdown of the
process.  Upstream pipe configurations should be examined.   Care should be
taken to use sample from stabilized process stream.

     Electrical:  115 VAC is needed for signal converter.
     Utility:Shall provide liquid sample tubes filled with liquids of
    .known suspended solids concentration for calibration.

APPLICATION:  Fouling and coating of the sensor affects the operation and
maintenance of the instrument greatly.  Sensor should be supplied with some
kind of continuous cleaning of the sensor for any wastewater application or
an arrangement should be made to wipe the optical surface periodically.

     Environment:  Ambient 0 to 120<>F (-20 to 50°C), 0 to 90% RH.
                                    339

-------
MAINTENANCE:

     Custodial:  Calibration check using a known sample once per week.
     Preventive:  The sensor should be withdrawn, inspected and cleaned
     every two months.  Moving parts and seals should be inspected for  wear
     and replaced if needed every two months.  The light source and the
     detector should be checked every two months.
     Frequency of Failure:  Estimated 3 to 4 times a year.

ACCESSORIES/OPTIONS:  Output— ma and volt and output relay contacts.   Local
readout.  NEMA 4 or other special enclosure.  Special length of probe.
Aeration baffles.
                                      340

-------
                  TEMPERATURE - RESISTANCE THERMOMETER (RTD)


OPERATING PRINCIPLE:  Change in electrical resistance of metal due to a
change in temperature.

     Primary Element:  It is a resistance wire wound in form of a coil and
     potted in a ceramic or other type insulating material.
     Transmitter/Signal Converter:  This uses a wheatstone bridge type cir-
     cuitry to measure the change in resistance because of the temperature
     change.

GENERAL SPECIFICATIONS:

     Primary Element:  Two widely used resistance metals are nickel and
     platinum.  Use three lead wire RTD's.
     Rangeability:  0 to 400°F or 0 to 2000°F (0 to 200°C or 0 to
     1000°C), depending on materials of construction.
     Accuracy:  + 0.5°F or better in the range.
     ReproducibiTity:  +_ 0.25°F in the range.

INSTALLATION:  Install in metal thermowell.  Precautions should be taken to
isolate the sensor from vibration.  Immersion length for the RTD will  vary
with application.  Available in different lengths.   Use copper conductor for
lead wires.

     Electrical:  The signal converters require a DC or 120 VAC external
     power source.

APPLICATION:  Specify thermowell  material as required by the process media.
Grease or other sticky substance  will  coat the thermowell.  This will
increase the system response time.

     Environment:  The signal converter will  operate normally in 32 to
     120°F (0 to 50°C) and 0 to 90% relative humidity.

MAINTENANCE:

     Custodial:  Check the signal converter calibration every 30 days.
     Frequency of Failure:  Estimated  once per year.

ACCESSORIES/OPTIONS:  Signal converter output— ma  or voltage.   Open circuit
or sensor failure detection.  RTD— materials of construction.   Thermowell —
size, material, termination head  and extensions.
                                    341

-------
                          TEMPERATURE - THERMOCOUPLE
OPERATING PRINCIPLE:  Seebeck's principle of thermoelectricity.

     Primary Element:  It can be one of the several combinations of pairs of
     dissimilar metal conductors welded together at the sensing end.
     Transmitter/Signal Converter:  Convert the millivolt signal generated
     by the thermocouple junction to a high level output.

GENERAL SPECIFICATIONS:

     Primary Element:  Commonly used thermocouples are constructed of
     chromel-alumel, iron-constantan, and copper-constantan.
     Rangeability:  Type J - Iron-Constantan 0 to 1200°F (-20 to 650°C)
                    Type K - Chromel-Alumel 0 to 2300°F (-20 to 1250°C)
                    Type T - Copper-Constantan -300 to +200°F (-160 to
                             100°C)
     Accuracy:  + 1% of NGE, dependent on type selected.
     Response Time:  1 second to 3 minutes for step change, depending on
     thermocouple construction.

INSTALLATION:  It is recommended that the thermocouple be installed in a
thermowell.  The thermocouple should be insulated with either ceramic or
magnesium oxide.  An immersion length of approximately ten times the
diameter of the thermowell is recommended.  Signal extension wires have to
be of the same material used for the thermocouple.

     Electrical:  The signal transmitter will require an external DC or 120
     VAC power source.

APPLICATION:  Used primarily where remote temperature indication, recording
or control  is required.

     Environment:  The signal converter will operate normally in 32 to
     140°F.

MAINTENANCE:

     Custodial:  Recommend calibration check of signal converter once per
     month  when used.
     Frequency of Failure:  Estimated once per year.

ACCESSORIES/OPTIONS:  Thermocouple construction— exposed junction, grounded
with metallic sheath or ungrounded with metallic sheath.  Thermowell— size,
material, termination head and extensions.  Signal converter output— ma or
voltage signal or alarm relay contacts with I/O isolation.  Open circuit or
sensor failure indication.
                                     342

-------
                   TOTAL ORGANIC CARBON ANALYZER - ON LINE
OPERATING PRINCIPLE:  Combustion and infrared detection.

     Analyzer:  The total organic carbon analyzer consists of a sampling
     module, inorganic removal section, sample injection module, combustion
     section, C02 analyzer and carbon analyzer.  The sampling module
     receives the raw sample and passes on predetermined amount to the
     inorganic removal chamber and overflows the sample excess.  A high
     efficiency sparger uses air to strip inorganic carbon from the measured
     sample.  The sample with organic carbon is injected into the combustion
     chamber, is oxidized and the organic carbon converted to 002-  A
     C02 analyzer, through infrared detection technique, determines the
     organic carbon present in the form of C02 and provides a proportional
     output signal.
     Enclosure:   All the modules described in the analyzer section are
     enclosed in one enclosure.  Normally designed for indoor application
     but can be obtained in NEMA 4 construction if required.

GENERAL SPECIFICATIONS:  The system should be able to use a sample from
pressurized or nonpressurized streams.

     Rangeability:  0 to 5,000 mg/1 and many intermediate ranges available.
     Sensitivity;  Minimum detection 1.0 mg/1.
     Accuracy:  + 5% of the span.
     Repeatability:  +_ 26 of the span.
     Linearity;   ~+_ 5% of the span.
     Cycle Time;  0 to 60 minutes,  adjustable in 5 minute intervals.
     Materials:"  316SS for all wetted parts.

INSTALLATION:  Enclosure is normally a free-standing unit.  Provide piping,
pumps, filters and valves to supply necessary sample to the analyzer.  For
sample size requirement, consult the manufacturer.

UTILITY REQUIREMENTS:

     Electrical:  120 VAC, 60 Hz, approximately 700 watts.
     Water:  For cooling, 1/2 gpm at maximum 80°F (25°C).
     Compressed Air:  Less than 1.0 cfm at 30 psia.
     Oxygen:200 cc/minute at 40 psia, commercial grade.
     Acid:  3 Normal Hydrochloric acid, 3 gallons/month.

APPLICATION:  Used for monitoring of organic loading in liquid process
streams.  Although the analyzers use proven techniques for determining the
organic carbon,  they have not been  widely acceptable in wastewater treatment
processes due to high maintenance requirements.
                                    343

-------
MAINTENANCE:

     Custodial:  Daily calibration check and visual inspection.
     Preventive:  Weekly clean filters and service pumps.   Replenish  the
     acid and oxygen supply every month and check tubing.
     Frequency of Failure:  Estimated 1 to 4 weeks.

ACCESSORIES/OPTIONS:  Output— ma, volts or relay output contacts.
Equipment failure alarm contacts.  Multiple stream sampler.
                                      344

-------
                                  SECTION 6
                                 BIBLIOGRAPHY
Baily, S. J., Varieties of Pressure Sensing Enrich Instrument Application,
Control Engineering. February, 1977.
Considine, D. M.. Process Instruments and Controls Handbook. McGraw Hill
Book Company, New York, NY, 1974.
D'Amelio, Peter and C. F. Guarino, Instrument Specifications:  The
Operator's View,  Deeds and Data.
Lawford, V. N., Differential Pressure Instruments:  The Universal
Measurement Tools,  Instrumentation Technology. December,  1974.
Liptak, B. G., Environmental Engineers'  Handbook.  Chilton  Book Co., Randor,
PA, 1974.
Manual on Installation of Refinery Instruments and Control System,  Section 1
- Flow, Section 2 - Level and Section 4 - Pressure. American Petroleum
Institute, Washington, DC.
Pearson, C. R.. Things to Remember About Pressure  Transmitters, Instruments
and Control Systems. August, 1977.
Process Instrumentation, a reprint from Chemical Engineering. McGraw Hill
Book Co.
Rogers, Alan J. and John W. Liskowitz,  Wastewater  Instrumentation,  Theory
and Application. August, 1975.
Scientific Apparatus Makers Association  Standard PMC20-2-1970.
Soisson, H. E., Instrumentation in Industry.  Wiley-Interscience Publication,
New York, NY, 1975.
                                    345

-------
                                  SECTION 7

                                 DESIGN GUIDE
INTRODUCTION
     This section includes a brief review and discussion of the considera-
tions involved in the selection and documentation of a control  system.  The
following aspects of control system design are addressed:

     Project familiarization—Plant processes, construction schedules and
operational philosophies should be reviewed in preparation for  the actual
design of the control system.

     Evaluation of control approaches—Alternate control system configura-
tions should be considered relative to the cost effectiveness of the system
and the needs of the user.

     Documentation of design guidelines—To assist in coordinating the
efforts of those involved in any work related to the control system design,
it is suggested that general control system design guidelines be estab-
lished, disseminated and updated as necessary.

     Design example—A hypothetical design is discussed to illustrate
instrumentation and control specification requirements for a return
activated sludge pumping system.

     Specification checklist—A list is provided to tabulate the items that
should be addressed in a control system specification.

     It is hoped that the information in this section will aid engineers,
treatment plant owners and regulatory agency personnel in understanding the
scope of work involved in control system design.

PROJECT FAMILIARIZATION

     The control system design engineer must completely familiarize himself
with all aspects of a wastewater treatment project.  This can be accom-
plished by reviewing the facility plan and by interviewing the process
design engineers.  It is important that involvement of the control system
engineer occur early in the Step II design phase for proper consideration of
maximum operation and control flexibility.
                                     346

-------
Process Considerations

     Plant process considerations involve identification of: 1) the plant
physical characteristics, and 2) new construction versus existing plant
facilities.

     The plant physical characteristics relate to the overall plant flow
train, the individual unit processes and any plant-related remote areas.
The plant flow train must be identified as to the normal flow routing,
alternate flow routing and any emergency bypass routing.  Unit process
considerations must include interrelated effects of processes, batch versus
continuous operation, and startup/shutdown requirements to meet variable
plant loadings.  Each unit process must be evaluated for its unique control
requirements.  Related remote areas (e.g. pump stations, lagoons,  remote
quality monitors) necessitate evaluation for special comnunication
requirements.

     New construction considerations may differ from those associated with
the retrofit of an existing facility.  In most cases, the control  system
will be supplied independent of new unit process equipment.  However, some
unit process specifications will be primarily based on performance, because
one supplier will provide the entire process package.  In this case,  the
supplier of the turnkey package may insist of furnishing his own control
system in order to guarantee the required performance (e.g. cryogenic oxygen
generation systems or incinerators).  In the retrofit of existing  facili-
ties, all existing control equipment must be evaluated for possible renewal,
replacement or additional interface requirements.   Special considerations
may be necessary for existing equipment which was  not initially designed for
automatic control.

Schedules

     Scheduling, as with all disciplines of engineering, is important.   The
plant construction schedule can be a factor in selecting the most  practical
plant control system.  If the construction is new  and all work is  to  be done
under one contract, the construction schedule is of importance in  that the
selected control system must be built and installed to coincide with  the
plant startup.

     In other cases, an existing plant is being upgraded to meet NPDES stan-
dards and construction is to be accomplished in phases.   Schedules of this
type are important because interim control systems  may be required to start
up and operate early phases prior to final completion of the entire plant.

Operational Philosophies

     The plant operational philosophy relates to several considerations:   1)
the required process control; 2) the desired operator interface; and,  3)  the
staffing limitations and/or regulations.  The operational approach must be
both desirable and practical.  The evaluation of the control  system's  needs
depends greatly on the operational philosophy.


                                    347

-------
     Each unit process must be examined for control requirements.  The com-
plexity of the process may determine areas that require automation.  From
the control requirements, the extent of the desired operator interaction can
be determined.  The operator interface must also be evaluated for local
versus centralized control interactions.

     From each unit process evaluation, the overall plant operational
requirements can be determined.  Thus, anticipated staffing needs and opera-
tion centers can be outlined and evaluated in view of staffing limitations
or regulations.  This evaluation may require changes to be made in
operational philosophy or staffing needs in order to satisfy both criteria.

     With this information, the control system designer can establish opera-
tional requirements, on a unit process basis and for the entire plant, which
must be satisfied by the plant control system.

EVALUATION OF CONTROL APPROACHES

     Having developed a thorough understanding of the processes involved,
the schedule for their construction and the requirement for their operation,
a control system design engineer can now begin evaluating different control
approaches to determine which are capable of meeting the pre-established
operation and performance criteria.  Each unit process must be examined to
determine the amount of manual and automatic control required and the infor-
mation that must be available for operator decision making.  This is then
reviewed against the overall plant operation philosophy and various control
approaches are selected.

     The various control approaches selected for further evaluation may be
manual, analog, digital or any combination of these.  Descriptions and
discussions on each of these control approaches are contained in Section 4.

     The various combinations of control approaches are then configured into
overall plant control systems.  Through prescreening based on other engi-
neering considerations, this field is then narrowed to the best two or three
possibilities for economic analysis.  The cost effectiveness of each system
is determined.  Detailed examples of how this is accomplished are shown in
Section 5.

     Having completed the cost effective analysis, the systems can be ranked
according to cost.  Now each must be evaluated in terms of less tangible
advantages/disadvantages as described in the Alternate Control Approaches
section.  These items might include user background or experience, reli-
ability, flexibility in accommodating future needs, user preference, etc.

     The control system engineer must summarize the findings of all
evaluations and make a recommendation as to which control system is most
desirable.  This information should then be presented to the project design
team and the user for a final control approach selection.
                                      348

-------
DOCUMENTATION OF DESIGN GUIDELINES

     The next step in generating a control system specification  is the
preparation of a design guideline.  The primary purpose of this  document  is
communication with all project team members whose work is involved with or
affected by the control system.  In the actual detailed desijn phase of a
project, many people other than members of the control system design team
are responsible for work that includes important elements of the control
system design.  For example, a separate consulting group is often responsi-
ble for the preparation of diagrams specifying the conduit and wiring
requirements for all plant equipment including the control system.  The
related nature of the design work being performed should indicate that if
the control system is to be successfully implemented, frequent conferences
must be held and communication among the various groups must be maintained
during the design phase.  When appropriate, a representative of the owner
should be involved.

