United States         Office of Municipal      EPA 430/09-87-007
             Environmental Protection     Pollution Control (WH-595)    September 1987
             Agency           Washington DC 20460
&EPA       Preliminary Treatment
             Facilities

             Design and Operational
             Considerations

-------
  PRELIMINARY TREATMENT FACILITIES:
DESIGN AND OPERATIONAL CONSIDERATIONS
           Project Officer

         Francis L. Evans III
     Wastewater Research Division
Water Engineering Research Laboratory
        Cincinnati, Ohio 45268
OFFICE OF MUNICIPAL POLLUTION CONTROL
           OFFICE OF WATER
 U.S. ENVIRONMENTAL PROTECTION AGENCY
        WASHINGTON, D.C. 20460

-------
     This document is based on an EPA research study of the design and
operation of preliminary treatment processes at municipal  wastewater
treatment facilities.  The document has been subjected to  the U.S.
Environmental Protection Agency's peer review.  The information contained
herein does not constitute EPA policy, guidance or directive, nor does
mention of trade names or commercial  products constitute endorsement or
recommendation for use.
                                     n

-------
                                  FOREWORD

     The continuing efforts to upgrade and improve the level  of municipal
wastewater treatment throughout the country have stimulated investigations
into the design and operation of various unit processes of wastewater treat-
ment facilities.  Based on a belief that preliminary treatment processes
are a very important, but often overlooked, part of a treatment facility,
a study of the design and operation of these processes was undertaken.
Preliminary treatment processes can have a significant impact on the
efficiency and effectiveness of downstream treatment processes, and should
be designed to operate economically and reliably in the removal of grit and
screenings from the wastewater flow.  The purpose of this investigation was
to document possible improvements to the design, operation, and maintenance
of preliminary treatment systems for the removal and handling of grit and
screenings.

     The information in this report is intended to supplement and qualify
information available from manufacturers and published literature.  The
report provides a basic understanding of preliminary treatment systems and
presents concise information on design considerations, operational charac-
teristics, and process and equipment problems and possible solultions.  The
report also addresses guidelines for the design of new grit handling and
screenings removal facilities and upgrading of existing facilities.  This
report will be useful to design engineers, governmental  agency review
personnel, municipal officials, operators, and others who are considering
installing preliminary treatment systems, or who are concerned with
optimizing the performance of existing preliminary treatment facilities.

     The information for this project was collected from various sources.
Available literature and review of previous surveys of wastewater treatment
plants established the data base.  This data base was updated and supple-
mented through the inhouse experience of the study contractor and with
information from manufacturers of grit and screenings collection and
handling equipment.   Site visits to wastewater treatment plants nationwide
and contacts with manufacturers provided most of the information regarding
problems, deficiencies,  and guidelines tor successful operation of prelimi-
nary treatment systems.
                                    i i i

-------
                                  CONTENTS

Foreword	    11|
Acknowledgements 	     V1

     1.  Introduction  	     1
              Screening Characteristics   	     ^
              Equipment and Process Problems  	     4
     2.  Preliminary Treatment Practice   	     6
              Common Equipment Types and Their Basis  of  Design  ...     6
              Disposal  Methods 	    13
     3.  Preliminary Treatment Performance  Data  	    14
              Grit Removal	    14
              Screens and Comminutors	    15
              Data Sources	    15
              Processes Subject to Impact  	    21
     4.  Case Studies	    31
     5.  Detailed Evaluation Requirements  	    39
     6.  Recommended Design Practices  	    47
              General Design Considerations  	    47
              Design Considerations tor  Small  Plants  	    52
              Design Considerations tor  Large  Plants  	    55
              Design Considerations tor  Retrofits  	    57

References	    59
English to Metric Units  	    63

-------
                              ACKNOWLEDGEMENTS

     This report is based on the results of a study conducted for the U.S.
Environmental Protection Agency by Metcalf & Eddy,  Inc.,  Wakefield,
Massachusetts under Contract No. 68-03-3208.  Additional  technical  material
was provided by Mr. James F. Kreissl,  EPA Task Assignment Manager for this
project.  Mr. Kreissl  and Mr.  Walter G.  Gilbert were responsible for
completion of the final  text of the report.

     Mr. Francis L. Evans III, EPA Project Officer, was responsible  for
overall project direction.  Mr. Walter G. Gilbert,  Office of Municipal
Pollution Control,  also  contributed to the conduct  of the study and  the
preparation of this report.

     Metcalf & Eddy staff participating in this project included:

          Allen F.  Goulart,  Project Director
          Thomas K. Walsh, Project Manager

     Paul F. Tomkavage                         Eugene Davis
     George Bloom                              William A. Martin
     David P. Bova                              Daniel N.  sommers
                                     VI

-------
                                 SECTION 1

                                INTRODUCTION


     Preliminary treatment processes are among the oldest in wastewater
treatment facilities.  Their main purpose is removal of grit and screenings
to prepare the wastewater for subsequent processing.  Inadequate removal  of
grit and screenings can result in damage to downstream equipment, disruption
of routine operation of that equipment, and disruption of many aspects of
plant operation.  While many of the nation's wastewater treatment plants
have been expanded and upgraded in recent years, in many cases insufficient
attention has been paid to preliminary treatment.  The information presented
in this report is based on an EPA sponsored study of preliminary treatment
processes in a variety of facilities throughout the country.  The report
provides basic information on common preliminary treatment processes and
their operational characteristics.  Process and equipment problems related
to preliminary treatment are discussed, along with possible solutions to
those problems.  Recommendations for the design of new grit handling and
screenings removal facilities, as well as for the upgrading of existing
facilities, are also presented.

     The report includes several cost curves for estimating the capital and
operation and maintenance (O&M) costs of various preliminary treatment
processes.  Selected case studies from the study are presented to help
illustrate the adverse impacts of inadequate preliminary treatment, the
possible solutions to preliminary treatment problems, and the benefits of
improved preliminary treatment on downstream processes.

     This report is not intended as a detailed design guide or as a replace-
ment for other design guides available from manufacturers or for technical
information contained in published literature.  The report should be regarded
as a summary of technical design and operational information, and should be
used as a supplement to other sources of information when making decisions
regarding the design, upgrading, or operation of preliminary treatment
processes.

GRIT CHARACTERISTICS

     Grit consists of sand, gravel, cinders or other heavy materials that
have specific gravities or settling velocities considerably greater than
those of organic putrescibles.  In addition to these materials, grit can
include eggshells, bone chips, seeds, coffee grounds and large organic
particles such as food wastes.  Generally, what is removed as grit is
predominantly inert and relatively dry.  However, grit composition can be
highly variable, with moisture content ranging from 13 to 65 percent, and
volatile content from 1 to 56 percent.  The specific gravity of clean grit
particles reaches 2.7 for inerts, but can be as low as 1.3 when substantial
organic material  is agglomerated with inerts.  A bulk density of 100 Ib/cf
is commonly used for grit.  Often, enough organics are present in the grit

-------
 so that it quickly putrifies if not properly handled after removal from the
 wastewater (2).  Particles 0.2 mm and larger have often been cited as the
 cause of most downstream problems, but documented evidence is very limited.
 The historical basis for 0.2 mm as the lower size limit can be traced to an
 early literature citation (7).  There has been little or no subsequent
 questioning or challenging of this value since that time.  Since the term
 "grit" is, therefore, somewhat arbitrary, the lower limit of grit size of
 concern will  depend upon the nature of the collection system and the waste-
 water treatment processes and equipment.

     Generally, most removed grit particles are retained on a No. 100 mesh
 (0.15 mm) sieve, reaching nearly 100 percent retention in some instances.
 However, grit can be much finer.  In the southeast, where fine sand known
 as "sugar sand" constitutes a portion of the grit, less than 60 percent of
 one city's grit was retained on a No. 100 mesh screen.

     Grit quantities removed vary greatly from one location to another and
 depend on collection system type (separate or combined); winter road sanding;
 condition of sewers; infiltration and inflow (I/I); presence of household
 garbage grinders; scouring velocities in sewers;  the growth of slimes on
 sewer walls;  and the efficiency of the grit removal system itself.  Ranges
 of from 1/3 to 24 cu. ft of grit per million gallons of flow have been
 reported (1).  Table 1 shows typical data on removed grit quantities in
 several  cities (2) (8).

     It is difficult to interpret grit removal  data because grit itself is
 poorly characterized and almost no data exist on  relative removal
 efficiencies.  The information on grit characteristics derives from what
 has been removed as grit.  Sieve analyses are not normally performed on
 grit chamber influents and effluents.  For these  reasons, the efficiencies
 of different  grit removal systems cannot be compared in relative terms,
 except in a side-by-side study.

 SCREENINGS CHARACTERISTICS

     Screenings are the material retained on screens.  The smaller the
 screen opening, the greater will be the quantity  of collected screenings.
While no precise definition of screenable material  exists, and no recognized
method of measuring quantities of screenings is available (3), screenings
exhibit  some  common properties.

     Coarse screenings (collected on racks or bars of about 1/2 inch or
 greater spacing) consist of debris such as rocks, branches, pieces of
 lumber,  leaves, paper, tree roots, food particles, fecal matter, rubber
products, plastics, and rags.  The rag content  can be substantial and has
been visually estimated at 64% and 70% of total screenings volume on coarse
 (>l-inch) screens (3).  Coarse screenings are highly organic in nature
 (70-90%  or more), have a nominal dry solids content of 10-20%, and density
of 40-70 Ib/cf (2, 17, 34, 35) Coarse screenings  amount to 0.5-5.0 cu.
ft/million gallons of flow, but can be much greater at plants served by
combined sewers, particularly during storm events.


                                    -2-

-------
                               TABLE 1
           QUANTITIES OF REMOVED GRIT AND SCREENINGS (2,8)
Plant
Norwalk, Connecticut
Portsmouth, Virginia
East Hartford, Connecticut
Oklahoma City, Oklahoma Southside
Taunton, Massachusetts
Uniontown, Pennsylvania
Fargo, North Dakota
Minneapolis-St. Paul, Minnesota
Waterbury, Connecticut
Bridgeport, Connecticut, East Side
Duluth, Minnesota
Marshal Itown, Iowa
Richmond, Indiana
Detroit, Michigan
E. Bay Mun. Utility., S.O. No. 1 WPCP
Chicago, Illinois, Northside
New York, New York, Jamaica WPC
New York, New York, Port Richmond WPC
New York, New York, North River WPC
San Francisco, California, Southeast
Boston, Massachusetts, MDC, Nut Island
St. Louis, Missouri, Lemay
Passaic Valley Treatment Plant
Allegheny Co., Pa., Alcosan WTP
Fort Worth, Texas, Village Ck. WTP
Hampton Roads Sanitary District,
Chesapeake-Elizabeth WPCP
Lamberts Point WPCP
County of Milwaukee, South Shore WTP
Twin Cities Metro, WTP
Santa Rosa, California, College Avenue
San Jose, California
Manteca, California
Santa Rose, California, Laguna
Seattle, Washington, West Point
Dublin-San Ramon, California
Los Angeles, California, Hyperion
Livermore, California
Gary, Indiana
Renton, Washington
Flow
MGD
11.75
9.7
4.0
25.0
3.5
3.0*
2.7*
134. *
15.
14. *
12.
4.0
6.2*
450. *
128.
333.
100.
60.
220.
30.
112.
167.
225.
200.
45.
12.

20.
120.
218.
—
143.
—
12.
125.*
--
420.
6.25
__*

Grit
cu. ft/mil gal
3.3
0.39
2.4
1.95
1.11
10.5
1.0
5.2
4.15
1.25
0.8
3.4
2.0
4.0
1.26
0.41
2.24
0.50
1.50
—
0.68
2.69
3.82
3.32
1.29
2.17

4.85
0.48
4.82
0.88
2.5
5.2
5.0
2.6
7.0
2.0
1.0
8.6
4.1
Screenings
cu. ft/mil gal
0.17
0.82
1.33
2.1
1.0
0.9
4.55
0.9
2.35
2.04
0.56
0.25
3.44
0.47
0.83
0.83
0.42
0.17
1.0
11.7
0.41
0.06
0.76
0.38
0.72
1.17

1.20
0.60
1.15
--
--
--
--
—
__
--
--
--

Large percentage of combined sewers
                                 -3-

-------
     Fine screens with openings from about 0.01 to 0.25 inches are normally
used downstream of coarse screens for protection from large objects in  the
wastewater stream.  Screens with 0.09-0.25-in.  openings remove 5-10% of
influent suspended solids, while those with openings of 0.03-0.06 inches
can remove 10-15% of suspended solids, although greater removals  have been
claimed (3).  Fine screenings have been reported to have volatile solids
contents of 68-99%.  Compared to coarse screenings, their bulk densities
are slightly lower and moisture contents are somewhat greater.  Because some
suspended solids are also collected with fine screenings, quantities are
considerable, ranging from 5-30 cu.ft/mil lion gallons,  or more (1).

     The amount of screenings captured by different sizes of screens has
not been ideally established since each wastewater has  its own unique
characteristics.  British studies (3, 36,  38, 39) have  yielded removal  curves
for specific and nominal  municipal wastewaters  at different screen openings.
U.S. screenings capture v_£ screen opening curves are also available (34,
35, 41).  Each curve differs owing to the nature of the wastewater, but all
indicate that finer screens remove more material than coarser ones and  that
for any given sewerage system removals increase with flow.  Therefore,
screens must be capable of handling the quantities of screenings  which  will
be generated during peak  flows.

     Because putrescible  matter, including pathogenic fecal material, is
contained within screenings, they must be carefully handled.  Fine screenings
contain substantial grease and scum, which require substantial care in
handling.

EQUIPMENT AND PROCESS PROBLEMS

     Recognition of the problems associated with inadequate preliminary
treatment has recently begun to emerge.  Among  the major findings are
that downstream and sludge system impacts related to inadequate grit and
screenings removal are extensive; that accelerated equipment wear due to
heavy grit loads can result in expensive and time-consuming repairs; and
that jamming or clogging  of process equipment by screenings can require
significant manpower efforts to remedy.  Some specific  studies include:

     EPA Surveys (4, 5) - A U.S. EPA survey of  287 municipal wastewater
     treatment plants revealed that preliminary treatment was rated the
     15th most common design deficiency adversely affecting plant
     performance.

     Environment Canada Survey (6) - A survey of Canadian treatment plants
     found that of the plants surveyed:

     - Thiry-two percent  experienced problems with grit removal.
     - Twenty-four percent experienced problems with comminution  equipment.
     - Fifteen percent had screening problems.

     British Survey (3) - The British Construction Industry and Information
     Association's (CIRIA) Technical Note 119,  Screenings and Grit in Sewage:
     Removal, Treatment and Disposal, listed a  number of impacts, including:

                                    -4-

-------
Many downstream problems are caused by ineffective screening or
comminution.  Unfortunately, these problems are often considered
normal or unavoidable by plant operating personnel.  These problems
include settlement of solids in pipes, channels and chambers;
collection on weirs and stilling boxes;  blockage of pumps, heat
exchangers, and pipelines; frequent repair and cleaning of anaerobic
digesters due to fouling of mixers and loss of available volume; and
blockage of small diameter pipe work associated with sludge dewatering
equipment.

Disintegration (grinding/comminution) of screenings and their return
to the plant flow causes more problems than screenings removal.

Fine screens are seen as a way to eliminate downstream problems.
However, capital and O&M costs are significantly higher due largely
to greater volumes of screenings.

Inefficient removal of grit causes fewer and less severe downstream
problems than inefficient removal of screenings.  However, downstream
problems most frequently cited as grit related are settlement in
wells, chambers, channels; accelerated wear of pumps; and buildup in
anaerobic digesters.
                              -5-

-------
                                 SECTION 2

                       PRELIMINARY TREATMENT PRACTICE


     Some form of preliminary treatment is provided at most wastewater
treatment plants in the United States.  The 1984 Needs Survey conducted by
the U.S. EPA  (33) shows over 15,000 wastewater treatment facilities now in
use with a variety of preliminary treatment processes.  That survey also
indicates that over 6000 treatment facilities are yet to be built.  Based
on these estimates, it can be anticipated that there will be a significant
level of activity in the design of preliminary treatment processes for new
plants, and in the upgrading and replacement of processes and equipment in
existing facilities.

