EPA 625/l-75-003a
                     PROCESS DESIGN MANUAL
                               FOR
                    SUSPENDED SOLIDS REMOVAL
            U.S. ENVIRONMENTAL PROTECTION AGENCY
                         Technology Transfer
                           January 1975

-------
                            ACKNOWLEDGEMENTS
This manual was prepared for the Technology Transfer Office of the U.S. Environmental
Protection Agency by Hazen and Sawyer, Engineers. The previous edition of this manual
(October,  1971) was prepared for Technology Transfer by Burns and  Roe, Inc. Major
U.S.VEjPA. contributors  and ; reviewers  were 'J.F.  Kreissl  and J.M.  Smith : of  the
U.S.EPA National Environmental Research Center, Cincinnati, Ohio, and D.J. Lussier of
Technology Transfer, Washington, D.C.
                                     NOTICE

The mention of trade names or commercial products in this publication is for illustration
purposes and does not constitute endorsement  or  recommendation for  use  by the U.S.
Environmental Protection Agency.

-------
                                    ABSTRACT

This manual surveys current practice in the removal of suspended solids in both traditional
and advanced treatment of  municipal wastewater. Specific processes are described, design
considerations are discussed and results are illustrated by data from actual installations.

Included are  processes  such  as sedimentation, straining and granular media filtration
which affect physical separation of solids as well as coagulation and flocculation processes
which alter  solids characteristics to facilitate such separation. Detailed information is also
provided concerning handling and application of coagulant chemicals.

Aspects  of  operation and maintenance pertinent  to  design are discussed and estimated
costs of construction and operation are provided for particular processes.
                                         in

-------
                          TABLEOFCONTENTS

CHAPTER                                                          PAGE

              ACKNOWLEDGMENTS                                 jj
              ABSTRACT                                             jjj
              TABLEOFCONTENTS                                  v
              LIST OF FIGURES                                      vii
              LIST OF TABLES                                        xjji
              FOREWARD                                            xv

    1          INTRODUCTION
                1.1      Purpose                                        1-1
                1.2      Wastewater Solids                               1-1
                1.3      References                                     1-3
    2          GENERAL DESIGN CONSIDERATIONS
                2.1      Applications of Suspended                        2-1
                        Solids Separation Processes
                2.2      Process Selection                               2-1
    3          FLOW VARIATIONS AND EQUALIZATION
                3.1      Flow Variation                                 3-1
                3.2      Performance vs. Flow Variation                   3-1
                3.3      Flow Equalization                               3-2
                3.4      References                                     3-2
    4          PRINCIPLES OF CHEMICAL TREATMENT
                4.1      Introduction                                    4-1
                4.2      Destabilization Mechanisms                      4-1
                4.3      Selection of Chemical Coagulants                 4-2
                4.4      Coagulation Control                             4-6
                4.5      References                                     4-16
    5          STORAGE AND FEEDING OF CHEMICALS
                5.1      General                                        5-1
                5.2      Aluminum Compounds                           5-1
                5.3      Iron Compounds                                5-16
                5.4      Lime                                          5-24
                5.5      Other Inorganic Chemicals                       5-34
                5.6      Polymers                                      5-50
                5.7      Chemical Feeders                               5-59
                5.8      References                                     5-66
    6          CHEMICAL MIXING, FLOCCULATION AND
              SOLIDS-CONTACT PROCESSES
                6.1      Introduction                                    6-1
                6.2      Chemical Mixing                               6-3
                6.3      Flocculation                                    6-5
                6.4      Solids-Contact Processes                         6-8
                6.5      References                                     6-12

-------
                   TABLE OF CONTENTS - Continued
CHAPTER                                                PAGE
    7        GRAVITY SEPARATION
     10
7.1 Introduction
7.2 Configuration of Sedimentation units
7.3 Basic Factors Affecting Settling Tank Design
7.4 Clarifier Design Considerations
7.5 Primary Sedimentation
7.6 Secondary Sedimentation
7.7 Chemical Sedimentation
7.8 Flotation
7.9 Shallow Settling Devices
7. 1 0 Wedge-Wire Settlers
7.1 1 References
PHYSICAL STRAINING PROCESSES
8.1 General
8.2 Wedge-Wire Screens
8.3 Microscreening
8.4 Other Screening Devices
8.5 Diatomaceous Earth Filters
8.6 Ultrafiltration
8.7 References
GRANULAR MEDIA FILTRATION
9.1 Introduction
9.2 Process Alternatives
9.3 Process Variables
9.4 Selection of Filtration Rate
and Terminal Headless
9.5 Filtration Media
9.6 Filter Control Systems
9.7 Filter Cleaning Systems
9.8 Filter Structures and General
Arrangement
9.9 Pilot Studies
9.10 Special Designs
9.11 Slow Sand Filters
9.12 References
COST ESTIMATES
10.1 Introduction
10.2 Curve Content
10.3 Operation and Maintenance Costs
10.4 Freight
10.5 How to Use Cost Curves
10.6 Curve Descriptions
10.7 References
7-1
7-1 •
7-2
7-9
7-14
7-15
7-22
7-23
7-27
7-32
7-35

8-1
8-1
8-9
8-31
8-33
8-36
8-43

9-1
9-3
9-9
9-20

9-26
9-31
9-35
9-45

9-46
9-48
9-52
9-54

10-1
10-1
10-1
10-2
10-2
10-2
10-16
                                 VI

-------
                             LIST OF FIGURES


Figure No.                                                              Page

   4-1                Jar Test Units With Mechanical (Top)                 4-8 .
                     and Magnetic (Bottom) Stirrers

   4-2               Six-Position Sampler                                4-9

   4-3               Settling Curves Frequently Obtained                  4-10

   4-4               Jar Test Results                                     4-12

   4-5               Zeta Potential Apparatus                            4-13

   4-6               Coagulation of Raw Sewage With Alum               4-15

   5-1               Typical Dry Feed System                            5-7

   5-2               Crystallization of Alum Solutions                     5-11

   5-3               Viscosity of Alum Solutions       "                   5-12

   5-4               Alternative Liquid Feed Systems                     5-14
                     For Overhead Storage

   5-5               Alternative Liquid Feed Systems                     5-14
                     For Ground Storage

   5-6               Freezing Point Curves For Commercial               5-17
                     Ferric Chloride Solutions

   5-7               Viscosity vs. Composition of Ferric                   5-18
                     Chloride Solutions at Various Temperatures

    5-8               Typical Lime Feed System                           5-31

    5-9                Lime Requirement For^pH > 11.0 as a                 5-35
                      Function of the Wastewater Alkalinity

    5-10             Viscosity of Soda Solutions                          5-37

    5-11             Viscosity of Caustic Soda Solutions                   5_40

    5-12             Typical Caustic Soda Feed System                   5-45
                                       vi i

-------
                           LIST OF FIGURES (continued)


Figure No.                                                              Page

   5-13            Typical Schematic of a Dry Polymer Feed System         5-57

   5-14            Typical Automatic Polymer Feed                       5-58
                   System for Large Plants

   5-15            Positive Displacement Pumps                           5-61

   5-16            Screw Feeder                                         5-63

   5-17            Positive Displacement Solid Feeder-Rotary              5-63

   6-1             Impeller Mixer                                        6-4

   6-2             Mechanical Flocculation Basin                          6-7
                   Horizontal Shaft-Reel Type

   6-3             Mechanical Flocculator Vertical                        6-7
                   Shaft-Paddle Type

   6-4             Solids Contact Clarifier Without                        6-9
                   Sludge Blanket Filtration

   6-5             Solids Contact Clarifier With Sludge                    6-10
                   Blanket Filtration

   7-1              Rectangular Settling Tanks                            7-3

   7-2            Typical Clarifier Configurations                        7-4

   7-3             Results of Salt-Injection Tests With                     7-6
                    Different Types of Sedimentation Tanks

   7-4            Schematic Representation of Settling Zones              7-10

   7-5            Sedimentation In a Secondary Settling Tank             7-10

   7-6             Dependence of MLSS Concentration on Secondary       7-17
                   Settling Tank Underflow Concentration

    7-7            Typical Flotation Unit                                 7-25

    7-8             Module of Steeply Inclined Tubes                       7-28
                                        Vlll

-------
                      LIST OF FIGURES (continued)



Figure No.                                                             Page

  7-9             Tube Settlers in Existing Clarifier                        7-29

  7-10           Plan View of Modified Clarifier                          7-29

  7-11            Tube Settler-Flow Pattern                              7-30

  7-12           Simple Wedge Wire Clarifier                            7-33

  7-13           Installation of Wedge Wire Panels in a Clarifier           7-33

  8-1             Hydrasieve Unit                                       8-3

  8-2             Schematic                                             8-3

  8-3             Screen Detail                                          8-4

  8-4             Curved Screen Bars                                    8-4

  8-5             Rotating Wedge Wire Screen at North                   8-8
                 Chicago ST.P.

  8-6             Typical Microscreen Unit                              8-10

  8-7             Micro-Matic® Strainer                                8-11

  8-8             Microscreen Capacity Chart                            8-16

  8-9             Microscreen Removal at Various Flow Rates             8-23

  8-10           Microscreening of Trickling Filter Plant Effluent          8-24

  8-11            Microscreen Unit With Pleated Outer Surface            8-27

  8-12           The Sweco Concentrator                               8-32

  8-13           Vertical Leaf Vacuum Filter                            8-34

  8-14           Vertical Leaf Pressure Filter                            8-35

  8-15           Schematic Flow Diagram of the Pikes Peak               8-37
                 Treatment & Reuse System
                                      IX

-------
                       LIST OF FIGURES (continued)


Figure No.                                                           Page

  8-16            "Storage Battery" Membrane Modules                 8-41

  9-1             Typical Rapid Sand Filter                             9-4

  9-2             Filter Configurations                                  9-5

  9-3             Cross Section of Upflow Filter                          9-6

  9-4             Typical Pressure Filter                                9-8

  9-5             Run Length vs. Filter Rate for Various                  9-21
                  Terminal Headlosses

  9-6             Net Production Rate vs. Filter Rate                     9-24
                  for Various Run Lengths

  9-7             Grain Size Curve                                     9-27

  9-8             Flow Control Systems                                 9-33

  9-9             Automatic Gravity Filter, Single Compartment          9-36

  9-10            Minimum Fluidization Velocity, Vmf, to Achieve         9-37
                  10 Percent Bed Expansion at 25 °C

  9-11            Effect of Temperature on Vmf for Sand and Coal         9-38
                  and on Absolute Viscosity of Water

  9-12            Underdrains                                          9-40

  9-13            Simater Filter                                        9-49

  9-14            Hydromation In-Depth Filter                          9-50

  9-15            Hardinge Travelling Backwash Filter                   9-51

  10-1             Flocculators-Flash Mixers                             10-3

  10-2             Chemical Feed Systems                               10-5

  10-3             Sedimentation                                       10-7

-------
                       LIST OF FIGURES (continued)









Figure No.                                                            Page



  10-4           Solids Contact                                         10-8




  10-5 .          Flotation                                              10-10




  10-6           Wire Septums and Settling Tubes                        10-11




  10-7           Wedge Wire Screens: Rotating and Stationary            10-13




  10-8           Microscreens                                          10-14




  10-9           Media Filters                                          10-15
                                       XI

-------
                              LIST OF TABLES
Table No.                                                             page

   2-1        Selected SS Separation Process Applications                  2-2

   4-1        SS Removal Performance For Chemical Coagulation           4-4
             Applications To Phosphate Removal

   5-1        Partial List of Alum Manufacturing Plants                    5-2

   5-2        Reactions of Aluminum Sulfate                              5-15

   5-3        Reactions of Ferric Sulfate                                  5-23

   5-4        Reactions of Ferrous Sulfate                                 5-24

   5-5        Partial List of Lime Manufacturing Plants                    5-25

   5-6        Reactions of Lime                                          5-34

   5-7        Partial List of Caustic Soda Manufacturing Plants             5-39

   5-8        COaYields of Common Fuels                                5-46

   5-9        Partial List of Carbon Dioxide Manufacturing Plants           5-47

   5-12       Partial List of Polymer Sources and Trade Names             5-51

   5-11       Types of Chemical Feeders                                  5-65

   7-1        Performance of Special Settling Tank Inlets                   7-13

   7-2        Typical Design Parameters For Primary Clarifiers             7-14

   7-3        Typical Design Parameters For Secondary Clarifiers           7-16

   7-4        Dissolved-Air Flotation Applications                         7-26

   7-5        Tube Settler Installations                                   7-31
                                     xin

-------
                         LIST OF TABLES (continued)

Table No.                                                              Page

  8-1        Physical Straining Processes                                 8-2

  8-2        Specifications of Hydrasieves                                 8-5

  8-3        Wedge-Wire Screens: Municipal Treatment Installations        8-7

  8-4        Data Sheet-Wedge Wire Screens                             8-9

  8-5        Microscreen Design Parameters                              8-13

  8-6        Microscreen Installations                                    8-17

  8-7        Municipal Microscreen Installations                          8-18

  8-8        Typical Microscreen Power and Space Requirements           8-28

  8-9        Diatomaceous Earth Filtration of Secondary Effluent           8-36

  8-10       Results of Ultrafiltration Installations                        8-38

  8-11       Summary of Pikes Peak Data                                8-39

  8-12       Typical Membrane Specifications                            8-42

  9-1        Results of Studies of Filtration of Effluents From               9-12
             Secondary Biological Treatment

  9-2        Results of Studies of Filtration of Chemically                  9-15
             Treated Secondary Effluent

  9-3        Results of Studies of Filtration Following Chemical             9-16
             Treatment of Primary or Raw Wastewater

  9-4        Expected  Effluent Suspended Solids From Multimedia          9-17
             Filtration of Secondary Effluent

  9-5        Typical Media Designs For Filters                            9-30

  9-6        Filter Gravel Design                                         9-43
                                      xiv

-------
                                    FOREWARD

The formation of the United States Environmental Protection Agency marked a new era
of environmental awareness in America.  This Agency's goals are  national in scope and
encompass broad  responsibility in  the areas  of  air and water pollution, solid  wastes,
pesticides, and radiation. A vital part of  EPA's national water pollution control effort is
the constant development and dissemination of new technology for wastewater treatment.

It is now clear that only the most effective design and operation of wastewater treatment
facilities, using the latest avaiable techniques,  will be adequate to  meet the future water
quality  objectives  and to ensure  continued  protection of  the  nation's  waters.  It  is
essential that  this new technology be incorporated into the contemporary design of waste
treatment facilities to achieve maximum benefit of our pollution control expenditures.

The purpose of this manual is to provide  the engineering community and related industry
a new source  of information to be used in the planning, design and operation of present
and future wastewater  treatment  facilities.  It  is  recognized that there  are a number  of
design manuals, manuals of standard practice,  and design guidelines currently available in
the field that adequately describe and interpret current engineering practices as related  to
traditional plant design. It is the intent of this manual to supplement this existing body
of knowledge  by describing new treatment methods, and by discussing the application  of
new techniques  for more effectively removing a  broad spectrum  of contaminants from
wastewater.

Much of  the  information  presented is based  on  the  evaluation and  operation  of  pilot,
demonstration and  full-scale plants. The  design criteria thus generated  represent typical
values.  These values should be  used as a  guide and  should  be  tempered  with sound
engineering judgment based on a complete  analysis of the specific application.

This manual is one  of several available through the Technology Transfer  Office of  EPA  to
describe recent technological advances and new information.  This  particular manual was
initially issued in October of  1971  and  this edition represents the first revision to the
basic text. Future editions will be issued as warranted by advancing state-of-the-art  to
include  new data  as it becomes  available,  and  to  revise  design  criteria  as additional
full-scale operational information is generated.

-------
                                    CHAPTER  1
                                 INTRODUCTION
 1.1  Purpose

 This manual is intended to provide:

     1.  A basis for selection of processes to meet specific suspended solids (SS) removal
        ^requirements

     2.  A basis for design of particular processes

     3.  A basis for selection of particular equipment configurations for a given process.

 Since  the emphasis is on information applicable to design or modifications of solids remov-
 al facilities, only those processes are included  for which reliable data from actual  appli-
 cations are available.

 1.2  Wastewater Solids

 The total solids in wastewater exist in a distribution of sizes from individual ions up to vis-
 ible particles. Specific analytical procedures (1)  have been established to distinguish the sus-
 pended fraction of the total  solids and to further distinguish  the settleable fraction within
 the SS. A typical concentration of SS for raw domestic wastewaters is 200 mg/ 1, but this
 can vary substantially from system to system (see below). The lower limiting size for the SS
 fraction (about 1.5  microns) is arbitrarily defined by the test procedures and it should be
 noted  that variations in test procedures themselves can also lead'to widely varying results, es-
 pecially at the low solids levels characteristic of treated effluents.

 Other workers (2) (3) (4) have applied procedures which distinguish  four solids fractions,
 and determine proportions of other wastewater characteristics such as COD, Nitrogen, Vol-
 atile (organic) matter in each fraction. For a New Jersey municipal raw wastewater, solids
 distribution in terms of these fractions was found to be as follows (2):
 Fraction
Soluble

Colloidal

Supra-
  Colloidal

Settleable
<0.001

  0.001-1

  1-100


>100
351

 31

 57


 74
Raw Wastewater
            Volatile
            Matter
           >mg/l

            116
                                              Secondary Effluent
                                                          Volatile
                                                          Matter
                                                          mg/1
             23

             43


             59
312

  8

 28


  0
62

 6

24


 0
                                         1-1

-------
The settleable  and supracolloidal fractions together are essentially equivalent to the sus-
pended fraction referred to above. Dividing lines between fractions again are somewhat ar-
bitrary depending on tests applied, and overall concentrations in different fractions can vary
substantially between systems depending on factors such as water use, travel time in sewers,
ground-water infiltration, and prevalence of home garbage grinding. Contributions of dis-
solved, colloidal and suspended solids from individual homes, multi-family dwellings or oth-
er point sources often have concentrations two or more times the average for a whole sys-
tem (5).

In addition to  particle size, specific gravity and strength or shear resistance of wastewater
solids  may affect solids separation performance. The three basic types of solids separation
processes—gravity separation, physical straining,  and granular media filtration are dis-
cussed in Chapters 7, 8 and 9, respectively. Wastewater solids characteristics can be altered
to enhance performance of the above separation processes. Chapters 4 and 6 discuss chem-
ical  treatment  (precipitation and/ or coagulation)  and  physical treatment (flocculation)
aimed  at  alteration of solids characteristics.  In addition, during the separation processes
themselves, agglomeration and compaction of solids generally continues, increasing separa-
tion efficiency  and reducing the volume of separated solids.

Biological wastewater treatment processes also affect solids characteristics and hence solids
separation.  Activated sludge solids have been  found (6) to have a distinct bimodal dis-
tribution with one mode in the supracolloidal to settleable range and another near the bor-
der between the colloidal and supracolloidal fractions. The concentrations and size limits in
each range  are affected by conditions in the biological reactor (Chapter 6). Dean (7) has
noted  that bacteria, cellular debris, etc. fall into the finer (colloidal-supracolloidal) range.
Agglomeration of these finer solids generally increases the efficiency of subsequent separa-
tion processes.
                                         1-2

-------
1.3   References

1.    Standard Methods for the  Examination of Water and Waste-water,  13th Edition,
     American Public Health Association, New York (1971).
                                                                 -i
2.    Rickert, David A. and Hunter, Joseph V., General Nature oj Soluble and Paniculate
     Organics in Sewage and Secondary Effluent, Water Research, 5, 421 (1971).

3.    Hunter, J. V., and Heukelekian, H.  The Composition of Domestic Sewage Fractions,
     JWPCF, 37, 1142 (Aug. 1965).

4.    Helfgott, T., Hunter, J. V. and Rickert, D., Analytic and Process Classification of Ef-
     fluents, Jour. SED, ASCE, 96,  779 (June 1970).

5.    Rawn, A. M., Some Effects of Home Garbage Grinding Upon Domestic Sewage, The
     American City, 66, 110 (Mar. 1951)

6.    Tchobanoglous, G., and Eliassen, R., The Filtration of Treated Sewage Ejjluent, Pro-
     ceedings of the 24th  Industrial Waste Conference, Purdue University Engineering
     Bulletin, Extension Series No. 135, 1323 (May, 1969).

7.    Dean, Robert B., Colloids Complicate Treatment Process, Env. Sci. and Tech., 3, 820
     (Sept. 1969).
                                       1-3

-------
Meaningful cost comparisons usually involve practically the entire process configuration of
the treatment facility, including processes for disposal of solid residues, and reflect how the
individual unit processes affect one another.

Cost data on individual processes for  suspended solids removal  are given in Chapter 10.
Outlined below are some additional factors not reflected in the unit process cost figures, but
which may warrant consideration in overall comparisons.


   1.   Sludge  Handling. Where chemical  treatment  is used  to  remove BOD  or phos-
       phates or improve SS removals, significant quantities of chemical sludge are pro-
       duced. The cost  of disposal of this sludge must be considered in process selection
       unless configurations being compared involve similar chemical treatment. The ac-
       tual cost involved will depend greatly on the particular method of sludge disposal to
       be used. For general guidance, in a 10 mgd plant using thickening, digestion, vacu-
       um filtration and landfill for sludge disposal, chemical addition of 200 mg/1 alum
       would increase the sludge disposal costs by almost 20 percent (from 9.7 cents/1000
       gal to 11.5 cents/ 1000 gal of plant flow on a 1972 cost basis) (Chapter 10,  Refer-
       ence  14).

       Where this difference appears significant in the comparison of alternatives  for SS
       removal, specific sludge disposal figures should be included in process comparisons.
       Information on expected sludge  quantities from particular chemical treatment pro-
       cesses is provided in Chapter 4.

   2.   Buildings. The need for housing specific unit process varies with climate and other
       local  conditions.  Where the housing requirements of alternative processes obviously
       differ widely under particular local conditions,  building cost should be considered
       in the selection.
                                   *,
   3.   Land Requirements..Generally, land requirements are a  small enough factor in
       overall cost that  the differences  for various process alternatives  are not significant.
       Where adequate  land is unavailable or very costly, however, area requirements of
       alternative processes should be compared in detail. Minimum  land  requirements
       may  be estimated, at between 1.20 (large plants) and 2.0  (small plants) times the
       area of the process units themselves.
            (
   4.   Head requirements. .Some of  the  processes employed for SS separation (sedimen-
       tation, microscreens, etc.) require relatively small head (only 2 to 3 ft. to overcome
       losses at inlet and effluent controls and in connecting piping). Others, such as gran-
       ular-media filters, and wedge-wire screens, require greater differential head (10 ft
       or more).  Differences in head requirements are most significant where they necessi-
       tate capital outlay for an extra pumping step. The costs for pumping, however, even
       with lifts above 10  ft. are usually not large in relation to the overall costs for treat-
       ment facilities.
                                       2-3

-------
                                    CHAPTER 2

                     GENERAL DESIGN CONSIDERATIONS

 2.1   Applications of Suspended Solids Separation Processes

 Processes for SS separation may fill three distinct functions in wastewater treatment.

     1.  Pretreatment  to  protect subsequent processes and reduce their loadings to  re-
        quired levels

     2.  Treatment to  reduce effluent concentrations to required standards

     3.  Separation of solids to produce concentrated recycle streams required to maintain
        other processes.

In the first two functions effluent quality is the prime consideration, but where the  third
function must be fulfilled along with one of the others, design attention must be given to
conditions for both the separated solids (sludge) and the process effluent.

Table 2-1 compares several SS separation  process applications selected to illustrate how
their performance  and their loading requirements are  functions of their applications.

Wedge-wire screens can operate at very, high hydraulic and solids loadings, but do not great-
ly reduce  SS. Hence,  wedge wire screens are limited to  pretreatment applications where
subsequent processes will assure production of a satisfactory final effluent. They can be
considered as an adjunct to primary sedimentation or, where conditions prescribe, as an
alternative.

Sedimentation units must operate at relatively low hydraulic loadings (large space require-
ments), but  can accept high solids  loadings. With  proper chemical  or biological pre-
treatment and design, they can produce good quality effluents.

Microscreens and  granular-media filters, operating at significantly higher hydraulic loads
than sedimentation units,  can produce an effluent  with lower SS than is possible with sedi-
mentation alone. In general they are not designed to accept high solids loadings, and  are
normally used following other processes which put out relatively low effluent SS concentra-
tions.

2.2  Process Selection

Selection  of one of the alternative processes can be based on cost only where all factors not
reflected in cost are equivalent. Direct cost comparison of individual solids removal pro-
cesses usually proves impossible because of  differences in  factors such as:  1) effluent quali-
ty, 2) pretreatment  requirements, 3) effects on sludge processing, 4) housing,  space and
head requirements.
                                        2-1

-------
           Type of Separation
                Process
           Straining
             Wedge Wire Screens
                                                                 TABLE 2-1

                                           SELECTED SS SEPARATION PROCESS APPLICATIONS
                            Application
                       Preliminary Treatment
                       of Raw Wastewater
                         Hydraulic
                         gpm/sq ft
                           10-30
Typical Loading Ranges

           InH. Solids
                                                                      mg/1     Ib/day/sq ft
      200
25-75
              Expected Effluent
                  SS(a)(b)
                    mgTl
150-190
                       Remarks
K)
             Microscreens
Gravity Separation
  Plain Sedimentation
  Chemical Coagula-
  tion and
  Settling

  Plain Sedimentation
     (secondary)
                       Polishing of Bio-             3-10     30          1-2
                       logical System Effluent
Primary Treatment        0.4-1.6     200        0.5-2
Chemical Treatment       0.3-1.0     200(c)      1-6
of Raw Sewage
(Phosphate Removal
Levels)
Separation of Solids       0.25-       2000-       4-40
after Activated            0.75       5000
Sludge Treatment
                                                                  5-15
                                                                                                    120-80
                                                                                                    20-60
                                                                                                    10-50
                                                    Chemical treatment for
                                                    90% + phosphorus removal.
                                                    Upper effluent quality
                                                    Limit may increase with poor
                                                    biological treatment.
                                                    Allowable solids loadings de-
                                                    pends on solids characteristics.
           Granular Media
           Filtration
                       Polishing of Bio-           4-8
                       logical Effluent or
                       Filtration of Chemi-
                       cally-Coagulated and
                       Settled Raw               3-5
                       Wastewater or
                       Secondary Effluent        3-5
                                    30



                                    40

                                    5-10
                  1-2



                  1-2

                  1-2
                  5-15



                  10-20

                  1-3
                                                                                                                   Secondary treatment may be
                                                                                                                   biological or by activated
                                                                                                                   carbon.
           (a)  Based on raw wastewater SS of 200 mg/1.
           Xb)  Performance is highly dependent on character of solids appli
            |)  Influent solids do not include chemical solids.
                                                              id hence on conditions in prior treatment.

-------
                                    CHAPTER 3

                   FLOW VARIATIONS AND EQUALIZATION
3.1   Flow Variation

Both the rate and characteristics of the inflow to most treatment plants vary significantly
with time. Diurnal cycles are found in all domestic discharges. Weekly and seasonal cycles
are common in municipal systems as are variations between wet and dry weather.

Even where only domestic flows are involved, the magnitude of variations can differ widely
between  different systems depending  on system  configuration, water  use  habits of the
population and opportunities for groundwater infiltration or direct inflow  of surface or
subsurface drainage. Industrial and institutional flows where significant, can further alter
domestic patterns.

Because  of these  wide differences, design of treatment facilities  should be based, whenever
possible, on measurements of actual flow variations in existing  systems. In projects being
submitted for federal construction grants, analysis of existing flows is required in any case
to identify "excessive"  infiltration/ inflow. Flows are considered excessive if they can be
eliminated more cheaply than they can be treated. Projected flow variations from existing
systems should reflect elimination of excessive flows.

Flows tend to be less variable in larger systems, due chiefly to differing times of travel from
different sections and to damping effects of flow storage in  large sewers.  Widely varying
relations have been reported between peak-to-average or minimum-to-average flow ratios
and system size (i.e. average flow or tributary population) (1) (2) (3) (4).  Care should be
taken in  using any of these relations for estimating flow variations in new systems or system
additions.  In terms of the  factors which affect flow  variations, applications should be
limited  to systems similar  to  those  for  which  the relation  was  originally developed.
Relations for which the basis is unclear should be disregarded.

3.2  Performance vs. Flow Variation

Variations in influent  flow  rate  and characteristics  affect performance of  all suspended
solids removal processes to some degree. Relations between performance and hydraulic or
solids loadings are discussed for individual processes in succeeding chapters. Magnitude and
character of significant  recycled flows resulting from  specific processes are also indicated.

Relations between performance and loadings are frequently developed in pilot units run
under steady flow  conditions, or from  data  from actual plants compiled  without close
attention to  short-term peaks. In using  such  relations for design decisions, care  must be
taken to allow for the  effects of short term flow variations on performance. Short term
would include any time span less than that for which  performance requirements are stated.
Typically requirements are on  a monthly average basis,  often  with a  less  stringent
requirement for the worst week or worst day within the month.

                                        3-1

-------
Designs based on maximum  24-hour flow, with allowance for diurnal peaks, provide some
margin  so that weekly  or monthly requirements  can be met even when other factors
affecting process performance are not optimum.

3.3  Flow Equalization

Equalization  storage can be used to reduce diurnal variations in flow and in concentration
of SS or other wastewater characteristics. Storage may also be used to handle peaks caused
by direct  inflow to the sewers  during wet  weather. Assuming that equivalent performance
can  be obtained  either  by  increasing the  size of treatment  facilities  or by  providing
equalizing basins, selection between these  approaches can be based  on their relative costs
and  environmental impacts.  In plants using processes involving  large, short-term recycle
flows—such  as  for backwashing granular media filters—equalization  is almost always
justified.

The  EPA Process  Design Manual for  Upgrading  Existing  Wastewater  Treatment  Plants
provides a basis for design of equalization facilities to achieve any given degree of equaliza-
tion  of  either peak flows or  peak flows and solid loadings (5). Material from the Design
Manual relevant to flow equalization only is also available in a separate publication (6).

3.4  References

1.  Smith, R., and Eiler, R. G., Simulation of the Time-Dependent Performance of the Ac-
    tivated Sludge Process Using the Digital Computer, U.S. EPA, National Environmental
    Research Center, Cincinnati, Ohio (October, 1970).

2.  Duttweiler, D.  W. & Purcell, L. T., Character and Quantity of Wastewater from Small
    Populations, Jour. WPCF, Vol. 34, pg. 63 (1962).

3.  Boyle Engineering and Lowry and Associates, Master Plan Trunk Sewer Facilities for
    County Sanitation District No. 3 of Orange County, California, (June, 1968).

4.  Design and Construction of Sanitary and Storm Sewers,  ASCE Manual of Engineering
    Practice  No. 37, WPCF Manual of Practice  No. 9  (1970).

5.  Process  Design Manual for Upgrading Existing Wastewater Treatment  Plants.  U.S.
    Environmental Protection  Agency,  Technology  Transfer,  Washington,  D.C. 20460
     (revised 1974).

6.  Flow Equalization, Technology Transfer Seminar Publication, U.S. Environmental  Pro-
    tection Agency, Washington, D.C. 20460 (May 1974).
                                       3-2

-------
                                    CHAPTER 4

                    PRINCIPLES OF CHEMICAL TREATMENT

 4.1   Introduction

 Chemical coagulation and decollation are accomplished by a combination of physical and
 chemical processes which thoroughly mix the chemicals with the wastewater and promote
' the aggregation of wastewater solids into particles large enough to be separated by sedimen-
 tation, flotation, media filtration or straining. The strength of the aggregated particles de-
termines their limiting size and their resistance to shear in subsequent processes.

 For particles in the colloidal and fine supracolloidal  size  ranges (<1  to 2 microns) natural
 stabilizing forces (electrostatic repulsion, physical separation by absorbed water layers) pre-
 dominate over the natural aggregating forces (van der Waals) and the natural mechanism
 (Brownian movement) which tends to cause particle contact. Coagulation of  these fine par-
 ticles involves both destabilization and physical processes  which disperse coagulants and in-
 crease  the opportunities for particle contact. Destabilization, the action of chemical coag-
 ulants, is discussed in this chapter. Physical processes, including chemical mixing, floccula-
 tion, and solids contact processes, are discussed in Chapter 6.

 Chemical coagulants used in wastewater treatment are generally the same as those used in
 potable water treatment  and include: alum, ferric chloride, ferric sulfate, ferrous chloride,
 ferrous sulfate and lime. The effectiveness of a particular coagulant varies in different appli-
 cations, and in a given application each coagulant has both an optimum concentration and
 an optimum pH range.

 In addition io coagulants themselves, certain chemicals may be applied for pH or alkalinity
 adjustment (lime, soda ash) or as flocculating agents (organic polymers). For full effective-
 ness chemical coagulation requires initial rapid mixing (Chapter 6) to thoroughly disperse
 the applied chemicals so  that they can react with suspended and colloidal solids uniformly.

 4.2  Destabilization Mechanisms

 The destabilizing action  of chemical coagulants in wastewater may involve  any of the fol-
 lowing mechanisms:

      1. Electrostatic charge reduction by adsorption of counter ions

      2. Inter-particle bridging by adsorption of specific chemical groups in polymer chains

      3. Physical enmeshment  of fine solids in gelatinous hydrolysis products of the coag-
         ulants.

 The significance of these mechanisms in design is considered  briefly below. Extensive dis-
 cussion of the mechanisms can be found  in the literature  (1) (2) (3) (4).
                                         4-1

-------
    4.2.1   Electrostatic Charge Reduction

Finely dispersed wastewater solids generally have a negative charge. Adsorption of cations
from  metal salt coagulants (in the case of iron and aluminum from their hydrolysis prod-
ucts), or from cationic polymers can reduce  or reverse this charge.

Where electrostatic charge reduction is a significant destabilization mechanism, care must
be taken not  to overdose with coagulant. This can cause complete charge reversal with res-
tabilization of the oppositely charged coagulant-colloid complex.

    4.2.2   Interparticle Bridging

When polymeric coagulants contain specific chemical groups which can interact with sites
on the surfaces of colloid particles, the polymer may adsorb to and serve as a bridge be-
tween the particles. Coagulation using polyelectrolytes of the same charge as the colloids or
non-ionic polymers depends on this mechanism. Restabilization may occur if excessive do-
sages of polymer are used. In this case all sites on the colloids may adsorb polymer mole-
cules without any bridging. Excessive mixing can also cause  restabilization by fracture or
displacement of polymer chains.

     4.2.3  Enmeshment in Preciptated Hydrolysis Products

Hydroxides of iron, aluminum or, at high pH, magnesium form gelatinous hydrolysis prod-
ucts which are  extremely  effective in enmeshing  fine particles of other material.  These
hydroxides are formed by reaction of metal salt coagulants with hydroxyl ions from the nat-
ural alkalinity in the water or from added alkaline chemicals such as lime or soda ash. Suf-
ficient natural magnesium is frequently present in wastewaters so that effective coagulation
is obtained merely by raising the pH with lime. Organic polymers do not form hydrolysis
products of significance in this mechanism.  At a pH value lower than that required to pre-
cipitate magnesium, the precipitates produced by lime treatment are frequently ineffective
in enmeshing the colloidal matter in wastewater. The remedy for this condition generally in-
volved addition of low dosage of iron salts or polymers as coagulant aids both to destabilize
and to increase the probability of enmeshment of colloids.

Coagulants may also react with other constituents of  the wastewater,  particularly anions
such as phosphate and sulfate, forming hydroloysis products containing various mixtures of
ions.  The chemistry of the  reactions is extremely complex and highly dependent on pH and
alkalinity. The presence of high concentrations of these anions may require increased doses
of coagulants or pH adjustment to achieve effective removals of  SS.

4.3   Selection of Chemical Coagulants

Design of chemical treatment facilities for SS removal must take into account: 1) the types
and quantities of chemicals to be applied as coagulants, coagulant aids and for pH control
and 2) the associated requirements for chemical handling and feeding (Chapter 5) and for
mixing and flocculation after chemical addition (Chapter 6). Reactions of specific coag-
                                        4-2

-------
 ulant chemicals are detailed in Chapter 5.

 Selection of coagulants should be based on jar testing of the actual wastewater (Section 4.5)
 to determine dosages and effectiveness, and on consideration of the cost and availability of
 different coagulants. Where expected changes in waste characteristics or market conditions
 may favor different coagulants at different times, chemical feed and handling should be set
 up to  permit a switchover. In developing a testing  program general information on ex-
 perience at other locations and on costs should be considered to aid in selection of processes
 and coagulants to be tested.

 Experience to date with improved SS removal  from chemical coagulation has been almost
 solely in systems designed to remove phosphorus. Guidelines for design and coagulant selec-
 tion for such systems are available in another manual  (5). Descriptive data and SS removal
 performance for several existing phosphorus removal installations are summarized in Table
4-1.

 Few cases have been reported involving chemical coagulation aimed at SS removal alone
 without phosphorous removal requirements. Anionic  polymers have been used  to increase
 SS removal  in primary treatment at Rocky River, Ohio (6). Doses of 0.3 mg/1 reduced SS
 from 107 mg/1 to 65 mg/1. Mogelnicki (7) reported use of anionic polymer at a dosage of 1
 mg/1 to improve primary clarifier SS removal from 43 percent to 76 percent.

 In discussing the favorable results sometimes obtained in polymer applications O'Melia (4)
 warns  that it can  be a  time-consuming  task  to  find the specific conditions  (pH, ionic
 strength, polymer type, molecular weight, degree of hydrolysis, etc.) which will  provide
 economy and effectiveness.

 Pilot work at Denver, Colorado (8) on coagulation of effluent from an activated sludge nit-
 rification system showed substantial reductions in SS, turbidity, BOD and other pollution
 parameters, using lime and alum doses well below those needed for effective phosphate re-
 moval. Lime dosages of 100 mg/ 1 were sufficient to reduce SS to below 15 mg/1 after settl-
 ing and 5 mg/ 1 after filtration. Phosphate reduction was  less than 80 percent. Alum do-
 sages of about 50 mg/ 1  were sufficient to reduce suspended solids and phosphorus concen-
 trations to similar levels. In direct filtration of alum-coagulated nitrified effluent, SS were
 reduced to less than 2 mg/ 1 with an alum dosage of 60 mg/ 1. Phosphate reductions at this
 alum concentration were only about 65 percent (6-7 mg/ 1  residual). This latter practice is
 accompanied by shorter filter runs due to significant increases in solids loading.

 Calgon Corporation investigated the use of ferric chloride with polymer addition for a small
 municipal wastewater treatment plant at Leetsdale, Pennsylvania (9). Ferric chloride do-
 sages were less than those necessary for 80 percent phosphate removal. Dosages and SS re-
 ductions are  shown below:
                                       4-3

-------
                                                 TABLE   4.1

                                      SS REMOVAL  PERFORMANCE  FOR
                     CHEMICAL COAGULATION  APPLICATIONS  TO PHOSPHATE REMOVAL
                                                                   SUSPENDED SOLIDS

LOCATION PROCESS

Lebanon, Ohio IPC
EPA, Blue Plains IPC
Plant, Wash-
ington, D.C.
Ely, Minn. Tertiary
S.LakeTahoe, Tertiary
California
Lebanon, Ohio Tertiary
Nassau County, Tertiary
New York
Salt Lake City, IPC
Utah



Leetsdale, Pa.
Key: FM - Flash Mix Unit
PLANT
SIZE
mgd
0.1
0.1
l.S
7.5
0.1
0.6
0.04-
0.1
0.05-
0.09
0.03-
0.18
0.6
AVERAGE
CHEMICAL FEED
mg/1
Lime ~ 250
Lime 460 ^
+FeCl3 sCb)
Lime 250-350 (a)
+Polymer .2(a)
+FeCl3 6d>)
Lime 400
275
Lime 220-270
Alum 20
Iron 34-41
Polymer 0-1.5
Alum 14
Polymer 0-0.25
Lime 270-586
FeCl3 100
+Polymer 0.5
BASIC
pH EQUIPMENT

9.5 1-stage SC
11.5(a) 2-stage
10.0(b) sc(a)
FM,FL,S(b)
ll.sCa) 2-stage
9.5(b) SC
11.3 FM.FL, S
10.5 SR
9.5 SC
SC
SC
SC

9.8- SC
11.0
6.5- S
.6.7

Inf.
mg/1
109
158
75
38
38
43.5
22.5
90
95

101
280
Settled
Eff.
mg/1
30
14.4
10
10
25
16.5
2.5
21.9
26.9

11.6
38
DATA OVERFLOW
PERIOD RATE COMMENTS
Months gpd/sq ft
45 1440 Acid pH adjustment.
Reference 19
6 500-1800 Two stages; with inter-
mediate recarbonation.
Reference 20
(a)
5 570 System designed for nearly
66o(b' complete (eff.SO.05 mg/1)
phosphorus removal.
Reference 21
21 400-600 Recarbonation
Reference 22
10 1440 Acid pH adjustment made.
Reference 23
72 860 Bulking 2-3 times a year.
Reference 24
2.5 360-1080 Reference 11
1 500-870

5 290-1800 H-S04; pH adjustment.
Primary Treatment, Cons-
tant Feed - Reference 9
Notes: Lime as Ca(OH)2
 FL - Flocculator
 SC - Solids Contact (Sludge Blanket)
 SR - Solids Recirculation
 S - Settling
IPC - Independent Physical Chemical
Alum as A1++*
Iron Salts as Fe++*
(a) First Stage
(b) Second Stage
Overflow rates are minimum and maximum where range appears.

-------
             Dosage                                      SS
                     Polymer
                       mg/1

      74                 0.5
      50                 0.6
      37                 0.08
Influent
mg/1
160
71
85
Effluent
mg/1
23
27
33
Reduction
Percent
86
62
61
Aluminum or iron salts tend to react with soluble phosphate preferentially so that substan-
tial phosphorus removal must be involved before organic colloids can be destabilized (10).
Required dosages will be affected by phosphorus content. Similarly lime treatment to a pH
at which coagulation is effective precipitates substantial phosphorus. Because chemical do-
sage and pH range for optimum SS removal may differ somewhat from those for optimum
phosphorus removal, coagulant requirements may be determined by the effluent criteria for
either pollutant, depending on wastewater characteristics and the choice of chemical.

    4.3.1   Sludge Production

Chemical coagulation increases sludge production in sedimentation units due both to great-
er removal of influent suspended  solids and  to insoluble  reaction products of the coag-
ulation itself. For phosphorus removal, data on sludge production and sludge characteristics
and sample procedures for estimating sludge quantities are presented elsewhere (5).

The weight of sludge solids can be estimated by calculation of the sum of the expected SS
removal and of the precipitation products expected  from  the coagulant dosages applied.
Usually jar tests can be employed to obtain the necessary information  for this calculation.

   4.3.2  pH Control and Alkalinity

The critical factor in the control of lime reactions is pH. The pH for optimum effectiveness
of lime  coagulation, determined  from jar testing and process  operating experience, can be
used as  a set point for a pH  control of lime dosing.

Alum and iron salt  coagulation are much less sensitive to pH. Testing can determine opti-
mum dosages for coagulation and whether natural alkalinity  is  adequate for the  reactions
(see Ch. 5). If supplemental  alkalinity is needed either regularly  or on an intermittent basis
(e.g. during high wet weather flows) provisions should be included for feeding necessary
amounts of lime or  soda ash.
                                        4-5

-------
   4.3.3  Points of Chemical Addition

In independent physical-chemical treatment or in phosphate removal in the primary clari-
fier ahead of biological treatment, chemicals are added to raw sewage. In tertiary treatment
for phosphate removal and SS reduction, they are  added to  secondary effluent.  In both
cases, proper mixing and flocculation units are needed. For phosphate removal or improve-
ment of SS capture in biological secondary treatment, chemicals are often added directly to
aeration units or prior  to secondary settling units, without separate mixing and flocculation.
In some phosphate removal applications coagulants have been added at multiple points, e.g.
prior to primary settling and as part of a secondary or tertiary treatment step.

   4.3.4  Supplementary Coagulants

Addition  of  the  hydrolyzing  metal coagulants to  wastewater often  results in a  small
slow-settling  floe or precipitate of phosphorus. Additional treatment is required to produce
a water with low residual suspended solids. Polymeric coagulants have proved to be quite
beneficial in  aggregating the precipitation products  to a settleable size and increasing  the
shear strength of the floe against hydraulic breakup (11). Data on  particular applications
appear in Table 4-1.

4.4  Coagulation Control

Because coagulation represents a group of complex  reactions, laboratory experimentation
is  essential to establish and maintain the optimum coagulant dosage and to determine  the
effects of important variables on the quality of coagulation of the wastewater under in-
vestigation. With alum and iron coagulants two procedures are generally followed for this
purpose: the jar test and measurement of zeta potential. Proper control of lime coagulation
may be maintained by  measuring the pH or automatically titrating alkalinity after lime  ad-
dition.

   4.4.1   Jar Test

The single, most widely used test to determine coagulant dosage and  other parameters is the
jar test. The equipment for this test and the directions for its proper  performance have been
published (12) (13) (14) (15). The jar test attempts  to simulate the  full scale coagula-
tion-flocculation process and has remained the most common control test in the laboratory
since its introduction in 1918. Since the intent  is to simulate an individual plant's condi-
tions, it is not surprising that  procedures may vary but generally have certain common ele-
ments. The jar test apparatus consists of a series of sample containers, usually six, the con-
tents of which can be stirred by individual mechanically-operated stirrers. Wastewater to be
treated is placed in the containers and treatment chemicals are added while the contents are
being stirred. The range of conditions, for example, coagulant dosages and pH, are selected
to bracket the anticipated optima. After a short, 1 to  5 minute period of rapid stirring to en-
sure complete dispersion of coagulant,  the  stirring rate is decreased and  flocculation is
allowed to continue for a variable period,  10 to 20 minutes or more,  depending on the simu-
lation. The stirring is  then stopped  and the floes are allowed  to settle  for a selected time.
The supernatant is then analyzed for the desired parameters. With wastewater the usual

                                        4-6

-------
analyses are for turbidity or suspended solids, pH, residual phosphorus and residual coag-
ulant.
If desired, a number of supernatant samples may be taken at intervals during the settling
period to permit construction of a set of settling curves which provide more information on
the settling characteristics of floe than a single sample taken after a fixed settling period. A
dynamic settling test may also be used in which the paddles are operated at 2 to 5 rpm dur-
ing the settling period. This type of operation more closely represents settling conditions in
a large horizontal basin with continuous flow.

It should be noted that simple jar tests cannot simulate the conditions in solids contact reac-
tors (Chapter  6) and may indicate somewhat higher coagulant  dosages than are actually
necessary when using these units for coagulation.

Several six-position stirrers are  available  commercially  for running jar tests; one from
Phipps  and Bird,  (Phipps and  Bird, Inc.,  Richmond, Va.),  another from Coffman  In-
dustries, (Coffman Industries,  Inc., Kansas City,  Ka.), are shown in Fig. 4-1. Standard
laboratory mixers have also been used; however, it is difficult to obtain reproducible mixing
conditions using different pieces of equipment. Various types of containers, usually beakers
or jars, are used to hold the samples. Improved mixing may be obtained by adding stationa-
ry plates in the containers as described by Camp and Conklin (15). The Coffman stirrer has
an attachment which makes it  possible to add coagulant  to all containers simultaneously.
Good results, however, can be obtained by rapidly adding coagulant from a large graduated
pipette to each jar in sequence.

A simple apparatus,  shown in  Fig. 4-2, can be constructed from tubing, rubber stoppers
and small aquarium valves to permit rapid sampling of supernatant. The unit is placed next
to the sample jars at the beginning of the settling period with the curved stainless steel tubes_
dipping into the jars. At desired intervals the vent valve is  covered with a finger, permitting
vacuum to draw samples into the small sample  bottles. The needle valves are adjusted so
that supernatant is drawn into all the bottles at the same rate. When sufficient sample is ob-
tained, the vent is uncovered and  the bottles are  replaced with empties. The maximum
sampling rate  is about once per minute.

Fig. 4-3 shows characteristic types of settling curves which may be obtained. Curve A  in-
dicates a coagulation which produced a uniformly fine floe so small that at the end of 1 to 2
minutes settling, the supernatant had a turbidity equal to  that of the starting water due, in
part, to the fine floe which resisted settling.  Settling was slow and the final turbidity was not
satisfactory. Curve B represents the most common type of  settling rate obtained. During  the
first 5  minutes, the settling rate was practically  a straight line on a semilog plot. Settling
was rapid and clarification was satisfactory. The coagulation represented by curve C  shows
that a  mixture of large rapid settling floe and small, slow-settling particles  was obtained.
Settling was rapid for the  first two minutes, but with little further clarification after that.
High residual  turbidity may also have resulted from incomplete coagulation. Curve D rep-
resents the ultimate in coagulation. Practically all of the floe particles were so large and
dense that 97  percent settled within three minutes. Sedimentation was essentially complete
within that time since only 0.5  percent additional floe settled in the next 27 minutes. Final


                                       4-7

-------
            FIGURE 4-1
JAR TEST UNITS WITH MECHANICAL (TOP)
  AND MAGNETIC (BOTTOM) STIRRERS
                 4-8

-------
     TO VACUUM SOURCE
VENT
                            TWO-HOLE
                            STOPPER
                                       SAMPLE

                                       BOTTLE
             SAMPLE

              TUBE
          U
          MAIN CONTROL VALVE

          PLUG
   PLUG

REGULATING VALVES
                      U.S EPA Headquarters Library
                            Mai! code 3404T
                      1200 Pennsylvania Avenue NW
                         Washington, DC  20460
                             202-566-0556
                        FIGURE 4-2

                 SIX-POSITION SAMPLER
                              4-9

-------
100
                         SETTLING TIME-MIN
                            FIGURE 4-3
               SETTLING CURVES FREQUENTLY" OBTAINED
                                4-10

-------
clarity of the supernatant was entirely satisfactory.
Measurement of turbidity provides the most rapid indication of the degree of solids removal
obtained. The recommended procedure for turbidity measurement by light scattering is giv-
en in the 13th edition of Standard Methods for Examination of Water and Wastewater;
however, other methods varying from simple visual evaluation to measurement of light
transmitted on a laboratory  spectrophotometer  can be used  for purposes of comparison.
Measurement of  residual suspended solids  is the  only procedure which gives the  actual
weight concentration of solids remaining, but the procedure is too slow for purposes of pro-
cess control. Where the  character of the solids  does not vary widely, their concentration
generally correlates well with measured turbidity.

A typical jar test  might be run as follows:

Wastewater samples are placed in containers and rapid mix is started at 100 rpm. Selected
dosages of  coagulant covering the expected range of the optimum concentration are rapidly
added to the containers and mixed for approximately 1 minute. If a polymer is to be used as
a coagulant aid, it is usually added to each jar at or just before the end of the rapid mix. The
paddles are then  slowed to 30 rpm  and mixing continues for 20 minutes. The paddles are
then  stopped and the sampling apparatus previously-described is placed in  position. At
settling times of 1,3,5,10 and possibly 20 minutes samples of supernatant are drawn for tur-
bidity measurement. After the  final turbidity sample is drawn, a larger volume of  super-
natant may be decanted  for more complete analysis. Results  are plotted as in Fig. 4-4 for
judgment as to the desired coagulant dosage.

If additional alkalinity is required to hold the coagulation in the optimum pH range, this
should be added to the samples ahead of the coagulant unless automatic titrators are set up
for  pH control.

Once an approximate optimum coagulant concentration has been determined, it may be de-
sirable to repeat the jar test using that optimum with varying quantities of added alkalinity
to give different pH values. Experience in coagulating a given wastewater provides the best
guide as to methods for controlling  the process.

   4.4.2   Zeta Potential

Measurement of particle charge is another procedure which may be useful for control of the
coagulation process (16) (17) (18).  The total particle charge is distributed over two con-
centric layers of water surrounding the particle:  an inner layer of water and ions which is
tightly bound to  the particle and moves with  it through the solution, and an outer layer
which is a part of the bulk water phase and moves independently of the particle. Charges of
these layers are not directly measureable,  but the  zeta potential, which is the residual charge
at the interface between the layer of bound water and the mobile water phase, can be deter-
mined indirectly with commercially-available instruments.

In the zeta potential measurement procedure, a  sample of treated water containing  floe is
placed in a special plastic cell under a microscope as shown in Fig. 4-5. Under the in-
                                       4-11

-------
   100
g

H
3
S
Q— 26 mg/1

I    I
A— 28mg/l
                     SETTLING TIME—MIN
                           FIGURE 4-4

                       JAR TEST RESULTS
                             4-12

-------
       FIGURE 4-5
ZETA POTENTIAL APPARATUS
          4-13

-------
fluence of a voltage applied to electrodes at the ends of the cell, the charged particles will
migrate to the electrode having a polarity opposite that of the particle. The velocity of mi-
gration will be proportional to the particle charge and to the applied voltage. The particle
velocity can be calculated by observing the time it takes a particle to travel a given distance
across an ocular micrometer. The  zeta potential can then be obtained from a chart which
combines the particle velocity with instrumental  parameters.  Detailed operating instruc-
tions are  supplied with the instruments. Because of uncertainties in the constants relating
charge and particle mobility, many test results are reported directly in terms of particle mo-
bility.

To control the coagulation by zeta potential, samples of water while being mixed are dosed
with different concentrations of coagulant. Zeta potentials are then measured and recorded
for floe in each sample. The dosage which produces the desired zeta potential value is ap-
plied to the treatment plant. Zeta potentials of  floe produced in the plant may also be mea-
sured as a means of control. The zeta potential value for optimum coagulation must be de-
termined  for a given wastewater by actual correlation with jar tests or with  plant perform-
ance as in Fig. 4-6. The control point is generally in the range of 0 to 10 millivolts.  If
good correlations can be obtained between some zeta potential values and optimum  plant
performance, then it is possible to make rapid measurements of particle charge to  com-
pensate for major variations in wastewater composition due to storm flows or other causes.
Short term variations such as those due to  sudden industrial waste dumps are still  beyond
control with  any present techniques because of the time lag between recognition of  a  prob-
lem with  coagulation and adoption of a satisfactory change of coagulation conditions.
                                        4-14

-------
+10
  0
   0
100
400
               200         300
           ALUM DOSAGE, (mg/1)
                FIGURE 4-6
COAGULATION OF RAW SEWAGE WITH ALUM
500
                            4-15

-------
4.5 References


 1.  Stumm W., and Morgan, J. J., Chemical Aspects of Coagulation, Jour, AWWA, 54,
     971 (1962).

 2.  Black, A. P., Basic Mechanism of Coagulation, Jour. AWWA, 52, 492 (1960).

 3.  O'Melia, C.  R., A Review of the Coagulation Process, Public Works, 100, 87 (May
    ;1969).

 4.  O'Melia, C. R., Coagulation and  Flocculation, Chapter in Physicochemical Pro-
     cesses for Water Quality Control (Walter J. Weber, Jr.) John Wiley and Sons (1972).

 5.  Process  Design Manual for Phosphorus Removal,  U.S.  EPA, Technology Transfer,
    Washington, D.C. (revised 1974).

 6.  Rizzo, J. L.  and Schade, R.E., Secondary Treatment -with Granular Activated Car-
     bon, Water and Sewage Works, 116, 307, (August 1969).

 7.  Mogelnicki, S.; Experiences in Polymer Applications to Several Solids-Liquids Sepa-
     ration Process, Proceedings, Tenth Sanitary Engineering  Conference-Waste Disposal
     from Water  and Wastewater  Treatment  Processes,  Univ. of Illinois (February 6-7,
     1968).

 8.  Linstedt, K.iD. and Bennett, E.  R.  Evaluation of Treatment for Urban Wastewater
     Reuse,  U.S.  EPA Office  of Research  and Monitoring,  Publication  U.S.  EPA
     R2-73-122 (July, 1973).

 9.  Bernardin, F.  E.,  Jr., Kusnirak, R., Chemical Treatment For Municipal Wastewater,
     WPCF  Deeds and Data (March, 1974).

 10.  Tenney, M. W. and Stumm, W. Chemical Flocculation of Micro-organisms in Biolog-
     ical Water Treatment, Jour. WPCF 37,  1370 (1965).

 11.  Burns, D. E. and  Shell, G.L. A New Approach to Phosphorus Removal by Chemical
     Treatment, Paper presented at 45th Annual WPCF Conference, Atlanta. Georgia(Oc-
     tober 9, 1972).