     The design guideline provides documentation of the selected control
approach for the plant and establishes basic control  system interface con-
cepts.  These are general guidelines to be followed by the control system,
electrical and equipment designers to insure compatibility between pack-
ages.  Since the guidelines may not be applicable in all cases,  a procedure
should also be established for handling special cases.  The design guideline
should become a controlled working document which is updated as design
details point out the need for modifying the general  approach.

Control Approach

     This portion of the design guideline describes the control  approach
selected, illustrates the basic configuration which the control  system
design will follow and discusses the various levels of control  which are to
be provided.  The levels of control can be classified as:  1)  field;  2)  area;
or, 3) central.

     Field control is also referred to as local control.   It  is  the  lowest
level of control because it is normally performed at  the operating equipment
location.  Examples include start/stop pushbutton stations  at a motor,  or a
hand crank on a pneumatic operated valve.  In multilevel  control  system con-
figurations, these are normally used for maintenance purposes and for opera-
tional backup should higher levels of control fail.

     Area control is characterized by a control panel in a process area
which includes monitoring information and control capabilities.   From this
area control panel, an operator can execute process  changes  by  controlling
equipment and adjusting process parameters.  Examples of area operating
functions through a control panel include:  primary treatment,  aeration  and
final clarification with sludge pumping.

     The highest level of control is central  control  wherein  information  is
presented to an operator who is remote  from the process  areas.   The  central
                                     349

-------
operator analyzes the process data and makes necessary adjustments to
achieve the desired operations.

     Most control systems will be a combination of two or all three levels
of control.  Early consideration of the unit processes involved and the
desired operations staff is necessary in establishing the preliminary
control system configuration.  As the various design phases are being
detailed, it may be necessary to revise the configuration.  However, plan-
ning early in the design phase is necessary for incorporation of the best
control system into the overall plant design.

Interface Design

     The term interface, in the context of this report, refers to the com-
patibility of connections between field instrumentation/equipment and a
remote area or central control center.  Typical interfaces should be estab-
lished as guidelines for those designing and specifying equipment monitored
or operated by the control system.  This eliminates the need to discuss each
item with the control system designer.

Signal Transmission-
     Basic signal levels for monitoring and control and provisions for
transmitting between points should be established (e.g. all pneumatic analog
signals will be.3 to 15 psig and transmitted via 1/4" plastic tubing).
Where practical, commonly accepted signals should be used.

     Following are examples of various signal types used in a control system
design with possible levels and methods of transmission between the field
and the remote control center.  For electronic analog signals, 4-20 maDC
transmitted on 18 AWG shielded, twisted pair is frequently used.  For status
and alarm information displayed at a remote control center, dry isolated
contacts are supplied in the field with 120 VAC sensing voltage provided at
the control console.  Connection between the control center and field is
made using 14 AWG twisted pair.

     It is important that the specifier of field mounted instrumentation and
equipment know the appropriate signal levels required for each signal type
to insure compatibility with the remote control center.  The electrical
designer must know the recommended type of wire for each signal type so the
required wire and conduit can be provided between the control center and
field mounted devices.

Equipment Monitoring—
     Guidelines for monitoring the conditions of electrical or mechanical
equipment should be established so the control designers can specify the
necessary sensors and who shall provide them.  Typically it is preferable to
have equipment monitoring sensors provided as part of the equipment assem-
bly.  These sensors include: vibration, bearing temperature, motor winding
temperature and breaker position transducers.  In most cases the equipment
manufacturer knows the preferred location for mounting the sensors.  In
                                      350

-------
addition, factory installation is more cost effective than retrofitting  the
equipment with sensors in the field.

     Guides or boundaries should be established for monitoring equipment
conditions.  The following is typical of the equipment monitoring that needs
to be stated in a complete design guide.

     "All electrical loads larger than five horsepower will be equipped  with
     run lights on the MCC and auxiliary contacts for remote run indica-
     tion.  Cranes, vent fans, air handling units, etc. are excluded from
     this list.  In addition, equipment one hundred horsepower and above
     will have current transmitters mounted in the motor starter compartment
     for remote monitoring."

     A special case of equipment monitoring is a unit consisting of the
operating equipment and associated controls supplied as a complete package.
Examples of this include large compressors, boilers and incinerators.  The
handling of alarm and shutdown conditions should be addressed.  The follow-
ing is an example guideline:

     "A common isolated contact shall be provided for remote alarming and
     annunciation of any shutdown condition occurring on a unit.  This is
     based on the premise that an operator will be dispatched to investigate
     the failed unit and that the first shutdown condition to occur will be
     indicated on the unit control box.  Impending shutdown conditions, such
     as high boiler pressure, should alarm individually at a remote location
     prior to final shutdown."

     These statements or monitoring guides should alert the equipment speci-
fier to the required monitoring points to be provided with the final equip-
ment.   In addition, a control system designer is informed that provisions
must be made for remote monitoring of the equipment.

Equipment Control-
     As with monitoring, the basic equipment control philosophy has to be
stated.  This equipment control relates to: 1) the level of control; 2)
safety considerations; and 3) modulating/two-state control.

     Using the previously established levels of control as a design con-
straint, priorities must be set and interlocked so conflicts of operation
cannot arise between levels.  Priority interlocking is accomplished by using
H-O-A (hand-off-auto), remote/local and computer/manual switches in appro-
priate combinations to achieve the desired result.

     Safety considerations involve emergency stopping of equipment.  When
working with rotating equipment in a remote control situation, as a protec-
tion to personnel and equipment, local stop and safety interlock controls
should be active at all times.  The local stop pushbutton will enable an
operator to locally shut down a piece of equipment any time an unsafe condi-
tion arises, without requesting that control be shifted from a higher level
                                     351

-------
remote source.  Remote automatic controls should recognize this as an abnor-
mal shutdown and lock out the equipment until the condition is cleared.  The
same holds true for equipment safety interlocking.  When working with higher
control levels involving a computer, the recognition of a safety shutdown
condition  and the execution of that shutdown should not be dependent on the
availability of a computer.  As an example,  a shutdown switch should be
hardwired  into the starter controls.

     Remote operation of equipment can be classified as two-state or modula-
ting control.  Examples of two-state devices include a constant speed pump
which can  be on or off and an isolation valve which is open or closed.
Control of this type of equipment can be accomplished using maintained or
momentary  control contacts.  Maintained contact controlled equipment
normally operates when the contact is closed and is off when  it is open.
With momentary contact control, a start contact closure occurs for a set
time interval and is sealed in with an auxiliary contact.  The stop function
is then accomplished by momentarily opening  a second set of contacts which
causes the auxiliary seal-in contact to drop out when the circuit is
de-energized.  Typical schematics are shown for each type of two-state
control (see Figures 7-1 and 7-2).  Generally, momentary type contacts
provide greater control system security.

     Modulating control is used for adjusting variable speed pumps, chemical
feeders, flow proportioning valves and other variable rate control devices.
Control of this type of equipment can be accomplished using positional or
incremental outputs.  The actuators associated with each of these equipment
items must be specified to receive a signal which  is compatible with the one
generated  by the remote control center.  The most commonly used signal for
position is 4-20 maDC.  Incremental modulating control is useful in a multi-
level control environment.  Incremental control uses a series of up/down or
open/close pulses to adjust a final control element.  Consideration should
be given to feedback of the controlled variable for indication at the
control center.

     For coordination of design activities it is important that preferred
methods of control be established and clearly stated.  In this way the
control equipment can be specified with a remote interface which is compati-
ble with the signal generated by the remote  control center.

Contract Responsibility—
     Whether the control system is being procurred for an entire plant or
for an  area, or  if  it  is being supplied and  installed as a subcontract or
separate contract, the demarcation of responsibilities between the control
system  and the field must be clearly defined.  The demarcation point may be
a terminal strip in a termination cabinet or control panel.  Responsibili-
ties can then be split with one contractor to provide the terminal strip and
all wiring on one side, and other contractors responsible for all wiring on
the other  side.  This  is also important to the design team so each member
knows how  far his work should be carried.
                                      352

-------
CO
CJ1
to
                                 UUUU
            OFF  ON
                     PUMP SAFETY
                               CR_,
                        MS. ©
MANUAL  CPU                            _   g)
             CPU START/STOP        STOP (§)
                 j-t3	fr-0  ' fl	(CR-I>
MANUAL
^^K
 e>      <
                                   (D
                                   START
                                   CR— 1
                   CPU
                     THERMAL SWITCH

               S.C.
                                               MOTOR
                                               STARTER

                                               MOTOR
                                               RUNNING
                                                       MOTOR
                                                       HOT
                                                                 PUMP
                                                                 FAILURE
                                                                                           VAC-
                                                                                       (7)  TWISTED 2/C NO. 18
                                                                                        120 VAC.
                                                                                       (3) 2/C  NO. 14

                                                                                       (2) TWISTED  2/C NO. 16
                                                                              INCOMING	
                                                                              POWER

                                                                              (3)  POWER  WIRES

                                                                              (2)  2/C  NO. 14
                                                                                        (I)  2/C  NO.I4
                                                                                                          REMOTE  AREA PANEL (3
                                                                                                              COMMUNICATION LINE
                                                                                                  DEVICE  INTERCONNECTIONS
                                                                                   LOCATED AT  PUMP/MOTOR

                                                                                   LOCATED AT  MCC

                                                                                   LOCATED AT  REMOTE AREA PANEL

                                                                                   LOCATED AT  COMPUTER  MUX
                                                                                   MUX  TERMINALS
     M.S. AUX
                             CR-3       CR— 4
                                           Ha
                      Figure 7-1.   Typical  constant speed  pump interface  (maintained contact control)

-------
CO
on
                                uuuu
                                           CPT'
                         r (T)  PUMP SAFETY
                               CIRCUITS   CRH.
                        M.S.I
                       Hh
                     CR-I
MANUAL COMPUTER' '
      {  ©   START     ©
      t-TJ-r-EH hB	
             IPUTER   (4)
                                          CR— 2
                H2H
              STOP
       MANUAL
        O      1
                        STOP
                      -O   O-
                 COMPUTER
                   THERMAL  SWITCH
              OL.

                                                    _
                                                      STARTER

                                                      RUNNING
START
MOTOR
STOP
MOTOR
                                                                HOT
                                                                PUMP
                                                                FAILURE
                                                                                                  COMPUTER
                                                                                      120  VAC-
                                                                                                                    -COMMUMCATION LINE
                                                                                                              MUX
                                                                            (7) TWISTED 2/C  NO.I8-
                                                                                      120 VAC-
                                                                                                      REMOTE AREA PANEL
                                                                                      (3) 2/C NO. 14
                                                                                      (2) TWISTED  2/C  NO. IB
                                                                                      INCOMING	
                                                                                      POWER
                                                                                      (3) POWER WIRES
                                                                                      (2) 2/C NO. 14
                                                                                      (I) 2/C NO. 14
                                                                                            DEVICE  INTERCONNECTIONS
                                                                                D LOCATED  AT PUMP/MOTOR
                                                                                D LOCATED  AT MCC
                                                                               D LOCATED  AT REMOTE  AREA PANEL
                                                                               @ LOCATED  AT COMPUTER MUX
                                                                               El MUX TERMINALS
    M.S. AUX
  EH
                             CR-3
                                       CR— 4
                       Figure  7-2.   Typical  constant speed  pump interface  (momentary contact  control)

-------
Site Planning

     Early consideration of where data acquisition enclosures, area panels
and other control system cabinets will be located is necessary so space will
be planned.  In addition to locations, special installation requirements
should be noted.  These would include power, environmental control, cable
routing, etc.

DESIGN EXAMPLE

     This section describes recommended procedures to be followed in detail
design of a specific control system for a wastewater treatment plant.  The
example used is a particular subprocess in a 30 mgd (1320 dm3/s) plant,
namely return activated sludge (RAS) withdrawal and pumping.  The example
includes a discussion of computerized control, however, the procedures are
applicable to conventional analog control as well.

Statement of Problem

     Activated sludge is to be returned from each of three secondary
clarifier batteries to three aeration batteries.  Each clarifier battery
consists of two clarifiers, one sludge underflow line per clarifier, a RAS
wet well and two variable speed RAS pumps.  The battery piping configuration
is shown in Figure 7-3.  Total RAS pumping capacity per battery is 10 mgd
(440 dm3/s).

     Each of the three batteries is to be controlled in an independent
mode.  Sludge is to be withdrawn from each clarifier by control of a modula-
ting valve and flowmeter on the associated underflow lines.  The sludge
flows by gravity to a wet well from which it is pumped to the associated
aeration battery.  A flowmeter and density meter are located on the common
pump discharge line.  The hydraulics are such that the wet well will not
overflow.

Control System Description

     A block diagram of the control system selected for this plant as a
result of a cost effective analysis is shown in Figure 7-4.  The plant is to
be centrally controlled by a digital computer system.   Data is concentrated
by multiplexers strategically located throughout the plant to minimize
wiring costs.  One such multiplexer is located in the secondary clarifier
area.

     Signals are routed to the multiplexer from a local control panel, which
is in turn wired to associated field elements and motor control centers.
The local panel is to provide manual and/or analog control for backup in the
event of computer system failure.  The panel  will also be used for startup
of unit processes before the computer system is brought into full  operation.
                                     355

-------
                                           SYMBOLS
                 MOTOR  OPERATED VALVE



                 CHECK  VALVE



                 MANUALLY  OPERATED  VALVE
[V/S)  VARIABLE  SPEED  PUMP



 DE    DENSITY  METER



       FLOWMETER
CO
01
                                                TO CHLORINE  CONTACT  TANK
                        MIXED  LIQUOR FROM
                        AERATION TANKS
                                                                                                           HAS TO AERATION  TANKS
                               Figure  7-3.   Secondary clarification  and  RAS  piping  schematic.