COMMON EQUIPMENT TYPES AND THEIR BASIS OF DESIGN

Characteristics of Grit Chambers

     Horizontal-flow grit chambers and aerated grit chambers represent the
two most commonly used grit chamber designs in the United States.  Recently,
devices in which the flow goes through a vortex motion have appeared, along
with fine static screens that have been used successfully to remove larger
grit particles.  Fine static screens are discussed more thoroughly under the
"Screens" portion of this Section.

     In horizontal  flow chambers, flow velocities of 0.5 to 1.0 ft/sec
and a chamber sized to provide a 1-minute detention are historic design
criteria (7).  The grit particles settle under gravitational force.  The
maximum velocity reached by the particle at which it continues to settle is
called the terminal  velocity.  The terminal velocity of a 0.2 mm diameter
particle (specific gravity of 2.65) has been experimentally established at
about 0.075 ft per second.  For particles of equal  specific gravity in the
same liquid, the theoretical  settling velocity varies as the square of the
diameter (Stokes1 law).  Consequently, a 0.1 mm diameter particle of the
same density would settle about one-fourth as fast.  For a given flow
velocity, a horizontal  flow chamber must either be about four times longer
at equal  depth, or one-fourth as deep at equal  length, to remove the smaller
diameter particle.   Although grit chambers are designed to allow grit to
settle and organics to remain in suspension, the settled grit can in fact
contain significant organic material, requiring separate grit washing (3).

     The two principal  types of horizontal  flow grit chambers are long,
velocity controlled channels and square, shallow sedimentation (detritus)
tanks.   Velocity controlled channels have been used since at least the
1920's.  The theoretical  length must be increased by as much as 50 percent
to accomodate inlet and outlet turbulence (1).   To maintain the design
velocity of 0.5 to 1.0 ft/sec, channels of rectangular cross section use
proportional  or sutro weirs;  parabolic channels use rectangular control


                                    -6-

-------
sections or a Parshall  Flume.  The weirs require a free discharge and hence
need relatively more head loss (approximately 36 percent of the flow depth).

     In larger plants,  grit is usually removed from channels by a conveyor
with scrapers, buckets  or plows.   Screw conveyors or bucket elevators are
used to elevate the removed grit  for washing or disposal.  Because velocity
controlled channels often serve older, big city plants with combined sewer
systems, the grit removal machinery is subjected to excessive wear due to
the larger grit load, and may be  hard to repair because of its age.  In
smaller plants, an additional grit chamber is usually provided so that one
unit may be taken out of service  and the grit removed manually with shovels,
clamshell buckets, or other devices.  Some older small plants may have long
grit channels without a section to control velocity.  These may experience
velocities significantly greater  than the design velocity of 1.0 ft./sec.

     Detritus tanks are shallow,  short detention sedimentation tanks that
have been in use for about fifty  years (9).  They are nominally designed to
remove 95% of 0.15 mm diameter (100 mesh) particles at peak flow.  Influent
is distributed over the cross-section of the tank by a series of vanes,
flows in straight lines across the tank, and overflows a weir in a free
discharge.  Settled grit is collected with a circular rake, and then may be
pumped through a hydrocyclone to separate remaining organic material and
concentrate grit.  The concentrated grit then may be washed again in a
classifier utilizing a submerged reciprocating rake.  A reciprocating rake
alone is also used to separate organic solids from the collected grit.  Head
loss, through the influent gates  and over the effluent weir, is modest.  As
with long channels, however, the grit removal and washing equipment is
subject to extensive wear.

     Aerated grit chambers remove grit by creating a spiral rolling motion
with diffused air. They are nominally designed to remove particles 0.2 mm
or larger, with 2-5 minute detention times.  Whereas horizontal flow grit
chambers are "once through", in aerated grit chambers, the spiral flow
results in a particle making two or three passes across the tank bottom at
maximum flow, more at lesser flow.  The size particle of a given specific
gravity which is removed is governed by the amount of air, which should be
adjustable, and the design of the basin, baffling, inlet and outlet.  The
roll maintains the lighter oryanics in suspension, allowing the longitudinal
flow to eventually carry them out of the tank.

     Aerated grit chambers have become very popular in the U.S. and Canada
in the last several decades; over 500 were in place 20 years ago, with many
more added since then (13).  Their popularity is due to more controllable
performance, benefits of preaeration and grit washing due to the introduction
of diffused air, and low headloss.

     The equipment used to remove grit from aerated chambers includes that
used with velocity controlled channels, as well as air lift pumps, grit
pumps, and vacuum trucks.  Tubular and screw conveyors reportedly discharge
the dryest grit (12, 13), while air lift pumps are particularly suitable
for small plants because of their low first cost.  Grit removal equipment


                                    -7-

-------
for aerated chambers is subject  to  the  same  wear  experienced  in  horizontal
flow chambers.

     Various configurations  for  aerated grit chambers  have  been  used  (12,
13), ranging from square tanks to  long, narrow  channels.  Although  there  is
some flexibility for site considerations,  overall  shape may not  be  as
important as proper placement of air  diffusers,  volume of air, and  adequate
influent and effluent baffling  (12).

     Grit is also removed in devices  with  a  vortex flow pattern.  The  two
principal devices are the PISTA™ and  the Teacup™,  both proprietary.

     In the PISTA™ grit trap, wastewater tangentially  enters  and  exits a
cylindrical basin.  At the bottom  of  the basin  is  an axial  flow  propeller,
and below this, a conical hopper.   The  propeller  maintains  constant  flow
velocity, and its adjustable pitch  blades  promote  separation  of  organics
from grit.  The action of the propeller produces  a toroidal flow path  for
grit particles.  The grit settles  by  gravity into  the  hoppers in  one
revolution of the basin's contents.  Solids  are removed from  the  conical
hopper either with an impeller-type grit pump,  or  an air  lift pump.   The
impeller-type pump operates  at  a higher head than  does the  air lift  pump,
and, in conjunction with a cyclonic concentrator,  can  remove  some of  the
remaining organics in the grit.

     The PISTA™ unit is sized to provide 30  seconds detention at  average
flow, and remove 0.15 mm (100 mesh) particles at  a minimun  of 0.25  in.
headloss.  Area requirements are minimal,  but the  unit is relatively  deep
(91 to 16')-

     About 400 units of this type  have  been  installed  in  over 300 plants  in
the U.S. (15).   The maximum unit size available is 70  mgd;  using six  of the
largest units.   All installations  are relatively  new,  hence operating
histories are short.  European  practice has  not been  fully  satisfactory (3).

     In the device known as the  "Teacup",  a  vortex is  generated  by  the flow
tangentially entering the top  of the unit.  Effluent  exits  the center of
the top of the unit from a rotating cylinder, or  "eye", of  fluid.  Centri-
fugal and gravitational forces  within this cylinder minimize  release  of
particles with densities greater than water.  Grit settles  by gravity to
the bottom of the unit, while  organics, including those separated from
grit particles by centrifugal  forces, exit principally with the  effluent.
Organics remaining with the settled grit are separated as the grit  particles
move along the unit floor.

     Headloss in the Teacup™ is  theoretically a function  of the  size  particle
to be removed and increases significantly  for very fine particles.   It is
claimed that 2 feet of headloss  is needed  to remove a  design  0.1 mm diameter
particle.  While grit removal  and  organics separation  improve at higher or
peak flows, the unit should not  be subjected to flows  in  excess  of  design
peak, or to wide fluctuations  in flow.   Pumps can level out flow variations,
and may be required in any event to provide necessary  operating  head.
Wedgewire screens have been used prior to  these units  to  remove  larger

                                    -8-

-------
solids that could clog the grit underflow drain.  However, the screens add
additional height and headloss to the unit.

     The oldest Teacup installation is only about four years old, so
operating experience is limited to new equipment at approximately a dozen
installations.  The manufacturer of the Teacup1" recommends pilot studies to
characterize the grit before final design of a unit, a procedure more
suitable for retrofits, or new installations with measureable influents
(10).  Thus far, units have only been developed to treat peak flows of
about 2 mgd, so a large installation would require many units and a means
of distributing flow to all such units and keeping flows to each operating
unit within prescribed limits.

Characteristics of Screens

     Screens used for preliminary treatment are broadly divided into coarse
screens and fine screens.  The purpose of coarse screens is to remove
potentially problem-causing debris, while fine screens are often used to
provide a level of treatment approaching primary as well as some degree of
grit removal.

     Coarse Screens.  Design criteria for coarse screens include bar size,
spacing, the angle from vertical, and fluid approach velocity.  Manually
cleaned bar racks typically have 1 to 2 inch openings, are inclined 30-45
degrees from the vertical, and require approach velocities of 1-2 ft/sec.
Mechanically cleaned screens usually have clear spacing between 1/2-inch
and 1-1/2-inches, are inclined from 0-30 degrees from the vertical, and
require approach velocities of 2-3 ft/sec (1).  To prevent penetration of
debris at peak flows, velocity between bars should not exceed 3 ft/sec.  An
approach velocity of at least 1.25 ft/sec is recommended to minimize solids
deposition in screen channels (16).  Head loss through partially clogged
coarse screens is typically limited to about 6 inches through operational
controls.

     Types of coarse screens include trash racks, manually cleaned screens,
and various types of mechanically cleaned screens.

     Trash racks consist of vertical or inclined bars with clear openings
from 2 to 6 inches.  They are designed to trap timbers and heavy debris.
They are often used in combined systems, and may be mechanically or manually
cleaned.

     Manually cleaned racks consist of inclined bars across a wastewater
channel.  Maximum water depth is about 3 to 5 feet in the channel to facil-
itate manual raking.  Screenings are manually removed using a rake with
tines matching the openings in the rack.  The screenings are pulled up to
the top of the rack onto a drainage plate, allowed to drain, then removed
to a container for disposal.

     An advantage of manually cleaned screens is that there is virtually no
equipment to maintain.  A disadvantage is that they are labor intensive and
prone to clog if not attended regularly (3).  They are found most frequently

                                    -9-

-------
at older small  (1 mgd and less)  plants,  but  also serve  as  bypasses  to
comminutors and to mechanically  cleaned  screens  in  larger  plants.   They  are
rarely specified for new plants  of any significant  size.

     Mechanically cleaned screens have been  extensively used  in  the U.S.
since the 1920's.  Spacing between bars  can  vary in size but  often  is  about
one inch.  Once used mainly in the larger plants, they  are now being used
in smaller plants as well.  Mechanically cleaned screens come in several
major varieties: chain operated  (the most prevalent),  reciprocating rake,
cable and catenary.

     Chain operated screens can  be divided into  categories based on whether
the screen rakes clean from the  front (upstream) side  or the  back  (down-
stream) side of the rack and whether the rakes return  into the flow from
the front or back.  Each type has its advantages and disadvantages, but
bacically the operation is similar.  In  general  front  clean,  front  return
screens are newer and more efficient in  terms of retaining captured solids,
but they may be more susceptible to jamming  by solids  that collect  at  the
rake's base.  With back cleaned  screens, the bars protect  the rake  from
damage by debris.  However, they may be  prone to solids penetration,
particularly as rake wipers wear out.  The rake  tines  which protrude through
the bars may bend and break.  All of the chain driven  screens share the
disadvantage of submerged sprockets that require frequent  operator  attention
and are difficult to maintain.  The heavy weight of chains and rakes causes
the mechanism components to wear rapidly and require labor for adjustment
and repairs.  The major disadvantage is  that the channel must be dewatered
for proper inspection and repair of submerged parts.

     A reciprocating rake screen imitates the movements of a  person raking.
A major advantage is that all parts requiring maintenance  are above water
and can be easily inspected and  maintained without  dewatering the channel.
The front clean, front return design minimizes solids  carryover.  They have
only one rake, instead of the multiple rakes on  chain  operated screens;
this may limit their capacity to handle heavy screenings  loads,  but may
offer the opportunity to improve removal efficiency by use of intermittent
cleaning or "matting" techniques during dry weather flow periods.   The high
overhead clearance required can  limit the instances in which  they can  be
retrofitted.

     In the front clean, front return catenary screen, the rake  is  held
against the screen by the weight of the chain.  An  advantage is  that the
driving mechanism has no submerged sprockets.  A disadvantage is the
relatively large amount of space needed to install  the screen.

     Cable driven screens are front cleaned, front  return  devices which  use
a pivoting rake that is raised and lowered on tracks by a  cable  and drum
drive.  The rake is lowered by gravity, pivots through the debris,  and  is
raised by the cable drive.  The rake itself is the only mechanical  part
entering the wastewater.  The device is limited in  its capacity  to remove
large debris from the screen, and operating experiences with this equipment
include problems such as slack cables, fouled cable reels, and improperly
operating brake mechanisms.

                                    -10-

-------
     Fine Screens.  Fine screens used for preliminary treatment are
distinguished by the small size of the clear openings - from less than 0.01
inches to about 0.25 inches.  They may be either fixed (static), or utilize
a rotating cylinder or drum.  They are designed on the basis of hydraulic
loading on the screen area.

     Static wedgewire screens with 0.01 to 0.06 inch openings are designed
for flow rates of 4 to 16 gpm/in. of screen width (10-30 gpm/sq ft of screen
area) and require 4 to 7 feet of headloss.  The screens require appreciable
floor area for installation, and must be cleaned (usually daily) with high
pressure hot water, steam and/or a degreaser to remove grease buildup
(14,18).

     Rotating wedgewire screens with the same openings have a hydraulic
capacity of 16 to 112 gpm/sq ft of peripheral area and require 2.5 to 4.5
feet of headloss.  In wedgewire screens, the headloss is attributable to
the wastewater entering the top of the unit and flowing down through it.
Rotating screens may be fed either from the top or internally.  The
cascading action of the water helps to clean the rotating wedgewire drum
(14,18), although high pressure cleaning is often needed with the rotating
screens.

     A fine screen should be placed after a coarse screen, such as a bar
rack, for protection in large systems (14).  They have been retrofitted
upstream of grit chambers, and have been used by themselves to remove grit.
The high headloss through fine screens means wastewater usually has to be
pumped prior to or after the unit.

Comminutors

     Comminutors retain solids (screenings) on a screen and shred them
until they become small enough to pass through slots (usually 1/4" to 3/8")
and proceed downstream.  They are in-channel  units.   While different manu-
facturers'  models share this common feature,  designs differ.  Units with
rotating drum screens operate with the drum nearly submerged.  Devices
with stationary semicircular screens operate with the screens about half
submerged.   In a barminutor, solids are retained on  a bar rack and shredded
by a device that rides up and down on the bars.  Headloss through a
comminutor usually ranges from several  inches to a foot,  and can reach
three feet  or more in large units at maximum flows.

     When introduced, it was envisioned that  Comminutors  would completely
eliminate the screenings removal  phase of preliminary treatment.  Further-
more, it was expected that the comminuted solids would be small  enough to
no longer cause problems with downstream equipment.

     Comminutors can theoretically eliminate  the messy and offensive task
of screenings handling and disposal.  This can be particularly advantageous
in a pumping station by eliminating the need  to handle and dispose of
screenings.   Installations are compact.   In cold climates, their use
precludes the need of preventing  collected screenings from freezing.

                                    -11-

-------
However, the comminuted solids often  present  downstream problems.   The
problems are particularly bad with  rags,  which  tend  to  recombine  after
comminution into ropelike strands.   These recombined rags  can  have  a
number of negative impacts,  such as clogging  pump  impellers, sludge
pipelines, and heat exchangers, and accumulating on  diffusers  and digester
mixers.  Experience in Great Britain, Canada  and elsewhere has been
generally unsatisfactory, resulting in their  elimination from  existing
plants and new designs (3,37,39,40,42).

     Locating comminutors downstream of a grit  chamber  reduces abrasion  on
wearing parts, but rag accumulations  in grit  chambers can  be severe.
Locating them upstream of the grit  chamber may  reduce the  rag  problem,  but
makes comminutor overhaul due to abrasion more  frequent, and grit can
accumulate in comminutor channels.   New units experience early wear on  the
lower cutting surfaces in contact with low initial  flows.

