&EPA
United States
Environmental Protection
Agency
Wastewater Technology Fact Sheet
Screening and Grit Removal
DESCRIPTION
Wastewater contains large solids and grit that can
interfere with treatment processes or cause undue
mechanical wear and increased maintenance on
wastewater treatment equipment. To minimize
potential problems, these materials require separate
handling. Preliminary treatment removes these
constituents from the influent wastewater.
Preliminary treatment consists of screening, grit
removal, septage handling, odor control, and flow
equalization. This fact sheet discusses screening
and grit removal.
Screening
Screening is the first unit operation used at
wastewater treatment plants (WWTPs). Screening
removes objects such as rags, paper, plastics, and
metals to prevent damage and clogging of
downstream equipment, piping, and appurtenances.
Some modern wastewater treatment plants use both
coarse screens and fine screens. Figure 1 depicts a
typical bar screen (a type of coarse screen).
Source: Qasim, 1994.
FIGURE 1 CABLE OPERATED BAR
SCREEN
Coarse Screens
Coarse screens remove large solids, rags, and debris
from wastewater, and typically have openings of 6
mm (0.25 in) or larger. Types of coarse screens
include mechanically and manually cleaned bar
screens, including trash racks. Table 1 describes the
various types of coarse screens.
Fine Screens
Fine screens are typically used to remove material
that may create operation and maintenance problems
in downstream processes, particularly in systems
that lack primary treatment. Typical opening sizes
for fine screens are 1.5 to 6 mm (0.06 to 0.25 in).
Very fine screens with openings of 0.2 to 1.5 mm
(0.01 to 0.06 in) placed after coarse or fine screens
can reduce suspended solids to levels near those
achieved by primary clarification.
Comminutors and Grinders
Processing coarse solids reduces their size so they
can be removed during downstream treatment
operations, such as primary clarification, where both
floating and settleable solids are removed.
Comminuting and grinding devices are installed in
the wastewater flow channel to grind and shred
material up to 6 to 19 mm (0.25 to 0.75 in) in size.
Comminutors consist of a rotating slotted cylinder
through which wastewater flow passes. Solids that
are too large to pass through the slots are cut by
blades as the cylinder rotates, reducing their size
until they pass through the slot openings.
Grinders consist of two sets of counterrotating,
intermeshing cutters that trap and shear wastewater
solids into a consistent particle size, typically 6 mm
(0.25 in). The cutters are mounted on two drive
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TABLE 1 DESCRIPTION OF COARSE
SCREENS
Screen Type Description
Trash Rack Designed to prevent logs, timbers,
stumps, and other large debris from
entering treatment processes.
Opening size: 38 to 150 mm (1.5-6 in)
Manually Designed to remove large solids, rags,
Cleaned Bar and debris.
Screen Opening size: 30 to 50 mm (1 to 2 in)
Bars set at 30 to 45 degrees from
vertical to facilitate cleaning.
Primarily used in older or smaller
treatment facilities, or in bypass
channels.
Mechanically Designed to remove large solids, rags,
Cleaned Bar and debris.
Screen Opening size: 6 to 38 mm (0.25 to 1.5
in).
Bars set at 0 to 30 degrees from
vertical.
Almost always used in new
installations because of large number
of advantages relative to other
screens.
Source: Design of Municipal Wastewater Treatment Plants,
WEF MOP 8, Fourth Edition, 1998.
shafts with intermediate spacers. The shafts
counterrotate at different speeds to clean the cutters.
Figure 2 depicts a channel wastewater grinder.
The chopping action of the grinder reduces the
formation of rag "balls" and rag "ropes" (an
inherent problem with comminutors). Wastewaters
that contain large quantities of rags and solids, such
as prison wastewaters, utilize grinders downstream
from coarse screens to help prevent frequent
jamming and excessive wear.
Grit Removal
Grit includes sand, gravel, cinder, or other heavy
solid materials that are "heavier" (higher specific
gravity) than the organic biodegradable solids in the
wastewater. Grit also includes eggshells, bone
chips, seeds, coffee grounds, and large organic
particles, such as food waste. Removal of grit
prevents unnecessary abrasion and wear of
mechanical equipment, grit deposition in pipelines
and channels, and accumulation of grit in anaerobic
digesters and aeration basins. Grit removal
facilities typically precede primary clarification, and
follow screening and comminution. This prevents
large solids from interfering with grit handling
equipment. In secondary treatment plants without
primary clarification, grit removal should precede
aeration(Metcalf&Eddy, 1991).