 12.  Cohen,  J. M., Improved Jar Test Procedure, Jour. AWWA, 49, 1425 (1957).

 13.  Black, A. P.,  Buswell, A. M., Eidsness, F. A., and Black, A. L., Review of the Jar
     Test, Jour, AWWA, 39, 1414 (1957).

 14.  Black, A. P.,  and Harris, R. J., New Dimensions for the Old Jar Test, Water &
     Wastes  Engrg., 6. 49 (Dec. 1969).
                                       4-16

-------
15.   Camp, T. R., and Conklin, G. F., Towards a RationalJar Test for Coagulation, Jour.
     AWWA, 84. 325(1970).

16.   Black, A. P. & Chen, C., Electrophoretic Studies of Coagulation and Flocculation
     of River Sediment Suspension with Aluminum Sulfate, Jour. AWWA, 57, 354 (1965).

17.   Riddick, T. M., Role of Zeta Potential in Coagulation Involving Hydrous Oxides,
     TAPPI, 47, 17A(1964).

18.   Riddick,  T.  M., Control  of Colloid Stability Through Zeta Potential,  Vol.  1,
    . Zeta-Meter, Inc., 1720 First Avenue, New York, New York  10028.

19.   Villiers, R. V., Berg, E. L. Brunner, C. A. and  Masse, A. N., Municipal Wastewater
     Treatment, Paper presented at 45th Annual WPCF Conference, Atlanta.  Georgia
     (October 9, 1972).

20.   Bishop, D. F., O'Farrell, T. P., and Stamberg, J. B.; Physical-Chemical Treatment of
     Municipal Wastewater, Jour. WPCF 44, 361 (March 1972).

21.   Brice, R. M., Shagawa Lake Project, Ely, Minnesota,  Personal Communication
    (August 1973).

22.   Evans, Wilson, Gulp, Suhr  and Mover; A Summary of  Plant Scale Advanced Waste
     Treatment Research  at  South Lake  Tahoe;  work  for  partial  fulfillment of  an
     U.S. EPA demonstration grant WPRD-52-01-67.

23.   Berg. E. L., Brunner, C. A. and William, R. T.; Single-Stage Lime Clarification, Wa-
     ter and Waste  Engineering Vol. 7, No. 3, pg. 42 (March  1970).

24.   Oliva, J.  A.,  Department of Public Health, County of Nassau, Personal  Commu-
     nication (March 1973).
                                     4-17

-------
                                   CHAPTER 5

                   STORAGE AND FEEDING OF CHEMICALS

 5.1  General

 This chapter surveys the chemicals most commonly used for suspended solids removal, with
 respect to their properties, availability, storage, transport, reactions and feeding.  All
chemical costs quoted in this chapter were  obtained from the latest  issues of
"Chemical  Marketing  Reporter" (Schnell Publishing Co.,  Inc., New York, N.  Y.)
 available during preparation of this manual.   Wide ranges in bagging  costs primarily
 reflect bag sizes  that  may  be  ordered. All chemical costs presented are for guidance
 only  and are subject  to  significant variations due  to  time and current  market con-
ditions.  Actually costs  for the chemicals being considered should be  carefully
 checked  prior to selection.

 5.2  Aluminum Compounds

The principal aluminum  compounds  that are commercially  available  and suitable  for
suspended  solids removal are dry and liquid  alum.  Sodium aluminate has been used in
 activated sludge plants, but  for phosphorus removal, and  its applicability for suspended
 solids removal is limited.

     5.2.1   Dry Alum

     5.2.1.1  Properties and Availability

 The commercial dry alum most often used in wastewater  treatment is known as  "filter
 alum." and has the  approximate  chemical  formula Al2(SO4)3*14H2Oand a molecular
 weight of about 600. Alum, is white to cream in color and a  1 percent solution has a pH of
 about  3.5.  The commercially available grades of alum and  their corresponding bulk den-
 sities and angles of repose are:

         GRADE           ANGLE  OF REPOSE          BULK DENSITY
                                                           Ib./cubic  feet
           Lump                  	                    62 to  68
          Ground                  43                      60 to  71
           Rice                     38                      57 to  71
           Powdered                65                      38 to  45

 Each of these grades  has a minimum alumium content of 27 percent, expressed as AlzOa,
 and maximum Fe20a  and soluble contents of 0.75 and 0.5 percent, respectively. Visosity
and solution crystallation temperatures are included in the subsequent section on liquid
 alum.

Since dry alum is only partially hydrated, it is slightly hygroscopic. However, it is relatively
stable when stored under the extremes of temperature and humidity encountered in the
United States.
                                       5-1

-------
The solubility of commercial dry alum at various temperatures is as follows:
                  Temperature'
                       32
                       50
                       68
                       86
                       104
                     Solubility
                     Ib/gal
                      6.03
                      6.56
                      7.28
                      8.45
                     10.16
Dry alum is not corrosive unless it absorbs moisture from the air, such as during prolonged
exposure to humid atmospheres. Therefore, precautions should be taken to ensure that the
storage space is free of moisture.

Alum is shipped in  100 Ib bags, drums, or in bulk (minimum of 40,000 Ib) by truck or rail.
Bag shipments may  be ordered on  wood  pallets if desired. Locations  of the major
production plants are listed in Table 5-1. At present, the price range for dry alum in bulk
quantities is $58 to  $63/ton. F.O.B. the point of manufacture. Freight costs to the point of
usage must be added to this. Bagging adds approximately $4 to 5/ton to the cost.

                                    TABLE 5-,l
            PARTIAL LIST OF ALUM MANUFACTURING PLANTS
Location
Manufacturer
Form of Alum Available
ALABAMA
    Coosa Pines
    Demopolis
    Mobile
    Naheola

ARKANSAS
    Pine Bluff

CALIFORNIA
    Bay Point (San Francisco)
    El Segundo (Los Angeles)
    Richmond (San Francisco)
    Vernon (Los Angeles)

COLORADO
    Denver
American Cyanamid
American Cyanamid
American Cyanamid
Stauffer
Allied
Allied
Allied
Stauffer
Stauffer
Allied
Liquid
Liquid
Liquid and Dry
Liquid
Liquid
Liquid and Dry
Liquid
Liquid
Liquid
Liquid and Dry
                                       5-2

-------
Location

FLORIDA
    Fernandina Beach
    Jacksonville
    Port St. Joe

GEORGIA
    Atlanta (2 plants)
    Augusta
    Cedar Springs
    Macon
    Savannah

ILLINOIS
    E. St. Louis
    Joliet

LOUISIANA
    Bastrop
    Baton Rouge
    Monroe
    New Orleans
    Springhill

MAINE
    Searsport

MARYLAND
    Baltimore

MASSACHUSETTS
    Adams
    Salem
                              TABLE 5-1 (continued)
Manufacturer
 Tennessee Corp.
 Allied
 Allied
 Burris, Allied
 Tennessee Corp.
 Tennessee Corp.
 Allied
 Allied
 Allied
 American Cyanamid
 Stauffer
 Stauffer
 Allied
 Allied
 Stauffer

 Northern


 Olin


 Holland
 Hamblet & Hayes
Form of Alum
     Available
Liquid
Liquid
Liquid
Liquid and Dry
Liquid and Dry
Liquid
Liquid
Liquid
Liquid and Dry
Liquid and Dry
Liquid
Liquid
Liquid
Liquid and Dry
Liquid

Liquid and Dry
 Dry


 Liquid
 Liquid and Dry
MICHIGAN
     Detroit
     Escanaba
     Kalamazoo (2 plants)

MINNESOTA
     Cloquet
     Pine Bend
 Allied
 American Cyanamid
 Allied, American Cyanamid
 American Cyanamid
 North Star
Liquid
Liquid
Liquid
Liquid
Liquid and Dry
                                      5-3

-------
                            TABLE 5-1 (continued)
Location

MISSISSIPPI
    Monticello
    Vicksburg

NEW JERSEY
    Newark
    Warners

NORTH CAROLINA
    Acme
    Plymouth

OHIO
    Chill icothe
    Cleveland
    Hamilton
    Middletown

OREGON
    North Portland

PENNSYLVANIA
    Johnsonburg
    Marcus Hook
    Newell

SOUTH CAROLINA
    Catawba
    Georgetown

TENNESSEE
    Chattanooga
    Counce
    Springfield
   Manufacturer
 American Cyanamid
 Allied
 Essex
 American Cyanamid
 Wright
 American Cyanamid
 Allied
 Allied
 American Cyanamid
 Allied
 Stauffer
 Allied
Allied
Allied
Burris
American Cyanamid
American Cyanamid
Stauffer
Burris
Form of Alum
  Available
Liquid
Liquid
Liquid and Dry
Liquid and Dry
Liquid
Liquid
Liquid
Liquid and Dry
Liquid and Dry
Liquid
Liquid and Dry
Liquid
Liquid and Dry
Liquid
Liquid
Liquid and Dry
Liquid and Dry
Liquid
Liquid
TEXAS
    Houston (2 plants)
Stauffer, Ethyl
Liquid and Dry
                                     5-4

-------
                            TABLE 5-1 (continued)
VIRGINIA
    Covington
    Hope well
    Norfolk

WASHINGTON
    Kennewick
    Tacoma (2 plants)
    Vancouver

WISCONSIN
    Menasha
    Wisconsin Rapids

Manufacturers and Addresses
Allied
Allied
Howerton Gowen
Allied
Stauffer, Allied
Allied
Allied
Allied
Liquid
Liquid
Liquid
Liquid
Liquid
Liquid and Dry
Liquid
Liquid
Allied Chemical Corporation
    Industrial Chemicals Division
    P.O. Box 1139R
    Morristown, New Jersey 07960

American Cyanamid Company
    Ind. Chem. Div.
    P.O. Box 66189
    Chicago, Illinois 60666

Burris Chemical Company
    Charleston, South Carolina

Essex Chemical Corporation
    1402 Broad Street
    Clifton, New Jersey 07015

Ethyl Corporation
    Houston, Texas

Hamblet & Hayes Company
    Colonial Road
    Salem, Massachusetts 01970
 Holland Chemical Company
     Adams, Massachusetts 01220
             Howerton Gowen Company, Inc.
                 Norfolk, Virginia

             Northern Chemical Industries, Inc.
                 Searsport, Maine 04974

             North Star Chemicals Inc.
                 P.O. Box 28-T
                 South St. Paul, Minnesota

             Olin Corporation,
                 Chemicals Division
                 745 Fifth Avenue
                 New, York, New York 10022

             Stauffer Chemical  Company
                 299 Park Avenue
                 New York, New York  10017

             Tennessee Corporation
                 Cities Service  Company
                 Industrial Chemicals Division
                 P.O. Box 50360
                 Atlanta, Georgia

             Wright Chemical Co.
                 Acme, North Carolina
                                     5-5

-------
   5.2.1.2   General Design Considerations

Ground and rice alum  are the grades most commonly used by utilities because of their
superior flow characteristics. These grades have less tendency to lump or arch in storage
and  therefore  provide  more  consistent feeding qualities.  Hopper  agitation is  seldom
required with these grades, and in fact may be detrimental to feeding because  of  the
possibility of packing the bin.

Alum dust is present in the ground grade and will cause minor irritation of the eyes and
nose on breathing. A respirator may be worn for protection against alum dust. Gloves may
be worn to protect the hands. Because of minor irritation in handling and the possibility of
alum dust  causing  rusting of adjacent  machinery, dust removal equipment is desirable.
Alum dust should be thoroughly flushed from the eyes immediately and washed from  the
skin with water.

   5.2.1.3   Storage

A typical storage and feeding system for dry alum is shown in Figure 5-1. Bulk alum can be
stored in mild steel  or concrete bins with dust collector vents located in, above, or adjacent
to the equipment room.  Recommended storage capacity is about 30 days. Dry alum in bulk
can be transferred with screw conveyors, pneumatic conveyors, or bucket elevators made of
mild steel.  Pneumatic conveyor  elbows should have a reinforced backing as the alum can
contain abrasive impurities.

Bags and drums  of alum should be stored in a dry location to avoid caking.  Bag or drum
loaded  hoppers should  have a nominal storage capacity  for eight hours at the nominal
maximum feed rate so that personnel are not required to charge the hopper more than once
per shift. Converging hopper sections should have a minimum slope of 60 degree to prevent
arching.

Bulk storage hoppers should terminate at a bin gate so that the feeding equipment may be
isolated for  servicing.  The bin gate should be followed by  a  flexible connection, and
transition hopper chute  or hopper which acts as a conditioning chamber over  the feeder.

     5.2.1.4   Feeding Equipment

The  feed system includes all of the components required for  the proper preparation of  the
chemical solution. Capacities and assemblies should be selected to fulfill individual system
requirements. Three  basic types of chemical  feed equipment are used: volumetric, belt
gravimetric,  and loss-in-weight  gravimetric.  Volumetric feeders are  usually used where
initial low cost and  usually lower capacities are the basis of selection. Volumetric feeder
mechanisms are  usually exposed to the corrosive dissolving chamber vapors which can
cause corrosion of discharge areas. Manufacturers usually control this problem by use of an
electric heater to keep the feeder housing dry or by using plastic components in the exposed
areas.
                                       5-6

-------
      ,-DUST COLLECTOR
        XA-FILL  PIPE  (PNEUMATIC)
  BULK  STORAGE
      BJN
                                  DAY HOPPER
                                  FOR DRY CHEMICAL
                                  FROM BAGS OR  DRUMS
              BIN .GATE
              FLEXIBLE
              CONNECTION

                 ALTERNATE SUPPLIES  DEPENDING
                          ON STORAGE
                                                     DUST COLLECTOR
                                                           XBAG FILL
                                                            -SCREEN
                                                             WITH BREAKER
                                                          ~\
                                                 SCALE OR SAMPLE CHUTE
DUST AND  VAPOR REMOVER
WATER

     f"1-!
            DRAIN
       SOLENOID  VALVE
     CONTROL
      VALVE -"ROTAMETER
PRESSURE REDUCING
      VALVE
                                                                     GRAVITY TO
                                                                     APPLICATION
                                                                  PUMP
                                                             TO APPLICATION
                                 FIGURE 5-1

                       TYPICAL DRY FEED SYSTEM
                                     5-7

-------
 Volumetric dry feeders presently in general use are of the screw type. Two designs of screw
 feed mechanism are available. Both allow even withdrawal across the bottom of the feeder
 hopper to prevent hopper dead zones. One screw design is the variable pitch type with the
 pitch'expanding unevenly to the discharge point. The second screw design is the constant pitch.
 type  expanding evenly to the discharge point. This type of screw design  is the  constant
 pitch-reciprocating type. This type has each half of the screw turned in opposite directions
 so that the turning and reciprocating motion alternately fills  one half of the  screw while the
 other half of the screw is discharging. The variable  pitch screw has one point of discharge,
 while the constant pitch-reciprocating screw has two points of discharge, one at each end of
 the screw. The accuracy of volumetric feeders is influenced by the character of the material
 being fed and ranges between ±;l;percenf for  free-flowing materials and =fc 7 percent for
 cohesive  materials. This accuracy is volumetric  and should  not be related  to accuracy by
 weight (gravimetric).

 Where the greatest accuracy and the most economical  use  of chemical  is desired, the
 loss-in-weight  type feeder should  be  selected. This  feeder  is limited to the  low and
 intermediate \feed  rates  up  to  a maximum  rate  of  approximately  4,000  lb/  hr.  The
 loss-in-weight type feeder consists of a material hopper and feeding mechanism mounted on
 enclosed  scales. The feed rate controller retracts the scale poise weight to deliver the dry
 chemical at the desired rate. The feeding mechanism must feed at this rate to maintain the
 balance of the scale. Any unbalance of the scale beam causes a corrective change in the
 output of the feeding mechanism. Continuous comparison of actual hopper weight with set
 hopper weight prevents cumulative errors. Accuracy of the loss-in-weight feeder is  ± 1 per-
 cent by weight of the set rate.

 Belt  type gravimetric feeders span the capacity ranges  of  volumetric  and loss-in-weight
' feeders and can usually be sized for all applications encountered in wastewater treatment
 applications. Initial expense is greater than for the volumetric feeder and slightly  less than
 for the loss-in-weight feeder. Belt type gravimetric feeders  consist of a basic belt feeder
 incorporating a weighing and control system.  Feed rates can be varied by changing either
 the weight per foot of belt, or the belt  speed, or both. Controllers in general use are
 mechanical, pneumatic, electric, and mechanical-vibrating. Accuracy specified for belt type
 gravimetric feeders should be within ±  1  percent of set rate. Materials of construction of
 feed equipment normally include mild steel hoppers, stainless steel mechanism components,
 and rubber surfaced feed belts.

 Because alum solution is corrosive, dissolving or solution chambers should be constructed
 of type 316 stainless steel, fiberglass reinforced plastic (FRP), or plastics. Dissolvers should
 be sized  for preparation of  the desired  solution strength.  The solution strength usually
 recommended is 0.5 lb of alum to 1  gal. of water, or a 6 percent  solution. The dissolving
 chamber is designed for a minimum detention time of 5 minutes at  the maximum feed rate.
 Because excessive dilution may be detrimental  to coagulation, eductors, or float valves that
 would  ordinarily be  used ahead of centrifugal pumps, are  not recommended. Dissolvers
 should be equipped with water meters and mechanical mixers so that the water to  alum ra-
 tio may be properly established and controlled.
                                        5-8

-------
   5.2.1.5  Piping and Accessories

FRP,  plastics  (polyvinyl  chloride,  polyethylene, polypropylene,  and  other similar
materials), • and rubber are general use and are recommended for alum solutions. Care
must be taken to provide adequate support for these piping systems, with close attention
given to spans between supports so that objectionable deflection will not be experienced.
The alum solution should  be injected into a zone of rapid mixing or turbulent flow.

Solution flow by gravity  to the point  of discharge is desirable. When gravity flow is not
possible, transfer components should be selected that require little or no dilution. When
metering pumps or proportioning weir tanks are used,  return of excess flow to a holding
tank should be considered. Metering pumps are discussed  further in the section on liquid
alum.

Valves used in solution lines should be plastic, type 316 stainless steel or rubber-lined iron or
steel.

   5.2.1.6  Pacing and Control

Standard instrument control and pacing signals are generally acceptable for common feeder
system  operation. Volumetric and gravimetric feeders are  usually adaptable to operation
from any standard instrument signals.

When  solution must  be pumped, consideration should be given to use  of  holding tanks
between the dry feed  system  and feed pumps, and the solution water supply should  be
controlled to prevent excessive dilution. The dry feeders may be started and stopped by tank
level probes. Variable control metering pumps can then transfer the alum stock solution to
the point of application without further dilution.

Means should be provided for calibration of the chemical feeders. Volumetric feeders may
be mounted on platform scales. Belt feeders should include a sample shute and box to catch
samples for checking actual delivery with set delivery.

Gravimetric feeders are usually furnished with totalizers only.  Remote instrumentation is
frequently used with gravimetric equipment, but seldom used with volumetric equipment.

   5.2.2.  Liquid Alum

   5.2.2.1  Properties and Availability

Liquid alum is shipped in rubber-lined or stainless steel, insulated tank cars or trucks. Alum
shipped during the winter  is heated prior to shipment so that crystallization  will not occur
during transit. Liquid Alum is shipped  at a solution strength of about 8.3  percent as  AhOa
or about 49 percent as Al2(SO4>3 • 14H2O. The latter solution weighs about 11 Ib/gal at
60 °F and contains about 5.4 Ib dry aium (17 percent  AhOa) per gal of liquid. This solution
will begin to crystallize at 30 °F and freezes at about 18 °F.
                                        5-9

-------
 Crystallization temperatures of various solution strengths are shown in Figure 5-2.

 The viscosity of various alum solutions is given in Figure 5-3.

 Tank truck lots of 3,000 to 5,000 gal are available. Tank car lots are available in quantities
. of 7,000 to 18,000 gal. Production locations of liquid alum are listed in Table 5-1. The cur-
 rent price range of liquid alum  on  an equivalent dry alum (17 percent AbOa) basis is about
 $45 to $507 ton,  F.O:B. the point of manufacture. Liquid alum will generally be more eco-
 nomical than dry alum if the point of use is within a 50 to 100 mile radius of the manufac-
 turing plant.

    5.2.2.2  General Design Considerations

 Bulk  unloading facilities usually must be provided at the treatment plant. Rail  cars are
 constructed  for .top unloading and therefore require an air supply system and  flexible
 connectors  to  pneumatically  displace  the alum  from  the car.  U.S.  Department of
 Transportation regulations concerning chemical tank car  unloading should be observed.
 Tank truck unloading is usually accomplished by gravity or by a truck mounted pump.

 Established practice in the treatment field has been to dilute liquid alum prior to application.
 However, recent  studies have shown that  feeding undiluted liquid  alum  results in better
 coagulation and settling. This is reportedly due to prevention of hydrolysis of the alum.

 No particular industrial hazards are encountered in handling liquid alum.  However, a face
 shield  and gloves  should be worn around leaking equipment. The eyes or skin should be
 flushed and washed  upon contact  with liquid alum. Liquid alum becomes very slick upon
 evaporation and therefore spillage should be avoided.

    5.2.2.3  Storage

 Liquid alum is stored without dilution at the shipping concentration. Storage tanks may be
 open  if indoors but must be closed and vented if outdoors. Outdoor tanks should also be
 heated, if necessary, to keep the temperature above 45°F to prevent crystallization. Storage
 tanks  should be  constructed of  type 316  stainless steel;  FRP; steel lined with  rubber,
 polyvinyl chloride, or lead. Liquid alum can be stored indefinitely without deterioration.

 Storage tanks should be sized according to maximum  feed rate, shipping time required, and
 quantity  of shipment.  Tanks  should  generally be sized  for P/2  times  the  quantity of
 shipments. A 10-day to 2-week  supply should be provided to allow for unforeseen shipping
 delays.
                                        5-10

-------
                  10   19  20  25  30  35  40
                   DEGREE BAUME AT 60°FARENHEIT
                                             45  50
                         FIGURE 5-2
CRYSTALLIZATION TEMPERATURES OF ALUM SOLUTIONS
             (Courtesy of American Cyanamid Co.)
                             5-11

-------
£
cs>
 60

 40


 20


 10
8.0
6.0

4.0


2.0
    1.0
    0.8
    0.6

    O.U
    0.2
    0.1
                                                                    I    I
                                 I    I   I   I    I    I   I
                                                         I   I    I
       30 40  50  60  70  80   90  100 110 120 130 140 150 160 170 180 190 200 210 220
                                TEMPERATURE,°F
                                  FIGURE 5-3

                      VISCOSITY OF ALUM SOLUTIONS
                        (Courtesy of Allied Chemical Co.)
                                       5-12

-------
   5.2.2.4  Feeding Equipment

Various types of gravity or pressure feeding and metering units are available. Figures 5-4
and 5-5 illustrate commonly used feed systems. The rotodip-type feeder or rotameter is
often  used for gravity feed and the metering pump  for pressure feed systems.

The pressure  or head available at the point of application frequently determines the feeding
system to be used. The rotodip feeder can be supplied from overhead storage by gravity
with the use  of an  internal  level  control valve, as shown by Figure 5-4. It may also be
supplied by a centrifugal pump. The latter arrangement requires an excess flow return line
to the storage tank, as shown by Figure 5-5. Centrifugal pumps should be direct-connected
but not close-coupled because of possible leakage into the motor, and should be constructed
of type 316 stainless steel, FRP, and plastics.

Metering pumps, currently available, allow a wide range of capacity compared  with the
rotodip  and rotameter systems. Hydraulic  diaphragm  type pumps are preferable to other
type pumps and  should be  protected  with an internal  or external relief valve. A back
pressure valve is  usually required  in the pump discharge to  provide efficient check valve
action. Materials of construction for feeding equipment should  be as recommended by the
manufacturer  for the  service, but depending on the type of system, will generally include
type 316 stainless steel, FRP, plastics, and rubber.


   5.2.2.5  Piping and Accessories

Piping systems for alum should be FRP, plastics (subject to  temperature limits),  type 316
stainless steel, or lead. Piping and valves used for  alum solutions are  also discussed in the
preceding section on dry alum.


   5.2.2.6  Pacing and Control

The feeding systems described above are volumetric, and the feeders generally available can
be adapted to receive standard instrument  pacing  signals. The signals can be used to vary
motor speed, variable-speed transmission  setting, stroke speed  and  stroke  length where
applicable.  A totalizer is  usually  furnished  with a rotodip-type  feeder,  and remote
instruments are available. Instrumentation is  rarely used with  rotameters and  metering
pumps.


   5.2.3  Reactions of Aluminum Sulfate

Reactions  between alum and the normal  constituents of wastewaters are influenced by
many factors, hence it is impossible to predict accurately the amount of alum that  will react
with  a  given amount  of alkalinity, lime or soda ash which  may have been added to the
wastewater. Theoretical reactions can be written which will serve as a general guide, but, in
general, the optimum dosage in'each case must be determined by laboratory jar tests.
                                       5-13

-------
                              OVERHEAD
                               STORAGE
                                TANK
               FLOAT
               VALVE
                                                                 tn
                                                                 V)
                              a.
                              ^
                              o
                                                       CJ
                                  ROTAMETER
       ROTODIP-TYPE FEEDER

GRAVITY FEED            GRAVITY  FEED
                     METERING PUMP


                              GRAVITY FEED
                               FIGURE 5-4
                ALTERNATIVE LIQUID FEED SYSTEMS
                      FOR OVERHEAD STORAGE
                       PRESSURE FEED
        -ROTODIP-TYPE FEEDER
^CONTROL VALVE

  ^ROTAMETER
\,


^~
r
YTVT^7
^-r |
1


1

f
VITY FEED 'i
TRANSFER PUMF


\

"h


xREC

••>







IRCULATION


GROUND
STORAGE
TANK



«
L
^ -
•< Q- S
s§ 2 ;
LL. Q. 0
_ o
ac. o z
^ a «
° UJ
^ * • 1
H * '
1 o
I s
<- >


- s
^ = PRESSURE
e a FE|
oe
Q.
L o
^ 2

^)


                             FIGURE 5-5

               ALTERNATIVE LIQUID FEED SYSTEMS
                       FOR GROUND STORAGE
                                 5-14

-------
The simplest case is the reaction of Al + with OH ions made available by the ionization of
water or by the alkalinity of the water.

Solution of alum in water produces:
                          :A12(SO4)3^±2A13+' +3(SO4)2~

Hydroxyl ions become available from ionization of water:
The aluminum ions (Al|3^) then react:
                           2A13+ +6OH':^±2 A1(OH)3

Consumption of hydroxyl  ions will  result  in  a  decrease  in the alkalinity. Where the
alkalinity of the wastewater is inadequate for the alum dosage, the pH must be increased by
the addition of hydrated  lime,  soda ash or caustic soda. The reactions of alum with the
common  alkaline  reagents are  shown in  Table 5-2. While  these  reactions are  an
oversimplification of  what actually takes place, they do  serve to indicate  orders of
magnitude and some by-products of alum treatment.

                                    TABLE 5-2

                     REACTIONS OF ALUMINUM  SULFATE

     A12 (SO4)3 + 3 Ca (HCO3)2— ^2 Al (OH)3J + 3 CaSO4 + 6 CO2f

     Ak (SO4)3 + 3 Na2CO3 + 3 H2O— ^2 AL (OH) 3\ + 3 CO2f

     Ah (SO4)3 + 3 Ca (OH)2— *-2 Al (OH)3| + 3 CaSO4

In terms of quantities, the reactions in Table 5-2 can be expressed as follows:
              f         •
     1 mg/ I of alum reacts with:

     0.50 mgV 1 alkalinity, expressed as CaCOs

     0.39 mg/ 1 95 percent hydrated lime as Ca (OH)2

     0.54 mg/ 1 soda ash as Na2COs

These approximate amounts of  alkali when added to wastewater will maintain the alkalinity
of the water unchanged when  1 mg/ 1 of alum is added. For example,  if no alkalinity is
added, 1 mg/ 1 of alum will reduce the alkalinity of 0.50 mg/ 1 as CaCOs but alkalinity can
be maintained unchanged if 0.39 mg/ 1 of hydrated lime is added. This lowering of natural
alkalinity is desirable in many cases to attain the pH range for optimum coagulation.

For  each mg/ 1  of alum  dosage,  the sulfate (SO4) content  of the water  will be increased
approximately 0.49 mg/ 1 and the CO2 content of the water will be increased approximately
0.44 mg/1.
                                      5-15

-------
5.3  Iron Compounds

Iron compounds have pH coagulation ranges and floe characteristics similar to aluminum
sulfate. The cost of iron compounds may often be less than the cost of alum. However, the
iron compounds are generally corrosive and often present difficulties in dissolving, and their
use may result in high soluble iron concentrations in process effluents.

   5.3.1  Liquid Ferric Chloride

   5.3.1.1   Properties and Availability

Liquid ferric chloride is a corrosive,  dark brown oily-appearing solution having a weight as
shipped and stored of 11.2 to 12.4 Ib/gal (35 to 45 percent FeCb) (1). The ferric chloride
content of these solutions, as FeCh, is 3.95 to 5.58 Ib/gal. Shipping concentrations vary
from summer to winter due  to the relatively high crystallization temperature of the more
concentrated solutions as  shown by Figure 5-6. The pH of a 1  percent solution is 2.0.

The molecular weight of ferric chloride is 162.22. Viscosities of ferric chloride solutions at
various temperatures are presented in Figure 5-7.

Liquid ferric chloride is shipped in 3,000 to 4,000 gal bulk truckload lots, in 4,000 to 10,000
gal bulk carload lots, and in  5 and 13 gal  carboys. Liquid ferric chloride is produced at the
following locations (2):

     Dow Chemical Co.
     Midland, Michigan

     Pennwalt Corp.
     Philadelphia, Pa. (Plant at Wyandotte, Mich.)

The current price  of liquid ferric chloride in bulk quantities is about $0.04 to $0.05/lb (as
FeCb), F.O.B. the point of manufacture.

Tank trucks and cars are  normally unloaded pneumatically, and operating procedures must
be closely followed to avoid spills and accidents. The safety vent cap and assembly (painted
red) should  be removed prior to opening the unloading connection to depressurize the tank
car or truck, prior to unloading.

   5.3.1.2   General Design Considerations

Ferric  chloride solutions are  corrosive to  many common materials and cause stains which
are difficult to  remove.  Areas which are subject to staining should be protected with
resistant paint or rubber mats.
                                       5-16

-------
   70



   60



   50



   40



   30



o"-  20

ul
|  10


oe
LU  0

I

--10



 -20



 -30



 -40



 -50
I    I    I    I    I    I    I    I    I    I    I    I    I    I    I    I
A  Agitated Solutions may  Degin to develop crystals
   below th i s 1i ne

B  Unagitated Solutions Degin to develop crystals when
   the Dulk solution temperature drops to about this line.
   Ice crystals  form Delow 33% FeCl, &
   Crystals form aoove 3$ Fed,.  d
  -60
         I    I    I    I    I    I    I    I
                                                    I    I    I    I    I    I
    10
           18
22
26
30
                                         Fed
34
38
                                                                    42
                                     FIGURE 5-6

                         FREEZING POINT CURVES FOR

                COMMERCIAL FERRIC CHLORIDE SOLUTIONS

                            (Courtesy of Dow Chemical Co.)
46
so
                                         5-17

-------
   100


    80



    60

    50


    40



oo   30
LU
ae

£
00   20

I—


S   15
o
CO
    10


     8



     6

     5


     4
           I    I     I     I    I     I    I    I     I    I     I

           (Aosolute Viscosity)=(Kinematic Viscosity)(Density)

           Centipoises = Cent i stokes x  Q"1
                                     cc
                    10
                             20
                                               40
                                                        50
                                %  Fed,
                         FIGURES-?


         VISCOSITY VS COMPOSITION OF FERRIC

            CHLORIDE SOLUTIONS AT VARIOUS

                      TEMPERATURES

                 (Courtesy of Dow Chemical Co.)
                              5-18

-------
Normal precautions should be employed when cleaning ferric chloride handling equipment.
Workmen should wear rubber gloves, rubber apron,  and goggles or a face shield. If ferric
chloride comes in contact with the eyes or skin, flush with copious quantities  of running
water and call  a physician. If ferric chloride  is ingested,  induce vomiting and  call  a
physician.

   5.3.1.3   Storage

Ferric chloride solution can be stored as shipped. Storage tanks should have a free vent or
vacuum relief valve. Tanks may be constructed of FRP, rubber lined steel, of plastic lined
steel. Resin-impregnated  carbon or graphite  are  also  suitable  materials for storage
containers.

It may be necessary in most instances to house liquid ferric chloride tanks in heated areas
or provide tank heaters or insulation to prevent crystallization. Ferric chloride can be stored
for long periods of time without deterioration. The total storage capacity should be  1 l/2
times the largest anticipated shipment, and should  provide at least a  10-day  to  2-week
supply of the chemical at the design average dosage.

   5.3.1.4   Feeding Equipment

Feeding equipment and systems described for liquid alum generally apply to ferric chloride
except for materials of construction and the use of glass tube rotameters.

It may not be desirable to dilute the ferric chloride solution  from its shipping concentration
to a weaker feed solution because of possible hydrolysis. Ferric chloride solutions may be
transferred  from underground storage to  day tanks with  impervious  graphite or  rubber
lined self-priming centrifugal pumps having teflon rotary and stationary seals. Because of the
tendency for liquid ferric chloride to stain or deposit, glass-tube rotameters should  not be
used for metering this solution. Rotodip feeders and  diaphragm metering pumps are often
used for ferric chloride,  and should be constructed of materials such as rubber-lined steel
and plastics.

   5.3.1.5   Piping  and Accessories

Materials for piping and transporting ferric chloride  should be rubber or Saran-lined steel,
hard rubber, FRP, or plastics. Valving should consist of rubber or resin-lined diaphragm
valves, Saran-lined  valves with teflon diaphragms,  rubber-sleeved pinch-type valves,  or
plastic ball valves.  Gasket material  for large openings such as manholes in storage tanks
should be  soft rubber;  all other gaskets  should be  graphite-impregnated blue asbestos,
teflon, or vinyl.

   5.3.1.6   Pacing and Control

System pacing and control requirements are similar to those discussed previously for liquid
alum.
                                        5-19

-------
   5.3.2   Ferrous Chloride (Waste Pickle Liquor)

   5.3.2.1   Properties and Availability

Ferrous chloride, FeCh, as a liquid is available in the form of waste pickle liquor from steel
processing. The liquor weighs between 9.9 and  10.4 Ib/gal and contains 20 to 25 percent
FeCb or about 10 percent available Fe2+. A 22  percent solution of FeCb will crystallize at
a temperature of-4 °F. The molecular weight of FeCb is  126.76. Free acid in waste pickle
liquor can vary from  1 to 10 percent and usually averages about  1.5 to 2.0 percent. Ferrous
chloride is slightly less corrosive than ferric chloride.

Waste pickle liquor is available in 4,000 gal truckload lots and a variety of carload lots. In
most instances the availability of waste pickle liquor will depend on the proximity to steel
processing plants. Dow Chemical Company produces a waste pickle liquor, having  an FeCb
content of about 22 percent at a price of $0.04/lb of FeCh in bulk car or truckload quan-
tities. F.O.B. Midland, Michigan.

   5.3.2.2   General Design Considerations

Since ferrous chloride or waste pickle liquor may not be available on a continuous  basis,
storage and feeding equipment should be suitable for handling ferric chloride. Therefore, the
ferric chloride section should be referred to for storage and handling details.

   5.3.3  Ferric Sulfate

   5.3.3.1   Properties and Availability

Ferric sulfate is marketed as dry, partially-hydrated granules with  the formula Fe2(SO4)3 •
X HaO, where X is approximately 7. Typical properties of one commercial product (2) are
presented below:


                 Molecular Weight                      526
                 Bulk Density                           56-60 Ib/cu ft
                 Water Soluble Iron Expressed as Fe     21.5 percent
                 Water Soluble Fe+3                    19.5 percent
                 Water Soluble Fe+2                     2.0 percent
                 Insolubles Total                         4.0 percent
                 Free Acid                               2.5 percent
                 Moisture @ 105°C.                      2.0 percent
Ferric sulfate is shipped in car and truck load lots of 50 Ib and 100 Ib moistureproof paper
bags and 200 Ib and 400 Ib fiber drums. Bulk carload shipments in box and closed hopper
cars are available. The major producer is Cities Service Company, with a plant located at
Copper Hill, Tennessee.
                                        5-20

-------
The current price of ferric sulfate (21.8 percent Fe) is about $397ton, F.O.B. Copper Hill,
Tennessee. The bagged form costs from $6 to $11/ton more than the bulk.

General  precautions should be observed when handling ferric  sulfate, such  as  wearing
goggles and dust masks, and areas of the body that come in contact with the dust or vapor
should be washed promptly.

   5.3.3.2   General Design Considerations

Aeration of ferric sulfate should be held to a minimum because of the hygroscopic nature of
the material, particularly in damp atmospheres. Mixing of ferric sulfate and quicklime in
conveying and dust vent systems should be avoided as caking and  excessive heating can
result. The presence of  ferric sulfate and lime in combination has been known to destroy
cloth  bags in pneumatic unloading  devices (3). Because  ferric  sulfate in  the  presence of
moisture will stain, precautions similar to those  discussed for  ferric  chloride should be
observed.

   5.3.3.3   Storage

Ferric sulfate is usually stored in the  dry  state either in the  shipping bags or in bulk in
concrete  or steel bins. Bulk storage bins should be as tight as possible to avoid moisture
absorption, but dust collector vents are permissible and desirable. Hoppers on bulk storage
bins should have a minimum slope of 36° however, a greater angle is prefered.

Bins may be located inside or outside and the material transferred by bucket elevator, screw
or air conveyors. Ferric sulfate stored  in bins usually absorbs some  moisture and forms a
thin protective crust which retards further absorption until the crust is  broken.

    5.3.3.4  Feeding Equipment

Feed  solutions are usually made up at  a water to chemical ratio  of 2:1  to 8:1 (on a weight
basis) with the usual ratio being 4:1 with a 20-minute detention time.  Care must be taken
not to dilute ferric sulfate solutions to  less than  1 percent to prevent hydrolysis and depos-
ition  of ferric hydroxide. Ferric sulfate is actively corrosive in solution, and dissolving and
transporting equipment  should be fabricated of type 316 stainless steel, rubber,  plastics, ce-
ramics or lead.

Dry feeding requirements are similar to those for dry alum except that belt  type feeders are
rarely used because of their open type of construction. Closed construction, as found in the
volumetric and  loss-in-weight-type  feeders, generally exposes  a minimum of operating
components to the vapor, and thereby  minimizes maintenance. A water jet vapor  remover
should be provided  at the dissolver to protect both the machinery and operator.
                                       5-21

-------
   5.3.3.5   Piping and Accessories

Piping systems for ferric sulfate should be FRP, plastics, type 316 stainless steel, rubber or
glass.

   5.3.3.6   Pacing and Control

System pacing and control are the same as discussed for dry alum.

   5.3.4   Ferrous Sulfate

   5.3.4.1   Properties and Availability

 Ferrous sulfate or copperas is a byproduct of pickling steel and is produced as granules,
 crystals, powder; and lumps. The most  common commercial  form of ferrous sulfate is
 FeSO4- VHaO, with a molecular weight of 278, and containing 55 to 58 percent FeSO4 and
 20 to 21 percent Fe. The product has  a bulk density of 62 to 66 Ib/cu ft. When dissolved,
 ferrous sulfate  is acidic. The composition  of ferrous sulfate may  be  quite variable  and
 should be established by consulting the nearest manufacturers.

 Bulk, drum (400 Ib) and bag (50 and 100 Ib) shipments are available from producers at the
 following locations:

     American Cyanamid Co.                               Savannah, Georgia
     Byproducts Processing Co., Inc.                         Baltimore,  Maryland
     Glidden Co.                                            Baltimore,  Maryland
     Cosmin Corp.                                          Baltimore,  Maryland
     NL Industries                                          St.  Louis, Missouri
     NL Industries                                          Sayreville, New Jersey

 The current price of ferrous sulfate in bulk carload and truckload quantities is about $187
 ton (21 nercent FeV  The happed cost is $247 ton
ton (21 percent Fe). The bagged cost is $247 ton.
 Ferrous sulfate is also available in a wet state in bulk form from some plants. This form is
 likely to be difficult  to handle and  the  manufacturer should  be  consulted  for specific
 information and instructions.

 Dry ferrous sulfate cakes at storage temperatures above 68°F, is efflorescent in dry air, and
 oxidizes and hydrates further in moist air.

 General precautions similar to those  for ferric sulfate, with respect to dust and handling
 acidic solutions, should be observed when working with ferrous sulfate. Mixing quicklime
 and ferrous sulfate produces high temperatures and the possibility of fire.
                                        5-22

-------
   5.3.4.2  General Design Considerations

The granular form of ferrous sulfate has the best feeding characteristics and gravimetric or
volumetric feeding equipment may be used.

The optimum chemical to water ratio for continuous dissolving is 0.5 Ib/gal. of 6 percent
with a detention time of 5 minutes in the dissolves Mechanical agitation should be provided
in the dissolver to assure complete solution. Lead, rubber, iron, plastics, and type 304 stain-
less steel can be used as construction materials for handling solutions of ferrous sulfate.

Storage, feeding and transporting systems probably should be suitable for handling ferric
sulfate as an alternative to ferrous sulfate.

   5.3.5  "Reactions of Iron Compounds

Ferric sulfate and ferric chloride react with the alkalinity of wastewater or with the added
alkaline  materials such  as  lime or soda ash. The reactions may be  written to show
precipitation of ferric hydroxide, although in practice, as with alum, the reactions are more
complicated than this. The reactions are  shown in Table 5-3, using ferric sulfate.

                                   TABLE 5-3
                      REACTIONS OF FERRIC SULFATE
       Fe2(SO4)3 + 3 Ca(HCO3)2—»-2  Fe(OH)3{ + 3 CaSO4 + 6 CO2f

       Fe2(SO4)3 + 3 Na2CO3  + 3  H2O—*-2 Fe(OH)3J  + 3 Na2SO4 + 3 CO2f

       Fe2(SO4)3 + 3 Ca(OH)2—»-2 Fe(OH)3{ + 3 CaSO4
  Ferric chloride can be substituted in these reactions.
  In terms of useful quantities, the reactions of Table 5-3 can be expressed as follows:

      1.   1 mg/ 1 of Fe2(SO4>3 • 7H2O reacts with:

          0.57 mg/ 1 alkalinity, expressed as CaCO3

          0.44 mg/ 1 95 percent hydrated lime as Ca(OH)2

          0.62 mg/ 1 soda ash as Na2CO3

     2.   1 mg/ 1 of anhydrous FeCL3  reacts with:

          0.92 mg/ 1 alkalinity expressed as CaCO3

          0.72 mg/ 1 95 percent hydrated lime as Ca(OH)2

          1.00 mg/ 1 soda ash as Na2CO3
                                      5-23

-------
Ferrous sulfate and  ferrous chloride  react with the  alkalinity of wastewater or with the
added  alkaline  materials  such as lime to  precipitate  ferrous hydroxide. The ferrous
hydroxide  is oxidized to ferric hydroxide by dissolved oxygen in wastewater.  Typical
reactions are shown in Table 5-4, using ferrous sulfate.
                                    TABLE 5-4

                      REACTIONS OF FERROUS SULFATE

                  FeSO4 + Ca(HCOa)2—^Fe(OH)2| + Ca SO4

                  FeSO4 + Ca(OH)2—*~Fe(OH)2j + Ca SO4

                  4 Fe(OH)2 + O2 + 2H2—*- 4 Fe (OH)3 (
Ferrous hydroxide is rather soluble and oxidation to the more insoluble ferric hydroxide is
necessary if high iron residuals in effluents are to be avoided. Flocculation with ferrous iron
is improved by addition of lime  or caustic soda at a rate of 1 to 2 mg/mg Fe to serve as a
floe conditioning agent. Polymers are also generally required to produce a clear effluent.

5.4  Lime

The term "lime" applies to a variety of chemicals which are alkaline in nature and contain
principally calcium, oxygen and, in some cases, magnesium. In this grouping are included
quicklime, dolomitic  lime, hydrated lime,  dolomitic  hydrated lime, limestone,  and
dolomite. This section is restricted to discussion of quicklime and hydrated lime, but the
dolomitic counterparts of these chemicals,  i.e.,  the  high-magnesium  forms,  are quite
applicable for wastewater treatment and are generally similar in physical requirements.

   5.4.1   Quicklime

   5.4.1.1 Properties and Availability

Quicklime, CaO, has a density range of approximately 55 to 75 Ib/cu ft, and a molecular
weight of 56.08. A slurry for feeding, called milk of lime, can be prepared with up to 45 per-
cent solids. Lime is only slightly soluble, and both lime dust and slurries are caustic in na-
ture. A saturated solution of lime has a pH of about 12.4.

Lime can be  purchased in bulk in both car and truck load lots. It is also shipped in 80 and
100 Ib multiwall "moistureproof"  paper bags. Lime is  produced at the locations indicated
by Table 5-5.
                                       5-24

-------
                                  TABLE 5-5
Location
PARTIAL LIST OF LIME MANUFACTURING PLANTS (4)
                                                          Form of
                   Manufacturer                        Lime Available
ALABAMA
    Allgood
    Keystone

    Landmark
    Montevallo
    Roberta

    Saginaw

    Siluria

ARIZONA
    Douglas
    Globe
    Nelson
              Cheney Lime & Cement Co.
              Southern Cement Co.
                   Div. Martin Marietta Corp.
              Cheney Lime & Cement Co.
              U.S. Gypsum Co.
              Southern Cement Co.
                   Div. Martin Marietta Corp.
              Longview Lime Co., Div.
                   Woodward Co., Div. Mead Corp.
              Alabaster Lime Co.
               Paul Lime Plant, Inc.
               Hoopes & Co.
               U.S. Lime Div., The Flintkote Co.
High Calcium
High Calcium

High Calcium
High Calcium
High Calcium

High Calcium

High Calcium
High Calcium
High Calcium
Hish Calcium
ARKANSAS
    Batesville
CALIFORNIA
    City of Industry
    Diamond Springs
    Lucerne Valley

    Richmond
    Salinas

    Sonora
    Westend

COLORADO
    Ft. Morgan

CONNECTICUT
    Canaan
               Batesville White Lime Co.,
                   Div. Rangaire Corp.
               U.S. Lime Div., The Flintkote Co.
               Diamond Springs Lime Co.
               Pfizer, Inc., Minerals,  Pigments
                  and Metals Div.
               U.S. Lime Div., The Flintkote Co.
               Kaiser Aluminum & Chemical Corp.
                  (currently captive  lime)
               U.S. Lime Div., The Flintkote Co.
               Stauffer Chemical Co.
               Great Western Sugar Co.
               Pfizer, Inc., Minerals, Pigments
                   and Metals Div.
High Calcium
High Calcium
High Calcium
High Calcium

High Calcium
Dolomitic

Dolomitic
High Calcium
High Calcium
Dolomitic
                                     5-25

-------
^Location
FLORIDA
     Brooksville
     Sumterville

ILLINOIS
     Marblehead
     McCook

     Quincy
     So. Chicago
     Thornton

INDIANA
     Buffington

IOWA
     Davenport

KENTUCKY
     Carntown

LOUISIANA
     Morgan City
     New Orleans

MARYLAND
     LeGore
     Woodsboro

MASSACHUSETTS
     Adams

     Lee

MICHIGAN
     Detroit
     Ludington
     Menominee

     River  Rouge

 MINNESOTA
     Duluth
     TABLE 5-5 (continued)

Manufacturer

Chemical Lime Co.
Dixie Lime and Stone Co.
Marblehead Lime Co.
Standard Lime & Refractories
    Div., Martin Marietta Corp.
Marblehead Lime Co.
Marblehead Lime Co.
Marblehead Lime Co.
Marblehead Lime Co.
Linwood Stone Products Co., Inc.
Black River Mining Co.
Pelican State Lime Corp.
U.S. Gypsum Co.
LeGore Lime Co.
S.W. Barrick & Sons, Inc.
 Pfizer, Inc., Minerals, Pigments
    and Metals Diy.
 Lee Lime Corp.
 Detroit Lime Co.
 Dow Chemical Co. (currently captive lime)
 Limestone Products Co., Div.
    C. Reiss Coal Co.
 Marblehead Lime Co.
 Cutler Magner Co.
   Form of
Lime Available

High Calcium
High Calcium
High Calcium
Dolomitic

High Calcium
High Calcium
Dolomitic
 High Calcium


 High Calcium


 High Calcium
 High Calcium
 High Calcium
 High Calcium
 High Calcium
 High Calcium

 Dolomitic
 High Calcium
 High Calcium
 High Calcium

 High Calcium
 High Calcium
                                      5-26

-------
Location
   TABLE 5-5 (continued)

        Manufacturer
    Form of
 Lime Available
MISSOURI
    Bonne Terre
    Hannibal
    Ste. Genevieve
    Springfield

NEVAD\
    Apex
    Henderson

    McGill
    Sloan
Valley Dolomite Co.
Marblehead Lime Co.
Mississippi Lime Co.
Ash Grove Cement Co.
U.S. Lime Div., The Flintkote Co.
U.S. Lime Div., The Flintkote Co.

Morrison-Weatherly Corp.
U.S. Lime Div., The Flintkote Co.
 Dolomitic
High Calcium
High Calcium
High Calcium
High Calcium
Dolomitic &
  High Calcium
High Calcium
Dolomitic &
  High Calcium
NEW JERSEY
    Newton

OHIO
    Ashtabula
    Carey
    Cleveland
    Delaware
    Geona
Limestone Products Corp. of America
Union Carbide Olefins Co.
National Lime & Stone Co.
Cuyahoga  Lime Co.
Marble Cliff Quarries Co.
U.S. Gypsum Co.
    Gibsonburg (2 plants) Pfizer, Inc., Minerals, Pigments
                            and Metal Div., National Gypsum Co.
    Huron               Huron Lime Co.
High Calcium
High Calcium
Dolomitic
High Calcium
High Calcium
Dolomitic
Dolomitic

High Calcium
    Marble Cliff
    Millersville
    Woodville

OKLAHOMA
    Marble City
    Sallisaw

OREGON
    Baker
    Portland
Marble Cliff Quarries Co.
J. E. Baker Co.
Ohio Lime Co., Standard Lime &
   Refractories Div., Martin Marietta Corp.
St. ClairLimeCo.
St. Clair Lime Co.
Chemical Lime Co. of Oregon
Ash Grove Cement Co.
High Calcium
Dolomitic
Dolomitic
High Calcium
High Calcium
High Calcium
High Calcium
                                     5-27

-------
                             TABLE 5-5 (continued)
Location

PENNSYLVANIA
  Annville
  Bellefonte (2 plants)
  Branchton
  Devault
  Everett
  Pleasant Gap

  Plymouth Meeting

SOUTH DAKOTA
  Rapid City

TENNESSEE
  Knoxville (2 plants)

TEXAS
  Blum
  Cleburne
  Clifton
  Houston
  McNeil
  New  Braunfels
  Round Rock
  San Antonio

UTAH
  Grantsville

  Lehi
Manufacturer
Bethlehem Mines Corp.
National Gypsum Co., Warner Co.
Mercer Lime & Stone Co.
Warner Co.
New Enterprise Stone & Lime Co.
Standard Lime & Refractories Div.,
    Martin Marietta Corp.
G. & W. H. Corson, Inc.
Pete Lien & Sons, Inc
Foote Mineral Co., Williams Lime
    Manufacturing Co.

Round Rock Lime Companies
Texas Lime Co., Div. Rangaire Corp.
Clifstone Lime Co.
U. S. Gypsum Co.
Austin White Lime Co.
U. S. Gypsum Co.
Round Rock Lime Companies
McDonough Bros., Inc.
 U. S. Lime Div., The Flintkote Co.
 Rollins Mining Supplies Co.
   Form of
Lime Available
High Calcium
High Calcium
High Calcium
Dolomitic
High Calcium
High Calcium

Dolomitic
High Calcium
High Calcium
High Calcium
High Calcium
High Calcium
High Calcium
High Calcium
High Calcium
High Calcium
High Calcium
Dolomitic &
  High Calcium
High Calcium
VERMONT
    Winooski

VIRGINIA
    Clearbrook
    Kimballton (2 plants)
 Vermont Assoc. Lime Industries, Inc.
 W.S. Frey Co., Inc.
 Foote Mineral Co., National Gypsum
     Company
High Calcium
High Calcium
High Calcium
                                      5-28

-------
Location

    Stephens City

    Strasburg

WASHINGTON
    Tacoma
                           TABLE 5-5 (continued)
Manufacturer
M.J. Grove Lime Co., Div. The
    Flintkote Co.
Chemstone Corp.
Domtar Chemicals Inc.
   Form of
Lime Available

High Calcium

High Calcium


High Calcium
WEST VIRGINIA
     Millville

     Riverton
WISCONSIN
     Eden
     Green Bay
     Knowles
     Manitowoc
     Superior
Standard Lime & Refractories Div.,
     Martin Marietta Corp.
Germany Valley Limestone Div.,
     Greer Steel  Co.
Western Lime'& Cement Co.
Western Lime & Cement Co.
Western Lime & Cement Co.
Rockwell Lime Co.
Cutler-LaLiberte-McDougall Corp.
Dolomitic

High Calcium
Dolomitic
High Calcium
Dolomitic
Dolomitic
High Calcium
Current prices for bulk pebble quicklime range from $187ton to $21/ton with the higher
prices generally in the  far west, and higher than average in the north. Bagging  adds
approximately $47 ton to the cost.

The CaO content  of commercially available quicklime can vary quite widely over an ap-
proximate range of 70 to 96 percent. Content below 88 percent is generally considered be-
low standard in the municipal use field (5). Purchase contracts are often based on 90 per-
cent CaO content with provisions for payment of a bonus for each 1  percent over and a pen-
alty for each 1 percent under the standard. A CaO content less than 75 percent probably
should be rejected because of excessive grit and difficulties in slaking.

Workmen should wear protective clothing and goggles to protect the skin and eyes, as lime
dust  and  hot slurry can cause severe  burns. Areas contacted by lime should be washed
immediately. Lime should not be mixed with chemicals which have water of hydration. The
lime  will  be slaked by  the water of  hydration causing excessive temperature  rise and
possibly explosive conditions. Conveyors and bins used for more than one chemical should
be thoroughly cleaned before switching chemicals.

    5.4.1.2  General Design Considerations

 Pebble quicklime, all passing a 3/4 in.iscreen and not more than 5 percent passingia. No.1100
 screen, is normally specified because of easier handling and less dust. Hopper agitation is
                                      5-29

-------
generally not required with the pebble form. Published slaker capacity ratings require "soft
or normally burned" limes which  provide fast  slaking and temperature rise, but poorer
grades of limes may also be satisfactorily slaked by selection of the appropriate  slaker
retention time and capacity.

   5.4.1.3   Storage

Storage of bagged lime  should be in a dry place,  and preferably elevated on pallets to avoid
absorption  of  moisture.  System  capacities  often make  the  use of bagged quicklime
impractical. Maximum  storage period is about 60 days.

Bulk lime is stored in air-tight concrete or steej bins having a 55 to 60 deg slope on the bin
outlet. Bulk lime can be conveyed by conventional bucket elevators and screw,  belt, apron,
drag-chain, and bulk conveyors of mild steel construction. Pneumatic conveyors subject the
lime to air-slaking and  particle sizes may be reduced by attrition. Dust collectors should be
provided on manually and pneumatically-filled bins.

   5.4.1.4   Feeding Equipment

A typical lime  storage and feed system is illustrated in Figure 5-8. Quicklime  feeders are
usually limited  to the belt or loss-in-weight gravimetric types because of the wide variation
of the bulk density. Feed equipment should have  an adjustable feed range of at least 20:1 to
match  the operating range of the associated slaker. The feeders should have an over-under
feed  rate alarm to immediately warn of operation beyond set limits of control. The feeder
                                              - * . .          •
drive should be instrumented to be  interrupted in the event of excessive temperature in the
slaker  compartment.

Lime slakers for wastewater treatment should be of  the continuous type,  and the  major
components should include one or  more slaking compartments, a dilution compartment, a
grit  separation compartment and a continuous grit remover. Commercial designs vary in
regard to the combination of water to lime, slaking temperature,  and slaking  time, in
obtaining the "milk of lime" suspensions.