     Comminutors must be maintained to function properly.   Comminutors  are
constructed with a bypass arrangement so that a manual  bar screen is  used
if the unit is down.  This arrangement permits  screening to continue  while
the unit is out.  In such cases, the high repair cost of comminutors  often
results in their being left  inoperative.

     Regardless of their size, comminutors should  be high  quality,  heavy
duty devices.  They should have cast iron frames with submerged parts
constructed of cast iron or  of corrosion resistant metal;  rotating  cutters
constructed of stellited, tungsten  carbide or equally hard cutting  edges;
stationary cutters constructed of hardened tool steel;  motors  with  ample
capacity to operate the comminutor  under maximum conditions of loading;
gearheads capable of continuous power transmission at required cutter
operating speeds; and a comminutor  shaft designed  for continuous  service
which operates in heavy-duty ball bearings and  employs  mechanical type
seals to prevent the entrance of liquid.  Motors should either be mounted
to avoid submersion under peak flow conditions, or should  be capable of
submersion without damage.

Sidestream Disintegration of Screenings

     This process is similar to comminution,  except that screenings are
first removed from the flow, then ground up and returned to it.  Devices
used include comminutor/macerators  and hammer mill grinders.  Downstream
processing disadvantages associated with sidestream disintegration  are
similar to those affecting in-channel comminution.  The disintegrators
themselves are often high maintenance items.

     Similar to sidestream disintegration is  the use of grinders  upstream
of sludge pumps.  Grinders are a "second line of defense" in that their
purpose is to reduce larger solids, not removed in the  headworks, that
could impact pump operation.  Grinders themselves  are high-maintenance  items,
                                    -12-

-------
DISPOSAL METHODS

     Grit and screenings are generally disagreeable materials to handle.
Disposal practice and need are often specific to the plant and its
surroundings and reflect character and quantity of those materials as well.

     Disposal of grit and screenings must comply with all applicable state
and Federal regulations, which are designed to minimize adverse impacts on
floodplains, surface and ground water, endangered species, and air quality.

     Grit Disposal.  Grit may be disposed of as fill and covered if it has
not been adequately washed.  Small treatment plants often bury grit on plant
grounds.  In large cities, grit is sometimes incinerated with sludge.
However, this practice can have adverse impacts on the operation of
incinerators, as discussed in the next chapter.  In larger coastal  cities,
grit is sometimes barged to sea.

     Screenings Disposal.  Screenings collected from small installations
may be buried on site or disposed of with municipal  refuse.   Large plants
may incinerate screenings.  However, unground screenings can  jam the feed
mechanism to the incinerator.  The grease and other unstabilized materials
contained in fine screenings require that disposal  be prompt  to avoid odor
problems.
                                   -13-

-------
                                 SECTION 3

                   PRELIMINARY TREATMENT PERFORMANCE DATA

     Many treatment facilities have downstream treatment problems  resulting
from inadequate preliminary treatment,  but the source of these  problems
often is not readily recognized.   In general,  downstream problems  are more
clearly noted with respect to screenings removal  than to grit removal.   It
appears that grit-related problems (equipment wear and grit build-up) take
longer to occur than screenings problems, and are considered more  routine
in nature.

     In order to identify and better quantify some of these problems  and
the costs related to them, some existing facilities were investigated.   A
total of 38 plants were contacted in 1985 and 1986 to characterize methods
of removing grit and screenings,  to identify equipment problems associated
with those methods, and to determine the role of preliminary treatment in
mitigating downstream impacts.  Most of the preliminary treatment  methods
described in the previous section were  included in the sample.   The
facilities ranged in size from less than one mgd to 900 mgd.  Tables  2 and
3 are summaries of the facilities contacted.  A number of plants on this
list were selected for detailed evaluation based upon their particular
experiences with preliminary treatment.  Case studies of these  facilities
are presented in a later section.  Additional  plants described  in  detailed
articles in the technical literature are also included as case  studies.

GRIT REMOVAL

     Most treatment facilities report that their grit removal  systems are
effective if they are not experiencing  grit chamber problems.   Since  there
is no readily available method by which an operator can quantify the
effectiveness of grit removal, the presumption is that if there are no
mechanical problems, the system is working well.

     Operators tend not to associate downstream operational  problems  with
poor grit removal.  This was so even when downstream impacts such  as  grit
deposition in channels, blinding ot vacuum filter cloth, or frequent  digester
cleanout were noted.  Impacts on plant  operation were never rated  "serious".
This indicates that operators may expect grit removal facilities to have
limited effectiveness, and to expect a  certain amount of downstream impacts
as routine.

     Table 2 represents a summary of information on grit handling  from the
plants contacted.  The plant ratings are subjective and are based  on  contact
with the plants' operating personnel.  Among these plants, aerated and
centrifugal grit chambers are good performers in the eyes of their operators.
Horizontal flow grit chambers have experienced problems with grit  removal
equipment, which has impeded their effectiveness.
                                    -14-

-------
SCREENS AND COMMINUTORS

     The effectiveness of screens and comminutors is more readily gauged in
terms of the presence or absence of downstream impacts than in the case  for
grit.  Perceived effectiveness is a subjective judgment of operators and
reflects quantities of screenings collected, as well as estimates of what
has passed through.  In general, effectiveness was considered "good" if
most of the incoming rags were believed to be removed by the screens. This
is so even in some cases where downstream impacts made operation difficult.
Downstream impacts from "screenings", generally rags, can jam machinery
and wrap around or block just about anything if they get through.  Rags
also tend to recombine after comminution, or if they are removed, ground
up, and returned to the flow.

     Table 3 presents the information and subjective ratings obtained on
screens and comminutors from the plants contacted.  The major findings of
this review are:

     - About 60 percent of the plants had some downstream impacts related
       to grit.

     - About 63 percent of the plants had some downstream impacts related
       to screenings (mainly rags).

     - Measuring the effectiveness of screens in terms of material retained
       versus what is passed is not performed.

     - Three of the fourteen plants contacted which have a history of
       comminutor use stopped using them, either abandoning and/or bypassing
       them, or removing and replacing them with a screen.

     - Three plants previously ground screenings and returned them to the
       flow but have stopped doing so.  At one of the two plants that still
       does this, the practice is being reviewed.  Personnel at the remaining
       plant counsel against the practice of reintroducing ground screenings
       to the flow due to operating problems downstream of the operation.

DATA SOURCES

Literature

     A computer aided key word search produced a list of 400 titles, from
which 150 abstracts relating to preliminary treatment were selected and
reviewed.  From these abstracts, over 40 articles were reviewed in detail.
In general, the articles were design-related or focused on descriptions  of
equipment; information on downstream impacts and quantification of them  was
minimal.  Two articles of particular note are entitled "Screenings and Grit
in Sewage - Removal, Treatment and Disposal - Preliminary and Phase 2
Reports".  These reports are Technical Notes No. 119 and 122 by the
Construction Industry Research and Information Association (CIRIA) and
Water Research Centre (WRC)  of the U.K.  (3,37).


                                    -15-

-------
            TABLE 2




PLANTS CONTACTED:  GRIT REMOVAL
Grit Chamber
Type
Horizontal Flow
Velocity Controlled*










Detritor






Aerated





Plant
Size
(MGD)

0.4
0.4
1.0
2.25
3.1
3.4
5.0
6.0
6.0
270
900
0.6
2.8
3.0
3.6
30
64
170
1.0
2.5
4.0
15.0
24.0
30.0
Plant
Type

Secondary
Secondary
Secondary
Secondary
Secondary
Primary
Secondary
Secondary
Secondary
Secondary
Secondary
Secondary
Secondary
Primary
Secondary
Secondary
Secondary
Secondary
Secondary
Secondary
Secondary
Secondary
Secondary
Secondary
Col lection
System

Separate
Separate
Separate
Separate
Separate
Combi ned
Separate
Separate
Combined
Combined
Combined
Separate
Separate
Separate
Separate
Separate
Combined
Combined
Separate
Separate
Separate
Separate
Separate
Separate
Effectiveness
of
Grit Removal

Fair
Fair
Fair
Poor
Good '
Good
Fair
Good
Poor
Good
Poor
Fair
Good
Good
Fair
Fair
Poor
Good
Good
Good
Good
Bypassed
Good
Good
Downstream
Impacts

Yes
Yes
Yes
Yes
No
No
Yes
No
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
No
Yes
No
No
Yes
Yes
Extent of Operational
Problems

Routine
Routine
Routine
Routine
Routine
Routine
Difficult
Routine
Difficult
Routine
Routine
Difficult
Routine
Routine
Routine
Routine
Difficult
Routine
Routine
Routi ne
Routine
Routine
Routine
Routine

-------
                                                TABLE  2  (continued)



                                          PLANTS CONTACTED:   GRIT REMOVAL
Grit Chamber
Type
Aerated (con'd)




Centrifugal
Forced Vortex (PISTA)





Free Vortex (Teacup)



Plant
Size
(MGD)
30.0
45.0
120
125
180


0.65
1.0
1.2
15
20
0.35
0.5
1.96
2.0
Plant
Type
Secondary
Secondary
Secondary
Primary
Secondary


Secondary
Primary
Secondary
Secondary
Secondary
Secondary
Secondary
Secondary
Primary
Col lection
System
Combined
Separate
Combined
Combined
Combined


Separate
Separate
Separate
Combined
Combined
Separate
Separate
Separate
Combined
Effectiveness
of
Grit Removal
Fai r
Good
#
Fair
Good


Good
Good
Good
Good
Good
Good
Good
Good
Good
Downstream
Impacts
Yes
Yes
Yes
Yes
Yes


Yes
No
No
No
No
No
No
No
No
Extent of Operational
Problems
Difficult
Routine
Difficult
Routine
Routine


Routine
Routine
Routine
Routine
Routine
Routine
Routine
Routine
Routine
*Plants below 6.0 mgd did not have sections to control  velocity.



^Seventy percent of flow enters after grit chambers.

-------
                                                          TABLE 3




                                         PLANTS CONTACTED:   SCREENS AND COMMINUTORS
CO
Screen
Type
Coarse
Manual ly Cleaned


Mechanically Cleaned










Comminutors
Screen in Series
Prior to Comminutor





Plant
Size
(MGO)

0.6
2.25
3.4
1.2
9.0
15.0
24.0
30.0
30.0
45.0
64.0
120.0
125.0
270.0

0.4
0.4
3.0
3.6
4.0
6.0
30.0
Plant
Type

Secondary
Secondary
Primary
Secondary
Primary
Secondary
Secondary
Secondary
Secondary
Secondary
Secondary
Secondary
Primary
Secondary

Secondary
Secondary
Primary
Secondary
Secondary
Secondary
Secondary
Col lection
System

Separate
Separate
Combi ned
Separate
Combined
Separate
Separate
Separate
Combined
Separate
Combi ned
Combined
Combined
Combined

Separate
Separate
Separate
Separate
Separate
Separate
Separate
Screen
Opening (In)

1.0
1.5
1.5
0.625
1.0
1.5
1.0
1.0
1.0
0.75
2.0
1.0
0.5
1.0

0.75
1.5
1.5
1.5
1.0
1.0
3.0
Effectiveness
of Screen/
Comminutors

Good
Good
Good
Good
Good
Good
Good
Good
Fai r
Good
Fai r
Good
Good
Good

Good
Good
Good
Good
Good
Fair
Fai r
Downstream
Impacts-
Yes/No

Yes
Yes
Yes
No
Yes
Mo
Yes
Yes
Yes
Yes
Yes
Yes
Yes
No

Yes
No
Yes
Yes
Yes
Yes
Yes
Extent of
Operational
Problems

Difficult
Routine
Routine
Routine
Routine
Routine
Routine
Routine
Difficult
Routine
Routi ne
Difficult
Routine
Routi ne

Routine
Routine
Routine
Routine
Routine
Routine
Routine

-------
 I
I—>
to
                                                      TABLE  3  (continued)


                                          PLANTS CONTACTED:   SCREENS  AND  COMMINUTORS
Screen
Type
Comminutors
w/ By-pass Screen
Fine
Static
Drum
Plant
Size
(MGO)
1.0
1.0
3.1
5.0
2.5
3.0
20.0
0.65
2.5
10.0
Plant
Type
Secondary
Secondary
Secondary
Secondary
Secondary
Secondary
Secondary
Secondary
Secondary
Secondary
Col lection
System
Separate
Separate
Separate
Separate
Separate
Separate
Separate
Separate
Separate
Separate
Screen
Opening (In)
1.0
3.0
1.25
1.5
0.06
0.06
0.06
0.03
0.06
0.02
Effectiveness
of Screen/
Comminutors
Good
Good
Good
Fai r
Good
Good
Good
Good
Good
Good
Downstream
Impacts-
Yes/No
No
Yes
Ho
Yes
No
No
No
No
No
No
Extent of
Operational
Problems
Routine
Routine
Routine
Difficult
Routine
Routine
Routine
Routine
Routine
Routine

-------
      These  reports  indicate the areas  in wastewater treatment plants where
 impacts  related to  inadequate preliminary treatment occur and conclude that
 grit  accumulation in anaerobic digesters and grit settlement in channels
 constitute  the extent of major 0/M problems associated with grit.  They
 also  conclude that  impacts caused by inadequate screenings removal or
 comminution  are more widely felt.  The reports present cost estimates for
 the aggregate savings achievable by implementing improved screening and
 comminution  in the  U.K., where sewage treatment practices and labor practices
 differ significantly from those in the U.S.

      Reports to the USEPA.  The following reports on sludge processing
 identified  significant impacts of grit and screenings on sludge handling
 processes.

      - Improving Design and Operation of Multiple Hearth and Fluid Bed
       Sludge Incinerators (21)

      - Improved Design and Operation of Recessed Plate Filter Presses (22)

      - Improved Design and Operation of Belt Filter Presses (23)

      - Achieving Improved Operation of Heat Treatment/Low Pressure Oxidation
       of Sludge (24)

      - Improved Design and Operation of Centrifuges (25)

     These  reports identified the impacts grit and screenings had on major
equipment items and on elements such as heat exchanger tubes, centrifuge
scrolls and nozzles, conveyors, mixers, grinders, pumps, and valves.

     Case Studies from the Literature.  A limited number of reports of
actual plant experiences are in the literature.  These reports  included
information on preliminary treatment improvements as well  as frequency and
cost of maintenance problems (11,19,27,36,38,39,47,50).

Plants Contacted and Visited

     Discussions with plant operators provided information such  as time
spent in deragging pumps of various types and how often  tasks like this had
to be performed.  In limited instances, information related to  equipment
repair costs could be obtained.

     In general, information on problems  and solutions related  to screenings
was obtained more readily than that related to grit, probably beacuse of
the more immediate impact poor screenings removal  has  on plant  operations.

Equipment Repair and Overhaul  Specialists

     Equipment repair specialists  and vendors provided limited  information
on the costs and frequency of repair of various common plant equipment.
                                    -20-

-------
PROCESSES SUBJECT TO IMPACTS

     Tables 4 through 8 summarize the impacts of grit  and screenings on
most wastewater treatment and sludge handling processes.   They are arranged
according to the following types of treatment:

     Liquid Processes  -  Primary and General Treatment Components
                       -  Secondary Treatment

     Sludge Processes  -  Sludge Conditioning
                       -  Sludge Dewatering
                       -  Sludge Disposal

     While the tables may not record every possible impact,  they do document
the following:

     - In a plant with primary and secondary sedimentation,  the primaries
       would experience more grit-related  impacts.   In a plant without
       primary sedimentation, relatively more grit  would appear in secondary
       units.

     - Grit can have a significant impact  on sludge processes, including
       equipment wear as well as accumulation.

     - Impacts related to inadequate screenings removal are  more of an
       operational nature and, in general, do not appear as  likely to result
       in equipment damage which would cause greater time out of service.

     - Grit-related impacts are more likely to result  in equipment repair
       or replacement than are impacts caused by inadequate  screening.