-------
Definition of  Control  Strategy

     The first step in the design procedure is the definition  of an overall
control strategy.   The problem should  be  jointly analyzed  by a combination
of process and control system engineers.   The process engineer provides the
knowledge of  how the particular system was physically designed to operate.
The control system engineer provides  the  expertise required to properly
instrument and control the process  to  accomplish the required  operation.
CENTRAL CONTROL
ROOM
1 CENTRAL COMPUTER
1
\
| MULTIPLEXER INTERFACE
/•
/
\
                                     PLANT
                                     WIRING
                TO OTHER
              PLANT AREAS
 \
  \

  TO OTHER
PLANT AREAS
                                       I
                                 MULTIPLEXER "B1
                                   LOCAL PANEL
          FIELD ELEMENTS
                                                      MOTOR CONTROL CENTER
                               SECONDARY CLARIFIER AREA
                  Figure  7-4.   Control system configuration,
                                      357

-------
     The process under-consideration can be subdivided into two sub-
processes; withdrawal of sludge from the secondary clarifiers and pumping of
RAS from the wet well to the aeration tanks.  The control strategy for with-
drawal will be to maintain a relatively constant sludge blanket in the
clarifiers.  The control strategy for pumping will be to return RAS to the
aeration batteries based on one of the following calculations:

     1.   Food/Microorganism Predictive

                Ki «U 
-------
Preparation of P&ID

     Once the basic control concepts are determined, the engineer should
proceed to develop a piping and instrument diagram (P&ID).  The purpose of
the P&ID is to illustrate the process piping, show all control devices,
primary sensors and local instrumentation, and (when applicable) define the
computer system input/output signal interface.

     A P&ID for the RAS process is shown in Figure 7-5.  The control system
engineer has used the piping schematic shown  in Figure 7-3 and the required
control strategy to determine the necessary instrumentation and computer
interface requirements.

     A standard set of symbols must be selected or developed for use in
preparing P&ID's.  A subset of those used for this project is shown in
Figures 7-3 and 7-6.

     Only controlled and/or monitored devices are shown on the P&ID.  Manual
valves shown on the piping schematic are not shown on the P&ID as these
valves are normally left open and are only used for maintenance purposes.
The only manual valves shown are those on the clarifier influent lines as
these will be monitored by the computer to determine whether a clarifier is
in service.

     The engineer has located a sludge blanket level transmitter in each
clarifier to be used by the computer for blanket control.  A local panel
indicator is also shown.  The sludge collector drive in each clarifier is
monitored for status and overload conditions.

     A local flow controller (FIC) is provided for automatic control of each
sludge underflow valve.  It will normally receive its setpoint from the com-
puter.  A valve fully closed limit switch (ZB) is provided to be used by the
computer for flow control.  The flow signal is wired to both the flow con-
troller and the multiplexer.  A computer/manual switch contact (YS) is
provided to indicate local control status to the computer (the switch is
physically located on the controller).

     A low level switch is located in the RAS wet well for pump protection
and alarm indication to the multiplexer.  Note that low suction cutout is to
be provided by hardware within the panel and is not dependent on computer
operation.  This rule should be applied for all safety interlocks.

     The controls for the RAS pumps include the following:

          Motor start control, both from the computer and from the panel
          (MN, HS)
          Motor status indication (MM)
          Motor speed control, both from the computer and from the panel
          (HIK, SC)
          Motor speed indication (ST)
                                      359

-------
CO
en
o
TYPICAL C
121,12!

M
> 1
                                                         TYPICAL OF TWO
                                                         106,107.116,117
                                                              T
TYPICAL OF TWO
102,103,112,113
TYPICAL
 OF TWO
MOAlH

   1   I
                                      TO CHLORINE
                                      CONTACT TANK
                                                                              RAS
                                                                             WET WELL
                    MIXED LIQUOR FROM
                    AERATION  TANKS
                                NOTE(S):

                                  I. ALL SIGNALS WIRED TO MULTIPLEXOR B

                                  2. STOP  PUMPS ON LOW LEVEL  (PANEL FUNCTION)
                                                                                                       RAS
                                                                                                       PUMPS
                                       Figure 7-5.   Secondary clarification and RAS P&ID.

-------
                                   SYMBOLS
4
1
I
1
ANALOG
INPUT
T
i
I
i
MODULATING
OUTPUT
A
i
I
i
i
DIGITAL
INPUT
Y
1
I
1
CONTROL
OUTPUT
o
FIELD MOUNTED
e
PANEL MOUNTED
                    INSTRUMENT  AND  DEVICE  LETTER  CODES
AI    ANALYSIS  INDICATOR




AT   ANALYSIS TRANSMITTER




DE   DENSITY ELEMENT




Dl    DENSITY INDICATOR




DT   DENSITY TRANSMITTER




FE   FLOW ELEMENT




Fl    FLOW INDICATOR




FIC  FLOW INDICATING CONTROLLER




FT   FLOW TRANSMITTER




HIK  HAND INDICATING CONTROL STATION




HS   HAND SWITCH




LLA  LOW LEVEL ALARM
LS   LEVEL SWITCH




MM   MOTOR ON/OFF




MMJ  MOTOR RUNNING LIGHT




MN   MOTOR START/STOP




MO   MOTOR OVERLOAD




MOA  MOTOR OVERLOAD ALARM




SC   SPEED CONTROL




ST   SPEED TRANSMITTER




VC   VALVE CONTROL (MODU-LATING)




YS   COMPUTER/MANUAL SWITCH




ZB   VALVE CLOSED LIMIT SWITCH




ZBJ  VALVE CLOSED LIGHT
                    Figure 7-6.  P&ID Symbol Legend.
                                   361

-------
          Computer/manual switch for each pump (YS)
          Flow and density measurements (FT, DT)
          Low level switch (LS)

     Manual control of the pumps is provided by a start/stop switch and a
manual loading station for speed control.  Note that both devices are inter-
locked through a computer/manual switch.  The rule applied is that all
manual controls associated with a particular device (or process) should be
activated by a single switch.  Switch status is indicated to the computer
for remote control activation/deactivation.

     Each "bubble" on the P&ID is assigned a specific number.  The numbers
are assigned on a device basis, e.g. all points associated with one RAS pump
are assigned number 121.  The pump itself is then number B121 where B is the
area, or multiplexer, designation.  By using the same numbering system for
instrumentation and equipment schedules, the mechanical and I&C drawings can
be linked together (see Figure 7-3, where the same device numbers are
used).  Similarly, instrument tag numbers are assigned and can be used for
all drawings.

     Note the use of typicals to avoid clutter.  This makes the drawing more
readable without loss of clarity.  Use of typicals can, however, be more
difficult with pure analog systems due to a greater number of "bubble"
interconnections.  The designer must consider whether use of typicals is
appropriate for each individual P&ID.

     Other attributes to be noted are the following:

          All process piping is shown.
          Process flow lines are drawn heavier than other lines.
          Electrical connections are shown by dotted lines.

     Any future equipment piping, etc. should also be shown on the P&ID's.
This will help insure that the vendor provides sufficient expansion
capabilities.

     The importance of the P&ID cannot be stressed too strongly.  It is the
link between the process and the instrumentation.

Input/Output Point Listing

     Once the P&ID's have been finalized, a complete list of input/output
(I/O) points should be developed.  The purpose of the I/O list is to pro-
vide, in one document, the information necessary to configure the computer
system multiplexer interface and system data base.  Another use of the I/O
list is to provide a check list for other specification sections (such as
electrical and mechanical) to insure that the field devices connected to the
computer system are accounted for and have been specified with the correct
interface.
                                     362

-------
TABLE 7-1.   RAS  INPUT/OUTPUT LIST
POINT
NAME
B101 ZB
B102 LT
81 03 MM
B103 MO
B106 FT
B107 VC
B107 YS
B107 ZB
Bill ZB
B112 LT
B113 MM
B113 MO
B116 FT
B117 VC
B117 YS
B117 ZB
B120 LS
B121 MN
B121 m
B121 SC
B121 ST
B121 YS
B122 MN
B122 MM
B122 SC
B122 ST
B122 YS
B125 FT
B126 DT
POINT
DESCRIPTION
SEC CLAR 1 INF GATE OPEN
SEC CLAR 1 SLUDGE LEVEL
SEC CLAR 1 DRIVE STATUS
SEC CLAR 1 DRIVE OJ..
SEC CLAR 1 SLUDGE FLOW
SEC CLAR 1 SLUDGE VLV CONTROL
SEC CLAR 1 SLUDGE VLV CTL MODE
SEC CLAR 1 SLUDGE VLV CLSD
SEC CLAR 2 INF GATE OPEN
SEC CLAR 2 SLUDGE LEVEL
SEC CLAR 2 DRIVE STAT
SEC CLAR 2 DRIVE O.L.
SEC CLAR 2 SLUDGE FLOW
SEC CLAR 2 SLUDGE VLV CONTROL
SEC CLAR 2 SLUDGE VLV CTL MODE
SEC CLAR 2 SLUDGE VLV CLSD
RAS WET WELL LOW LEVEL
RAS PUMP 1 CONTROL
RAS PUMP 1 STATUS
RAS PUMP 1 SPEED CONT
RAS PUMP 1 SPEED
RAS PUMP 1 CTL MODE
RAS PUMP 2 CONTROL
RAS PUMP 2 STATUS
RAS PUMP 2 SPEED CONTROL
RAS PUMP 2 SPEED
RAS PUMP 2 CTL MODE
RAS FLOW TO AERATION
RAS DENSITY
TYPI
DI
AI
DI
DI
AI
MO
DI
DI
DI
AI
DI
DI
AI
MO
DI
DI
DI
CO
DI
MO
AI
DI
CO
DI
MO
AI
DI
AI
AI
* AI Analog Input CO = Control Output
MO Modulating Output DI Digital Input
                                                 RANGE
                                                 2-10 ft.
                                                 0-5 MGD
                                                 0-100%
                                                2-10 ft.
                                                0-5 MGD
                                                0-100%
                                                0-100%
                                                0-100%
                                                0-100-%
                                                0-100%

                                                0-10 MGD
                                                0-3%
                 363

-------
     The list of points associated with RAS pumping is shown in Table 7-1.
Included are point names, point description, type and range.  The point name
is derived from the P&ID's.  Each point name follows a specific format.  As
an example, consider the first point, B101ZB.  The first character (B)
refers to the plant area/multiplexer into which the point is to be wired.
Characters two, three and four (101) are a numeric designation for a field
device.  The fifth and sixth characters (ZB) designate the variable and the
computer function, in this case a full closed limit switch.

     The I/O list must be accompanied by a list of areas and abbreviations
used in the data base, along with an explanation of the format.  The
complete I/O list should also include items such as scan times, alarm
limits, deadband values, maximum rate of change and scaling information for
each point.  The points may also be sorted in several different ways, e.g.
by point type, by plant area, and/or by control loop.

     Future points which can be firmly identified should be included in the
I/O list to insure that the system is sized correctly,

Interface Drawings

     Clear specification of interface details is essential to control system
success.  Some which are particularly important are:

          Modulating valves
          Open/close two-state valves
          Constant speed motors
          Variable speed drives
          Local control panel
          Manual loading stations
          Controllers

     An example of an  interface drawing for the variable speed RAS pumps is
shown in Figure 7-7.  The level of detail shown is typical of that required
for each interface.  The drawing shows interconnection between panels and
motor control centers.  All interlocks and local control logic, as well as
signal interface to the multiplexer are shown.  An incremental interface is
specified for speed control for fail-safe operation, i.e. loss of signal
will not cause a change  in speed.

     An interface drawing for a controller  is shown in Figure 7-8.  Recall
that flow controllers  were specified for backup on the sludge underflow
control loops  (FIC 107, FIC 117).  The interface between analog controllers
and computer systems  is one of the most critical.  Bumpless transfer from
one control mode to another is crucial to avoid process upsets.  The  inter-
face shown in Figure 7-8 is for an incremental, or velocity type, DDC
controller.  Similar to the interface shown for the RAS pumps, the incremen-
tal approach is inherently fail-safe.  Loss of the computer or multiplexer
will cause the controller to revert to the  local automatic mode without
                                     364

-------
                                                     4TRANSFER
                                                      CONTACTOR
CO
a\
en
                                                                      Q LOCATED AT PUMP/MOTOR

                                                                      (D LOCATED AT MCC

                                                                      © LOCATED AT LOCAL AREA PANEL

                                                                              DEC   0      INC
                                                                         HSH Ha-r-KH
                                                      STARTER
MOTOR REQUIRED
ON VARIABLE SPEED

MOTOR RUNNING
AT CONSTANT SPEED
                                                                               DEC ($)
                                                                            L±±
                                     INC
                                                                             CR-I
                                                                           [1
                                                         (4) LOCATED  AT THE MUX

                                                         (5) LOCATED AT VARIABLE SPEED
                                                                                  DRIVE
                                                         5Q MUX TERMINALS
                                                                   I2O VAC
                                                                                              (3)TWtSTED 2/CNO.I6
                                                                                              (D2/C NO.I4/(l)2/CNp.l4
                                                                                              LINE
                                                                                              POWER
                                VARIABLE
                                  SPEED
                                CONTROL
                     SIGNAL
                  TRANSDUCER
                      81   /71
                  INDICATOR V»
                                              (I) TWISTED 2/C NO 16


                                              120 VAC


                                              (8) TWISTED 2/C NO. 16


                                              120 VHC
                                                   V8D  PANEL
                                                                                                    O) POWER WIRES
                                                                                                    (J) POWER  WIRO
                                                                                                               DEVICE INTERCONNECTIONS
                                                                                      MOTOR SPEED
                                                                                      SIGNAL (RPM)
                                                                                      tTRANSMfTTER (1
                                                                          SPEED SIGNAL TO CPU
                                                                               (4-20 MA)
                                              Figure 7-7.   Variable  speed  pump interface.

-------
            Al
                                 MULT PLEXER
            MO
                                    CO     Dl     Dl
           t
                           Y
PROCESS
VARIABLE
NCREASE/DECREASE
    PULSES
                  COMPUTER
                  FAILURE
                  CONTACT

               i
COMPUTER-AUTOMAT IC-MANUAL
               I
               I
                                   C-A-M
                                    0
                              OPEN
                               0
               CLOSE
                 O
                                T— '
                                  [COMPUTER/LOCAL
                                      STATUS
                    INCREASE/DECREASE
                        PULSES
REVERSING
  MOTOR
                                                   FULLY CLOSED
                                                   LIMIT SWITCH
        Figure  7-8.   Typical  incremental controller interface.
                                366

-------
bump.  In the computer mode, the increase/decrease signals are passed
straight through to the valve, without controller action.  This is the
advantage of a DDC type controller.

     With an incremental controller, there is no need to wire the controller
output back to the computer.  If a positional, or maintained type output
(e.g. 4-20 ma) is used in lieu of incremental, the controller output must be
wired to the computer multiplexer.  In order to provide bumpless transfer
from local automatic to computer with a positional controller, the software
must track the controller output.

     These are a few examples of interface considerations which must be
taken into account.  The control system engineer must carefully address the
interface to every controlled and/or monitored type of device in the plant.