The  "paste-type" slaker admits water as required to  maintain  a desired mixing viscosity.
This viscosity therefore sets  the  operating retention time of the slaker.  The paste  slaker
usually operates with a low  water to lime ratio (approximately 2:1 by  weight),  elevated
temperature, and five-minute slaking time at maximum capacity.

The  "detention" type slaker admits water to maintain a desired ratio with the lime,  and
therefore the lime  feed rate sets the retention  time  of  the slaker.  The detention  slaker
operates with a wide range of water to lime ratios (2.5:1 and 6:1), moderate temperature,
and  a  10 minute slaking time at maximum capacity. A water to lime ratio of from 3.5:1 to
4:1 is most  often used. The operating temperature in lime slakers is a function of the water
to lime ratio, lime quality, heat transfer, and water temperature. Lime slaking evolves heat
in hydrating the CaO to  Ga(OH)2 and therefore, vapor removers are required for  feeder
protection.

                                        5-30

-------
NOTE:  VAPOR REMOVER
HOT SHOWN FOR CLARITY
                                              COLLECTOR

                                                FILL PIPE  (PNEUMATIC)
                       BULK STORAGE
                           BIN
                                      \
                         SCALE
                                         FEEDER
   SOLENOID
     VALVE,,
            OR  SAMPLE CHUTE

   ROTAMETERS

SLAKING WATER
 DILUTION WATER
BIN GATE
FLEX IBLE
CONNECTION
        FLOW RECORDER
        WITH PACING
        TRANSMITTER^
                                                            pH RECORDER
                                                            CONTROLLER
       ROTOD IP-TYPE
          FEEDER

     GRAVITY FEED
      RECIRCULATION
                                 HOLDING
                                  TANK
                                         METERING
                                           PUMP-
                          'BACK
                         PRESSURE
                          VALVE
                                     FIGURE 5-8

                         TYPICAL LIME FEED SYSTEM
                                         5-31

-------
    5.4.1.5  Piping and Accessories

 Lime slurry should be transported by gravity in open channels wherever possible. Piping
 channels, and accessories may be rubber, iron,  steel, concrete, and plastics. Glass tubing,.
 such as that in rotameters, will cloud rapidly and therefore should not be used. Any abrupt
 directional changes in piping should include plugged tees or crosses to allow rodding-out of
 deposits. Long sweep elbows should be provided to allow the piping to be cleaned by the use
 of a cleaning "pig". Daily cleaning is desirable.

 Milk of lime transfer pumps should be of the open impeller centrifugal type. Pumps having
 an  iron body and  impeller with bronze trim are suitable for this purpose. Rubber-lined
 pumps with rubber-covered impellers are also frequently used.  Make-up tanks are usually
 provided ahead of centrifugal pumps to ensure a flooded suction at all times. "Plating-out"
 of lime is minimized by the use of soft water in  the make-up tank and slurry recirculation.
 Turbine pumps and eductors should be avoided in transferring milk of lime because of
 scaling  problems.

    5.4.1.6  Pacing and Control

 Lime  slaker water proportioning is  integrally-controlled or paced  from  the feeder.
 Therefore, the feeder-slaker system will follow pacing controls applied to the  feeder only.
 As  discussed previously, gravimetric  feeders  are  adaptable  to  receive  most  standard
 instrumentation pacing signals. Systems can be instrumented to allow remote pacing with
 telemetering of temperature and feed rate to a central panel for control purposes.

 The lime feeding system may be controlled  by an instrumentation system integrating both
 plant flow and pH  of the  wastewater after lime addition. However, it should be recognized
 that pH probes require daily maintenance in this application to  monitor the pH accurately.
 Deposits tend to build up on the probe and  necessitate  frequent maintenance.  The low pH
 lime treatment  systems (pH 9.5 to 10.0) can be more readily  adapted to this method of
 control than high-lime treatment systems (pH 11.0 or greater) because less maintenance of
 the  pH equipment is required.  In  a closed-loop pH-flow control system,  milk of lime is
 prepared  on a batch basis and  transferred  to a holding tank with variable output feeders
 set by the flow and pH meters to proportion  the feed  rate. Figure 5-8  illustrates such a
 control system.

    5.4.2  Hydrated Lime

    5.4.2.1 Properties and Availability

 Hydrated lime, Ca(OH)2, is usually a white powder (200 to 400 mesh); has a bulk density of
 20  to 50 Ib/cu ft; contains 82 to 98 percent Ca(OH)2; is slightly  hydroscopic; tends to flood
 the feeder, and will arch in  storage bins if packed. The molecular weight is 74.08.  The dust
.and slurry of hydrated lime are caustic in  nature. The cost of bulk hydrated lime varies
 from $18 to $22/ton. Bagged lime is available but increases the cost from $4  to  $167ton.
 The availability of hydrated lime may be determined by contacting manufacturers listed in

                                        5-32

-------
Table 5-5. The pH of a saturated, hydrated lime solution is the same as that given for quick-
lime.

   5.4.2.2   General Design Considerations

Hydrated lime  is slaked lime  and needs only enough water added to form milk of lime.
Wetting or dissolving chambers are usually designed to provide 5 minutes detention with a
ratio of 0.5  Ib/gal of water or 6 percent slurry at the maximum feed rate. Hydrated lime
is usually used  where maximum feed rates do not exceed 250 lb/hr., i.e., smaller plants.
Hydrated lime and milk of lime will irritate the eyes, nose, and respiratory system and will
dry the skin. Affected areas should be washed with water.

   5.4.2.3   Storage

Information given for quicklime  also applies to hydrated  lime except  that bin agitation
must be provided. Bulk bin outlets should be provided with non-flooding rotary feeders.
Hopper slopes vary from 60 to 66 deg.

   5.4.2.4   Feed Equipment

Volumetric or gravimetric feeders may be used, but volumetric feeders are usually selected
only for installations where comparatively  low feed rates are required.  Dilution does not
appear to be important, therefore, control of the  amount of water used in  the  feeding
operation is not considered necessary. Inexpensive hydraulic jet agitation may be furnished
in  the wetting chamber of the feeder as an alternative to  mechanical agitation. The jets
should be sized for the available water supply pressure to obtain proper mixing.

   5.4.2.5   Piping and Accessories

Piping and accessories as described for quicklime are also appropriate for  hydrated lime.

   5.4.2.6   Pacing and Controls

Controls as listed for dry alum  apply  to hydrated lime.  Hydraulic jets  should  operate
continuously and only shut off when the feeder is taken out of service. Control of the  feed
rate with pH as well as pacing with the plant flow may be  used with hydrated lime as well as
quicklime.

   5.4.3   Reactions of  Lime

Lime is somewhat different from the hydrolyzing coagulants. When added to wastewater it
increases pH and reacts with the  carbonate alkalinity to precipitate calcium carbonate.. If
sufficient lime is added to reach a high pH, approximately 10.5, magnesium hydroxide is also
precipitated. This latter precipitation enhances clarification due to the flocculant nature of
the Mg(OH)2. Excess calcium ions at high pH levels may be precipitated by the addition of
soda ash. The above reactions are shown in Table 5-6.

                                       5-33

-------
                                   TABLE 5-6

                              REACTIONS OF LIME

          Ca(OH)2 + Ca(HCO3)2— ^2 CaCOaf + 2H2O

          2 Ca(OH)2 + Mg(HCO3)2— +- 2 CaCOaj + Mg(OH)2) + 2H2O

          Ca(OH)2 + Na2CO3— *• CaCOa { + 2 NaOH

Reduction of the resulting high pH levels may be accomplished in one or two stages. The
first stage of the two-stage method results in the precipitation of calcium carbonate through
the addition of carbon dioxide according to the following reaction:
                         Ca(OH)2 + COa— »-CaCO3  + H2O
Single-stage pH reduction is generally accomplished by the addition of carbon  dioxide,
although acids have been employed. This reaction, which also represents the second stage of
the two-stage method, is as follows:

                          Ca(OH)2 + 2 CO2 — ^Ca(HCO3)2

As noted for the other chemicals, the above reactions are merely approximations to the
more complex interactions which actually occur in wastewaters.

The lime demand of a given wastewater is a function of the buffer capacity or alkalinity of
the wastewater. Figure 5-9 shows this relationship for a number of different wastewaters
(6).

5.5  Other Inorganic Chemicals

In addition to aluminum and iron salts and lime a number of other inorganic chemicals
have been used in wastewater treatment. Only three are discussed in this section, i.e.,  soda
ash, caustic soda, and carbon dioxide, but others have been and will be employed.  Mineral
and other acids are prime examples. For information on any of these chemicals, the  local
supplier or manufacturer should be contacted.

   5.5.1   Soda Ash
    5.5.1.1   Properties and Availability.

 Soda ash, Na2COa, is available in two forms. Light soda ash has a bulk density range of 35
 to 50 Ib/cu ft and a working density of 41 Ib/cu ft. Dense soda ash has.a density range of
 60  to 76 Ib/cu ft and a working density of 63 Ib/cu ft. The pH of a 1 percent solution of
 soda ash is 11.2. It is used for pH control and in lime treatment.
                                       5-34

-------
   500
5


o
§
tu,

Q
W
«


I
w
a:

w
   400
   300
   200
w
   100
       0
                 100         200         300         400


                  WASTEWATER ALKALINITY mg/1 - CaCO3
500
                               FIGURE 5-9


          LIME REQUIREMENT FOR pH£ 11.0 AS A FUNCTION OF THE

                           WASTEWATER ALKALINITY
    U S EPA Headquarters Library

          Mail code 3404T

    1200 Pennsylvania Avenue NW

       Washinaton, DC 20460

           202-566-0556
                                 5-35

-------
The molecular weight of soda ash is 106. Commercial purity ranges from 98 to greater than
99 percent NaaCOs. The viscosities of sodium carbonate solutions are given in Figure 5-10.
Soda ash by itself is not particularly corrosive, but in the presence of lime and water, caus-
tic soda is formed which is quite corrosive.

Soda ash is available in bulk by truck, box car and hopper car, and in 100 Ib bags from  the
following partial list of manufacturers.
          Location
Manufacturer
         CALIFORNIA
              Bartlett
              Trona

              Westend
PPG Industries, Inc.
American Potash and Chemical
    Corp.
Stauffer Chemical Co.
          GEORGIA
              Brunswick
Allied Chemical Co.
          LOUISIANA
              Baton Rouge
              Lake Charles
Allied Chemical Co.
Olin Chemicals
          MICHIGAN
              Wyandotte

          NEW YORK
              Solvay

          OHIO
              Barberton
              Painesville
Wyandotte Chemicals Corp.
Allied Chemical Co.
PPG Industries, Inc.
Diamond Shamrock Chemical Co.
         TEXAS
              Corpus Christi

         WEST VIRGINIA
              Moundsville
PPG Industries, Inc.
Allied Chemical Co.
          WYOMING
              Green River (3 plants)
Allied Chemical Co, FMC Corp., and
Stauffer Chemical Corp.
                                      5-36

-------
             (COURTESY   PPG INDUSTRIES  INC-, CHEMICAL DIV.)
CO
LU
co
p

>-
    6.0
    5.0
    4.0
   3-0
    2.0
    1.0
10
15
20
                                                      25
                                       30
                           FIGURE 5-10

              VISCOSITY OF SODA ASH SOLUTIONS
                                5-37

-------
The current price for soda  ash  ranges from  $40  to  $50/ ton,  F.O.B. the  point  of
manufacture,  however, prices  vary substantially between manufacturers and should  be
obtained  from the  closest manufacturers or  local distributors.  Bagging  may add
substantially to the cost of the chemical.

   5.5.1.2  General Design Considerations

Dense soda ash is  generally  used in  municipal applications  because of superior handling
characteristics. It has little dust, good flow characteristics, and will  not arch in the bin  or
flood the feeder. It is relatively hard to dissolve and ample  dissolver capacity must  be
provided.  Normal  practice calls for 0.5 Ib of dense soda ash per gal. of water or a 6 percent
solution retained for 20 min in  the dissolver.

The dust and solution are irritating to the eyes, nose, lungs and skin and therefore general
precautions should be observed and  the affected areas should be washed promptly with
water.

   5.5.1.3  Storage

Soda ash  is usually stored in steel bins and where pneumatic  filling equipment is used, bins
should be provided  with  dust collectors.  Bulk  and bagged soda ash  tend to absorb
atmospheric  CO2  and water,  forming  the  less active sodium  bicarbonate  (NaHCOa).
Material recommended for unloading facilities is steel.

   5.5.1.4  Feeding Equipment

Feed equipment as described for dry  alum is suitable for  soda  ash. Dissolving of soda ash
may be hastened by the use of warm  dissolving water. Mechanical or hydraulic jet mixing
should be provided in the dissolver.

   5.5.1.5  Piping and Accessories

Materials of construction for  piping and accessories should  be iron, steel,  rubber, and
plastics.

   5.5.1.6  Pacing and Control

Controls as discussed for dry alum  apply also to soda ash equipment.

5.5.2  Liquid Caustic Soda

Anhydrous  caustic soda (NaOH) is available  but  its  use is generally not considered
practical in water and wastewater treatment applications.  Consequently, only liquid caustic
soda is discussed below.
                                        5-38

-------
   5.5.2.1   Properties and Availability.

Liquid caustic soda is shipped at two concentrations, 50 percent and 73 percent NaOH. The
densities of the solutions as shipped are 12.76 Ib/gal for the 50 percent solution and 14.18
Ib/gal for the 73 percent solution. These solutions contain 6.38 Ib/gal NaOH and 10.34 lb/
gal. NaOH, respectively. The crystallization temperature is 53°F for the 50 percent solution
and 165°F for the 73 percent solution. The molecular weight of NaOH is 40. Viscosities of
various caustic soda solutions are presented in Figure 5-11. The pH of  a 1 percent solution
of caustic soda is  12.9.

Truck load lots of 1,000 to  4,000 gal  are available in the 50 percent  concentration only.
Both shipping concentrations can be obtained in 8,000, 10,000 and 16,000 gal car load lots.
Tank cars can be unloaded through the dome eduction pipe using air  pressure  or through
the bottom valve by gravity or.by using air pressure or a pump. Trucks are usually unloaded
by gravity or  with air pressure or a truck mounted pump.

Major producers of caustic soda and their respective plantlocations are listed in Table 5-7.
The current price  for liquid caustic soda ranges from $76/ton @ 50 percent and  $81/ton @
73 percent, (NaOH), F.O.B. the point of manufacture.

                                   TABLE 5-7

      PARTIAL LIST OF CAUSTIC  SODA MANUFACTURING PLANTS

         Location                                       Manufacturer

         ALABAMA
              Lemoyne (Mobile)                          Stauffer
              Mclntosh                                  Olin
              Muscle Shoals                             Diamond Shamrock

         CALIFORNIA
              Pittsburg                                  Dow

          DELAWARE
              Delaware City                             Diamond Shamrock

         GEORGIA
              Augusta                                   Olin
              Brunswick   ,                             Allied

         KANSAS
              Wichita                                   Vulcan

         KENTUCKY
              Calvert City (2 plants)                       Pennwalt, Goodrich

                                      5-39

-------
CO
LU

CO


o
   200
   iOO

    80


    60



    40
20
                 (COURTESY  OF  HOOKER  CHEMICAL  Co-)
;=   10
Z    8
o    ,
!2    2
     i

   0.8

   0.6


   0-4
   0.2
                       I    I    I
                                  T   I    I    I    I    I
          I	I
              I
J	I
I    I    I    I
I
     60  70   80   90   100  110  120  130  140  150  160 170  180  190  200



                          TEMPERATURE, °F
                            FIGURE 5-11


             VISCOSITY OF CAUSTIC SODA SOLUTIONS
                               5-40

-------
Location
                                             Manufacturer
LOUISIANA
    Baton Rouge
    Geismar
    Lake Charles (2 plants)
    Plaquemine
    St. Gabriel
    Taft
Allied
Wyandotte
PPG, Olin
Dow
Stauffer
Hooker
MICHIGAN
    Midland
    Montague
    Wyandotte (2 plants)

NEVADA
    Henderson
Dow
Hooker
Pennwalt, Wyandotte
Stauffer
NEW JERSEY
     Linden
 GAP
 NEW YORK
     Niagara Falls (3 plants)
     Solvay

 NORTH CAROLINA
     Acme
 Hooker, Olin, Stauffer
 Allied
 Allied
 OHIO
     Barberton
     Cleveland
     Painesville
 PPG
 Harshaw
 Diamond Shamrock
 OREGON
     Portland
 Pennwalt
 TENNESSEE
     Charleston
 Olin
 TEXAS
     Corpus Christi
     Deer Park (Houston)
     Freeport
     Port Neches
 PPG
 Diamond Shamrock
 Dow
 Jefferson
                             5-41

-------
        Location
               Manufacturer
        VIRGINIA
            Saltville

        WASHINGTON
            Tacoma (2 plants)
            Olin
            Hooker, Pennwalt
        WEST VIRGINIA
            Moundsville
            Natrium
            South Charleston
            Allied
            PPG
            FMC
Manufacturers and addresses

Allied Chemical
    Solvay Process Division
    40 Rector Street
    New York, New York  10006

Diamond Shamrock Chemical Co.
    300 Union Commerce Building
    Cleveland, Ohio 44115

Dow Chemical Co.
    Abbott Road
    Midland, Michigan 48640

GAF Corp. Chemical  Division
     140 West 51st Street
    New York, New York  10019

FMC Corporation
    Inorganic Chemicals Div.
    633 Third Avenue
    New York, New York  10017
B.F. Goodrich Chemical Co.
    3135 Euclid Avenue
    Cleveland, Ohio 44115

 Harshaw Chemical Co.
     1945 East 97th Street
    Cleveland, Ohio 444106

 Hooker Chemical Corp.
    P.O. Box 344
    Niagara Falls, New York 14302

 Jefferson Chemical Co.,  Inc.
    3336 Richmond Avenue
    Houston, Texas 77006

 Olin Corporation
    Chemicals Division
    745 Fifth Avenue
    New York, New York 10022
                                    5-42

-------
Pennwalt Corporation
    Pennwalt Building
    Three Penn Center
    Philadelphia, Pa. 19102

PPG Industries, Inc.
    Chemical Division
     1 Gateway Center
     Pittsburgh, Pa. 15222

Stauffer Chemical Co.
     Industrial Chemical Div.
     299 Park Avenue
     New York, New York 10017
Vulcan Materials Co.
    Chemicals Division
    P.O. Box 545-T
    Wichita, Kansas 67201

Wyandotte Chemicals Corp.
    Michigan Alkali Division
    Wyandotte, Michigan 48192
   5.5.2.2  General Design Considerations

Liquid caustic soda  is received in bulk shipments, transferred to storage and diluted as
necessary for feeding to the points of application. Caustic soda  is  poisonous and is
dangerous to handle.  U.S. Department of Transportation Regulations for "White Label"
materials must be observed.  However, if handled properly caustic soda poses no particular
industrial hazard. To  avoid accidental spills, all pumps, valves, and lines should be checked
regularly for leaks. Workmen should be thoroughly instructed in the precautions related to
the handling of caustic soda. The eyes  should be  protected by goggles  at all times when
exposure to mist  or splashing is possible. Other parts of the body should be protected as
necessary to prevent  alkali burns. Areas exposed  to caustic soda should be washed with
copious  amounts  of  water  for  15  min  to 2 hr. A physician should be called when
exposure is severe. Caustic soda taken internally should be diluted with water or milk and
then neutralized with dilute  vinegar or fruit juice.  Vomiting may occur spontaneously but
should not be induced except on the advice of a physician.

   5.5.2.3  Storage

Liquid caustic soda may be stored at the 50 percent concentration. However, at this solu-
tion strength, it crystallizes at 53°F. Therefore, storage tanks must be located indoors or
provided with heating and suitable insulation if outdoors. Because of its relatively high crys-
tallization temperature, liquid caustic soda is often dilluted to a concentration of about 20
percent NaOH for storage.  A 20 percent solution  of NaOH has a crystallization tempera-
ture of about -20°F. Recommendations for dilution of both 73 percent and 50 percent solu-
tions should be obtained from the manufacturer, because  special considerations are neces-
sary.

Storage  tanks for liquid caustic soda should be provided with an air vent for gravity flow.
The storage  capacity should be equal to 1 l/2 times  the largest expected delivery,  with an
allowance for dilution water,  if used,  or  2-weeks  supply  at  the anticipated feed rate,
whichever is greater. Tanks for storing 50 % solution at a temperature between 75°F and

                                       5-43

-------
 140°F may be constructed of mild  steel. Storage temperatures above 140°F require more
 elaborate materials selection and are not recommended. Caustic soda will tend to pick up
 iron when stored in steel vessels for extended periods. Subject to temperature and solution
 strength limitations, rubber, 316 stainless steel, nickel, nickel alloys, or plastics may be used
 when iron contamination must be avoided.

   5.5.2.4   Feeding Equipment

 Further dilution of liquid  caustic soda below the storage strength may be desirable  for
 feeding by volumetric feeders. Feeding systems as described for liquid alum generally apply
 to caustic soda with appropriate selection of materials of construction. A typical system
•schematic is shown in  Figure 5-12. Feeders will usually include materials such as ductile
 iron, stainless steels, rubber, and plastics.

   5.5.2.5  Piping and Accessories

 Transfer lines from the shipping unit  to  the storage tank should  be spiral-wire-bound
 neoprene or rubber hose, solid  steel pipe with swivel joints, or steel hose. Because caustic
 soda attacks glass, use  of glass materials should be avoided. Other miscellaneous materials
 for use with liquid caustic soda feeding and handling equipment are listed below (7):
 Components

 Rigid Pipe
 Flexible Connections

 Diluting Tees
 Fittings
 Permanent Joints
 Unions
 Valves—Non-leaking (Plug)
      Body
      Plug
 Pumps (Centrifugal)
      Body
      Impeller
      Packing
 Storage Tanks
 Recommended Materials
 for Use With 50 % NaOH
	Up to  14QQF	

 Standard Weight Black Iron
 Rigid Pipe with Ells or Swing Joints,
 Stainless Steel or Rubber Hose
 Type 304 Stainless Steel
 Steel
 Welded or Screwed Fittings
 Screwed Steel

 Steel
 Type 304 Stainless Steel

 Steel
 Ni-Resist
 Blue Asbestos
 Steel
                                         5-44

-------
                                -TRUCK FILL LINE
DILUTION
 WATER
                      SODIUM HYDROXIDE
                        STORAGE TANK
                                               VENT,  OVERFLOW
                                               AND  DRAIN
VENT,  OVERFLOW
AND DRAIN

MIXER
                                         SAMPLE TAP
                                     SODIUM HYDROXIDE
                                         FEEDER
                         POINT OF
                        APPLICATION
                         FIGURE 5-12
          TYPICAL CAUSTIC SODA FEED SYSTEM
                               5-45

-------
   5.5.2.6   Pacing and Control

 Controls as listed for liquid alum also apply to liquid caustic soda equipment.

   5.5.3  Carbon Dioxide

   5.5.3.1  -Properties and Availability

 Carbon dioxide, ,CO2, is available for use in wastewater treatment plants in gas and liquid
 form. The molecular weight of CO2 is 44.  Dry COa is not chemically active at normal
 temperatures and is a  non-toxic safe  chemical; however, the gas displaces oxygen and
 adequate ventilation of closed areas should be provided. Solutions of CO? in water are very
. reactive chemically and  form carbonic acid. Saturated solutions of CO2 have a pH of 4.0 at
 68°F.

 The gas form may be produced on the treatment plant site by scrubbing and compressing
 the combustion  product of lime  recalcining  furnaces, sludge furnaces, or generators used
 principally for the production of COa gas only. These generators are usually fired with
 combustible'gases, fuel oil, or coke and have COa yields as shown in Table 5-8 (8).
                                  TABLE 5-8

                      COa YIELDS  OF COMMON FUELS
    Fuel                             Quantity                       . COa Yield
                                         I                          !     Ib
    Natural Gas                     1,000 cu ft                         115
    Coke                            1 Ib                                  3
    Kerosene                        1 gal.                               20
    Fuel Oil (No. 2)                  1 gal.                            '   23
    Propane                         l.,000 cu ft                         141
    Butane                          l.OOOcuft                         142
 The gas forms, as generated at the plant site, usually have a COa content of between 6 per-
 cent and 18 percent depending on the source and efficiency of the producing system.

 The liquid form is available from commercial suppliers in 20 to 50 Ib cylinders, 10 to 20 ton
 trucks and 30 to 50 ton rail cars. The commercial liquid form has a minimum COa content
 of 99.5 percent.

 Current prices range from $307 ton for 3,000 tons per year and  over, to $687 ton for a
 quantity of 150 tons/year. These prices include an allowance for freight within  a  100 mile
 radius of the point of manufacture. Another $6/ton may be added for each additional 100
 miles to the point of destination. Major producers of commercial  COa are listed  in Table
 5-9.
                                        5-46

-------
                            TABLE 5-9
PARTIAL LIST OF CARBON DIOXIDE MANUFACTURING PLANTS
  Location
  CALIFORNIA
      Watson (Los Angeles)
      Oakland
      Brea
      Lathrop
      Ventura ,
      Taft
  GEORGIA
      Augusta

  ILLINOIS
      Morris
      Chicago
  INDIANA
      Jeffersbnville

  IOWA
      Clinton
      Ft. Madison
      Ft. Dodge
      Muscatine
  KANSAS
      Dodge City
      Lawrence
      Lawrence
  KENTUCKY
      Doerun (Brandenburg)
  LOUISIANA
      New Orleans
      Luling
  MASSACTUSETTS
      Tewksbury

  MISSISSIPPI
      Yazoo City

  MISSOURI
      Kansas City
      Le May (St. Louis)
Manufacturer

Liquid Carbonic
Liquid Carbonic
Airco
Airco
Cardox
Standard Oil


Liquid Carbonic


Cardox
Airco

Cardox


Airco
Liquid Carbonic
Liquic Carbonic
Publicker

Liquid Carbonic
Airco (1972)
Cardox

Olin

Liquid Carbonic
Airco

Liquid Carbonic


Airco
Airco
Cardox
                                 5-47

-------
 Location
         Manufacturer
NEW JERSEY
     Paulsboro
     Belleville
     Deepwater

NEW MEXICO
     Bueyeros
     Solana
     Mosquero
NEW YORK
     Olean
OHIO
     Toledo
     Oregon (Toledo)
     Lima
     Huron
PENNSYLVANIA
     Philadelphia
     Thermice (Philadelphia)
TENNESSEE
     Woodstock (Memphis)
TEXAS
     Texas City
     Dallas
     Dumas
VIRGINIA
     Hopewell
     Saltville
WASHINGTON
     Finley
         Olin
         Liquid Carbonic
         Airco


         SEC
         SEC
         SEC

         Airco

         Cardox
         Liquid Carbonic
         Airco
         Cardox

         Liquid Carbonic
         Publicker

         Cardox

         Liquid Carbonic
         Cardox
         Diamond Shamrock

         Airco
         Olin

         Airco
Manufacturers and Addresses

Cardox Div. of Chemetron Corp.
    Dept. TR
    840 N. Michigan Avenue
    Chicago, Illinois 60611

Airco Industrial Gases Div.of
    Air Reduction Co.
    575 Mountain Avenue
    Murray Hill, N.J. 07974
Olin Corporation
    Chemicals Division
    745 Fifth Avenue
    New York, New York 10022

Publicker Ind., Inc.
    Walnut & Thomas
    Philadelphia, Pa.
                                 5-48

-------
 Liquid Carbonic Corporation                   Diamond Shamrock Chemical Co.
     Dept. TR                                      300 Union Commerce Building
     135 S. LaSalle                                  Cleveland, Ohio 44115
     Chicago, Illinois 60603
                                               Standard Oil Company of California

     1033 Humble Place                             or    •     /^  IT     n*,nA
     „. „     ~     —oc-,                           San Francisco, California 94104
     El Paso, Texas 79987

   5.5.3.2   General Design Considerations

Recovery of CO2 from recalcining furnaces or incinerators is the least expensive source, but
maintenance of stack gas systems is likely to be extensive because of the corrosive nature of
the wet gas  and the presence of particulate matter. Scrubber systems are required to clean
the stack gas and specially designed gas compressors are necessary to provide the process
injection pressure.

Pressure generators and submerged burners require less  maintenance because the  system
pressure  is  established by  compressors  or  blowers handling  dry  air  or gas.  On-site
generating  units have  a limited range of CO2 production  as  compared  with the liquid
storage and feed system, and therefore may require multiple  units.

The liquid  COa storage  and  feed system  generally  includes a temperature-pressure
controlled,  bulk storage tank,  an  evaporation  unit, and a  gas  feeder to meter the gas.
Solution feeders, similar in construction to chlorinators, may also be used to feed COa.

    5.5.3.3   Storage

This section applies only to use  of  commercial  liquid COa.  Liquid system capacities
encountered in wastewater treatment usually  require on-site bulk storage  units. Standard
pre-packaged  units are available,  ranging in size  from 3/s  to 50  tons capacity, and are
furnished with temperature-pressure controls to maintain  approximately 300 psi  at 0°F
conditions.  The typical package unit contains refrigeration, vaporization, safety and control
equipment. The units are well insulated and protected for outdoor  location. The gas from
the evaporation unit usually passes  through two stages of pressure reduction before entering
the gas feeder to prevent the formation of dry ice.

    5.5.3.4   Feeding Equipment

Feeding systems for the stack gas source of COa consist of simple valving arrangements, for
admitting  varying  quantities  of make-up  air to the suction side of the constant volume
compressors, or for venting  excess gas on the compressor discharge.  A typical system is
described elsewhere (9).

Pressure generators and submerged burners are regulated by valving arrangements on the
fuel  and  air supply. Generation of COz  by combustion is usually difficult to control,
requires frequent operator  attention and demands considerable maintenance over the life of


                                        5-49

-------
the equipment, when compared with liquid CO2 systems.

Commercial  liquid carbon dioxide is becoming more generally used because of its high
purity, the simplicity and range  of feeding  equipment, ease of control, and smaller, less
expensive piping systems. After vaporization, the COa with suitable metering and pressure
reduction may be fed directly to the point of application as a gas. However, vacuum
operated,-solution type gas feeders are often preferred. Such feeders generally include safety
devices and operating controls in a compact panel housing, with materials of construction
suitable for CO2 service. Absorption of CO2 in the injector water supply approaches 100%
when a ratio  of 1.0 Ib of gas to 60 gal of water is maintained.

    5.5.3.5  Piping and Accessories

Mild steel piping and accessories are suitable for use with cool, dry, carbon dioxide. Hot,
moist gases, however, require the  use of type 316 stainless steel or plastic materials. Plastics
or FRP  pipe are generally used for solution  piping  and diffusers.  Diffusers  should be
submerged at least 8 ft, and preferably deeper, to assure complete absorption of the gas.

   . 5.5.3.6   Pacing and Control

Standard instrument signals and control components can be used to pace or control carbon
dioxide feed systems.

Using stack gas as the source of COa, thejfeed rate can be controlled by proper selection^and
operation of compressors, by manual control of vent or bleed valves, or  by  automatic
control of such valves by a pH meter-controller system.

In commercial CO2 feed systems, solution feeders may function as controllers and can be
paced by instrument signals from pH monitors and plant flow meters.

In feeding commercial CO2 directly to  the point of  application as  a gas, a differential
pressure  transmitter and a control valve may function as the primary elements of a control
system. Standard instrument signals may be used to pace or control the rate of CO2 feed.

CO2  generators  are  difficult to pace  or  control  other than by  manual  or  automatic
operation of vent or bleed valves  that waste a portion of the produced gas according to the
plant requirements.

    5.6.   Polymers

Polymeric flocculants are  high  molecular weight  organic  chains with ionic or other
functional groups incorporated at intervals along the chains. Because these compounds have
characteristics of  both  polymers and electrolytes, they  are frequently called
polyelectrolytes. They  may be of  natural or synthetic origin.

All synthetic polyelectrolytes can be classified "on the basis of the type of  charge on the
                                       5-50

-------
polymer chain. Thus polymers possessing negative charges are called  anionic while those
carrying positive charges are cationic. Certain compounds carry no electrical charge and
are called  nonionic polyelectrolytes.

Because of the great variety of monomers available as starting material and the additional
variety that can be obtained by varying the molecular weight, charge density and ionizable
groups, it is not surprising that a great assortment of polyelectrolytes  are available to the
wastewater plant operator. A partial listing of manufacturers is shown in Table 5-10. This
list is based mainly  on three  major sources  (10) (11) (12) and  does not purport  to be a
complete list.

Extensive  use of any specific polymer as a flocculant is of necessity determined by the size,
density  and ionic charge  of the colloids  to  be coagulated. As other  factors need  to  be
considered, i.e. coagulants used, pH  of the system,  techniques and  equipment  for
dissolution  of  the  polyelectroyte, etc.; it is  mandatory that extensive jar  testing  be
performed to determine the specific polymer that will perform its function most efficiently.
These results should  be verified by plant-scale testing.

   5.6.1   Dry Polymers

   5.6.1.1   Properties  and  Availability

Types of  polymers vary widely in characteristics.  Manufacturers  should be consulted for
properties, availability, and cost of the polymer being considered. References are available
that indicate  the types and characteristics  of polymers available  (10) (11) (12). Bulk
shipments are generally not desirable. Polymers are available in a variety of container or
package sizes.
                                TABLE 5-10

                             PARTIAL LIST OF
               POLYMER SOURCES AND TRADE NAMES
       Source                                                Trade Name (s)

       Allied Colloids, Inc.                                    Percol
       One Robinson Lane
       Ridgewood, N.J. 07450

       Allstate Chemical Co.                                  Allstate
       Box 3040
       Euclid, Ohio 44117
                                       5-51

-------
                            TABLE 5-10 (continued)
Source

Allyn Chemical Co.
2224 Fairhill Rd.
Cleveland, Ohio 44106

American Cyanamid Co.
Berdan Ave.
Wayne, N.J. 07470

Atlas Chemical Ind., Inc.
Wilmington, Dela.  19899

Berdell Industries
28-01 Thomson Ave.
Long Island City, NY  11101

Betz Laboratories, Inc.
Somerton Red.
Trevose, Pa. 19047

Bond Chemicals, Inc.
1500 Brookpark Rd.
Cleveland, Ohio 44109

Brennan Chemical Co.
704 N. First St.
St. Louis, Mo. 63102

The Burtonite Company
Nutley, N.J. 07110
 Trade Name (s)

 Claron
Superfloc
Magnifloc
Sorbo
Atlasep

Berdell
Betz
Polyfloc
Bondfloc
Brenco
Burtonite
Calgon Corporation
P.O. Box  1346
Pittsburgh, Pa.  15222

Commercial Chemical
11 Paterson Ave.
Midland Park, N.J. 07432
Cat-Floe
Speedifloc
                                     5-52

-------
                    TABLE 5-10 (continued)
Source

Dearborn Chemical Div.
W.R. Grace & Co.
Merchandise Mart Plaza
Chicago, 111.  60654

Dow Chemical USA
Barstow Building
2020  Dow Center
Midland, MI. 48640
Drew Chemical Corp.
701 Jefferson Rd.
Parisippany, N.J. 07054

Du Bois Chemicals Div.
W.R. Grace & Co.
3630 E. Kemper Rd.
Sharonville, Ohio 45241

E.I. DuPont de Nemours & Co.
Eastern Laboratory
Gibbstown, N.J. 08027

Environmental Pollution Investigation
    &  Control, Inc.
9221 Bond St.
Overland Park, KS. 66214

Fabcon International
1275 Columbus Ave.
San Francisco, Calif. 94133
j'rade Name (s)

 Aquafloc
 Dowell
 PEI
 Purifloc
 Separan
 XD

 Drewfloc
 Amerfloc
 Flocculite
 Du Pont
 Dynafloc
 Zuclar
 Fabcon
 Henry W. Fink & Co.
 6900 Silverton Avenue
 Cincinnati, Ohio 45236

 Gamlen Sybron
 321 Victory Avenue
 S. San Francisco, Calif. 94080
 Kleer-Floc
 Gamafloc
 Gamlose
 Gamlen
                            5-53

-------
                    TABLE 5-10 (continued)
Source^

Garrett-Callahan
111 Rollins Rd.
Millbrae, Calif. 94030

General Mills Chemicals
4620 N. 77th Street
Minneapolis, Min.  55435

Hercules, Inc.
910 Market St.
Wilmington, Dela.  19899

Frank Herzl Corp.
299 Madison Avenue
New York, N.Y. 10017

ICI America, Inc.
Wilmington, Dela.  19899

Illinois Water Treatment Co.
840 Cedar St.
Rock ford, 111. 61102

Kelco Company
8225 Aero Dr.
San Diego, Calif. 92123

Key Chemicals
4346 Tacony
Philadelphia, Pa. 19124

Metalene Chemical Co.
Bedford, Ohio 44014
Trade Name(s)

Garrett-Callahan
Supercol
Guartec
Hercofloc
Perfectamyl
Atlasep
Illco
Kelgin
Kelcosol
Key-Floe
 Metalene
The Mogul Corporation
20600 Chagrin Blvd.
Cleveland, Ohio 44122

Nalco Chemical Co.
6216 W. 66th Street
Chicago, 111. 60638
 Mogul
 Nalcolyte
                            5-54

-------
                    TABLE 5.-10 (continued)
Source
                                           Trade Name (s)
Narvon Mining & Chemical Co.
Keller Ave. & Fruitville Pike
Lancaster, Pa. 17604

National Starch & Chemical Corp.
1700 W. Frront St.
Plainfield, N.J. 07063

O'Brien Industries, Inc.
95 Dorsa Avenue
Livingston, N.J. 07039

Oxford Chemical Div.
Consolidated Foods Corp.
P.O. Box 80202
Atlanta, Ga. 30341

Reichhold Chemicals, Inc.
RCI Building
White Plains, N.Y.  10602

Standard Brands Chem. Ind., Inc.
P.O. Drawer K
Dover, Dela. 19901

A.E. Staley Mfg. Co.
P.O. Box 151
Decatur, 111. 62525

Stein, Hall & Co., Inc.
605 Third Avenue
New York, N.Y. 10016

Swift & Company
Oakbrook, 111.60521

James Varley & Sons, Inc.
1200 Switzer Ave.
St. Louis, Mo. 63147

W.E. Zimmie, Inc.
810 Sharon  Dr.
Westlake, Ohio 44145
Sink-Floe
Zeta-Floc
Floe-Aid
Natron
O'B Floe
Oxford-Hydro-Floc
Aquarid
Tychen
 Hamaco
 Hallmark
 Jaguar
 Polyhall

 Swift
 Varco-Floc
 Zimmite
                                5-55

-------
    5.6.1.2  General Design Considerations

Dry Polymer and water must be blended and mixed to obtain a recommended solution for
efficient action. Solution concentrations vary from fractions of a percent up.  Preparation
of the stock solution involves wetting of the dry material and usually an aging period prior
to application. Solutions can be very viscous, and close attention should be paid to piping
size and length  and  pump selections. Metered solution is usually diluted just prior to
injection to the process to obtain better dispersion at the point of application.

    5.6.1.3   Storage

General practice  for storage of bagged dry chemicals should be observed. The bags should
be stored in a dry,  cool, low humidity area and used in proper rotation,  i.e.,  first in, first
out.

Solutions are generally stored in type 316 stainless steel, FRP, or plastic lined tanks.

    5.6.1.4  Feed Equipment

Two types of systems are frequently combined to feed polymers. The solution preparation
system  includes a manual or automatic blending system with the polymer dispensed  by
hand or by a  dry feeder to a wetting jet and then to a mixing-aging tank at a controlled
ratio. The aged polymer is transported to a holding tank where metering pumps or rotodip
feeders dispense  the polymer to the process. A schematic  of such a system  is shown  by
Figure  5-13. It is  generally advisable to  keep the holding or storage time of polymer
solutions to a minimum, 1 to 3 days or less, to prevent deterioration of the product.

     5.6.1.5  Piping and Accessories

Selection must be made after determination of the polymer,  however, type 316 stainless
steel or plastics are generally used.

     5.6.1.6  Pacing and Controls

Controls as listed for liquid alum apply to the control of liquid dispersing feeders.

The solution preparation system may be an automatic batching system,  as shown by the
schematic on Figure 5-14, that fills the holding tank  with aged polymer as required by level
probes. Such  a  system is usually provided only at large plants.  Prepackaged solution
preparation units are available, but have a  limited capacity.
                                        5-56

-------
WATER SUPPLY-

                                           — DRY
                                            FEEDER
                                           DISPERSER
                                           MIXER
                                           DISSOLVING-AGING
                                                 TANK
                                           -HOLDING TANK
                                           -SOLUTION FEEDER
                               POINT OF
                              APPLICATION
                      FIGURE 5-13
     TYPICAL SCHEMATIC OF A DRY POLYMER
                    FEED SYSTEM
                            5-57

-------
c/1
I
oo
        HOT
       WATER
              SOLENOID
             / VALVE
                                                         .SCALE
   COLD,
   WATER

 BLENDER
NOTE:   CONTROL & INSTRUMENTATION
       WIRING IS NOT SHOWN
                  SOLUTI ON
                  FEEDERS
  POINT OF APPLICATION
                                                 MIXER
                                                                                                              DISPERSER
                                                                                              LEVEL
                                                                                              PROBE
                                                                                     LEVEL PROBE
                                                                   MIXER
                                      MIXING-AGING
                                          TANK
                                                                                                       MIXING-AGING
                                                                                                           TANK
                                                                                       -oo-
                                                        TRANSFER  PUMP


                                                         O—LEVEL  PROBE
                                             HOLDING TANK
                                      FIGURE 5-14
                    TYPICAL AUTOMATIC DRY POLYMER FEED
                                        SYSTEM

-------
                                                          U.S  EPA Headquarters Librai.
                                                                Mail code 3404T
   5.6.2  Liquid Polymers                                1200 Pennsylvania Avenue NW
                                                            Washington, DC 20460
   5.6.2.1   Properties and Availability                            202-566-0556

As with dry polymers, there is a wide variety of products, and manufacturers should be
consulted for specific information.

   5.6.2.2   General Design Considerations

Liquid systems differ from the dry systems only in the equipment used to blend the polymer
with water to prepare  the solution. Liquid solution preparation is usually a ha.nd batching
operation with manual filling of a mixing-aging tank  with water and polymer.

    5.6.2.3.  Feed Equipment

Liquid Polymers need no aging and simple dilution is the only requirement for feeding. The
dosage  of liquid polymers may be accurately controlled  by metering pumps or rotodip
feeders.

The balance of the process is generally the same as described for dry polymers.

5.7  Chemical Feeders

Chemical feed systems must be flexibly designed to provide for a high degree of reliability
in light  of the  many  contingencies  which may affect their operation.  Thorough waste
characterization in  terms.of flow extremes and chemical requirements should precede the
design of the chemical  feed system. The design of the chemical feed system must take into
account the form of each chemical desired for feeding, the particular physical and chemical
characteristics  of the chemical,  maximum waste flows and the reliability of the feeding
devices.

In suspended and colloidal solids removal from wastewaters the chemicals employed are
generally in liquid or solid form. Those in solid form  are generally converted to solution or
slurry form  prior to introduction to the wastewater stream;  however, some chemicals are
fed in a dry form. In any  case, some type of solids feeder is usually required. This type of
feeder has numerous different forms due to wide ranges in chemical characteristics, feed
rates  and degree of accuracy required.  Liquid  feeding  is  somewhat  more  restrictive,
depending mainly on liquid volume and viscosity.

The capacity of a chemical feed system is an important consideration in both storage and
feeding. Storage capacity design must take into account the advantage of quantity purchase
versus the disadvantage of  construction cost  and chemical deterioration with  time (13).
Potential delivery delays and chemical use rates are  necessary factors in the total picture.
Storage tanks or bins for solid chemicals must  be designed  with proper consideration of the
angle of repose of the chemical and its  necessary  environmental  requirements,  such as
temperature and humidity. Size and slope of  feeding lines are important along with their
materials of construction with respect to the corrosiveness  of the chemicals.
                                       5-59

-------
 Chemical feeders must accommodate the minimum and maximum feeding rates required.
 Baker (13) indicates that manually controlled feeders have a common range of 20:1, but this
 range can be increased to about 100:1 with dual control systems. Chemical feeder control
 can be manual,  automatically proportioned to flow, dependent on some form of process
 feedback or a combination  of any  two  of these. More sophisticated control systems  are
 feasible if proper sensors  are available. If manual control systems are specified  with  the
 possibility of future-automation, the feeders selected should be amenable to this conversion'
 with a minimum of expense. An example would be a feeder with an external motor which
 could easily be replaced with a variable  speed motor or drive when automation is installed!
 (13). Standby or backup units should be included for each type of feeder used. Reliability
 calculations will be necessary  in larger plants  with a  greater multiplicity of these units.
 Points of chemical addition  and piping to them should be capable of handling all possible
.changes in  dosing patterns in order to have  proper flexibility  of  operation. Designed
 flexibility in hoppers, tanks, chemical feeders and solution lines is  the key  to maximum
 benefits at least cost (14).

 Liquid feeders are generally in the form of metering pumps  or orifices.  Usually these
 metering pumps are of the positive-displacement variety, plunger or diaphragm type. The
 choice of liquid feeder is highly dependent on the viscosity, corrosivity, solubility, suction
 and discharge heads, and internal  pressure-relief requirements (10). Some examples  are
 shown in Figure 5-15. In some cases control  valves and  rotameters may be all that is
 required. In other cases, such as lime slurry feeding, centrifugal pumps with open impellers
 are used with appropriate  controls  (9).  More  complete descriptions  of  liquid feeder
 requirements can be found in the literature and  elsewhere (14).

 Solids characteristics vary to a great degree and the choice of  feeder must be considered
 carefully, particularly in the smaller-sized facility where a single feeder  may be used for
 more than  one chemical. Generally, provisions should  be made to keep all chemicals cool
 and  dry. Dryness is  very important, as  hygroscopic (water absorbing) chemicals may
 become lumpy, viscous or even rock hard; other chemicals with  less affinity for water may
 become sticky from moisture on the particulate  surfaces, causing  increased arching in
 hoppers. In either case, moisture will affect the density of the chemical and  may result in
 under-feed.  Dust  removal  equipment  should  be  used at shoveling locations, bucket
 elevators, hoppers and feeders for neatness,  corrosion prevention and safety  reasons.
 Collected chemical dust may often be used.

 The simplest method for feeding solid chemicals is by hand. Chemicals may be preweighed
 or simply  shoveled or poured  by  the bagful  into  a  dissolving tank. This  method is of
 economic necessity limited  to very small operations,  or to chemicals  used  in very weak
 solutions.

 Because of the many factors, such as moisture content, different grades and compressibility,
 which can affect chemical density (weight to volume ratio), volumetric feeding of solids is
 normally restricted to smaller plants, specific types of chemicals  which are reliably constant
 in composition and low rates of feed. Within these restrictions several volumetric types are
                                        5-60

-------
                                          DISCHARGE VALVE
                       PLUNGER
     PLUNGER PUMP (Courtesy of Wallace & Tiernan)
.SUCTION VALVE
            DISCHARGE VALVE
           SUCTION VALVE
                                       -DIAPHRAGM
DIAPHRAGM PUMP  (Courtesy of Wallace & Tiernan)
                FIGURE 5-15
      POSITIVE DISPLACEMENT PUMPS
                     5-61

-------
available. Accuracy of feed is usually limited to ± 2 percent by weight but may be as high
as ±15 percent.

One type of volumetric dry  feeder uses a continuous belt of specific width moving from
under the hopper to the dissolving tank. A mechanical gate mechanism regulates the depth
of material on the belt, and the rate of feed is governed by the speed of the belt and /or the
height of the gate opening. The hopper normally is equipped with a vibratory mechanism to
reduce arching. This type of feeder is not suited for easily fluidized materials.

Another type employs a screw or helix  from the bottom of the hopper through  a  tube
opening slightly larger than the diameter of the screw or helix. Rate of feed is governed by
the speed of screw or helix rotation. Some screw-type designs are self-cleaning, while others
are subject to clogging. Figure 5-16 shows a typical screw-feeder.

Most remaining types of volumetric feeders generally fall  into the positive-displacement
category. All designs of this  type incorporate some form of moving cavity of a specific or
variable size. In operation, the chemical falls by gravity into the cavity and is more or less
fully enclosed and separated  from the hopper's feed. The size of the cavity, and the rate at
which the cavity moves and is discharged, governs the amount of material fed."The positive
control of the chemical may place a low limit on rates of feed. One unique design is the
progressive cavity metering pump, a non-reciprocating type.  Positive-displacement feeders
often  utilize air  injection to improve  the  flow  of  the  material.  Some examples of
positive-displacement units are illustrated in Figure 5-17.

The basic drawback of volumetric feeder design, i.e., its inability to compensate for changes
in the density of materials, is overcome by  modifying the volumetric design to include a
gravimetric or  loss-in-weight controller.  This  modification  allows for weighing  of the
material as it is fed. The beam balance type  measures the actual mass of material.  This is,
considerably more accurate,  particularly over a long period of time, than the less common
spring-loaded gravimetric designs. Gravimetric feeders are  used where feed accuracy of
about  1 % is required for economy, as in large scale operations and for materials which are
used in small, precise quantities. It should be noted, however, that even gravimetric feeders
cannot compensate for weight added to the chemical by excess moisture. Many volumetric
feeders may be converted to  loss-in-weight function by placing the entire feeder on a
platform scale which is tared to neutralize the weight of the feeder.

Good housekeeping and  need for accurate feed rates dictate that the gravimetric feeder be
shut down and thoroughly cleaned on a regular basis. Although many of these feeders have
automatic or semi-automatic devices which  compensate to some degree for accumulated
solids  on  the weighing mechanism, accuracy is affected, particularly on humid days when
hygroscopic  materials are fed.  In some  cases, built-up chemicals can actually jam the
equipment.

No discussion of feeders is complete without at least passing reference to dissolvers, as any
metered  material must be mixed with water to  provide  a chemical  solution of desired
strength.  Most  feeders, regardless of type, discharge  their  material to a small dissolving
                                        5-62

-------
                       FIGURED-1#
                      SCREW FEEDER
 MOTOR AND
 GEAR REDUCER
 SOLUTION
CHAMBER
                                                       HOPPER
                                                       ROTATING &
                                                       RECIPROCATING
                                                       FEED SCREW
                                                      SOLUTION
                                                       LEVEL
                                                        JET MIXER
                            ' FIGURE 5-17
                 POSITIVE DISPLACEMENT SOLID
                       FEEDER—ROTARY (15)
                             5-63

-------
tank which is equipped with a nozzle system and/or mechanical agitator depending on the
solubility of the chemical  being  fed. Solid materials, such  as  polyelectrolytes,  may  be
carefully spread into a vortex spray or washdown jet of water immediately before  entering.
the dissolver. It is essential that  the surface of each particle become thoroughly wetted
before entering the feed tank to ensure accurate dispersal and to avoid clumping, settling or
floating.

A dissolver for a dry chemical feeder is unlike a chemical feeding mechanism, which by
simple adjustment and change of speed  can vary its output tenfold. The dissolver must be
designed for the job to be  done.  A dissolver suitable for a rate of 10 lb/ hr may not be
suitable for dissolving at a rate of 100 Ib/hr. As a general rule, dissolvers may be oversized,
but dissolvers for commercial ferric sulfate  or lime slakers do not perform well if greatly
oversized.

It is essential that specifications for dry chemical feeders include  specifications on dissolver
capacity. A  number of factors need to be considered  in designing dissolvers of proper
capacity. These include detention times and water  requirements, as well as other factors
specific to individual chemicals.

The capacity of a dissolver is  based on detention  time, which is directly related to the
wettability or rate of solution of the chemical. Therefore, the dissolver must be large enough
to provide the necessary detention for both the chemical and the water at the maximum rate
of feed. At lower  rates of feed, the strength of solution or suspension leaving the dissolver
will be less, but the  detention time will be approximately the same unless the water supply
to the dissolver is reduced. When the water supply to any dissolver is controlled for the
purpose  of forming a constant strength  solution,  mixing within the dissolver  must be
accomplished by mechanical means, because sufficient power will not be available  from the
mixing jets  at  low rates  of flow.  Hot water  dissolvers are also available in  order to
mimimize the required  tankage.

The foregoing descriptions  give  some indication of the wide variety of materials  which may
be handled. Because of this variety,  a modern facility may contain any number of a variety
of feeders with combined  or multiple  materials capability.  Ancillary equipment to the
feeder also  varies according to the material to be  handled.  Liquid feeders encompass a
limited number of design principles which account for density and viscosity ranges. Solids
feeders,  relatively  speaking,  vary considerably due to the  wide range of physical and
chemical characteristics, feed rates and the degree of precision and repeatability required.

Table 5-11  describes several types  of  chemical feeders  commonly  used in  wastewater
treatment.
                                        5-64

-------
      Type of Feeder

Dry feeder:
  Volumetric:
     Oscillating plate .
     Oscillating throat (universal)

     Rotating disc	


     Rotating cylinder (star) ....
     Screw. .

     Ribbon.

     Belt . . .
   Gravimetric:
     Continuous—belt and scale
   Loss in weight
Solution feeder:
   Nonpositive displacement:
     Decanter (lowering pipe) .. .
     Orifice	
     Rotameter (calibrated valve)
     Loss in weight (tank with
       control valve).
   Positive displacement:
     Rotating dipper	
   Proportioning pump:
     Diaphragm	
     Piston 	
Gas feeders:
   Solution feed .
   Direct feed.
                                                       TABLE 5-11    "
                                        TYPES OF CHEMICAL  FEEDERS
                                                  Use
Any material, granules or
  powder.
Any material, any particle
  size.
Most materials including NaF,
  granules or powder.

Any material, granules or
  powder.

Dry, free flowing material,
  powder or granular.
Dry, free flowing material,
  powder, granular, or lumps.
Dry, free flowing material up
  to I'/z-inch size, powder or
  granular.

Dry, free flowing, granular
  material, or floodable
  material.
Most materials, powder,
   granular or lumps.
Most solutions or light slurries
Most solutions	
Clear solutions	
 Most solutions
 Most solutions or slurries ...

 Most solutions. Special unit
   for 5% slurries.1	
 Most solutions, light slurries.
 Chlorine	
 Ammonia	
 Sulfur dioxide .
 Carbon dioxide
 Chlorine	
 Ammonia	
 Carbon dioxide

General



loader for
arching.




agitator to
maintain
constant
density.


No slurries . .
No slurries . .









Limitations
Capacity
cu ft/hr
001 to 35
002 to 100
001 to 1 0
8 to 2,000
or
7.2 to 300
0.05 to 18 	
0.002 to 0.16. . . .
0 1 to 3 000 ... .
0 02 to 2
0.02 to 80 	
0.01 to 10 	
0 1 6 to 5
0.005 to 0.1 6
or
0.01 to 20
0.002 to 0.20 	
0.1 to 30 	
0.004 to 0.15 	
0.01 to 170 	
8000 Ib/day max
2000 Ib/day max
7600 Ib/day max
6000 Ib/day max
300 Ib/day max
1 20 Ib/day max
10,000 Ib/day max

Range
40 to 1
40 to 1
20 to 1
10 to 1
or
100 to 1
20 to 1
10 to 1
10 to 1
or
100 to 1
100 to 1
100 to 1
100 to 1
10 to 1
10 to 1
30 to 1
100 to 1
100 to 1
20 to 1
20 to 1
20 to
20 to
20 to
10 to
7 to
20 to
 1 Use special heads and valves for slurries.
                                                                   5-65

-------
     5.8 References

 1.   BIF,  data sheets, Chemicals Used  in  Water  and Wastewater  Treatment-Ferric
     Chloride,  Wat. and Wastes Eng., Vol. 7, No. 3, pg. 65 (1970).

 2.   Ferric-Floe Jor  wastewater  treatment.  Cities  Service  Co.,  Industrial Chemical
    'Division, Atlanta, Ga. (1972).

 3.   Schworm, W.B., Iron Salts Jor Water and Waste Treatment, Public Works, Vol. 94,
     No. 10, pg. 118(1963).

 4.   National Lime Association, Personal Communication (May, 1971).

 5.   Lime for Water and Waste-water Treatment,  BIF Reference No. 1.21-24, BIF
     Industries, Providence, Rhode Island (June, 1969).