     - Sludge dewatering equipment, with its associated grinders, pumps,
       valves and small diameter piping, presents numerous opportunities for
       grit and screenings to cause problems.
                                    -21-

-------
                                                               TABLE  4

                          IMPACTS OF GRIT AND SCREENINGS ON  PRIMARY TREATMENT  4  GENERAL  TREATMENT  COMPONENTS
           Treatment
           Component
           Primary
           Sedimentation
to
Problem
                             Cause
Slippage of chains on
sprocket collector
mechanisms

Jamming of screw
collectors In primary
tank cross collectors

Sludge collectors
wear out prematurely

Sludge collectors jam
                                 Blockage  of center  feed
                                 Sludge pump  impellers
                                 wearing S  replaced
                                 prematurely

                                 Excessive  wear on
                                 plunger-type sludge
                                 pumps, and centrifugal
                                 pumps
                                 Collection  on weirs

                                 Ball  check  valves on
                                 plunger  pump discharge
                                 do not seat properly
Screenings collect
on mechanisms
                                                              Screenings
Abrasion due to
grit accumulation

Blocked by volume
of accumulated grit

Accumulation of rags
due to poor screening
and comminution

Abrasion due to grit
                             Abrasion due to grit
                             Screening  accumulation

                             Accumulation  of  rags
                             Impact
Breakdown of mechanism;
take unit off-line to
repair

Jamming prevents removal
of sludge; take units
off-line to clean

Greater frequency of
replacement

Requires clean out of tank
                                                                                           Labor  to  remove
                                                          Greater frequency of major
                                                          repair - (new impellers, etc.)
                             Frequent repacking of pumps.
                             Remachining required
                             eventually at 40-50% of
                             replacement cost; Impeller
                             replacement

                             Labor for removal

                             Affects capacity of pump;
                             labor to remove rags

-------
                                                       TABLE 4 (continued)

                        IMPACTS OF GRIT AND SCREENINGS ON PRIMARY TREATMENT & GENERAL TREATMENT COMPONENTS
         Treatment
         Component
Problem
Cause
Impact
i
ro
         Primary
         Sedimentation
         (continued)
Increased scum volume
Screenings accumulation
Fouling of level  probes      Screenings accumulation
                               Settlement in channels,
                               etc.
                             Grit & screenings
                             accumulation
 Odors, aesthetic problems;
 increased removal 0/M

 Labor for removal

 Labor for removal
                               Excessive pump wear
                             Grit accumulation
                             Major repairs more frequent

-------
                                                    TABLE  5

                             IMPACTS OF  GRIT  AND  SCREENINGS ON  SECONDARY  TREATMENT
Secondary Treatment
Component	
Problem
Cause
Impact
Activated
Sludge - Aeration
Oxygen transfer efficiency
of diftusers reduced
                      Excessive wear  of  surface
                      aerators  and  accumulations
                      on submerged  turbine  or
                      propeller blades
                      Automatic  shutdown
                      of surface aerators

                      Uneven  distribution ot
                      flow to aeration  tanks
                      Grit accumulation  in
                      aeration  tanks  of  plants
                      without primary  clarltiers
                      Floatables  in  final
                      effluent without primaries
Rag buildup on diffusers
                             Rag  buildup
                             Rag  buildup  on  impellers
                             trips  overload  switches

                             Settlement of grit  In
                             distribution channels
                             and  accumulation of rags
                             on weir  plates

                             Grit passing through
                             preliminary  treatment
                             Screenings passing
                             through  secondary
 Impairs  oxygen transfer,
 especially for fine bubble
 diffusers; requires more
 power and possible draining
 of tank  for removal

 Loss  of  02 transfer
 efficiency and increased
 power required.   More
 frequent replacement
 cost.  Labor to remove
 rags;  may have to drain
 tank  as  last resort

 Labor  to remove  rags
                             Labor to remove
                             Possible 02 transfer efficiency
                             reduction;  eventually need to
                             have aeration tanks out of
                             service and clean

                             Unaesthetlc; permit violations

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

                             IMPACTS  01- GRIT AND SCREENINGS ON SECONDARY TREATMENT
Secondary Treatment
Component	
Problem
Cause
Impact
Trickling Filters
RBC
Oxidation Ditches
Secondary
Sedimentation
Ponding, odors,  unaesthetlc  screenings passage through
appearance due to            distributors
accumulation ot  debris  on
media
Blockage of distributor
arms

Fouling of media and
tank accumulations
Return sludge pumping/
piping clogging

Blockage of draft tubes
in rapid sludge removal
type clan tiers
                                                   Plugging  by  rags
Screenings and grit
accumulation
Screenings accumulation
Plugging by rags - post
A.S. only (w.o. primaries)
                      Wear of casing  and  Impeller   Grit  abrasion  -  post
                      rings of centrifugal  sludge   A.S.  only  (w/o primaries)
                      pumps
                      Clogging  of  centrifugal
                      sludge  pumps and check
                      valves
                             Rag accumulation -
                             post A.S.  only (w/o
                             primaries)
                             Labor tor cleanup of media
                             surface
                             Must flush lines to remove
 Potential  reduction in
 treatment  efficiency;
 labor to clean

 Labor to clean
 Affects sludge removal  rate;
 must flush to remove
                                                          More frequent replacement
                                                          costs
                             Labor to remove rags

-------
                                                                TABLE 6

                                           IMPACTS OF GRIT AND SCREENINGS ON SLUDGE PROCESSING
             Conditioning
             Method
                      Problem
                                                   Cause
                            Impact
to
Heat Treatment        Plugging ot various
                      components
                       - Valves - (grit &  rags)
                       - Sludge transfer pump
                       - Heat exchange tubes,
                      particularly 1n sludge to
                      sludge exchanger with
                      stabilizers In annular space

                      Premature wear of some
                      components, especially
                      180 degree elbows 1n heat
                      exchangers, pressure-
                      reducing valves, and rubber
                      "boots" of progressive
                      cavity pumps

Aerobic Digestion     Operational difficulties
                                                               Rags cause plugging.
                                                               More severe when comminuted
                                                               rags recombine and form
                                                               rope-like strands
Abrasion due to grit,
compounded by turbulence in
the case of the elbows and
valves
             Anaerobic
             Digestion
                      Reduction of effective
                      digester volume
                                                               Rags accumulate on air
                                                               diffusers or on mechanical
                                                               aerator shaft and impeller

                                                               Grit accumulation
Gradual buildup of grit
in digesters
                             Shut system down to clean;
                             shutdown and restart waste
                             energy.  Cleaning can be
                             hazardous and labor
                             intensive
Shut system down to clean;
parts and labor for repair.
Progressive cavity pumps
have needed new rotors and
stators after 1500 hours
of operation
                             Labor to remove rags
Labor to drain, remove and
dispose; must shut digester
down during cleaning

Reduced HRT in digester
and reduced stabilization;
Reported costs vary from
$<1 to >b/lUO gallons of
capacity

-------
                                                     TABLE 6 (continued)

                                     IMPACTS OF GRIT AND SCREENINGS ON SLUDGE PROCESSING
Conditioning
Method
                             Problem
Cause
Impact
Anaerobic
Digestion
 I
ro
                             Floating debris
                             accumulation
                             Premature wear of
                             digester transfer
                             pumps

                             Heat exchanger
                             problems
Rags
Grit abrasion
Rags; screenings
 Efficiency of internal  gas
 diffusers and mixers
 impaired.  Labor to remove;
 access may be difficult,
 unsafe, and costly.

 More frequent repair
                                                                                Labor to remove;  frequency
                                                                                of cleaning increases during
                                                                                storm periods for combined
                                                                                sewers

-------
      Dewatering
      Method
      Vacuum Filters
      Centrifuges
no
oo
                                                          TABLE  7

                                    IMPACTS OF  GRIT  AND  SCREENINGS  ON SLUDGE DEWATERING
Problem
Cause
Impact
Non uniform performance
(coil  filters)

Early cake fall-off;
Sludge cake will  not
form or stick

Excessive scroll  wear
                            Excessive vibration
                            and noise

                            Sludge feed tube  and
                            nozzle erode
                            Bowl  and scroll  bind
                            on solid bowl
                            centrifuge

                            Downtime of progressing-
                            cavity sludge  feed  pumps

                            Poor performance of
                            discharge conveyor
Rags accumulate under
coils

Excessive grit in sludge
feed
Grit abrasion
                             Unbalancing due to rags
                             Grit abrasion
                             Rags,  hair,  scum fill
                             space  between  bowl  and
                             scroll

                             Grit abrasion
                             Grit  erodes  parts  and
                             clogs screw  conveyors
 Shut down unit; labor to
 remove

 Reduces cake dryness
 Rebuilt scroll  costs $3-50,000,
 depending on size, plus labor
 to replace

 Shut unit down; labor to remove
 rags

 Replace nozzles and feed tubes;
 labor to replace for each
 centrifuge

 Major labor requirement to
 disassemble and clean
                             Parts and labor to rebuild pumps.
                             Labor to clean; parts and labor
                             for bearings

-------
         Dewatering
         Method
         Belt F1 Iter
         Presses
         Recessed Plate
         Filter Presses
IS3
                                                             TABLE  7  (continued)

                                       IMPACTS  OF  GRIT  AND  SCREENINGS ON  SLUDGE  DEWATERING
Problem
Cause
Impact
                               Failure of belt roller
                             Screenings  reach  belt  and
                             then stress rollers
Cracking of polypropylene     Rags  get  into  space
plates                       between plates
                               Clogging of In-line  mixers    Rags which  carry  through
                                                            to dewaterlng system
                               Failure of drives  on
                               progressive cavity type
                               feed pumps

                               Blockage by grit of
                               vertical sludge  discharge
                               line
                             Rags  bind  rotor
                             Grit accumulation
                             "exacerbated"  by  non-use
                             periods
                             Replacement ot roller is expensive
                             and time consuming, and requires
                             temporary shutdown of equipment

                             Replacement of plate is
                             expensive and time consuming
                             and requires temporary equip-
                             ment shutdown

                             Reductions 1n sludge flow
                             and poorly conditioned
                             sludge; can result in higher
                             OSM costs for frequent
                             cleaning

                             Cost of repair at one plant
                             was $2,000; occurred after
                             ZbOU hours

                             Three days required to remove
                               line, unplug, and reinstall
                             at one site

-------
                                                              TABLE 8

                                         IMPACTS OF GRIT AND SCREENINGS ON SLUDGE DISPOSAL
          Disposal
          Method
Problem
Cause
Impact
          Incineration
          Systems
CO
o
Generation of black smoke
when grit is burned in
multiple-hearth furnace
Premature wear of sludge
feed conveyors

Clogging of sludge feed
mechanisms
Uneven feed of grit
Abrasion caused by grit
contained in sludge

Collected screenings
jam mechanisms
 Exerts high demand on
 combustion air, causing
 reduced efficiency of
 combustion

 Replace auger; requires
 equipment shutdown

 Interruption in feeder
 operation to clean

-------
                                 SECTION 4

                                CASE STUDIES


     In this section, 12 case studies of treatment facilities that have
experienced problems related to poor grit and screenings removal  are
presented.  The studies demonstrate the nature of the problems encounterd.
In some cases, they also illustrate the efficacy of improved preliminary
treatment practices.

     Plants rarely report retrofits of grit removal systems.  Where grit
removal problems are recognized, plants normally make modifications to
their existing grit system.  In some cases simple revisions to operating
procedures, such as adjustment of the aeration rate in aerated grit chambers,
have been adequate.

     In cases in which poor screenings removal is a problem, equipment
substitutions are often made.  A number of case studies are presented in
which substitutions of screening and comminution equipment have been made.

Case History No. Gl

     A 0.5 mgd secondary treatment plant serves an area with sandy soils.
The plant consists of an aerated lagoon, a final j;larifier, and a chlorine
contact chamber.  Extensive maintenance was required to remove grit buildup
in the lagoon.  The plant was upgraded in 1982 by installing a static screen
and Teacup™ grit removal facility before the aerated lagoon.

     The grit removal facility prevents the buildup of grit in the aerated
lagoon.  The screenings consist largely of grit and are collected in metal
cans, dewatered, and disposed of in a landfill.  The grit removed in the
teacup is collected in a separation box, dewatered, and disposed of in a
landfill.  The system removes approximately 15 cubic feet of grit per
million gallons of flow.

Case History No. G2

     A 30 mgd primary treatment plant serves an old combined sewer system.
Hilly terrain results in a heavy grit load from winter road sanding.  Prior
to upgrading, grit removal facilities included velocity controlled grit
chambers, chain and flight mechanisms, and a grit hopper.  Operating problems
with the grit system included maintenance difficulties with the chain and
sprocket drive collectors and grit carry-over into the primary settling
tanks during high flows.

     Three years ago, aerated grit chambers were selected for grit removal
as part of a plant expansion and upgrading program.  During the plant
upgrading, the existing chain and flight mechanisms completely failed.  The
plant was operated without grit removal facilities for a year.  During this

                                    -31-

-------
period, excessive wear on the primary  sludge pumps  required them  to  be
repacked every two weeks.

     Since the aerated grit chambers have been put  on  line,  the plant
has not experienced any operating problems due to grit and  sludge pump
maintenance has returned to a more routine schedule.

Case History No. G3

     The grit removal system at a west coast 36 mgd activated sludge treat-
ment plant consists of two aerated grit chambers, 12  grit pumps,  and tour
cyclone grit separators.  Prior to improvements in  this system, problems
included excessive loading and clogging of some of  the grit pumps due to
grit deposition at the entrance to the chambers, blockage of the  separators
due to the accumulation of large or light objects in  the separators, grit
carry-over into the primary settling tanks, and excessive wear on primary
sludge pumps caused by grit.

     Actions to correct these problems included the installation  of  manually
cleaned bar screens ahead of the grit separators.   Reducing the aeration
rate and tapering the amount of aeration along the  length of the  grit
chamber distributed grit more evenly throughout the chamber and increased
grit removal by an estimated 400 to 500 percent.  Manually  cleaned bar
screens were installed on two cyclonic grit separator overflows.   This
reduced grit pump clogging from 2 to 3 times per shift to virtually  none  by
catching rags, sticks and plastics that were rejected by the cyclones  and
found  their way back to the grit pump intakes.

     Grit carry-over into the primary sludge has been reduced by  approxi-
mately 28,800 cubic  feet per year.  Due to the large quantity of  grit in
the cyclone overflows, however, the manually cleaned bar screens  will  be
replaced by mechanically cleaned screens  in the future.

Case History No. GA-

     A 64 mgd activated  sludge plant currently has an average daily flow of
35 mgd and a peak flow of 200 mgd. The plant serves a combined sewer system.
Grit is removed at the plant  in two degritor-type tanks with rake arms and
grit pumps.  These facilities were installed in the 1950s.  They  are
difficult to maintain  due to  the age of the grit removal mechanism  and are
frequently out of service for maintenance.  The grit pumps are frequently
plugged.  When  one unit  is  out of  service, the  grit removal capacity is
limited to handling  only dry  weather flow.

     Poor grit  removal  results in  grit carry-over into  the primary  settling
tanks.  The grit  causes  excessive  wear on  the  sludge collectors  and sludge
pumps  and causes  blockages  in the  aeration  tank air diffusers.

These  units will  be  replaced  with  aerated  grit chambers within two  years.
                                     -32-

-------
Case History No. G5

     Prior to upgrading, a 180 mgd tertiary wastewater treatment plant had
four covered aerated grit chambers, constructed in 1957.  This grit handling
system consisted of chain and flight collectors, screw conveyors, and bucket
elevators.  The system was inaccessible for maintenance.  Inadequate venti-
lation of the grit chambers resulted in a corrosive environment above the
water surface which attacked the concrete, the air diffuser headers, and
the influent and effluent sluice gates.  To remedy this situation, the
concrete covers over the grit chamber were removed and new concrete walls
were built around the existing grit chambers.  The new grit chambers were
constructed with clamshell buckets mounted on a traveling bridge crane.  To
further improve the operation of the grit handling system, mechanically
cleaned screens were installed upstream of the grit chambers.   With these
modifications, the performance of the grit facilities has substantially
improved.

Case Study No. SI

     A 0.8 mgd extended aeration plant treats 0.3 mgd of domestic wastewater
by means of a detritor, aeration tanks, and secondary clarifiers.  Sludge
is stabilized in aerobic digesters and dewatered on belt filter presses.  A
collection system pump station, equipped with a manually cleaned bar rack
and comminutor, pumps all flow into the plant.