Many types of grit removal systems exist, including
aerated grit chambers, vortex-type (paddle or jet-
induced vortex) grit removal systems, detritus tanks
(short-term sedimentation basins), horizontal flow
grit chambers (velocity-controlled channel), and
hydrocyclones (cyclonic inertial separation).
Various factors must be taken into consideration
when selecting a grit removal process, including the
quantity and characteristics of grit, potential adverse
effects on downstream processes, head loss
requirements, space requirements, removal
efficiency, organic content, and cost. The type of
grit removal system chosen for a specific facility
should be the one that best balances these different
considerations. Specifics on the different types of
grit removal systems are provided below.
Aerated Grit Chamber
In aerated grit chambers, grit is removed by causing
the wastewater to flow in a spiral pattern, as shown
Source: WEF, 1998.
FIGURE 2 WASTEWATER GRINDER:
CHANNEL UNIT
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in Figure 3. Air is introduced in the grit chamber
along one side, causing a perpendicular spiral
velocity pattern to flow through the tank. Heavier
particles are accelerated and diverge from the
streamlines, dropping to the bottom of the tank,
while lighter organic particles are suspended and
eventually carried out of the tank.
Helical liquid
flow pattern
Outlet weir
Trajectory ol
grit particles
- Air
Air ditfuser
Source: Crites and Tchobanoglous, 1998.
FIGURES AERATED GRIT CHAMBER
Vortex-Type Grit Chamber
The vortex-type grit chamber consists of a
cylindrical tank in which the flow enters
tangentially, creating a vortex flow pattern. Grit
settles by gravity into the bottom of the tank (in a
grit hopper) while effluent exits at the top of the
tank. The grit that settles into the grit hopper may
be removed by a grit pump or an air lift pump.
Detritus Tank
A detritus tank (or square tank degritter) is a
constant-level, short-detention settling tank. These
tanks require a grit-washing step to remove organic
material. One design option includes a grit auger
and a rake that removes and classifies grit from the
grit sump.
Horizontal Flow Grit Chamber
The horizontal flow grit chamber is the oldest type
of grit removal system. Grit is removed by
maintaining a constant up stream velocity of 0.3 m/s
(1 ft/s). Velocity is controlled by proportional
weirs or rectangular control sections, such as
Parshall flumes. In this system, heavier grit
particles settle to the bottom of the channel, while
lighter organic particles remain suspended or are
resuspended and transported out of the channel.
Grit is removed by a conveyor with scrapers,
buckets, or plows. Screw conveyors or bucket
elevators are used to elevate the grit for washing or
disposal. In smaller plants, grit chambers are often
cleaned manually.
Hydrocyclone
Hydrocyclone systems are typically used to separate
grit from organics in grit slurries or to remove grit
from primary sludge. Hydrocyclones are sometimes
used to remove grit and suspended solids directly
from waste water flow by pumping at a head
ranging from 3.7 to 9 m (12 to 30 ft). Heavier grit
and suspended solids collect on the sides and
bottom of the cyclone due to induced centrifugal
forces, while scum and lighter solids are removed
from the center through the top of the cyclone.
APPLICABILITY
Because various types of screening and grit
removal devices are available, it is important that
the proper design be selected for each situation.
Though similarities exist between different types of
equipment for a given process, an improperly
applied design may result in an inefficient treatment
process.
Screening
As discussed above, most large facilities use
mechanically cleaned screening systems to remove
larger materials because they reduce labor costs and
they improve flow conditions and screening capture.
Typically, only older or smaller treatment facilities
use a manually cleaned screen as the primary or
only screening device. A screening compactor is
usually situated close to the mechanically cleaned
screen and compacted screenings are conveyed to a
dumpster or disposal area. However, plants
utilizing mechanically cleaned screens should have
a standby screen to put in operation when the
primary screening device is out of service. This is
standard design practice for most newly designed
plants.
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The use of fine screens in preliminary treatment has
experienced a resurgence in the last 20 years. Such
screens were a common feature before 1930 but
their use diminished because of difficulty in
cleaning oils and grease from the screens. In the
early 1980s, fine screens regained popularity
because of improved materials.