Review of Electrical & Mechanical Specifications

     Often, the specifications for electrical and mechanical equipment are
written by departments, or even companies, separate from the control system
engineer's domain.  These specifications, however, must contain the "hooks"
for the control system.  Typical examples of items which must be called out
are:

          Auxiliary contacts for motor starters
          Motor starter 120 VAC pilot circuits, with provisions for remote
          control
          Spare limit switches for valves
          Valve actuators
          Air compressors
          Pneumatic piping
          Control panels supplied with various package equipment items
          Instrument wiring for interconnection between panels
          Communication wiring

     Each item must be carefully reviewed and coordinated to insure that the
process equipment can be controlled and monitored.

Instrumentation Specifications & Schedules

     Following completion of the P&ID's, each instrument should be listed in
schedules and specified in detail from a performance standpoint.  The first
step is the preparation of an instrument summary list for all sensors in the
plant (see Table 7-2).  This list includes tag numbers (from P&ID's), multi-
plexer or area designation, equipment description, application, P&ID drawing
cross reference and specification reference.
                                     367

-------
CO

00
     TAG NO.


     LT  102
     LT  112
     HIK 121
     HIK 122
     FIC 107
                  TABLE 7-2.   INSTRUMENTATION SUMMARY LIST

MUX    EQUIPMENT                          APPLICATION


B      SLUDGE LVL TRANSMITTER           SEC. CLARIFIER #1
B
      FE/FT  116    B
      FE/FT  125    B
     DE/DT 126    B
B
B
B
SLUDGE LVL TRANSMITTER
      FE/FT  106    B       FLOW TRANSMITTER  (MAG)
FLOW TRANSMITTER (MAG)


FLOW TRANSMITTER (MAG)


DENSITY TRANSMITTER


MANUAL LOADING STATION


MANUAL LOADING STATION


FLOW CONTROLLER
SEC. CLARIFIER #2
                                        SEC. CLAR. #1 UNDERFLOW
                                        SEC. CLAR. #2 UNDERFLOW
                                        RAS
                                        RAS
RAS PUMP 1 SPEED CONTROL
RAS PUMP 2 SPEED CONTROL
                                                                   P&ID   -
                                                                   DWG.   SPEC- REF-
PI-110


PI-110


PI-110



PI-110



PI-110



PI-110



PI-110


PI-110
SEC. CLAR. #1 UNDERFLOW CONTROL   PI-110
     FIC 117
B
FLOW CONTROLLER
                                                          SEC. CLAR. #2 UNDERFLOW CONTROL   PI-110

-------
     For each type of sensor, the engineer should prepare a performance
specification and an instrument schedule.  For information of specific
devices and on instrument specifications in general, refer to Section 6,
Available Instrumentation.  It is suggested that a standard outline similar
to the Construction Specification Institute (CSI) format be used to maintain
uniformity in writing the specifications.  An example outline of a specifi-
cation of this type is provided on the following page.

     A typical instrument schedule is shown in Table 7-3 for the magnetic
flow meters used in the RAS control example.  For each meter, the schedule
includes tag numbers, application, size, liner material, range and indicator
requirements.

     In addition to the specifications, a mounting detail should be provided
for each instrument on the plans.  Proper mounting is crucial to proper
instrument operation.  The engineer should clearly show the contractor
exactly how each instrument is to be mounted.  A typical mounting detail for
an optical density meter is shown in Figure 7-9.

     During the past several years, many new sensors have been developed,
especially for analysis of the physical and chemical characteristics of
wastewater.  Examples include TOC analyzers, ultrasonic sludge blanket level
detectors (continuous output type), nitrogen analyzers and new types of flow
measuring devices.  Before specifying these, the engineer should carefully
investigate the design and operating principles and, if possible, user
references.  Frequency and duration of maintenance requirements  are espe-
cially critical.  A good way to check out a new instrument is to convince
the vendor to loan one for trial by the engineer or his client in an opera-
ting plant.  The continuous reading sludge blanket detector shown on the RAS
P&ID is a good example of a sensor which should be carefully evaluated
before being specified.

Control Panel Design

     Control panels are required for manual, analog and digital  control
systems.  Function and interface are the key words in panel  design.  Inter-
face has already been addressed.  Function requires that the panel  be laid
out with the operator in mind.  Components should be arranged in an orderly
and logical manner.  Controls associated with a specific process or device
should be grouped together.

     A typical control panel for our RAS example is shown in Figure 7-10.
The controls for the clarifiers are grouped together, as are the controls
for the RAS pumps.  The specifications for the panels should include a list
of panel mounted instruments complete with tag numbers (again from  the
P&ID's), description and indicator scale requirements.
                                      369

-------
                     SECTION
                     (INSTRUMENT OR DEVICE)







PART 1  - GENERAL




1.01     WORK INCLUDED




1.02     RELATED WORK SPECIFIED ELSEWHERE




1.03     SUBMITTALS




   A.    Shop Drawings




   B.    O&M Manuals




1.04     SPECIAL TOOLS AND EQUIPMENT




   A.    Spare Parts




   B.    Test Equipment




1.05     STANDARDIZATION




1.06     RESPONSIBILITY AND COORDINATION




1.07     MANUFACTURER'S SERVICES




1.08     GUARANTEE




PART 2 - PRODUCTS




2.01     (INSTRUMENT OR DEVICE)




   A.    Specification




         1.  Operating principle




         2.  Accuracy




         3.  Repeatability




         4.  Linearity




         5.  Indicator accuracy




         6.  Environmental requirements (e.g., temperature range)




         7.  Enclosure




         8.  Power requirements
                           370

-------
CO
•vl
                                TABLE  7-3.   MAGNETIC  FLOWMETER  AND TRANSMITTER

                                                                        CALIBRATED
        TAG  NO.       APPLICATION              SIZE      LINER MATERIAL     RANGE      INDICATOR   NOTES
        FE/FT  106       Secondary  Clarifier        8"       Polyurethane     0-5  MGD        Yes      Indicator
                       No.  1  Underflow                                                         Panel  Mounted
        FE/FT  116       Secondary  Clarifier        8"       Polyurethane     0-5  MGD        Yes      Same  as
                       No.  2  Underflow  "                                                       FE/FT 106
        FE/FT  125       RAS                       12"       Polyurethane    0-10  MGD      Yes      Same  as
                                                                                               FE/FT 106

-------
      I  WATER
      VALVE FOR
      SAMPLING
CO
*vl
ro
                                                                2" NPT WELDOLET TO BE USED IF
                                                                PIPE WALL  CANNOT SUPPORT
                                                                SENSOR RIGIDLY
                                                                          WIRING  CABLE B PLUG
                                                                          ASSEMBLY RUN TO ADJACENT
                                                                          JUNCTION BOX OR
                                                                          TRANSMITTER/1NDICATIOR
                                                                  CORPORATION  VALVE
              NOTES;
                I. IF SENSOR IS MOUNTED  ON A HOR4ZONTAL PIPE RUN,
                  THE SENSOR  AXIS SHOULD  LIE IN A HORIZONTAL  PLANE.

                2. ALL SIZING  OF CONDUITS,  CABLE LENGTHS, DIMS ETC. TO BE IN
                  ACCORDANCE  WITH MFG'S.  RECOMMENDED INSTALLATION PRACTICES.
                         Figure 7-9.  Typical mounting detail-sludge density analyzer.

-------
CO
CO

| CLARIFIER 1 |

^•A

[_ SLUDGE LEVEL J
-©; $;
[ RUNNING ] | OVERLOAD )

| DRIVE STATUS

C-A-M
©
0 O
OPEN CLOSE
SLUDGE FLOW
CONTROL ,-

| CLARIFIER 2 |

r-f^

\ SLUDGE LEVEL |
f ^ ' . \
[ RUNNING' | OVERLOAD |

DRIVE STATUS

C
C-A-M
©
O O
OPEN CLOSE
SLUDGE FLOW
^ CONTROL f

^

( RAS FLOW ]

j RAS PUMP 1
s~^\ s <
RUNNING ] LOW SUC
CUIOl
H-O-A
0
1
C-M
©
O O
INC DEC
SPEED
-x, CONTROL >^

--/-


RAS DENSITY |


RAS PUMP 2 ]
[ jg(
TION [ RUNNING

H-O-A
0
C-M
G>
0 0
INC DEC
SPEED
/ CONTROL





O ^ ,wx
LAMP

TEST. ALARM

S.ILENCE COMPUTER f

'A 1 LURE


Figure 7-10.  RAS local control panel.

-------
     Important items to be considered in specifying panels include the
following:

          Sheet metal material and workmanship
          NEMA type (for each panel)
          Paint specification
          Device specifications for all components
          Nameplates
          Power supplies
          Panel wiring

     The engineer may also choose to do schematic drawings of panel wiring.
These should be done functionally to allow the vendor to adhere as closely
as possible to his standard procedures to minimize costs.  The requirement
for detail schematics and/or loop drawings may be more important for a pure
analog system, as the degree of panel complexity will typically be greater
than for a digital system.  This is a design decision which must be made
early in the project.

Control Loop Description

     The specifications must include a complete description of control
requirements for each loop.  For analog systems, the description will encom-
pass functions to be performed by hardware; for computer systems, the
functions will be implemented by software.

     The  loop description should include the following:

          Overall control strategy diagram and description
          Different modes of control required
          Supervisory control requirements, e.g. flow control
          Sequence control requirements, e.g. motor start/stop
          Interface to other control loops
          Contingencies (Failure/restart considerations and effects of
          device failures)
          CRT displays (computer systems only)
          List of input/output points

     For our example, automatic control is implemented by a software pro-
gram, often called an algorithm.  The algorithm details the strategy to be
used for sludge withdrawal and pumping.  The highest level of control is
based on the RAS calculations, sludge blanket measurements and sludge flow
and density measurements.  A second level of control is specified to be used
by the operator to set flows and pumping rates from his CRT console.  A
basic level of control should always be specified to be used during system
startup or in the event of sensor failure.

     One of the more difficult and frequently neglected tasks is to attempt
to anticipate all of the things that can go wrong with a process and the
associated equipment (contingency analysis).  In each situation identified,
it must be decided whether the control system is to initiate automatic


                                     374

-------
response actions or if the situation is to be handled by manual interven-
tion.  In the RAS control example, some of the contingency situations that
might arise include:

          Power failure (momentary and long-term)
          Failure of one or both pumps
          Flow meter failure
          Density meter failure
          Sludge blanket level detector failure
          Clarifier drive failure
          Clarifier out of service

     The above contingencies are in addition to events encountered in normal
operation such as high and low wet well level, and high and low sludge blan-
ket level.  The latter must be handled by the control system as a minimum.

     The control loop description should also call for certain performance
requirements.  In our example, a typical requirement would be to maintain a
sludge blanket within a certain level range and to maintain a certain RAS
sludge density range.  The performance requirements should be realistic and
measurable.

     The method of documenting control loops should provide enough detail to
give the vendor a clear understanding of the requirements.  Drawings should
be used to illustrate the control strategy and CRT screens (if applicable).
The loop may be described verbally, or a combination of description and flow
charts may be used.  The important point is functional detail.  Many control
systems have not functioned properly because the vendor was not given
sufficient information to properly implement the control strategy.

Computer System Specification

     The specification of a computer system can be accomplished by preparing
either a detailed design or a performance document.  A detail design is on
the Kllnuts and bolts" or component level for the computer, its related equip-
ment and the programs which will make the system work, while the emphasis of
a performance specification is on function or capabilities of the computer
system.  For the purpose of this design example, a performance specification
will be used.

     Detailed computer designs have few advantages compared to a performance
specification.  Detail is here referred to as the internals of how things
are done in the computer system.  This type of design can lead to many
problems.  If they are prepared without regard for the commercial availa-
bility of the items specified, there is the risk that no supplier or
contractor can furnish the specified system.  Another risk with this type of
specification is that the system supplied per the contract documents is
likely to be a first time product which has not been field tested and, as
such, it may be difficult to obtain maintenance  and service.  Of course, the
specifying engineer concerned about the commercial availability of  a
                                      375

-------
computer system could, in preparing a detailed design, tailor the specifica-
tions around a particular system or equipment.  In so doing, competition  is
restricted and the engineer risks being faced with a protest regarding
proprietary specifications.  As a result, the engineer may be forced to
rewrite the specification.

     The disadvantages presented for a detailed computer system design
should not de-emphasize the need for detail in the design of a control
system.  In addressing the interface between a computer system and its data
acquisition hardware to operations, processes or equipment external to the
computer system, clarity of the interface requirements and functions is a
must.  This clarity can only be achieved by providing the detail as
described for the input/output point listing, interface schematics and
control loop descriptions.

     A performance oriented specification for a computer system encourages
competition and insures receipt of a representative number of bids.  Also,
there are numerous ways of building a computer system to perform the same
task.  Few standards have been established by the computer industry, conse-
quently there are as many ways of achieving the same end as there are
suppliers of hardware and software.  Also, a performance specification
allows more flexibility for innovation in the approach to a control system
design.  However, the engineer must still be realistic in specifying per-
formance requirements and in evaluating new technological approaches to a
design.

     From a supplier standpoint, a performance specification allows more use
of off-the-shelf equipment and programs to meet the specification require-
ments.  This is advantageous because it aids in keeping down the system
price by minimizing new product development.  Not only is a cost savings of
benefit to the user, but thoroughly tested and field proven hardware and
software is likely to be supplied.  This will help minimize system startup
difficulties and there is assurance that long-term support will be available.

Computer Hardware Specification—
     The hardware specifications should establish minimum equipment to be
supplied, and minimum performance criteria.  A basic configuration should be
defined, identifying the functions and primary capabilities to be performed
by the computer system.  This would include requirements for redundant or
backup equipment, however, specific items to be supplied and their inter-
connection are the responsibility of the control system supplier.

     Each of the major components are then specified individually by estab-
lishing guidelines for sizing and performance.  These guidelines are
considered minimum based on experience and knowledge of requirements for
other systems similar in size and complexity.  The final sizing and speed of
equipment furnished shall be the responsibility of the system supplier.  To
make sure that the system is flexible to meet today's demands and those of
tomorrow, known future expansion capabilities need to be defined plus a
reasonable unassigned spare capacity.
                                     376

-------
Computer Software Specification—
     The computer software can be generally classified as operating system
software, development software and application software.  General require-
ments for each group and the computer programs which comprise them must be
defined along with the purpose of each program, what it is supposed to
accomplish and how it can be used.  Clear definitions are necessary when
specifying programs because with few exceptions, program types and capabili-
ties are vendor dependent with proprietary names.

     The operating system is normally a program produced and supported by
the computer.  Since this program or package of programs can be considered
as the central nervous system of the system, it is recommended that it be
specified as an unmodified standard product of the computer manufacturer.
The operating system keeps track of what is going on in the computer,
responds to requests from equipment external to the computer and schedules
other programs to run when they are required.