     Several operating problems are caused by rags recombining in the plant.
Rags accumulate on the influent slide gates to the aeration tanks and must
be removed daily.  The build-up of rags on the impellers of the surface
aerators in the aeration tanks and on the submerged turbine aerators in the
aerobic digesters frequently trip overload switches.  The tanks must be
dewatered and the rags manually unwound from the impellers to  remove them.
Rags also accumulate in the secondary clarifiers, clogging the return
activated sludge pumps.  These pumps are cleaned daily, which  takes approx-
imately one hour per pump.

     The plant staff is planning to install a manually cleaned bar rack
after the detritor in an effort to remove rags upstream of the aeration
tanks.

Case Study No. S2

     A midwestern 1.2 mgd activated sludge plant was put into  operation
1-1/2 years ago.  The plant is equipped with a front cleaned/front return
(chain and sprocket operated) screen with clear openings of 3/4-inch.  The
screen is installed in a channel  and is pivoted so that it can be swung out
of the channel for inspection and maintenance, leaving the channel in opera-
tion.

     The screen was installed outside so that cold weather caused the
screenings to freeze and shear pins to break.  To alleviate these problems,
a shed was constructed around the screen to protect it from the weather.
The plant has not experienced any significant difficulties from rags.

                                    -33-

-------
Case Study No. S3

     A 2.25 mgd extended aeration facility has  a  headworks  consisting  of
manually cleaned bar racks, velocity controlled grit  chambers,  and
comminutors.  The influent waste is primarily domestic  but  does contain
some industrial wastes with rags.  In the past, these rags  accumulated on
the floating surface aerator impellers,  causing excessive wear  on this
equipment, and clogged the return activated sludge pumps which  had  to  be
manually cleaned.

     To correct these problems, manually cleaned  bar  racks  were installed
in the return activated sludge channel  to the aeration  tanks to collect the
rags.  This reduced pump blockages.  Installation of  mechanically cleaned
screens is under consideration.

Case Study No. S4

     A 3 mgd collection system pump station is  equipped with a  comminutor/
macerator, which has been in operation for one  year.  The wastewater consists
of normal domestic wastes with minor industrial flow  contributions.  The
unit has worked well, requiring only routine maintenance.   Only one operating
problem has occurred.  The comminutor/macerator motor was submerged and had
to be rebuilt due to failure of a pump which caused the wet well to overflow.

     A similar incident occurred with a comminutor/macerator at a  pump
station in another location.  The wet well level  rose,  submerging  the motor
which, consequently, had to be repaired.  This  comminutor/macerator had
been in operation for two years with only routine maintenance required.

     Installation of three new units at other  pump stations is  planned.
These units will be equipped with submersible  motors  to prevent damage from
wet well overflows.

Case Study No. S5

     A midwestern 10 mgd contact stabilization  plant  experienced operating
problems with screening operations. The system consisted of influent
pumping, a bar rack with 1-inch clear openings, two externally  fed  submerged
rotating drum screens with 1/8-inch openings,  and a grit channel.   Operating
problems included flooding and mechanical breakdowns of the screen  drives
and seals, clogging of screens by grease and oil, and freezing  of screenings.
This resulted in poor screening removal efficiency, allowing floating debris
to pass through the plant to the aeration tanks, digesters, and thickener,
and causing odor problems at the screenings dumpsters.

     This system was replaced by internally fed rotating drum screens,
constructed of stainless steel wedgewire with  clear openings of 0.020-inch,
mounted above the floor.  Because the influent  wastewater has non-emulsified
greases and oil, which interrupt the screening operation,  a multi-staged
centrifugal pump was installed to wash the drum screens.  The pump supplies
tap water at  300 psi at short duration (60-120 second)  cleaning cycles 10
to 15 minutes apart to keep the screens clean.   This cycle is controlled  by

                                    -34-

-------
timers.  Enclosing the screenings dumpsters in a building prevents the
screenings from freezing and provides a drainage area for the perforated 16
cubic yard dumpsters.  Because of high landfill  fees, a screenings press
was installed to reduce their volume.

     These improvements have increased screenings removal from 3 to 12
cubic yards per day with a dry solids content up to 15 percent.  This
improvement in screenings removal has decreased  process air requirements,
improved sludge quality, significantly reduced deposits in the aeration
tanks and piping, and reduced pump blockades.  Removal of BOD and suspended
solids by the screens is over 10 percent and floating material is also
effectively removed by the screens.  The enclosed dumpster area and
screenings press have substantially reduced odors.

Case Study No. S6

     In 1975, a west coast 15 mgd activated sludge plant replaced existing
comminutors with comminutor/macerator devices to improve grinding efficiency,
The comminutor/macerators worked well and ground most solids except for
spherical objects.  However, the grindings recombined, forming rags which
accumulated on the air diffusers in the aerobic  digesters.  The units were
replaced in 1978 to increase the treatment capacity of the headworks.  Two
mechanically cleaned front cleaned/back return (chain and sprocket) screens
and a manually cleaned bypass bar rack were installed.  Although the waste-
water was 90 percent domestic flows, these screens experienced severe
corrosion and became unuseable within four years.  During this period, a
hammermill grinder was installed to grind the screenings and return them to
the flow.  This grinder failed within six months and was replaced with one
of the comminutor/macerator devices formerly installed in the headworks.
The comminutor/macerator worked well in grinding the screenings.

     In 1982, the screens were replaced by two rack and pinion driven
reciprocating rake screens which are located outside.  Screen openings are
3/4-inch in 4-foot channels with a water depth of 2.5 to 3.5 feet.  Initial
difficulties in operating the spring assisted wiper bar shock absorbers
were solved by replacing them with a hydraulic oil shock absorber, which
has worked satisfactorily.  Approximately 8 cubic yards of screenings are
removed weekly by the rake screens and carried by a belt conveyor to a dump
truck to be hauled to a landfill.  The rake screens have worked well for the
past 2-1/2 years.

     In order to minimize clogging in sludge pumps, a comminutor/macerator
was installed upstream of the primary sludge pumps.  These are used to pump
co-settled grit, primary and waste activated sludges to belt filter presses.
Originally, the comminutor/macerator was installed in a vertical position,
which resulted in grit accumulations that damaged the bottom seals.  The
unit was then installed in a horizontal position which has greatly extended
the life of the bottom seal.
                                    -35-

-------
Case Study No. S7

     A 30 mgd advanced treatment  plant  with  primary  lime  treatment,  nitri-
fication, denitrification,  and dual  media filtration processes  has three
screening units located before aerated  grit  chambers.   One  unit is a front
cleaned/back return screen, which was  installed  in  1957.  The other  two
screens are front cleaned/front return  chain and sprocket operated screens,
installed in 1975.  Clear openings are  3/4-inch.  Screenings are removed
from the flow, ground up, and then returned  to the  flow.

     Prior to improvements, screenings  problems  included  poor capture and
removal, plugging of grit and sludge pipelines and  pumps  due to rags, and
difficult maintenance due to the  location of the screens.  Turbulent flow
in the channel to the screens reduced  screenings capture.  Removal of the
screenings by belt conveyors was  difficult because  they were located 20
feet below ground level.  Rags recombined and plugged  grit  and  sludge
pipelines and pumps.  Inspection  and maintenance of  the lower sprockets of
the chain operated screens was difficult because they  were  below the
wastewater flow and required dewatering of the channel.

     A recent improvement is the  installation of a  comminutor/macerator,
and planned improvements include  catenary screens and  a screenings press.
A comminutor/macerator was installed in the primary  sludge  line to reduce
plugging of the line and sludge pumps  by rags.  This unit has worked well
with little required maintenance.  Two  new catenary  screens will be  installed
to replace existing screens.  The catenary screens  will be  able to handle
the wide range of flows that the  plant  experiences  (up to 150 mgd).   A
screenings press is planned to be installed to remove the screenings from
the flow, which will eliminate the problems of rags  in the  plant.

Case Study No. S8

     An east coast 31 mgd activated sludge plant, constructed  in the 1950s,
is undergoing a second upgrading.  The  headworks are being  rebuilt to reduce
inefficient screenings removal presently accomplished by  three  5-foot wide
screening/comminuting devices (barminuters)  which are located  before velocity
controlled grit chambers.  The barminuters replaced mechanical  rakes, having
1/2-inch clear openings, and grinders which returned the  screenings  to the
flow.  This original system resulted in high screenings load and wear on
the grinders due to the grit.  The barminutor equipment was ineffective in
removing the screenings and was very maintenance intensive  because  of the
complex machinery.  Frequent problems and blockages were  encountered with
the sprockets and chain drives.  In order to perform maintenance on  the
equipment, the channel had to be dewatered which caused operational
difficulties due to unequal flow distribution to the other  channels.

     Downstream operating problems were mainly with the primary sludge pumps
where  rags recombined to form long strings which clogged  the pump impellers.
Sludge pumps had to be inspected weekly and flow in the sludge  pumps was
reversed daily to unwind rag accumulations.  Rags accumulated  on any obstruc-
tion,  particularly at the influent channel weirs at the primary settling
tanks.  Rags also caused blockages in the sludge lines to the  digesters.

                                    -36-

-------
      The  barminutor  equipment was  replaced by catenary  screens which have
 been  operating  for six  months with  good  screenings capture.  Maintenance on
 these screens is  expected  to be  considerably less than  on the barminutors.
 Blockage  problems in the sludge  treatment processes are not as frequent.

 Case  Study  No.  S9

      In 1982, a study of the existing  screen facilities in a major city was
 undertaken  to determine their condition, reliability, and operational
 performance and to make recommendations  for the repair, modification, or
 replacement of  the screens.  Ten facilities with a total of 57 screens and
 average flows of  20  to  300 mgd were studied.  The screens were either front
 cleaned/back return  or  front cleaned/front return chain operated devices
 installed between 1970  and 1978.

      The  study  concluded that inefficient screening capture was caused by
 excessive velocities in the screen  channels forcing screenings through the
 bar racks,  by flapper plate malfunction  in back return  screens which allowed
 screenings  to pass under the bar rack, and by screenings being re-deposited
 in the sewage flow downstream of the screens due to inefficient operation
 of the wiper mechanisms.   In addition, the existing chain and sprocket
 operated  screens were maintenance intensive and permitted screenings to
 bypass and  carry-over to downstream processes.

      Inefficient screening of raw wastewater was the cause of many opera-
 tional problems due  to  rag accumulation  in downstream processes.  These
 problems  included the following:

      - Slippage of sludge collector chains and breakdown of the mechanisms
       in primary and secondary settling tanks

      - Jamming  and overloading of screw-type sludge collector mechanisms in
       the  primary settling tank cross collectors

      - Reduced  oxygen transfer efficiency, equipment breakage, and high
       cleaning costs of aeration tank diffuser tubes,  piping, and headers

      - Reduced  reliability of sludge concentration tanks from pump clogging
       and  scraper mechanism blockages

      - Inefficient operation of anaerobic sludge digesters'  internal  gas
       diffusers and mechanical  mixers.

      Short term corrective measures taken to increase screenings removal
 included plates with new flappers and replacement of lower chain sprockets
 and shafts.   Operational changes instituted  were bi-weekly flapper plate
 inspection,  daily adjustment of screen components,  and operation of
 additional screens,  as necessary, to reduce  velocities through the screens.

     After evaluation of available  screening devices,  the  recommendation
was to replace  the existing screens with  rack  and pinion reciprocating


                                    -37-

-------
screens as existing screens reached their service  life  limits.   Several  of
these new units have been installed and are working well  with  increased
screenings capture and reduced maintenance.
                                     -38-

-------
                                 SECTION 5

                      DETAILED EVALUATION REQUIREMENTS

     To  fully evaluate the costs and benefits of improved preliminary
 treatment,  i.e., to perform a cost-effective analysis, the engineer must
 have a variety of  information available.  This information should include
 present  operation  and maintenance  (O&M) costs for both the preliminary
 treatment and downstream wastewater treatment processes and sludge treatment
 and disposal steps.  These total and individual costs must be compared to
 either other plants with similar wastewaters and treatment components or to
 available information which identifies these same costs in terms of manpower
 or dollars  for similarly designed  facilities.  Sources of the latter infor-
 mation can  be found in references  3,5,14,29,30,34,37,45,47,48,49,50, 51,
 and 52.  However,  few of these sources are comprehensive in nature, especia-
 lly when dealing with the impacts  of preliminary treatment on downstream
 O&M requirements.  Sidwick (37) illustrated and quantified downstream O&M
 tasks in typical British trickling filter facilities resulting from inade-
 quate screening.   The most time consuming and costly task was found to be
 the cleaning of trickling filter distributors.

     To  perform this cost-effective analysis, quantified information is
 required on the time required to perform O&M tasks resulting from inadequate
 preliminary treatment, the cost of labor and tools to perform these tasks,
 and the external costs for contract specialists, replacement parts and
 other direct costs to the treatment facility.  One relatively straightfor-
 ward example of cost-effective analysis would be pumps.  After the effect
 of grit  abrasion or partial screening blockages has become significant,
 the efficiency of the pump may drop by 40-60X, resulting in significant
 increase in energy consumption.  When the plant staff become aware of this
 problem, the pump will be removed for repair.  The cost of repair for
 different types of pumps varies.  Progressive cavity pumps worn by grit
 may require a repair cost nearly equal  to the original  price.  Plunger
 pumps which "bell out" from grit abrasion can be resurfaced with ceramics
 for about 50% of replacement cost.   Centrifugal  pumps worn from grit also
 require about 50% of their replacement cost to repair.   To all of these
 external  costs must be added the plant labor costs to remove and replace
 the units.

     The USEPA conducted a study to evaluate the mechanical  reliability of
 treatment plant components (20).  An early  finding of this study was that
 only a tew large plants kept proper records to perform such an analysis.
 The data that was available showed  a ratio  of almost 5  to 1  of service
 life to down time for failures of pumps at  those facilities,  and the primary
cause of centrifugal  pump failure was  worn  wear rings or plates due to
 grit abrasion.   Mean times to repair,  availability,  and maintenance times
for certain  components were also recorded.

     Data of the above type must be developed for all downstream components
                                    -39-

-------
in order tor engineers and plant managers  to perform the  necessary cost-
effective analyses for preliminary  treatment processes.   Some other data
that have been developed in addition to the above  study include:

     - Comminutors normally require overhaul  every 3 years  at a cost of 50-
       60% of the purchase price, in addition to labor costs  for  removal
       and replacement.  Teeth sharpening  at 1-to  3-month intervals and
       replacement every 6 months have been reported (3).

     - Grit removal  equipment requires overhaul  every 4 to  5 years at
       10-15% of total grit chamber installation cost.

     - Digester cleaning should not be required  more often  than every  5-10
       years, but excessive grit has caused this frequency  to be  as often
       as once every 1 to 3 years (3,19).   The USEPA (53) reports that
       the most frequent cleaning interval of plants contacted was every 3
       years.  Costs of digester cleaning  can vary from less than 1 to more
       than 5 cents per gallon of digester capacity (19,53).  The length of
       time required for repair is  a function of digester size, available
       facilities, and need for contract services.

     - Centrifuge scroll wear from grit abrasion may result in rebuilding
       costs of $3,000 to $50,000,  depending on centrifuge  size.   Similar
       grit abrasion of sludge feed tubes  and nozzles require replacement
       as often as 6 times per year at $400 to $1,400, plus labor (minimum
       of one day).

     Other information of a similar nature must also be gathered  to  develop
an adequate picture of normal treatment plant component service  life  between
repairs and the costs of those repairs.  To accomplish this the  following
must occur:

     - Plant owners/operators must become aware of the potential  impacts
       of preliminary treatment on subsequent unit processes.   Many  O&M
       requirements presently considered to be inherent or routine in these
       processes may result from inadequate preliminary treatment.

     - Plant record keeping must be upgraded.  Frequency  of repair and time
       requirements tor labor and materials to perform 0*M tasks  resulting
       from pretreatment process inadequacy must be documented in order to
       properly quantify their relationship to improved performance.