Communitors and Grinders
Comminutors and grinders are used primarily at
smaller treatment facilities (less than 5 MOD) to
process material between 6 and 19 mm (0.25 to
0.75 in) (WEF, 1998). This shredded material
remains in the wastewater and is removed in
downstream treatment processes.
Grit Removal
When selecting a grit removal process, the quantity
and characteristics of grit and its potential to
adversely affect downstream processes are
important considerations. Other parameters to
consider may include headloss requirements, space
requirements, removal efficiency, organic content,
and economics.
ADVANTAGES AND DISADVANTAGES
Advantages
Screening
Manually cleaned screens require little or no
equipment maintenance and provide a good
alternative for smaller plants with few screenings.
Mechanically cleaned screens tend to have lower
labor costs than manually cleaned screens and offer
the advantages of improved flow conditions and
screening capture over manually cleaned screens.
Communitors and Grinders
A major advantage of using Communitors and
grinders is that removal of grit reduces damage and
maintenance to downstream processes.
Comminutors and grinders also eliminate
screenings handling and disposal, which may
improve the aesthetics of the plant, reducing odors,
flies, and the unsightliness associated with
screenings. Some recently developed grinders can
chop, remove, wash, and compact the screenings.
The use of Comminutors in cold weather eliminates
the need to prevent collected screenings from
freezing. Comminutors and grinders typically have
a lower profile than screens, so cost savings can be
significant when the units must be enclosed.
Grit Removal
Aerated Grit Chamber
Some advantages of aerated grit chambers include:
Consistent removal efficiency over a wide
flow range.
A relatively low putrescible organic content
may be removed with a well controlled rate
of aeration.
Performance of downstream units may be
improved by using pre-aeration to reduce
septic conditions in incoming wastewater.
Aerated grit chambers are versatile,
allowing for chemical addition, mixing, pre-
aeration, and flocculation.
Vortex-Type Grit Chamber
These systems remove a high percentage of
fine grit, up to 73 percent of 140-mesh (0.11
mm/0.004 in diameter) size.
Vortex grit removal systems have a
consistent removal efficiency over a wide
flow range.
There are no submerged bearings or parts
that require maintenance.
The "footprint" (horizontal dimension) of a
vortex grit removal system is small relative
to other grit removal systems, making it
advantageous when space is an issue.
Headloss through a vortex system is
minimal, typically 6 mm (0.25 in). These
systems are also energy efficient.
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Detritus Tank
Grit Removal
Detritus tanks do not require flow control because
all bearings and moving mechanical parts are above
the water line. There is minimal headloss in this
type of unit.
Horizontal Flow Grit Chamber
Horizontal flow grit chambers are flexible because
they allow performance to be altered by adjusting
the outlet flow control device. Construction is not
complicated. Grit that does not require further
classification may be removed with effective flow
control.
Hydrocyclone
Hydrocyclones can remove both grit and suspended
solids from wastewater. A hydrocyclone can
potentially remove as many solids as a primary
clarifier.
Disadvantages
Screening
Manually cleaned screens require frequent raking to
avoid clogging and high backwater levels that cause
buildup of a solids mat on the screen. The
increased raking frequency increases labor costs.
Removal of this mat during cleaning may also cause
flow surges that can reduce the solids-capture
efficiency of downstream units. Mechanically
cleaned screens are not subject to this problem, but
they have high equipment maintenance costs.
Communitors and Grinders
Comminutors and grinders can create problems for
downstream processes, such as increasing plastics
buildup in digestion tanks or rag accumulation on
air diffusers. In addition, solids from comminutors
and grinders will not decompose during the
digestion process. If these synthetic solids are not
removed, they may cause biosolids to be rejected
for reuse as a soil amendment.
Grit removal systems increase the headloss through
a wastewater treatment plant, which could be
problematic if headloss is an issue. This could
require additional pumping to compensate for the
headloss.
The following paragraphs describe the specific
disadvantages of different types of grit removal
systems.
Aerated Grit Chamber
Potentially harmful volatile organics and odors may
be released from the aerated grit chamber. Aerated
grit chambers also require more power than other
grit removal processes, and maintenance and control
of the aeration system requires additional labor.
Vortex-Type Grit Chamber
Vortex grit removal systems are usually of a
proprietary design, which makes
modifications difficult.
Paddles tend to collect rags.
