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-
------- |