 6.   Mulbarger, M.C., Grossman, E., Dean, R.B., and Grants O.L., Lime Clarification,
     Recovery, Re-use and Sludge Dewatering Characteristics, JWPCF, Vol. 41, pg. 2070C
    '(1969).

 7.   Caustic Soda, PPG Industries, Inc., Chemical Division, Pittsburgh, Pa. (1969).

 8.   Haney,  P.D. and Hamann, C.L.,  Recarbonation and Liquid  Carbon  Dioxide,
     JAWWA, Vol. 61, No. 10, pg. 512 (1969).

 9.  Gulp,  R.L.,  and  Gulp,  G.L.,  Advanced  Wastewater Treatment,  Van
     Nostrand-Reinhold Company, New York (1971).

10.   Russo, F. and Carr,  R.L., Polyelectrolyte Coagulant aids and Flocculants:  Dry and
     Liquid, Handling and Application, Wat. and Sew. Works Vol. 117, pg. R-72 (1970).

 11.  U.S. EPA OAWP, Report on Coagulant Aids for  Water Treatment (July 1973).

 12.  Carr,  R.L., Polyelectrolyte  Coagulant Aids-Dry  and Liquid Handling and
     Application, Wat. and Sew. Works—Reference No., 114: R.N., p 4-64, (1967).

 13.  Baker, R.J.,  Chemical Feed Systems Determine Plant  Efficiency and Reliability,
     Water & Sew. Wks.,  Vol. 116, pg. R-21 (November 1969).

 14.  R.P. Lowe, Chemical Feed Systems, 10th Annual Water Conference of Eng Soc. of
     W. Penna. (Oct.  17-19, 1949).

 15.  Design manual, civil engineering, Navdocks DM-5; Department of the Navy, Bureau
     of Yards and Docks, Washington, D.C. (1972).
                                      5-66

-------
                                    CHAPTER 6

             CHEMICAL  MIXING, FLOCCULATION AND SOLIDS-
                             CONTACT PROCESSES

6.1   Introduction

Chemical mixing and flocculation or  solids-contact are important mechanical steps in the
overall coagulation process described  in Chapter 4. Application of the processes to waste-
water generally follows standard  practices  and employs basic equipment used for years in
the  water-treatment field. Chemical mixing thoroughly disperses  coagulants or their hy-
drolysis products so the maximum possible portion of influent colloidal and fine supracol-
loidal solids are absorbed and destabilized. Flocculation or solids contact processes increase
the  natural rate of contacts between particles. This makes it possible, within reasonable de-
tention periods,  for destabilized colloidal and fine supracolloidal solids to aggregate into
particles large enough for effective separation by gravity processes or media filtration.

All  the processes discussed in this chapter depend on fluid shear for coagulant dispersal and
for  promoting particle contacts. Shear is most commonly introduced by mechanical mixing
equipment.  In certain solids-contact  processes shear  results from fluid passage  upward
through a blanket of previously settled particles. Some designs have utilized shear resulting
from energy losses in pumps or at ports and baffles.

Numerous theoretical descriptions of  the flocculation process have been developed (1) (2)
(3)  (4) (5) (6). Almost  all relate to experience in water treatment but can be applied to
wastewater coagulation with proper attention to significant differences in the nature of sol-
ids.

All  theoretical approaches recognize the importance of time (t in sec.) and velocity gradient
(G,  a measure of shear intensity in fps/ft or secrl) as controlling parameters in determining
performance of mixing and flocculation processes. It should be noted that chemical mixing
and flocculation  differ only in intensity and duration and that some aggregation takes place
in the chemical mixing stage.

In addition to velocity gradient and time, expressions for the rate of aggregation in floccula-
tion or solids contact processes involve parameters reflecting the total volume and the size
and number of floe particles. When destabilized, particles in the fine colloidal range rapidly
aggregate under natural conditions to form small floes of  fine supracolloidal size,  about 1
micron diameter (5), often termed primary sized particles. In developing mathematical rela-
tions this is generally the assumed initial size of particles to be further aggregated.

The rate of aggregation is commonly taken as a function of the dimensionless product GCt
where C is the ratio of the volume of floe to total volume of suspension and G and t are as
defined above.

The floe volume concentration resulting from a given coagulant dosage depends, among
                                       6-1

-------
other things, on the amount of water entrained in the floe. Hudson (7) and Camp (1) have
shown that more water is entrained and higher floe volumes result when flocculation takes
place at lower values of G.

The value of C may be increased greatly by recirculation of settled solids. This is used in
certain types of solids contact reactors (Section 6.4) and has been applied at Lake Tahoe as
part of a conventional coagulation system with separate rapid mix, flocculation and sedi-
mentation basins (8).

Design of rapid mix and flocculation units generally involves the choice of detention and G
value and selection of configurations, of mixing equipment, tanks, piping, etc. Unless the
designer provides for direct control of floe volume concentration through solids recircula-
tion, operating values of this parameter are determined indirectly through the chemical do-
sage and choice of G value  and detention. Special attention should be given to avoiding ex-
cessive focalized shear and reducing short circuiting. Pretreatment should assure that waste-
water  is free of debris (rags, sticks,  etc.) which could damage mixing equipment. Special
considerations in design of  solids-contact units are presented in Section 6.4.

G represents the root mean square velocity gradient (fps/ft) over the mixing basin. For me-
chanically-stirred basins it can be calculated from the relation:

                                            P  \^
                                     G='  — 1 2
        Where: P  = power applied to stirring (ft-lb/sec = HP x 550)
               V  = reactor volume (cu ft)
               •u  = viscosity of fluid (Ib-sec/sq ft)

        Viscosity varies with temperature as follows:
                        T
                        °C                       Ib-sec/sqft

                         1                       0.361 x 10-4
                         5                       0.316 x 10-4
                        10                       0.273 x 10-4
                        15                       0.239 x 10-4
                        20                       0.210 x 10-4
                        25                       0.187x 10-4
                        30                       0.167x lO'4
 Formulas for calculating G from  head losses in baffled basins or in conduits are given by
 Camp (9).
                                         6-2

-------
6.2  Chemical Mixing

Chemical mixing facilities should be designed to provide a thorough and complete dispersal
of chemical throughout the wastewater being treated to insure uniform exposure to pollu-
tants which are to be removed.

The intensity and duration of mixing of coagulants with wastewater must be controlled to
avoid overmixing or undermixing.

Overmixing excessively disperses newly-formed floe and may rupture existing wastewater
solids. Excessive floe dispersal retards effective flocculation and may significantly increase
the flocculation period needed to obtain good settling properties. The rupture of incoming
wastewater solids may result in  less  efficient removals of pollutants associated with those
solids (2) (4).

Undermixing  inadequately disperses coagulants resulting  in uneven dosing. This in turn
may reduce efficiency  of solids  removal  while requiring unnecessarily high coagulant do-
sages.

In water treatment practice several types of chemical mixing units have been used.  These in-
clude high-speed mixers, in-line  blenders and pumps, and baffled mixing compartments or
static in-line mixers (baffled piping sections). The high-speed mixer, as shown in Figure 6-1,
has been the most common choice for  water treatment. Designs usually call for a 10 to 30
second detention time and approximately  300  fps/ft velocity  gradiant (10). Hudson and
Wolfner (11) recommend variable-speed mixers to  allow for varying requirements for opti-
mum mixing. In solids-contact reactors the G values in the immediate mixing zone approxi-
mate those for high-speed mixing (See  section 6.4).

High speed mixers designed  on the  above basis should be equally satisfactory for waste-
water applications. Gulp, et al, (12)  recommend providing two parallel units with a some-
what larger detention:  2 minutes at total design flow with both units. It has been  question-
ed, however, whether in-line blenders (with G values as high as 5000 fps/ft) should be used
for wastewater in view of the possibility of rupturing organic solids (4). Where flows must
be pumped just  prior to coagulation, addition  of  chemicals at the pumps is feasible. The
pump selection should take into  account possible effects on organic solids of shear in centri-
fugal units. Where  problems are anticipated, lower speed units such as  screw  pumps
should be used. Baffled compartments or in-line static mixing devices are limited in their ef-
fectiveness as chemical mixing devices  whether in water or wastewater treatment because:

     1. Head losses of up to 3 ft are  required.
     2. G cannot be changed to meet  requirements, but  rather is a function of  flow rate
       through  the units.

In mineral addition to biological wastewater treatment systems, coagulants may  be added
directly to mixed  biological  reactors such as aeration tanks  or rotating  biological con-
                                        6-3

-------
             DRIVE MECHANISM
                MOTOR
SUPPORT BEAMS
    OVERFLOW
    IMPIUER
                                            FEED
                         FIGURE 6-1
                     IMPELLER MIXER
                        6-4

-------
tactors.

Based  on typical power  inputs per unit tank volume, mechanical  and diffused aeration
equipment and rotating fixed-film biological  contactors produce average shear intensities
generally in the range suitable for chemical mixing. Parker (13) indicated that an analysis of
datajfor 14 activated sludge plants revealed that G ranged'from 88 to 220 fps/ft with an aver-
age of 136 fps/ft. Localized maximum shear  intensities vary widely depending  on speed of
rotating equipment or on bubble  size for diffused aeration. Camp (9) presented bases for
relating localized maximum shear intensities  to bubble size in diffused aeration. For fine
bubble diffusion (1.5 mm bubbles) maximum  intensity reaches 1500 fps/ft with higher val-
ues for coarse bubbles.

No similar development has been located for rotating mechanical aerating equipment, but it
appears that  maximum localized  shears range from little more than the basin  mean value
for large, low-speed devices such as rotating biological contactors, to perhaps as high as 50
times the mean for high speed (1800 rpm) mechanical aerators. Questions have been raised
about detrimental effects of high speed aerators on settling of activated sludge.

When  using polymers, manufacturer's recommendations should be sought on the mixing
conditions which optimize their effectiveness, and these should be supplemented by jar tests,
if  possible. When coagulant aids are  employed, provisions  for multiple addition points
should be made at the rapid mixing basin and in the flocculator to  optimize the perform-
ance of the coagulant.

6.3  Flocculation

The proper measure of flocculation effectiveness is the performance of subsequent solids
separation units in terms of both effluent quality and operating requirements, such as filter
backwash frequency. Effluent quality depends greatly on the reduction of residual primary
size particles during flocculation,  while operating requirements relate more  to the  floe vol-
ume applied to separation units.

For water treatment using alum or iron coagulants and flow-through flocculation (as op-
posed to solids-contact units) traditional designs have been based on G* of up to 100 fps/ft
andiGt values of 0.3 to 1.5 x 10$ (10) and GCt values of  10-100 (3). The  wide ranges of
these parameters may reflect genuine differences between waters  (or wastewaters) but may
also reflect different design approaches. Hudson (7) has suggested use of Gt values of 2 x
105 which he claimed would produce high density floe with settling velocities equivalent to
those of larger lower density floe produced  at low G values. Camp (1) has suggested use of
higher G values and resulting lower floe volumes to get equivalent primary particle agglo-
meration but with lower solids loadings on subsequent separation units.

Values in the ranges above are certainly ample for wastewater flocculation in flow through
units. Because of the larger coagulant doses commonly used in wastewater treatment (espe-
cially with phosphorus removal) detention times and Gt values can generally be lower. Gulp
et al (12) recommend a maximum of 15 minutes detention for wastewater coagulation. Gulp
and Gulp (8) recommend using paddle speeds of 0.5 to 1.0 fpm.

                                       6-5

-------
Tapered flocculatiqn  in  which the flow is  exposed to decreasing G  values as it passes
through the unit, can provide a rapid build-up of small dense floe with subsequent agglome-
ration  at  lower 73  into  larger  but still  dense  particles. (9) (10) (11).  Use  of mul-
ti-compartment flocculators not only permits tapered flocculation, but also greatly reduces
the high short-circuiting associated with single-compartment units. A wide variety of physi-
cal layouts are possible to achieve series flow through multiple compartments (10).

Flocculation units should have multiple compartments and should be equipped with adjust-
able speed mechanical stirring devices  to permit meeting changed conditions.  In  spite of
simplicity  and low maintenance, non-mechanical, baffled basins are undesirable because of
inflexibility,  high head losses and large space requirements.

Mechanical flocculators may consist of rotary, horizontal-shaft reel units as shown in Fig.
6-2, rotary vertical shaft turbine units as shown  in Fig. 6-3  or other  rotary or recipro-
cating equipment. Features of these various type units are discussed and compared else-
where (9) (10) (11).

Tapered flocculation may be obtained by varying reel or paddle size on horizontal common
shaft units or by varying speed on units with separate shafts and drives. A typical series of G
values for successive compartments would.be  100, 50 and 20 fps/ft. In  most cases, equip-
ment should provide overall Gt values up to 2 x 105 at maximum drive  speed. Speed vari-
ation over a range of  1:3 or 1:4 should be possible (10).

.G values are determined from the hp  actually transmitted to the fluid (water hp). This
should be distinguished from the total input hp which includes losses in motors, drives,
bearings, etc. It should be  noted that G is a mean value for the entire flocculator volume.
Practical limits are set to localized high values at the flocculator blades or paddles by speci-
fying peripheral speeds below about 2 fps.

. In applications other than coagulation with alum or iron salts, flocculation parameters may
be quite different. Lime precipitates are granular and benefit little from prolonged floccula-
tion or very low terminal G values. At Lake Tahoe a detention of 4.5 min. proved adequate.
Gulp and Gulp (8) recommend a minimum of 5 min. but as much as 10 min may be needed
to assure complete dissolution and reaction of CaO. As in water softening practice, G val-
ues of 100 or more are desirable.

Polymers which already have a long chain structure may provide a good floe at low Gt val-
ues. Often the turbulence and detention in the clarifier inlet distribution  is adequate.

Settling and effluent clarity in the activated sludge process can frequently be improved by'
controlled flocculation between the aeration tank  and clarifier. Parker, et al, (14) showed
that flocculation at G = 40 to 60 fps/ft and detention of 20 to 30 min. could reduce the SS
in aerator effluent (after settling) by some 45 to 55 percent. The benefits of flocculation de-
pend on the level of turbulence in the aerator, and on the sludge age which affects the natu-
ral flocculating characteristics of the sludge. In the above study sludge with a sludge age of
10 days was better destabilized and benefited more from flocculation than did sludge with a
                                         6-6

-------
                                               carrot «um
                                                    JL
     n
             _u	u_
              1!   K
                           II   II   II
                                                     J
                         FIGURE 6-2
            MECHANICAL FLOCCULATION BASIN
              HORIZONTAL SHAFT-REEL TYPE
                                 MOTORIZED SPEED REDUCER
    GUIDE BEARING
WATER PRESSURE LUBRICATED
                      'FIGURE 6-3
             MECHANICAL FLOCCULATOR
            VERTICAL SHAFT—PADDLE TYPE
                 (Courtesy of Ecodyne Corp.)
                          6-7

-------
sludge age of 3 to 4 days or 12 days.

This behavior may be interpreted in light of the observation by Dean (15) that activated
sludge contains an excess  of natural anionic  polymers. As sludge age increases these pol-
ymers are  reduced—first to levels where destabilization is better—but then to levels below
the optimum.


6.4  Solids-Contact Processes

Solids-contact processes combine chemical mixing, flocculation and clarification in a single
unit designed so that a large volume of previously-formed  floe is retained in the system. The
floe volume may be as much as 100 times that in a "flow-through" system. This greatly in-
creases the rate of agglomeration from particle contacts (11), and may also speed up chem-
ical destabilization reactions.

Solids contact units are of  two general types: slurry-recirculation and sludge-blanket. In the
former, the high floe volume concentration is maintained by recirculation from the clari-
fication to the  flocculation zone, as illustrated in Fig. 6-4. In the latter, the floe solids are
maintained in a fluidized blanket through which the wastewater under treatment flows up-
ward after leaving the mechanically stirred-flocculating compartment, as depicted in Fig.
6-5. Some slurry-recirculation units can also.be operated  with a sludge blanket.

Solids-contact  units have  become  popular  in water treatment and are being increasingly
considered in advanced wastewater treatment because of the following advantages:

     1.   Reduced size and lower cost result because flocculation proceeds rapidly at  high
         floe volume concentration.

     2.   Single-compartment flocculation is practical because high reaction rates and the
         slurry effects overcome short circuiting.

     3.   Units are available as compact single packages,  eliminating separate units.

     4.   Even distribution  of inlet flow and  the vertical flow pattern in the clarifier improve
         clarifier performance (16).

Equipment typically consists  of concentric  circular compartments for mixing, flocculation
and settling. Velocity gradients (G) in the mixing and flocculation compartments are devel-
oped by turbine pumping  within the unit and by velocity dissipation  at baffles. For ideal
flexibility it is desirable to independently control intensity of mixing (G) and sludge scraper
drive speed in the different compartments. Ives  (3) indicates that slurry-recirculation solids
contact reactors in the water treatment field operate with  a velocity gradient in the range of
60 to 120 fps/ft. Hudson and Wolfner (11) indicate that in water treatment solid-contact re-
actors.with variable-speed turbine-type agitators apply velocity gradients of 300 fps/ft in
the mixing zone while reaction zone values may vary from 100 fps/ft near entrance to  20
fps/ ft at the settling  zone boundary. Comparably proportioned units are being  used  in

                                        6-8

-------
                                    RAPID MIXING AND RECIRCULATION
                                                                               SLOW MIXING AND FLOC FORMATION
                                                                                               CHEMICAL INTRODUCTION
               TREATED WATER
               EFFLUENT
ON
               CLEAR WATER
               SEPARATION
                                            SLUDGE RECIRCULATION
                                                                                                                  CLARIFIED
                                                                                                                  WATER
                                                                                                                  RAW WATER
                                                                                                                  INFLUENT
                                                                                                            SEDIMENTATION
                                                                                SLUDGE REMOVAL
                                                                FIGURE 6-4
                                SOLIDS CONTACT CLARIFIER WITHOUT SLUDGE BLANKET FILTRATION
                                                       (Courtesy of Ecodyne Corp.)

-------
EFFLUENT COLLECTOR FLUME



,





!


AGITATC
' f^ |i
r i] •/ '-j
!| ' ' "
..'.., „ 	 , 	 il. - *+-.£.-^ ,

R
CHEMICAL fflrO INltTS
f . r
. INFIUFNT
>dfflS§3
jji^*1* 	
                                                                  SKIMMING
                                                                    SLOT
• »»' 00000  OO   O   O
                                                         o   ooooooooo»«i
                                                                              SLUDGE
                                                                             BLOW OFF
                                                                               LINE
                                                                          SAMPLE CONNS.J
                                  AGITATOR
                                    ARM
             SLUDGE
          CONCENTRATOR
SWING SAMPLE
  INDICATOR -
                                                          PRECIPITATOR DRAIN
                                      FIGURE 6-5
              SOLIDS CONTACT CLARlFlER WITH SLUDGE BLANKET FILTRATION
                                (Courtesy of the Permutit Co.)

-------
wastewater treatment, but with little explicit consideration of G values.

Experience with solids-contact units in wastewater treatment has up to now been limited to
slurry-recirculation units. Gulp and Gulp  (8) have expressed concerns  about the use of
sludge-blanket  units: septicity and uncontrolled blanket upsets under varying-load condi-
tions. Slurry-recirculation units not requiring sludge blankets  or with  minimum  blanket
depths are not  very sensitive to such upsets. Units equipped with scrapers have operated
without septicity problems treating secondary effluent at Nassau County, N.Y. (17) and at
Ely, Min. (18).

Operation of slurry-recirculation solids-contact units is typically controlled by maintaining
steady levels of solids in the reaction zone. For lime treatment of wastewater at  Ely  and
Blue Plains a solids concentration of 10 to 12 percent by weight was found most effective
(for phosphorus removal) (18) (19). For tertiary alum treatment at Nassau Gounty 45 per-
cent floe volume concentration proved most satisfactory (17).

Design features of solids-contact clarifiers should include:

    1.  Rapid  and complete mixing of chemicals, feedwater and slurry solids must be pro-
         vided.This should  be comparable to conventional flash mixing capability  and
         should provide for variable control of Gt values, usually by adjustment of recir-
         culator speed.

    2.  Mechanical means  for controlled circulation of the  solids slurry must be provided
         with at least a 3:1 range of speeds. The maximum peripheral speed of mixer blades
         should not exceed 6 ft/sec. Rushton and Mahoney (20) offer means of estimating
         pumping capacity of mixers.

    3.  Means should be provided for measuring and varying  the slurry concentration in
        the contacting zone up to 50 percent by volume.

    4.  Sludge discharge systems should allow for easy automation  and variation of vol-
         umes  discharged. Mechanical scraper tip speed should be less than 1 fpm with
         speed  variation of 3:1.

    5.  Sludge-blanket levels must be kept a minimum of 5 feet below the water surface.

    6.  Effluent launders should be  spaced so as to minimize the horizontal movement of
        clarified water.

Most'Of the above requirements are based on those cited  in Water Treatment Plant Design
(10). Further considerations  include skimmers and weir overflow rates. Skimmers should be
provided on all units since even secondary effluents  contain  some  floatable solids  and
grease. Overflow rates and  sludge scraper design should conform to the requirements of
other clarification  units.
                                        6-11

-------
6.5  References

1.   Camp, T. R., Floe Volume Concentration, Jour. AWWA, 60, 656 (1968).

2.   O'Melia, C. R.,  The Coagulation Process: Application oj Research to Practice, Re-
    port submitted to ASCE (Oct. 1969).

3.   Ives, K. J.,  Theory oj Operation of Sludge Blanket Clarijiers, Proc. ICE (Br.), 39, 243
    (Feb. 1968).

4.   Weber, W. J., Jr., Physicochemical Processes Jor Water Quality Control, John Wiley
    & Sons, Inc., New York (1972). '

5.   Harris, H. S., et al, Orthokinetic Flocculation in Water Purification, J. San Eng. Div.,
    ASCE, 92, SA6, 95 (Dec.  1966).

6.   Parker, D. S. et al, Floe Breakup in Turbulent Flocculation Processes, J. San. Eng.
    Div. ASCE, 98, SA1, 79-99 (Feb. 1972).

7.   Hudson, H.  E.,  Jr.,  Physical Aspects of Flocculation, Jour. AWWA, 57, 885 (July,
    1965).

8.   Gulp, R. L. and Gulp, G.  L., Advanced Waste-water Treatment, Van Nostrand Rein-
    hold Company, New York (1971).

9.   Camp, T. R., Flocculation and Flocculation Basins, Trans. ASCE, 120, (1955).

10.  Water Treatment Plant  Design, American Water Works Ass'n, Inc.,  New York
     (1969).

11.  Hudson, H. E., Jr., and  Wolfner, J.  P., Design oj Mixing and Flocculating Basins,
     Jour. AWWA, 59,1257 (Oct.  1967).

12.  Gulp, G.  L.,  et  al.,   Physical-Chemical  Wastewater  Treatment  Plant  Design.
     U.S. EPA Technology Transfer Seminar Publication (August 1973).

13.  Parker, D. S., Effect  of Turbulence on Activated Sludge Effluent  Clarity, Paper
     Presented at 12th Annual Northern Reginal Conference, Calif. WPCA, (Oct.  1970).

14.  Parker, et al, Physical Conditioning of Activated Sludge Floe, JWPCF, 43, 9, pg.
     1817, (Sept. 1971).

15.  Dean, R.  B., Colloids  Complicated  Treatment Processes, Environmental Science &
     Tech. pg. 820, (Sept. 1969).
                                        6-12

-------
16.  Aitken, I.M.E., Reflections on Sedimentation Theory and Practice—Part I, Eff, and
    Water Treatment Jour. (Br.), 74, 226 (Apr. 1967).

17.  Oliva, J. A. Department  of Public Health, County of Nassau,  Personal Commu-
    nication. (March 1973).

18.  Westrick, J. J., U.S. EPA,  NERC, Cincinnati, Ohio, Personal communication (April,
     1973).

19.  Kreissl, J. F., U.S.E.P.A., National Environmental  Research Center,  Cincinnati,
    ,Ohio, Personal Communication, (April 1973).

20.   Rushton, J. H. and Mahoney, L.  H., Mixing  Power & Pumpage Capacity, Annual
     Meeting of AIME, New York (Feb. 15, 1954).
                                     6-13

-------
                                    CHAPTER 7

                             GRAVITY SEPARATION
7.1 Introduction

Gravity separation refers to the removal of SS whose specific gravity difference from that
of water  causes  them  to settle or rise during  passage through  a tank or  basin  under
quiescent conditions. Separation by settling is termed sedimentation; separation by rising is
termed flotation. The size of particles determines the fluid drag retarding this separation.
For a  given specific gravity, smaller particles having greater surface area encounter more
drag and hence are more difficult to separate.

The factors affecting separation efficiency are discussed in depth for sedimentation, and
separate sections cover each of its major applications. The section on flotation indicates
special considerations pertaining to this process and deals with its applications. Finally two
sections deal with devices which enhance the performance of sedimentation basins.

7.2 Configuration of Sedimentation Units

The tanks or basins in which sedimentation is carried out (also frequently termed clarifiers)
may be classified as horizontal flow or vertical-flow according to the predominant direction
of the flow path  from inlet to outlet.  It should be noted that, depending on placement of
inlet? and outlets, certain designs—particularly small radial flow tanks—may have a flow
path with significant components in both horizontal and vertical directions.

    7.2.1 Vertical-Flow Units

Vertical-flow applications in the U.S. have generally been limited to settling compartments
in  flocculation-clarifiers, solids-contact  units and activated sludge  systems of similar
configuration (Aero-Accelator, Rapid  Block, etc.).  In Europe, vertical-flow basins have
been used extensively. Kalbskopf has illustrated several European designs (1).

Vertical-flow  units  may  be  annular or rectangular, and  are generally  narrower at the
bottom than at the top. In annular designs, the flow is distributed at the bottom along the
circumference of the tank and  rises to peripheral or radial effluent weirs or launders. Flow
in rectangular tanks is distributed at the bottom along the length of the tank and rises to
longitudinal or transverse effluent weirs or launders.

Annular units have been built  with outside diameters to 150 ft, but the width from inner
wall to outer wall is much less. Figures 6-4 and 6-5 illustrate annular,  vertical-flow units.

    7.2.2 Horizontal-Flow Units

In the U.S. horizontal-flow units, both rectangular and circular, are most  often used for


                                        7-1

-------
sedimentation applications. Tank proportions, inlet and  outlet arrangements and types of
sludge and scum collecting equipment are summarized and discussed in the ASCE/WPCF
Manual  for Sewage  Treatment Plant  Design  (2). Individual bays of  rectangular tanks
should have a length to width ratio of at least four.

Flow through  rectangular  tanks  enters at  one  end, passes  a  baffle arrangement,  and
traverses the length of the tank to  effluent weirs.  In narrow tanks, longitudinal collectors
scrape sludge to single or multiple hoppers at one end  (Figure 7-1). In tanks with multiple
wide bays, the longitudinal collectors scrape sludge to a cross collector which then moves
the sludge to  a central hopper. Circular designs  employ three inlet/ outlet  configurations
with corresponding flow paths as shown in  Figure 7-2.  In configurations 7-2 (a) and  (c),
sludge is removed by  mechanical scraping to a central hopper or draw-off. In configuration
7-2 (b) a hydraulic suction sludge removal system is employed.

7.3 Basic Factors Affecting Settling Tank Design

     7.3.1 Hydraulic Loading

The basic parameter  to which settling tank  performance is related is the surface hydraulic
loading (Q/A). This  is the inflow (Q) divided by the surface  area (A) of the basin, and is
commonly expressed  in units of gpd/sq ft.

Hazen (3) showed that under  the following assumptions performance  is a function of
surface loading alone:

     1.   Quiescent or non-turbulent flow

     2.   Uniform distribution of velocity over all sections normal to general flow direction

     3.   Discrete non-interacting particles
                                                        . 5 A.

     4.   No resuspension of particles once they reach the floor of the basin

Under these conditions all particles whose settling velocity (Vs) exceeds Q/A are removed.
In addition, in horizontal flow tanks  particles  of  lower settling  velocities are partially
removed in the proportion Vs/(Q/A).

In actual basins conditions depart in many respects from those assumed in Hazen's original
analysis. The  most significant of these departures are:

     1.   Currents induced by inlets, outlets, wind and density differences may cause short
         circuiting or dead spaces within the tank.

     2.   Turbulence due to forward velocity or currents in the tank retards settling.

     3.   Flocculent solids may agglomerate into larger particles during passage through the
        .basin.

                                        7-2

-------
     INFLUENT
                           DRIVE SPROCKET
                                                                         -ADJUSTABLE WEIRS
                                        .WATER LEVEL
                       .   RECESS FOR
                       /   DRIVE CHAIN
                FLOW

             SKIMMING
                         CHAIN a FLIGHT
                         CROSS COLLECTOR


                         SLUDGE HOPPER
AVERAGE
 WATER
 DEPTH
                                                  J     I     J	-—^t^
                                                  	f T	IT-	:-^r4
                                                     D
                                                                                    EFFUJENT
                      2"x 6" FLIGHTS
                                       PIVOTING FLIGHT-1
                          A. WITH CHAIN AND FLIGHT COLLECTOR
           TRAVELING
            BRIDGE
                                          BRIDGE
                                          TRAVEL
                           COLLECTING   •• 'HMYCL. ^  SKIMMING
                                                       SCUM
                                                       TROUGH
                        WATER LEVEL
SLUDGE
DRAWOFf
      SLUDGE
      HOPPER
                                                            SKIMMING POSITION
                   SLUOOE COLLECTION POSITION
                 - SCREW CROSS
                  COLLECTOR
B. WITH TRAVELING BRIDGE COLLECTOR
                                          FIGURE 7-1

                          RECTANGULAR SEDIMENTATION TANKS
                                    (Courtesy of FMC'Corp!)
                                              7-3

-------
                                                  EFFLUENT
            SLUDGE
   INFLUENT
               (a)CIRCULAR CENTER-FEED CLARIFIER WITH
                 A SCRAPER SLUDGE  REMOVAL SYSTEM
INFLUENT
                                                   EFFLUENT
                                                SLUDGE
        (b)CIRCULAR RIM-FEED,  CENTER  TAKE-OFF CLARIFIER  WITH A
             HYDRAULIC SUCTION  SLUDGE REMOVAL SYSTEM
           -i-
                jrf


1±
            ^g
INFLUENT

 EFFLUENT
                            I
       SLUDGE
             (clCIRCULAR RIM-FEED,  RIM  TAKE-OFF CLARIFIER



                          FIGURE 7-2

             TYPICAL CLARIFIER CONFIGURATIONS
                              7-4

-------
    4.  Sludge may be scoured and resuspended at high forward velocities.

    5.  When influent solids concentrations are high, particles settle as a mass rather than
        discretely.

The subsections below indicate how investigators, most notably Camp (4),  have attempted
to account for these departures by relating  performance  to  additional parameters. The
relationships are not generally adequate to permit prediction of performance from design
values of the parameters, but they do provide insights helpful in deciding a  number of tank
features such as shape, depth, inlet type, etc. In addition, such relationships offer guidance
in translating settling test results into sizing for full scale tanks. Procedures for conducting
and interpreting such tests have been outlined by O'Connor and Eckenfelder (5) and others
(6) (7) (8).

To account for departures of full scale tanks from ideal or test conditions scale-up factors in
the following ranges have been suggested (5):


               Sizing Parameter                      Scale-Up Factor
                 •"••"Area"                           1.25 to 1.75
                   Volume                           1.5 to 2.0
These  scale-up factors are not intended to cover  extreme variations in flows or solids
loadings, or to allow for operation at temperatures significantly different from those in the
tests. Neither do  they  include standby capacity as needed for units critical to overall plant
performance.  Smith (9) has discussed the use of excess capacity  factors to provide for
standby and to cover expected variations in loadings.

     7.3.2 Short Circuiting

Short circuiting can greatly reduce the removal efficiency of a settling  tank.  Effects are
most critical for  flocculent suspensions whose  removal is affected by detention time (Sec.
7.3.4), but depending  on the current pattern, removal of discrete particles may also be
affected. Short circuiting is accentuated by high inlet velocities, high outlet weir rates, close
placement of inlets and outlets, exposure of tank surface to strong winds,  uneven heating of
tank contents by sunlight,  and density  differences between inflow and  tank contents.
Density-induced  short  circuiting can be  a significant factor  in  secondary settling tanks
handling activated sludge mixed liquor (10). Inlet and outlet conditions, tank geometry, and
density differences due to influent  SS concentrations  produce steady short circuiting,
whereas effects of other factors are generally intermittent and  unpredictable.

The degree of short circuiting can be measured using tracer studies. Figure 7-3 shows results
of such studies on four types of settling tanks (11), where short circuiting was due primarily
to inlet and outlet conditions and tank geometry. Studies of this type have confirmed that
such short circuiting is minimized in narrow, rectangular, horizontal-flow tanks and is most
serious in circular horizontal  flow tanks. Although upflow  tanks show  the  least short

                                        7-5

-------
r»
O
n
o
z
O
5
              Radial flow in Circular
                   Tank.

              Horizontal RowinWiifa
               Rectangular Tank.

              Horixpnfal Flow in Narrow
                Roctonqulor Tank.

              Vertical Flow in Upflow
              Ctaiqn of Tank.
                                                       \oo'/t
                            PERCENTAGE  OF  NOMINAL  DETENTION  PERIOD
                                           FIGURE 7-3

                           RESULTS OF SALT-INJECTION TESTS WITH v
                        DIFFERENT TYPES OF SEDIMENTATION TANKS

-------
circuiting, practical problems in obtaining uniform initial flow distribution have limited their
use to small diameter units.
The degree of short circuiting in circular units can vary considerably, however, depending
on the type of inlet used. Inlet conditions have been shown to be more critical than those at
the  outlets (12).  For activated sludge final  settling  tanks,  peripheral  feed  and certain
special-design center  feed  inlets have been  shown  to cause  less  short circuiting  than
conventional center feed inlets (10) (13) (14).
Even where the degree of short circuiting can be measured or predicted, techniques for
evaluating the effect on tank performance (1) (15) are questionable as to  their utility and
accuracy. Hence, the best design approach is to avoid  short circuiting as far  as possible,
thus minimizing  uncertainty as to its effects. The most important factors to consider in
controlling short circuiting are dissipation of inlet velocity, protection of tanks from wind
sweep and uneven heating, and reduction of density currents associated with high inlet SS
concentrations (13).
Such density current short curcuiting is a particular problem in settling tanks for activated
sludge. Fitch (10) has presented estimates of the velocities of such currents as a function of
SS concentration, and  has compared two fundamental approaches to preventing  short
circuiting from  this source. These are dynamic stabilization as proposed by Camp (4) and
density stabilization.  Dynamic stabilization requires shallow basins  with high forward
velocities. (Froude numbers of 5 x 10 5 or greater). The  resulting friction losses, in theory,
counteract stratification and instability of flow. Density stabilization essentially establishes
an upflow type pattern by introducing the dense feed at low velocities and close to the tank
bottom.  Fitch  showed that low inlet  velocities  are essential  to  successful  density
stabilization, and proposed a novel center inlet design to achieve such velocities (see Section
7.4).
     7.3.3  Turbulence

Turbulence levels in a settling basin are normally difficult to estimate. The only exception is
turbulence due to  drag from net forward velocity. Camp (4)  has presented a basis for
estimating turbulence  from this source and for compensating for its effects by increasing
tank area. Required increases vary directly with forward velocity in the tank and with the
desired removal rate.
Good design practice is to minimize other sources of turbulence such as inlet, outlet, wind
and density currents.  These sources produce unpredictable  levels of turbulence and  may
increase short circuiting. Even where the degree of turbulence during sedimentation can be
definitely measured the effect on  removal of flocculent  particles is not easily predicted,
because agglomeration induced by turbulence can alter particle sizes and localized settling
velocities.

                                        7-7

-------
     7.3.4   Particle Agglomeration

For  the flocculent suspensions  handled in wastewater treatment,  particle contact and
agglomeration continues during sedimentation. Two mechanisms produce particle contacts:
velocity gradients within the settling tank,  and differential settling rates; each of which
permit faster moving particles to  overtake  slower ones. Depending on the nature of the
influent suspension, either  mechanism can  significantly affect both the size and  settling
velocity of  floe and  the  fraction of fine, unsettleable particles remaining in suspension.
Regardless  of surface loading on a settling  tank,  attachment  of  smaller, unsettleable
particles onto larger  ones of separable size  is essential  in  attaining high SS  removal
efficiences.  In any case, these larger particles must have the opportunity to agglomerate to
sizes which will be  removed at  the  maximum surface  loadings applied to the  tank.
Otherwise massive  failure of the separation process will occur with significant loss of SS in
the effluent.

Camp (4) asserted that the rate of particle contacts due to differential settling depends only
on the  characteristics of the  suspension. Fitch (16) (17)  maintained that the rate also
increased  with tank depth. In either case, the total  number of contacts occurring due to
differential  settling is a direct function of detention time, which at a given surface hydraulic
loading is, in turn, a function of settling tank depth: In contrast, the rate of particle contacts
due to velocity gradients increases  with forward velocity and hence decreases with depth.

For  the  10 to  15  ft  tank  depths normally used  in wastewater  treatment  in the  U.S.,
agglomeration depends mainly on differential settling. For wastewaters such as raw  sewage,
which agglomerate slowly under differential settling, detention time can have a significant
effect on settling tank performance (See Section 7.5).

Camp (4) urged the use of much shallower settling tanks, theorizing that the higher  velocity
gradients  would accelerate particle agglomeration  sufficiently to more than offset the
reduction in detention time. Fitch (16) disputed this noting  that-in stirred settling tests
velocity gradients comparable to those proposed  by Camp provided little flocculation. In
any  case,  common U.S. practice has remained to design fairly deep tanks with low  forward
velocities  (about 1  fpm at mean flow)  and to depend on some other means than gradients
due  to  forward velocity  to achieve  desired flocculation. Kalbskopf (1) indicated that in
Europe it is common, to design shallower (3  to  10 ft depth) primary settling tanks with
higher forward velocities (2.5 fpm at  mean flow and up to 7.5  fpm at maximum  flow).
Studies for the Emscher Mouth treatment facility  (18) showed only minor variation of
primary effluent SS with  forward velocity. Performance  related much more to surface
loading. For any given surface loading, however, the best performance was at a velocity in
the range between  1.6 and 2.5  fpm.

It is well recognized that increasing velocity  gradients by stirring the inlet zone of a settling
tank can  often improve performance  (1) (4)  (19).  Essentially this  combines mechanical
flocculation and settling in a  single tank. Compartmentation is desirable to reduce short
circuiting.  The major advantage  of such combined units is that  a suspension can be
flocculated  at decreasing G values  (see Section 6.1) down to very low levels and then
delivered  to sedimentation without  subjecting the suspension to the shearing effects of

                                        7-8

-------
collection and redistribution. Recognizing this advantage, several equipment manufacturers
offer combined units designed on this basis. Where flocculation is to be used to upgrade
performance of existing settling tanks,  the possibility of locating flocculation mechanisms
directly in the tanks should be considered.  (See U.S. EPA,  Process Design Manual for
Upgrading Existing Wastewater Treatment Plants).

    7.3.5   Bottom Scour

Where high forward velocities are  used, the possibility  of scouring  previously deposited
sludge should be analyzed. As a rule of  thumb, forward velocities should be limited to from
9 to 15 times the settling velocity of critical size solids to avoid scour  (20).


     7.3.6   Hindered Settling and Compaction

When a  concentrated  suspension such  as  activated  sludge mixed  liquor  settles under
quiescent conditions, a distinct interface develops almost immediately between the sludge
and the clarified liquid above it (21). As illustrated in Figure 7-4, this  interface subsides for
a time at a constant rate. This  rate is termed  the initial settling velocity of the  sludge.
Because  the accompanying upward  displacement of liquid reduces this settling velocity to
below that of discrete particles of the sludge, the process  is termed hindered settling.

As the sludge mass continues to  settle an interparticle structure develops  in the  more
concentrated lower layers and the subsidence rate slows further. Figure 7-5 illustrates this
compaction or thickening of sludge in a full scale tank. If high sludge  concentrations are to
be  obtained, thickening rather than  solids separation may control the tank  sizing. Sizing
secondary settling tanks for activated sludge  to meet thickening requirements is discussed in
Section 7.6.

7.4  Clarifier Design Considerations


    7.4.1   General

In selecting the particular tank shape, proportions, equipment, etc. the designer should:

     1.   Provide  for even  inlet  flow  distribution in a  manner  which minimizes  inlet
         velocities and short circuiting.

    2.   Minimize outlet  currents and  their  effects by limiting weir loadings (see Sec.  7.5
         and 7.6) and by  proper weir placement.

    3.   Provide sufficient sludge storage depths to permit desired  thickening of sludge.

    4.   Provide sufficient wall height to give a minimum of 18 inches of freeboard.

    5.   Reduce wind effects on open tanks by providing wind screens and by limiting fetch
         of wind on tank  surface with baffles,  weirs or launders.

                                        7-9

-------
              CLEAR  WATER ZONE
            HINDERED SETTLING
         A\ CONSTANT COMPOSITION
                    TRANSITION. ZONE
                     VARIABLE COMPOSITION
                               COMPRESSION ZONE
                         ULTIMATE CONCENTRATION
                          TIME
                                                  CYLINDER
                     FIGURE 7-4

SCHEMATIC REPRESENTATION OF SETTLING ZONES
             o.
             UJ
             (9
             UJ
             
-------
     6.  Consider economy  of alternative  layouts  which  can be  expected to  provide
        equivalent performance.

     7.  Maintain equal flow to parallel units. This is most important and often  forgotten.
        Equal flow  distribution between settling units is generally obtained by designing
        equal resistances into  parallel  inlet flow ports or by flow splitting in symmetrical
        weir chambers.

     7.4.2  Inlet Design

Inlet design for rectangular  tanks, where the distance from inlet to outlet is large, is less
critical than for circular tanks where there is generally little separation between inlet and
outlet.

In rectangular tanks flow is distributed over the width of the tank by provision of multiple
inlets. Size and spacing vary considerably from  one design to another. Small openings are
avoided in wastewater applications because of the possibility of fouling. Maximum spacings
are generally less than  10 ft. Target baffles are commonly provided to help dissipate the
velocity of the  inlet jets.  Distribution to multiple  inlets in  a  rectangular  tank  usually
involves a manifold  conduit. A method, developed by Dobbins, for design of  inlets  and
manifold conduits, is presented elsewhere (22).

The common  type of  center feed for circular tanks depends on symmetrical baffling to
distribute flow equally in all  radial directions. The high degree of short circuiting with such
inlets has led manufacturers to  develop several special inlet designs for circular tanks—both
center and peripheral feed.

Figure 7-2b and c show peripheral feed  units.  In these units,  inlet ports discharge outside a
deep peripheral  baffle  and flow passes under  this baffle to enter the tank.  In a peripheral
feed unit manufactured by Lakeside Equipment Corp., the inlet line to the tank discharges
tangentially into a tapered  race located behind a similar skirt baffle. The manufacturer
claims that the tangential motion imparted to the tank contents reduces short circuiting. In
model studies, the latter type of peripheral unit showed significantly higher removals of iron
floe than a similarly loaded  center feed unit (23). This was attributed to better conditions
for particle agglomeration in the peripheral feed model.

A  center  feed   inlet  manufactured  by   Dorr-Oliver,  Inc:   has   two  races   with
tangentialiy-introduced flows rotating in opposite directions. Shear between these  rotating
flows dissipates the energy of the inlet velocity before the inflow leaves the  feedwell (10).

The modular  Energy  Dissipating (MED) Feedwell, manufactured by Envirotech Corp.,
forces all flow to pass through honeycombs of small tubes, mounted vertically around the
entire feedwell periphery. The  manufacturer claims that the honeycomb creates a  laminar
flow pattern with uniform radial velocities and  that  periodically reversing  the modules on
their pivot mountings(changing flow direction through them) will clear the honeycomb of
any accumulated solids.

                                        7-11

-------
Available data comparing performance of primary and secondary clarifiers using special
and conventional inlets are presented in Table 7-1.
    7.4.3  Economy

The two major elements in settling tank cost are the structure and the sludge collection
mechanism. Installed cost for the mechanism is typically 30 to 40 percent of the structural
cost.  Structural costs for multiple- rectangular and  circular tanks (horizontal flow)  are
comparable,  provided common-wall construction is used for the rectangular units (1) and
liquid depths are not more than about 10 ft. At greater depths, circular units with tank walls
designed as hoops show increasing savings. Single circular units are less expensive than the
same  size rectangular basin. Where tanks must be covered, costs  may favor rectangular
units  because of their shorter roof spans. European  data (1) indicate structural costs for
vertical flow  units may run 50 percent higher than horizontal units of the same volume, but
the vertical tanks have deep conical bottoms eliminating the need for costly sludge collector
mechanisms.
 Rotary collectors for circular tanks generally cost 20  percent less than chain-and-flight
 collectors for comparable rectangular units. In addition, maintenance requirements for the
 rotary  units are  decidedly lower. Travelling-bridge  collectors  for  rectangular tanks
 apparently  compete favorably  with  rotary circular  collectors  in  cost and  ease  of
 maintenance. They  are common  in Europe but until  recently have not found widespread
 application in the U.S.  A recent comparison  (27) for secondary tanks showed a floating
 travelling bridge collector with siphon sludge drawoffs to be decidedly cheaper than either
 chain-and-flight or circular mechanisms.
     7.4.4   Skimming

 In rectangular tanks with chain-and-flight collectors, skimmings are moved toward their
 discharge point by return travel of the flights at the tank surface (Figure 7-1 A). In circular
 tanks skimmings are moved by travel of a surface arm attached to the  rotary collector
 (Figure 7-2). A surface arm can be similarly used in rectangular tanks with travelling-bridge
 collectors. Discharge of scum from the  settling tank may be continuous  or intermittent
 depending on quantity produced. Skimmer mechanisms are of two types: dipping-weir and
 sloping-beach. In the first, a slotted tilting pipe or other weir device is positioned during
 skimming so that scum overflows from the tank together  with considerable water. In
 sloping-beach  units  scum  is raked mechanically  up a beach leaving most  of the water
 behind. The latter are simple to provide on circular tank mechanisms where they are almost
 standard.  For rectangular  tanks  .with chain-and-flight  collectors, however,  a separate
 mechanism is required  to move scum up the sloping beach. In this application, use of sloping
 beach rather than weir type  it is desirable to minimize the moisture content in the scum and
 facilitate subsequent handling. Where scum is to be pumped  away from the tanks the less
 expensive weir-type skimmers are generally preferable.

                                        7-12

-------
                TABLE 7-1
PERFORMANCE OF SPECIAL SETTLING TANK INLETS
                           Loading
Effluent SS
Special Inlet Type and
Application

Peripheral Feed (Rex-
Nord) — Activated
Sludge Final Clarifier

Peripheral Feed
(Lakeside Equipment)
Primary Clarifier
Modular Cell Inlet
Feed well (Enviro-
tech)
Location

Ann Arbor,
Mich.
Sioux Falls,
S.D.
Ewing-Lawrence,
N.J.
Odgen, Utah
Test
Period

5/22/61
to
6/22/61
8/1 2/58 to
9/27/58
6/70 to
12/70
10/70 to
5/71
Special
Inlet
gpd/sq ft
1408
2015
1000±
520-715
850-950
950-1150
Conventional
Center Feed
gpd/sq ft
951
401
1000±
520-715
850-950
950-1150
Special
Inlet
mg/1
11
30
67
31
31
31
Conventional
Center Feed
mg/1
14
30
81
46
48
54
Refer-
ence

24
24
25
26
26
26

-------
7.5  Primary Sedimentation

In theory, sizing of primary tanks may be regarded as a question of economics. Successive
increments of tank area (providing lower loadings and longer detentions) typically yield
diminishing returns in performance. At some point it becomes more economical to accept
higher loads in subsequent units rather than provide more  primary tank capacity. Some
designs have even omitted primary tanks entirely. Von der Emde (28) has indicated this may
be advantageous if one or more of the following conditions apply:

     1.   Sludge from the facility is  to be pumped away for treatment elsewhere

     2.   Problems are expected with odors in  primary tanks  or poorly settling sludge in
         secondary tanks

     3.   Aerobic digestion or extended aeration processes are to be used.

As a  practical  matter,   performance-loading relationships  adequate for  use  in  cost
optimization studies  can  currently be obtained only  by  extensive testing  of the actual
wastewater in existing full scale or pilot facilities,  taking  into  account  variations in flows
and  characteristics. The generalized performance-loading curves for sedimentation units
available in the literature  (2) (8) (19) (29)  are unsatisfactory even as a basis for predicting
performance at particular design loadings much less for  cost  optimization studies. Such
curves are based  on average daily plant flows from diverse  sources, and ignore effects of
diurnal  flow variations and of major in-plant flows such as waste secondary sludge, which
may be recycled to the primary tanks. The effect of  su'ch  unaccounted-for factors may be
seen in the  wide  scatter of removal-loading data  plotted in the WPCF/ ASCE  Sewage
Treatment Plant Design Manual (2).

In the absence of reliable performance-loading relations, primary tank  designs may be
based on the typical parameters shown in Table 7-2.

                                    TABLE 7-2

        TYPICAL DESIGN PARAMETERS FOR PRIMARY CLARIFIERS

                                          Hydraulic Loading
     Type of Treatment                 Average	Peak                Depth
                                               gpd/sqft                         ft
  Primary Settling Followed
  by Secondary Treatment            800-1,200       2,000-3,000               10-12

Primary Settling  with Waste
  Activated Sludge Return            600-800          1,200-1,500               12-15

Sizing should be calculated for both average and peak conditions (if flow equalization is not
used) and larger size used.
                                       7-14

-------
These parameters are applicable to normal municipal wastewaters primarily of domestic
origin and should provide SS removals of 50 to 60 percent.


Weir loading limitations between  10,000 and 30,000 gpd/ ft (24-hr basis) have been  sug-
gested for primary tanks (19) (29). At usua.l surface loadings, up to 1200 gpd/sq ft, round
tanks with single peripheral weir fall in this range for all but very large diameters (> 100 ft).
Thus normal practice is  to provide only the  single weir.  In contrast, at surface loadings
as low as 600 gpd/sq ft  rectangular tanks with single transverse weirs across the effluent
end exceed this range if the tank length is over 50 ft. Although rectangular tanks with  weir
rates of  more than 100,000 gpd/ft have shown SS removal  in the normal range (30), rec-
tangular tanks are commonly equipped with  multiple weir troughs to provide loadings of
30,000 gpd/ft or less. However, weir loadings are not as critical for primary tanks as  they
are for secondary clarifiers.
Sludge  solids can be estimated directly from the expected  SS removal,  making sure to
include waste activated sludge returned to the primary tank in the solids loading. The sludge
volume can be calculated based on expected concentration. If sludge is properly thickened
in the primary tank and pumping is carefully controlled to avoid pulling excess water, solids
concentrations of 2 to 7 percent may be obtained.  On this basis typical primary sludge vol-
umes for domestic sewage would range from 0.2 to 0.5 percent of plant flow. The concen-
tration  used in  particular estimates should be based on actual plant experience or at least on
settling/thickening tests. Quantities of skimmings are quite variable. On a sustained  basis
few plants average over 1 cu ft/mg (free water decanted) but scum handling facilities should
be capable of moving peak loads of perhaps six times this amount.
 7.6   Secondary Sedimentation

     7.6.1   Tank Sizing—General

 The approach to sizing secondary clarifiers varies with the type of biological process they
 serve.


 7.6.1.1  Tank Sizing For Trickling Filter Effluent

 Clarifiers following trickling filters are basically sized on hydraulic loading. Solids loading
 limits are not involved in this sizing. Where further treatment follows sedimentation, cost
 optimization may be considered in sizing the settling tanks, but the effort of developing ade-
 quate performance—loading  relations  is seldom  justified. Typical design parameters for
 clarifiers following  trickling filters are presented  in  Table 7-3. In applying the hydraulic
 loading values from the table to design, sizing should be calculated for both peak and aver-
 age conditions and the largest value determined should be used. At the indicated hydraulic
 loadings, settled effluent quality is limited primarily by the performance of the biological
 reactor not of the settling tanks.

                                        7-15

-------
                                     TABLE 7-3
       TYPICAL DESIGN PARAMETERS FOR SECONDARY CLARIFIERS
Type of Treatment


Settling Following
  Trickling Filtration

Settling Following
  Air Activated
  Sludge (Excluding
  Extended Aeration)

Settling Following
  Extended Aeration

Settling Following
  Oxygen Activated
  Sludge with
  Primary Settling
 Hydraulic Loading
Average	Peak
    ' gpd/sq ft
400-600     1,000-1,200
400-800     1,000-1,200
200-400
800
400-800  ,   1,000-1,200
               Solids Loading*
             Average	Peak
             Ib solids/day/sq ft
              20-30
20-30
             50
50
                                    10-12
         12-15
12-15
              25-35
             50
         12-15
*Allowable solids loadings are generally governed by  sludge thickening characteristics
associated with cold weather operations.

  7.6.1.2  Tank Sizing For Activated Sludge Mixed Liquor

Activated sludge settling tanks have two distinct functions: solids separation and production
of a concentrated return  flow to sustain  biological treatment. Figure 7-6 illustrates how
important final  tank underflow concentration is in maintaining the level of active solids
(and hence treatment) in the aerator mixed liquor.

As  indicated in Section  7.3.6,  the  initial separation of  activated  sludge solids  involves
hindered rather  than discrete settling.  For this type of settling, tanks must be sized so the
maximum surface hydraulic loading is less than the minimum initial settling velocity (ISV)
expected  at maximum  mixed liquor concentration and  at minimum temperature. If the
hydraulic loading exceeds the  ISV  massive failure  and  overflow  of solids will  result.

To  perform properly while producing a concentrated return flow, activated sludge settling
tanks must be designed to meet thickening as well as solids separation requirements. The
critical  element  in thickening is the rate at  which solids are transported downward and
removed in the tank underflow.  This is termed the solids transport or solids flux capacity,
generally expressed  in the units of  solids loading. Ib/sq ft/day. When the actual solids
loading applied to a tank exceeds its transport capacity, solids are being added faster than
                                       7-16

-------
Q+R
Q=WASTEFLOW V C=MLSS
i
f 0-1.
'•*«t"AERATloN°**oo^
i%"*. *V JANK * o »*•! « V
R« RECYCLE
C= Cu
R) Ul Aft = D X r,i
Q-f R , V
r - SECONDARV
SETTLING
MAHK -
^ 	 "5=
<
V
<
r* — ^
C=Cu
iQ
1 - WASTE
; = cu
     MLSS =
             Q + R
                    Cu
              FIGURE 7-6

DEPENDENCE OF MLSS CONCENTRATION ON
      SECONDARY SETTLING TANK
      UNDERFLOW CONCENTRATION <31)
                 7-17

-------
they are being removed. If this condition persists the blanket of solids in the tank will build
up and eventually overflow with drastic effects on effluent quality. If significant solids are
lost  from the system, biological treatment efficiency will be impaired. Tank depth may be
important in containing blanket buildup from diurnal peaks in solids loading.

Dick (31) has analyzed solids transport capacity assuming both solids and (vertical) under-
flow velocity uniformly distributed  over the plan area of the tank. Although these condi-
tions are approximated only in moderate size circular tanks, this analysis provides major in-
sight into the thickening process and represents the only rational and straightforward ap-
proach currently available for estimating solids transport capacity. Under the conditions as-
sumed, solids transport capacity depends on only two factors: the thickening characteristics
of the  solids (i.e., the relation between subsidence rate and concentration within the sludge
blanket) and the tank underflow rate. To get a concentrated underflow requires a low sludge
return rate  which in turn means low solids transport  capacity. High underflow rates  have
been resorted to for handling poorly compacting sludges. This is only partly effective since
while increasing solids  transport capacity, higher underflow rates also increase solids load-
ings due to  the higher sludge recycle.

Methods for developing hydraulic and solids loading  parameters from tests of settling and
thickening characteristics are discussed in Section 7.6.3. Typical design parameters for clar-
ifiers in activated sludge systems treating domestic waste are given in Table 7-3. In applying
hydraulic and  solids loading values from this table,  sizing should be calculated for both
peak and average conditions and the largest value determined  should be used.

Settling tests provide worthwhile guidance in selecting design loadings. They should certain-
ly be included wherever pilot study of biological treatment is warranted by unusual waste
characteristics or treatment requirements. Testing is essential in any case where proposed
loadings go beyond the upper limits shown in Table 7-3.