     Development software is comprised of all programs which can be used for
modifying existing programs or generating new ones.  Most commonly this
would consist of assemblers and compilers.  Other programs useful to a user
would permit adding and deleting control system monitoring and control
points, development of new programs, generating operating reports and modi-
fying operation displays.  These programs make the system flexible to accom-
modate future expansion or changes with a minimum of difficulty.  Therefore,
when specifying these programs, the use and the degree of flexibility has to
be clearly stated.

     Applications software includes all process control programs, operating
reports, special operator console displays and all special software which is
unique to the project.

Man/Process Interface (MPIF) —
     The MPIF is an operators console containing equipment by which an
operator can retrieve information regarding plant status and can also per-
form operations.  The equipment to accomplish these tasks consists basically
of a cathode ray tube (CRT) and keyboard.  Configuration of the operators
console may consist of multiple CRT's and keyboards, however, the operation
is the same for each.

     Both hardware and software are involved in defining the MPIF.  However,
the separation between the two is not always clear.  For this reason, when
specifying a MPIF, the emphasis is placed on operations and the steps to
perform them.  Of utmost importance is simplicity for an operator to perform
any given task.  First, the presence of the computer should be transparent
to the operator; in other words, the operator should not be concerned that  a
computer is involved.  Next, an operator trained on the console and with a
few weeks on-the-job experience should be able to retrieve any process
information and perform all operation tasks without a need to reference data
listing or manuals.  A system with five hundred or more input/output points
is obviously too large for a person to remember coded point names associated
                                      377

-------
with each piece of information.  Therefore, a hierarchial design with com-
puter directed prompting should be required as a method for directing the
operator to the desired point.

Testing—
     Thorough testing of the computer system at key points during the
project  is necessary to determine whether the hardware and software are
being correctly integrated.  As a minimum, testing should be performed
before the system is shipped from the contractor's facility, upon completion
of field installation and for a period of extended operation.

     Before delivery to the job site, the hardware and software as a total
system must be debugged.  This is done to minimize problems during startup.
If the debugging is postponed until on-site installation, the number of
problems likely to occur can weaken the confidence of operations personnel
in the ability of the system to ever work.  It is inefficient to perform all
debugging on site as it will unnecessarily prolong the entire startup proce-
dure.  The factory test should thoroughly exercise the system using test
panels to simulate process conditions with reports generating and operator
tasks being performed simultaneously.  Each input and output point should be
tested to determine if  it is wired and performing correctly.

     The field test is  conducted following installation of the system on
site.  All input/output points are wired into the plant equipment, control
loops are tuned and operating, reports can be printed and the plant
personnel are ready to  take over operations via the computer.

     Since the computer system availability is necessary for normal plant
operations, reliability is an important factor.  It is not possible during a
few days of actual testing to experience many of the combinations of events
that can occur during day-to-day operations.  Therefore, it should be a
specification requirement that the system operate for an extended period of
time while meeting a minimum level of performance and on-line operation.
The duration and performance level for the extended test should be deter-
mined based on the configuration, i.e. redundant equipment and the dependen-
cy on the control system to maintain smooth operations with a normal staff.
An example would be to  require a 98% availability for a period of 30
consecutive days with a single computer system that controls six loops.
Longer periods and higher performance may be required for more complex
systems with multiple levels of backup.  However, 30 days should be a
minimum period for any  test.

System Support—
     For continuous long term operation, the user must be prepared to take
over the support of the system.  The specification must address several
items which may differ  from the normal requirements associated with the
purchase of equipment.  These include:

          Warranty
          Maintenance contracts
          Test equipment


                                     378

-------
          Spare parts
          Operations training
          Maintenance training
          Maintenance manuals
          Software documentation
          Program source code

     Even if the user plans to enter into a maintenance contract, all of the
support items should be addressed by the specification.  In this way the
user has the option of entering into a contract with an independent service
organization for long-term support.

SPECIFICATION CHECKLIST

     There are a large number of control options open to the designer of a
control system.  The level of complexity varies from low, such as a start/
stop station at a motor, to high, consisting of all field sensors and
equipment monitored and controlled by a computer with multiple levels of
backup.  At first, the wide array of factors contributing to the design of a
control system may appear confusing.  To help the control system designer
sort out the items that contribute to the specification, the following
checklist has been prepared to aid in selection of pertinent topics which
the contract documents should address.
                                     379

-------
                           SPECIFICATION CHECKLIST
1.   P&ID'S
     A.   Symbology clearly defined
     B.   One per major process (minimum)
     C.   All process piping shown
     D.   All control devices shown
     E.   All primary sensors included
     F.   All panel mounted devices shown
     G.   Interface to computer system or master control panels clearly
          defi ned

2.   I/O POINT LIST

     A.   Numbering system defined
     B.   All analog and digital points included
     C.   Logically linked to P&ID's
     D.   Point names and descriptions specified
     E.   Point type indicated for each point
     F.   Signal ranges, scan times, alarm limits, deadbands, rate of change
          and scaling information given for each point
     G.   Points listed by multiplexer and/or plant area
     H.   Points listed by control loop
     I.   Points listed by point type

3.   INTERFACE DRAWING

     A.   Constant speed motor interface
     B.   Variable speed motor interface
     C.   Open/close valve interface
     D.   Modulating valve interface
     E.   Local panel/multiplexer
     F.   Manual loading stations
     G.   Controllers
     H.   Major equipment items (e.g. blowers, chemical feeders, etc.)

4.   ELECTRICAL & MECHANICAL SPECIFICATIONS

     A.   Valves/actuators/limit switches
     B.   Motor starters
     C.   Package control panels
     D.   Instrumentation wiring
     E.   Communication wiring
     F.   Power wiring
                                     380

-------
5.   INSTRUMENTATION

     A.   Instrument summary list
     B.   Instrument schedule for each instrument  type
     C.   Operating principles
     D.   Enclosures
     E.   Environmental requirements
     F.   Accuracy
     6.   Repeatability
     H.   Linearity
     I.   Indicator accuracy
     0.   Power requirements
     K.   Spare parts and tools
     L.   Mounting details

6.   CONTROL PANELS

     A.   Panel layouts
     B.   Enclosures
     C.   Size limitations
     D.   Future expansion provisions
     E.   Paint
     F.   Nameplates
     6.   Schematic drawings
     H.   List of components for each
     I.   Component specifications:

          1)   Relays
          2)   Indicators
          3)   Recorders
          4J   Switches
          5)   Controllers
          6J   Manual loading stations
          7}   Power supplies

7.   CONTROL LOOP SPECIFICATION

     A.   Overall control strategy diagram and description
     B.   Modes of control
     C.   Supervisory control
     D.   Sequence control
     E.   Failure/restart considerations  i
     F.   Sensor/device failure considerations
     G.   Contingencies
     H.   Interface to other control loops
     I.   CRT displays
     J.   I/O points
     K.   Performance requirements
                                     381

-------
8.   COMPUTER SYSTEM SPECIFICATIONS

     A.   Performance

          1)   Computer system availability
          2)   Failure response

     B.   Hardware specifications

          1)   Configuration
          2)   Computer

               a)  Size
               b)  Speed

          3)   Peripheral equipment

               a)  Disk/drum drives
               b)  Communications links
               c)  High speed printer
               d)  Magnetic tape recorder
               ej  Failover detection module
               f)  Low speed data logger

          4)   Process input/output controller/multiplexer
                a)  Analog  input
                   DC voltage
                   Resistance measurement

                b)  Digital input
                   Binary  coded decimal (BCD)
                   Pulse duration
                   Status/event

                c)  Data integrity
                   Common  mode rejection (CMR|
                   Normal  mode rejection (NMR)

                d)  Analog  outputs
                   DC voltage
                   DC current

                e)  Digital outputs
                   Solid state
                   Electrical hold
                   Timed
                   Current rating

                f)  Expandability
Thermocouple
AC voltage
Binary
Pulse train
Accuracy
Other performance conditions
Relay
Latching
Voltage rating
                                      382

-------
     5)   Uninterruptible power supply (UPS)

C.   Software specifications

     1)   Operating system
     2)   Peripheral drivers
     3)   System development

          a)  Compilers
              Interpreters
              Assemblers
     4)   Diagnostic and test
     5)   Operating reports
     6)   Regulatory agency reports
     7)   Data acquisition
     8)   Process control
     9)   Special application
D.   Man/process interface

     1)   Operator station

          a)  Panel
              Keyboards
              CRT
              Entry security

     2)   CRT displays
          a)  Data points
          bj  Process graphics
          c)  Lab data entry
          d)  Time/date
          e)  Data summary
          f)  Alarm summary
          g)  Process parameter entry
     3)   Operation
          a)  Start/stop equipment
          b)  Display response
          cj  Activate/deactivate automatic controls
          d)  Display request
          e)  Display update
                                383

-------
     4)   Alarm handling

          a)  Setpoint adjustment
          b}  Alarm reporting
          c)  Alarm silence/acknowledge
          d)  Number of alarm limits
          e)  Alarm annunciation
          f)  Alarm clear

E.   Testing

     1)   Factory
     2)   Field

F.   System support

     1)   Warranty
     2)   Maintenance contract
     3)   Test equipment
     4)   Spare parts
     5)   Training
          Submittals
          Final documentation

G.   Site planning/install ation

     1)   Computer room location and size
     2)   Control room location and size
     3)   Cable routing provisions
     4)   Grounding provisions
     5)   Power availability
     6)   Remote located equipment
     7)   Remote equipment communication lines
     8)   Electrical environment
     9)   Ambient environment
                                384

-------
                                  SECTION 7
                                 BIBLIOGRAPHY


How to Specify Valve Actuators, Instrument & Control Systems.  January, 1976,

Selecting an Air Dryer, Instrument & Control Systems. April,  1976.

Kessler, Evan, Checklist for Data Logging/Acquisition Equipment, Instrument
& Control Systems. February, 1978.

McKenzie, R. D., Working With a Construction Contractor	How to Handle
Control System Projects, Instrumentation Technology. April,  1977.

Reinarts, Thomas M., EM & C Systems - How to Design and Specify Them,
Specifying Engineer. February, 1978.
                                     385

-------
                                   SECTION 8

                        RECOMMENDED FUTURE ACTIVITIES
     The basic purpose of this document  is to serve as a guide to consulting
 engineers  and governmental officials  involved in the design and/or specifi-
 cation of  instrumentation and automation for activated sludge treatment
 plants.  This document thus presents  the present technological status of the
 application of instrumentation and  automation in activated sludge plants.
 The  information basically reflects  the observations which were made on the
 field visits and the  general knowledge and experience of the authors and the
 EPA  officials involved in the project.

      It was found that the application  of instrumentation and automation in
 the  field  was significantly below that which was technologically achiev-
 able.  The reasons for this discrepancy  are discussed below under the
 general terms of Design Problems, Process Control Problems and Instrumenta-
 tion Problems.  This  discussion  is  an expansion of the technology based
 problems mentioned in Section 1  of  this  handbook.  In order to overcome
 these problems, a series of research, demonstration and technology transfer
 based activities is recommended  here.

     In addition, a hindrance in the  fruitful application of instrumentation
 and  automation was surfaced by this study.  The problem is a lack of appli-
 cation of  effective modern management techniques to municipal wastewater
 control organizations.  This is  not a new problem per se, but a new realiza-
 tion of the adverse effects of this problem on the proper application of
 available  technology.  Because of its importance, the management problems
 and proposed solutions will be discussed prior to the technical problems
 mentioned  above.

 Management and Organizational Problems

     Similar types of management and  organizational problems appeared with
 great regularity.  This general  observation is, of course, not applicable to
 all the organizations visited; however, management problems were observed
 both in small and large organizations with enough frequency to be a cause of
 concern.  Often these problems resulted  in under-utilization of available
 instrumentation and poor performance of the treatment facility.  Management
problems generally fell into the categories discussed below.
                                     386

-------
Ineffective Management Execution—
   In general,  an adequate hierarchial  organizational  structure  was
   observed in  the management of wastewater  organizations.   However,  the
   use of the chief executive's  time was  observed  to  be  random and unor-
   dered.  The  day-to-day administrative  problems  associated with running
   a growing, new organization dominated  his actions.  Union problems,
   personnel problems and fiscal problems were  observed  to  have  a much
   greater impact on his time than  operations or maintenance planning.   At
   the lower levels of the organization,  there  were many situations where
   an operator  would receive direction  from  more than  one person,  leading
   to confusion.  Management execution  and time management  must  be
   improved. A starting point would be improved policies and procedures.

Policies and Procedures-
   There was a  lack of specifically defined  actions and  reactions for line
   managers and operating staff. Policies and  procedures make better use
   of all managers time because  basic issues do not have to be presented
   or discussed, they are simply executed in accord with policy.  In  this
   way the actions are more consistent.  Policies  and  procedures also
   enhance the  concept of hierarchial communication which was observed  to
   need improvement.

Communication—
   There appeared to be many road blocks  to  good communication at most
   organizations.  Regular staff meetings at the executive  level were
   infrequent.   Line managers often complained  that it was  difficult  to be
   heard because other problems  were always  more critical.   At the  lower
   levels, operators very rarely heard  from  management,  other than  an
   occasional memorandum.  This  lack of management/staff communication
   often caused a definite attitude problem. Regular communication  is  a
   must and should become better with improved  management discipline.

Discipline-
   Management actions and reactions should be consistent and predictable.
   During field visits, managers and operators  commented that policies,
   procedures and schedules were often  not followed.   This  lack  of  disci-
   pline through an organizational  hierarchy often led to the management
   philosophy of "everyone doing their  own thing," causing  operational
   consistency  to be reduced.  In every instance where a solid management
   discipline did not exist, crisis management  reigned.   Discipline must
   start with management taking  time to listen  (feedback) and act
   (followthrough).

Lack of Feedback—
   On numerous  occasions plant operators  expressed a  need for management
   feedback. Management, on the other  hand, only  rarely expressed  a  need
   for operator feedback.  Feedback, on a consistent  basis, improves  both
   parties and  is a necessary element for management  success. Field
   visits indicate that often the only feedback management  hears is  union
   grievances.   This situation must improve.  Effective  feedback should
   lead to effective followthrough.


                                   387

-------
  Lack of Followthrough—
     Operators very frequently commented that management and/or their
     supervisors lacked the time or the desire to follow through on instruc-
     tions.  Line managers would "expect" something to be done and not
     "inspect;" therefore, operators would not take verbal instructions
     seriously.  Without followthrough, the management cycle is incomplete.
     Followthrough is effective management execution.