Vortex units usually require deep excavation
due to their depth, increasing construction
costs, especially if unrippable rock is
present.
The grit sump tends to clog and requires
high-pressure agitation using water or air to
loosen grit compacted in the sump.
Detritus Tank
Detritus tanks have difficulty achieving
uniform flow distribution over a wide range
of flows because the inlet baffles cannot be
adjusted.
This type of removal system removes large
quantities of organic material, especially at
low flows, and thus requires grit washing
and classifying.
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Grit may be lost in shallow installations
(less than 0.9 m [3 ft]) due to the agitation
created by the rake arm associated with this
system.
Horizontal Flow Grit Chamber
It is difficult to maintain a 0.3 m/s (1 ft/s)
velocity over a wide range of flows.
The submerged chain, flight equipment, and
bearings undergo excessive wear.
Channels without effective flow control will
remove excessive amounts of organic
material that require grit washing and
classifying.
Head loss is excessive (typically 30 to 40
percent of flow depth).
High velocities may be generated at the
channel bottom with the use of proportional
weirs, leading to bottom scour.
Hydrocyclone
Hydrocyclones require energy because they use a
pump to remove grit and suspended solids. Coarse
screening is required before these units to remove
sticks, rags, and plastics.
DESIGN CRITERIA
Screening
Screening devices are classified based on the size of
the material they remove (the screenings). The
"size" of screening material refers to its diameter.
Table 2 lists the correlation between screening sizes
and screening device classification.
In addition to screening size, other design
considerations include the depth, width, and
approach velocity of the channel; the discharge
height, the screen angle; wind and aesthetic
considerations; redundancy; and head loss.
Table 3 lists typical design criteria for mechanically
cleaned bar rack type screens.
TABLE 2 SCREENING DEVICE
CLASSIFICATION
Screening Device Size Classification/Size
Classification Range of Screen Opening
Bar screen
Manually Cleaned Coarse/25-50 mm
(1-2 in)
Mechanically Cleaned Coarse/15-75 mm
(0.6-3.0 in)
Fine bar or perforated coarse screen (mechanically
cleaned)
Fine Bar
Perforated Plate
Rotary Drum
Fine Coarse/3-12.5 mm
(0.1-0.5 in)
Fine Coarse/3-9.5 mm
(0.1-0.4 in)
Fine Coarse/3-12.5 mm
(0.1-0.5 in)
Fine screen (mechanically cleaned)
Fixed Parabolic
Rotary Drum
Rotary Disk
Fine/0.25-3.2 mm
(0.01-0.13 in)
Fine/0.25-3.2 mm
(0.01-0.13 in)
Very fine (micro)/0.15-0.38 mm
(0.01-0.02 in)
Source: Crites and Tchobanoglous, 1998.
The use of fine screens produces removal
characteristics similar to primary sludge removal in
primary sedimentation. Fine screens are capable of
removing 20 to 35 percent suspended solids and
BOD5. Fine screens may be either fixed or
movable, but are permanently set in a vertical,
inclined, or horizontal position and must be cleaned
by rakes, teeth, or brushes.
Communitors and Grinders
Figure 4 depicts a typical comminutor. When
designing a comminutor, headloss should be
considered. Headloss through a comminutor is
usually in the range of a few centimeters to 0.9 m (3
ft). Therefore, the manufacturer's ratings should be
decreased by 70 to 80 percent to account for
clogging of the screen, since manufacturer's
headloss characteristics are usually based on clean
water flow (Crites and Tchobanoglous, 1998).
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TABLE 3 DESIGN CRITERIA FOR
MECHANICALLY CLEANED BAR
SCREENS
Design Criteria
Item
Metric Units English Units
Bar width
Bar depth
Clear spacing
between bars
Slope from
vertical
Approach
velocity
Allowable
Headless
5-15 mm
25-40 mm
15-75 mm
0-30 degrees
0.6-1.0 m/s
150 mm
0.2-0.6 in
1.0-1. 5 in
0.6-3.0 in
0-30 degrees
2.0-3.25 ft/s
6 in
Source: WEF, 1998.
When a comminution device is installed upstream
of a grit removal device, the teeth of the
comminutor are subject to high wear and tear.
Rock traps are recommended to prolong the life of
the comminutor. In addition, a bypass manual bar
rack should be installed in the event that flow rates
exceed the comminutor capacity or there is a
mechanical failure.