Sizing activated sludge settling tanks according to proper hydraulic and solids loading  para-
meters  protects against massive failure, but does not by itself'guarantee high quality
effluent. After separation of the mass of activated sludge solids, significant quantities of small,
slowly settling particles may still be left in the clarified liquor. The amount and character of
such residual suspended solids logically relate to the loading and  operating conditions in the
aeration tank, but few spe'cific studies have  explored  such relations. A study in Baltimore,
Md. (32) indicated that sludges with poorer thickening characteristics left lower residual
solids in the effluent. This was confirmed by studies covering a number of plants in Sweden
(33).

As noted in Chapter 6, the concentration of these residual solids can be reduced by floccula-
tion of the mixed  liquor between aeration and settling,  or by use of recirculation-type or
sludge-blanket-type solids  contact reactors. Finally, although rates  are lower, flocculatiori
in the clear water  zone still appears to be a significant mechanism in removal of solids not
already entrapped  in the sludge mass as it settles. This indicates that basin depth and deten-
tion are important in getting effluent SS down to low levels. Mixed liquor settling tests run
at several treatment. rUants,ia,Sweden (33) showed  that residual turbidity above the  sludge
         Mail code 3404T
  1200 Pennsylvania Avenue NW          7-18
     Washington, DC 20460
          202-566-0556

-------
interface dropped significantly over the first hour.

    7.6.2  Development of Loading Parameters from Mixed Liquor Settling Tests


    7.6.2.1   Surface Hydraulic Loadings

Initial settling velocity (ISV) at actual mixed liquor concentration may be determined in a
single test simply by plotting the height of the sludge-liquid interface vs. time and noting the
slope of the straight line portion of the plot. The critical minimum ISV value for a particu-
lar system may be estimated from results of a number of individual tests. The designer
should attempt to establish relations between ISV and biological process parameters such
as mixed liquor concentration and organic loading. The selected ISV value should then re-
flect conditions most unfavorable to  settling including correction for minimum expected
temperature. Finally a capacity factor as discussed  in Section 7.3.1 should be applied to
convert the critical ISV to a hydraulic loading.

The resulting maximum surface hydraulic loading should not be exceeded by any sustained
maximum flow (say 4-hr duration). Initial settling velocities for mixed liquor from air acti-
vated sludge systems have been reported to  range from 3 ft/hr to over 20 ft/hr (4) (21) (34).
For good settling (non-bulking) air activated sludges from municipal wastewaters the  fol-
lowing design relation between the ISV and the mixed concentration has been suggested
(34):
                                  Vi = 22.5e -338Cl

    Where:
                             Vi  = settling velocity in ft/hr
                             Ci  = concentration in Ib/lb

Bulking sludges will show ISV values well  below those indicated by this  line. Sludges with
superior settling qualities may show considerably higher values.

   7.6.2.2  Sludge Volume Index

The sludge  volume index  (SVI)  widely  used to guide operating control of the activated
sludge process, provides an approximate indication of sludge compaction characteristics.
The index is calculated by dividing the initial mixed liquor  SS concentration (percent) into
the settled volume (percent of initial volume)  occupied by the solids after one half hour
of settling.

The reciprocal of the SVI is often taken as an approximate indication of the maximum re-
turn sludge concentration which can be obtained with a given mixed liquor (100/SVI = per-
cent solids). The index has been  used as a guide to  sizing  return sludge pumping require-
ments to maintain different mixed liquor concentrations (2). Although  the SVI does not
give a direct indication of solids  transport capacity, it has been suggested that for index
values of less than  100,.underflow concentrations below 1 percent and mixed liquor concen-
trations below 3000 mg/1, hydraulic rather than loadings will govern clarifier sizing (2).

                                       7-19

-------
    7.6.2.3  Solids Loading

Based on the analysis discussed in Section 7.6.1, Dick (31) has proposed a method for deter-
mining  limiting solids transport capacity as a function of underflow rate, given a curve or
equation  defining the relation of settling velocity to concentration. Dick and Young (35)
have  formulated the method into a series  of equations, assuming that the settling veloc-.
ity-concentration curve could be represented in the form:

                                      V = a.c-n

    where
                  V is settling velocity
                  c is concentration
    and
                  a and n are appropriate  constants for the units used.

The  most  serious  problem  in applying the method is determining the  settling veloc-
ity-concentration relation. Dick suggests developing the relation, from a series of ISV tests
on the same mixed liquor at different initial concentrations (obtained by settling, decanting
clear liquid and resuspending the solids). There is a serious question whether a curve devel-
oped from such tests really represents the behavior of the solids in the sludge blanket of a
clarifier.  Nevertheless  this approach is the best presently available for estimating  solids
transport capacity fom settling tests. Others  suggested (6) are  open to even more serious
objections.

In translating solids transport capacity to an allowable solids loading some safety  factor
may  be needed to allow for possible critical conditions (temperature, poor thickening char-
acteristics, etc.) not reflected in the test work.

In a design application trial solutions at different return sludge rates may be justified  to de-
termine the effect on tank sizing of the different solids loadings and capacities that result at
the various underflow rates. Sizing should be based on peak solids loadings associated with
sustained maximum flows unless specific testing has justified a reduction taking advantage
of storage of peak  solids by  increases in  sludge blanket height. Such storage  should be
avoided with  nitrifying sludges (34).
    7.6.2.4  Settling Test Procedures

 Although it has been demonstrated (36) (37) that factors such as column diameter, sludge
 depth,  dissolved oxygen and  application of stirring can significantly affect the results of
 settling tests, standard  values for such factors or standard allowances for their variations
 have not been adopted. Dick (36) has detailed test procedures and indicated (38) preference
 for use of-sludge depths of 3 ft. column diameters of 3.5 in. or more and slow stirring at tip
 speeds of 10 in./ min.

                                         7-20

-------
   7.6.3   Flow Stabilization and Density Currents

Two approaches to preventing short circuiting from density currents, were described in Sec-
tion 7.3.2: dynamic stabilization and density stabilization. An exhaustive study of shallow
activated  sludge settling tanks has been made in Sweden (33). Included were tracer studies
on  tanks  under actual operating conditions and parallel quiescent and stirred settling col-
umn tests on the mixed liquor. These tanks, although designed for dynamic stabilization,
showed serious short circuiting. Due to flocculation in the tanks, however, effluent quality
was better than predicted from the quiescent settling results and actual detention times.

In  the U.S. where final settling tanks commonly have design depths  10 ft or more, flow
stabilization depends totally on density.  Unfortunately studies of the type conducted in
Sweden have apparently not been run  on tanks designed for density stabilization. In the
side-by-side performance  tests (Section 7.4) comparing  special inlet designs for circular
tanks with conventional center feedwells, density stabilization could have been important,
but no data were  taken to show the degree of short circuiting. Neither were any parallel
settling column tests run. Tracer studies of these special inlets generally have been run on
clear water, so they fail to show any effects of density stabilization. Even without these ef-
fects, special inlets displayed less short circuiting (13) (14).

In an attempt to minimize undesirable  density current effects, several designs have varied
the placement of sludge drawoffs and effluent weirs in relation to the inlet.  Sawyer (39)
pointed out the "submerged waterfall effect" that occurs when the density current reaches
the tank floor in its initial downward sweep. In conventional rectangular or circular tanks
where the sludge drawoff is located below the inlet the impact of the "waterfall" can dilute
the collected sludge and resuspend a  portion of it. Peripheral-feed circular tanks avoid this
problem as do those equipped with suction-type mechanisms which remove  sludge from the
entire tank floor. Even in tanks with centerfeed and center sludge drawoff, use of deep feed-
wells discharging at low velocities can minimize the problem (10). Rectangular tanks have
been constructed with  sludge drawoffs located  away from the inlet. Excellent results have
been obtained at New  York City with sludge drawoffs at mid-length of the  tanks (30). This
arrangement uses the density current to speed sludge removal but prevents  the density cur-
rent from  entering the outlet zone of the tank. Rectangular tanks may also be equipped with
suction-type sludge removal on traveling-bridge  mechanisms.

In one special rectangular tank arrangement, effluent weirs are distributed over the length
of the tank, with baffles in the upper part of the tank to impede counter currents induced by
density current below and force vertical flow to the weirs. In peripheral feed circular tanks,
tests have shown that  units with  peripheral drawoffs located just inside the inlet channel
produce better effluent than units with weirs located more toward the tank  center (40).

Weir hydraulic loadings of 15,000 gpd/ft at average design flows are  suggested in the Ten
State Standards (29), with allowances of up to 20,000 gpd/ft where weirs are located so that
density  currents do not upturn below them.  Loadings of up  to 100,000 gpd/ft have been
used without apparent problems in designs such as those of New York City where weirs are
well separated from density current effects (30).

                                        7-21

-------
    7,6.4  Sludge and Skimmings Removal

Suction-type sludge removal should be considered wherever sludge detention in the tanks is
critical and  doubt exists about conveyance time for other mechanisms. Desirable features
in suction-type mechanisms include independent flow controls for each suction drawoff and
visible gravity sludge discharges.

Federal guidelines (41) require skimming equipment on secondary settling tanks to remove
floating sludge and any oily materials not separated in previous treatment. Scum quantities
generally  are small in  relation to  those from  primary tanks (0.1 cu ft/mg).  Where  no
primary tanks are included in plant process, scum quantities from secondary tanks could be
conservatively estimated on the same basis as for primary tanks.  Effluent weirs should be
laid out to permit skimming the maximum possible portion of the tank surface.

Maximum practical concentrations of underflow  from secondary clairifiers  in  activated
sludge systems range from 0.5 Jo 2.0 percent solids, depending on settling and compaction
characteristics of the  sludge. Actual concentrations depend on the return sludge pumping
rate. Sludge concentrations of 3 to 7 percent solids may be obtained from secondary clari-
fiers iff trickling filter systems.
   7.7  Chemical Sedimentation

 Sedimentation of chemically coagulated or precipitated wastewaters is similar to sedimen-
 tation of wastewaters without chemicals. The design of tanks can proceed on essentially the
 same basis, provided special consideration is given to the effects of chemical treatment on
 settling characteristics, sludge quantities, resistance of the sludge to movement by collecting
 and pumping equipment,  and the special maintenance problems  encountered with  lime
 coagulation. Few data have been reported concerning settling characteristics of chemically
 precipitated floe in wastewater treatment. Some data are available on chemical precipitation
 in water treatment using similar chemicals (42).

 From the literature it is apparent that actual surface loadings vary  considerably from one
 application  to another (2) (43) (44) (45) (46) (47) (48) (49). This wide variation emphasizes
 the importance of testing and pilot work in designing chemical precipitation facilities. In the
 absence of testing indicating higher figures to be satisfactory, the following typical surface
 loading rates may be used for sizing tanks (47) (50):

                       Chemical                     Peak Surface Loading
                                                          gpd/sq ft

                       Alum                               500-600
                       Iron                                 700-800
                       Lime                               1400-1600

 In general, these design rates may be used for primary, secondary or tertiary applications.
 It should be noted, however, that they are based on limited data and may be revised when
 more experience is available.

                                       7-22

-------
Sludge quantities from chemical precipitation can be estimated from the SS removal and
the stoichiometry of chemical reactions involved. Volumes depend on sludge concentrations
which are highly variable (1  to  15 percent) and are best determined by actual test. Equip-
ment suppliers should be consulted about strength  and power of  collector equipment to
handle the dense sludges expected from lime precipitation.  Extra smooth piping glass-lined
or PVC, should be  used for lime sludges. Average sludge productions determined from raw
wastewater coagulation by lime, iron and alum are 6,500, 1740 and 1120 Ib/mg respectively
(47). Average sludge volumes for the same locations for lime and iron are 10,000 and 13,000
gal/mg, respectively (47). Brown (51) observed a  sludge production of 1894 Ib solids/mg
(6275 gal/mg) using alum for precipitating trickling filter effluent."

7.8  Flotation

    7.8.1  Applications

This section deals  with flotation induced by introduction  of fine gas bubbles into waste-
water. Since most  SS in municipal wastewater have specific gravity values only slightly
above 1, adhesion of the gas  bubbles to the solids particles readily makes them buoyant.

For flotation of solids in municipal wastewater, gas bubbles must be quite fine (.01 to 0.1
mm); otherwise, their own rise rate prevents  significant adhesion to the solid particles.
Three  methods of introducing gas bubbles have been shown to  create bubbles sufficiently
fine for flotation of municipal wastewater SS. Vacuum flotation and dissolved-air flotation
(DAF) both create conditions in which the wastewater  is supersaturated with air at some
pressure. Upon reduction of  that pressure, air comes out of solution as finely-divided bub-
bles.  Auto-flotation can  occur in  algae suspensions  if they become sufficiently super-
saturated with dissolved  oxygen from photosynthesis. Vacuum flotation and autoflota-
tion are not often used because the former is expensive and the latter can only operate under
limited conditions of warm weather and bright sunshine (52). Diffused or submerged tur-
bine aerators create bubbles much too coarse for flotation of municipal wastewater solids.

Pressure and  vacuum  flotation  units have found only limited application in treatment of
municipal wastewater. It has been difficult to justify using these units in conventional appli-
cations such as primary SS removal or mixed liquor clarification because sedimentation is
ordinarily cheaper, simpler and  often provides better results.

Advantages which might favor use of flotation in special applications include: 1) higher sur-
face loadings, hence smaller tanks sizes  (important where space is critical); 2)  ability  to
handle peak seasonal  loads  or  storm  flows (in some designs flotation may be used  inter-
mittently to increase capacity of settling tanks); 3) effectiveness in removing solids which
. are difficult to settle.

Dissolved-air  flotation has been suggested for separation of grit and scum in a single treat-
ment unit (2) (53) (54). Performance data for such applications are lacking, however. Be-
cause  it can produce a float  of much thicker consistency than settled activated sludge, dis-
solved-air flotation has been tried for mixed liquor solids  separation. Full scale studies  at

                                        7-23

-------
Manassas, Va. (55) indicated that this application was not economically competitive with
conventional systems.

   7.8.2  Dissolved-Air Flotation

As shown in Figure 7-7, dissolved air flotation (DAF) units commonly employ rectangu-
lar tanks  with separate chain-and-flight scum and sludge  collectors. Circular units are also
commercially available. The widest application  for these units has been  as thickners for
waste activated sludge. Units used for SS separation are similar, but design parameter val-
ues  vary according to  the  application. To avoid  fouling of  pressurizing  and  pres-
sure-regulating equipment and excessive shearing of influent solids, a stream of recycled ef-
fluent is usually pressurized. Upon pressure  release, this stream is blended with the inflow
to be treated. Other methods include pressurizing all or part of the influent stream.

Design of DAF units involves selection of values for a  number of parameters including per-
cent recycle flow, operating pressure, pressurization retention time, air flow, and surface
hydraulic loading,  solids loading (area basis) and float detention period. Variables reflec-
ting  influent characteristics include flow, solids loading,  liquid temperature and type and
quality  of influent  solids.  Investigators have attempted to  relate  flotation performance to
the air to  solids  ratio and a number of other variables with  a  limited amount of success.

Mulbarger and Huffman (56) noted that float concentrations depend more on float deten-
tion  time  than solids loading. They related capture in  flotation to a parameter equal to the
air to solids ratio divided by the product of surface hydraulic loading and dynamic viscosity.

Values of specific parameters used in  actual applications vary widely. Typical ranges cited
are as follows (2) (50) (56) (57) (58) (59) (60) (61):
              Parameter:                                   Range:

              Pressure, psig                                25 to 70
              Air to Solids Ratio, Ib/lb                     0.01 to 0.1
              Float Detention, min                          20 to 60
              Max. 24-hr
              Surface Hydraulic Loading, gpd/sq ft          500 to 4000
              Recycle, percent                              5 to 120

Available data from specific applications are summarized in Table 7-4.

In flotation equipment special attention must  be given  to the  inlet, outlet and  collector
mechanism configurations. The flotation tank must permit aggregate rise with a minimum
of interference in the form of turbulence or obstructions and provide for removal of floated
froth, settled  sludge and treated effluent. Effluent ports must be sufficiently submerged to
prevent interference with the froth on the surface. The inlet conditions of the flotation tank
are critical to proper performance. Baffles, walls, and other obstructive energy-dissipating
devices tend to destroy aggregate bonding with resulting loss in flotation efficiency. Also,

                                        7-24

-------
                      SLUDGE REMOVAL MECHANISM
EFFLUENT
  ^WRECIRCULATION PUMP
FEED
                                       RETENTION TANK
                                       AIR DISSOLUTION
                                                         INFLUENT
RECYCLED
FLOW  —5
                                                                     SLUDGE
                                                                     DISCHARGE
                             REAERATION PUMP
                             FIGURE 7-7
                  SCHEMATIC OF A DISSOLVED-AIR
                          FLOTATION UNIT
                     (Courtesy of Komline-Sanderson)
                                7-25

-------
                                                               TABLE 7-4
10'-
o\'
                                               DISSOLVED-A1R FLOTATION APPLICATIONS
                                                                              Flotation

Plant

Aker, Sweden
Klagerup, Sweden
Salemstaden,
Sweden
Bara, Sweden

Kungsor, Sweden
Design ,
Flow
mgd
0.4
0.12
2.16

0.08

1.93
Type of
Municipal
Wastewater

Primary Effluent
Primary Effluent
Primary Effluent

Aerator Mixed
Liquor
Unsettled Trick-
Chemical
Treatment

Coagulant

Alum(a)
Alum(a>
AIum(a)

Alum

AJum(a)

Dose
mg/1
100
159
175

-

145
Surface
Hydraulic
Loading
gpd/sq ft
2360
1180
2540

2480

4480
Detention
Time
hrs.
0.31
0.37
0.26

0.24

0.20
SS Performance Data

Inf. Eff. Removal
mg/1 mg/1 percent
71(0
77(0
60(O

_ _ _

97(d)

Ref. Remarks

(62)
(62)
(62)

(62)

(62)
        Flen, Sweden       2.54
        Prince William      1.0
          County, Va.

        Bellair, Texas       (b)

        Stockton, Calif.     (b)
  ling Filter
  Effluent
Unsettled Trick-
  ling Filter
  Effluent
Aerator Mixed
  Liquor

Aerator Mixed
  Liquor
Lagoon Effluent
Alum(a)
None
Cationic    8
  Polymer  30
Alum      75-225
3360
 360
0.35
3.4
           0.32
       30
       to
       100
2000   70
       17
94     12
to     to
152    20
                       87
                       (62)
(56)   Limiting solids
       loading 15 Ib/lb
                 SF
(63)

(52)   Includes filtration
       (a)  30-60 min. of flocculation provided before flotation,  (b) Pilot Plants.
       (c)  BOD removal; no SS data given,    (d)  P removal; no SS data given.

-------
turbulence in the. region of the froth will result in losses of floated solids. Ettelt (58) report-
ed several different designs of inlet  structures in his prototype units. His tangential flow in-
let appears to offer considerable promise where such designs are compatible with the entire
structure.

Further discussion of design features of dissolved-air flotation units may be found in the lit-
eVature (56) (58) (59) (60).

7.9  Shallow Settling Devices

The potential advantages of multiple tray shallow settling devices have long been recognized
(3) (4), but early prototypes of such equipment were unsuccessful due to practical problems
of flow distribution and sludge removal. In recent  years, shallow  settling  devices of im-
proved design, such as tube settlers, have been applied to water and wastewater treatment.
Tube settlers consist of bundles of small plastic tubes with hydraulic radii ranging from one
inch upward and lengths of 2 ft or more, depending upon the particular application. Square
tube sections are most common but hexagonal and other shapes have been used by various
manufacturers.

Tubes are commonly inclined  steeply (60°) to horizontal and fabricated in modules, as
shown in Figure 7-8. These modules have beam  strength which permits their installation in
settling tanks, as shown in Figures 7-9 and 7-iO. Clarifier influent is introduced beneath the
tube modules. The flow passes upward through the modules with the solids moving  counter-
currently by gravity (Figure 7-11) and falling from the tube bottoms into the sludge collec-
tion zone beneath. The clarified effluent is collected above the tube modules.

Free standing package units with tubes only slightly inclined (5°) have found some appli-
cation in small chemical clarification/ filtration systems for tertiary wastewater treatment.

Tube settlers promote sedimentation in three ways: 1) the multiple tubes stacked one above
another provide an effective  settling area several times that of the projection in plan of the
modules;  2) the small hydraulic radius of the tubes  maintains  laminar flow  and promotes
uniform flow distribution; 3) in steeply inclined  tubes, the movement of sludge against the
direction of flow favors particle contact and agglomeration. This additional flocculation off-
sets the reduction in their horizontal projected area caused by inclining the tubes. For alum
floe suspensions a given length.tube has been shown to provide most effective removal at an
inclination of about 45 degrees, and performance even at 60 degrees was comparable to that
when horizontal (64).

Tube settlers have been promoted both for reducing required size of settling tanks and for
improving their performance, but manufacturers presently tend to emphasize  improved per-
formance and recommend the same surface hydraulic loadings for tanks equipped with tube
settlers as for conventional tanks.

Comparative data on  performance of tanks with  and  without  tube settlers (either
side-by-side or before-and-after)  are  shown in Table 7-5. The data are quite limited and

                                       7-27

-------
^1

NJ
OO
                                                    FIGURE 7-8

                                         MODULE OF STEEPLY INCLINED TUBES

                                            (Courtesy Neptune Microfloc, Inc.)

-------
                   FIGURE 7-9
       TUBE SETTLERS IN EXISTING CLARIFIER
SUPPORT MODULE
 TUBE  SETTLER
    MODULES
                   FIGURE 7-10 .
         PLAN VIEW OF MODIFIED CLARIFIER
                      7-29

-------
DIRECTION OF
   FLOW
                 TO SLUDGE
                 COLLECTION
           FIGURE 7-11
  TUBE SETTLERS - FLOW PATTERN
              7-30

-------
                                                           TABLE 7-5
                                                 TUBE SETTLER INSTALLATIONS
  Plant Location

Hopewell Township,
  Pennsylvania
Trenton.
  Michigan
Lebanon.
  Ohio
Operational Data
Using Tube Settlers

Type

Activated
Sludge
Activated
Sludge
Activated
Sludge
Plant

Design
mgd
0.13

6.5

0.75

Flow

Actual
mgd
0.13

5.6

1.25

Existing Facility
Location

Secondary
Clarifier
Secondary
Clarifier
Secondary
Clarifier
Overflow Rate
gpm/sq ft
0.34
-
-

0.61

Effluent SS
mg/1
60-70
-
-

61

Tube Over-
flow Rate
gpm/sq ft
-
2
0.56

0.85

Tank Over-
flow Rate
gpm/sq ft
—
0.68
0.29

0.61


Effluent SS
mg/1
—
27
8

30


Reference

65

66

67


-------
rather inconclusive as to the benefits obtained from the tube settlers.

Tube settlers have found  wider application  in water treatment  than  in wastewater. For
wastewater, tube settlers may find their best applications in tertiary coagulation and settl-
ing. They also may be of help in upgrading performance of units with serious short circuit-
ing problems.

When installed, settling tubes usually cover one half to two thirds of the basin area. To pre-
vent fouling of tubes, the remaining area between the inlet and tube area is arranged to pro-
vide scum removal. The portion of the basin equipped with settlers should have collecting
weirs at  15 ft or closer spacing to induce an even vertical flow distribution and reduce short
circuiting. '

Tube settler installations require a support grid (usually designed  to support one man) and
surface baffles to separate  the tube settler and scum collection area. Minimum basin depths
should be 10 to 12 ft.

Long term studies have revealed that in wastewater treatment the upper surface of the settl-
ing tubes becomes coated  with  sludge (68). Long  term fouling of the tubes with grease  or
rags has  not been a problem, but in order to maintain a high degree of  solids removal, it is
generally necessary to install an air grid and periodically interrupt flow to introduce air to
remove the sludge build-up on the tubes.

A typical air  wash cycle consists of draining the tank to  the level of the tubes and then  al-
lowing air to rise up through the tubes. The air is supplied either  from a fixed  grid or a
scour system attached to the rake arm. After the air wash, approximately 15 to 25 min is  re-
quired for the effluent SS to return to normal, e.g., from 60 mg/ 1  to  10 mg/1 before the
unit can  be returned to service (68). A short quiescent period of no flow may also be needed
between  the drain down and the air wash(69). Generally, required  cleaning frequency varies
from one week to several  months (70). Where serious sludge carryover conditions are ex-
perienced, however, it has proven difficult to prevent fouling with even  the highest cleaning
frequency (71).

7.10   Wedge-Wire Settler

Wedge-wire settlers are wire matrices installed in clarification basins similarly to tube set-
tlers. They are designed to improve the quality of  settled effluent from activated  sludge or
trickling filter treatment.

The equipment can be installed in any conventional clarifier configuration. The settling de-
vice consists of a matrix constructed from parallel wires (See Figure  7-12) suspended be-
neath and parallel with the surface of the water so that the wastewater must pass upward
through  the  mesh before  reaching  the  effluent  weir.  Over 200 secondary clarifier  in-
stallations in  England are  equipped with wedge-wire settlers (72).

Typically, the wire in the matrix is triangular in cross-section and arranged with apex point-

                                        7-32

-------
                                 TANK WALL
     INFLUENT
                                                FINAL EFFLUENT  OUT
                                            WEDOE WIRE PANELS
                                    FIGURE 7-12 ;

                          SIMPLE WEDGE WIRE CLARIFIER
                   4x3 TIMBER FRAME
                   SECURED BY RA6BOLTS TO
                   WALLS.
                                                  WEDGE WIRE PANELS.
          2x2 ANGLE
          IRON.
METAL BAFFLE PLATE.
 4 1/2" WALL BUILT
 UP FROM TANK
 FLOOR.
OUTLET WEIR
                                    FIGURE 7-13

                     INSTALLATION OF WEDGE WIRE PANELS
                                        7-33

-------
ed downward, to provide 0.125 to 0.250 mm openings at the top surface. The openings com-
prise about 15 percent of the total area. Construction is of either stainless steel or aluminum
and the wires must be  rigidly fixed. The wire "rack" is supported within the tank about
6-inches below the  water surface on steel angles or other similar structural grid-work (See
Figure 7-13).

Used in conjunction with effluent launders at spacings of 15 ft or less, wedge-wire settlers
distribute flow quite uniformly over the entire area of the basin, producing  a nearly ideal
upflow clarification zone above the wire (65). Finer particles which settle in this zone even-
tually coalesce into a sludge blanket which aids in removal of near-colloidal particles. When
the blanket builds in thickness and nears the water surface (2 to 4  days) cleaning is neces-
sary. The basin level is  lowered below  the wire level and the wire is  hosed down to clean off
the accumulated solids. Total clean  up time is about one hour (72).

Wedge-wire settler applications have been limited to relatively low surface hydraulic load-
ings, i.e.,  600 gpd/sq ft with peaks  to 800 gpd/sq ft. Effluent quality has been roughly re-
lated to flow rate (73). Under stable conditions an effluent SS of 15  to 20 mg/1 could be ex-
pected at  600 gpd/sq ft and 10 mg/1 at 300 gpd/sq ft. Results of side by side  tests of secon-
dary clarifiers treating trickling filter effluent (150 mg/ 1  SS) indicated that standard clari-
fiers produced effluents from  7  to 77 mg/ 1 with an average of 41  mg/ 1 while identical
wedge-wire settlers produced  effluents of 1.6 to 18 mg/ 1 with an average of 8 mg/1 (74).
All units  operated at a  relatively low rate of 300 gpd/sq ft. This low hydraulic loading ap-
peared significant to the wedge-wire settler performance, but it had little effect on the ef-
fluent quality of the conventional settling unit.

In activated sludge clarification use of the wedge-wire settlers reduced effluent SS to 8 to 16
mg/ 1 compared with 30 to 40 mg/  1 from the same conventional clarifier at a  rate of 600
gpd/sq ft (74).

Pullen (75) cited 5 clarifiers of small size (18,000 to 170,000 gpd), which were equipped with
wedge-wire screens, experiencing a 50 percent improvement in effluent SS quality.

Wedge-wire  settlers are limited to multiple tank installations so that during shut down for
washing flow can be diverted to other  clarifiers. Drain and wash down flow can  be recycled
to pretreatment units or to sludge handling systems.
                                        7-34

-------
 7.11   References
 1.   Kalbskopf, K. H., European Practices in Sedimentation, in Water Quality Improvement
     by Physical and Chemical Processes, Univ. of Texas Press, Austin, Texas (1970).

 2.   Sewage  Treatment  Plant Design, WPCF  Manual  of  Practice No. 8, ASCE Manual
     No. 36(1959).

 3.   Hazen, A., On Sedimentation, Trans. Amer. Soc. Civ. Eng.,  53, 45 (1904).

 4.   Camp, T. R., Sedimentation and Design oj Settling Tanks, Trans., Am. Soc. Civil
     Engrs., Ill, 895(1946).

 5.   O'Connor, D. J. and Eckenfelder W. W., Evaluation of Laboratory Settling Data for
     Process Design in Biological Treatment of Sewage and Industrial Wastes, Vol. 2,  Rh'ein-
     hold Publishing Co.  (1958).

 6.   Eckenfelder, W. W.  and  Ford, D. L., Water Pollution Control, Jenkins Book Publishing
     Company, Austin, Texas (1970).

 7.   Weber, W. J., Physicochemical Processes for Water Pollution Control, Wiley—  Inter-
     science (1972).

 8.   Metcalf,  L.,  and Eddy,  H. P.,  Sewerage and Sewage Disposal, McGraw-Hill Book
     Company, New York (1930).

 9.   Smith, R., Cost of Conventional  and Advanced  Treatment of Wastewater, Jour.
     WPCF, 40, 1546 (Sept. 1968).

10.   Fitch,  E. B.  and  Lutz,  W. A., Feedwells for Density Stabilization, JWPCF  32,  147
     (1960).

11.   Aitken, I.M.E., Reflections on  Sedimentation Theory and Practice—Part  I, Eff. and
     Water Treatment Jour. (Br.), 7,  No. 4, pg. 226 (Apr. 1967).

12.   Geinopolis, A. and  Katz,  W. J., United States Practice in Sedimentation of Sewage
     and Waste Solids,  in  Water Quality Improvement by  Physical and Chemical Pro-
     cesses, Univ.  of Texas Press, Austin, Texas (1970).

13.   Katz, W. J. and Geinopolis, A.,  A Comparative Study of Hydraulic  Characteristics of
     Two Types of Circular Solids Separation Basins  in Biological Treatment of Sewage
     and Industrial Wastes, Vol. 2 Rheinhold Publishing Company (1958).

14.  Dague, R. R.  and Baumann, E. R., Hydraulics of Settling Tanks Determined by Mod-
    els, Presented at 1961  Annual Meeting of  Iowa Water Pollution  Control Association
    (Reprinted by Lakeside Equipment Corp.)

                                      7-35

-------
15.  Villemonte, F.R., Rohlich, G.A., et al, Hydraulic and Removal Efjiciencies in Sedimen-
    tation^asins -Third International Conference on Water Pollution Research, Munich,
    Section II, Paper 16(1966).

16.  Fitch, E.  B. Sedimentation Process Fundamentals,  in Biological Treatment of Sewage
  : and Industrial Wastes, Vol. 2, Rheinhold Publishing, Co.  (1958).

17.  Fitch, E. B., Significance  oj Detention in  Sedimentation,  Sewage  and Industrial
    Wastes, 29, 1123 (Oct. 1957).

18.  Knop, E., Design Studies for the Emscher Mouth Treatment Plant, JWPCF 38, 1194
    (July 1966).

19.  Fair, G.  M.,  Geyer, J. C. and Okun, D. A.;  Water and Waste-water  Engineering, 2,
    John Wiley & Sons, New York: (1968).

20..' Ingersoll, A. C., McK.ee, J. E. and Brooks, N. H., Fundamental Concepts of Rectangu-
    lar Settling Tanks Trans ASCE 121,  1179 (1956).

21. Eckenfelder  W.  W. and  Melbinger,  N., Settling and Compaction Characteristics of
    Biological Sludges, Sewage and Industrial Wastes 29, 1114 (Oct. 1957).

22.  Naval Facilities Engineering Command, U.S.  Navy, Pollution Control Systems, Ch. 10
    iri Civil Engineering Design Manual, DM-5.

23. Cleasby,  J. L., Baumann, E. R., and  Schmid, L., Comparison of Peripheral Feed and
    Center Feed Settling Tanks  Using Models, Report by Iowa Engineering Experiment
    Station, Ames, Iowa, (February, 1962).

24. Reed, R. V.,  Rexnord, Personal Communication^ (April 1973).

25. Hikes, Burd,  Lakeside Equipment Corp., Personal Communication (May, 1973).

26.- Envirotech, Municipal Equipment Division, Eimco Modular Energy Dissipating Clari-
    fier Feedwell, Technical Brochure: MED 121, 10/72.

27. Mercer,  R. H., Rectangular vs. Circular Settling Tanks, The American City, 98, (Oct.
    1973).

28. Von der  Emde, W., To What Extent  are Primary Tanks Required?, Water Research, 6,
    395(1972).

29. Recommended  Standards for Sewage Works, Great Lakes—Upper Mississippi River
    Board of State Sanitary Engineers (1968).'
                                      7-36

-------
30.  Gould, R., Wards Island Plant Capacity Increased by Structural Changes, Sewage and
    Industrial Wastes, 22, 997 (1950).

31.  Dick, Richard I., Role of Activated Sludge Final Settling Tanks, J. SED, ASCE, 96,
    423 (Apr. 1970).

32.  Keefer, C. E., Relationship of the Sludge Density Index to the Activated Sludge Pro-
    cess, Jour. WPCF, 35, 1166 (1963).

33.  Fischerstrom, C.N.H., Isgard, E. and Larsen, I., Settling of Activated Sludge in Hori-
    zontal Tanks, J. SED, ASCE, 93,  SA3, 73 (June 1967).

34.  Suggested Peaking Considerations for Activated Sludge, Sanitary Engineering  Staff
    Report, Iowa State University (197)).

35.  Dick, R. I. and Young, K. W., Analysis of Thickening Performance of Final Settling
    Tanks, Proceedings of 27th  Purdue Industrial Waste Conference, (1972).

36.  Dick, R. I. and Ewin, B. B., Evaluation of Activated Sludge Thickening Theories, J.
    SED, ASCE, 93, SA4, 9 (Aug. 1967).

37.  Veselind, P.  A.. Discussion  of Evaluation of Activated Sludge Thickening Theories, J.
    SED ASCE  94, SA1, 185 (Feb. 1968).

38.  Dick, R. I.,  Thickening in Water Quality Imrovement by Physical and Chemical Pro-
    cesses, Univ. of Texas Press, Austin, Texas (1970).

39.  Sawyer, C.  N., Final Clarifiers and Clarifier Mechanisms, Biological Treatment of
    Sewage and Industrial Wastes, Vol. l,Reinhold Publishing  Co, New York (1956).

40.  Fall,  E. B.,  Jr., Redesigning Existing Facilities  to Increase Hydraulic and Organic
    Loading, Vol. 43, pg. 1695,  Jour. WPCF (1971).

41.  Federal Water Quality Administration, Federal Guidelines for Design, Operation and
    Maintenance of Waste-water Treatment Facilities (September, 1970).

42.  Geinopolos,  A., Albrecht, A.E., and Katz, W. J.,  The Character of Suspended Solids
    and Basin Hydraulics are Key Factors in the Clarification of Water and Waste-water,
    Industrial Water Engrg., 3,  10, 19 (Oct. 1966).

43. Green, O., Eyer, F.,  and Pierce, D., Studies on Removal of Phosphates and Related
    Removal of  Suspended Matter and BOD at Grayling, Michigan,  Distributed by Dow
    Chemical Co.

44.  Hennessey, J., Keilinski, R., Beeghly, J. H., and Pawlak, T. J., Phosphorous Removal
    at Ponliac, Michigan, Presented at U.S. EPA, WQO Design Seminar, Cleveland, Ohio
    (Apr. 1971).
                                      7-37

-------
45. Oliva, J. A., Dept. of Public Health, County of Nassau, Personal Communication
    (March, 1973).

46. Gulp, R. and Gulp, G.,.Advanced Wastewater Treatment, Van Nostrand Reinhold Co.
    (1971).

47. Kreissl, J. F. and Westrick, J. J., Municipal Waste Treatment by Physical-Chemical
    Methods, U.S. EPA, National Environmental Research Center, Cincinnati, Ohio.

48, Water Treatment Plant Design, American Water Works Association, Inc., New York
    (1969).

49. U.S.  EPA, Advanced Wastewater Treatment as Practiced at South Tahoe, Proj. No.
    171010 ELQ08/71 (August 1971).

50. Metcalf and Eddy, Inc.,  Wastewater Engineering,  Collection, Treatment, Disposal,
    McGraw-Hill, New York (1972).

51. Brown, James C.,  Alum  Treatment of High-Rate Trickling Filter Effluent, Chapel
    Hill, 'North^Carolina, Supplemental Information for U.S. EPA Technology Transfer
    Design Seminar  on Upgrading Trickling Filters,  Salt Lake City, Utah, Nov. 13-15
    (1973).

52. Parker,Denny S.,'et al, Algae Removal  Improves Pond Effluent, Water and Wastes
    Engineering,  10, 1, pp. 26-29 (January, 1973).

53. Wahl,A.J., Larson, C.C., et al,  1963 Operator's Forum, Jour. WPCF, 36, 401 (April,
    1964).

54. Katz, W.J.,Solids Separation Using Dissolved-Air Flotation, in Air Utilization in the
    Treatment of Industrial Wastes, University of Wisconsin (1958).

55. Mulbarger, M. C., et al, Manassas Va., Adds Nutrient Removal to Waste Treatment,
    Water and Wastes Engineering, 6, 4, pp. 46-48 (April 1969).

56. Mulbarger, Michael C., and Huffman, Donald D., Mixed Liquor Solids Separation By
    Flotation, Jour. SED, ASCE, 96, SA4, pp. 861-871 (Aug. 1970).

57. Levy, R. L., White, R. L., Shea,T. G., Treatment of Combined and Raw Sewages with
    the Dissolved Air Flotation Process, Water Research, Pergamon Press, Great Brittain,
    Vol. 6, pp. 1487-1500(1972).

58. Ettelt, G. A., Activated Sludge Thickening by Dissolved Air Flotation, Proc. 19th Pur-
    due Ind. Wastes Conf. (1964).
                                      7-38

-------
59.  Vrablik,  E.  R.,  Fundamental Principles of Dissolved Air Flotation of Industrial
    Wastes, Proc. 14th Purdue Ind. Wastes Conf. (1959).

60.  Masterson, E. M.,and Pratt, J.W., Application of Pressure Flotation Principles to Pro-
    cess Equipment Design, in  Biological Treatment of Sewage and  Industrial Wastes, Vol
    II, Reinhold Pub., Co., New York (1958).

61.  Ort. J. E., Lubbock WRAPS It Up. Water and  Wastes Engineering,  Vol. 9, No.  9, pp.
    63-66 (Sept.  1972).

62.  Experience of Chemical Purfication, National Swedish Environment Protection
    Board  (1969).

63.  Anderson, L., The Permutit Company, Personal  Communication (January 1973).

64.  Gulp,  G., Hansen, S.,  Richardson, G., High-Rate Sedimentation in Water Treatment
    Works, Jour. AWWA, 60, 681 (June, 1968).

65.  Hansen, S. P., Gulp, G. L.  and Stukenberg, J. R., Practical Application of Idealized
    Sedimentation Theory in Wastewater Treatment, Journal Water Pollution Control
    Federation, 41, No. 8, pp. 1421-1444 (1969).

66.  Neptune Mocrofloc  Incorporated, City of Trenton Sewage Treatment Plant, Case  His-
    tory No.  27(1971).

67.  Oppelt, E. T., Evaluation of High Rate Settling of Activated Sludge, Interim U.S..
    EPA  Internal  Report, Advanced Waste Treatment  Laboratory, Cincinnati, Ohio
    (1973).

68.  Slechta, A. F., Conley, W.  R. Recent Experiences in  Plant-Scale Application of the
    Settling Tube Concept Jour. WPCF, 43, 1724 (August 1971).

69.  Neptune MicroFLOC, Inc.Application Criteria for Tube Settling  In Activated Sludge
    Plant  Secondary Clarifiers, Technical Release No. 3.

70.  Slechta, A.,  Neptune Micro-FLOC,  Inc., Personal Communication (May, 1973).

    Hennessey, T.  L., City Engineer, Trenton, Michigan, Personal  Communication (April,
71.  1973).

72.. Sparham, V. R., Improved Settling Tank  Efficiency by Upward Flow Clarification,
    Journal Water Pollution Control Federation, 42, 801 (May 1970, Part 1).

73.  Sparham, V. R., Personal Communication.
                                      7-39

-------
(74).Crockford, J. B., Sr., Sparham, V. R., Developments to  Upgrade Settlement Tank
    Performance, Screening, and Sludge De-watering Associated with Industrial  Waste-
    water Treatment, Proc. 27th Purdue Ind. Wastes Conference (May 3, 1972).

75.  Pullen, K.  G., Methods of Tertiary Treatment, Pebble and Wedge Wire Clarifiers, Lich-
     field Rural District Council, Pollution Monitor, October/November (1972).
                                        7-40

-------
                                    CHAPTER 8

                      PHYSICAL STRAINING PROCESSES

8.1   General

Physical straining processes are defined for the purpose of this manual as those processes
which remove solids by virtue of physical restrictions on a media which has no appreciable
thickness in the direction of liquid flow.

Physical straining devices may be grouped according to the nature of their straining action.
(See Table 8-1).

8.2   Wedge Wire Screens

    8.2.1   Inclined  Screens

Inclined screens, are typified by the Hydrasieve, (Figure 8-1). made by C-E Bauer, Division
of Combustion Engineering Inc., or the Hydroscreen made by Hydrocyclonics Corporation.
These devices were  originally developed in  1965 for the pulp and paper industry to dewater
and classify pulp slurries having solids contents of 6 percent or less (1). The units operate by
gravity  and function as an inclined drainage board with a screen of wedge wire construction
having openings running transverse to the flow.

The first full scale municipal application of Hydrasieves was at the Ohio Suburban Waste-
water Treatment Plant at Huber Heights in 1967  treating raw wastewater(l).

    8.2.1.1   Equipment Details

The screen consists of three sections with successively flatter slopes on the lower sections.
(Figure 8-2). The screen wires  are triangular in cross section as shown in Figure 8-3,  and
usually  spaced 0.06 in.  apart  for  raw wastewater screening applications. In the Bauer
unit, these wires  bend in the plane of  the screen, as illustrated in  Figure 8-4. They are
straight and transverse to the flow in the Hydrocyclonics unit.

Above the  screen and running across its width is a headbox; Figure 8-2 shows two possible
inlet designs.  A light-weight hinged baffle at the top portion  of the  screen reduces flow
turbulence  in the  Bauer unit. To collect the solids coming off the end of the screen several
arrangements can be used, including a trough with a screw  conveyor.

Inclined screening units are generally constructed entirely  of stainless steel. Lighter units
with a fiber glass housing and frame costing about 25  percent less (1) may also be obtained.
Dimensions and capacities for hydrasieve units are given in Table 8-2.
                                       8-1

-------
                                                                        TABLE 8-1
                                                            PHYSICAL STRAINING PROCESSES
oo

Principal
Applications
Pretreatment &
Primary Treat-
ment

n tt



n a

Secondary and
Tertiary SS
Removal
it it

it it


Device
Inclined
wedge-wire
stainless
steel screens
Rotary
stainless steel
wedge wire
screens
Centrifugal
screens
Micro-
Screens

Diatomite
filters
Ultra-
Filters


Hydraulic
Capacity
High flow rates
4-16 gpm/in of
screen width

16-112gpm/sqft



40-100gpm/sqft

Medium flow
rates 3 to 10
gpm/sq ft
Medium flow rates
0.5-1.0 gpm/sq ft
Low flow rates
5 to 50 gpd/sq ft


Straining
Surface
Coarse
.01 to .06 in
(250-1500
microns)
Coarse
.01 to .06 in
(250-1500
microns)
Medium 105

Medium (")
1 5-60 microns

N/ACb)

Fine 99

                           (a)  These values typify the range of solids filtered by the media. Removals are a function of media thickness and
                               not media opening sizes.
                           (b)  Straining occurs through particulate mat of solids on screening surface.

-------
           FIGURE 8-1
HYDRASIEVE SCREENING UNIT
   Courtesy C-E Bauer
                       SLUDGE
                                                       FEED
                                                           HEAOBOX
                                                 DRAIN (OR
                                              ALTERNATE  FEED)
                                          FIGURE 8-2
                                   HYDRASIEVE SCHEMATIC
                             8-3

-------
     FIGURE 8-3
   SCREEN DETAIL
      FIGURE 8-4
CURVED SCREEN BARS
    Courtesy C-E. Bauer
          8-4

-------
Width
~Ti
2
3.5
4.5
5.5
6.5
7
14
21
28
35
Depth
"IT"
3.5
4
5
5
5
9.5
9.5
9.5
9.5
9.5
Height
ft
5
5
7
7
7
7.3
7.3
7.3
7.3
7.3
                                     TABLE 8-2
                        SPECIFICATIONS OF HYDRASIEVES
     OVERALL DIMENSIONS	
                                                                   Capacity
                                                                      mgd
                                                     350             0.2
                                                     550             0.4
                                                     650             0.9
                                                     800             1.2
                                                    1000             1.5
                                                    1800             2.9
                                                    3600             5.8
                                                    5400             8.7
                                                    7200             11.6
                                                    9000             14.5

   8.2.1.2   Process Description and Design
Influent wastewater enters and overflows the headbox, on to the upper portion of the screen.
On the screen's upper slope most of the fluid is removed from the influent. The solids mass
on the following slope, because it is flatter, and additional drainage occurs. On the screen's
final slope the solids stop momentarily, simple drainage occurs, and the solids are displaced
from the screen by oncoming solids (2).
In test  studies and actual installations hydrasieves have been  operated  satisfactorily  at
loading  capacities of 4 to 16 gpm  per in. of screen width (1).  This hydraulic capacity is a
function of the viscosity (which is a function of the temperature of the fluid), the solids load-
ing,  and the spacing  of  the individual  slots. Slot  width is selected by actual  tests  using
sample  screens. Once the slot opening  has been chosen the screen's capacity per foot  of
width can be determined  from empirical relationships. Since work to date has not been suf-
ficiently extended to  actual municipal wastewater  conditions, pilot studies should be the
prime basis for design.
Little quantitative work  has been done on the solids loading capacity of a hydrasieve but
generally speaking, for good performance, the influent should be dilute enough for smooth
flow over the weir.  Unit sizes designed to accommodate  more  than  1  mgd are available;
however, for pilot studies a  6-in.  wide by 22 in. long screen can be used provided flow
rates are limited to 5 to 10 gpm (1).

   8.2.1.3   Operating Experience

At the 3 mgd Huber Heights plant in Ohio hydrasieves ahead of trickling  filters have
effectively replaced primary clarifiers. Using 3 hydrasieves 72 in. wide and 54 in. long with
a  slot opening  of  0.06 in., an average  suspended  solids  removal  of 25 percent was
obtained while the units operated over a flow range of 1.5 to 4.5 mgd. Roughly 1 cubic yard
of solids was removed per million gallons of wastewater with an average solids content of  12
to,15 percent (3) (4).
                                        8-5

-------
Although inclined screens cannot remove SS to the same extent as a sedimentation tank,
they have been  favorably received by  operators because  they  do an excellent job of
removing trashy materials which may foul  subsequent treatment of sludge handling units.
Their  ability  to  remove fine  grit is  limited  by size  openings. Separate grit removal
equipment, if needed, should be installed after the inclined screens. In a pilot study at South
Buffalo  Creek Sewage  Treatment Plant  at  Greensboro,  North  Carolina,  hydrasieve
suspended solids removals ranged from 10 to 30 percent with an average removal of 20 per-
cent (5).

At the U.S. EPA Blue Plains pilot study in Washington,  D.C. (6) hydrasieves were installed
in an effort to eliminate operational problems of debris collection on the mixers and plugg-
ing of recycle and waste discharge lines. Although the screens eliminated these problems,
suspended solids  removals varied from  only 7 to 11 percent. The low removals were attrib-
uted to the wastewater's  age (24 to 48 hours) (7).

An installation list  is included  (Table  8-3). Some of these installations are temporary;
hydrasieves are being used for short term alleviation of excess solids and flows coming into
plants  which are to be abandoned when new facilities are built.

Operating experience  in these  installations  varies  as to  cleaning  and  maintenance
requirements.  Generally, a daily washing  of the screen surfaces,  which takes about 5
minutes, is sufficient for good screen performance. Washing is normally done with steam or
hot water to remove grease which accumulates and blinds the screen preventing passage of
wastewater through the screen, and resulting in  poor separation (5).

Daily steam cleaning proved necessary at Freehold, N.J. (8) but other installations such as
Huber Heights required only monthly steam cleaning (3). Grease build-up requiring steam
cleaning appears to  be related to low air and wastewater temperatures, exposure of units
and high grease content in wastewater.

Incidental to the removal of suspended  solids in this process is the aeration of the separated
water. At the  Huber Heights plant raw wastewater impinging  on a Bauer screen has been
found  to be aerated  up to a level of 2 or more mg/ 1 of  dissolved  oxygen (3). A noticeable
reduction in odors from the grit removed in the subsequent chamber has also been claimed
along with the elimination of scum in the digester (3).

   8.2.2  Rotating Wedge Wire Screens

Hydrocyclonics,  to  overcome grease  blinding problems of its own wedge wire  screen,
developed a rotating wedge wire screen which backwashes  itself (Figure 8-5). Wastewater
passes  vertically downward from the  outside to the inside of the drum by gravity. The
screened  wastewater then  passes out through the lower half  of the drum to a collection
trough.
                                       8-6

-------
                                  TABLE 8-3

    WEDGE WIRE SCREENS MUNICIPAL TREATMENT INSTALLATIONS

All units below are Bauer units unless otherwise indicated:

PLANT & LOCATION                      REMARKS

Ohio Suburban Water Co.                     3-4 mgd
Huber Heights, Ohio

Rochelle Treatment Plant
Rochelle, Illinois

Prophetstown Treatment Plant
Prophetstown, Illinois

Corinna Treatment Plant
Corinna, Maine

Bucks County Sewage Authority
Mr. MacNamara Executive Director
(Hydrocyclonics Units)

Upper Gwynedd
Towamencin Municipal Authority
Lansdale, Pa.

Irvine Ranch Water District
Irvine, California

Hercules—AWT Div.
Freehold Township, N.J.
Mr. Ron Lee, Engineer

Montgomery County Commissioners            10 mgd
Moraine, Ohio

STP Rogersville, Tennessee
(Hydrocyclonics Units)

Blue Plains  Pilot Study
Washington, D.C.

S. Buffalo Creek STP                         0.03 mgd
Greensboro, N. Carolina
                                     8-7

-------
                       FIGURE 8-5

ROTATING WEDGE WIRE SCREEN AT NORTH CHICAGO S.T.P.
               (Courtesy of Hydrocyclonics Corp.)

-------
Solids are retained on the outside of the drum and are removed by a fixed scraper blade. A
screen spacing of 0.06 inches is recommended for service on raw wastewater. In comparison
with static screens, the manufacturer claims the rotating units require  less maintenance,
lower operating head and smaller space and produce dryer solids (9). Table 8-4 shows com-
parative design data for rotating and stationary wedge wire screens. (10).

                                    TABLE 8-4
Parameter

S.S. Removal, percent

Flow rate



Wire Spacing
DATA SHEET-WEDGE WIRE SCREENS

              Inclined                      Rotary

              5 to 25                       5 to 25

              4 to 16 gpm                  15 to  112 gpm/sq ft
              per inch of
              screen width

              .01 to .06 in.                  .01 to .06 in.
Percent solids by wt.

Volume of solids
produced
              12 to 15

              1  to 2 cu yd of
              solids per million
              gallons of waste-
              water
16 to 25
8.3  Microscreening

    8.3.1   General Description

As shown in Figure 8-6 in its usual configuration a microscreen unit consists of a motor
driven rotating drum mounted horizontally in  a rectangular chamber. A fine  screening
media covers the periphery of the drum. Feedwater enters the drum interior through the
open end and passes radially through the screen  with accompanying deposition of solids on
the inner surface of the screen. At the top of the drum pressure jets of effluent  water are
directed onto the  screen  to  remove the mat of deposited  solids. The dislodged  solids
together with that portion of the backwash stream which penetrates the screen are captured
in a waste hopper  as shown  in Figure 8-7. Solids flushed from the unit are sent to sludge
handling systems  or  recycled to the head  of  the  plant. Units may be equipped with
ultraviolet lights to control biological growth on  the screen media. Effluent passes from the
chamber over control weirs oriented perpendicular to the drum axis.
                                       8-9

-------
>
                                                             FIGURE 8-6
                                                   TYPICAL MICROSCREEN UNIT
                                                (Courtesy of Cochrane Division, Crane Co.)

-------
                         BACKWASH  HOPPER
                         DRUM  LIFT
                                                         SPRAY  SYSTEM
-
                                                                           MESH SYSTEM
                                                                            FIGURES-?

                                                                     MICRO-MATIC®STRAIM R
                                                                     (Courtesy of Zurn I ml.. Inc.)

-------
    8.3.2  Functional Design

The functional design of a microscreen unit involves:

     1.   Characterization of suspended  solids in feed  as to concentration and degree of
         flocculation,  as these  factors  have been shown to affect microscreen capacity,
         performance and back washing requirements (9)(11)

    2.   Selection of unit design parameters which will assure sufficient capacity to meet
         maximum hydraulic loadings with critical solids characteristics, and provide the
         required performance  over the expected range of hydraulic loadings and solids
         characteristics.

    3.   Provision of backwash and supplimental cleaning facilities to maintain the design
         capacity.

Table 8-5 shows typical  values for microscreen and backwash design parameters for solids
removal from secondary effluents. Similar values would apply to direct microscreening of
good quality effluent from fixed film biological reactors such as trickling filters or rotating
biological contactors,  where the microscreens replace secondary settling  tanks  (9).  This
application is not widely practiced, however.

Microscreening has been used for the removal of algae from uncoagulated lagoon effluents.
At  Bristol, England, algae reductions of  1565 to 450 algae per ml and 989 to 168 algae per
ml  were achieved on astrerionella, cyclolella  and synedra  (12). However many classes of
algae, e.g. chlorella, are  too small to be removed, even on fine screens (23 microns) and ex-
cessive loadings (up to 2 x 106 algae per ml) make this application a limited one.

The parameters of mesh size,  submergence, allowable headless and  drum speed [rpm =
peripheral  speed/ -^ (diameter)j are  sufficient  to  determine the  flow capacity of  a
microscreen with given suspended solids characteristics (13).
                                        8-12

-------
                                   TABLE 8-5
                   MICROSCREEN DESIGN PARAMETERS
Hydraulic
    Loading
Typical Value

20-25 microns

75 percent of height
66 percent of area

5-10 gpm/sq ft
of submerged drum
surface area
                                                      Remarks
                                                      Range 15-60 microns
Head-loss (Hi.)
    through Screen
3-6 in
Maximum under extreme con-
dition:  12-18 in. Typical
designs provide for overflow
weirs to bypass part of flow
when head exceeds 6-8 in.
Peripheral
    Drum Speed
Typical Diameter
    of Drum
Backwash Flow
    and Pressure
15 fpm at 3 in. (Ht)
125-150 fpm at
6 in. (Ht)

10ft
2 percent of throughput
at 50 psi
5 percent of throughput
at 15 psi
Speed varied to control
extreme maximum speed
150 fpm

Use of wider drums
increases backwash require-
ments.
Among these parameters peripheral speed, hydraulic loading and major variations in mesh
size also affect performance on a given feed flow. In addition, drum speed and  diameter
affect the wastewater flows and pressures needed to effect proper cleaning of the screen.
                                     8-13

-------
    8.3.3   Hydraulic Capacity

The Jilterability index developed by Boucher (14) quantifies the effect of the feed solids
characteristics on the flow capacity of a particular  fabric. Boucher assumed that at any
constant laminar flow rate the headless, AP in ft, through any given strainer fabric would
increase exponentially with the volume passed per unittarea (V. in cu, ft/sq ft ):

                                    AP  _ PIV
In the above relation the filterability index is the exponential rate constant I (in I/ ft).