     To  illustrate the  information required to complete  a cost-effectiveness
analysis, examples of two typical facilities were created.   The first is a
typical  small  (1 mgd) municipal wastewater treatment  facility employing a
manually cleaned bar screen, a controlled-velocity grit  removal  chamber,
comminution, oxidation  ditch, and  final clarifier with waste activated
sludge thickening, aerobic digestion and sand drying  beds.   For the compon-
ents provided  in  this  typical facility, Table 9 was  then developed from
literature references,  contacts with treatment plant  operations staff, and
interviews with contract operating staff  of Metcalf  and  Eddy.  If, for
example, the O&M  program of  this facility were adequate, the high and low

                                    -40-

-------
 ends  of the table's  O&M  costs may  be  considered  the  difference between
 this  plant's  operational  costs and that of a  plant with excellent prelimin-
 ary  treatment processes.   The plant manager or engineer could then calculate
 the additional  O&M costs  incurred  due to inadequate  preliminary treatment.
 These additional  costs could then  be  compared to the estimated capital and
 O&M costs  of  improved preliminary  treatment.

      Similarly, Table 10  shows the ranges for annual O&M costs resulting
 from  various  efficiencies  of preliminary treatment for a typical large (30
 mgd)  treatment  plant consisting of primary clarifiers, conventional diffused-
 air aeration, and secondary clarifiers, sludge thickening (primary by
 gravity and waste activated by centrifuge), belt-filter-press dewatering of
 combined sludge and  incineration.  An engineering analysis similar to the
 one described above  would  also be  possible for this  facility if the same
 assumptions were made.

      Several potentially important issues are not addressed by Tables 9 and
 10 which may further add to the total  cost of preliminary treatment impacts.
 These  include the cost of  increased power as  hydraulic components,  e.g. pumps,
 are reduced in efficiency either prior to the decision for major repairs or
 as a  result of  inadequate  frequency of corrective maintenance.  Sidwick
 (37)  reported energy losses of 40% to 60% at one plant due to this  problem.
 Therefore,  the cost-effective analysis must consider these and other site-
 specific issues which are less obvious effects of inadequate preliminary
 treatment  in developing the total  annual  cost due to this problem.

     Sidwick (37)  provides an excellent analysis of the labor costs  resulting
 from inadequate screening in typical  trickling filter plants in  Great
 Britain.  These analyses  resulted  in  the  conclusion that the greatest
 benefits from improved screening accrue to small  plants with part-time
 operation, rather  than to larger facilities.   However,  significant  benefits
 can be derived from such  improvements  in  the form of power savings  at larger
 plants.

     It should be  restated that  the normal  British  design  practices  for
 smaller treatment  plants, i.e.,  primary clarifiers  and  slow-rate trickling
 filters, are different from U.S.  practice.   The  applicability of these
British conclusions  to U.S. practice,  therefore,  may  be limited to similarly
designed facilities.
                                   -41-

-------
                                                               TABLE  9

                                 HYPOTHETICAL  IMPACTS OF GRIT AND  SCREENINGS  ON A TYPICAL SMALL PLANT
no
 i
IMPACT
Grit abrasion on
comminutor


Raa accumulation on
REMEDY
-Set and sharpen
teeth
-Overhaul
comminutor
-Remove screenings
COST
LABOR
40

160

10
PER OCCURRENCE
($)
EQUIPMENT TOTAL
	

7,500 7,

-,--
40

660

10
OCCURRENCES
PER YEAR
3-6

0.33-0.5

4-52
COST PER
YEAR ($)
120-140

2,550-3,825

40-520
surface aerators,
weirs and baffles in
oxidation ditch or
aeration tank

Grit Accumulation
in oxidation ditch
      Draft  tubes  in
      secondary settling
      tanks  plug with rags

      Rag accumulation on
      mixers in secondary
      settling tank scum
      wells

      Clogging and wear
      of the two centri-
      fugal  return/waste
      sludge pumps
-Drain  and  remove
 grit with  vacuum
 track

-Flush  to  remove
 screenings
                       -Remove screenings




                       -Remove screenings

                       -Rebuild the pumps

                       -Repacking and
                         resealing
 20



 20




 60

160

 15
                                5,000

                                  100
   20



   20




   50

5,160

  115
   4-24



   1-6




   4-24

0.14-0.33

   4-12
250-1,000



 80-480



 20-120




240-1,440

740-1,740

460-1,380

-------
                                                      TABLE  9  (continued)

                              HYPOTHETICAL  IMPACTS  OF  GRIT AND SCREENINGS  ON  A  TYPICAL  SMALL  PLANT
00
 I
IMPACT
Clogging and wear
of two plunger-
type thickened sludge
pumps and valves


Clogging of pipeline
REMEDY
-Remove screenings
from bal 1 checks
-Replace balls
-Repacking and
resealing
-Resurface plungers
due to belling
-Flush or rod line;
COST
LABOR
5
10
15
160
500
PER OCCURRENCE
($)
EQUIPMENT TOTAL
400
100
400
—
5
410
115
560
500
OCCURRENCES
PER YEAR
100-700
0.33-1
4-12
0.067-0.2
0.2-1.0
COST PER
YEAR ($)
500-3,500
140-410
460-1,380
40-110
100-500
      due to accumulation
      of grit and screen-
      ings (eg. - line
      between gravity
      thickener and
      thickened
      sludge pumps)

      Rag accumulation on
      diffusers in aerobic
      digester

      Grit build up in
      aerobic digester
 worst case  -  remove
 line and  rod
-Remove screenings
-Remove when tank
 is down
 10
400
400
            10
800
                12-24
0.10-1
                                                                              120-240
                                                                               80-800
      Note:  Labor costs based on $10.00/hr.

-------
                                                      TABLE 10

                        HYPOTHETICAL IMPACTS OF GRIT AND SCREENINGS ON A TYPICAL LARGE PLANT
IMPACT
REMEDY
   COST PER OCCURRENCE ($)
LABOR   EQUIPMENT    TOTAL
                           OCCURRENCES
                            PER YEAR
                  COST PER
                  YEAR ($)
Blockage of aerated
grit chamber
diffusers due to
rag accumulation

Grit accumulation
in channel feeding
primary clarifier

Wear on chain and
flight in rectangular
primary clarifier due
to grit accumulation

Rag accumulation on
chain and fight or
other collection
equipment usually
accompained by
grit buildup

Buildup on mixer
system in scum wells

Clogging and wear of
the four plunger-type
primary sludge pumps
-Remove rags
-Shovel out
 80
480
-Replace chain and  10,000
 cross flights
-Drain tank remove     160
 buildup and replace
 flights as needed
-Remove screenings
 20
-Remove screenings      20
 from ball checks
                        -Replace balls          20

                        -Replace shear pins     10
            80
           480
         40,000   50,000
            200      360
            20


            20



2,400    2,420

    2       12
  1-4
0.1-0.5
                            0.1-0.2
                              2-4
  6-24


180-365



0.5-1

  8-12
   80-320
   48-240
               5,000-10,000
                 720-1,440
  120-480


3,600-7,300



1,210-2,420

  100-140

-------
                                                TABLE 10 (continued)

                        HYPOTHETICAL IMPACTS OF GRIT AND SCREENINGS ON A TYPICAL LARGE PLANT
IMPACT
Clogging and wear of
the four plunger-type
primary sludge pumps
(continued)
Blockage of draft
tubes in rapid sludge
removal secondary
clari fiers
Clogging and wear of
the four centrifugal
return sludge pumps

REMEDY
-Resurface plungers
-Repacking and
resealing
-Flush to remove
screenings
-Remove screenings
-Rebuild the pumps
-Repacking and
COST
LABOR
320
40
60
10
320
30
PER OCCURRENCE ($)
EQUIPMENT
1,000
200
"
10,000
200
TOTAL
1,320
240
60
10
10,320
230
OCCURRENCES
PER YEAR
0.2-0.5
1-2
12-52
26-52
0.067-0.1
2-4
COST PER
YEAR ($)
260-660
240-480
720-3,120
260-520
690-1,032
460-920
                        resealing

Grit abrasion on       -Replacement of        480      20,000   20,480             0.17-0.33       3,400-6,800
scroll surface of       scroll edges
solid-bowl centrifuge

Rag accumulation on    -Empty tank and         20         -         20               12-52           240-1,040
sludge blending tank    remove rags
shaft.  Causes
vibration and can      -Replace bolts as       20          50       70                2-4            140-280
snap motor mount bolts, needed

Grit accumulation in   -Remove grit when    5,000         -      5,000              0.1-0.2          500-1,000
sludge blend tank       tank is down
Note:  Labor costs based on $10.00/hr.

-------
                                               TABLE  10  (continued)



                        HYPOTHETICAL  IMPACTS OF GRIT AND  SCREENINGS ON  A  TYPICAL  LARGE  PLANT



1
O1

IMPACT
Clogging and wear
of the progressive
cavity blended
sludge pumps.

REMEDY
-Remove rags and
screenings buildup
-Replace rotor and
stator
COST
LABOR
10

320

PER OCCURRENCE ($)
EQUIPMENT TOTAL
10

2,000 2,320

OCCURRENCES
PER YEAR
50-150

1-2

COST PER
YEAR ($)
500-1,500

2,320-4,640

Note:  Labor costs based on $10.00/hr.

-------
                                  SECTION 6

                         RECOMMENDED DESIGN PRACTICES


       In this Section, recommended practices for the selection and design
 of grit and screenings removal systems are discussed.  In selecting such
 systems a number of factors must be considered, including operability,
 safety, cost, and specific local conditions.

 GENERAL DESIGN CONSIDERATIONS

      Engineering practice dictates that the designer consider preliminary
 treatment in every facility served by conventional sewerage.  Most state
 design regulations are typified by the commonly known "Ten State Standards"
 (16).  In essence these standards require the use of coarse bar racks or
 screens for protection of pumps, comminutors and other equipment.  Grit
 chambers are required for combined sewers and are suggested for all
 facilities.  Grit removal, when employed, should precede  comminutors, and
 the latter should not be used where fine screens or primary clarifiers are
 present.  Such  requirements eliminate uncertainties as to whether to use
 specific preliminary treatment processes and in what order to apply  them.

 Screening

      Although a  coarse screen  can  be  commonly  accepted as protection  for
 downstream equipment from  physical  damage by  large objects,  its  removal
 capability for other problematic  screenings  such  as rags  has  until recently
 been  unquantified save for numerous  references  as  to the  quantity  of
 screenings per given  volume  of flow at  various  locations  (1  2  17  34  35)
 The primary  factors  which  affect the  type and quantity  of screenings  removed
 are the  raw  wastewater characteristics,  the size  of screen  openings,  velocity
 of  flow  through  the  screen and the method of screen operation.  The designer
 can control  the  last three factors and  should be aware  of the  first orior
 to design.                                                          K

     Given a specific  wastewater with known flow variation and screenings
 content, smaller  screen openings will remove more material per unit of
 flow.  Therefore, the  question  arises as to "how fine is  fine enough" as  it
 pertains to  screen openings.   A recent British study has  demonstrated that
 a 0.5-in.  opening will remove  all the screenings likely to cause downstream
 impacts  136).  Material captured on smaller subsequent screens was observed
 to be plentiful   (an additional  30% by weight on a 0.3-in.  screen), but of a
 nature which would have no downstream impact.  Ten State Standards limits
 the minimum opening to 0.6 in.  (16).

c*   Jh5 V?!^ity °f flow throu9h the screen is limited by the Ten State
Standards (16)  to 3.0 ft/sec "to prevent forcing material  through the
openings.   A similar figure (3.3 ft/sec) is proposed by Sidwick for  British


                                    -47-

-------
practice (3).   Therefore,  the engineer must determine  the  required  screen
area by application of this  figure  to  peak  flow conditions.

     The method of screen operation can significantly  affect the removal
efficiency of screens.  With better control  of mechanical  cleaning  rates,
many facilities have improved efficiency by use of "matting" techniques
where deliberate methods are employed  which use previously retained screen-
ings to remove finer materials from subsequent flows.   These techniques
are generally applied only at larger plants (54).

     The one major design factor which cannot be controlled but should be
known by the designer is the wastewater characteristics.   Flow variation
has already been noted as a major design factor; however,  the screenings
content as it relates to the flow of the wastewater also can have a major
effect on the screening facility.  One phenomenon often noted in practice
is the increase in screenings content  during higher flow periods.  Although
this relationship is primarily related to combined sewers, it can also be
significant in separate sanitary sewers.  Recent British work has shown
that one combined sewer with flow peaks of less than 3 to  1 over the average
flow yielded screenings peaks of as high as 73 times the average, although
a screenings capture peak of 29 times  the average was  more common (36).
Therefore, designers must take these peaks into account in choosing both
the mechanical raking systems for the  screens as well  as the screenings
handling facilities.  The ability to deal with the excessively high screen-
ing content would be enhanced by use of mechanically cleaned screens with
two speeds of raking and improved control systems (3).

     Although several sources (3,17,41) contain graphs of  screen opening
sizes v£ average and peak screening quantities, such graphs must be used
with cafiJtion owing to variations in sewer and other service basin charac-
teristics which may cause a significant change in the  quantities present.

     All major references discourage new facilities from grinding of screen-
ings and returning them to the flow, an enigma in light of the general
approval of in-line grinding/disintegration (comminution).  Therefore,
removal of screening from the wastewater flow and disposal by other means
is now generally recommended such as,  burial or landfilling of screenings,
either on the grounds or at a local landfill, and incineration.  The first
method is by far the most popular,  and is almost exclusively used in smaller
treatment plants with screens.  The second method, incineration, has been
used by some of the larger treatment plants for many years.  Because of
the need to prevent putrefaction, screenings must be washed and dewatered
quickly prior to separate or co-incineration with sludge cake.  Although
this latter method may be cost-effective for larger facilities, the equip-
ment is expensive, and labor requirements and odors may be significant.
Recently, British engineers have been  investigating several new screenings
handling devices, recognizing both that present equipment  is less than
satisfactory and that the trend toward finer screens will  accentuate the
need for better equipment (3,34,37,38,39,42,43, 44, 45).   Among these
devices are washers, dewaterers, baggers, incinerators and maceration
systems.
                                    -48-

-------
 Comminution

     The Ten State Standards  (16) recommend the use of commlnutors In plants
 that do not have primary clarlflers, fine screens or mechanically cleaned bar
 screens.  Since this scenario describes thousands of small U.S. municipal
 wastewater treatment plants, the number of comminutors Installed is signi-
 ficant.  Contrary to this recommendation, however, is the growing concern
 over the performance history of these devices.  British studies conclude
 that truly effective comminuting devices have yet to be developed and note
 their removal from existing facilities (3,39,42,45).  A Canadian study (47)
 of extended aeration plants showed that malfunctioning comminutors were a
 major cause of the failure of over 50% of these plants to achieve effluent
 quality requirements.  The Province of Ontario recommends against the use
 of comminutors in municipal wastewater facilities (40).  Several of the
 facilities contacted in this study and the case histories described earlier
 appear to reinforce the experience of the British and Canadians.

     The original purpose of comminution in smaller treatment facilities
 was to overcome the manpower-intensive task of manually cleaning screens
 by grinding those screenings for settlement in a subsequent primary clarifier
 for disposal with the sludge (41).  Although as a purpose this goal was
 laudable and reflected the practice of wastewater treatment in the 1930's,
 the use of these devices should be reevaluated in light of present practice
 by both the design profession and the design guidance authorities.  Based
 on performance, the utility of comminutors should be reconsidered.


 Grit Removal
     The designer must evaluate several factors during design.  The first
issue is whether grit removal is required.  British authors have recently
reviewed their practice and concluded that the common practice of not
requiring grit removal at small plants (<5,000 population served) was
basically sound since most facilities employ primary treatment and do not
employ anaerobic digestion of sludge (3).   It was also noted that detritus
tanks might be advantageously employed for plants serving more than 2,000
population, and improved methods of grit removal  from constant-velocity
channels were needed (45).  The latter conclusion has also been noted in
the U.S. and elsewhere (13,42,46).   Before any designer were to decide
against grit removal it would be necessary to characterize the wastewater
for gnt.  If this  is not feasible  the designer must evaluate each of the
following:

     - Type of collection system.   If a system is combined,  grit removal  is
       required.  Most separate (sanitary) sewers will  also  require grit
       removal.