Grit Removal
With respect to grit removal systems, grit is
traditionally defined as particles larger than 0.21
mm (0.008 in) (65 mesh) and with a specific gravity
of greater than 2.65 (U.S. EPA, 1987). Equipment
design was traditionally based on removal of 95
percent of these particles. However, with the recent
recognition that smaller particles must be removed
to avoid damaging downstream processes, many
modern grit removal designs are capable of
removing up to 75 percent of 0.15 mm (0.006 in)
(100 mesh) material.
Aerated Grit Chamber
Aerated grit chambers are typically designed to
remove particles of 70 mesh (0.21 mm/0.008 in) or
larger, with a detention period of two to five
minutes at peak hourly flow. When wastewater
flows into the grit chamber, particles settle to the
bottom according to their size, specific gravity, and
the velocity of roll in the tank. A velocity that is too
high will result in lower grit removal efficiencies,
while a velocity that is too low will result in
increased removal of organic materials. Proper
adjustment of air velocity will result in nearly 100
percent removal of the desired particle size and a
well-washed grit.
Design considerations for aerated grit chambers
include the following (WEF 1998):
Motor and Gear Drive
Influent
Source: Reynolds/Richards, 1996.
FIGURE 4 TYPICAL COMMINUTOR INSTALLATION
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Air rates typically range from 0.3 to 0.7
m3/m»min (3 to 8 ftVftrmin) of tank length.
A typical minimum hydraulic detention
time at maximum instantaneous flow is two
minutes.
Typi cal 1 ength-to-wi dth rati o i s 2.5:1 to 5:1.
Tank inlet and outlet are positioned so the
flow is perpendicular to the spiral roll
pattern.
Baffles are used to dissipate energy and
minimize short circuiting.
Vortex-Type Grit Chamber
Two designs of vortex grit units exist: chambers
with flat bottoms and a small opening to collect
grit; and chambers with a sloping bottom and a
large opening into the grit hopper. Flow into a
vortex-type grit system should be straight, smooth,
and streamlined. The straight inlet channel length
is typically seven times the width of the inlet
channel, or 4.6 m (15 ft), whichever is greater. The
ideal velocity range in the influent is typically 0.6 to
0.9 m/s (2 to 3 ft/s) at 40 to 80 percent of peak
flow. A minimum velocity of 0.15 m/s (0.5 ft/s)
should be maintained at all times, because lower
velocities will not carry grit into the grit chamber
(WEF, 1998).
Detritus Tank
of the target grit particles and the flow control
section-depth relationship. An allowance for inlet
and outlet turbulence is added. The cross sectional
area of the channel is determined by the rate of flow
and the number of channels. Allowances are made
for grit storage and grit removal equipment. Table
4 lists design criteria for horizontal flow grit
chambers.
TABLE 4 HORIZONTAL FLOW GRIT
CHAMBER DESIGN CRITERIA
Design Criteria
Item
Range Metric
(English)
Typical
Metric
(English)
Detention time
Horizontal velocity
Settling velocity1:
50-mesh
100-mesh
Headless (% of
channel depth)
Inlet and outlet
length allowance
45-90 s
0.24-0.4 m/s
(0.8-1.3 ft/s)
60s
0.3 m/s
(1.0 ft/s)
2.8-3.1 m/min
(9.2-1 0.2 ft/min)
0.6-0.9 m/min
(2.0-3.0 ft/min)
2.9 m/min
(9.6 ft/min)
0.8 m/min
(2. 5 ft/min)
30-40%
25-50%
36%2
30%
11f the specific gravity of the grit is significantly less than 2.65,
lower velocities should be used.
2For Parshall flume control.
Source: Crites and Tchobanoglous, 1998.
Detritus tanks are designed to keep horizontal
velocity and turbulence at a minimum while
maintaining a detention time of less than one
minute. Proper operation of a detritus tank depends
on well-distributed flow into the settling basin.
Allowances are made for inlet and outlet turbulence
as well as short circuiting by applying a safety
factor of 2.0 to the calculated overflow rate.
Horizontal Flow Grit Chamber
Horizontal flow grit chambers use proportional
weirs or rectangular control sections to vary the
depth of flow and keep the velocity of the flow
stream at a constant 0.3 m/s (1 ft/s). The length of
the grit chamber is governed by the settling velocity
PERFORMANCE
The use of screening and grit removal systems is
well documented. The performance of bar screens
varies depending on the spacing of the bars. Table
5 lists typical screening quantities for various screen
sizes.