From the filterability index concept Mixon (13) developed hydraulic capacity relations for
continuous operation of a rotating drum microscreen, which can be expressed as follows:
 Where:
    H  = mean flow velocity through submerged screen area (fps)
     Q  = total flow through microscreen (cfs)
     A  = submerged screen area (sq ft)
     P  = pressure drop across screen (ft)
     CF = fabric resistance coefficient (ft/fps  or sec) (clean fabric headless at 1 fps
             approach velocity)
     I  = filterability index (1 / ft)
     0  = decimal fraction of screen area submerged
     R  = drum rotational speed (rpm)

 The expression'AP/CF represents the initial flow velocity through the clean screen as it en-
 ters submergence.CF is a particular characteristic of  the screen fabric, varying inversely
 with mesh opening size as follows:

                 Mesh Size                    Fabric Resistance   CF
                     mu                              ft/fps

                      15                                 3.6
                     23                                 1.8
                     35                                 1.0
                     60                                 0.8
                                        8-14

-------
Limits on A P reflect screen fabric mechanical strength and expected operating conditions
for the unit. A typical value is 0.5 ft at normally-expected maximum flow.
The relation of parameters in the expression (I<£/R) shows that the effect of a higher index
or faster buildup of headless on the screen may be offset by maintaining a higher drum
rotational speed.

Figure 8-8 is a graphical representation of the  above relation which Mixon obtained by
plotting Q/A against A.P/CF for various values of the parameter I0/R.

The graph shows lines of constant value for the ratio

                                    E=   Q/A
which is the ratio of the mean velocity through the screen to the initial velocity when the
screen enters submergence. Recognizing the effect of drum speed on performance Mixon
suggested   selecting 107 R to keep the ratio E below 0.5. Above this limit he assumes that
insufficient opportunity is given for a mat to form on the drum and solids removal efficien-
cy is likely to suffer.

The filterability index may be determined by Boucher's laboratory procedure (14), by  field
testing with an apparatus available from Crane Co. (11)  or by analysis of test data from
pilot microscreen units using relations such as those proposed by Mixon or Boucher. In
some cases where numerous  values of T have been obtained, a relationship to influent SS
loading may be obtained (15). Solids loading limitations such as those noted by Lynam, et
al,  (16) would have broader  applicability if they were related to a maximum filterability
index under which a particular microscreen could maintain a given capacity.

   8.3.4   Performance

Suitable relationships have not been developed for quantitative predictions of microscreen
performance from knowledge of influent characteristics and key design parameters. Where
performance must  be  predicted  closely,  pilot  studies  should  be made.  Where  close
prediction is less critical, performance data from other  locations with generally similar
conditions may serve  as a guide.

Table 8-6 provides  performance data  for a number  of microscreen installations for SS
removal  from secondary effluents including the first  such  installation made  at Luton
Sewage  Works, England,  in the early  1950's. Table  8-7 lists  additional  American
installations provided by two  manufacturers.
                                       8-15

-------
O.004
O.002
   0.01
          O.O2
                  O.04 0.06O.080.I
                    AP CF FT/SEC
                                          0.4   O.6 O.8 I.O
             TYPICAL DESIGN RANGE
                  5-10 GPM/SQ  FT
                  FABRIC
                 MARK  0
                 MARK  I
MESH- MU
   23
   35
                       FIGURE 8-8
           MICROSCREEN CAPACITY CHART(13)
                            8-16

-------
              8-6
MICROSCREEN INSTALLATIONS
Drum
Dia.
SrTApn
LOCATION

Luton
Bracknell

Hambledon R.D.C.
Elmbridge
Leighton-
Linslade U.D.C.
Fleet U.D.C.
Esher U.D.C.
Hatfield R.D.C.

The Borough of
Bury St. Edmonds
Franklin Township
STP Murraysville,
Pa.
Letchworth

Basingstoke

Euclid, Ohio



Euclid, Ohio



Euclid, Ohio

Lebanon, Ohio

Hanover Park, 111.

MSD North Side
STP Chicago, 111.
Country

England
England

England

England

England
England
England

England

U.S.A.


England

England

U.S.A.



U.S.A.



U.S.A.

U.S.A.

U.S.A.

U.S.A.

Influent Source

Trickling Filter
Trickling Filter

Trickling Filter

Trickling Filter

Trickling Filter
Trickling Filter
Trickling Filter

Trickling Filter

Trickling Filter
Final-Settling
Tanks
Act. Sludge Final
Clarifiers
Act. Sludge Final
Clarifiers
Act. Sludge with
Chem.Precip.in
Primary Clarif.
(Fed)
Act. Sludge with
Chem.Precip.in
Final Clarif.
(Fed)
UNOX Final
Clarifiers
Act. Sludge
Final Clarif.
Act. Sludge
Final Clarif.
Act. Sludge
Final Clarif.
Width
ft


7.5x5,




" ~ 1

t

10x10,
10x10,
7,5x5,
,
10x10,

10x10,


5x3,

10x10,

2.5x2,



2.5x2,



2.5x2,

5x1,
5x1,
10x10,

12.5x30

Mesh
mu
60
35

35
23
35

35
-
.

23

23


23

23

23



23



23

35
23
23

, 23

Hydraulic
Load on
Submerged
Mo of Area Plant Flow
Units Max. Avg. Max. Avg.
gpm/sq ft mgd mgd
9.0-3
2 6.3 2.2 6.3 2.2

10.8 2.5 2.5 0.6
10.8 2.5 2.5 0.6
6.8 3.9 2.0 1.2

2 6.0 2.0 3.6 1.2
3 - 10.8 (Design Flow) .
3 - 2.55 (Design Flow)
9.0 (Design Flow)
5 - - 5.3 (Max. Flow to
Date)
2 7.8 - 4.0


1 4.3 3.3 Pilot Study

S 1.5 1.0 3.2 2.2

1 2.5 1.25 Pilot Study
40 gpm 20 gpm


1 2.5 1.25 Pilot Study
40 gpm 20 gpm


1 2.5 1.25 Pilot Study
40 gpm 20 gpm
1 1
1
1 S.3 2.6 1.5 0.8

15 (Design Flow)

Average
Suspended

Solids
Influent
mg/1
14
20
16.2
14
14
29

15
19
14

28

37


17

13.1

54



38



65

27
17
6-28

10

Effluent
mg/1
8
11
6.9
8
7.7
11

6
9
8

7

6


6.6

3.9

8



10



21

7
2
4-11

3

Removed Reference
percent
45 19
45 19
57
45 19
45
60 19

60 9,19
60 9
43 9,19

75 9

83+10 9


62 9,19,20

70 9,19

85 17



74 17



68 17

73 15
83 15
55(ave.) 16

67 9


-------
                                    TABLE 8-7
               MUNICIPAL MICROSCREENER INSTALLATIONS
Location

Arthur Bloom Apts.
Lancaster, Pa.
Ecological Utilities
North Miami, Fla.
City of Murfreesboro
Murfreesboro, Tenn.
MSD Chicago
Lemont, 111.

City of Cookeville
Cookeville, Tenn.               7.2

City of Dayton
Dayton, Tenn.                  5.4
Department of Public Works
Erie, Pa.                       45.0

Village of Pepperpike
Pepperpike, Ohio               1.0

Borough of Bellefonte
Bellefonte, Pa.                  3.0

Good Samaritan Hospital
Islip, New York                0.2
Plant
Flow
mgd~
0.1

2.7

4.0

Units
No.
Courtesy
2

1

2

Unit
Sizes
D x L, ft
Zurn Industries
4x2

10x10

10x10

Straining
Media*
microns

20
(polyester)
20
(polyester)
20
(polyester)
10x10
10x10
10x10
10x15
 6x6
 6x6
 4x4
21


21


20
(polyester)


21


21


21


21
* All fabrics stainless steel unless otherwise indicated.
                                      8-18

-------
             TABLE 8-7 (continued)
MUNICIPAL MICROSCREENER INSTALLATIONS

Location

Borough of Carroltown
Carroltown, Pa.
Cincinnati Dept. of Sewers
Cincinnati, Ohio
Muncie Mall
Muncie, Indiana
Opalaka Sewage Plant
Chesterland, Ohio
I.B.M.
Essex Junction, Vt.
Hot Springs Village
Hot Springs, Arkansas

Union "76" Oil Co.
Clarion, Pa.
Deer Creek State Park, Ohio
Oakbourne Hospital,
Westchester, Pa.
Sugar Creek STP,
Greene County, Ohio
Little Miami STP,
Greene County, Ohio
Westminster, Md.
Hammond, Ind.
East Wheatfield Township, Pa.
Plant
Flow
mgd

1.0

0.5

0.2

0.3

0.2

0.3
(Courtesy

0.1
0.3

0.1

6.0

6.0
3.0
1.5
0.1

Units
No .

2

1

1

1

1

1
of Crane Co.)

1
1

1

2

2
1
1
1
Unit
Sizes
DxL, ft

4x4

4x4

4x4

4x4

4x4

4x4


5x1
5x3

5x1

10x10

10x10
10x10
7.5x5
5x1
Straining
Media*
microns

21

21

21

21

35

21


23
23

23

23

23
23
23
23
                    8-19

-------
              TABLE 8-7 (Cont'd)
MUNICIPAL MICROSCREENER INSTALLATIONS
Location
Hartville, Ohio
Louisville, Ky.
Browntown, Minn.
Salisbury, N.C.
Allen County, Ohio
Fairmont, Minn.
Parkway, Md.
Penn State University, Pa.
Bolingbrook, Illinois
Park Forest, Illinois
Sugar Creek, Ohio
Union Oil Co.
Harrisburg, Pa.
Chicago Metro Sanitary District
Chicago, Illinois
Ursuline Academy, Ohio
Jackson Township, N.J.
FWPCA Research Project, Phila.
Akron STP, Ohio
Wm. Henry Apts., Dowington, Pa.
Plant
Flow
mgd
2.5
0.8
0.1
6.0
2.5
6.0
15.0
0.3
0.5
2.0
0.3
0.1
2.0
0.3
0.1
0.2
3.0
0.2
Units
No.
2
2
1
2
2
2
5
1
1
2
1
1
1
1
1
1
1
1
Unit
Sizes
D x'L, ft.
7.5x5
5x3
5x1
10x10
7.5x5
10x10
10x10
5x3
7.5x5
7.5x5
5x3
5x1
10x10
5x3
5x1
5x3
7.5x5
5x1
Straining
Media*
microns
23
60
23
23
23
23
23
23
60
23
23
23
23
35
35
23
60
35
                    8-20

-------
TABLE 8-7 (continued)
Location

Franklin Twp., Pa.
Hempfield Twp., Pa.
Chelsea Ridge Apts., N.Y.
Harpeth Valley, Tenn.
University School
Cleveland, Ohio
Willoughby Hills
Cleveland, Ohio
Lionville, Pa.
Margate STP, Fla.
Lauderhill STP, Fla.
Bel-Aire STP,
Miami, Florida
Homestead, Fla.
Upper Sandusky, Ohio
Petosky, Michigan
Ravenna, Ohio
Bolingbrook STP, 111.
Commonwealth Edison, 111.
Lucas County, Ohio
Pymatunihg State Park, Pa.
Plant
Flow
mgd
4.0
6.0
0.2
1.5
0.1
0.1
0.7
3.5
3.5
0.9
0.2
1.5
2.5
1.5
0.5
2.0
0.2
0.1
Units
No.

2
2
1
1
1
1
2
2
2
1
2
. 2
2
2
1
1
2
1
Unit
Sizes
D x L, ft
10x10
10x10
5x3
7.5x5
5x1
5x1
5x3
10x10
10x10
7.5x5
5x1
7.5x5
10x10
7.5x5
7.5x5
10x10
5x1
5x1
Straining
Media
microns
23
35
23
23
23
23
23
23
23
23
23
23
23
23
60
23
23
35
          8-21

-------
Figure 8-9 shows average operating results from a number of British tertiary microscreen
installations with various hydraulic loadings.

Figure 8-10 presents the results of three extended British studies on microscreening of
trickling filter and activated sludge secondary effluents.

Some general conclusions can be made about the microscreen as a device for removing SS
from secondary effluents:

     1. Under best operating conditions microscreen units can reduce solids to as low as
       5mg/l.

    2. Although the SS removal pattern is irregular, performance tends to be better at
       lower hydraulic loadings (Figure 8-10a).

    3.  Increases  in influent  suspended  solids are  reflected  in the effluent  but  with
        noticeable damping of peaks (Figure 8-10).

    4. Microscreens are applicable in place of clarifiers to polish effluent from low  rate
       trickling filters, if the solids are generally low in concentration and well flocculated
       (Figure 8-10b).

Other investigations provide insights beyond those cited above:

Data from Lebanon, Ohio, (15) show better removal with smaller mesh sizes.

Commenting  on how solids characteristics affect microscreen performance,  one British
study (8) notes that a clear non colloidal secondary effluent containing a reasonable amount
of suspended solids  would result in  a better effluent than colloidal effluent containing less
suspended solids.

Chemical  application  can  unfavorably  alter solids characteristics: Lynam,  et al.  (16)
reported poor removals when applying an alum flocculated secondary effluent directly to a
microscreen. In contrast, at Euclid, Ohio (17) a microscreen  removed 74 to 85 percent of
the_ solids from the settled effluent of an  activated sludge pilot plant with  mineral  addition
for phosphorus removal. The better performance at Euclid could be attributed to a tougher
biological  nature of the effluent solids.

Microscreen suppliers  (9) (18) stress the  importance of  minimizing shearing action on
microscreen influent to avoid breaking up flocculated particles. This is advanced as a reason
for settling limits on drum peripheral speed and for avoiding pumping ahead of microscreen
units.

Lynam,  et. al, (16) indicate that microscreening at lower drum speeds yields better quality
effluent. This is attributed to better straining action through the thicker mat of solids which
builds up at low speeds.
                                        8-22

-------
      •   PLANT OPERATED AT MAXIMUM
          FLOW  RATES OF  9.2  TO 10.8 gpm/s.f.
      O   PLANT OPERATED AT MAXIMUM
          FLOW  RATES OF  6.0  TO 6.8 gpm/s.f.
   20
CO
V)
u.
u.
Ill
    10
                 10         20         30
                  INFLUENT  SS   Mg/l
40
                       FIGURE 8-9

 MICROSCREEN REMOVAL AT VARIOUS FLOW RATES
                (After Isaac and Hibbert (19))
                          8-23

-------
               CONCENTRATION OF SUSPENDED SOLIDS
            8OK A A                ~~ Filter Effluent
               1  ft A                - — Humus Tank Effluent
                                       1 Microstrainer Effluent
  SUSPENDED
  SOLIDS
  (p.p.m.)
             OLa
              7 21 5 19 2 1630142811 25 8 22 6 2O 3 17 1 15291226
              April May June  July Aug  Sept Oct  Nov Dec  Jan
                                1962                  1963
                A.  HAftPCNOEN  SEWAGE   WORKS
                         (Courtesy of Crane Co.)
§    30
     20
     10
    400
    300
u. B
  I  °7
                           SUSPENDED  SOLIDS (influent)
                                FLOW
 INDICATES TIMES WHEN MICF.OSTBAINER    WASHWATER JETS CHANGED
'TREATED WITH HYPOCHLORITE
                          SUSPENDED SOLIDS (effluent)
                              •v
—i- t—'-->_!_•> '	I I- L i i-i J I  LI t I I  I __ ]_ . r ] j |  I I i i i  I I ] i I	| j | | LI |  11 j 111 I (
 21  5 19  2  16 30 14 28  II  25 8 22 6 20 3  17  I  15 29 12  26 9 23 9 23
APR. MAY JUNE JULY   AUG. SEPT. OCT. NOV.  DEC. JAN.  FEB. MAR.

                   1966                            1967
                                                            -400"6
                                                             300^
                                                             200 g
                                                                u.
B.    LETCHWORTH  WATER  POLLUTION  CONTROL  WORKS

         Data were obtained from unulysi.s ot continuous records, each point representing
         weekly mean hourly readings. Broken lines indicate break in operation of plant
                               FIGURE 8-10

             MICROSCREENING OF TRICKLING FILTER
                           PLANT EFFLUENT
                                    8-24

-------
SUSPENDED 25
SOLIDS
(mg E)    20
                         MICROSTRAINER
                           EFFLUENT
           Sept Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec

                  1968               1969

                           C.  FLEET   U- D-C.

                         (Courtesy of Crane Co.)


                           FIGURE 8-PO (Continued)
                                8-25

-------
As  shown in Figure  8-8 under  peripheral speed  and  hydraulic  loading  limits set by
manufacturers and regulatory authorities, the ratio E for most microscreen designs actually
falls below 0.1, a lower limit suggested by Mixon for  most efficient utilization of equipment.

The wide range  of suggested design values underlines the need for developing quantitative
relations between  removal efficiency and key design  parameters.

     8.3.5  Microscreen Construction

The basic screen  support structure  is a  drum shaped,  suitably stiffened rigid  frame
supported  on bearings to allow rotation. Designs using water lubricated  axial bearings or
greased bearings located on the  upper inside  surface  of  the rotating drum allow
submergence well  above the central axis.

Both plastic (polyester) and stainless steel are used for the microscreen media itself. Greater
mechnical strength, especially at higher temperatures, is the prime advantage stated for
stainless  steel (9) (18).  Greater economy  and  chemical  resistance  are pointed to as
advantages of plastic (18).

Depending on manufacturer, screen fabric is supplied either in small sections (8 in. x 12 in.)
supported  by and fastened directly to the drum frame or in larger (18 in. x 24 in.) panels
consisting of fabric integrally bonded to a grid like supporting mesh of stainless steel. These
panels are attached to the drum frame.

One manufacturer offers a microscreen unit with an accordion or pleated outer surface to
achieve up to 30 percent more filtering surface within the same general  dimensions of regu-
lar designs. The unit is 12.5 feet by 30 feet and has  a rated  capacity of  15 mgd. This unit is
shown in Figure 8-11.

In  the past cast bronze and cast carbon steel were used as drum and frame construction
materials. The present practice is to use fabricated carbon steel. Generally, smaller units are
factory assembled in steel tanks while large units are placed in concrete structures.

Table 8-8  illustrates the approximate size and power requirements for various microscreen
units.

     8.3.6   Screen Fabric and Operating Headless

Microscreen  fabrics normally are  woven  of  stainless  steel or plastic (polyester  with
polypropylene supporting grid) with openings in  the range of 15 to 60 microns.  ..  .

Plastic fabric is less subject to chemical attack by strong chlorine or acid cleaning solutions.
Stainless steel can better withstand temperatures encountered in  steam  cleaning..'  '

Suggested operating headloss limits for microscreens are based on observation of the effect
of  differential head on screen life. Standard design calls for a 3 in. headloss at average flows
                                        8-26

-------
                                FIGURE 8-11


          MICROSCREEN UNIT WITH PLEATED OUTER SURFACE
                     (Courtesy of Cochrane Div., Crane Co.)
U S EPA Headquarters Library
      Mail code 3404T
1200 Pennsylvania Avenue NW
  " Washifiaton, DC 20460
       2o2-bbb-0556
                                   8-27

-------
                                  TABLE 8-8
   TYPICAL MICROSCREEN POWER AND SPACE REQUIREMENTS
        Drum. Sizes
Source
Code
A
A
A
A
Diam.
It
5.0
5.0
7.5
10.0
Length
ft
1.0
3.0
5.0
10.0
Floor Space
Width Length
   ft   ft
                            8    6
                            9    14
                            11   16
                            14   22
      Motors
Drive
BHP

0.50
0.75
.2.00
5.00
Wash Pump
BHP

 1.0
3.0
5.0
7.5
Approx. Ranges
of Capacity
mgd

0.07-0.15
0.2 - 0.4
0.5 - 1.0
1.5 -3.0
B
B
B
4.0
6.0
10.0
4.0
6.0
10.0
7 15
10 17
14 22
                                           0.75
                                           2.00
                                           5.00
                           1.0
                           1.5
                           5.0
                         0.2 - 0.4
                         0.5- 1.0
                         1.5-3.0
                 Code A  Courtesy Crane Co., Cochrane Div.
                 Code B  Courtesy Zurn Industries
and 6 in. for normally-expected maximum flows (9) (18). For occasional peaks (less than 3
percent of time) headlosses up to 24 in. can be tolerated. Crane Co. indicates that stainless
steel screens operated under the above conditions would have a life of 10 years; if operated
continuously at a 24 in. headloss  the same screen might only last 6 months. (9)
                                      8-28

-------
    8.3.7  Hydraulic Control

Hydraulic control of microscreening units is effected by varying drum speed in proportion
to the differential  head across the  screen. The controller is commonly set  to  give a
peripheral drum speed of 15 fpm at  3 in. differential and 125 to 150 fpm  at 6 inches (9)
(18). In addition, backwash  flow rate  and pressure may be increased when the differential
reaches a given level (9) (18).

The operating drum submergence is related to the effluent water level and headless through
the fabric. The minimum drum submergence value for a given installation is the level of
liquid inside the drum  when there  is no flow over the effluent weir. The  maximum drum
submergence is fixed by a bypass weir which permits flows in excess of unit capacity to be
bypassed; at maximum submergence the  maximum drum differential should never exceed
15 inches.

Effluent and bypass weirs should be designed as follows:

     1.   Select drum submergence level (70 to 75 percent of drum diameter) for no flow
         over the effluent weir.

    2.   Locate top of effluent weir at selected submergence level.

    3.   Determine maximum flow rate.

    4.   Size effluent weir to limit liquid depth in effluent chamber  above the weir  to 3 in.
         at the maximum flow rate.

     5.   Position the bypass weir 9 to 11 in. above effluent weir. (3 in. head on effluent weir
         maximum flow plus  6 to 8 in.  differential on drum at maximum drum speed
         and maximum flow).

     6.   Size bypass weir length to prevent the level above effluent weir flow exceeding 12
         to 18 in. at peak maximum flow or overflowing the  top of the  backwash collec-
         tion hopper.

   8.3.8  Backwashing

Backwash jets  are directed  against the outside of the  microscreen  drum as it  passes the
highest point in its rotation.  About half the flow penetrates the  fabric,dislodging  the mat of
solids formed on the inside (15). A hopper inside the drum .receives the flushed-off solids.
The hopper is positioned to  compensate for the trajectory that the solids  follow at  normal
drum peripheral velocities.

Microscreen  effluent is  usually used for backwashing. Straining is required to avoid
clogging of  backwash nozzles. The  in-line strainers used for this purpose will  require
periodic  cleaning; the frequency  of  cleaning will  be  determined by  the  quality of the
backwash water.

                                       8-29

-------
The backwash system used by Zurn employs two header pipes; one operates continuously at
20 psi, while the other operates at 40 psi when the unit receives a high solids loading. The
Crane system also uses two sets of jets but both  operate continuously at pressures from 15
to 55 psi. Under normal operating conditions  these jets operate at 35 psi. Once a day they
are operated at 50 psi for l/2 hour to keep  the jets free  of slime build-up. Should this
procedure fail to keep the jets clean, the pressure is raised to 55 psi. At this pressure the
spring loaded jet mouth widens to allow for more effective cleaning.

Backwash pressure is also increased to compensate for heavy solids loadings which require
the higher pressure for thorough cleaning. Crane reports that no major problems have been
encountered with this jet design (9).

Prior to  1967 Crane designed backwash systems to operate only at 15 psi. A pilot study in
Letchworth England (20) showed the superiority of the higher pressure system. Results of
this study showed:

     1.   Operation at 50 psi, as opposed  to  15 psi, increased the  process flow capacity
         30 percent.

    2.   Suspended solids concentration in the  backwash increased from 260 mg/ 1 at 15
         psi to 425 mg/ 1 at 50 psi.
     3.   Water consumption of the jets as a percent of process effluent decreased  from
         5  percent at 15 psi to 2 percent at 50 psi.

In general, backwash systems are operated at as low a pressure as possible consistent with
successful cleaning. High pressure operation incurs added system maintenance, particularly
jet replacement, and is used only as needed.

  8.3.9  Supplemental Cleaning

Over a period of time screen fabrics may become clogged with algal and slime growths, oil,
and  grease. To  prevent clogging, cleaning methods in addition to backwashing are neces-
sary.

To reduce clogging from algae and slime growth, Crane Company recommends  the use of
ultraviolet  lamps placed  in close proximity to the screening fabric and monthly removal of
units from  service to permit screen cleaning with a mild chlorine solution. While most liter-
ature sources say ultraviolet lamps are of  value, one authority (21)  feels these lamps are
uneconomical because they require frequent  replacement. Zurn Industries claims that, be-
cause their screening fabric is completely bonded to the supporting material, crevices where
algae become lodged are eliminated and backwashing alone is sufficient to remove algal and
associated  slime growths (18).
                                       8-30

-------
Where oil and grease are present, hot water and/or steam treatment can be used to remove
these materials from the microscreens. Plastic screens with  grease problems are cleaned
monthly (9) with hot water at 120° F (18) to prevent damage  to the screen material. Down
time for cleaning may be up to 8 hours.

   8.3.10  Operation

In starting a  microscreening unit care should be taken to limit differential water  levels
across the  fabric to normal design ranges of 2 to 3  inches.  For example, while the drum is
being filled it should be kept  rotating and the backwash water should be turned on as soon
as possible. This  is done to limit the formation of excessive differential heads across the
screen which would stress the fabric during tank fill-up.

Leaving the drum standing in dirty water should be avoided  because suspended matter on
the inside screen face which is above the water level may dry and prove difficult to remove.
For this  reason  introducing  unscreened  waters,  such as  plant  overloads,  into  the
microscreen effluent compartment should also be avoided (18).

If the unit is to be left standing for any length of time the  tank should be drained and the
fabric cleaned to prevent clogging from drying solids.

8.4  Other Screening Devices

Conventional  mesh  screens  have not  been  used with success in municipal wastewater
treatment.  Recently,  however,  a  centrifugal  screen,  the Sweco  Concentrator,  has
demonstrated its  effectiveness.  In this unit,  influent  is  directed  against the  inside
of a  rotating cylindrical screen  cage  (See Fig. 8-12). It is claimed  that the rotational
speed (centrifugal  force of 3  to 6 gravities) increases hydraulic capacity and, together with
the impingement  angle, permits  separation of solids finer than the screen  openings (150 to
165).

Separated  solids and the rejected portion of  the liquid flow are removed  from the bottom
of the unit while effluent is taken off at the periphery. Screen blinding is cleared  by timer-
actuated spray cleaning systems which direct water jets against both the  inner and outer
screen surfaces.

At Contra Costa County, California, a 60 inch unit treats  0.9 mgd wastewater containing
about 200 mg/ 1 SS  (22). The waste flow is split into two streams,  a small volume (15 per-
cent) concentrated stream and a supernatant (85 percent of influent) stream. The concen-
trated stream is settled in the conventional primary basin (formerly overloaded but now ca-
pable of good solids reduction at the lower hydraulic loading). The supernatant is treated by
flotation (the Sweco concentrator does not specifically remove floatables but thoroughly
aerates the  wastewater aiding subsequent flotation) and finally, flotation  and settled con-
centrate effluents are mixed and chlorinated before disposal. Overall SS reductions for the
system including concentration, flotation, settling and chlorination are reported as 70 to 80
percent.  Similar removals are claimed for settled secondary effluent from aeration pro-
cesses (23). Design flow rates are claimed  to range  from  40 to 100 gpm/sq ft. The con-
                                       8-31

-------
                          Influent Distribution Pans
                        Removable Screen Panels
X

ui
i j
                            Effluent Discharge
                                                                       Concentrate Discharge
                                                                                                                Rotating Screen Cage
                                                                                                                        Concentrate Collector
                                                                                                                            Influent
                                                                                                                    Effluent Collector


                                                                      •    FIGURE 8-12


                                                               THE SWECO CONCENTRATOR

                                                                   (Courtesy of SWECO, Inc.)

-------
centrator is of epoxy-lined steel and screen construction is of stainless steel or polyester in
plastic frames. During operation the outside of the screen is washed every 20 minutes with
cold clarified water and the inside with 5 to 7 gpm hot water.

8.5  Diatomaceous Earth Filters

Diatomaceous  earth  (DE)  filters  have  been applied  to  the  clarification of secondary
effluents at pilot scale. No full-scale installations have been characterized in the literature.
They produce a  high quality  effluent but  appear unable to  handle  the  solids  loadings
normally expected in this application.

DE filtration utilizes a thin layer of precoat formed around a porous septum to strain out
the suspended solids in the feedwater which passes through the filter cake and septum. The
driving force can be imposed by vacuum from the  product side or  pressure from the feed
side. As  filtration proceeds, headless through the cake increases due to solids deposition
until a maximum is reached.  The cake and associated solids  are  then  removed by flow
reversal and the  process  is repeated. In  the cases where secondary effluents have been
treated by this process, a considerable amount of diatomaceous earth (body feed)  has been
required  for continuous feeding  with the  influent in order to prevent  rapid buildup of
headlosses.  Generally,  the DE  filtration  process  is  capable of excellent  removal  of
suspended solids but not colloidal matter.

A wide variety of diatomaceous earth (diatomite) grades are available for use. As  might be
expected, the coarser grades have greater permeability and solids-holding capacities than do
the finer grades which will generally produce a better effluent. Some grades of diatomite are
pretreated to change  their characteristics  for improved performance. A  number  of vessel
configurations are available, with open-basin vacuum and vertical pressure designs most
common.  (See Figures 8-13, and 8-14.)

Design criteria for diatomite filters have been discussed by Bell (24). The filtration cycle can
be divided into two phases, run time and down time. Down time includes the periods when
the dirty cake is dislodged from the septum and  removed from the filter and when the new
precoat is formed. Run time commences when the feed is introduced to the filter  and ends
when a limiting headless is reached. The single most important factor in secondary effluent
filtration by DE filters is the amount of body feed required during the filtration or run time.'
The body feed rate is the largest operating cost factor and strongly affects the operating
economics of the  process.  Similarly, it is related to cycle time between backwashing which
determines the installed filtering area, hence the capital cost economics.  A ratio  of 5 to 6
mg/ 1 of  body feed per JTU  of influent  turbidity was required at San  Antonio and the
possible need for a higher ratio was suggested (25). Another pilot study  used a variety of
ratios and filtration  rates and used both pressure and  vacuum  systems for secondary
effluent filtration (26). Some results from this study are shown in Table 8-9. Both studies
indicate that a precoat of about 0.1 Ib/sq ft of filter area and greater than 6 mg/ 1 of body
feed per JTU of turbidity should.be used.

An English  study with a reversible-flow DE unit also resulted  in uneconomical operating
conditions  due  to excessive  body feed,  short  filter runs  and  high  backwash  water
                                       8-33

-------
X
                            CORROSION PROOF
                             FILTER ELEMENTS
CORROSION PROOF
     TANK
           FILTERED WATER
               OUTLET
          ANGLE IRON
           FRAME
                        RAW WATER
                          INLET
                                     BAFFLE
   FILTERED WATER
     MANIFOLD
                                                           DRAIN
                                           FIGURE 8-13

                                    VERTICAL LEAF VACUUM FILTER
                                       (Courtesy of Johns-Manville)

-------
                  FIGURE 8-14
VERTICAL LEAF PRESSURE FILTER, VERTICAL TANK
            (Courtesy of Johns-Manville)
                   8-35

-------
                                     TABLE 8-9

     DIATOMACEOUS EARTH FILTRATION OF SECONDARY EFFLUENT
                                       Turbidity	
Flow Rate       Body Feed        In             Out         Run Length       Type
gpm/sq ft          mg/1          JTtJ           JTU             hr

  0.53              42             5.5             0.8             19.5         Vacuum
  0.75              33             5.2             0.8             10.7         Vacuum
   1.0               19             4.4             0.4              5.4         Vacuum
  0.50              50             8.2             3.1             50.0         Pressure
  0.81              42             8.3             3.9             28.4         Pressure
   1.0              45             7.5             3.0             31.0         Pressure
 requirements (27). Acceptable operation was possibly only with very low influent solids (3
 to 13 mg/1). None of these studies considered the possibility of  recovering DE filter aid,
 which could reduce the estimated costs significantly (28).


 8.6    Ultrafiltration

     8.6.1   General

 Ultrafiltration (UF) is the title given to a form of membrane separation which employs
 relatively coarse membrane separation at relatively low pressures. The process should be
 differentiated from  reverse  osmosis which is a similar process used for dissolved solids
 separation  using  fine membranes  and  high pressures.  Ultrafiltration,  using a thin
 semi-permeable polymeric membrane is reported most successful in  separating suspended
 solids as well as large-molecule colloidal solids (0.002 to 10.0p.) from wastewater (29).
 Fluid transport and solids retention are achieved by regulating pore size openings. Thus,  the
 Ultrafiltration process is a physical screening through molecular-sized openings rather than
 one controlled by molecular diffusion.

 A system  employing high-MLSS aeration followed by UF operated at  Pikes Peak since
 1970 has proven capable of removing virtually 100 percent of the suspended matter and 93
 to 100 percent of the associated BOD, COD and TOC from aerated mixed liquors (30).

      8.6.2  Application

 Several installations (29) (30) (31)  have proven  the ability of the  activated
 sludge-ultrafiltration process to  remove all SS and almost all bacteria  and  BOD. These
 systems are typified by Figure 8-15. Results of such installations are given in Table 8-10.
 Because UF installations have all produced zero SS effluents, other parameters are given to
 illustrate process capability. More detailed data on the Pikes  Peak  installation is given in
                                        8-36

-------
                           POTABLE WATER SUPPLY
               POTABLE

                 USE
oo
i
t^j
-j
                                                                        DISCHARGE
                                                     DISINFECTION-
 NON-
POTABLE
 USE
                                           ULTRA-
                                         FILTRATION
                                            CELL
                                                                                    MEMBRANE
                               HOLDING
                                  TANK
                     GRINDER
  HIGH-SOLIDS
ACTIVATED SLUDGE
   PRESSURIZED
    REACTOR
     (AERATOR)
                                                                                      en
                                                                                      H
                                                                                      CO
                                                                                      IT"
                                                FIGURE 8-15
                      SCHEMATIC FLOW DIAGRAM OF THE PIKES PEAK TREATMENT & REUSE SYSTEM

-------
Table 8-11. Coliform counts in all instances were above zero but were attributed to outside
contamination rather than passage through the membranes (30) (31).
                                  TABLE 8-10
            RESULTS OF ULTRAFILTRATION INSTALLATIONS

                        Effluent                                       Superficial
             Capacity   BOD    Coliform     Flux         Pressure     Flow Rate
               gpd      mg/1    No./100 m sq ft final        psi.   >       fps

Fabric Fire
    Hose,     3,600     2-15     1-100*     18     8        22-27          4-6
Sandy Hook            (av. 5)

Pikes Peak    21,000    1        0-11**     30     6        50             5-6
                                           16     9
* includes deliberate upset tests
**attributable to external contamination
The major drawback of-ultrafiltration is the high capital and operating costs. Phosphate
and color removal are both negligible, but they may not be necessary in many places. The
high cost may be offset by compactness where space is a critical factor, such as on a ship or
a mountain top. A 6800 gpd shipboard installation was designed to occupy a volume of 6 x
8x9ft(31).

     8.6.3   Design

The most important design considerations are:

     1.   Membrane area

     2.   Membrane configuration

     3.   Membrane material

     4.   Membrane life

     5.    Driving force.
                                      8-38

-------
                                                    TABLE 8-11

                                            SUMMARY OF PIKES PEAK DATA
oo
I

OJ
Parameter
BOD (mg/1)
COD (mg/1)
TOC (mg/1)
TSS (mg/1)
FLUX (gpd/sq ft)
MLSS (mg/1)
Fecal Coliform
(per 100ml)
CSSTP '70 Summit '70
72 days 22 days
Inf. Eff. Inf. Eff.
382 1 285 1
678 20 547 32
192 7.5 136 6.6
323 0 129 0
30 to 6
3954
0 0
Summit '71
49 days
Inf. Eff.
362 1.3
738 52
197 9.8
172 0
16 to 9
4156
6
Weighted
Summit '72 Average
83 days 226 days Percent
Inf. Eff. Inf. Eff. Removal
426 6.2 384 3 99.2
839 536 737 40.5 94.6
185 8.1 95.6
263 0 249 0 100
—
11 9

-------
   8.6.3.1   Area

Membrane area is a function of flux which is determined by membrane construction and the
fouling characteristics of the wastewater. A flux of 8 gpd/sq ft has proven satisfactory at
the Pikes Peak installation and can be used  as  a  normal design figure in calculating
necessary area. Membrane flux tends to decrease with time due to surface fouling.  It has
been  found that physical  elimination of foulants, mostly  organic acids and  polarized
materials, lessens their flux-reducing effect (29). By operating the process at liquid velocities
of 3  to  10 fps parallel  to  the membrane  surface, scouring of contaminants can be
accomplished  and a more stable flux achieved (29). At such velocities, with normal
membrane  fluxes,  single-pass design would require  impractically  large membrane area.
Therefore,  the wastewater is  recirculated as shown  in Figure 8-15. Some blowdown of
concentrated waste results to prevent excessive solids buildup. The blowdown can be
intermittent, at a rate sufficient to keep the MLSS within acceptable ranges, usually 4,000
to 15,000 mg/ 1 (29) (32).

    8.6.3.2   Membrane Configuration

This  aspect of  design  concerns  the  amount of membrane surface area which can be
incorporated into a module. Because of low membrane fluxes it is imperative to design the
module  to  maximize  membrane surface  area.  One configuration adopted  solely for
ultrafiltration is the storage battery configuration, as shown in Figure 8-16. The membrane
is cast on both external  faces of a hollow  plate. A number of these plates are arranged in a
parallel array. The edges  of these plates  face the incoming stream of solids and act as a
coarse screen which can be backwashed by reversing the direction of the approaching flow.
Other designs include tubular  support elements  over which a membrane is wound helically
or in which the membrane is enclosed in a continuous spiral.

   8.6.3.3   Material

The membrane itself is made  up  of two basic layers:


   1.   Surface—an  extremely  thin  homogenous  polymer  of 0.1 to  10.00  microns
       (typical, 5.0 microns).

   2.   Surface support—an open cell of  5 to 10 mil thickness
The membrane,  in turn, is supported on a porous sheet (paper) for added mechanical
support.

The thin  surface layer controls the transport and rejection properties of the membrane.
Numerous means and types are available and can be tailored to the particular application.
Typical membrane specification ranges are listed in Table 8-12.
                                       8-40

-------
                                                                  Outlet
oo
                Recirculating
                flow inlet
                  Ultrafiltrate
                                                                     Ultrafiltration
                                                                     cartridge
ili
                                                                                 Cover
                                              FIGURE %-\6
                                  "STORAGE BATTERY" MEMBRANE MODULES
                                          (Courtesy of Dorr-Oliver).

-------
                                    TABLE 8-12

                    TYPICAL MEMBRANE SPECIFICATIONS

Material                                                  Most organic polymers
Water Permeability                                        7-290 gpd per sq ft at 30 psi
Molecular weight                                          340-45,000
Retentivity                                                60-100 percent
Maximum Operating
  Temperature                                            50-120°C
Water permeability is used to characterize the porosity of the membrane, but does not
represent the stabilized, long-term flux on  a  process fluid. In the waste treatment field,
fluxes of 7 to 10 gallons per square foot of membrane surface per day are typical (31).

Given the current state of membrane manufacturing technology,  almost any set of clean
water performance characteristics, without  consideration for fouling can be produced. A
few of the leading manufacturers of ultrafiltration membranes are Romicon Corp., Abcor,
Inc. and Dorr-Oliver, Inc. Catalogues offering a wide variety of membranes are available.

    8.6.3.4  Membrane Life

Membrane life is  a function of fouling  and  required  flux rates. A  membrane may be
considered acceptable for a life span of 6 months in continuous operation with an initial flux
of  18 and a final flux  of 8 gpd/sq ft. A  plant must be designed for the lower figure and
membrane replacement made when the design figure is reached.

    8.6.3.5   Driving Force

The driving force for transport  of  water through the membrane is pressure. Operation is
achieved  at pressure gradients  of  approximately 25 psi. Total system pressures do not
exceed 50 psi. Very recent work has shown that vacuum extraction of the product can be
used advantageously in certain applications (31).
                                       8-42

-------
8.7 References
1.  Ginaven, M.E., The Hydrasieve—A New Simplified Solids-Liquid Separator; Paper
    Trade Journal (January 19, 1970).

2.  Ginaven, M.E., A New Low Cost Device for Solids Recovery from Effluent; Cost En-
    gineering; 16,4, pp 4-10 (Oct., 1971).

3.  Wittenmyer, James D., A Look at the Future Now, Presented at the Ohio Water Pol-
    lution Control Conference (June 20, 1961).

4.  Wittenmyer J.D., Operating Experience, Ohio Water Pollution Conference (June  16,
    1972).

5.  Wong, Alan, Personal Communication, Hazen and Sawyer.

6.  Bishop, D.F. Jr.,  U.S.  EPA Internal Monthly  Report,  May,  1972, Contract No.
    68-01-0162.

7.  O'Farrell, Tom, Personal Communication, Sanitary Engineer, U.S. EPA Blue Plains
    Pilot Study.

8.  Lee, R., Personal Communication, Hercules, Inc., AWT Division.

9.  Hydrocyclonics Corp., Rotostrainer Bulletin.

10.  Diaper, E.W.J., Personal Communication, Crane Co. (April, 1973).

11.  Diaper, E.W.J., Tertiary Treatment by Micro straining. Water and Sewage  Works,
    776, pg. 202 (June 1969).

12.  Diaper, E.W.J.,  Oxidation  Pond Effluent Improvement, 49th Texas Water and Sew-
    age Works Associations's Short School, College Station, Texas (March 1967).

13.  Mixon, P.O., Filterability Index and Microscreener Design, Jour. WPCF, 42, pg. 1944
    (Nov. 1970).

14.  Boucher, P.L., A New Measure of the Filterability of Fluids with Application to Wa-
    ter Engineering, ICE Jour.  (Brit.), 24, 4, pg. 41'5 (1947).

15.  Bodien, D.G.,  and Stenburg, R.L., Microscreening Effectively  Polishes Activated
    Sludge  Effluent, Water and Wastes Engineering, 3, pg. 74 (Sept. 1966).

16.  Lynam, B., Ettelt,  G., and McAloon, F., Tertiary Treatment at Metro  Chicago by
    Means of Rapid Sand Filtration and Microstrainers, Jour., WPCF, 41, pg. 247 (Feb.
    1969).

17.  Havens and Emerson Ltd. Consulting Engineers, Report on Wastewater Treatment Pi-
    lot Plant for the City of Euclid, Ohio (April 30, 1971).

                                       8-43

-------
18.  Sapper, John, Personal Communication, Zurn Industries (April, 1973).

19.  Isaac, C.G. and Hibberd, R.L. The Use of Microslrainers and Sand Filters for Ter-
    tiary  Treatment. Water Research, (Britain) 6, pg. 465, (1972).

20.  Trusdale, G.A., and Birkbeck, A.E., Tertiary Treatment of Activated—Sludge Ef-
    fluent, Metropolitan and Southern Branch  International Filtration Exhibition and
    Conference, The Institute of Water Pollution Control (Brit.), 67, pg. 483 (1968).

21.  Microscreening Plant for Effluent Polishing, Effluent and Water Treatment Jour., Per-
    mutit, (July 1971).

22.  Saucer, Victor, Concentrator and Floatation Cell Increase Wastewater Plant Capacity
    Fourfold, American City 88, 4, pg. 49, (April,  1973).

23.  Falby, W.J.,  Personal  Communication,  SWECO Inc.,  Los Angeles, Calif.  (May,
    1973).

24.  Bell,  G.R., Design  Criteria for Diatomite Filters, Jour. AWWA, 54, pg. 1241 (Oct.
    1962).

25.  Wells, W.N. and Davis, D.W., Filtration of Activated Sludge Plant Effluent. Public
    Works, 98, 4, pg. 94 (Apr. 1967).

26.  Summary Report—Advanced  Waste Treatment  Research Program (1964-1967),
    U.S.D.I., FWPCA Publication WP-20-AWTR-19 (1968).

27.  Guiver, K., and Huntington,R., A Scheme for Providing Industrial Water Supplies by
    the Re-use of Sewage Effluent. Water Pollution Control (G.B.),  70, pg. 74 (1971).

28. Filter and Recovery Systems Bulletin FF 206. Celite Technical Data, Celite Div.,
    Johns-Manville Corp.  (1971).

29.  Smith, C.V. Jr., Di Gregario, D. and Talcott, R.M., The Use of Ultrafiltration Mem-
    branes for Activated Sludge Separation, Purdue Ind. Waste Conf. 5/7/69. Water Re-
    novation of Municipal Effluents by Reverse Osmosis, Water Quality Office, U. S. EPA.
    Project EPA  17040 EOR (Feb. 1972).

30. Bemberis,  Ivars,  Personal  Communication, MST  Marketing,  Dorr-Oliver Inc.
    (March, 1973).

31. Bemberis, Hubbard and Leonard, Membrane Sewage Treatment Systems. American
    Soc.  of Agric. Engrs.,  1971 Winter Meeting.

32. Schwartz, Warren A., Record Memorandum,  Office of Program Co-ordination, U.S.
     EPA, (July 6,  1972).
                                         8-44

-------
                              CHAPTER 9

                 GRANULAR MEDIA FILTRATION

9.1   Introduction

    9.1.1  Applications and General Description

This process, long applied in treatment of municipal  and industrial water supplies, is
becoming widely used for  waste water treatment both in upgrading existing  conventional
plants and in designs of new advanced treatment facilities. Next to gravity sedimentation it
is the most widely used process for separation of wastewater solids. The following specific
applications have been noted (1):

      1.  Removal of residual biological floe in settled effluents from secondary treatment
         by trickling filters or activated sludge processes.

      2.  Removal of residual chemical-biological floe after alum, iron, or lime  precipitation
         of phosphates in secondary settling tanks of biological treatment processes.

      3.  Removal of  solids remaining after the chemical coagulation of wastewaters in
         tertiary or independent physical-chemical waste treatment.

In  these  applications  filtration may  serve both as an  intermediate'process to prepare
wastewater for further  treatment (such as  carbon  adsorption, cli-noptilolite  ammonia
exchange columns,  or reverse osmosis) and  as a final polishing step  following  other
processes.

Granular media, filtration involves passage of water through a bed of granular material with
resulting deposition  of  solids. Eventually the  pressure  drop across the bed  becomes
excessive or the ability of the bed to remove suspended solids is impaired. Cleaning is then
necessary to restore operating head and effluent quality to acceptable levels. Most filters
operate on a batch basis, the entire unit  being removed from service for periodic cleaning.

The time in service between cleanings is termed the  run length. The head loss at which
filtration is interrupted for cleaning is called the terminal head loss.

    9.1.2  General Design Considerations

Filter design involves selection of the following filter characteristics (1):

       1. Filter configuration

      2. Media sizes and depths and materials

       3. Filtration rate (gpm/sq ft)
                                          9-1

-------
       4. Terminal head loss (ft of water)

       5. Method of flow control

       6. Backwashing design features

The major goal  in design is to  achieve effluent  quality  objectives  at  low capital  and
operating  costs. The  most important characteristic in determining capital costs  is the
filtration  rate,  which fixes the  filter size.  Operating costs  are affected  primarily by
filtration  rate, terminal  head loss, media characteristics and backwash design. The  first
three  filter characteristics determine  the cost  of power for operating  head and the
production of the filter per run. The backwash design determines the cost per cleaning of
operator attention, washwater pumping, air scouring (compressor operation) and treatment
of dirty washwater. The cost of cleaning per unit volume treated (cost per cleaning divided
by production per run) depends on all four factors.

Section 9.3 discusses  the interrelation and effects  on performance of process variables
describing the characteristics of filters and of influent wastewaters.  Many investigators have
attempted to relate filter performance quantitatively to these variables (2) (3) (4) (5) (6).
Unfortunately,  such  relations are of little help in  predicting performance except  under
specific conditions already explored in pilot work. In part, this is due to the wide variations
in the filtering characteristics  of wastewater. solids  and to the dearth of  reproducible
objective data from well-conceived studies of wastewater filtration. In part, however, it may
be  inherent in  the nature of the  filtration  process that any  fully general quantitative
relations  describing it would be too  complex  for  practical  use.  Nevertheless existing
theoretical relations  are useful  in providing:  1) general  insight  into  filter  behavior, 2)
frameworks for analysis of data from pilot investigations and 3) bases for comparing cost
effects of alternatives in  specific applications.

     9.1.3  Basis for Design

Wherever  possible, designs should be based on  pilot filtration studies of the actual waste
(Section 9.9).  Such studies are the only way to assure:

     1. Meaningful cost  comparisons between different filter designs capable of equivalent
       performance (7), i.e., producing the  same output  quantity and  quality  over  the
        same time period.

     2. Most  economical  selection of filter rate, terminal  head loss and run  length for a
        given media application.

     3. Definite effluent quality performance for a given media application.

 Pilot  studies are  also  useful for determining  effects of pretreatment variations  or for
 characterizing Jilterability in terms of performance attainable with a specific filter design.
 Where there is no opportunity for pilot studies, parameters for workable designs can still be
                                         9-2

-------
determined  from the discussions of wastewater and filter characteristics in the sections
below. The  parameters will necessarily be conservative and will tend to give more costly
designs and less assurance of effluent quality than parameters based on testing. Facilities
designed without pilot testing  are  likely to be small ones, for which the design should
provide long filter runs and minimize required operating attention.

Another approach to obtaining economical facilities is to prepare a functional specification
which  will  permit  competitive  bidding between  suppliers of  alternative filter systems.
Functional requirements should include:

     1.  Guarantees of  specified performance; both capacity and effluent quality.

    2.  Guarantees of  proposed values of all factors which affect operating costs such as
        head or power  requirements and backwash volume to be recycled.

Bids should be evaluated based on total present  worth including operating costs, which
should be calculated by a predetermined formula using factors in the guarantee.

This approach will work best when bidders are  supplied test results characterizing the
filterability  of the waste flow. In any case, bidders should  be given full information on the
wastewater and treatment to  be provided ahead  of filtration plus the opportunity of testing
effluents from such treatment, where already in operation.

9.2  Process Alternatives

Filter  units generally  consist of a containing  vessel,  the granular  media,  structures to
support or retain the  media, distribution and collection devices for influent, effluent and
washwater flows, supplemental  cleaning devices, and necessary controls for flows, water
levels or pressures.

Some of the more significant alternatives in filter layout are discussed below.

     9.2.1   Alternative Flow Directions

Most filter  designs  employ  a  static bed with vertical flow either downward or  upward
through the bed.  The downflow designs  traditionally  used  in potable  water treatment
(Figures 9-1  and 9-2  (a) (d) and (e)) are  most common,  but  recently  a  number of
installations have been designed for upward  flow (Figures 9-3  and 9-2 (b)). The  Eurdpean
biflow design (Figure  9-2  (c))  employs both  flow  directions with the effluent withdrawn
from the  interior of the  bed.  Upflow  washing is used regardless of the operating flow
direction. Two special  filter designs employ horizontal radial flow through an annular bed.
Media is cycled downward through the bed, withdrawn at the bottom, externally washed,
and returned to the top. (See Section 9. 10).
                                        9-3

-------
                                                                  Operating-.
                                                                    table
                                                                                         Rate of flow and loss
                                                                                         of head gages
vo
                                                        Operating
                                                          floor
                                                    Pipe gallery
                                                       floor
                                                          Filter drain
                                                        Filter to waste
                                                   Perforated
                                                    laterals
                                                          Cast-iron/
                                                           manifold
                                                                                       -Filter
                                                                                        floor
Filter bed wash-
'water troughs
                                                                                                                                            Influent to filters
           Concrete filter .
               tank

        Pressure lines to
        hydraulic valves from
        operating tables
                                                                                                                                          Effluent to
                                                                                                                                          clear well
                                                                                                                                  Drain
                                                                                                 FIGURE 9-1
                                                                                     TYPICAL RAPID SAND FILTER

-------
                EFFLUENT
              INFLUENT
     30-40'i-

UNDERDRAIN —
 CHAMBER
                                     OVERFLOW TROUGH
                                             GRID TO
                                             RETAIN
                                             SAND
                                               EFFLUENT
EFFLUENT
INFLUENT
                                                STRAINER
                                              UNDERDRAIN
                                               CHAMBER
                                                                               INFLUENT
(a), CONVENTIONAL FILTER     (b), UPFLOW FILTER


                               SINGLE MEDIA FILTERS
                                             (c), BI-FLOW FILTER
            30-40"
                         ANTHRAFILT
            (d), DUAL MEDIA FILTERS
                     —COARSE MEDIA—
    — INTERMIX<
        ZONE  \
   •—FINER MEDIA ^


     FINEST MEDIA
                                        CHAMBER
                                                     ...fc».  •• ..• -.
                                                     ANTHRAFILT
                                                                \
                                                          28-48"
                                                                  GARNET SAND
                                    (e),MIXED-MEDIA FILTERS

                                      (TRIPLE MEDIA)
                                      FIGURE 9-2


                             FILTER CONFIGURATIONS
                                           9-5

-------
  COVER OPTIONAL
(FOR CLOSED SYSTEM)
       "GRID"

    DEEP SAND LAYER
      GRAVEL LAYERS
            1
  INLET RAW WATER
WASH WATER
                                        FILTRATE OUTLET
                                SAND "ARCHES"
                                       SPECIAL VENT
                                              AIR FOR
                                        SANDFLUSH CLEANING
                            FIGURE 9-3

                  CROSS SECTION OF UPFLOW FILTER
                             9-6

-------
    9.2.2  Gravity vs. Pressure Filtration

Filters may be designed with closed vessels permitting influent pressures above atmospheric
(Figure 9-4)  or with open vessels where only the hydrostatic pressure over the bed is
available to overcome filter headlosses (Figure 9-1). Pressure units are generally preferable
where high terminal headlosses are expected or where the additional head will permit flow
to pass through downstream units without  repumping (1) (8). They are most commonly
used  in small-to-medium-sized treatment  plants where  steel-shell package units  are
economical (8).

    9.2.3  Media Alternatives

Figure 9-2 shows schematically  a number of different filter configurations using fixed bed
media. The beds shown are all graded during upflow washing so that the finer material of a
given  specific gravity is on top. It should be noted that the conventional single media filter
used in potable water treatment (Figure 9-2(a)) is generally unsatisfactory for wastewater
treatment because the  wastewater solids cause  a  high headloss buildup at the fine surface
layer.

In upflow designs, flow passes first through the coarser media which  for a given head loss
buildup has greater  capacity  for  retaining filtered  solids.  This  is  advantageous  in
lengthening filter runs and increasing output. Dual and multi-media (Figure 9-2 (d) and (e))
obtain the same effect under  downflow operation by  placing coarser  layers of lighter
material over finer denser material. An  alternative downflow single-media configuration
not shown attempts to get the same advantage from  use of beds of  uniform-sized coarse
media with depths of 60 in. or more. The effects of significant  media characteristics such as
size gradation,  specific gravity and depth, on filter performance are  discussed in  detail in
Section 9.3.

In filters using  external wash, the media  is  not vertically graded; particle size distribution
tends to be the  same throughout the bed.

    9.2.4  Batch vs. Continuous Operation

It is normal practice to design filters to operate on a batch basis with entire units taken out
of service for  cleaning  according to schedule or as required. Several special  designs,
however,  provide more or  less  continuous  cleaning,  either externally with  media cycled
through the bed, or in-place with techniques  such as traveling backwash or air pulsing of the
bed and air mixing of the liquid above it.