     -  Degree of  sewer system corrosion. Grit may include products of
       hydrogen  sulfide  corrosion.

     -  Scouring velocities in the sewers.   If scouring  velocities  are not
       regularly  maintained,  grit will  build  up in  the  sewers.   During  peak

                                     -49-

-------
       flows, the grit may be resuspended and conveyed  to  the  treatment
       plant.

     - Presence of open joints and cracks in  the  sewer  system.   These  permit
       soil  around the pipes to enter the sewers.   This effect  also  depends
       upon  soil  characteristics and groundwater  levels.

     - Structural failure of sewers.  Such failures can deliver enormous
       amounts of grit to the wastewater system.

     - Characteristics of industrial wastes.

     - Degree to which household garbage grinders  are used.

     - Amount of septage or other trucked wastes.

     - Occurrence of construction in the service  area or at  the treatment
       plant.

This evaluation may be necessary where design standards reflect the  Ten
State Standards, which require grit removal  facilities  for all  combined
sewers, but  not for separate sanitary sewers.  If the evaluation is  at all
marginal, grit chambers should be installed.

     It is important to recognize that extreme variations  occur in grit
volume and quantity.  A generous safety factor should be used  in calcula-
tions involving the storage, handling or disposal  of grit  (35).  In  a  new
system where there are separate sanitary sewers and favorable  conditions
such as adequate scouring velocities, an allowance of 5 to 15  cubic  feet
per million  gallons should suffice for maximum flows.  For combined  sewers
15 to 30 cubic feet or more per million gallons may be  necessary with
conventional removal systems.  The use of high efficiency  techniques which
capture smaller grit sizes may increase these estimates.

     Sandy areas have the potential to introduce large quantities of grit,
even in separate sanitary systems.  In several instances,  vortex flow
devices have demonstrated the ability to meet performance  criteria for
removal of fine grit ("sugar sand" from the Southeast;  "blow sand" from
Northwest beaches), based on removals in full-scale and pilot  units  (15,
27).  The design for grit handling facilities must take into account
significantly greater volumes of grit captured.

     Although numerous references (1,7,12,17,34,35,40)  exist which provide
excellent information on grit chamber design, the basis of most grit chamber
design methods is removal of particles of 0.2 mm size with a specific
gravity of 2.65.  This criterion can be traced to Camp's 1942  paper (7)
which stated:
                                    -50-

-------
      "Experience has indicated that if the chamber will remove all
      sand 0.2 mm in size and larger (material retained on a 65-mesh
      sieve) it will remove most of the grit which gives trouble in
      treatment plants."

Consequently, grit chamber design has been driven (16,17,35) by this
criterion which was introduced without scientific basis.  Treatment plant
operators in several areas of the country where fine sands dominate the
local soil characteristics will  attest to the inadequacy of this criterion.
However, no better criterion has been determined scientifically.  Annen (56)
did similarly note that grit particles of less than 0.1 mm size do not dis-
turb  sludge treatment, but failed to provide data to prove this contention.

      With grit removal, the designer is usually faced with few options.
Location is generally specified by guidance (16) to precede all other plant
unit  processes, except tor coarse screening.  New plant designs generally
provide either constant velocity channels or aerated grit chambers, although
detritus tanks are still fairly common in older facilities and a few centri-
fugal devices and fine screens are in use.  Most small plants are equipped
with  constant-velocity channels, while aerated grit chambers have generally
been  employed by larger facilities.

Operability of Grit and Screenings Removal Systems

      Grit and screenings removal equipment is subject to the same impacts
that  affect downstream equipment: abrasion, clogging and jamming of submerged
moving parts.  Hence, ease of maintenance is of paramount concern.  For
example, the channel in which mechanically cleaned, chain-driven screens
with  submerged moving parts are installed will require dewatering for access
to these parts.  Grit removal equipment is subjected to extremely harsh
conditions and must be accessible for maintenance and repair.  To remove
rags, cleanouts should be provided at inlets to progressive cavity or
plunger pumps, and at chemical mixing tanks with paddle type mixers.   The
ability to flush rags from secondary clarifier draft tubes, particularly
in plants without primary clarifiers,  should be provided.  In a previous
section the general  advantages and disadvantages of the various methods of
removing grit and screenings were described.

      Headworks and pump stations can be hazardous environments, as the
possibility exists for corrosive or explosive gas concentrations to be
present.  Therefore, operator safety should be a major concern in design
and operation of preliminary treatment systems.

     The need tor housing the screening equipment is dependent on the
equipment and the climate.   If housing is required,  adequate ventilation
will  provide a safer environment and prolong the life of the equipment by
preventing the accumulation of gases and moisture.   Grit chambers should be
designed to provide  safe access  to the chamber and  mechanical  equipment.
Stairway access is required for  units  located in deep pits.
                                    -51-

-------
     In one of the case studies,  the grit handling system serving four
covered aerated grit chambers  consisted of chain  and flight  conveyors,
screw conveyors and bucket elevators,  and was inaccessible for maintenance.
Corrosion in the enclosed, moist  environment enhanced deterioration  of
parts above the water surface. The covers were removed,  and clamshell
buckets mounted on a traveling bridge  crane were  installed for grit  removal.

     Working clearances around equipment should be sufficient for maintenance
purposes and conform to state  and local codes.   Platforms on and around
machinery at locations not easily reached from floor level should be provided.
Designs should provide operating  personnel with railings  around channels and
shrouds over moving equipment  (chains, sprockets, or rakes).  Electrical
equipment should be explosion  proof and kept to a minimum because of the
damp corrosive conditions.

DESIGN CONSIDERATIONS FOR SMALL PLANTS

Staffing

     Municipal wastewater treatment plants that are less  than 1 mgd in size
are distinguished by small staffs and non-specialized personnel that often
have less specialized training than those found in larger plants.  These
plants may also be intermittently staffed.  In these plants, preliminary
treatment systems should be mechanically simple,  operate  reliably, and
require only part-time attention.  Although automatic operation is desirable,
instrumentation to allow such  operation must be kept simple.

     Because approximately 80  percent of the facilities to be built in the
next 15 years are to be less than 1 mgd in size,  there is a growing need
for appropriate small plant preliminary treatment systems.

Special Design Considerations  for Small Plants

     The first small community design  issue is the type of sewer.  Although
most small communities are served by conventional gravity separate sewers,
a large number of recently constructed facilities employ  alternative  sewers.
For those systems preliminary treatment issues are significantly different.
Sewer  systems which employ on-lot septic tank pretreatment include septic
tank effluent pumping  (STEP) systems and  small-diameter gravity  (SDG)
systems.  The wastewaters which enter  the treatment  plant from these  sewer
systems do not require preliminary  treatment since the grit,  screenings
and grease which these processes are designed to remove have  already  been
removed by the septic  tanks and, if the sewers are properly designed  with
no or  just a few manholes at critical  junctions, there is no  means of
reintroduction during wastewater transmission  to the plant.
                                    -52-

-------
      Grinder-pump  (GP)  pressure  sewers  are  different  in that the wastewater
 has  only  been  ground  (comminuted) prior to  transmission to the treatment
 plant and has  minimal opportunity for dilution due to  infiltration.  There-
 fore,  grit will  still be present in the wastewater, while solids have been
 comminuted.  Since  the  comminutors in this  case are an integral part of the
 wastewater pumping  process, the designer may have some flexibility in pre-
 treatment system design, although conventional guidance (16) would dictate
 that normal pretreatment processes should be employed.  Given the normal
 tendency  for comminuted solids to recombine, a mechanically cleaned screen
 with relatively  fine  openings would appear  prudent.  Since GP wastewater
 tends  to  be high in solids and organics due to the lack of infiltration,
 designs incorporating preliminary treatment or lagoon technology are
 desirable.

      Vacuum sewers  provide a wastewater which is very  similar in character
 to conventional  sewers  and should be subject to similar design guidance.
 The  major difference  is the hydraulic violence which occurs in transmission
 which  may result in screenings of somewhat  reduced size, suggesting the use
 of screens finer than used in conventional  applications.

     The  wastewaters delivered to the plant by alternative sewers have
 certain other  characteristics of importance to the designer.  These sewers
 minimize  infiltration and thereby result in lower average flows to the
 plant.  Peak flows  tend to be similar to normal dry weather flow peaks,
 even during storms  if the sewer is properly constructed.  Pressure sewers
 (STEP  and GP systems) and SDG sewers have high sulfide contents which must
 be taken  into  account by the designer to minimize odors and corrosion
 problems  at the  treatment facility.

     Where the collection system is a mixture of sewer technologies the
 designer  should assume  the worst characteristics of each type of collec-
 tion technology.  For example, if conventional  sewers and GP pressure
 sewers convey the wastewater to the plant, the designer should assume
 that the  wastewater will exhibit the higher flows and flow variation of
 conventional  sewers and the potential  for odor and corrosion of the GP
 sewer.  In practical terms this would mean that preliminary treatment
 processes should be designed for the higher peak  flows of  the conventional
 sewer, but with preliminary treatment process choices which minimize turbu-
 lence  (potential hydrogen  sulfide stripping) and  corrosion-resistant
 materials of construction.

     Another significant design consideration is  climate.   In climates
where freezing occurs, screenings can  freeze,  damaging equipment and becom-
 ing difficult to handle.  In such climates bar racks should be located in
a heated enclosure.  In  one case  study  a mechanically cleaned screen
 installed outside experienced shear pin breakage  due to screenings  that
froze.  Construction of  a  heated  enclosure alleviated these problems.

     Small treatment plant technologies for accomplishing  required  levels
of treatment  will also impact the preliminary, treatment system design.
Since the majority  of small  treatment  systems  are  lagoons  or lagoon-based


                                    -53-

-------
technologies, grit removal  may be applied only  in special  cases,  such  as
those where fine grit concentrations  are high  in  the  incoming wastewater.
Many such areas exist in the U.S. where fine "blow"  or "sugar" sands could
fill lagoons in a relatively short period unless  removed  at  the headworks
(preliminary treatment location).  The grit removal  systems  provided must
be capable of removing these very small  grit particles.   Therefore, conven-
tional designs do not provide the necessary degree of grit removal, unless
modified.  Trickling filters and long-solids retention time  (SRT) activated
sludge systems are common types of small community treatment systems.   The
former type generally employs primary treatment,  while the latter does not.
Historically, control!ed-velocity channels have been the  dominant grit re-
moval process for these small trickling filter plants and for the long-SRT
facilities, when grit removal is employed.  This design choice is consistent
with the operational needs of these treatment systems.  In recent years,
designers have been applying aerated grit chambers to smaller facilities
where wide fluctuations in flow cause the control 1ed-velocity channel  to
yield variable performance.  However, the preceding screen performance
requirements increase with the aerated chamber, necessitating finer,
mechanically cleaned screens.

     A British preliminary treatment survey of eight Water Authorities (3)
indicated that there was a unanimous response as to the unacceptability of
comminution, as  historically practiced with currently available equipment.
There was some sentiment on the part of the respondees that better (more
heavy-duty)  devices might have beneficial application in smaller treatment
plants in the future.  U.S. and Canadian experience is consistent with that
of  the British.

     The  need to improve screenings  removal in small package  plants,   usual-
ly  high-SRT  activated sludge systems without primary clarification, is also
clear.   Such improvement requires  finer  screens which must employ mechanic-
al  cleaning  or frequent manual attention.   Since most small  treatment plant
designs  attempt  to  minimize  O&M  requirements,  the designer must  evaluate
the tradeoffs between increased  preliminary treatment efficiency and  its
O&M requirements with reduced  O&M  requirements downstream.  Finer screens
increase  capital  costs over  coarser  manually cleaned designs,  with or with-
out comminutors, but  the difference  must be compared against  the savings
in  reduced  O&M  tor  downstream  treatment  and sludge handling processes.
For example, if  the downstream process  is  a facultative  lagoon,  there is
little  concern  over the efficiency of  the  screen  since downstream effects
are primarily of an aesthetic  nature.   However,  if  surface  aerators are
employed,  some  tradeoffs will  exist  in  reduced O&M of the aerator.  With
trickling filters,  properly  designed primaries would  be  expected to capture
almost  all  of the screenings and tradeoffs  will  be found  in  reduced skimm-
 ings handling,  clarifier mechanism O&M and sludge pumping and handling
O&M.  Since land spreading  is  a  likely  sludge  disposal method for these
 plants,  improved acceptability of the sludge may also  be a  tangible trade-
 off (3).   With  long-SRT activated sludge systems,  reduced O&M requirements
 for aerators, secondary clarifier scum and sludge collection, return  sludge
 pumping and other waste sludge handling and disposal  must be  weighed  against
 increased preliminary treatment costs.


                                     -54-

-------
 Specific  Recommendations  for  Small  Plants

      -  Mechanically  cleaned screens with fine  (0.5-to 0.75-in.) openings
        should be  considered where mechanical  (trickling  filter and  activated
        sludge)  treatment  systems are  employed.

      -  Preliminary treatment  designs  must  be  compatible  with  the collection
        system and downstream  processes  employed.

      -  Special  cases where very fine  grit  concentrations would interfere
        with  downstream  treatment may  benefit  from new vortex  or fine screen
        technologies, on which more  design,  performance and cost data are
        required.

 DESIGN  CONSIDERATONS FOR  LARGE PLANTS

 Staffing

      Large municipal wastewater treatment  plants should  have  round-the-
 clock staffing, with specialized, highly skilled personnel.   They may have
 well-equipped machine shops as well.  The  greater quantities  of wastes
 handled means labor-related economies of scale can be achieved; consequent-
 ly,  processes and equipment tend to be  more mechanically complex than for
 smaller plants.

 Design  Considerations for Large Plants

     There are  fewer plants with treatment capacities greater than  one mgd,
 but  their combined capacities treat the great majority of the wastewater
 volume  treated  in the U.S. (31,33).   For the most part the previous discus-
 sion of this  section pertains to facilities which generally have a  diversity
 of processes  that can be adversely affected by inadequate preliminary
 treatment performance.

     Systems  for  removing grit and screenings from large treatment  plants
 must be suited to the type of collection system (separate or combined),
 specific characteristics of the waste stream, and subsequent downstream
 and  sludge handling processes.

     Combined sewers will bring in much greater quantities of grit and
 screenings during periods of  storm flushes than will  separate systems.
 Designs must  recognize  greater ratios of peak to average flow and even
 greater ratios of peak to average quantities of screenings and grit.  There
 appears to be no  available guidance on these latter ratios.   Wastewater
 characterization  information  is vital  for the designer to evaluate the
 adequacy of screenings and grit handling equipment.   Some references (1,3,
 17,36) provide data  on the ratio of peak flow screenings  to  average flow
 screenings quantities.   The two U.S. sources (1,3)  appear to be reasonable
 estimates tor separate domestic sewers in that they  indicate rather small
 (<2 to  1) peak to average values.   The two British  references (17,36)  are
more typical  of combined sewers where  ratios of 2b  or more to 1 are
 experienced.   The amount of screenings to be expected at  the treatment

                                    -55-

-------
plant will  also vary with the length (and time of travel)  of  sewer,  the
flow rate,  and the climatic  records  of  the preceding  period.

     Some industries can influence waste stream characteristics  for  both
grit and screenings.  Plastic sheet  material  or fabric  sweepings  from
textile mills can magnify the screenings problem.  If such sweepings are
fine, they  will pass through any coarse screen and will  enter the sludge
streams.  In such cases, finer bar screens should be  considered.   Another
method which should be considered is the use  of a screen or heavy-duty
grinder in  the sludge line prior to  the sludge pump as  a "second line  of
defense".  This additional screening or grinding could  reduce pump and
pipeline clogging and additional downstream problems.  In  one case study,
screens were installed in the return sludge lines of  an activated sludge
plant to intercept rags.  This reduced  aerator ragging  and pump  blockages
caused by the inability of comminutors  to properly eliminate  these problems.