The quantity of screenings depends on the length
and slope of the collection system and the presence
of pumping stations. When the collection system is
long and steep or when pumping stations exist,
fewer screenings are produced because of
disintegration of solids. Other factors that affect
screening quantities are related to flow, as quantities
generally increase greatly during storm flows. Peak
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TABLE 5 SCREENING REMOVAL
QUANTITIES
Screen Size
Screenings Quantity
m3/106m3 ft3/Mgal
13 mm (0.5 in)
38 mm (1.5 in)
60
11.2
8
1.5
Source: Reynolds and Richards, 1996.
daily removals may vary by a 20:1 ratio on an
hourly basis from average flow conditions.
Combined collection systems may produce several
times the coarse screenings produced by separate
collection systems.
Given the complexity of collection systems and
types of materials that may be considered "grit," the
quantity and characteristics of grit removed from
wastewater will vary. Grit quantity is influenced by
the type and condition of the collection system, the
characteristics of the drainage area, garbage
disposal methods, the slope of the collection
system, and the efficiency of the grit removal
system. The quantity of grit may vary from 0.004
to 0.21 m3/103m3 (0.5 to 30 ft3/Mgal) (Crites and
Tchobanoglous, 1998). The performance of a grit
removal system may be enhanced if actual plant
data is used when designing a new grit removal
system.
Table 6 depicts quantities of screenings and grit
from various wastewater treatment plants. There
are no obvious trends associated with design flow
through a plant and grit and screenings removal
quantities. Differences in wastewater characteristics
and equipment efficiencies make a correlation
between flow and quantities of screenings and grit
removed nearly impossible.
OPERATION AND MAINTENANCE
Screening
Manually cleaned screens require frequent raking to
prevent clogging. Cleaning frequency depends on
the characteristics of the wastewater entering a
plant. Some plants have incorporated screening
devices, such as basket-type trash racks, that are
manually hoisted and cleaned. Mechanically
cleaned screens usually require less labor for
operation than manually cleaned screens because
screenings are raked with a mechanical device
rather than by facility personnel. However, the rake
teeth on mechanically cleaned screens must be
routinely inspected because of their susceptibility to
breakage and bending. Drive mechanisms must also
be frequently inspected to prevent fouling due to grit
and rags. Grit removed from screens must be
disposed of regularly.
TABLE 6 GRIT AND SCREENINGS REMOVAL QUANTITIES AT VARIOUS PLANTS
Plant Location
Uniontown, Pennsylvania
East Hartford, Connecticut
Duluth, Minnesota
Lamberts Point Water Pollution Control Plant,
Norfolk, Virginia
Village Creek Wastewater Treatment Plant,
Ft. Worth, Texas
County of Milwaukee, Wisconsin, South Shore
Twin Cities Metro Wastewater Treatment Plant,
Minnesota
Chicago, Illinois (Northside)
Flow, m3/d
(MGD)
11,400(3.0)
15,100(4.0)
45,400(12.0)
75,700 (20.0)
170,000(45.0)
454,000(120.0)
825,000(218.0)
1,260,000(333.0)
Grit, m3/103m3
(ft3/Mgal)
0.074(10.5)
0.017(2.4)
0.006(0.8)
0.034 (4.85)
0.009(1.29)
0.003 (0.48)
0.034 (4.82)
0.003(0.41)
Screenings,
m3/103m3(ft3/Mgal)a
0.006(0.9)
0.009(1.33)
0.004 (0.56)
0.009(1.20)
0.005 (0.72)
0.004 (0.60)
0.008(1.15)
0.006 (0.83)
aft3/Mgal=cubic feet per million gallons
Source: WEF, 1998.
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Communitors and Grinders
Comminutors can create operation and maintenance
problems in downstream processes. While
shredding solids eliminates the problem of handling
screening materials at the head of the plant,
problems inherent to the use of communitors, such
as the decreased quality of digested biosolids and
the accumulation of rags on air diffusers, have
lessened the popularity of this technology.
Comminutors are generally avoided in new designs
and are being removed from many existing plants.