    9.2.5  Normal Design

Most of this chapter will relate to fixed bed systems with intermittent upflow washing. Both
upflow  and downflow designs will be  included as normal design. Proprietary designs using
non-fixed beds  or special washing systems are discussed in Section 9.10.
                                        9-7

-------
SIG PRESSURE
VESSEL-^
A

1 COUPLING
AIR RELEASED


L/IC
1

V
1 \
MEDIA
,
al p Q o o o o
V
9 9 9 9' 9 9 99 909

B 9/
-4
I2"x 16" MANHOLE
ON VERTICAL
  OF TANK
                                                    FLANGE INFLUENT
                                                 BACKWASH WASTE
                                                     2 FLANGE
                                                     SURFACE WASH
                                                     JO"FLANGE EFFLUENT
                                                     AND BACKWASH
                                         \
                                          X2" FILTER DRAIN
                       FILTER SUPPORTS
                       AT 1/4  POINTS
                       ELEVATION
DISTRIBUTOR-

     s
                        8'-O" O.D.
                                         I2"x 16" MANHOLE
                                         SURFACEWASH
                                                MIXED  MEDIA
                                     LATERALS
                       S ECTI 0 N


                        FIGURE 9-4

                TYPICAL PRESSURE FILTER
              (Courtesy of Neptune Microfloc, Inc.)
                                                SUPPORT GRAVEL

                                                CONCRETE
                             9-8

-------
9.3  Process Variables

     9.3.1  Performance Relations

The measures of filter performance are output quality and quantity. The variables which
determine or limit performance fall into two major groups: influent characteristics and the
physical characteristics of the filter. The latter include media characteristics, filtration rate,
available and applied operating head and the design and operating parameters of the filter
cleaning system.

In determining the fundamental limits on quality performance.the characteristics of prime
importance  are those  of the  influent solids: concentration,  strength,  size,  and the
physical-chemical properties governing adhesion of particles to each other or to the media
surfaces. Commonly a  number of filters with different physical characteristics can come
close to  the limiting  quality performance  for  a given influent.  In contrast,  quality
performance of given filters can vary widely  for different solids characteristics.

In determining output quantity from filters, the influent solids characteristics—especially
floe strength  and  solids  concentration—are again  very  inportant,  but  the  physical
characteristics of the filter become significant too.

At run lengths of 24 nr or more, output depends almost totally on filter rate. As run lengths
become shorter, however, the effects of downtime and washwater recycle during cleaning
become increasingly important (See Section 9.4). The  washwater recycle volume depends
on the backwash flow rates and the wash cycle duration needed for adequate cleaning (see
Section 9.7). Factors governing backwash system design include:

     1.  Size distribution, depth and specific gravity of media

     2.  Nature of  solids removed, principally their  adhesion to the media and their
        tendency to compact in a dense layer at the media surface

     3.  Type of supplementary cleaning provided.

Run length may be limited either by available head or by deterioration of effluent quality as
the  filter bed becomes filled with solids (breakthrough). Which factor governs depends on
the  interaction of several variables including:

     1.  Influent solids characteristics (all those which affect quality performance)

     2.  Flow rate

     3.  Temperature and viscosity of the wastewater

     4.  Media characteristics

     5.  The amount of head available.
                                        9-9

-------
Headless in a clean bed varies directly with  filter rate and inversely with grain  size.  In
determining head loss buildup, the  most significant  media characteristic  is the grain  or
pore size at the influent surface of the bed (or in some cases within finer denser layers of
multi-media filters). In downflow filtration through a graded bed, influent solids particles
larger  than about 7 percent of the  minimum  grain size (9) will be  removed by straining
provided their strength is sufficient to withstand the shear at the surface. Shear varies with
filter rate and liquid viscosity.

In surface straining, head loss increases exponentially with time  or solids accumulation (See
Chapter 8). Where significant solids loads are removed predominantly by surface straining,
head loss buildup will be rapid, filter runs short and backwash  frequency high. In addition
the solids removed at the surface tend to be compressed into a dense mat which is  difficult
to remove in backwashing.

Removal of solids within the bed rather than just at the surface is termed depth filtration.
Both  surface  and depth  filtration  are usually  involved  to  some  degree in any  given
application.

In  depth filtration  head  loss  tends  to build up  linearly  with  time  or  with  solids
accumulation. Compression of the solids removed is limited by the granular structure of the
bed. For downflow filtration within a single media, the farther solids  penetrate into  the bed,
the slower will be the rate of head loss buildup, but the sooner solids  will breakthrough into
the effluent.

The factors which determine breakthrough for a single media are the media size and depth,
the flow rate and the resistance of deposited materials to shear within the bed. Hudson (10)
suggested characterizing the resistance of solids to breakthrough by an index, K, calculated
from  the physical characteristics of the  filter and the head loss at which breakthrough
occurs. The expression for the index is:

                                     K = Vd3H/L

     Where:

                        V = filtration rate—gpm/sq ft
                        H = head loss at breakthrough—ft
                        d = effective size of media—mm
                       i L = bed depth—ft

     9.3.2   Influent Characteristics

The influent  characteristics of prime importance  in  determining filter performance are
those  of  the solids  to be removed.  The only significant characteristic of the wastewater
liquid—as opposed  to the solids—is viscosity which varies  with temperature. Its effects on
development of filter head loss  are generally small, however, in  comparison to the effects of
solids  accumulations or filter rates.
                                        9-10

-------
 The  characteristics of wastewater solids  which govern  or limit  filter performance are
 determined by the treatment processes ahead  of filtration (see Section 9.1).  In direct
 filtration of  secondary biological  effluent the residual  solids applied to  the  filter are
 predominantly biological floe grown in the treatment process.  In filtration of effluent fol-
 lowing tertiary coagulation for phosphate removal the residual solids are predominantly
 chemical floes. In filtration of chemically precipitated raw wastewater or primary effluent,
 the solids consist of inorganic chemical floe with varying quantities of precipitated organics.

 Loading, media and  performance data for filter applications of the above three types are
 shown in Tables 9-1, 9-2 and 9-3. Most data are for full scale installations but a few large
 pilot facilities are included. The data  are  those ordinarily  recorded in tests of  filter
 installations. These data show only that the systems filtering physical-chemical  floe  tend to
 use somewhat lower filter rates  and  somewhat finer  media  (with dual  or multi-media
 configurations almost standard) and  that  effluent results for  any given type  of influent
 source may vary considerably.

 Compiling data from a  number of filter installations treating biological effluents,  Kreissl
 (33)  found  removals ranging from 50  to  90 percent with  a  mean of about  70 percent,
 provided influent solids were below 35 mg/ 1. Included  were data for  a variety of loading
 rates media configurations and types of prior  biological treatment. Subsequent compilation
 of similar data for effluents from chemical treatment systems showed mean solids removals
 of  only  60  percent (34), indicating that on  the average  chemical  floe tends  to be more
 difficult to filter.

 The only influent solids characteristic  included in routine filter testing is the concentration,
 perhaps because it is the  only one that is easily measured.  A few special studies  have
 attempted  to  take  into  account  other characteristics such  as  floe strength, particle size
 distribution (concentration  vs. size) and properties governing adhesion of particles to each
 other or to the media. Some other studies have tried  to distinguish  differences in  filter
 performance according to parameters of the treatment prior to filtration.  Outlined below
 are a few significant additional insights into wastewater  filtration provided  by these special
 studies.

    9.3.2.1   Floe Strength

Biological floes tend to be significantly stronger or more  resistant to shear than chemical
floes, at least those from alum or iron coagulants (2). Consequently, in filtering biological
floes, surface  straining is generally significant and runs are almost always terminated by
excessive head  loss. Breakthrough is rarely observed. In one study head losses as high as 30
ft were applied without deterioration of effluent quality (35). This contrasts with alum and
iron (hydroxide) floes which have been shown to penetrate readily  into filters and  to
breakthrough  at relatively low heads  ranging from 3 to 6 ft (2) (10) (36). In  an isolated
instance where breakthrough of biological floe was observed, the index  K was found to be
 13.7,  far above the range of  0.3 to 3.6 cited for alum or iron floe in water treatment (37).  In
contrast to floes  from  other common coagulants, calcium  carbonate precipitates are
strongly removed at the filter surface where they may form a dense compressed layer hard
                                       9-11

-------
                        TABLE  9-1
                  RESULTS OF STUDIES OF
FILTRATION OF EFFLUENT FROM SECONDARY BIOLOGICAL TREATMENT
LOCATION

Luton, G.B.



•Hanover Park,
Illinois









Walled Lake-Novi,
Michigan
Louisville, Ky.
(Hite Creek)
Coldwater,
Michigan

i i r L. wr
FILTER

Gravity
Downflow
Gravity
Downflow
Gravity
Downflow

Pressure
Upflow

Pressure
Downflow



Gravity
Downflow
Pressure
Horizontal
Pressure
Downflow

i n r LI u u 11 i
SOURCE

Activated
Sludge
Trickling
Filter
Activated
Sludge

Activated
Sludge

Activated
Sludge



Activated
Sludge
Activated
Sludge
Trickling
Filter

MEDIA
type
Sand

Sand

Coal
Sand
Garnet
Sand


Coal
Sand



Mixed
media
Coal
Sand
Garnet
Coal
Garnet
Garnet






1


1


1
0



0

1
0
0
0
0
1
SIZE
mm




.2-1.3
.8- .9
.4- .8
-2


.4-1.8
.8-1 .0



.25-2.0

.0 -1.2
.45-0.55
.2-0.3
.8
.4-0.6
.2
DEPTH
in
36

36

30-]
12 r
6J
60


2 4~1
12J



30

16. Si
9 1
4.5J
20~]
20 f»
9 J
11 i ur\n\j LI j. i*
LOADING
gpm/ft2
1.6-4.0

3.4
5.0
2.0
4.0

2.0
4.0
5.0
2
4
6
8
10
3-4

3.4
4.9


IN
mg/1
25-50

28-35
13
14
16

14
15
13
16
15
16
13
18
7

27
21


OUT
rag/1
3-6

9-10
8
4
4

7
6
6
7
5
6
6
8
3

3
8


REMOVAL
percent
72-91

67-74
40
57
67

SO
67
54
56
67
62
54
55
57

89
62


LENGTH
hr
12

_
-
106
27

150
17
7
90
15
22
31
12


-
2.5-8


REFERENCE

(11)

(12)

(13)


(13)

(14)





(15)

(16)
(15)


                         9-12

-------
                                              TABLE 9-1  (CONTINUED)

                                              RESULTS  OF STUDIES OF

                         FILTRATION OF  EFFLUENT FROM SECONDARY BIOLOGICAL  TREATMENT
     LOCATION  '
Bedford Township,
Michigan

Ventura, Cali-
fornia


West Hertfordshire,
G.B.
Ann Arbor,
Michigan

State  College,  Pa,
Springfield, Ohio
1 I ft UJ-
FILTER

Pressure
Downf low
Gravity
Deep Bed
Downf low
Immedium
Up flow
Pressure

Downf low

Pressure
Downf low
Gravity
Downf low

INf LUtN 1
SOURCE

Activated
Sludge

Trickling
Filter
Activated
Sludge


Activated
Sludge
Activated
Sludge
Contact
Stabi-
lization
MEDIA SIZE
type mm
Multi-
media

Sand 1-2

Gravel
Sand 1-2


Mu 1 1 i -
media
Sand

Sand 0.45


DEPTH
in
.


-

261
60J


-

84

10


HI LIKMULil,
LOADING
gpm/ft2
.


6

2.2
4.0
5.0
6.0
6

3-12

5.3


IN
mg/1
15


18

44
37
55
37
42

6

14


OUT
mg/1
3


7

2
4
7
10
5

1

5


REMOVAL LENGTH
percent hr
80 IS


61 6-18

95
90
87
73
88

85 6

64


(IS)



(17)


(18)




(19)


(20)


(21)
                                                       9-13

-------
                  TABLE   9-1  (CONTINUED)
                  RESULTS OF  STUDIES OF
FILTRATION OF EFFLUENTS FROM SECONDARY BIOLOGICAL TREATMENT
LOCATION

Letchworth, England

Upper Stour Main
Drainage, Freehold
IVorks ,' England
Harpenden, U.D.C.

Rodbourne Works,
Swindon, England
Derby, England

Tharaeside, England



Thameside, England



Thameside, England



Ashton-Under-Lyne,
England
i i r c ur
FILTER

Pressure
Upflow
Gravity
Downf low

Gravity
Downf low
Gravity
Downf low
Simater
Radial Flow
Iramedium
Pressure
Upflow

Permutit
Upflow


Simater
Radial Flow


Iramedium
Upflow
i t\ r L U C N 1 	
SOURCE MEDIA SIZE
type mm
Activated Sand 1-2
Sludge
Activated Sand 0.5-2.5
Sludge

Trickling Sand 1 . 1
Filter
Trickling Sand 1.5-3
Filter
Trickling Sand 1-2
Filter
Activated Sand 1-3
Sludge


Activated Sand 0.60-1.20
Sludge


Activated Sand 0.5-1
Sludge


Trickling Sand 1-2
Filter
— — — — — niUKHULlL
DEPTH LOADING
in gpra/ft2
60 5.3

1.2-2.4


1-3

1.6-3.2

4-6

63 3.3
3.3
5.0
5.0
57 3.3
3.3
5.0
5.0
3.3
3.3
5.0
5.0
60 4.5-5.0

IN
mg/1
17

12


20

21

22

9
46
8
37
9
32
11
28
11
51
11
24
30

OUT
mg/1
7

5


5

5

9

2
8
6
10
1
7
4
5
3
7
4
10
8

	 nun
REMOVAL LENGTH
percent hr
60

58


75

75

60

74
84
20
74
86
78
60
83
74
86
62
58
80

REFERENCE

(22)

(22)


(22)

(22)

(22)

(23)



(23)



(23)



(24)

                    9-14

-------
                    TABLE   9-2

               RESULTS  OF  STUDIES OF

FILTRATION  OF  CHEMICALLY TREATED SECONDARY EFFLUENT
LOCATION

Piscataway, Md .

Ely, Minnesota


Jefferson Parish,
La.

Nassau County,
N.Y.
Lake Tahoe,
California
lYFt Uh
FILTER

Pressure
Downf low

Gravity
Downf low


Upflow

Gravity
Oownf low
Two -Stage
Pressure
INhLUhNl
SOURCE

A.S . +2-Stage
Lime Cla- •
rif icat ion
High Rate
+2-Stage
Lime Cla-
rifica-
tion
T.F. tin-
Line Alum
Inj ection
A.S .+Alum
Clarifica-
tion
A.S.+Lime
Clarifica-
tion-*- Ammonia
MEDIA
type
Coal
Sand

Coal
Sand


Sand

Coal
Sand
Coal
Sand
Garnet
SIZE DEPTH
mm in
1.0 1 2~L
0.5 6 J

24~1
1?J


-

0.9min. 36~1
0.35 nin. 12_J
18T
12 f
6 J
LOADING IN OUT REMOVAL LENGTH 1 REFERENCE
gpm/ft2 mg/1 mg/1 percent hr
3 12 8 33 50 (263

2.3 8 <2 >75 24 (27)


3 40 21 48 2.5-6.5 (28)

2.5-3.5 2-10 0-2 80-90 16-48 (29)
2.8-4.0 9-15 0-1 93 4-60 (30)
 Stripping
 and Recar-
 bonation
                      9-15

-------
                             TABLE   9-3

                        RESULTS OF  STUDIES  OF

FILTRATION FOLLOWING CHEMICAL  TREATMENT OF PRIMARY OR RAW WASTEWATER
LOCATION

Washington, D.C.

Lebanon, Ohio
Washington, D.C.
Washington, D.C.
lire ur
FILTER

Gravity
Downf low

Gravity
Downf low
Gravity
Downf low
Gravity
Downf low
inrLucn i
SOURCE

Two-Stage
Lime Cla-
rification
Single
Stage
Lime Cla-
rification
Two-Stage
Lime Cla-
rification
Single
Stage
MEDIA
type
Coal
Sand

Coal
Sand
Coal
Sand
Coal
Sand
SIZE
mm
0.9
0.45

.75
.46
1.2-1.4
0.6-0.7
1.2-1.4
0.6-0.7
— — — — n i u R/\ u L i t, 	
DEPTH LOADING IN
in gpm/ft2 mg/1
IB"! 1.7-6.3' 14
6J

IS"^ 2.0 30
6_
24~1 2.4-4.4 139
24~l 2.3-4.3 123
I'/
	 rvuii
OUT REMOVAL LENGTH REFERENCE
ing/1 percent hr
6 70 12-50 (32)

10 67 - (31)
33 74 ^.24 (25)
23 81 >24 (25)
           Lime Cla-
           rification

-------
to remove during washing (36). Comparative data are lacking on the strength of floes from
precipitation of phosphates  in wastewater using alum, iron or lime.  It is reasonable  to
assume, however, that they are similar to aluminum or ferric hydroxide floe.

Polymer filter aids may be added to the filter influent to strengthen weak chemical floes
thereby permitting operation at higher rates without breakthrough. Doses of 0.1 mg/ 1 or
less are often adequate (8).  Polymers  added  as coagulant aids in upstream settling or
flocculating units may similarly strengthen the residual floe applied to the filters. Ample
head loss must be available to meet losses due to the tougher floe, and doses must be kept as
low as possible to avoid excessive head loss.

    9.3.2.2  Particle Size

Floe particle sizes in settled biological effluent tend to be bimodally distributed. Mean sizes
for the two modes in one study were 3 to 5 microns and 80 to 90 microns (2). About half of
the weight was in each mode. Theoretical work (38) suggests that particles in the lower size
range are much less effectively removed by filtration than those in the higher range. Hence
for the best  quality performance from  filtration, the proportion of smaller size  particles
must be reduced to a minimum by proper flocculation.

    9.3.2.3   Filterability

The filterability of residual solids from secondary settling varies with solids retention time
and with liquid contact time in the biological process. For biological systems with higher
solids  retention times and longer liquid contact times, filtered effluents tend to have lower
suspended  solids. Gulp and Gulp (8) indicated the expected performance of multi-media
filters  for plain filtration in secondary effluents as shown in Table 9-4.

                                     TABLE 9-4

EXPECTED EFFLUENT SUSPENDED  SOLIDS FROM  MULTI-MEDIA
                   FILTRATION OF  SECONDARY EFFLUENT

           Effluent Type                             Effluent SS
                                                       mg/1
           High Rate Trickling Filter                   10-20
           2-Stage Trickling Filter                      6-15
           Contact Stabilization                         6-15
           Conventional Activated Sludge               3-10
           Extended Aeration                            1-5

It is significant that the solids in extended aeration effluents filter particularly well, in  as
much as they often settle poorly, leaving  high concentrations in the secondary effluent. This
behavior may be  understood from the flocculation studies of Parker, et al, (39) who found
that sludges with high solids  retention times lose their tendency to agglomerate into larger
easily settleable particles, but increase in strength so that fewer are broken up into particles
of a size not  readily filtered.
                                        9-17

-------
    9.3.2.4  Headloss Buildup vs. Solids Capture

While effluent  quality reflects  the solids which pass  through  the  filters, headless
development reflects the amount and location of solids which deposit in the bed. Both solids
loading (solids concentration  times flow  rate) and filter efficiency are  important  in
determining the buildup of headloss with increasing solids capture Various studies relating
headloss buildup to solids capture  show widely different  results. This would be expected in
view of the wide range of solids characteristics, media characteristics and filter rates, and
the very different headloss patterns that result from surface and depth filtration. Baumann
and Cleasby (1) cite specific solids  capture values (average over the filter run) ranging from
0.035 to 0.35 Ib/sq ft/ft of headloss. The variation was mainly in activated sludge effluent.
The trickling filter  data, from a single plant in Ames, Iowa, showed values close to 0.07 lb/
sq ft/ft of headloss for a wide range of media sizes and filter rates. British data for trickling
filter  effluent,  however,  showed specific capture values  averaging  0.35 lb/ sq ft/ ft  of
headloss over a filter run with initial values as high as 0.6 Ib/sq ft/ft of headloss (40). For a
fine (0.5 mm)  media  with low  solids loadings, Tchobanoglous and Eliassien (2) reported
values an order of  magnitude lower than the smallest cited by  Baumann and Cleasby. It is
reasonable to  expect  the highest values of specific  capture where the filter and influent
solids characteristics permit depth filtration and extremely low values where they promote a
high degree of  surface straining.

    9.3.2.5  Properties of Solids Affecting Adhesion

Available measures of the properties which affect adhesion of solids particles to other solids
or to media grains are limited to Zeta potential or the related electrophoretic mobility. Very
few studies have included such measures  or attempted to relate performance  to them.

Tchobanoglous (6) reported that reduction of natural negative electrophoretic mobility of
wastewater solids  using  cationic  polymers improved filter quality  performance. Where
sufficient polymer  was added to reverse  the negative charge of the particles performance,
though excellent at first, deteriorated rapidly  after the  first  hour.  With charge reversal,
initial performance apparently was aided by electrostatic attraction between  the negatively
charged filter media  grains and the positively charged wastewater solids. After an hour,
however,  the  grains  were coated  with positively charged  solids,  and  the resulting
electrostatic repulsion interfered with filtration of further solids applied.

    9.3.3  Physical Characteristics of the Filter

Most wastewater filter designs  employ media configurations and loadings which minimize
surface straining and promote depth filtration (Section 9.4). A few special designs with fine
media (Section  9.6)  are intended to remove  solids primarily by  surface filtration or
straining. These designs include provisions  for overcoming the adverse effects of rapid
headloss buildup. Where surface  filtration predominates, the media characteristics have
little  effect on quality performance or  head loss. In addition, removal of solids is quite
independent of filter rate or influent solids concentration (1). Hence the effects of physical
characteristics  of filters are discussed below only in  relation to depth filtration not surface
straining.
                                        9-18

-------
     9.3.3.1  Media Characteristics

The most important media characteristic in determining performance is size. Studies using
uni-size media have clearly demonstrated that finer media have greater removal efficiency
(2) (6) (7) (35) (41).  Various investigators have  related percent  removal to powers of
diameter ranging from -1 to -3 (3).  In finer media headloss per unit of removal (Ib/cu in. of
media) is also higher (2).

In  a  media graded from  fine  to coarse in the  direction  of flow,  the highest  solids
concentration is applied to the layers with the greatest removal  efficiency.  As a result,
removal is concentrated in a small  depth with accompanying high headlosses.

In media graded from coarse  to fine in the direction of flow, substantial penetration occurs
but most of the solids are removed in the coarser media where less head loss buildup results.
The finer layers, protected from heavy  solids  loadings, are available for  polishing and to
prevent breakthrough as the coarser layers become filled with solids.

Media depth  is most  significant in coarse uniform  beds. Because of  the uniformity, the
efficiency of removal (as a percent of the solids applied to each depth) is nearly constant for
all layers of the filter. Penetration is substantial and extra depth is relied upon for polishing
and to retard breakthrough.

Size and specific gravity of media together are significant in determining expansion during
backwash and the degree of intermixing in multi-media beds.

       9.3.3.2   Filter Rates

The effect of filter rates on quality performance can vary widely depending on application.
In  filtering biological floe at reasonably low  influent solids concentration, the effect on
effluent quality of rates up to 10 gpm/sq ft is not very significant (24) (37) (42). In a study
of ultra high rate filtration (43), operation at up to 32 gpm/sq ft still provided 50 percent
removals compared to 75 percent at 8 gpm/sq ft. With weaker chemical floes  or with high
influent concentrations of biological solids (usually indicating poorly functioning biological
treatment) filter effluent quality tends to degrade at filter rates above about 5 gpm/sq ft
(33). Sudden changes  in filter  rates  may  affect  effluent  quality  more adversely  than
sustained higher rates.

Higher filter  rates tend  to increase solids penetration. In cases  where this  significantly
reduces surface removal, head loss buildup per unit volume filtered may actually be less at
.higher rates.  This was  illustrated  in  studies  at Iowa State  University involving settled
trickling-filter  and  lime-softening effluents (35) (36). For  the trickling filter effluent,
production per run (at a given terminal head loss) increased slightly with filter rate  over the
range tested up to 6 gpm/sq ft. For the lime effluent, production per run increased with
filter rate up to 5 gpm/sq ft and then decreased (36). Existence of an optimum rate, as in
the latter study, has been suggested as typical of combined surface and depth filtration (1).
It has also been suggested that the advantages of using a coarse top media layer may be lost
if the filter rate is not high enough to force solids into the bed and limit surface straining (7).
                                        9-19

-------
       9.3.3.3  Cleaning System Variables

In addition to upflow  washing, some form of auxiliary scouring of the media appears
essential to adequately clean  wastewater filters. If cleaning  is inadequate, two serious
problems will develop:  filter bed cracking and mud ball formation. Cracks  open in filter
beds because of compression of excessively thick coatings on the filter grains. The resulting
localized heavy penetration of solids may both lower effluent quality and contribute to mud
ball formation.

Mud balls are compressed masses of filtered solids large and dense enough to remain in the
bed during backwashing. If conditions favoring their formation persist, mud balls tend to
increase in size and to sink  deeper  in the  bed. Their presence increases head loss and may
lead to loss of effluent quality.

Both air scrubbing and surface or internal water jets have been used for auxiliary scouring
of the  media. Air injected below the media produces shear as the bubbles rise through the
bed. Water jets, positioned at the top  of the expanded bed,  produce high shear around the
surface media, which is the  most heavily loaded with solids. In multi-media beds, jets may
be similarly provided at the expanded  height of the media interface.

The main  upflow  wash  and the  auxiliary  scouring systems should  be  controlled
independently to permit use together  or separately. Washing procedures are discussed in
Section 9.7. The key parameters for design of the cleaning system are the upflow wash rate
capacity and the air scour rate or surface wash rate capacity. Typically, upflow wash rates
are about 20 gpm/sq ft. The maximum capacity is selected to provide the desired degree of
fluidization and expansion of the media under critical high temperatures (See Section 9.7).
Capacities for auxiliary scouring  are generally established empirically. Air scour rates
typically range from 3 to 5 scfm/sq ft, and surface wash rates from 1 to 3 gpm/sq ft.

9.4  Selection of Filtration Rate and Terminal Headless

Given  adequate information on  performance,  the  filter rate and  terminal headless for a
particular media design should be selected by making economic tradeoffs between filter
size, operating head requirements and run length, all within the limits dictated by effluent
quality requirements. This section  outlines procedures for such tradeoffs and provides an
alternative basis for selection where specific performance information is lacking.
     9.4.1   Information for Economic Tradeoffs

Adequate  information for making economic  tradeoffs can be obtained only from pilot
studies of the specific media application. (See Section 9.9). Pilot studies should indicate the
buildup of  headloss with  time  for various filter rates and for average and peak influent
solids concentrations.  Results may be indicated in a form similar to Figure 9-5. With this
information it is possible to estimate the filter run length, the net production and the capital
and operating costs of the  filter for the given influent solids concentrations and for different
combinations of filter rate and terminal headloss (See Sections 9.1 and 9.3).
                                        9-20

-------
   50-
   40-
co
CE
13
O
X
O
z
UJ
30-
    20-
    10 -
       i
       0
           CO
           CO
           o
       UJ
       a:
       UJ
               l
              2
                                   PEAK SOLIDS  LOADING
                       	AVERAGE  SOLIDS  LOADING
l
4
l
6
i
8
i
10
                     FILTER  RATE  (gpm / sq ft )


                              FIGURE 9-5

                   RUN LENGTH VS. FILTER RATE FOR
                   VARIOUS TERMINAL HEAD LOSSES
                                 9-21

-------
In determining net production, allowances must be made, for downtime during cleaning and
for recycle of washwater through the treatment plant. The downtime effects are calculated
from the cleaning frequency, cleaning cycle duration and the number of individual filters.
Washwater recycle effects are calculated from the cleaning frequency and the backwash
rate and duration. Washwater recycle has no effect on net production if filter influent is
used for washing. Net production may be expressed as volume (filter rate x run length) or as
an average rate (gpm/sq ft) over one filter cycle (run length plus  cleaning time). The net
production rate is almost the same as the filter  rate for runs of 24-hr  or more. For run
lengths below 10 to 12-hr the differences become  significant (1);  below 6 to 8-hr the effect
on production may be critical.

Short  term peak loadings  due  to  down time or recycle  during  backwash need not be
considered directly in economic tradeoffs.  After the design filtration rate  and terminal
headloss are determined, however,  the  design should be  checked to assure  that it  can
accommodate these peaks within the available headloss and effluent  quality limits. If  not,
peak effects should be reduced or eliminated by increasing the number of filter units or by
providing equalizing storage for  the backwash  and wastewater flows.

The design should also be checked for its ability to handle the sustained peak loads imposed
when a unit is taken  out of  service for repairs. If the resultant  shorter run lengths do
not  provide  enough  capacity,  the  design  may  be revised as  follows: peak hydraulic
loadings should first be reduced by increasing the number of filters keeping the total area
the same;  if this reduction is not  sufficient,  the area  should be increased beyond that
determined in the original design.

Before cost tradeoffs are made, the  following must be defined:
     1.  Maxium flows and solids loadings for various durations up to 24-hr. A tentative
        decision is required on the use of equalization to limit maximum wastewater flows.

     2.  Run length limits. The lower limit should be 6 to 8 hr to maintain reasonable net
        production.  The  upper  limit  should  be 36  to 48  hr to  avoid anaerobic
        decomposition of solids in the filter (1).

     3.  Head loss limits. For gravity filters allowable head losses generally are below 10 ft.
        Use  of  heads much above this  commits the design to pressure filters. Use of
        pressure filters  would  be favored  where  pressurized discharge  to  following
        facilities is needed (1). Gravity filters would be favored where the extra head for
        pressure filters would require intermediate pumping but head for the gravity units
        is available without such pumping.

     4.  Backwash design  and expected  cost per cycle. Manpower costs should  reflect
        whether the operation is to be automated. Backwashing costs should include costs of
        treating recycled backwash in units  ahead of the filters,  and the recycled  flow
        should be deducted in determining the net productio
                                                               Mail cotib 3404T
                                                        1200 Pennsylvania Avenue NW
                                        9-22                Washington, DC  20460
                                                                202-566-0556

-------
     5.   Space limitations. These may force use of higher filter rates.

     6.   Number  of  filter  units.  This  should  be tentatively selected  to facilitate  cost
         estimates, but may be varied with little effect on the tradeoff calculations provided
         labor is not a major factor in the operating cost per backwash. For reliability and
         economy, a minimum of four to six units  should generally be provided,  with at
         least two in even the smallest installations. Above these minimums, the number of
         filter units, depends on the actual size of individual units. The practical maximum
         size of  gravity filters is about 800 sq ft.


In addition to limits indicated above, pilot testing may reveal: 1) upper limits on headloss or
rate required to  avoid  solids breakthrough and effluent quality deterioration,  2)  an
optimum  filter  rate for  minimizing headloss buildup. No  filter  rates lower  than the
optimum should be considered in the tradeoffs.

     9.4.2   Tradeoff Procedures

 The following procedures are suggested for determining the most cost effective filter sizing,
 design terminal head loss and  run length. Figure  9-6, relating net production to filter rate
 and run length, has been prepared to facilitate the analysis. The figure should be modified
 before application if backwashing conditions differ significantly from those assumed in its
 development.

     1.  From filter  test  data  for  average  and  maximum design influent  solids con-
        centration, prepare a headloss development plot (see example plot Figure 9-5).

     2. Assume  initial trial values for terminal headloss and filter run length. (See Item 12).

     3. For the assumed terminal headloss and run length determine the filter rate from the
       headloss development plot for maximum solids concentration.

     4. For this filter rate and the assumed run length determine the net production rate
       from Figure 9-5.

     5. Determine filter sizing based on  this net production rate and the maximum design
       flow for a duration equal to the filter cycle time (run length plus downtime  for
       cleaning).

     6. Estimate capital costs for filters based on above sizing and the design terminal head-
       loss.

     7. Determine average net production by dividing average flow by filter area.

     8. Construct  a plot of net production vs. filter rate based on run lenths to reach  the
       trial value of terminal headloss at various filter rates with average solids concentra-
                                           9-23

-------
    10
     8
I    6
UJ
cc
     5
z
o
o
§    «
cc
Q.
UJ
2    3
                                   INFINITE RUN LENGTH
                                                  71
                                20 HOUR RUN  LENGTH^
                              10 HOUR RUN  LENGTH
                           6 HOUR RUN LENGTH
                        6 HOUR  RUN   LENGTH
NET PRODUCTION  RATE  AT AVERAGE  SOLIDS
LOADING AND IO FT. TERMINAL HEADLOSS
                        I
                              I
I
                       345
                        FILTER RATE
                              6    7
                           (GPM/SQ. FT.)
     8
IO
                               FIGURE 9-6
               NET PRODUCTION RATE VS. FILTER RATE
                    FOR VARIOUS RUN LENGTHS
                                  9-24

-------
       tions. (See Example Figure 9-6).

    9. From  the  plot  in  8. determine  filter  rate and run  length to provide  average
       net production.

    10. Calculate operating costs based  on the assumed terminal headloss and  the  run
       length for average flow and solids loading.

    11. Convert operating costs to present worth and add to capital cost to determine total
       present worth.

    12. Repeat above analysis assuming  different values for terminal head  loss and filter
       runs.  The objective is to find assumptions which minimize present worth, within
       technological constraints.

       It is suggested that a conservative initial value of 8 ft be assumed for terminal head-
       loss with a run length  of at least 8 hr at maximum solids concentrations. Subsequent
       trials  would explore use of higher headloss values to permit either longer runs or
       higher filter rates whichever  appears more advantageous.  Judgement must be ap-
       plied to minimize amount of calculation required.

     9.4.3   Selection Without Pilot Testing

Where it is impossible to test proposed filter media on the actual influent, guidance may be
obtained from results with the same media treating similar influents. In the absence of spe-
cifically applicable test results, filter rates and headloss allowances should be very con-
servatively selected, based on ample estimates of influent solids concentrations. To assure
adequate capacity it is suggested that, as a minimum, sufficient filter area be provided to
handle the 24-hr design flow at 4 gpm/sq ft or the 4-hr maximum  design flow at 6 gpm/sq
ft, whichever is more stringent. For predominantly chemical floe, the surface media should
be no finer than 1 mm and allowance should be made for a terminal headloss of 10 ft.  For
filtration of biological solids in secondary effluent the following procedures are suggested in
selecting terminal headloss and final  filter sizing:

     1.  For  the  minimum filter area as  determined above, estimate headloss  buildup
        based  on  expected  solids  removals  and  the  following  values of  specific
        capture:

              Minimum Media Size
               at Influent Surface                  Specific Capture
                      mm                  Ib of solids removed/sq ft/ft of
                                                  headloss increase

                       1.8                               0.07
                       1.3                               0.035
                                          9-25

-------
   2.  Avoid use of any finer surface media. Surface media coarser than 1.8 mm may per-
       mit higher specific captures but problems of adequate cleaning must be considered
       (See Section 9.7).

   3.  For the minimum filter area calculate the required head for 24-hr  run length at aver-
       age solids loading and for 8-hr run length at maximum (8-hr) solids loadings.

   4.  Provide for terminal headloss on the more critical basis above  or use more than
       minimum filter size and recalculate solids loadings and headloss requirements..

Designs based on the criteria above should be as flexible as possible to permit use of higher
rates or lower heads if operating experience shows this is possible. Flexibility to increase
rates is most valuable where  capacity  is to be increased in  future  stages.  Flexibility in
pumping and control  systems  will permit head to  be  reduced  to what proves necessary in
actual operation.

9.5   Filtration Media

     9.5.1  Materials

Media commonly used in water and wastewater filtration  include  silica sand (sp gr 2.65),
anthracite coal (sp gr 1.4 to 1.6) and in special multi-media designs  garnet (sp  gr 4.2) or
ilmenite (sp gr 4.5).

As they occur in nature these materials are not of uniform size but instead typically have a
grain size distribution such  as that shown in Figure 9-7.  Fair and Geyer (44) discuss size
measures for irregular particles, equivalent diameters, shape effects, etc. Natural grain size
distributions frequently are close  to geometrically  normal, i.e.plot  as  a straight line on log
probability paper. As shown in Figure 9-7, grain size distributions are often  characterized
by two points, the 10 percent  and 60 percent  size  (dio and deo). These  are sizes such that
the weight of all smaller  particles constitutes respectively 10 or 60 percent  of the  whole.
Media is frequently specified in terms of effective  size (d 10) and the uniformity coefficient
(deo/d 10).

It is possible to change the  characteristics of a given media material by removing certain
size  fractions. Coarser fractions may be seived out  while finer fractions may be removed by
"scalping" (removing surface layers) after hydraulically grading  material during  upflow
washing. Fair and Geyer (44) present a method of calculating the size fractions which must
be removed to convert from one size distribution to another.

The most important modification for most media is to remove any very fine particles—say
those less than 80 percent of the effective size. Such fine  material never constitutes more
than a small fraction of the media volume but, if not removed, may cause headlosses  far
greater than would be expected for the given effective size.

With sufficient effort in size separation,  it  is possible to produce almost uniform  media.
                                           9-26

-------
                                                             U.S. Standard Sieves
K)
                                  1" 3/4"   3/8"     4   6    10
20      40    60    100    200
                                                         Grain Diameter in Millimeters

                                                                  FIGURE 9-7
                                                             GRAIN SIZE CURVE
                                                                                                0.1
                                                0.01

-------
Such media are frequently used in experimental investigations, but most designers have not
considered the extra cost justified in full scale installations. Important exceptions should be
noted, however: One equipment supplier, Dravo, emphasizes use of uniform coarse media in
deep beds; Baumann (7) recommends use of uniform anthracite and sand in dual media
filters pointing out that the extra cost is probably not more than 1 percent of the overall
cost of the filters.

     9.5.2  Dual and Multi  Media

Upflow washing stratifies a bed  in accordance with the settling velocities of the media
particles as determined by their size, shape and specific gravity (See Section 9.7). In a dual
or tri-media bed, although  each media  component is  still  graded fine to coarse  in a
downward direction, lighter coarser media can be maintained above finer denser media.
This makes it  possible to approximate a coarse-to-fine gradation in down flow filtration
units. Another advantage of dual or multi-media over a single medium is that mud balls
formed in the filter remain above the coal-sand interface where they are subject to auxiliary
scrubbing action (1).

The maximum settling velocities of media particles also determine the minimum wash rate
required for adequate fluidization of the bed during backwash. Hence, for a given media
size  at the top of the bed, lower wash rates can be used if each media component is more
uniform and the top portion of the filtration is of anthracite rather than a  heavier material.
Baumann and  Cleasby (1) recommend that dual or multi-media be sized so the coarsest
(dgo) sizes of each component have about the same minimum fluidization velocity.

     9.5.3  Pore Size and Intermixing of Media Components

The hydraulic behavior and filtration performance of any given  media are more properly
related to pore size  than to grain size. For single media component, pore  size is directly
proportional to grain size, and the porosity (percent of volume represented by pores) is a
constant depending only on  media shape. Coal which tends to be  angular  has a porosity of
almost 0,5 whereas  sand porosity is  closer to 0.4. In water  treatment applications, coal
media, because of its greater porosity, has been found to give poorer removals but lower
pressure losses than  sand of the same grain size (45).

The pore size in multi-component filter media depends on the degree of intermixing of the
components. With no mixing, pore size distribution simply follows that of the components.
With intermixing,  however,  the finer layers of the denser material below are dispersed into
the voids of the coarser layers of lighter material above. No precise methods have as yet
been demonstrated for calculating actual pore size, or even the degree of intermixing, from
the characteristics of media components.  Where such information is of interest it may be
obtained from test columns or from experience with specific combinations of components in
other installations. Limiting size ratios have  been proposed to control intermixing and to
avoid the extreme  where lower density coarse media is overtopped by very fine high density
media. Camp (46) has hypothesized that for dual media filters with an interface size  ratio
(coarsest coal/finest sand)  of 2.8 no  intermixing should result, whereas for a ratio of 4.0
                                          9-28

-------
intermixing would occur over a depth of about 5 inches. Baumann (7) indicates only limited
intermixing (6 inches) and no overtopping with highly uniform coal and sand having a size
ratio of 3.35.

Gulp  and  Conley (47) indicate  that  to  avoid  overtopping in dual  media  beds,  the
effective size of the coal grains must be  no more than about three times the effective
size   of the   sand.    Whatever  the  exact   limiting  ratio,  designs  using  coarser
anthracite  to  accept   higher  solids  loadings  at  lower  head  losses  must  also  use
correspondingly   coarser  sand.   Where    stringent   effluent    quality   or   weak
floe  conditions  require a finer media component  than the sand, garnet can  be  used.
Coal/garnet size ratios as high as five will not result in overtopping.

The significance and desirability of intermixing in dual or multimedia beds is a subject of
debate. Camp  (46) has reported deliberate  selection  of dual media sizes to minimize
intermixing, whereas some manufacturers actively promote intermixing as advantageous in
three and  four component  media, claiming that controlled  intermixing approximates a
"theoretically ideal" coarse to fine gradation of voids in the direction of filtration.

In side by side tests at Washington D.C. (See  Table 9-3) on chemically treated effluent,
mixed (tri) media filters  did show slightly better effluent quality performance than dual
media  filters.  However there  was  no  evidence  to demonstrate  that  this  was due to
intermixing rather; than just to  the fine,  high specific  gravity garnet  present  at the
bottom of the filters.

    9.5.4  Specific Media Designs

Table 9-5 lists typical  characteristics for several specific media configurations which have
been used in normal design of wastewater filters. All provide initial filtration through coarse
media either by upflow filtration or by downflow filtration using dual, tri or deep, uniform,
media.  Configurations using  fine  single  media and hence  requiring  special cleaning
provisions, are not included. Typical application conditions (floe strength, solids load) are
shown for  each design. The dual-media  designs for the most part employ depths ranging
from 30 to 36 inches. To allow  for level variations due to uneven backwashing, sand depths
are set at 12 to 15 inches even though only the top few  inches significantly affect removals
Minimum  depths  for the  anthracite  are  15 "to 18 inches. Greater depths may be necessary
where solids loads are heavy.

In tri-media designs the overall depths and the minimum depths of anthracite and  of the
combined finer media are in the same range as in dual media designs.

The single media configurations employ depths of 60 inches or more. In downflow filtration
this great depth is intended  to improve efficiency, while in upflow units it has an additional
purpose of adding weight to restrain the bed from uplift due to differential pressures during
operation. Where uplift exceeds the submerged weight of the media it will either fluidize the
bed or lift  it in a "piston" effect (small diameter filters).
                                          9-29

-------
                                                           TABLE 9-5
                                               TYPICAL MEDIA DESIGNS FOR FILTERS
Ul
o
                             Coal
Sand
Garnet
Media
Design

Single
Single
Dual
Dual
Tri
Tri
^
Dia.
mm
—
—
0.9
1.84
1.0-1.1
1.2-1.3
Unif.
Depth Coeff.
in
• — —
—
36 <1.6
15 <1.1
17 1.6-1.8
30

Dia.
mm
1-2
2-3
0.35
0.55
0.42-0.48
0.8-0.9

Depth
in
60
72
12
15
9
12
Unif.
Coeff.

1.2
1.11
<1.85
< 1.1
1.3-1.5
—

Dia.
mm
—
—
—
—
0.21-0.23
0.4-0.8

Depth
in
—
—
—
—
4
6
— — — — i ypicai
Unif. Application
Coeff. Conditions

A
A
B
A
1.5-1.8 B
C

Reference

13
48
29
7
25
13
          NOTES:    A = Heavy Loadings, Strong Floe.
                     B = Moderate Loadings, Weaker Floe.
                     C = Moderate Loadings, Strong Floe.

-------
Some of the theoretical advantage of upflow coarse-to-fine filtration is lost because min-
imum grain sizes must be  coarse enough to avoid excessive uplift.

Additional resistance to uplift is provided in many upflow designs by placing a restraining
grid on top of the media. The spacing between bars of the grid must be large enough to pre-
vent upward bed movement during filtration. Although these two requirements appear con-
tradictory, arching of the  grains takes place between the bars, allowing a  reasonably large
spacing, in the range of 100 to 150 times the diameter of the smallest grain size in the beds.

    9.5.5   Selection of Media

Pilot  testing is indispensable to provide the information necessary for meaningful com-
parison of different media designs or to assure the  effluent quality performance of  any
media design selected. Without pilot testing, the designer should select a  media which, on
the basis of experience with  similar influents, may be expected to provide good solids re-
moval with low head loss buildup. In general, any such media would include an ample depth
of coarse media followed by  fine  media in the  size ranges indicated for  dual  media con-
figurations in Table 9-5.

Pilot testing to guide media selection should define headless  development  vs time for each
media design, under all test conditions. Suggested  ranges for test conditions  are given in
Section 9.9. If one media design clearly gives lower head buildup at all times and under all
test conditions it may be selected directly provided its backwash  requirements are not ex-
traordinary. If different media provide essentially the same headloss development over the
range of test conditions, selection may be based on other factors. Where different media ap-
pear significantly better under different conditions,  selection  should be based on cost com-
parison of the alternative designs each at its most favorable rate, terminal headloss  and  run
length, determined  as indicated  in  Section 9.4.  Significant differences in  backwash flows
should be taken into account.

9.6  Filter Control Systems

Major filter functions requiring monitoring and/or control are:

      1. Head Loss

      2. Effluent quality

      3. Initiation of backwash where automatic

      4. Flow rate through the filters

      5. Backwash sequence, rate and duration.

An important tool in performance control is the automatic turbidimeter which can contin-
uously monitor the filter feed and product. This allows the operator to anticipate difficulties
                                           9-31

-------
from changes in feed quality, and rapidly remedy process failures. In addition, these devices
allow the operator to rapidly evaluate the effects of changes in process variables and pro-
vide a continuous record of plant performance. All turbidimeters operate on the pinciple of
measurement of scattered or transmitted  light. A variety of commercial instruments  are
available.

Filter installations should be equipped with appropriate loss of head and flow indicators. In-
dividual filters should have multiple taps for pressure readings if full scale  experimental
testing is desired.

Provisions for automatic or remote initiation of backwash by timers or based on headloss
or turbidity monitoring may be justified to reduce the need for operating attention.

Three types of flow control systems are used for filters:

       1. Effluent rate control

       2. Influent flow splitting

       3. Variable declining rate control.

Features of these systems and of automatic backwash systems are described below.

     9.6.1   Effluent Rate Control

This system, common in traditional water treatment plant designs, maintains a set flow for
each filter by throttling the effluent (see Figure 9-1). The throttle valve may be controlled
directly by mechanical linkage to a venturi controller or indirectly by a set point controller
linked to a pneumatic or hydraulic valve operator. The direct acting system is unsuitable if
flows to individual filters must vary over the day. The indirect system  is complex and both
may be  troublesome in maintenance. The system is also wasteful of  head since available
head not needed in a clean filter is lost in the controller. In addition, control valves may
produce high frequency surges in the filter bed with  accompanying loss of efficiency (10).


    9.6.2  Influent Flow Splitting

In this system flow is evenly divided among filters in a splitter box located at or above  the
level of  the top of the filter boxes (see Figure 9-8a). The boxes themselves are made deep so
that the  water level in them can build up to provide the maximum operating head  needed
when the filter bed is dirty. A  weir on the filter outlet maintains a constant back pressure or
minimum water level to prevent accidental dewatering of the bed.

Advantages of influent flow splitting include:

     1.  Rate controllers with attendant maintenance and surging problems are  eliminated.
                                           9-32

-------
                        Flow Splitting Tank

Inlet
Valve
Open
O
o
Closed
Backwash
Outlet
Valve


s


Filter Cell
No. 1

Wash Trough

nnj-Ln.



-
Filter Cell
No. 2

Wash Trough

n_rLn_n.


t

-*—


•+
Open
o

-------
    2.  Flow variations are distributed to filters automatically.

    3.  Head loss may be read directly from water levels in filter boxes.

    4.  Only a single master flow meter is needed.

    5.  Changes in filter rate are gradual because of time required for head to build up in
       filter boxes.

Disadvantages are:

    1.  The head not needed for filter operation is lost in the drop between the splitter and
       the filters.

    2.  Capital cost of filter box construction is increased by the greater depth.

      9.6.3  Declining Rate Filtration

This system requires multiple filters. All operate under the same head but at different flow
rates depending on the degree of clogging. Under constant head the output from a single fil-
ter declines as the run progresses. The filter selected to be  backwashed  is always the one
which  has been on line the longest and is most clogged.  Total output from all filters is con-
trolled by varying the head applied. Figure 9-8b shows a variable declining rate filter.

The head on the filters  may be controlled by varying either the upstream or downstream
water  level (10) (49). With downstream  water level control, an equalizing chamber must be
provided to limit the rate of change of head and hence of flow, when filters are taken off line
or restored to service. It is common to apply maximum design loadings to the filters as a
group  and to limit maximum rates on individual clean filters to from 20 percent to 40 per-
cent above these design loadings.

Advantages  cited  for declining rate filtration  (10) (49) include  better  effluent  quality,
absence  of  surges, and  significantly lower total  head requirements. Less head is needed
because:

     1. There is no loss due to throttling or due to free fall after flow splitting.

    2. As rate declines turbulent head losses (underdrains, valves, etc.) reduce rapidly (in
       proportion to second power of flow) making head available to overcome resistance
       of clogged filter (proportional to flow).

For proper operating control, flow rates should be measured individually for each declining
rate filter. Only single indicators are needed for inlet and outlet levels on head  loss, since
these are the same for all filters.

The chief disadvantage of this method of flow control is the need  for a large volume  of
                                          9-34

-------
water storage upstream of the filter.

    9.6.4   Backwash Control

Programmed backwash systems are widely used in current designs. Such systems consist of
interlocked controllers  and timers programmed  to open and close valves, make or break
siphons, start and stop pumps and blowers and limit backwash flows to control the rate, du-
ration and sequence of activities during backwash. Even where backwash is manually in-
itiated, the rest of the control system may be entirely automatic.

Proprietary systems, with various features are available from different manufacturers. One
such system, designed to operate with only a single control valve is shown in Figure 9-9.

9.7  Filter Cleaning Systems

    9.7.1   Upflow Washing

Accumulated solids are removed from  filters by a rapid upflow of washwater. The waste
flow is then recycled to some prior treatment unit, usually primary settling. Washwater
sources may  include filter  influent,  filter effluent or effluent from subsequent treatment
units. Storage of washwater supply may be  needed if rates required exceed the flow avail-
able. Recycled spent washwater flows should be equalized by storage so they do not disrupt
prior treatment processes. Backwash rates for most effective cleaning vary with media size
and density.

Baumann  and Cleasby (1) recommend providing upflow wash capacity adequate to fluidize
the 90 percent finer size of each media component at the warmest expected water tempera-
ture. Figures 9-10 and 9-11 may be used to  determine minimum upflow velocities or wash
rates to fluidize  coal, sand, and garnet  media of various sizes. Rates should be variable to
compensate for changes in temperature, viscosity, and hence bed expansion. Maximum hy-
drodynamic shear and most efficient cleaning have been shown to occur when the porosity
of the expanded media is about 0.7 (1). To reach this porosity in the surface layers of a
non-uniform  sand (effective size = 0.4, uniformity coefficient =  1.47) requires almost 50
percent expansion (7). Because of its higher unexpanded porosity coal requires only 20 to 25
percent expansion to reach a porosity of 0.7 in the surface layers.  In practice the backwash
rates generally used in filter designs, range  up to 20 gpm/sq ft, and do not provide more
than 15 to 30 percent expansion except for very fine  media (50). This  means that  higher
backwash durations (5 to 10 or at the extreme 15 minutes) and somewhat higher washwater
consumption are required than at rates which would provide the most efficient washing.

Whatever wash rate and duration are expected, design of piping, valves, pumps and storage
tanks should provide extra  capacity of at least 25 percent.

Methods are available for predicting expansion  of sand beds accurately (51), but have not
yet been satisfactorily extended to other media components.  Expansion of multi-component
media can be rapidly obtained,  however, from backwash tests in pilot columns.
                                         9-35

-------
                      FFFLUEN1
c

-.
-
                     AIR
                                                                          BACKWASH WATER
                                                                              INFLUENT
                                                                            ANTHRACITE
                                                                             SAND



                                                                             ROTO SCOUR

                                                                             UNDERDRAIN
                                                 FIGURE 9-9


                              AUTOMATIC GRAVITY FILTER, SINGLE COMPARTMENT

                                            (Courtesv^of Ecodyne Corp.)

-------
0.12


0.10


0.08


0.06


0.04


0.02


 0.0
   0.
                              COAL p= 1.7
          = 1.73
                                    o
                                   -o-
I
I
                                    p=  .
                              SILICA SAND p = 2.65  o
                              GARNET SAND p= 4.13 A

                                 I      I     I __ I
60


50


40


30


20


10
                                                            .ST
25  0.5  0.75
1          1.5         2

  MEAN SIEVE SIZE, mm

      FIGURE 9-10
             MINIMUM FLUIDIZATION VELOCITY, Vmf,
           TO ACHIEVE 10 PERCENT BED EXPANSION
                         AT 25°C(1).
                     2.5
                               9-37

-------
0.04
0.03
0.02
o.oi
                            1.0
                            0.5
               .1
               'o
               Q.
                                                                     0)
                                                                     u
                                                                     (5
                                                                     u
                                                                     U
                                                                     o

                                                                     CO
                     10
                               TEMP,
20

°C
30
                             FIGURE 9-11

                EFFECT OF TEMPERATURE ON Vmf FOR

           SAND AND COAL AND ON ABSOLUTE VISCOSITY

                            OF WATER (1)
                                9-38

-------
Several manufacturers offer coarse media filter systems with wash rates too low to fully
fluidize the bed. Auxiliary air scouring is provided, but its long term effectiveness in main-
taining the media in good condition has not been proven. Studies at Iowa State University
(52) indicate that air scour and simultaneous upflow washing at full fluidization can free a
dirty filter bed of mud balls which accumulate during multiple cycles using air scour follow-
ed by washing. Further work will study effectiveness of air scour and simultaneous upflow
washing at less than full fluidization.

     9.7.2  Underdrains

Underdrains should distribute washwater as uniformly as possible over the area of the filter.
Excessive variation in washwater rate results in uneven and ineffective cleaning. Moreover,
the accompanying excessive jet action can lead to lateral displacement of gravel and clog-
ging of the underdrains with filter media.

In general, underdrain systems developed  for water filtration may also be used in waste-
water applications. One of the first systems employed in water filtration consists of several
layers  of graded gravel surrounding manifold piping positioned  on the filter floor. Orifices
in the manifold piping provide preliminary distribution of the washwater as shown in Figure
9-12a.  The final distribution is accomplished as the water moves upward through the gravel.
Fair and Geyer (44) provide rules of thumb for sizing lateral header systems and a basis for
hydraulic analysis.

Several commercial systems are available which employ patented false bottom  distributors
in place of the manifold. The gravel is then placed on top of  the false bottom.  Leopold
Block  andWheeler Filter  Bottom systems are illustrated in Figure 9-12b and c. A system
manufactured by Dravo (Figure  9-12e) distributes  both air during airwash and washwater
during normal backwash.

Table 9-6 shows the standard gravel design used in water filtration together with a modified
design  suggested by Baylis (45) for use with higher backwash rates.
                                           9-39

-------
                                                                               A. HEADER   LATERALS
                                                                                 (COURTESY  OFTHEAWWA)
5/52" DIA. DISPERSION ORIFICES
APPROX  49 PER  SO  FT.-
6/8* OIA CONTROL ORIFICES
APPKOX. tPER 80. FT.
                                                   COMPENSATIN6 LATERAL
                                                   (•ECONDARY126.B SO IN&

                                                   FEED LATERAL(PRIMARV)
                                                       90 8 80 IN.
                                   B.   LEOPOLD    BLOCK   SYSTEM

                                          (Court«»y  F. B-  Leopold  Co-)
                                              tuenx I/*"-
                                                 at* 1/4"-
                                                LAYER I A"-1/4"
                                WOOD STRIPPH4C TO
                                PRgVPfTMIOUT I
                                        WTO FUMC
                                                     <•-,  •„  .;>v*- •
                                                     -.-•'  »-- ***-
                                  - PLUME AREA '1.5 [FILTER AREA)
                                    (MM)      I  UO FT) J

                                  I«T RCCOMMOOCD
                                  If* MAXIMUM
                                                  FIGURE 9-12

                                                UNDERDRAINS
                                                    9-40

-------
        C.  WHEELER  FILTER  BOTTOM
               (Courtesy of B I F )
!_*"-•'     . " " '*'-  ** ' - * / '  * t - ..A ' *  _      iJ*^
           D. SUBFILL-LESS  STRAINERS
                (Courtesy of  Ecodyne Corp.)