Recommended Solution

     Managers need ongoing management training specifically aimed at this
industry.  The American Management Association, the Water Pollution Control
Federation and the Environmental Protection Agency can provide this
training.  A handbook should be developed by a management consultant
(supported by EPA) specifically for this industry.  This handbook must
involve all facets of management and address the real-life problems of
managing a process organization where a profit motive does not exist.

TECHNOLOGY BASED PROBLEMS

Design Problems

     Frequent complaints from management and operations people at the
facilities visited indicated that inadequate design hampered their ability
to maintain good, consistent treatment.  In some cases, problems which were
due to improper design or construction were wrongly attributed to poor man-
agement and communication.  These design problems can be categorized as
follows.

  Inoperable Process Equipment--
     In many situations process equipments were found to be either incapable
     of performing as specified or requiring too much operation/maintenance
     and hence were shut down.

  Hydraulic Control —
     At many of the plants visited, the operational personnel indicated they
     could not adequately control routing of the flow to the multiple tanks
     of the various unit processes through the plant.  This difficulty led
     to severe control problems in trying to maintain consistent treatment.

  Distributed Control Centers-
     Distributed control, whether it be executed with manual pushbuttons or
     automatic controllers, was a consistent complaint of management person-
     nel.  There were often as many as ten or eleven local control centers
     at a plant which led to poor communication and poor process consistency.

  Physical Engineering—
     Often equipment was installed in such a way that it could not be
     accessed.  Control panels are often not logically laid out and fre-
     quently instrumentation mounting was such that it was difficult to
     maintain.


                                     388

-------
  Instrumentation Specifications —
     Often specifications were written primarily by a vendor and incorpo-
     rated in a general equipment specification by a consultant.  The lack
     of an independent design and specification by the consultant often
     resulted in poor instrumentation of the plant and, consequently, poor
     process control.  This occurred even when the vendor who wrote the
     specifications was awarded the procurement.

  Control Strategies {Lack of and nonapplication) —
     In Section 3 of this report a large number of process and unit opera-
     tion control strategies in use in the field were detailed.  Unfortu-
     nately, for some important treatment steps such as activated sludge and
     sludge dewatering, truly adequate control strategies have not been
     developed.  The control strategies given in Section 3 must be consid-
     ered sub par, and process efficiency and cost effectiveness are
     seriously impaired as a result.  Fundamental understanding of these
     processes upon which a rational control strategy can be based is not
     available.  In addition, for many other treatment steps, the control
     strategies used are inadequate.  For most, a superior alternate has
     been devised but has not yet been applied at other than pilot scale.

Process Control Problems

     During the field investigations included in this report many operators
and management people were interviewed.  The concept of process control was
discussed in every case and questions were asked relative to how control of
the plant was achieved.  The following widespread problems were evident from
these interviews.

  Conceptual Understanding of Process Control—
     The concept of process control on a real-time basis was not generally
     understood by personnel responsible for management and operation of
     wastewater treatment plants.  The most important concept which was not
     understood is the need to continuously control a flow, a pressure, a
     level or any of a number of other process variables.  The opinion was
     that processes in treatment plants are inherently stable and do not
     need to be controlled.  In addition, these people are unfamiliar with
     the procedures for implementing process control.  Often inappropriate
     techniques were applied to a process control problem.  Needless to say,
     these situations resulted in "automation failures.'"

  Process and Control Dynamics —
     An understanding of the dynamic nature of the flow into the plant and
     the dynamic nature of the treatment process did not exist.  This
     problem amplifies the effect of the lack of conceptual understanding of
     process control.
                                     389

-------
  System Approach—
     Because of the use of distributed control centers throughout the plant,
     the approach of looking at a treatment plant as one system is not
     utilized.  The control procedures used are based on a particular unit
     process, typically with little regard to the impact of a change on
     upstream or downstream processes.

  Crisis Control —
     In an alarming number of cases, operators would not apply process
     control on a predetermined basis but-would control on a crisis basis.
     A plant often would be controlled from one failure to another.  If
     solids started bulking from a clarifier, that problem would be dealt
     with-.  If the mixed liquor suspended,,solids got to an alarmingly low
     level, that problem would be dealt with.  Problems would be handled
     when they reached a critical level, not early in the game when it would
     be easier to cope with.  This is a result of a lack of appreciation of
     process control concepts and the dynamic nature of wastewater treatment
     systems.

  Unclear Objectives—
     In many cases, control objectives were not specifically clear to
     operators.  Often, they felt their job was to keep the process running
     rather than to attain a certain efficiency with a minimum level of
     energy, chemical use and labor.

  Changing Strategies —
     Because procedures were not typically nailed down and objectives not
     clearly called out, shift operators would tend to use their own experi-
     ence or prejudice in controlling a particular unit process.  This would
     lead to changing strategies on a shift basis and cause many problems
     with process consistency from shift to shift.

Instrumentation Problems

     In general, the field investigations generated a good deal of negative
comments relative to instrumentation.  From field observations, the negative
comments are well founded and can be traced to either management problems,
design problems or process control problems.  The most repetitive problems
with instrumentation are as follows.

  Misapplication-
     Misapplication of instrumentation was the dominant cause of mal-
     function.  In many instances an instrument with an inappropriate span
     was being used to measure a quantity.  Also, many instruments were
     poorly mounted such that the sample was not meaningful or the analyzer
     could not receive proper maintenance.  Finally, some instruments could
     not adequately measure the parameter that they were purchased to
     measure.   This points up the need for improvements in instrumentation
     specifications.
                                    390

-------
  Underutilization—
     Because of the process control problems and the lack  of a conceptual
     understanding, instrumentation which in some instances  was properly
     installed and specified was not utilized because it was not thought of
     as necessary or of any value for process control.

  Maintenance--
     For whatever reason (management problems, design mounting problems,
     process control understanding problems), a large portion of the instru-
     mentation was found to be inoperable because it had not received the
     appropriate maintenance.  This led to the inoperability of the equip-
     ment and helped develop the negative attitude toward  instrumentation.

PROPOSED SOLUTION TO TECHNOLOGY BASED PROBLEMS

     The problems described above are complex and fall  in  several cate-
gories.  Some problems are due to a lack of understanding  or lack of
knowledge.  Potential solutions to some problems are available but have not
been tested.  Other problems are well understood and solutions have been
demonstrated but the results of the latest technology simply have not been
distributed to the industry.

     Hence, proposed solutions must take three forms:

     •    Research to develop a better understanding and better knowledge.

     •    Demonstrations to show the industry that theories  can be success-
          fully applied to process control with use of instrumentation.

     t    Technology transfer to distribute state-of-the-art information to
          the industry in a timely fashion.

Recommended Research

     To develop a better understanding of wastewater treatment technology,
the following research should be undertaken as soon as possible.

  Activated Sludge Processes-
     Many versions of the activated sludge processes were  observed in the
     field.  There are also numerous theories with regard  to controlling
     activated sludge processes in terms of F/M, SRT, etc.  The research
     which is recommended involves the development of a practical integrated
     control strategy utilizing the degrees of freedom offered by the acti-
     vated sludge process (RAS, WAS, DO control and location of influent or
     sludge return).

     The complicating factor is that these parameters are  not completely
     independent.  Over several hours that sludge which is not returned to
     the aerator must be wasted because of the limited storage capacity of
     the secondary clarifier for sludge.  The position of  feed and recycle
     in the aerator (step aeration) can have a marked temporary effect on


                                     391

-------
   the solids load to the final clarifier, which in turn will effect both
   the effluent solids and concentration of sludge in the RAS.  In addi-
   tion, any attempt to effect a control strategy which will keep the rate
   of feed of organics and the rate of return of sludge in proper balance
   will produce hydraulic transients in the system unless a good deal of
   off-line sludge storage is provided.  In many situations these tran-
   sients will temporarily overload the final clarifier yielding a poor
   effluent.  The strategy developed must deal with the physical limita-
   tions of the system and the interrelationships of these control
   parameters.

   A research study is needed to establish and document an integrated
   operational control algorithm based on theoretical or empirical
   relationships which can be presented to the industry as a standard.

Sludge Collection, Removal and Thickening—
   On frequent occasions, plant superintendents and operators commented on
   the difficulty of collecting and removing sludge from clarifiers.  This
   seemingly very simple requirement turns out to be a difficult task.
   Removal problems lead to adverse effects on effluent water quality.  A
   study is needed to develop improved methods of collecting, removing and
   thickening sludge so that a more consistent material can be routed to
   downstream unit processes.

Dewatering Processes —
   The most frequent observation at the plants visited with regard to
   dewatering was labor intensity, irrespective of the dewatering
   process.  Basic research is needed to establish a practical measurement
   which can determine or predict dewaterability.  This measurement would
   replace presently used manual techniques.  This parameter and reasons
   for its variation must be studied so that its measurement can be
   utilized for automated control of dewatering processes.

Man/Process Interface Research Needs —
   In both digital and/or analog control systems, the relationship of the
   operator to the process control equipment seems to be a weak point.
   Large analog systems are difficult for an operator to use in a moment
   of crisis.  Some computer systems have cumbersome methods for an
   operator to access and enter data, therefore, they are not convenient
   to use.  The advantages and disadvantages of various methods of
   operator interface to different types of control systems need to be
   examined to determine the simplest and easiest method for an operator
   to interact with the process.  A research grant is needed to study the
   human engineering aspects of automation to prepare the wastewater
   industry for-the future.

Energy Management Research Needs-
   One of the most disturbing observations at many of the treatment plants
   visited was the lack of energy conservation efforts.  Because the
   plants are often operated in the crisis mode, conservation of energy is
                                   392

-------
     a low priority.  Consumption of energy,  chemicals and materials repre-
     sents over 50% of a typical  operating budget.   A study of how automa-
     tion and instrumentation techniques could reduce energy consumption at
     treatment plants is needed.   This research could then be the basis  of  a
     demonstration grant to implement the plan and  document the results.

  Profit Motive Research Needs-
     Many of the plant superintendents and operators mentioned the need  to
     get positive motivation into the water pollution control program.
     Fines for not meeting effluent standards simply are not enough to moti-
     vate plant management to operate the process more efficiently with  less
     utilization of energy, materials and labor.  A study is necessary to
     examine the practical aspects of implementing  positive incentives in
     the water pollution control  program.  The document could serve as an
     initial plan for promoting such a program to Congress should the
     research study illustrate justification.

Demonstrations

     A number of control strategies which are discussed below may be better
than the control strategy presently used in the field.  Demonstrations
should be conducted at full scale to illustrate relative efficiencies.


Strategy #1 - Constant velocity and/or constant overflow rate controlled
              grit chambers—


     Advantages:    Grit separation could possibly  be improved over that
                    achieved in aerated grit  chambers.

     Requirements:  Multiple parallel grit chambers of any of the following
                    types:

                    a)  Parbolic crossection

                    b)  Rectangular crossection

                    c)  Rectangular crossection with discharge through sutro
                        weirs

                    d)  An automated system to bring grit chambers on and off
                        line so as to maintain velocity within a certain
                        range must be supplied.
                                     393

-------
Strategy #2 - Primary sludge pumping on a continuous basis to prevent sludge
              from collecting in the primary clar.ifier—
     Advantages:


     Requirements:
Minimize odors, minimize floated sludge, improve pumping
capability, less maintenance on sludge pumps.

A clarifier and thickener with the appropriate pumping
facilities.  Clarifier should be equipped with either a
sludge level detector or a suspended solids analyzer on
the sludge discharge.  The strategy is to pump sludge
continuously and stop pumping if the sludge level or
concentration fell below a minimum requirement.
Strategy #3 - SRT control of activated sludge wasting rate by directly
              wasting mixed liquor-
     Advantages:


     Requirements:
More direct method of controlling SRT, ease of
automation.

An activated sludge system with the necessary valves,
pumps and meters enabling wasting of mixed liquor on an
automated basis.  Also needed would be a thickener or
chamber for direct receipt of the mixed liquor wasted.
Strategy #4 - The use of activated sludge respiration rate to proportion
              available reactor volume between sludge reaeration and contact
              in a contact stabilization or step aeration system-
     Advantages:


     Requirements:
More efficient use of activated sludge system (higher
loading), improved stability.

An activated sludge system with a compartmentalized
reactor and provision to return sludge and.feed waste-
water independently to any combination of compartments.
Some method of measuring sludge respiration rate is
required.
Strategy #5 - Use of the secondary clarifier only for clarification, not for
              sludge compaction or storage.  The sludge will be continuously
              withdrawn from the clarifier to maintain a very low blanket
              level.  Sludge will be stored off line in an intermediate
              storage chamber so it can be returned to the reactor as needed
              to meet process requirements—
                                     394

-------
     Advantages:    No septicity or sludge carryover in the clarifier,  a
                    more controlled return rate,  avoidance of hydraulic
                    overloads,  a more consistent  process performance.

     Requirements:  Clarifier sludge level and return activated sludge  sus-
                    pended solids measurements with separate return acti-
                    vated sludge storage.   All necessary control  devices
                    such as variable speed drives and modulating  valves must
                    be provided.

Strategy #6 - Pace chemical coagulants based  on feedforward measurements and
              feedback measurements to optimize chemical  use in process
              performance —
     Advantages:
     Requirements:
The demonstration that money can be saved due to
optimized addition of chemicals utilizing a load
following technique with feedback trim.  More consistent
removals of coagulated materials could also be demon-
strated as a result of a controlled chemical dose.

Necessary instrumentation to allow measurement of load
variables and measurement of resultant process perform-
ance, a mechanism for pacing the chemical feed over a
sufficient range compatible with the loads imposed on
the process, and computation capability to allow imple-
mentation of an optimized strategy would be required.
Strategy #7 - Optimization (for energy conservation)  of  ozonation  based  on
              feedforward parameters  such  as  flow,  suspended  solids,  soluble
              organics,  nitrite and  ammonia,  and  the  feedback parameter  of
              effluent COD--
     Advantages:
     Requirements:
A demonstration could show significant reduction in
energy use due to optimized ozonation.  Note:  Ozonation
is a very high power user and over-disinfection must be
practiced because of the difficulty of determining
bacteriological kills.

Available instrumentation for measurement of the
necessay feedforward or load parameters would be
required.  In addition, the necessary sensing instru-
mentation for feedback measurement and computation capa-
bility allowing the execution of an optimized strategy
would also be necessary.  A potential demonstration
location is Springfield, Missouri.
                                     395

-------
Strategy #8 - Optimization and control of chlorination with a load following
              feedforward strategy utilizing measurements of flow, soluble
              organics and suspended solids and a feedback mechanism of
              chlorine residual measurements at various points in the
              contact chamber-
     Advantages:    The demonstration could illustrate a significant
                    reduction in use of chlorine as well as a more consis-
                    tent effluent residual and hence a more consistent
                    bacteriological kill.  Field results have shown that
                    chlorination is typically a difficult process to opti-
                    mize.  The problems involve the large time constants in
                    the contact chamber and the difficulty in making the
                    necessary measurements.