     The choices of specific screening  designs for large wastewater treat-
ment plants tend to reflect local conditions  and preferences.  With  regard
to screening, most large facilities  employ trash racks  (very  coarse  screens)
for equipment protection followed by mechanically cleaned  screens for  actur-
al removal  of rags and other troublesome materials.  Although this practice
appears appropriate, there is much disagreement with  regard to the best
mechanically cleaned screen design.   British engineers  prefer backraked
designs (3), while some cities prefer front-raked screens  (54).   The only
concepts which appears to engender agreement are the  need to  have all
moving parts above the water level to facilitate maintenance  and the need
to have better control of raking speeds, such as two-speed raking controls.

     There is a clear trend toward reducing the size  of mechanically cleaned
screen openings in Great Britain where numerous recent studies have investi-
gated the potential benefits and disadvantages of this  change (3,36,37,38,
39,43,45).   Although such efforts are yet negligible  in the U.S., the
potential benefits of this design change are clearly  apparent and quanti-
fiable.

     As discussed earlier, grit  quantities are  subject to dynamic influences
in addition to soils of the service area and other natural and man-made
conditions.  The greater  grit quantity variation in  larger plants, especial-
ly those served by combined sewers, has led the design profession to wider
use  of  aerated grit chambers with variable and  controllable air  supplies.
British practice has tended more toward detritus tanks, but aerated grit
chambers and  vortex-separation  types have gained in  popularity in recent
years  (3).  Grit removal  systems have been maintenance intensive  owing to
the  inherent  difficulty of  handling this  abrasive material.  Odors  tend  to
be minimized with well-operated  aerated grit chambers, but can be signifi-
cant with  other  types of  grit removal  systems,  and odor control  systems
may  need to be included.  Grit  disposal for large  plants may be  to  land-
fills,  incineration or  ocean  disposal,  depending on  the location  and local
circumstances.
                                     -56-

-------
 Specific Recommendations for Large Plants

      - Openings tor mechanically cleaned bar screens should be as small  as
        0.5 inches.

      - All bar screen mechanical equipment should remain above the water
        line in the screening chamber to facilitate maintenance.

      - Mechanical cleaning of bar screens should have at least two speeds
        to handle peak screenings loads as well  as to optimize removals
        during lower flow periods.

      - Grit removal by aerated grit chambers is well-suited to larger
        treatment plants when supplied with proper baffling, chamber design,
        and controllable air supplies.

      - Vortex-type grit removal  devices and improved methods of  settled
        grit collection should be considered if  sufficient justification
        data can be provided.

 DESIGN CONSIDERATIONS FOR RETROFITS

      Many municipal wastewater treatment plants are  presently in  need of
 upgrading and  many relatively new plants will be at  some  time in  the  future.
 Consequently  the need for retrofitting grit and screening removal  devices
 is  significant and will  persist.

      Many of  the considerations  that  apply  in designing a new facility also
 are applicable in  retrofits.   However,  a number of constraints are  more
 influential:  hydraulic profiles  are established,  space may  be limited, and
 plant personnel  are accustomed to  existing  conditions.

      When retrofitting devices for grit and screenings removal, the following
 must be considered:

      - Reliability  and need  for  redundancy.

      - Maximum use of  existing equipment  if present equipment is adequate
       for  subsequent  needs.

      - Need for odor control or other ancillary  systems.

When  replacing screens, space considerations can direct the decision.  For
example, reciprocating rake screens consume little channel length, but
require relatively high vertical  clearance.  Catenary type screens are not
as  tall as reciprocating rake devices, but  require more horizontal space.

     Odor control systems for preliminary treatment facilities should be
capable of effectively removing hydrogen sulfide and methyl mercaptan in
addition to plant-specific compounds.   Acid-alkali scrubbing has  not been
found effective, but activated carbon  and ozonation have.   Small  plants


                                    -57-

-------
should consider soil/compost or iron oxide/woodchip contact units  for
simplicity.

     Conversion to aerated grit chambers should eliminate  the  need for  a
separte grit washing step.  Chain  and bucket collection  of removed grit
should be avoided, while screw conveyors and air-lift pumps have  had success.

     Fine screens and teacup-type  grit chambers may require pumping to
satisfy their need for head differential.  In retrofits, use of these  units
may necessitate additional pumping which may not be physically possible or
economically feasible.

OPERATION AND MAINTENANCE CONSIDERATIONS

     The heavy wear on equipment in grit and screenings  removal systems
requires that sufficient preventive maintenance be practiced.   Regular
maintenance of the preliminary treatment system will also help to minimize
downstream impacts due to poor grit and screenings removal.

     Preventive maintenance required by preliminary treatment systems is
generally small and limited to observation, good housekeeping and
oil/lubrication needs.  Total O&M requirements include labor and power.
For mechanically cleaned  (MC) screens operation is usually intermittent,
varying from 5 to 15 minutes per hour, but may be full-time during storm
events.  Electrical requirements for MC screens in smaller plants vary from
about  1.7 kWh/day at 0.1 mgd to 11 kWh/day at 100 mgd, while aerated grit
chambers require from 30 to 740 kWh/day over this size range (48).  Non-
aerated grit chambers trade power savings for increased labor requirements.
Generally, the O&M labor  requirements for preliminary treatment constitute
about  10% of the  plant total (49).  However, the time required to operate
and maintain preliminary  treatment systems and impacted downstream processes
during a major storm event may be significantly greater (55).
                                     -58-

-------
                                 Ktl-tRtNCES
 1.  Metcalf & Eddy, Inc., Wastewater Engineering, McGraw-Hill, New York,
     1972.

 2.  U.S. Environmental Protection Agency, "Process Design Manual  for
     Sludge Treatment and Disposal," EPA-625/1-79-011, September 1979.

 3.  Sidwick, J.M., "Screenings and Grit and Sewage:  Removal, Treatment
     and Disposal - Preliminary Report," Construction Industry Research
     and Information Association Technical Note 119, Westminster,  UK, 1984.

 4.  Hegg, B.A., Rakness, K.L., and Schultz, J.R., "Evaluation of  Operation
     and Maintenance Factors Limiting Municipal Wastewater Treatment Plant
     Performance," USEPA Report No. 600/2-79-034, June 1979.

 5.  Smith, J.M., Evans, F.L. Ill, and Bender, J.H., "Improved Operation and
     Maintenance Opportunities at Municipal  Treatment Facilities," 7th
     US/Japan Conference on Sewage Treatment Technology Proceedings, USEPA
     Report No. 600/9-80-047, December 1980.

 6.  Azan, K.M. and Boyko, B.I., "Identification of Problem Areas  in Water
     Pollution Control  Plants," Environment Canada, Research  Report No. 15,
     1973.

 7.  Camp, T.R., "Grit Chamber Design,"  Sewage Works Journal,  Vol. 14,  March
     1942.

 8.  Metcalf & Eddy, Inc., "Report to National Commission on  Water Quality
     Assessment of Technologies and Costs for Publicly Owned  Treatment
     Works," Volume 2,  September 1975.

 9.  Dorr-Oliver Corporation, Personal Communication.

10.  Environmental  Resources Management,  Inc., "Assessment of  Field Test
     Candidates - Teacup Solids Classifier," EPA Contract No.  68-01-7108,
     December 1985.

11.  Lenhart,  Charles  F. and McGarry, James  A., "Screening Makes Downstream
     Life Simpler,"  Water/Engineering and Management. June 1984.

12.  Morales,  L.  and Reinhart,  D.,  "Full  Scale Evaluation of Aerated Grit
     Chambers," Journal  Water Pollution  Control  Federation. Vol. 56,  No.  4,
     April  1984.                                        ~~

                                    -59-

-------
13.  Neighbor,  J.B.  and Cooper,  T.W.,  "Design  and  Operation of Aerated Grit
     Chambers," Water and Sewage Works,  December 1965.

14.  U.S.  Environmental Protection  Agency,  "Innovative  and Alternative Tech-
     nology Assessment Manual,"  EPA-430-9-78-009,  February 1980.

15.  Smith and Loveless Corporation,  Technical  Literature.

16.  Great Lakes - Upper Mississippi  River  Board of  State Sanitary  Engineers,
     "Recommended Standards  for  Sewage Works,"  Health Education Service,
     Inc., Albancy,  N.Y., 1978 Edition.

17.  Water Pollution Control  Federation,  Wastewater  Treatment Plant Design,
     Manual of Practice No.  8, 19/7.

18.  U.S.  Environmental Protection  Agency,  "Process  Design Manual for Sus-
     pended Solids Removal,"  EPA-625/l-75-003a, January 1975.

19.  Finger, R.E. and Parrick, J.,  "Optimization of  Grit Removal  at a Waste-
     water Treatment Plant,"  Journal  Water  Pollution Control Federation,
     Vol.  52, No. 6, August  1980.

20.  Schultz, D.W. and Parr,  V.B.,  "Evaluation and Documentation  of Mechani-
     cal Reliability of Conventional  Wastewater Treatment Plant Components,"
     EPA-600/S2-82-044, August 1982.

21.  Metcalf & Eddy, Inc., "Improved  Design and Operation of Multiple Hearth
     and Fluid Bed Sludge Incinerators," EPA Contract No. 68-03-3208,
     September, 1985.

22.  Metcalf & Eddy, Inc., "Improved  Design and Operation of Recessed Plate
     Filter Presses," EPA Contract No. 68-03-3208, September 1984.

23.  Metcalf & Eddy, Inc., "Improved  Design and Operation of Belt Filter
     Presses," EPA Contract No.  68-03-3208, September  1984.

24.  Metcalf & Eddy, Inc., "Achieving Improved Operation of  Heat  Treatment/
     Low Pressure Oxidation  of Sludge,"  EPA Contract No. 68-03-3208,
     September 1984.

25.  Metcalf & Eddy, Inc., "Improved  Design and Operation  of Centrifuges,"
     EPA Contract No. 68-03-3208, August 198b.

26.  U.S.  Environmental Protection Agency,  "Recommendations  from  Value
     Engineering Studies on Wastewater Treatment  Works," EPA-430/9-80-010,
     September 1980.

27.  Wilson, George E.,  "Is There Grit in Your Sludge?," Civil  Engineering/
     ASCE,  April 1985.

28.  U.S.  Environmental  Protection Agency,  "Electrical  Power Consumption for
     Municipal Wastewater Treatment," EPA-R2-73-281, July 19/3.

                                    -60-

-------
29.  Iowa State University, "Part I.  - Staffing Guidelines for Conventional
     Municipal Wastewater Treatment Plants less than 1.0 MGD, Estimating
     Staffing and Cost Factors for Small  Wastewater Treatment Plants less
     than 1.0 MGD" EPA Grant No. 5P2-WP-195-0452, June 1973.

30.  U.S. Environmental Protection Agency, "Total Energy Consumption for
     Municipal Wastewater Treatment," EPA-600/2-78-149, August 1978.

31.  U.S. Environmental Protection Agency, "1978 Needs Survey:  Cost Esti-
     mates for Construction of Publicly-Owned Treatment Plants," EPA-430/9-
     79-001, February 1979.

32.  Metcalf & Eddy, Inc., "Assessment of Technologies and Costs for Public-
     ly Owned Treatment Works under Public Law 92-500," National Commission
     on Water Quality, Volumes 1-3, September 1975.

33.  U.S. Environmental Portection Agency, "1984 Needs Survey, Report to
     Congress:  Assessment of Needed Publicly Owned Wastewater Treatment
     Facilities in the United States," EPA-430/9-84-011, February 1985.

34.  Keefer, C.E., Sewage-Treatment Works, McGraw-Hill, New York, 1940.

35.  Babbitt, H.E., Sewerage and Sewage Treatment, John Wiley & Sons, New
     York, 1947.

36.  Page, S.J., "Quantifying the Screenings Problem at Sewage-Treatment
     Works," Water Pollution Control (U.K.), Vol. 86, No. 4,  1986.

37.  Sidwick, J.M., "Screenings and Grit  in Sewage:  Removal, Treatment  and
     Disposal - Phase 2," Construction Industry Research and  Information
     Association, Westminster, U.K., 1985.

38.  Little, R.W., and Symes, G.L., "Screening:   A New Approach," Waterv
     Pollution Control (U.K.), Vol. 83, No. 2, 1983.

39.  Hubbard, A.M., and Crabtree, H.E., "Some Observations on Screening  and
     Screenings Treatment at Sewage-Treatment Works," Water Pollution Con-
     trol (U.K.), Vol. 86, No. 4, 1986.

40.  Ontario Ministry of the Environment, "Guidelines for the Design of
     Water Treatment Plants and Sewage Treatment Plants," July 1984.

41.  Morgan, P.F., "The Comminution of Sewage Solids," Sewage Works  Journal,
     Vol. 13, No. 1, January 1941.

42.  Noone, G.P., and Brade, C.E., "Principles of Sewage Works Process
     Design," Water Pollution Control  (U.K.), Volume 85, No.  4, 1985.

43.  Newman, G.D., "Operational  Experiences with a Screenings Washer-
     Dewaterer, Bagger and Incinerator,"  Water Pollution Control (U.K.),
     Volume 86, No. 4, 1986.
                                    -61-

-------
44.  Keefer, C.E.,  "Pulverizing  of  Sewage  Screenings  at Baltimore, Mary-
     land," Transactions of ASCE, September  1929.

45.  Lowe, P., and  Sidwick, J.M., "Towards More  Efficient Treatment - Pre-
     liminary and Primary Treatment,"  Water  Pollution Control  (U.K.),
     Volume 87, No. 2, 1987.

46.  Eliassen, R.,  and Coburn, D.F.,  "Versatility  and Expandability of
     Pretreatment," Journal ASCE-EED,  Volume 95, No.  SA2, April  1969.

47.  Guo, P.H.M., Thirumurthi, D.,  and Jank, B.E.,  "Evaluation of Extended
     Aeration Activated Sludge Package Plants,"  Journal Water  Polluton
     Control Federation, Volume  53, No. 1, January  1981.

48.  Wesner, G.M.,  Gulp, G.L., Lineck, T.S., and Hinrichs,  O.J.,  "Energy
     Conservation in Municipal Wastewater  Treatment," USEPA Report No.
     430/9-77-011,  March 1978.

49.  U.S. Environmental Protection  Agency,"Estimating Staffing for Municipal
     Wastewater Treatment Facilities," Contract  No.  68-01-0328,  March 1973.

50.  Ettlich, W.F., "A Comparison of Oxidation Ditch  Plants to Competing
     Processes for Secondary and Advanced  Treatment  of Municipal  Wastes,"
     USEPA Report No. 600/2-78-051, March, 1978.

51.  Benjes, H.H.,Jr., "Small  Community Wastewater Treatment Facilities  -
     Biological Treatment Systems," in USEPA Design  Seminar Handout Small
     Wastewater Treatment Facilities,  July,  1979.

52.  Ross, S.A., Guo, P.H.M.,  and Jank, E.E., "Design and  Selection of
     Small Wastewater Treatment  Systems,"  Environment Canada Report No.
     EPS 3-WP-80-3, March, 1980.

53.  USEPA, "Operations Manual:  Anaerobic  Sludge Digestion," USEPA Report
     No. 430/9-76-001, February, 1976.

54.  Melbinger, N.R. and Schaaf, R.M., "The  Benefits of  10 Years of New
     York City's Innovative Grit and Screenings  Removals,"  Paper presented
     at 59th Water Pollution Control  Federation  Conference, October,  1986.

55.  Leschak, P.M., "Rainy Night in the Sewage  Plant,"  Operations Forum  -
     WPCF, February, 1986.

56.  Annen, G.W., "Efficiency of a  Grit Chamber",  Water  Research, Volume 6,
     pages 393-394, 1972.
                                    -62-

-------
             ENGLISH TO METRIC UNITS CONVERSION TABLE
English
Conversion
Factor
Metric
Equivalent
Foot, ft                     0.305

Feet per second, ft/sec      0.305


Million gallons per day,
mgd                          43,800

Cubic feet per minute, cfm   0.472
Pounds per cubic foot,
Ib/cf

Cubic feet per mllion
gallons, cf/mg

Gallons per minute per
square foot, gpm/ft2
0.016


7.48 x 10~6


0.679
Meter, m

Meters per second,
m/s

Milliliters per
per second, mL/s

Liters per second,
L/s

Kilograms per
liter, Kg/L

Liters per liter,
L/L

Liters per second
per square meter,
Lps/m2
                                -63-

-------