                FIGURE 9-12 (Continued)
                        9-41

-------
        RAW
     WATER
 STABILIZING
      LAYER
     MEDIUM
SUPPORTING
     LAYER
        AIR
DISTRIBUTION
      PIPES
CLEAR WATER
     a
  WASH WATER
    CONDUITS
 CLEAR
 WATER
 FILTERING
                                                                      FILTER BED
FINE
SUPPORTING
LAYER
COARSE
SUPPORTING
LAYER
                                                                      M-BLOCKS
COVER
PLATES
           FILTRATE   OUT <—
                                                     *-  RAW  WATER IN
                       E.  DRAVO  M-BLOCK  SYSTEM

                            (COURTESY  OF DRAVO CORR )



                                FIGURE 9-12 (continued)
                                     9-42

-------
                                   TABLE 9-6
                           FILTER GRAVEL DESIGN
           Standard
2-1/2
3-1/2
3-1/2
2
4
6
1/12 to 1/8
1/8 to 1/4
1/4 to 1/2
l/2to3/4
3/4to 1-1/2
1-1/2 to 3-1/2
                                               Baylis
Depth
  in.

5
2
2
4
2
2
4
1 to 2
1 / 2 to 1
1/4 to 1/2
1/8 to 1/4
1/4 to 1/2
1/2 to 1
1 to 2
 Baylis found that with the standard design the upper layers of gravel could fluidize under
 high backwash rates (above 20 gpm/ sq ft), and proposed placing a final layer of heavy
 gravel over the finer gravel to prevent this fluidization. A 3-inch layer of coarse (1 mm) gar-
 net or ilmenite above the gravel has also been suggested to overcome this problem (8).

 Some systems eliminate the gravel layers entirely by using nozzles or porous plates. Advan-
 tages and disadvantages of these systems are discussed elsewhere (1) (8). One system, manu-
 factured by Graver Water Conditioning Co., utilizes a cast concrete false bottom with plas-
 tic or metal strainers on about 12-inch centers as shown in Figure 9-12d.

     9.7.3  Washwater Troughs

 Except for small units,  filters are commonly equipped with washwater troughs spaced on
 about 6-ft centers. Hydraulic design of these devices is discussed in Fair and Geyer (44).

 Troughs are installed with the aim of aiding uniform distribution of washwater and avoid-
 ing dead spots which may retard the removal of dislodged solids from the filter box.

     9.7.4  Auxiliary Cleaning

 The function of auxiliary cleaning by air scour or surface wash is to loosen accumulated de-
 posits  from the filter media. The slimes and organic particulates encountered in wastewater
 filtration cannot be completely loosened by normal backwash flow.

 Typical surface wash equipment consists of either fixed or rotating pipe distributors fitted
 with nozzles which  are placed about 1 to 2 inches above the top of the bed. While the sur-
 face wash is on, the backwash expansion is set at a lower rate than after the surface wash is
 terminated. Surface washwater is  supplied at 50 to 100 psi at rates approximating 1 to 3 gal/
 min/sq ft of bed.
                                       9-43

-------
With the  widespread use of media permitting deeper floe penetration, the ability of the
above type of surface wash to clean mid-and lower portions of the bed has been questioned.
Wash jets positioned at lower levels in the bed may help to alleviate this problem.

Air scour systems have been increasingly used in an attempt to reduce washwater require-
ments and to effect cleaning of the deeper portions of the bed. Some concern  has been ex-
pressed  concerning loss of finer lighter media particles when air washing is used (8). Where
this is a problem,  air scour should be applied separately from the backwash, with liquid in
the filter box drawn down below the  washwater overflow level  so that no overflow occurs
during air wash. Allowance must be made for 6 to 9 inches of water level rise due to air lift
(7). Although  some of the lighter media may remain on  the surface of the water and sub-
sequently be lost, the rate of such loss should generally be negligible.

To prevent air scour from disrupting gravel placement, air is usually injected through a grid
above the under-drain gravel. It may go directly into the underdrains where no gravel is
used.

     9.7.5  Backwash Sequence

The  cleaning cycle time (total downtime during one cleaning operation) includes time for
valve openings and closings, time for  drainage of inflow from the filter and time  for the ac-
tual  upflow washing and auxiliary cleaning. Unless influent in the filter box is  to be wasted,
drainage time should be calculated at normal filter rates.  Valve openings and closings to
start and stop backwash and air scour should be gradual  to keep from upsetting the media
gradation and structure.

A typical sequence for backwash with auxiliary surface wash is:

     1.  Shut influent and permit water level to drain down to top of the washwater troughs
        or other washwater control weir.

    2.  Apply surface wash for 1 to 3 minutes.

    3.  Apply upflow wash and surface  wash together 5 to 10 minutes as needed to flush
        out solids.

    4.  Shut  off auxiliary wash  and  apply  backwash  alone  for 1 to  2 minutes at rate
        needed to classify the bed.

    5.  Return bed to service.

A typical sequence for cleaning using upflow wash and air scour is:

     1.  Stop influent and lower the water level to a few inches above bed.

     2.  Apply air alone at 2 to  5 cfm/sq ft for 3 to 10 minutes.
                                       9-44

-------
    3.  Apply water backwash at 2 to 5 gpm/sq ft with air on until water Is within one foot
       of wash water trough.

    4.  Shut air off.

    5.  Continue water backwash at normal rate for usual period of time.

    6.  Apply backwash for 1 to 2 minutes at a rate required to insure hydraulic classifica-
       tion of the filter media.

    7.  Return  bed to service.

9.8  Filter Structures and General  Arrangement

A typical wastewater filter consists  of a tank or filter box containing  an underdrain system,
media  and sufficient overall depth to contain media during backwash. In gravity units, the
overall depth must  also provide for  operating submergence  and freeboard.  Influent, ef-
fluent, washwater and waste connections are  provided. In addition, all wastewater filters
should have provisions for auxiliary cleaning.

Underdrains are designed to properly distribute the washwater during cleaning (see Section
9.7). During  normal operation,underdrains~collect filter effluent (downflow operation) or
distribute influent (upflow operation). Washwater troughs  and filter inlets (downflow) or
effluent launders (upflow) are  located in the submerged zone above the media.

The recommendations for general arrangement and special structural features of concrete
filters  presented in Water Treatment Plant Design (53) are fully applicable to wastewater
treatment applications.

For gravity filters of concrete construction, filter boxes are usually arranged in rows along
one or two sides of a common pipe  gallery, narrow side toward the gallery. This maximizes
common wall construction and minimizes piping runs. Gravity filters may be of concrete or
steel shell construction. Concrete units are generally square or rectangular and steel  units
circular.  Sizes  of gravity  concrete units are limited to about  1000 sq ft (8); steel units are
generally smaller.

For filters using influent flow splitting (see Section 9.6) multiple filter boxes have been con-
structed as compartments in a single round or  square tank (concrete or steel) with common
influent and waste piping located  above the center of the tank  and common washwater and
effluent piping  around the outside base.

Steel shell package  pressure filters are cylindrical units with either horizontal or vertical
axes. To  minimize piping runs horizontal units are usually placed in rows with common pip-
ing along the ends. Vertical units are arranged in either rows  or clusters. Horizontal  pres-
sure units are less restricted in size than the vertical pressure units and hence are normally
used for  plant capacities above 1 to 1.5 mgd (8). Where pressure units are used it is essential
                                       9-45

-------
that manholes be provided for interior access both above the bed and below the under-
drains.  Pressure filters should  also be provided with a means for hydraulic removal of all
the filter media, and with sight glasses for observation of the bed.

In municipal water filtration plant designs it is common to totally enclose the pipe gallery
and to locate controls in an enclosed superstructure above the gallery and overlooking the
filters.  In northern climates the filters  themselves are  usually included under the super-
structure.

Wastewater effluent temperatures are generally somewhat higher than the local natural wa-
ters, so in a given locality there is less justification for housing wastewater filters than water
filters.  Piping and valves need to be protected in climates where  freezing occurs either by
housing or by  insulation  and heating. Controls  need housing or  weather protected enclo-
sures in any climate. Local controls for each  filter should be placed in a location from which
the backwashing filter can be observed.

9.9   Pilot Studies

Specific pilot study objectives  may include:

     1.  Comparing performance of different media designs in a given application.

     2.  Establishing relations between flow  rate, headloss and run length for a particular
        media and application.

     3.  Establishing limiting headlosses and rates  to assure required effluent quality and
        no deterioration due to breakthrough.

     4.  Characterizing wastewater variability  in terms  of performance variations with a
        specific media.

     5.  Studying effects of variations in  chemical coagulation, flocculation, biological pro-
        cesses or other pretreatment.

In  all cases the variation of influent and effluent quality and headloss buildup with time
should  be noted. Studies aimed at media selection may be limited to variations of size and
depth for a particular configuration or may  involve parallel  operation of quite different
media configurations.

Media  selection tests should cover the range of operating conditions which may occur in the
actual design: typically flow rates of at least 6 to 8 gpm/sq ft..headlosses to 30 ft (urilesslT
lower limit is  imposed by constraints)  and runs  to at least 24-hr (if headloss or break-
through do not  limit). Effects of influent  quality variations  are revealed by conducting
long-term tests on the same influent. It is important however to include influent quality con-
ditions representative of the entire range expected  in operation.
                                        9-46

-------
Studies to select filter rates and terminal headlosses for a particular media will generally in-
clude more rates and cover a somewhat wider range than a study involving media selection.
Also extra effort should be given to including critical influent conditions (e.g. solids load-
ings and floe strength).

Studies aimed at characterizing the filterability of a waste (see Section 9.1) would generally
be run at a low rate (2 to 4 gpm/sq ft) using a media expected to give high effluent quality
but without excessive surface filtration. Changes  in quality and  headloss with elevation in
the bed should be observed. The widest possible range of influent conditions should be in-
cluded.
Studies to test pretreatment generally would employ a specific media and a single filter rate,
but would attempt to vary influent conditions by adjustment of pretreatment.

In judging what are critical influent conditions, variations in operating records of existing
pretreatment facilities should be thoroughly studied. Where facilities do not exist, records
for similar treatment at other locations should be considered.

Where  existing biological pretreatment is inconsistent, with frequent periods of upset, it
would be prudent to determine and eliminate the causes of these upsets rather than count on
filtration as a cure-all for resulting problems (33).

Standard  methods for conducting filtration pilot studies have not been established. The fol-
lowing is a list of typical equipment and practices.

     1.  Multiple filter tubes of transparent material with a minimum diameter of 4 to 6 in.
        are utilized.

    2.  The tubes are fitted for either gravity or pressure operation.

    3.  A false bottom underdrain is utilized with either a gravel covered plate or strainer
        backwash system.

    4.  Flow to filter units is set by a combination of positive displacement  metering
        pumps, weirs,  control valves, etc. Declining rate control or influent flow splitting
        may be used where important to test for design.

    5.  Sample taps are provided above and below the media, as well as at other locations
        within the bed. Tap locations  are generally located  near  the top of each type of
        media used.  If the effects of media depth variations are  to be studied, however,
        taps should be located at 3 to 4 in. intervals down the column. As  an alternative,
        parallel multiple columns of different lengths may be used.

Additional details of pilot filters are given in various references on filtration studies (7) (54)
(55).
                                        9-47

-------
Pilot studies as outlined above cannot adequately determine effects of cleaning system de-
sign parameters. Cleaning of the filter bed is difficult to simulate in pilot scale because of
the small surface area of the beds utilized. The small area makes it impossible to study sur-
face wash and air  scour. Results of water backwash may not be representative because of
the wall effect.

Information on cleaning performance can be obtained only in long term studies using large
pilot installations with filtering areas of 4 sq ft or more. Because such studies are expensive,
it  may be desirable to design cleaning systems based upon experience from other studies
(50) (51) (52).

9.10  Special Designs

    9.10.1   Radial Flow Filters

Some recently developed  filters  employ horizontal radial flow through media contained in
the anular space between concentric vertical cylinders. The inner cylinder acts as distributor
and draw off points are distributed around the periphery of the outer cylinder which forms
the filter vessel.

In the Simater unit (Figure 9-13) developed in  England and marketed in the U.S. by Dravo,
the sand media is continuously moved downward, drawn off, washed in a separate tank and
returned to the top of the bed.  A  filter developed by Hydromation Corporation (Figure
9-14) has a batch external wash  to clean its synthetic resin media.
A Simater filter was tested on biologically treated wastewater at the Essex River Authority
in England. The unit was run in parallel with two upflow units (23). Media size for the Sim-
ater filter was 0.5 to  1.0 mm, comparable to one of the upflow filters but much finer than
the other.  Rates were not stated for the radial flow unit but were apparently in the same
range as for the upflow units—4 to 6 gpm/sq ft. All the filters gave SS reductions of 60 to
80 percent and effluent SS concentrations below 10 mg/ 1.

The Simater showed marked tolerance for short slugs  at high influent solids. Prolonged op-
eration at high concentration resulted in clogging of the outlet screens, but this could "be pre-
vented by using higher rates of media washing.

     9.10.2  Travelling Backwash Filter

Hardinge Corporation furnishes a fine media (0.48 mm sand), multi-compartmented filter
in which each compartment (8-in  width) can be backwashed without stopping filtration in
the remainder of the filter. (See Figure 9-15). Backwash rates are similar to those for other
filters (up to 15 gpm/sq ft), but the backwash time for individual compartments is as low as
45 seconds. Hence, each compartment can be backwashed every  1  to 2 hours without  ex-
cessive washwater use.

Each compartment has its own underdrain section.  Media is supported on 1-in. porous
                                        9-48

-------
 H EAD TANK (OPT)  *
      FEED
   ISLUDGE]
FILTER INLET

   |FILTRAfE
   [FILTRATE I
                                  AIRLIFT TUBE
                                       FIGURE 9-13
                                      SIMATKR ULTl R
                                    (Courtesy ol DravoCorp.)
                        9-49

-------
           FIGURE 9-14
HYDROMATION IN-DEPTH FILTER
               9-50

-------
A. Influent line.
B. Influent ports.
C. Influent channel.
D. Compartmented  filter  bed.
E. Sectionalized under-drain.
F.  Effluent and  backwash ports.
G. Effluent channel.
H. Effluent discharge line.
I.  Backwash valve.
J.  Backwash pump assembly,
K.  Washwater hood.
L  Washwater pump assembly.
M. Washwater discharge pipe.
N. Washwater trough.
O. Washwater discharge.
P. Mechanism drive  motor.
O. Backwash support retaining springs.
R. Pressure control springs.
S. Control instrumentation.
T. Traveling backwash mechanism.
                                                                                  FIGURE 9-15
                                                         HARDINGE AUTOMATIC BACKWASH FILTER
                                                                         (Courtesy  Koppers Co., Inc.)

-------
plates over the underdrain. Flow from the underdrain sections discharges through individual
ports to a common effluent channel.

The travelling backwash consists of a rolling bridge carrying two pumps and equipped with
a hood extending over the length of a single compartment. The backwash pump draws wa-
ter from the effluent chamber and discharges it into the underdrain section for the com-
partment where the bridge is stationed. The wash  water pump withdraws  backwash flow
from the hood positioned over the compartment and discharges it to waste. Initiation  of a
backwash cycle is controlled  either by timer or by headless sensors.

Lynam (56) reported 68 percent removal of SS in uncoagulated activated sludge effluent by
Hardinge filters at an SS loading of 0.5 lb/ sq ft/ day and 11.5 inches headless. At 4.4 inches
of headless the removal at 0.4 Ib/sq  ft/day was 75 percent. At 11.5 in. headloss the max-
imum hydraulic loading was 6.0 gpm/sq ft compared with 2.5 gpm/sq ft at 4.4 in. In the
same study, coagulation with alum did not improve performance.

     9.10.3   Filter with On-Line Surface Scouring

Hydro-Clear Corporation offers a fully automatic, shallow bed, fine-media sand  filter for
tertiary wastewater treatment. The media consists of 10 inches of 0.45 mm sand with a uni-
formity coefficient of 1.5 supported on a wire mesh  above the underdrain system. This filter
combines air mixing of the water above the bed with air surging upward through the bed to
prolong run length. Influent  flow splitting controls the flow to parallel units.

Typically, a filter run consists of a preset number of filter cycles. Each cycle  begins by filter-
ing secondary effluent until a preset headloss is developed. Air mixing is then started in the
liquid above the bed to resuspend the  solids collected on the media surface. After additional
 headloss buildup, air trapped in the vented underdrain system is forced upward through the
 bed for a short period.  Solids removed by the air are resuspended by the air mixing, and the
 cycle begins again. After the predetermined number of cycles, the filter is backwashed, en-
 ding the run.

Data from Clark County, Ohio,  indicate an average filtrate SS concentration of 4.8 mg/ 1
using effluent from a 0.2 mgd contact stabilization  plant at 1.2 gpm/sq ft (57).

9.11   Slow Sand Filters

Slow sand filters consist of a layer of sand supported on graded gravel with an underdrain
system but no backwash system. The  depth of the sand layer ranges up to 42 inches, and the
effective size  is 0.25 to 0.35 mm with a uniformity coefficient of 2 to 3 (53). Secondary
effluent is applied, generally at a  rate  of about 3 gph/sq ft (8), and the filter is used until the
wastewater rises  to the top of the  filter  wall. The filter  is then  removed from  service,
drained, permitted  to dry and then cleaned by manually removing the filtered solids.

Truesdale and Birkbeck (58)  report only 60 percent SS removal for slow sand filters and a
cleaning frequency of once or twice per month. Rapid clogging  of slow sand  filters has been
                                        9-52

-------
observed (59). Slow sand filters require large land areas and therefore, are not normally em-
ployed for large installations. Sand that is lost during cleaning must eventually be replaced.

Filters of the same construction, operated intermittently, have been used as combined phys-
ical-biological treatment  for secondary effluent  polishing. Intermittent operation permits
aerobic digestion of solids reducing somewhat the required frequency of maintenance. Area
requirements are  still quite large,  however,  and  generally limit  applications to small
plants. The fact that maintenance is only required on an intermittent basis makes this type
of filter a viable process for upgrading existing  lagoons which cannot meet effluent stan-
dards. Further discussion of this application can be found in the U.S. EPA manual, Up-
grading Existing Wastewater Treatment Plants and elsewhere (60).
                                       9-53

-------
9.12  References

 1.  Baumann, R.E., and Cleasby, J. L., Design oj Filters for Advanced Wastewater Treat-
    ment, Engineering Research Institute, Iowa State University, Ames,  Iowa (October
    1973).

 2.  Tchobanoglous, G. and Eliassen R., Filtration oj Treated Sewage Effluent. JSED,
    ASCE, 96, 243 (April 1970).

 3.  Ives, Kenneth J. and Sholji,  Ihsan, Research on Variables Affecting Filtration, Jour.
    SED, ASCE, 91,  1 (Aug. 1965).

 4.  Hsiung, Kou-ying and Cleasby, J. L., Prediction of Filter Performance. Jour. SED,
    ASCE, 91, 1 (June, 1965).

 5.  Ives, Kenneth J.,  Filtration:  The Significance of Theory. Journal of The Institution of
    Water Engineers. 25,  13 (Feb.  1971).

 6.  Tchobanoglous, G., Filtration  Techniques in Tertiary Treatment. Journal WPCF, 42,
    4, 604 (April, 1970).

 7.  Baumann, E. R.,  Design of Filters for Advanced Wastewater Treatment. Presented at
    the Technology Transfer Design Seminar sponsored by the U.S.  EPA, at Syracuse,
    N.Y. (May 31, June  1, 2, 1972).

 8.  Culp,  R. L.,  and  Gulp,  G. L., Advanced Wastewater Treatment,  Van  Nos-
    trand-Reinhold Co., New York (1971).

 9.  Baumann, E. R., and Oulman, C.S., Sand and Diatomite Filtration Practice. Water
    Quality Improvement by Physical and Chemical Processes, University of Texas Press,
    Austin, Texas (1970).

10.  Hudson, H.E. Jr., Declining Rate Filtration, Journal AWWA, 51, 1455 (Nov., 1959).

11.  Convery, J.  J., Solids Removal Processes. Nutrient Removal and Advanced Waste
    Treatment Symposium.  Presented by  Federal Water  Pollution Control Adminis-
    tration,  Cincinnati, Ohio (April 29-30, 1969).

12.  Naylor, A. E., Evans, S. C.,  and Dunscombe, K. M., Recent Developments on the Ra-
    pid Sand Filters at Luton. Water Poll. Control Jour. (Brit.) 66, 309 (1967).

13.  Zenz, D.  R., Weingarden, M. J., and Bogusch, E. D., Hanover Park Experimental
    Bay Project  (March 8, 1972).

14.  Zenz, D. R., Lue-Hing, C.,  and Obayashi, A., Preliminary Report on Hanover Park
                                       9-54

-------
    Bay Project, U.S. EPA Grant #WPRD 92-01-68 (R2) (November, 1972).

15.  University of Michigan short course, January 25-26, 1973. Reported by Thomas Hoo-
    gerhyde, Michigan Department of Health.

16.  Private Communication with J. Wiley Finney, Jr., Treatment results Hite Creek Ter-
    tiary Plant, Louisville, Kentucky (April, 1973).

17.  Ventura, California, East Side STP Test Report, Technical Bulletin, Dravo Corp., Wa-
    ter and waste Treatment Division, Pittsburgh, Pa.

18.  Wood, R., Smith, W. S. and Murray, J. K., An Investigation Into Upward Flow
    Filtration, Water Pollution Control (British) 67: 421-426. (1968).

19.  Private communication with H. M. Mueller, Jr., Neptune Microfloc (April,  1973).

20. Ripley, P. G., and Lamb, G. L., Filtration of Effluent from a Biological-Chemical
    System, Water and Sewage Works, 12, 67 (February, 1973).

 21. Performance Data  Contained in Hydroclear Corporation Catalogue,  Avon  Lake,
    Ohio, as tested by the Clark County Utilities Department, Springfield, Ohio (May,
    1969).

22. Isaac, P. C. G. and Hibberd, R. L., The Use of Microstrainers and Sand  Filters for
    Tertiary Treatment, Water Research, Pergamon Press,  6, p. 465-474.

23. Guiver, K. and Huntingdon, R., A Scheme for Providing Industrial Water Supplies by
    the Re-Use of Sewage  Effluent, Water Pollution Control Journal, 70, 1,  p. 75, 1971.

24. Michaelson, A. P., Under the Solids Limit at Ashton-Under-Lyne, Water  Pollution
    Control (Brit.), p. 533 (1971).

 25. U.S. EPA  Blue Plains  Pilot Plant, Washington,  D.C.  Contract  No. 6801-0161,
    Monthly Reports (1972).

 26. U.S. EPA Internal Monthly Reports, Piscataway, Md. (March-September, 1973).

 27. U.S. EPA Internal Monthly Reports. Ely, Minn. (April-December,  1973).

 28. Study of Upflow Filter for Tertiary Treatment. U.S.  EPA Project  No. 17030 DMA
    (August, 1972).

 29. Oliva, J.  A.,  Department  of Public Works,  Nassau  County,  New York, Personal
    Communication (March, 1973).

 30. U.S. EPA,  Advanced  Wastewater Treatment As Practiced At South Tahoe.  Project
                                       9-55

-------
     17010 ELQ (WPRD 52-01-67) (August, 1971).

31.  Villiers, R. V., Berg, E. L., Brunner, C. N. and Masse, A. N., Municipal Waste-water
     Treatment By Physical and Chemical Methods, Water and Sewage Works, R-62
     (1971).

32.  Bishop, D. F., O'Farrell, T. P., and Stamberg, J. B., Physical Chemical Treatment of
     Municipal Waste-water. Presented before the 43rd Annual Meeting, WPCF, Boston,
     Mass. (Oct., 1970)

33.  Kreissl, J. F., Granular Media Filtration of Waste-water: An Assessment, Presented at
     Seminar Filtration of Water and Wastewater, Ann Arbor, Mich. (Jan.,  1973).

34.  Kreissl, J. F., U.S.  EPA NERC, Cincinnati, Ohio, Personal Communication.

35.  Baumann, E. R. and Huang,  J. C. Granular Filters for  Tertiary Wastewater Treat-
     ment.  Accepted for publication, Journal of the Water Pollution Control Federation.
     (Iowa State University, ERI 72051, Preprint) (February,  1972).

36.  Cleasby, J. L. and Baumann,  E. R. Selection of Sand Filtration Rates. JAWWA 54,
     579 (May, 1962).

37.  Misaka, Yasunao, et. al., Filtration of Activated Sludge Secondary Effluent Through
     Sand and Anthracite-Sand Beds. The University of Wisconsin Water Resources Center
     (1969).

38.  O'Melia, C. R. and Stumm, W., Theory of Water Filtration. Jour. AWWA, 59, 1393,
     (Nov. 1967).

39.  Parker, D. S. et. al., Floe Breakup in Turbulent Flocculation Processes, J. San. Eng.
     Div. ASCE 98, SA1, 79-99 (Feb. 1972).

40.  Oakley,  H. R. and Cripps, T. British Practice in the  Tertiary  Treatment of Waste-
     water, JWPCF, 41, 36 (Jan. 1969).

41.  Diaper, E. W. J. and Ives, J.  J., Filtration Through Size-Graded Media. Jour. SED,
     ASCE, 91, 89 (June, 1965).

42.  Tebbutt, T.H.Y., An Investigation Into Tertiary Treatment By Rapid Filtration, Wa-
     ter Research (Brit.), Pergamon Press, 5, p. 81 (1971).


43.  Nebolsine, R., Poushine, I. and Fan, C. Y., Ultra High Rate Filtration of Activated
     Sludge Plant Effluent,  U.S. EPA  Environmental  Protection Technology  Series,
     EPA-R2-73-222 (April, 1973).

44.  Fair,  G., and Geyer, J., Water Supply and Waste Water Disposal, Chapter 24, John
                                      9-56

-------
    Wiley & Sons, Inc., New York (1954).

45. Water  Quality  and Treatment,  American  Water Works Association, Inc.,
    McGraw-Hill, Inc., New York (1971).

46. Camp,  T. R., Discussion of Anthracite Sand  Filters. (Walter  R. Conley), Jour.
    AWWA,  53, 1478 (Dec.  1961).

47. Gulp, G. L. and Conley, W. R., High Rate Sedimentation and Filtration, Water Qual-
    ity Improvement by Physical and Chemical Processes. University of Texas Press, Aus-
    tin, Texas (1970).

48. Dravo Corporation, Effluent Polishing With Deep-Bed Filtration, Technical Bulletin,
    7/WWT19.

49. Cleasby, J.  L., Filter  Rate Control Without Rate Controllers.   Jour. AWWA, 61, 4,
    181-185 (April, 1969).

50. Cleasby, J.  L., Filtration, Chapter 4 in Physicochemical Processes for  Water Quality
    Control, Weber, W. J., Jr.,  Editor, Wiley-Interscience,  New York, (1972).

51. Amirtharajah, A. and Cleasby, J. L.,  Predicting Expansion of Filters  During Back-
    washing, Jour. AWWA,  64, 1, 52-59 (Jan. 1972).

52. Cleasby, J.  L., et. al., Optimum Backwash of Granular Filters. Engineering Research
    Institute,  Iowa State University, September 1973. (Presented  at WPCF Conference
    Cleveland, Ohio (Oct. 1973).

53. Water Treatment Plant Design, American Water Works Association, Inc., New York
    (1969).

54. Kreissl, J. F., and Robeck, G. G., Multi-Media  Filtration:  Principles  and Pilot Ex-
    periments, Bulletin No.  57, School of Engineering and Architecture,  University of
    Kansas, Lawrence, Kansas (1967).

55. Summary Report, Advanced Waste Treatment. WP-20-AWTR-19, U.S. Dept. of the
    Interior, FWPCA (1968).

56. Lynam, B., Ettelt, G., and McAloon, T. J.,  Tertiary Treatment at Metro Chicago by
    Means  of Rapid Sand Filters and Microstrainers, Jour. WPCF, 41, 247 (Feb.  1969).

57. Rogers, E., Clark County Utilities Department, Springfield,  Ohio, Personal Commu-
    nication (May, 1972).
                                      9-57

-------
-58.  Truesdale, G.  A. and  Birkbeck,  A. E., Tertiary  Treatment Processes Jor Sewage
      Works Effluents. Water Poll. Control Jour. (Brit.) 66, 371 (1967).

 59.  New England  Interstate Water Pollution Control Commission, A Study of Small,
     Complete  Mixing, Extended Aeration  Activated  Sludge  Plants  in  Massachusetts,
      (1961).

 60.  Marshall, G. R., and Middlebrooks, E. J., Intermittent Sand Filtration To Upgrade
     Existing Wastewater  Treatment Facilities, Utah Water Research Laboratory, PRJEW
     115-2 Utah State University, Logan, Utah (February 1974).
                                        9-58

-------
                                    CHAPTER 10

                                 COST ESTIMATES  .

10.1  Introduction

The cost curves included in this chapter are based on: 1) actual installations, 2) projections
from pilot studies and other literature, and 3) manufacturers' information. In general, larg-
er capacity units show economy in both capital and operating costs. Since the added econo-
my of scale for plants greater than 100 mgd size is small,  unit figures for this flow (or the
total area required  at this  flow) can be applied to larger  plants. Costs for plants smaller
than 1  mgd vary too widely to permit effective use of general curves.

Cost calculations were based on an EPA-STP Index of 175 (July 1972 for U.S. Average).
The procedure for adjustment of costs to another cost index is outlined in Section  10.5.

10.2  Curve Content

The curves shown include all equipment and controls necessary for a working unit process.
Construction is assumed to include excavation and backfill in good soil on a level site. In
general cost curves  do not include:

     1.  Buildings
    2.  Land
    3.  Pumping between  processes
    4.  Sludge disposal
    5.  Yard piping
    6.  Special site conditions requiring pile foundations, rock excavation, etc.
    7.  Chemical feed equipment (given as separate curve)
    8.  Automated control (except as noted)
    9.  Engineering, Legal and Fiscal Costs.

10.3  Operation and Maintenance Costs

These are presented as curves or as a percentage  of total capital costs and include normal
repairs  expected during operation  of  units, but not breakdowns  resulting  from mis-
application or overloading. Chemical costs are not included in operating costs  of chemical
feed systems, and should be allowed  for separately based on actual cost and dosage. O&M
costs include power for normal operation but do not include external power costs for pump-
ing between units.

EPA Regulations, Title 40, Chapter  1, Part 35, Appendix A (Federal Register, 38, No. 174,
September, 1973), specify the useful  life of various structures and equipment items to be ap-
plied in cost-effectiveness analysis. The regulations also specify use of 7 percent annual in-
terest in cost comparisons. In general, structural items have lives of 30 to 50 years, process
equipment 15 to 30 years,  auxiliary equipment 10 to 15 years and electrical equipment 8 to
 10 years. More specific information than given in the regulations may, in some cases, be ob-
tained from product manufacturers.


                                       10-1

-------
10.4,  Freight

Costs include freight allowances for equipment based on typical distances in the mid-west
and eastern parts of the country. For western installations costs may be up to 2 percent
higher due to extra shipping costs.

10.5  How to Use Cost Curves

     1.  Select capacity of unit(s) based  on plant design capacity plus desired standby
        capacity in unit(s).

    2.  Enter curve with capacity of unit(s) and read total cost.

    3.  Correct  unit cost to current local cost index. EPA publishes current treatment
        plant construction cost indices for 20 cities in the U.S. The current cost index
        should be corrected for  time and geographic location. After selecting the proj-
        ected cost index use the following equation to compute corrected costs:
           Corrected Total Cost =
                  ($1000)
[Current Cost Index 1 x [Total Cost!
        175       J   L  ($1000)  J
    4.  The costs arrived at in Step 3 should be modified for the following items wherever
        applicable:

           a. Special instrumentation for automation or computer control

           b. Special site preparation such as pile foundation, rock excavation, housing and
             landscaping

           c. Architectural requirements

10.6  Curve Description

The curves are primarily intended for preliminary cost comparisons between  processes at
any flow or equipment size in the  range covered. For actual designs a detailed  specific cost
analysis should be made.  Further  data on costs  for suspended solids removal processes are
available (1)  (2) (3) (4) (5) (6).

     10.6.1   Flocculators—Flash  Mixers

Figure  10-1 presents capital and O&M cost estimates for flocculators and flash mixers.
                                       10-2

-------
  100
o
O
O
Q
UJ
(O
O

I
   10 ,_
                               .  FLO CGULATORS
                                  CAPITAL  COST FLOCCULATORS
             & M.   FLASH MIXER
                                        CAP.ITAL   COST

                                       I FLASH  MIXERS
                                       j- r _...-.-.   .  T T ..  t rr rr  T n i n I
                                                                          O.I
                                                                          _  cr

<  o.

7  CO
                                                                                 O
                                                                               (T
                                                                               Ul
                                                                               Q.
                                                                               O
                                                                          O-OOI
                                       10

                             CAPACITY  MGD


                                 .FIGURE 10-1

                         FLQCCULATORS—FLASH MIXERS	
                     'COSTS ADJUSTED TO EPA—STP INDEX 175
                                                                        100
                                           10-3

-------
Capital costs ($1000) and O&M costs (
-------
1000-

0eooJ
o
0
-W- 900
INSTALLED
o 9
p p
i-
co
o
o
_i 80~
H
H
20-
lO



t
'





f
}•



rf>





S






•*



**



iiiSljiUM
	 EJ 	 I
	 S 	 	 	 	
	 " — ; 	 	 'l



,
	 - ? ' 	
— — ^ 	 	 pi!. -
^ ™ 	 	 _ . i !.......
	 ._ _ 	 — •* — -•• • • - 1 • i
	 	 ;»•' = 	 I
^ *
^ ^
^, 3*




2 3
iilimmHiliH:^



LEGEND : CAPJT)

= =fci= 3=
\L CO
Q ALUM — 200 mg/l
OR IRON — 100 mg/l
i ^ LIME — 300 mg/ 1
• Q POLYMER— 1.0 mg/l
i, ~~jT~ T 	 j 	
'"> I 1
" .. .. (.\ .


1 " 	 ^~ - "2 ( * •* 	 ' ' 	 •" 	 1 	
i|EEiEEEi;:||;;j j —
O fi M 1 FDR A LI J
v/iVi i»i. \ i vyr\ <™« ^w










^

r/








/

r
1







>


i
M
II







/


^
L
*






___..,
^<...


>
M==*
^ — I — 1— _ B
ONC






456789 2
10


— s 	 -. i — --,
-- 	 •---:;'? 	 j'!
_ 	 	 , ! 	 __, i . .
l'' '
>' I1'
' i'''t I
i 	 	 ! ii_. ._
)R POLYMER)






345
IMplI-10
§ ::::: :!::£: :3
:: ::::: ::: :: X
.. ','.. ..... ,E
•ft
	 -•• |i ••}
_...., 	
>\ 1 	


l;PJ::l-">
r:::li;:: i: >
'•'-'•'• ! ;: 	 ?
i;;;|;;;;!!!

f 	







6 7 8 9 ' w
100
                                                         UJ
                                                         o
                                                         UJ  _l
                                                         I-  <
                                                            ui
                                                            o
                                                         oc
                                                         UJ
                                                         o.
                                                         o
        CAPACITY  MGD


             .FIGURE 10-2
       CHHMICAL FEED SYSTEMS
COSTS ADJUSTED TO EPA—STP INDEX 175
                    10-5

-------
For different chemical dosages calculate cost based on an equivalent plant flow calculated
as follows:
                 equivalent plant flow.= , I design I
                                       [ flowj
expected dosage
dosage used to
develop cost curve
Each system includes: 1) a minimum of two volumetric or gravimetric automatic propor-
tioning feeders sized to provide 50 percent excess feed capacity; 2) pumps to deliver chem-
ical feed  solutions to the process; and 3) 30-days bulk storage. Prices cover installed equip-
ment suitably corrosion-protected for the intended chemical service. Not included in capital
costs are buildings (except for dry  chemical storage space), land, sludge disposal and ex-
ternal piping.

O&M costs include manpower for operation and normal maintenance, and power costs for
pumping, but do not include chemical costs.
     10.6.3   Sedimentation Basins

Figure 10-3 presents estimates of capital and O&M costs for sedimentation basins.

Capital costs ($1000) and O&M costs ($1000/yr) are given for installations requiring total
basin surface areas of 1000 to 100,000 sq ft. Costs are based upon installations using two or
more units.

Included in capital costs are inlet appurtenances and sludge-collecting mechanisms circular
or rectangular tanks (steel or concrete), skimmers, scrapers, supports and walkways, and
sludge draw-off, all completely installed. Curves are applicable for end-feed, center-feed or
peripheral-feed designs for primary or secondary treatment applications.

O&M costs include manpower for operation and  normal maintenance but do not include
chemical or pumping costs.

Not included in capital costs are land,  buildings, covers, chemical feed equipment, pump-
ing, sludge pumping and disposal and external  yard  piping.  Instrumentation is  limited to
automatic sludge blowdown valves and lines.


     10.6.4  Solids Contact

Figure 10.4 presents estimates of capital and O&M costs for solids-contact units.

Capital costs ($1000) and O&M costs ($1000/yr) are given for installations requiring total
basin surface area of 1000 to 100,000 sq ft. A minimum of two operating units was used to
estimate costs.
                                        10-6

-------
     1000-
O
o
o
(0

O

o
0.

<

0
      100-
       10-




























9J
^
































































__ ""_. -PA Dl "
i V^Ar 1









( i '
( * '
0 !
„*"
?
























r A l PflCT i ii
1 ML. \j vO 1 v

::::::::::: = = ::::;;*
^ ^
^
, "
, »'
....... ^
	 ... _ f ...

	 .. ., *_ 5 _ . _ _
.1" ^ , n
,.' S
^MM-|Wc
— 5 	



























r




^

































t



*






























	 ..T.. 	 j
	 	 ._..._ 4! . -^ —


	 -_ 	 	 ,8 	
t '
^ *
£ '
^






a M.!EEE;:!!!!EEE!
ro
















.
	 1 	 j<-- -too


....... ^ ^ 	 .. Ill

-L C^
i
! ; "!E ::gr ~ " ~" "i "~ ^t
• 	 	 	 ^ ^ ^J

. ::... ...( 2 	 :. f^ u?
::S!?::::: _" = "| — UJ
:::::::::: :::::: LU

Q.

<^*>
iiiiimiiiiiii-miriu 8
- •- J- £•• 	 	 Q

^? "
^^
........... ------- Q jfr


	 	 _ 	 QL
EEEEEE;EEEE-EEEEEE o









| 2 3456789) 2 34567891 1-w
1 10 100
^SURFACE AREA 1000 SQ. FT.
                                    FIGURE 10-3

                                  SEDIMENTATION

                       COSTS ADJUSTED TO EPA—STP INDEX 175
                                      10-7

-------
   10
     • I»I»B =========== =====sz=:rz::i::::::=s===s=::: ::::::;::
                                                                                     1000
o
o
o

40-
STALLED
 t-
 V)
 o
 O
                                                                        5678 9
                                                                                   ioo
                               SURFACE AREA, 1000 SO. FT
                                           FIGURE 10-4

                                        SOLIDS CONTACT

                            COSTS ADJUSTED TO EPA—STP INDEX '75
                                           10-8

-------
Included in the capital costs are concrete or steel tanks and  slabs, turbine recirculators,
sludge scrapers, skimmers, inlet and  outlet distributors,  supports and walkways,  sludge
drawoff, internal baffles, piping and accessories, all fully installed. Prices are applicable for
upflow-type solids-contact units with integral flash mixing  and flocculating provisions.

O&M costs include manpower for operation, turbine mixer power, and normal mainte-
nance but do not include chemical or pumping power costs.

Not included in capital costs are buildings, land, chemical-feed equipment, external pump-
ing, sludge disposal and external yard piping. No flow, turbidity, conductivity, or other as-
sociated instrumentation is included.

     10.6.5  Flotation

Figure  10-5 presents estimates of capital and O&M costs for dissolved-air flotation pro-
cesses.

Capital costs ($1000) and O&M costs  ($10007yr) are given for installations requiring total
basin surface areas of 300 to 50,000 sq ft.

Included  in the capital cost are all tanks and  internals,  air-pressurizing equipment, rec-
ycie-pumping equipment, operating valves and piping, all  fully installed. No special corro-
sion protection is included except for normally-painted items.

Assumed O&M costs were 3 percent of capital costs including manpower for operation and
normal maintenance, power for normal  pumping and air pressurization, but not including
chemicals.

Not included in capital costs are buildings, land, chemical feed equipment,  sludge disposal
and external yard piping.  Instrumentation is limited to pressure-sensing controls for normal
operation of the units.

     10.6.6  Settling Tubes and Wire Septums

Figure  10-6 presents estimates  of capital costs for tube settlers and wire septums.

Capital costs are given as $1000 for total required screen areas of 10 to 2000 sq ft. The esti-
mated operating labor requirement is 0.5 man-hrs/day mgd.

O&M costs for tube settlers can be estimated at 2 man-hr per basin per week. O&M costs
for wire septums can be estimated at 1 man-hr per  basin per day.

Included in the capital costs of tube settlers are  plastic tubes with 60° inclination and 21 in.
deep plus steel supports and additional effluent collector weirs. Wire septum costs include
stainless steel wires, all fully installed.
                                        10-9

-------
cuyuuu






10
O*
•
O"
T,
r>
° 6
-«*- 9
4
0
UJ ,
-1
?
CO
z •


H 1000
CO 9
Oa .
•
07.

6:
< 5
H 4
0.
<-m.
O
2



IOO!

























• * 	 	


ft
g •
*




•












	 0







	 9
	 ,,9
-,ii!
.'




































a M







g^









4

P*'*

























^
^ ^

i

0 '

























/

























t 	 	 -- j
	 	 ; i —
\ •
i '
--s^--j •* 	 	
' '
(-,<- 	 ---




.
	 _ ^ E - - . 	 ..... 	 U




	 	 	 _... —












!lE!|!:l===E|ii
:»;;;;;;; ==5«!;i:=

! 	 	 	 	 —

j
----- ^
I
	 	 ... __!.,.-
j
j -

g i '
2 ' ^
J!!::i!;iiii^|C/J























/,






F


















^



/







>l











,i
_ 	 ,,!:..
	 4\ 	
. '

^
., '_. 	 !
r T


'^- 	 I' '








TAL COS





j ::::: :±i 2OO
	 . ^B. . .
T j|
, w
j *


	 	 too


:::::::::: Ul

]:::::":" o
i^E|=g; z
J:|5i^ <
i^»?;;;;: u
^l""f * <
jiffiMffl « ^
— 	 ^* >•
mm a«:
. 	 	 O Q^
z
....:..:.. ^ o
	 10 o
iiiiijiniiii o
— 	 z "~

".'.".'.' ".'".' O '^*'
;T EEEEEEE; H
)[[-[ J L J (E
UJ
::::::::::: Q.
::i::i:::::: Q



	 I.O
I    4  967091
0.3            1.0
4967 Vo'o
3  4 is.'o
                SURFACE  AREA, 1000 SQ.  FT.
                          FIGURE 10-5
                          FLOTATION
             COSTS ADJUSTED TO EPA—STP INDEX 175
                        10-10

-------
o
o
o
o
UJ
O)
CO
o
0.
<
O
10,000 m
9 •



» :
4 •

» ::
2 •




1000
9. ..
0



6 •
• •
4 ••

••
2 <•







100-
9 '•


, ,!

1 .
5 ::
* I!


». ..
2 •






10- .

































J
w
1



it
\ ~












































'



^













































, *

>
4
jjj

SC









































-
r
t





^
-^ -*































\
H







^
'


/



X

































WIRE SEPTU
1
I1
	 	 ~ , - . . ,t . . . -
	 . ! 	 .4
	 ,! 	 .,l.

---?- — ,>'
A - *' 1 '
> ' t> I1
J ' I1
* ('
,.2- -_.<('..
x^ ..'
i'
	 J r
— r
r





















SETTLING Tl
(CIRCULAR B







^

^^
j
MS l^ljj,^::;
I"t LU»ff1 PM Lkf J4^
_--.(t....T| ? 	 3
~a'" T4" " '^*! 	 ' '"
/ i *
— -;z.!-|-?Z---^
	 ^ 	 . - r i — 	 	
-£-- — "jiff" 	
	 ?*---& 	 JJ-J


t — --- i
\





























BE
AS





^
i



7
(i








c
II





/



/










5
4




/



/











S



/



(^



^








). 	 S 	

, -. r .... -
i:^..s 	 	
	 . . _ 	 .1 _
" • - j * • •---• —
- _ - . ! 	
	 ~t 	 ^
--3"- 	 !•'•-
3 1
2 «' /
>' i1
__.^ 	 .(!.._
/ •'
-3 	 -tl 	 -
	 -^- 	 -
_,2 	 	 _
9
*



SETTLING TUBE
RECTANGULAR B
























































































































A
	 -»_-. .g!.. 	
-- 	 |! 	 	
_.j: 	 _^
	 i:__. 	 _2 _
i 4
t . *
? . i
:;:: ::j !:::::: *-q
(-_-.. , i . _
--^ 	 i--- 	
!--..,! 	 3
i
, '












- - -^- J~'
'S —

AS INS)

" '
























. *



- I -
jj _ _
r
f
f











































^




;'












































 •J  6789^

0.5      1.0
                       2    3   456789
                                                 2    3   45678
                                         10
100
                       INSTALLED AREA, 1000 SQ.FT.
                                 FIGURE 10-6
                      WIRE SEPTUMS AND SETTLING TUBES
                     COSTS AUJUS1 tD TO EPA—STP INDEX 175
                                    10-11

-------
Prices for wire septums are applicable to circular or rectangular designs. Prices for tube set-
tlers are given separately for circular and rectangular tank designs. No special corrosion pro-
tection is included except for normally-painted items.

Not included  in capital costs are buildings, tanks, cleaning devices (air grids), sludge dis-
posal and external yard piping.

     10.6.7  Wedge-Wire Screens

 Figure  10-7 presents estimates of capital costs for wedge-wire screens. O&M costs are cal-
 culated as described  below.

 Capital costs are given as $1000 for total required screen areas of 10 to 2000  sq ft. The esti-
 mated operating labor requirements is 0.5 man-hrs/ day/ mgd.

 Included in the capital cost are screens and screen supports. All screens are stainless steel
 construction,  but supports, baffles and distributors are steel. Rotating screens include mo-
 tor drives. Prices are applicable for rotating screens and stationary screens. No special cor-
 rosion protection is included except for normally-painted items.

 Not included in capital costs are land, buildings, pumping, sludge handling and external
 yard piping.

     10.6.8 Microscreens

 Figure 10-8 presents estimates of capital and O&M costs for microscreen equipment.

 Capital costs ($1000) and O&M costs ($10007yr) are given for installations  requiring total
 screen areas of 100 to 10,000 sq ft. One unit for 1 to 2 mgd flow, two units for 3 to 4 mgd,
 and three units for flows of more than  5 mgd were used to estimate costs.

 Included in the capital costs are tanks, drums, screens, backwash equipment, drive motors
 and all accessories for automatic operation, all fully installed. Prices are applicable for con-
 crete or  steel tank construction.  No special corrosion protection is included except for nor-
 mally-painted items.

 O&M costs  include  operation,  normal  maintenance,  and  power for rotation and  for
 spray-water pumping, but do not include chemicals  or external pumping power costs.

 Not included in capital costs are buildings, land, pumping (except spray system), sludge dis-
 posal and  external yard piping. Instrumentation is limited to automatic valves and time
 cycle or pressure sensing backwash control suitably panel-mounted.
                                        10-12

-------
IOOO-"
8
7
6
6
4
3
2
0
0
o
•«*- 100 -
9
8
0 7
W 6
-I 6
J 6
» 4
CO
z 8
CAPITAL COST 1
o
at O -a os to 1 10
4
3
2
1.0
l(






































=j
<-











































?



















































	 T 	 	














I

" 	 ... ( ROT








— 	 "~:
- r
T 2
.
t '
, '
^ '"/

j. .._..., _j__


j )
_^_ _. 	 ..... 	
^ 	 . 	 .... _ _






2 3
3
















-\
ATING \--~,~~-'-'.~


'-.---.~::-lt\-: — - = -*:': =
~ ' ~ '[ 	 	 ~ £ 	 ~~



1 	 .-•-•/ 	

g '
^
4 '
j n
<































?
^

























•1 — HI niiriiiiii 1 1 1






_.:"::; •;::::::: ::^
^
r
i '
1 1
__ 	 (i!..., ...
p 	 j i 	 __*
1 j ~
P
. *
A 	 ...-1..
4 «'
	 	 a - - - - 	 	
/ -- 	 	 	
\- 	 	 --"
= -t= = ioTAT OKI
	 -w 1 " 1 1 w IN























4B6789 2 3
100





:::::::::: :_::::::g
__ 	 ... 	 f
_.,_... .. _ — a''-
	 -,.-.. 	 -t 	
_ ... j ...... ~ j---..
(... 	 - -a - 	 --
^fiiiiiiiiiKiiriiin

. *
^ *
. t








A R V -H-il -\-\4 444 'M




























v












































— j
^












































--:j







































4 6 6789.
1000
TH

^ '
t
- t -
*--







































1
2000
           SCREEN  AREA, SQ. FT.

                 FIGURE 10-7
WEDGE WIRE SCREENS: ROTATING AND STATIONARY
      COSTS ADJUSTED TO EPA—STP INDEX 175
                     10-13

-------
f
3
2
1000-
O 9
O 6
O 7
•w-
5
s «
_J
_J 3
<
»-
(/) 2
z
h-
C/>
O 100
» a9
<
1- 5
Q- 4
<
°
2
10 -
O.I




































£_

x












































(*"









































/


4
e.





























































_ . 	 .X
	 T
T
	 . I
- 	 ±


















_. -- g*
t
"f. "~
^l-z-----
i
t *
	 ^ C 	 +
^ 	 . .







2













_
.
::::::: = — :::!£:
	 	 +-
CAPITAL









('
j
S
... ..j* .
t
t '
: i' 	
)


	 	 j" 	 3
„.,!:.. 	 ..
,i ' 	 	 ..








3 4















COS




2
	 x
^
.i'
i







	 _2
2
,'
.,,.
























T















/
































Z ..









_ 1 _
2














5 6 7 8 <















L











/
^










































y


































/








<


































/








,
^1


































?'








"

































! '
(j'
(?__ 	

	 . - . - 	
	 -(' 	 -

-v 	 -

2
!_ 	 _


._-\. ._ i, ..
-•^oa M
rmmr
— 	 f —
T
	 t













) 2 3
1.0



EEEE;^:|JEEEEE
	 	 	 ^
- 	 --->-
	 x** 	

,,•'
/
— , i 	 	 _
. '
,j 	 	
> (
?
f
__,_._._..!... 	 ___
	 .1 	 	
<'




























4 5678



:::^~




.
. . . ^ _
^ '










































<*











































/








































2
^
<_
/






































9
10

»'


v



i 100



















--• 10















1 0
•
20
                                                    UJ
                                                    O
                                                    z
                                                    <
                                                    z
                                                    UJ
o:
<
UJ
                                                       cr
                                                       UJ
                                                       Q.

                                                       o
                                                       o
                                                       o
                                                    or
                                                    UJ
                                                    a.
                                                    o
  SCREEN AREA,  1000 SQ.  FT.
             FIGURE 10-8

           MICROSCREENS
COSTS ADJUSTED TO EPA—STP INDEX 175
                  10-14

-------
O
O
o
o
UJ
O
o
a.
<
o
4 1 1 Mil



2 	


10,000 	
9 	




5 	
4 	

2 	
. j '
( i


1000 -::::
9 	
Q 	



6 --• j
5 	
4 	

2 £--•
0 1
1 •

100 	














fffflr0 a

,!
•;,i'- 	
i'










J:: = = = = :&


... 	 _(
,!
... « - 	 ...
,i I




2 34














M;; =
. ,!. -
1 ' ....










ftpm

::::;i-































\

f.



















" ~ *
0










L (







5 6 7 e














- 1 n
., R











X)S
2




















^











T
^




















^












^

















?
	 	 .1 '! _,
>i
S 	 ._ 	 _











f
	 1:: j!!::::: =
	 jl-_, 	 _







9 23




::::::: :::;
	 	 ,*'-.
t*
,,i'
	 . - l" 	 	 .
	 I " . 	 	

- '







^
.s
,> ^
F

i
, !
• •













2
«L_











-. 2
i •





















^











?<






















><











.
e.






















Tl











^l





















ILU











"" ~ ^ ?
-^ ---



















4 S 6 7 S 9
                                                              300
    UJ
    o
    z

    z
    UJ
    I-
    z

100  <
 '   *

    o

    <
                cr
                <
                UJ
                                                                     o
                                                                     o
                                                                  or
                                                                  UJ
                                                                  a.
                                                                  o
                                                              1.0
                          1.0
10
                   SURFACE AREA, 1000  SQ.  FT.
                             FIGURE 10-9

                           MEDIA FILTERS

                 COSTS ADJUSTED TO EPA—STP INDEX 175
                             10-15

-------
      10.6.9   Media Filters

  Figure 10-9 presents estimates of capital and O&M costs for filtration equipment.

  Capital costs ($1000) and O&M costs ($1000/yr) are given for filter installations with total
  required surface areas of 200 to 10,000 sq ft. A minimum of three operating units and filter
  bed depths of 4 to 6 ft. with sand and/or coal media were used to estimate costs.

  Included in the capital cost are filter tanks, internals, media, operating valves and piping
  and automatic backwash controls, all fully installed. The curve shown is an average curve
  for upflow or downflow, gravity or pressure (up to 60 psig) designs of either concrete or
  steel construction. Pressure filters are usually less expensive than gravity units below 3 to 6
  mgd, but are considerably more expensive at larger flows.

  10.7  References

   1. Smith,  R.,  Cost of Conventional and Advanced  Treatment  of  Wastewater,  Jour.
     WPCF Vol. 41, pg.  1546 (1968).
                               *
   2. Smith,  R., and McMichael, W. F.,  Cost and Performance Estimates for Tertiary
     Wastewater Treating Processes, USDI, FWPCA Report No. TWRC-9 (June 1969).

   3. Evans, D. R., and Wilson, J.  C., Actual Capital and Operating Costs for Advanced
     Waste Treatment, paper presented at WPCF 43rd Annual Conference, Boston, Mass.
     (Oct. 1970).

   4. Black & Veatch Engineers, Estimating Costs and Manpower Requirements for  Con-
     ventional Wastewater Treatment Facilities, prepared for U.S.  EPA, Project No. 17090
     DAN (Oct. 1971).

   5. Lynam,  B.,  Ettelt,  G., and McAloon, T.,  Tertiary Treatment at  Metro Chicago  by
     Means of Rapid Sand Filtration and Microstrainers,  Jour. WPCF, Vol. 41 pg. 247
     (Feb. 1969).

   6. Inder  Jit Kumar, Clesceri, N. L. Phosphorus Removal from  Wastewaters: A Cost
     Analysis, Water and Sewage Works, Vol. 120, pg. 82 (March, 1973)

   7. Gulp,  R. L., and Culp, G.  L., Advanced Wastewater Treatment, Van Nostrand  Rein-
      hold Company, New York (1971).

   8. U.S. EPA, Advanced Wastewater Treatment As Practical At South Tahoe,  Proj. No.
      171010 ELQ08/71 (August 1971).

   9. Sewage Treatment Plant Design, ASCE Manual of Practice 36 (WPCF M.O.P.  8),
     New York (1972).

U S  EPA Headquarters Library
      Mail code 3404T
1200 Pennsylvania Avenue NW
   Washington, DC  20460               10-16
       202-566-0556

-------
10.  Diaper, E. J., Personal Communication, Crane.Company, Cochrane Division, King of
    Prussia, Pa. (Nov. 1972).

11.  Ginaven, M. E.,  Personal Communication, The Bauer  Bros. Co., Springfield, Ohio
    (April, 1973).

12.  Carpenter, D. A., Personal Communication, Komline-Sanderson Engineering Corp.
    Peapack, N.J. (April,  1973).

13.  Johnson, R., Personal Communication, E1MCO, Envirotech Corp., Salt Lake City,
    Utah (April, 1973).

14.  Dvorin, R., Personal Communication, Graver Water Conditioning Company, Union,
    N.J. (April, 1973).

15.  Diaper, E.W.J. Tertiary Treatment by Micro-Straining, Water and Sewage Works,
    Vol.-116, pg. 202 (June  1969).

16.  Eilers  Richard, G., Condensed one page cost estimates for wastewater treatment U.S.
    EPA,  NERC, Cincinnati, Ohio (Nov. 1970).

17.  Graham, A.,  Personal Communication, Neptune Microfloc, Inc., Corvallis, Oregon
    (April, 1973).
                                     10-17

-------