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).

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

Source: Qasim, 1994. (0.25 in). The cutters are mounted on two drive

FIGURE 1 CABLE OPERATED BAR

SCREEN
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 jetinduced 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

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

Cleaned Bar

Screen

Designed to remove large solids, rags,

and debris.

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

Cleaned Bar

Screen

Designed to remove large solids, rags,

and debris.

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.
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.

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 upstream 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 wastewater 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.

Source: Crites and Tchobanoglous, 1998.

FIGURE 3 AERATED GRIT CHAMBER
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 MGD) 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, preaeration, 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.
Detritus Tank

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

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. 
• 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.

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).

TABLE 2 SCREENING DEVICE

CLASSIFICATION

Screening Device

Classification

Size Classification/Size

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 Fine Coarse/3-12.5 mm

(0.1-0.5 in)

Perforated Plate Fine Coarse/3-9.5 mm

(0.1-0.4 in)

Rotary Drum Fine Coarse/3-12.5 mm

(0.1-0.5 in)

Fine screen (mechanically cleaned)

Fixed Parabolic Fine/0.25-3.2 mm 

(0.01-0.13 in)

Rotary Drum Fine/0.25-3.2 mm 

(0.01-0.13 in)

Rotary Disk Very fine (micro)/0.15-0.38 mm

(0.01-0.02 in)

Source: Crites and Tchobanoglous, 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):

TABLE 3 DESIGN CRITERIA FOR

MECHANICALLY CLEANED BAR

SCREENS

Item

Design Criteria

Metric Units English Units

Bar width 5-15 mm 0.2-0.6 in

Bar depth 25-40 mm 1.0-1.5 in

Clear spacing

between bars 15-75 mm 0.6-3.0 in

Slope from

vertical 0-30 degrees 0-30 degrees

Approach

velocity 0.6-1.0 m/s 2.0-3.25 ft/s

Allowable

Headloss 150 mm 6 in

Source: WEF, 1998.

Source: Reynolds/Richards, 1996.

FIGURE 4 TYPICAL COMMINUTOR INSTALLATION
• Air rates typically range from 0.3 to 0.7

m

3

/m•min (3 to 8 ft3/ft•.min) of tank length.

• A typical minimum hydraulic detention

time at maximum instantaneous flow is two

minutes.

• Typical length-to-width ratio is 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

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

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.

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

TABLE 4 HORIZONTAL FLOW GRIT

CHAMBER DESIGN CRITERIA

Item

Design Criteria

Range Metric

(English)

Typical

Metric

(English)

Detention time 45-90 s 60 s

Horizontal velocity 0.24-0.4 m/s

(0.8-1.3 ft/s)

0.3 m/s

(1.0 ft/s)

Settling velocity1:

50-mesh 2.8-3.1 m/min

(9.2-10.2 ft/min)

2.9 m/min

(9.6 ft/min)

100-mesh 0.6-0.9 m/min

(2.0-3.0 ft/min)

0.8 m/min

(2.5 ft/min)

Headloss (% of

channel depth) 30-40% 36%2

Inlet and outlet

length allowance 25-50% 30%

1

If 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.
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 m

3

/103m

3

 (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 5 SCREENING REMOVAL

QUANTITIES

Screen Size

Screenings Quantity

m3/106 m3ft3/Mgal

13 mm (0.5 in) 60 8

38 mm (1.5 in) 11.2 1.5

Source: Reynolds and Richards, 1996.

TABLE 6 GRIT AND SCREENINGS REMOVAL QUANTITIES AT VARIOUS PLANTS

Plant Location Flow, m3

/d

(MGD)

Grit, m3/103m3

(ft3/Mgal)

Screenings,

m3/103m3 (ft3/Mgal)a

Uniontown, Pennsylvania 11,400 (3.0) 0.074 (10.5) 0.006 (0.9)

East Hartford, Connecticut 15,100 (4.0) 0.017 (2.4) 0.009 (1.33)

Duluth, Minnesota 45,400 (12.0) 0.006 (0.8) 0.004 (0.56)

Lamberts Point Water Pollution Control Plant,

Norfolk, Virginia 75,700 (20.0) 0.034 (4.85) 0.009 (1.20)

Village Creek Wastewater Treatment Plant,

 Ft. Worth, Texas 170,000 (45.0) 0.009 (1.29) 0.005 (0.72)

County of Milwaukee, Wisconsin, South Shore 454,000 (120.0) 0.003 (0.48) 0.004 (0.60)

Twin Cities Metro Wastewater Treatment Plant,

Minnesota 825,000 (218.0) 0.034 (4.82) 0.008 (1.15)

Chicago, Illinois (Northside) 1,260,000 (333.0) 0.003 (0.41) 0.006 (0.83)

a

ft3/Mgal=cubic feet per million gallons

Source: WEF, 1998.
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

Collected grit must be removed from the chamber,

dewatered, washed, and conveyed to a disposal site.

Some smaller plants 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 hydrocyclone 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

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

Other Related Fact Sheets

Sewer Lift Station

EPA 832-F-00-073

September 2000

Sewer Cleaning & Inspection

EPA 832-F-99-031

September 1999

0.01

0.1

1

10

0.1 1 10 100

Wastewater Flow MGD

Construction Cost-Millions of Dollars

0.001

0.01

0.1

1

Annual O&M

Construction Cost Annual O&M

Source: Martin, 1991.

FIGURE 5 COST CURVE FOR

HORIZONTAL SHAFT ROTARY SCREEN
For more information contact:

Municipal Technology Branch

U.S. EPA

Mail Code 4204

1200 Pennsylvania Avenue, NW

Washington, D.C. 20460

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. Crites, 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 of Municipal 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, IL 60103

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

EPA 832-F-03-011

June 2003