Grinders are greatly affected by grit and other
solids. As such, they require routine inspection
every six months and replacement of bearings and
cutter teeth every one to three years.
Grit Removal
equipment, and applicability of various technologies
to different situations.
Graphs can be used to relate average wastewater
flow through a plant to a specific technology.
Figure 5 shows a graph relating wastewater flow to
the cost of a horizontal shaft rotary screen. Costs
include construction, operation, and maintenance.
Contractor bids on a recent wastewater project
ranged from $150,000 to $400,000 for Rotary Drum
Screenings Removal and from $150,000 to
$208,800 for Vortex-type Grit Removals.
Generally, equipment costs will be close for each
bid. However, the overall costs vary for each
treatment process/project because of differences in
construction approaches by the contractors.
REFERENCES
Collected grit must be removed from the chamber,
dewatered, washed, and conveyed to a disposal site.
Some smaller pi ants use manual methods to remove
grit, but grit removal is usually accomplished by an
automatic method. The four methods of automatic
grit removal include inclined screw or tubular
conveyors, chain and bucket elevators, clamshell
buckets, and pumping. A two-step grit removal
method is sometimes used, where grit is conveyed
horizontally in a trough or channel to a hopper,
where it is then elevated from the hopper to another
location.
Aerated grit chambers use a sloped tank bottom in
which the air roll pattern sweeps grit along the
bottom to the low side of the chamber. A
horizontal screw conveyor is typically used to
convey settled grit to a hopper at the head of the
tank. Another method to remove grit from the
chamber floor is a chain and flight mechanism.
Once removed from the chamber, grit is usually
washed with a hydrocy clone or grit classifier to ease
handling and remove organic material. The grit is
then conveyed directly to a truck, dumpster, or
storage hopper. From there, the grit is taken to a
landfill or other disposal facility.
COSTS
The cost of screens and grit removal systems varies
depending on the type of technology used, ancillary
Other Related Fact Sheets
at
O
Wastewater Flow MGD
Construction Cost
Source: Martin, 1991.
FIGURE 5 COST CURVE FOR
HORIZONTAL SHAFT ROTARY SCREEN
Sewer Lift Station
EPA 832-F-00-073
September 2000
Sewer Cleaning & Inspection
EPA832-F-99-031
September 1999
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Screens
EPA 832-F-99-040
September 1999
Other EPA Fact Sheets can be found at the
following web address:
http://www.epa.gov/owm/mtb/mtbfact.htm
1. C rites, R. and G. Tchobanoglous, 1998.
Small and Decentralized Wastewater
Management Systems. The McGraw-Hill
Companies. Boston, Massachusetts.
2. Martin, E.J. and E.T. Martin, 1991.
Technologies for Small Water and
Wastewater Systems. Van Nostrand
Reinhold. New York, New York.
3. Qasim, S., 1994. Wastewater Treatment
Plants: Planning, Design and Operation.
Technomic Publishing Co., Lancaster,
Pennsylvania.
4. Reynolds, T. and P. Richards, 1996. Unit
Operations and Processes in Environmental
Engineering. PWS Publishing Company.
Boston, Massachusetts.
5. Urquhart, L., 1962. Civil Engineering.
Costs include construction, operation, and
maintenance. Specific cost data from
contractor bids.
6. Water Environment Federation, 1998.
Design ofMunicipal Wastewater Treatment
Plants. Water Environment Federation.
Alexandria, Virginia.
ADDITIONAL INFORMATION
H.I.L. Technology, Inc.
94 Hutchins Drive
Portland, ME 04102
Lakeside Equipment Corporation
1022 E. Devon Ave.
Bartlett, IL60103
National Small Flows Clearing House
at West Virginia University
P.O. Box 6064
Morgantown, WV 26506
Parkson Corporation
2727 NW 62nd Street
P.O. Box 408399
Fort Lauderdale, FL 33340-8399
U.S. Filter
Link-Belt Headworks Products
100 High Point Drive - Suite 101
Chalfont, PA 18914
The mention of trade names or commercial products
does not constitute endorsement or recommendation
for use by the U.S. Environmental Protection
Agency (EPA).
Office of Water
EPA832-F-03-011
June 2003
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
1200 Pennsylvania Avenue, NW
Washington, D.C. 20460
MTB
Excellence in compliance through optimal technical solutions
MUNICIPAL TECHNOLOGY BRAN^fff
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