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
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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.
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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
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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:
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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.
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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
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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.
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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.
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Flotation Thickening
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33
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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.
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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
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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
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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
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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
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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.
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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.
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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
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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
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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
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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
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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
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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.
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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
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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.
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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.,
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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.
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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
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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
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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.
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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
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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
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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.
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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.
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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.
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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
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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
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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
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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.
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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.
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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).
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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
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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.
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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.
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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.
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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.
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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.
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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
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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.
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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.
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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(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
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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
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(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
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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
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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
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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
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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
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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
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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
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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.
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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
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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.
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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.
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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.
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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.
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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
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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
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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
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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.
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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
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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.
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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.
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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
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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
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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
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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
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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
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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
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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
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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.
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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.
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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.
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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.
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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.
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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.
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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
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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
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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.
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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
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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
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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
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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).
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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
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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
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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
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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
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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
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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
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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.
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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
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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
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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.
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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
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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
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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
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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
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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
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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.
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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.
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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.
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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.
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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.
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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.
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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
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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
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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.
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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—
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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.
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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.
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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—
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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,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.
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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
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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
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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
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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
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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
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