     Requirements:  The instrumentation to measure the necessary load
                    variables must be available.  In addition, the configu-
                    ration of the chlorine contact tank and the chlorine
                    residual analyzers in the chlorinator itself must be
                    such that the time constants involved are minimized.
                    Necessary feedback instrumentation is required as well
                    as the computation capability allowing the execution of
                    an advanced strategy.
Strategy #9 - Optimization of gravity thickening based on maintaining a
              stable sludge blanket and consistent underflow sludge
              concentration —
     Advantages:
     Requirements:
It can be expected that a consistent operation of a
gravity thickener will improve the operation of down-
stream dewatering and stabilization processes.  A stable
blanket should maximize capture in the gravity thick-
ener, thus lowering the impact of the recycle liquors on
the  liquid process.  A limited demonstration of these
advantages has been shown at Minneapolis-St. Paul.

The  necessary instrumentation for consistently control-
ling the feed flow to the unit and the underflow from
the  unit would be necessary.  Instrumentation for
control of dilution water and for measurement of sludge
blanket level and underflow concentration, indication or
measurement of overflow conditions, and the necessary
computation capability for implementing this strategy
would also be necessary.
                                      396

-------
Strategy #10 - Automate a flotation thickener to maintain a constant
               air-solids ratio while dosing polymer on the basis of feed-
               forward mass flow of solids and feedback of thickener
               overflow suspended solids with limits to assure overdosing is
               avoided—
     Advantages:    Improved solids capture and a more consistent floated
                    cake can be achieved through automation of flotation
                    thickener.   This process rarely receives the operational
                    attention required because of the disagreeable environ-
                    ment in the process area.

     Requirements:  A flotation thickener would have to be equipped with
                    control of influent flow as well as the sensor instru-
                    mentation for measurement  of the concentration of  the
                    feed sludge.  Control of the recycle flow and control of
                    recycle pressure,  instrumentation for control of polymer
                    addition and computation capability allowing implementa-
                    tion of this strategy would also be necessary.  Measure-
                    ment of the blanket thickness and measurement of
                    overflow suspended solids  as well as thickened sludge
                    suspended solids would be  advantageous.


Strategy #11 - Adjust the flocculator  speed automatically to maintain  a root
               mean square velocity gradient (G) in a restricted range-


     Advantages:    Improved performance of the flocculator and hence
                    improved process performance could be expected.

     Requirements:  The capability of  varying  the speed or applied power of
                    a mixer-flocculator as flow varies, and measuring  the
                    power transmitted  to the liquid would be required.


Strategy #12 - Automate the complete cycle of  a continuous filter press
               operation.  The  feed cycle will be terminated based on
               pressate volume  which will initiate the cake removal  cycle.
               Automation of the removal cycle should be demonstrated  as
               well as readying the press for  the next charge cycle—
                                     397

-------
     Advantages:
     Requirements:
Field investigations have shown that the plate press is
a very labor intensive operation.  If complete automa-
tion could be demonstrated, significant savings could be
achieved in fairly menial operational labor functions.
In addition, complete automation could lead to
programmed utilization of the plate press based on load
which would lead to solids inventory control capa-
bility.  Finally, ending of a press cycle based on
volume of pressate is a more discrete and specific means
of determining when the charge cycle has ended.

A plate press equipped with automatic sludge removal and
properly instrumented with a means of measuring pressate
volume would be required.  Measurement of influent
sludge concentration and flow as well as pressate sus-
pended solids concentration, control  of polymer addition
(if utilized) and execution capability to enable imple-
mentation of the advanced strategy would also be
required.
Strategy #13 - Inventory Scheduling and Control in the Solids Process Train
               Including Thickening, Holding, Stabilization and Final
               Disposal —
     Advantages:
     Requirements:
Field investigations indicate that planning and consis*-
tently executing control of solids inventory is one of
the most difficult problems in wastewater treatment.
Solids inventory control would enable optimized utiliza-
tion of the various solids treatment processes and would
lead to better management of a very difficult treatment
area.  Reduced costs for labor, chemicals and power can
be expected from a unified strategy of inventory
scheduling and controlled solids processing.

A treatment plant equipped with a solids processing
train would require centralized control to implement an
inventory scheduling strategy.  This would include the
necessary instrumentation and control devices, as well
as execution capability for programmed inventory
scheduling.
Strategy #14 - Control of anaerobic digestion by:

               a)   Feeding in response to daily methane production per unit
                    of volatile solids fed.
               b)   Daily measurement of alkalinity and volatile acids with
                    addition of alkaline material to keep alkalinity at
                    least 100 mg/1 less than volatile acids.
                                     398

-------
               c)   Maintain temperature at setpoint _+ loc.   Recommended
                    setpoint is in range of 33 to 37oc.
               d)   Feed schedule to insure reasonable mixing of new feed
                    with digester contents.  If digester is  well  mixed,  feed
                    can be done over a one to two-hour period with low
                    solids cutoff on feed pumps and volume of feed not to
                    exceed the detention time setpoint.   Recommended
                    detention time setpoint is 10 to 30 days.
               e)   Withdrawal of digested sludge based on average setpoint
                    detention time.   Sludge removal prior to feed cycle.
               f)   Monitor mixer gas flow or torque to insure mixing is
                    taking place.


     Advantages:     Improve consistency of gas production in terms of volume
                    as well as composition leading to more effective energy
                    management.  More consistent volatile destruction within
                    the digester leading to more consistent  dewaterability.
                    A demonstration  of a unified control strategy that can
                    lead to consistent operation of anaerobic digesters.

     Requirements:   An anaerobic digester equipped with the  necessary
                    instrumentation  to identify the volume and composition
                    of the charged materials as well as the gas flow and
                    composition and  allowing for the addition of  supplemen-
                    tary alkalinity.  The necessary program and interface
                    capability to allow execution of an automated strategy
                    as well as recording of results in a consistent manner.


Strategy #15 - Optimize the operation of a continuous low pressure oxidation
               unit to demonstrate reduced steam utilization and  consistent
               dewaterability without the use of chemicals-
     Advantages:
     Requirements:
A better understanding of the low pressure oxidation
unit process would be forthcoming as well as demonstra-
tion that pressure/temperature stabilization can be
effectively executed yielding a reduction in dewatering
chemical usage.  As a result of the optimized control,
improved energy management of the low pressure oxidation
process would be forthcoming.

A low pressure oxidation treatment train equipped with
the necessary sensor instrumentation, centralized
control and computation as well as data retention capa-
bility to implement and monitor the results on a
continuous basis would be necessary.
                                     399

-------
Strategy #16 - Automate a vacuum filtration operation based on a real-time
               measurement of sludge of specific resistance.   Control vacuum
               level, drum speed and submergence and chemical  feed to
               produce a constant solids level in the cake--
     Advantages:
     Requirements:
A more consistent sludge cake would lead to better
energy management in the final disposal of the sludge
through incineration.  More consistent operation based
on the predetermined strategy could lead to reduced
chemical usage.  More consistent strategy and demonstra-
tion of automation will lead to less operational
intensity and the associated cost savings.

An instrumented vacuum filter would be necessary
allowing centralized control of feed as well as removal
including the chemical additions.  The mechanism for
programming the automated strategy and recording the
results on a continuous basis would also be necessary.
Note:  This demonstration is presently taking place at
Minneapolis-St. Paul under an EPA Grant.
Strategy #17 - Automation of sludge incineration to:

               a)   Maintain a minimum excess oxygen  content in the
                    discharge gases by adjusting combustion air.
               b)   Control temperature profile by adjusting rabble arm
                    drive speed.
               c)   Control hearth temperatures on a  closed loop basis--
     Advantages:
     Requirements:
Automation of this unit process will lead to substantial
reductions in energy utilization either of fuel oil or
natural gas.  In addition, because of the volatile
nature of the process, labor intensity is high and auto-
mation of the process could lead to substantial savings
in operational costs.

The necessary instrumentation for measuring the load and
the process temperature and pressure measurements as
well as centralizing the control capability of the
entire process is necessary.  The necessary computation
capability and data retention in reporting capability is
also necessary.
                                     400

-------
Strategy #18 - Recycled liquors handling and treatment.   Two strategies
               should be demonstrated.   One strategy involves storing the
               return liquors  for  return to the  influent plant during times
               of low plant  load.   The  second strategy  involves treating the
               return liquors  prior to  return to the head end of the plant--
     Advantages:
     Requirements:
                    The advantages to be expected are improved liquid
                    process performance on a consistent basis.  Field
                    observations indicate that the hydraulic, solids and
                    organic loads imposed by return liquors are often the
                    cause of many process problems.

                    For the two strategies discussed above, two equipment
                    configurations are required for demonstration.  A plant
                    must have a large vessel capable of routing return
                    liquors and the appropriate control devices and instru-
                    mentation to allow programmed release of the material on
                    a control basis to the head end of the plant.  The
                    second strategy involves having a treatment plant speci-
                    fically for return liquors handling and treatment where
                    it can be demonstrated that the load (predominantly
                    organics and solids) can be reduced significantly
                    through treatment.  A demonstration should include
                    improvement of process performance due to the treatment
                    of return liquors on a continuous basis.  The Metro
                    Plant of the Metropolitan Minneapolis-St. Paul Waste
                    Control Commission or the Mill Creek Plant of Greater
                    Cincinnati  MSD could be used for this purpose.

Technology Transfer

     Publications are needed on a continuing basis to transfer information
to the industry regarding instrumentation and automation.  It is recommended
that the following technology transfer documents be published as soon as
possible and that others be planned on a continuing basis.

  Case History of Automation Benefits—
     A frequently heard comment was that automation is more trouble than it
     is worth.  Studies of automated plants should be conducted to document
     the control system operation in these projects to serve as models for
     other potential users and  should be published as Technology Transfer
     manuals.  Case studies of  situations where automation resulted in
     improvement in plant performance and/or reduction in costs would docu-
     ment the actual benefits of automation at wastewater treatment plants.
                                     401

-------
Instrumentation Specifications-
   Instrumentation and control very frequently-received bad ratings.  Poor
   reliability and excessive maintenance requirements were often given as
   reasons.  Lack of training and a feeling of not knowing the purpose of
   instrumentation were other very prominent causes of lack of acceptance
   of  instrumentation.  To  improve the reliability of instrumentation
   applied in future wastewater treatment facilities, sample instrument
   specifications should be published including information on applica-
   tion, installation, maintenance and the like.

Combustion Processes—
   Many variations of final sludge disposal were observed.  Frequently
   these included combustion, in one form or another.  Incineration of
   sludge is not only a labor intensive process, but in most of the
   installations observed, an expensive, energy inefficient process.  A
   Technology Transfer grant is needed to develop a "sludge combustion
   control manual" for the wastewater industry.

Hydraulics-
   Plant operations personnel and superintendents often mentioned that
   hydraulics appear to cause many of their process problems.  On numerous
   occasions, it was stated that the concept of the hydraulic design was
   to  split flow evenly with weirs or splitter boxes throughout the
   plant.  Repeatedly flow was found not to split equally, causing over-
   loading on some clarifiers and washouts in aeration tanks.  The reason
   why flow is not controlled is because design techniques used to cope
   with load variations from design conditions are not adequate.  What is
   needed is a full theoretical and practical analysis of the dynamics of
   flow splitting to multiple unit processes, illustrating anticipated
   comparative results between weirs, splitter boxes, valves and meters.

Control and Centralization Review-
   Observations of various installations illustrated that operations were
   conducted from centers of control distributed around the plant and that
   it was desired to centralize these functions.  The usual reason cen-
   tralization of information and control was not implemented was the cost
   involved for such a system.   A preliminary economic analysis (see
   Section 5) indicates that with modern equipment, centralization should
   be cheaper than distributed control.   A case history is needed to docu-
   ment the advantages of centralization and the techniques available to
   the designer for implementing centralization.

Automation and Process Control  Training—
   A training program should be developed to familiarize operators with
   automation systems and training them to function with this new tech-
   nology.  This  will alleviate their fear of the unknown, and prevent the
   establishment  of an adversary relationship between man and machine.
                                  402

-------
                                   GLOSSARY
access time:  1. The time it takes a computer to locate data or an
     instruction word in its storage section and transfer it to its
     arithmetic unit where the required computations are performed.  2. The
     time it takes to transfer information which has been operated on from
     the arithmetic unit to the location in storage where the information is
     stored.

actuator:  A mechanism for translating a signal into the corresponding
     movement or control.  Typically the actuator moves a valve.  See also
     final control element.

algorithm:  A prescribed set of well-defined rules or processes for the
     solution of a problem in a finite number of steps, for example, a full
     statement of an arithmetic procedure for evaluating sin X to a stated
     precision.

alphanumeric:  Pertaining to a character set that contains both letters and
     digits and usually, other characters such as punctuation marks.

amplifier:  A device that enables an input signal to control power from a
     source independent of the signal and thus be capable of delivering an
     output that bears some relationship to, and is generally greater than,
     the input signal.

analog:  Pertaining to representation of numerical quantities by means of
     continuously variable physical characteristics.  Contrast with digital.

analog control:  Implementation of automatic control loops with analog
     (pneumatic or electronic) equipment.

analog device:  A mechanism which represents numbers by physical quantities,
     i.e. by lengths, as in a slide rule, or by voltage or currents as in a
     differential analyzer or a computer of the analog type.

analog signal:  An analog signal  is a continuously variable representation
     of a physical quantity, property, or condition such as pressure, flow,
     temperature, etc.  The signal may be transmitted as pneumatic,
     mechanical or electrical energy.

analog-to-digital (A/D):  The conversion of analog data to digital data.
                                    403

-------
analog-to-digital converter (AD):  Any unit or device used to convert analog
     information to approximate corresponding digital information.

annunciator:  A visual or audible signaling device and the associated
     circuits used for indication of alarm conditions.

ANSI:  Abbreviation for American National Standards Institute, Inc.
     Formerly called USASI.

ASCII:  Abbreviation for American Standard Code for Information
     Interchange.  Also known as USASCII, Q.V.

asynchronous:  Pertaining to a lack of time coincidence in a set of repeated
     events where this term is applied to a computer to indicate that the
     execution of one operation is dependent on a signal that the previous
     operation is completed.

asynchronous transmission:  Transmission in which each information character
     is individually synchronized, usually by the use of start and stop
     elements.

automatic control system:  A control system which operates without human
     intervention.

batch processing:  1. Pertaining to the technique of executing a set of
     programs such that each is completed before the next program of the set
     is started.  2. Loosely, the execution of programs serially.

bias:  1. The departure from a reference value of the average of a set of
     values; thus, a measure of the amount of unbalance of a set of
     measurements or conditions.  2. The average DC voltage or current
     maintained between a control electrode and the common electrode in a
     transistor.

binary coded decimal (BCD):  Describing a decimal notation in which the
     individual decimal digits are represented by a group of binary bits,
     e.g., in the 8-4-2-1 coded decimal notation each decimal digit is
     represented by a group of four binary bits.  The number twelve is
     represented as 0001 0010 for 1 and 2, respectively, whereas in binary
     notation it is represented as 1100.

bit:  1. An abbreviation of binary digit.  2. A single character in a binary
     number.  3. A single pulse in a group of pulses.  4. A unit of
     information capacity of a storage device.  The capacity in bits is the
     logarithm to the base two of the number of possible states of the
     device.  Related to storage capacity.
                                     404

-------
buffer:  1. An internal portion of a data processing system serving as
     intermediate storage between two storage or data handling systems with
     different access times or formats; usually to connect an input or
     output device with the main or internal high-speed storage.  2. An
     isolating component designed to eliminate the reaction of a driven
     circuit on the circuits driving it, e.g. a buffer amplifier.

byte:  1. A generic term to indicate a measurable portion of one or more
     contiguous binary digits, e.g., an 8-bit or 6-bit byte.  2. A group of
     binary digits usually operated upon as a unit.

capacity:  The ability to store material or energy which acts as a buffer
     between the input and the output of a control loop element.

cascade control:  The use of two conventional feedback controllers in series
     such that two loops are formed, one within the other.  The output of
     the controller in the outer loop modifies the setpoint of the
     controller in the inner loop.

cathode ray tube (CRT):  1. An electronic vacuum tube containing a screen on
     which information may be stored for visible display by means of a
     multigrid modulated beam of electrons from the thermionic emitter,
     storage effected by means of charged or uncharged spots.   2. A storage
     tube.   3. An oscilloscope tube.  4. A picture tube.  5. A computer
     terminal using a cathode ray tube as a display device.

central processing unit (CPU):  A unit of a computer that includes circuits
     controlling the interpretation and execution of instructions.

closed-loop:  A signal path formed about a process by a feedback measurement
     signal (input to a controller) and the signal delivered to the final
     control element (controller output signal).

compile:   To prepare a machine language program from a computer program
     written in another programming language by making use of the overall
     logic  structure of the program, or generating more than one machine
     instruction for each symbolic statement, or  both,  as well as performing
     the function of an assembler.

compiler:  A computer program more powerful than  an assembler.  In addition
     to its translating function which is generally the same process as that
     used in an assembler,  it is able to replace  certain items of input with
     series of instructions,  usually called subroutines.  Thus,  where an
     assembler translates item for item, and produces as output the same
     number of instructions or constants which were put into it, a compiler
     will do more than this.   The program which results from compiling is a
     translated and expanded  version of the original.
                                    405

-------
computer:  1. A data processor that can perform substantial computation,
     including numerous arithmetic or logic operations, without intervention
     by a human operator during the run.  2. A device capable of solving
     problems by accepting data, performing described operations on the
     data, and supplying the results of these operations.  Various types of
     computers are calculators, digital computers and analog computers.

control:  1. Frequently, one or more of the components in any mechanism
     responsible for interpreting and carrying out manually initiated
     directions.  2. In some application, a mathematic check.  3.
     Instructions which determine conditional jumps are often referred to as
     control instructions, and the time sequence of execution of
     instructions is called the flow of control.

control mode:  A specific type of control action such as proportional,
     integral or derivative.

control sequence:  The normal order of selection of instructions for
     execution.  In some computers one of the addresses in each instruction
     specifies the control sequence.  In most other computers, the sequence
     is consecutive except where a transfer occurs.

control system:  A system in which deliberate guidance or manipulation is
     used to achieve a prescribed value of a variable.

control valve:  A final controlling element through which a fluid passes,
     which adjusts the size of flow passage as directed by a signal from a
     controller to modify the rate of flow of the fluid.

controller:  A device which operates automatically to regulate a controlled
     variable by comparing a measurement of the variable with a reference
     value representing the desired level of operation.

dead band:  A specific range of values in which the incoming signal can be
     altered without also changing the outgoing response.

dead time:  The interval of time between initiation of an input change or
     stimulus and the start of the resulting observable response.

derivative action:  A controller mode which contributes an output
     proportional to the rate of change of the error.

differential gap control:  Two-position control in which the controller
     output changes only after the controlled variable passes through a dead
     band range.

digital:  Pertaining to representation of numerical quantities by discrete
     levels or digits conforming to a prescribed scale of notation.
                                     406

-------
,direct  acting  controller:  A controller  in which the  value of  the  output
      signal  increases as the value of the input (measured variable)
      increases.

 direct  digital control  (DDC):  A control technique  in which a  digital
      computer  can be used as the sole controller and  its output can be
      connected directly to the final control element.  Used to distinguish
      from  analog control.

 disturbance:  A change  in the operating condition of  a process, most commonly
      a  change  in input  or output loading.

 error:  The  difference  between the setpoint reference value and the value of
      the measured signal.

 feedback:  The signal in a closed-loop system representing the condition of
      the controlled variable.

 feedback control:  Control in which a measured variable is compared to  its
      desired value to produce an actuating error signal which  is acted  upon
      in such a way as to reduce the magnitude of the  error.

 feedforward  control:  Control in which information  concerning  one  or more
      conditions that can distrub the controlled variable is converted,
      outside of any feedback loop, into corrective  action to minimize
      deviations of the  controlled variable.

 final control element:  The device used to directly change the value of the
      manipulated variable.

 instrumentation:  A collection of instruments or their application for  the
      purpose of observation, measurement or control.

 integral action:  A controller mode which contributes an output proportional
      to the  integral of the error.

 integral time:  The time required after a step input  is applied for the
      output of a proportional plus integral mode controller to change by an
      amount equal to the output due to proportional action alone.

 loop  gain:  The ratio of the change in the return signal to the change  in
      its corresponding  error signal at a specified  frequency.  Note:  the
      gain of the loop elements is frequently measured by opening the loop,
      with  appropriate termination.  The gain so measured is often  called the
      open  loop gain.

 loop  gain  characteristics:  Of a closed loop, the characteristic curve  of
      the ratio of the change in the return signal to  the change in the
      corresponding error signal for all real frequencies.
                                     407

-------
main frame:  1. The central processor of the computer system.  It contains
     the main storage, arithmetic unit and special register groups.
     Synonymous with CPU and central processing unit.  2. All that portion
     of a computer exclusive of the input, output, peripheral and in some
     instances, storage units.

manipulated variable:  The process variable that is changed by the
     controller to reduce or eliminate error.

manual reset:  Elimination of offset by adjustment of the output bias level
     of a proportional controller.

multi-element (multi-variable) control system:  A control system utilizing
     input signals derived from two or more process variables for the
     purpose of jointly affecting the action of the control system.

multi-processing:  Pertaining to the simultaneous execution of two or more
     programs or sequences of instructions by a computer or computer network

multi-programming:  Pertaining to the concurrent execution of two or more
     programs by a single computer.

multi-tasking:  The facility that allows the programmer to make use of the
     multi-programming capability of a system.

multiplexer (MUX):  A device which samples input and/or output channels and
     interleaves signals in frequency or time.

offset:  The steady-state deviation of the controlled variable from the
     setpoint caused by a change in load.

on/off control:  A system of regulation in which the manipulated variable
     has only two possible values, on and off.

open-loop:  A signal path without feedback.

optimization:  A process whose object is to make one or more variables
     assume, in the best possible manner, the value best suited to the
     operation at hand, dependent on the values of certain other variables
     which may be either predetermined or sensed during the operation.

primary element:  The device which converts a portion of the energy of the
     variable to be measured to a form suitable for amplification and
     retransmission by other devices.

priority:  Level of importance of a program or device.
                                     408

-------
priority interrupt:  The temporary suspension of a program currently being
     executed  in order to execute a program of higher priority.  Priority
     interrupt functions usually include distinguishing the highest priority
     interrupt active, remembering lower priority interrupts which are
     active, selectively enabling or disabling priority interrupts,
     executing a jump instruction to a specific memory location, and storing
     the program counter register in a specific location.

process:  1. The collective functions performed in and by industrial
     equipment, exclusive of computer and/or analog control and monitoring
     equipment.  2. A general term covering such items as assemble, compile,
     generate, interpret and compute.

process control:  Descriptive of systems in which controls are used for
     automatic regulation of continuous operations or processes.

process I/O:  Input and output operations directly associated with a
     process, as contrasted with I/O operations not associated with the
     process.  For example, in a process control system,  analog and digital
     inputs and outputs would be considered process I/O whereas inputs and
     outputs to bulk storage would not be process I/O.

proportional action:  A controller mode which contributes an output
     proportional to the error.

proportional band:  The range of the controlled variable that corresponds to
     the full range of the final control element.

range:   The region between the limits within which a quantity is measured,
     received or transmitted, expressed by stating the lower and upper range
     values.

rate time:  For a linearly changing input to a proportional plus derivative
     mode controller, it is the time interval by which derivative action
     advances the effect of proportional action.

ratio control:   Control in which a secondary input to a process is regulated
     to maintain a preset ratio between the secondary input and an
     unregulated primary input.

repeats per minute:  Controller integral mode adjustment units.  The
     inverse of integral  time.

reset windup:  In a controller containing integral action, the saturation
     of the controller output at a high or low limit due to integration of a
     sustained deviation  of the controlled variable from the setpoint.

reverse acting controller:   A controller in which the value of the output
     signal decreases as  the value of the input (measured variable)
     increases.
                                     409

-------
setpoint:  A reference source which represents the desired value of the
     controlled variable.

signal:  1. The event or phenomenon that conveys data from one point to
     another.  2. A time dependent value attached to a physical phenomenon
     and conveying data.

simulation:  The representation of certain features of the behavior of a
     physical or abstract system by the behavior of another system, for
     example, the representation of physical phenomena by means of
     operations performed by a computer or the representation of operations
     of a computer by those of another computer.

software:  A set of programs, procedures, rules and possibly associated
     documentation concerned with the operation of a computer system, for
     example, compilers, library routines, manuals, circuit diagrams.

steady-state:  A characteristic of a condition, such as value, rate,
     periodicity, or amplitude, exhibiting only negligible change over an
     arbitrary long period of time.  It may describe a condition in which
     some characteristics are static, others dynamic.

supervisory control:  A control technique in which a digital computer is
     used to determine and fix setpoints for conventional analog
     controllers.  Used to distinguish from direct digital control.

synchronous transmission:  Transmission in which the sending and receiving
     instruments are operating continuously at substantially the same
     frequency and are maintained by means of correction, in a desired phase
     relationship.

telemetering:  The transmission of a measurement over long distances,
     usually by electromagnetic means.

throttling control:  Control which directs a final control element to
     intermediate points within its operating range; distinguished from
     on/off control.

time constant:   The time required for the output of a single capacity
     element to change 63.2 percent of the amount of total response when a
     step change is made in its input.

transducer:  An element or device which receives information in the form of
     one physical quantity and converts it for transmission, usually in
     analog form.  This is a general definition and applies to specific
     classes of devices such as primary element, signal transducer and
     transmitter.
                                    410

-------
transfer function:  A mathematical, graphical or tabular statement of the
     influence which a system or element has on a signal or action compared
     at input and output terminals.

transmitter:  A transducer which responds to a measured variable by means of
     a sensing element, and converts it to a standardized transmission
     signal which is a function only of the measured variable.

watchdog timer: ,An electronic internal timer which will generate a priority
     interrupt unless periodically recycled by a computer.   It is used to
     detect program stall or hardware failure conditions.

word:  A character string or a bit string considered as an entity.
                                     411

-------
                                   TECHNICAL REPORT DATA
                            (Please read Instructions on the reverse before completing)
1. REPORT NO.
 EPA-600/8-80-028
2.
                             3. RECIPIENT'S ACCESSIOI*NO.
4. TITLE AND SUBTITLE

  DESIGN HANDBOOK FOR AUTOMATION OF ACTIVATED
  SLUDGE WASTEWATER TREATMENT PLANTS
                             5. REPORT DATE
                               August 1980
              (Issuing Date)
                             6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
  Alan W. Manning
  David M. Dobs
                             8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
  EMA, Inc.
  St. Paul, Minnesota   55101
                              10. PROGRAM ELEMENT NO.
                                 1BC821
                              11. CONTRACT/GRANT NO.
                                 68-03-2573
 12. SPONSORING AGENCY NAME AND ADDRESS
  Municipal Environmental Research Laboratory-
  Office of Research and Development
  U.S. Environmental Protection Agency
  Cincinnati, OH   45268
                  -Cinn., OH
13. TYPE OF REPORT AND PERIOD COVERED
   Final Report 7/77-2/79
                              14. SPONSORING AGENCY CODE
                                 EPA/600/14
 15. SUPPLEMENTARY NOTES
  Project Officer:  Irwin J. Kugelman    (513-684-7633)
 16. ABSTRACT
       This report is a systems engineering handbook for the automation of activated
  sludge wastewater treatment processes.   Process control theory and  application are
  discussed to  acquaint the reader with  terminology and fundamentals.   Successful
  unit process  control strategies currently in use are discussed.   Alternative methods
  of control  implementation are presented  where other considerations  such as
  reliability or  flexibility are important.   A method for preparing a cost effective
  analysis is detailed through the use of  examples.  Currently available instrumentatior
  is reviewed to  serve as a guide for the  selection of instruments  for specific
  applications.   The design guide section  reviews some of the aspects of control system
  design and  includes examples of documentation required to convey  the engineer's and
  user's requirements.  The concluding section presents recommendations for further
  studies which will advance the application of automation in wastewater treatment.
17.
                                KEY WORDS AND DOCUMENT ANALYSIS
                  DESCRIPTORS
                b.lDENTIFIERS/OPEN ENDED TERMS  C.  COSATI Field/Group
  Automation, Automatic Control,
  Automatic Control  Equipment,
  Data Processing, Digital Computers,
  Instruments, Waste Treatment ,
  Wastewater Process Control,
  Centralized Control
                   Activated Sludge
                   Process Control  Theory
                   Treatment Plant  Design
                13B
18. DISTRIBUTION STATEMENT

  Release to Public
                 19. SECURITY CLASS (ThisReport)
                   Unclassified
                                                                         21. NO. OF PAGES
                   426
                 20. SECURITY CLASS (Thispage)
                   Unclassified
                                           22. PRICE
EPA Form 2220-1 (9-73)
                                            412
                                                                   OU.S GOVERNMENT PRINTINe OFFICE: 1980-657-165/0141

-------