United States

Environmental Protection

Agency

Biosolids Technology Fact Sheet

Gravity Thickening

DESCRIPTION

Thickening is the process by which biosolids are

condensed to produce a concentrated solids product

and a relatively solids-free supernatant. Thickening

wastewater solids reduces the volume of residuals,

improves operation, and reduces costs for

subsequent storage, processing, transfer, end use, or

disposal. For example, thickening liquid-solids

(slurry) from 3 to 6 percent will reduce the volume

by 50 percent. 

There are several different methods for thickening

biosolids, including dissolved air floatation (DAF),

centrifugal thickening, gravity belt thickening, and

gravity thickening. Gravity thickening uses the

natural tendency of higher-density solids to settle

out of liquid to concentrate the solids. 

Gravity thickeners consist of a circular tank

(usually with a conical bottom) that is fitted with

collectors or scrapers at the bottom. Primary and/or

secondary solids are fed into the tank through a

center well, which releases the solids at a low

velocity near the surface of the tank. The solids

settle to the bottom of the tank by gravity, and the

scrapers slowly move the settled, thickened solids

to a discharge pipe at the bottom of the tank. A vnotch weir located at the top of the tank allows the

supernatant to return to a clarifier. In addition,

many systems also use a skimmer to collect and

remove any floatables and grease that have

accumulated at the top of the tank.

The biosolids concentration and thickening that

occurs in the tank is achieved through three

different settling processes, which include gravity

FIGURE 1 GRAVITY THICKENER

Source: U.S. EPA, 1987.
settling, hindered settling, and compaction settling.

Gravity settling occurs when solid particles travel

downward due to their weight. Settlement

continues as solids begin to concentrate near the

bottom of the tank, but the settlement rate decreases

as the solids concentrations increase. This is

known as “hindered settling.” Compaction settling

occurs when bottom solids are further concentrated

due to the pressure of solids on top of them. 

Solids at the bottom of the tank can reach as high as

15 percent total solids (TS). A more typical result

is 4 to 6 percent TS. Liquid at the surface of the

tank is nearly clear, with suspended solids

concentrations as low as 200 mg/L. The transition

point between clear liquid and thickening solids

that develops in the middle of the tank is called a

"solids blanket." 

APPLICABILITY

Solids thickening can prove beneficial at most

wastewater treatment plants. However, solids

thickening characteristics are unique to each

wastewater treatment plant and facilities should

develop a biosolids management plan to evaluate

thickening technology options, including gravity

thickening, DAF, centrifuge thickening, and gravity

belt thickening. All of these technologies are

commonly used, but they vary in performance

criteria, such as solids concentration achieved,

solids capture, odors, power demand, labor

requirements, sensitivity to changing temperatures,

and solids characteristics. In addition, pilot testing

should be performed prior to design. Evaluation of

gravity thickening as an option should include

evaluation of the solids to be removed, as discussed

below.

Primary solids are more easily thickened using

gravity than are secondary solids. Primary solids

tend to settle quickly and form a thick solids layer.

The settled solids can be pumped out of the tank

and do not require the addition of chemicals for

extra thickening. Many field and laboratory tests

have shown that introduction of waste activated

solids with primary solids can aid in the settling

process by increasing the retention time before the

solids start to produce gas and rise to the surface.

One potential disadvantage of this practice is that

biosolids odor testing at some plants showed

greater odors when primary and waste activated

solids were co-thickened. 

In contrast to primary solids, secondary solids have

a large surface area per unit mass, which results in

low settling rates and resistance to compaction.

Thus, the use of gravity thickening alone usually

cannot achieve the required thickening. Therefore,

gravity thickening is usually accompanied by some

other method of thickening for secondary solids. 

Gravity thickening reduces the downstream

requirements for further sludge processing, and thus

it is often used prior to anaerobic digestion or lime

stabilization. Historically, thickening was not

employed prior to aerobic digestion because it was

difficult to supply enough oxygen to the digestion

process when the total solids content was greater

than 2 percent. Recent improvements in aeration

equipment have allowed some plants to aerobically

digest 3-4 percent TS. Thickening prior to storage

or transportation offsite is also common.

ADVANTAGES AND DISADVANTAGES

Advantages

• Gravity thickening equipment is simple to

operate and maintain. 

• Gravity thickening has lower operating

costs than other thickening methods such as

DAF, gravity belt or centrifuge thickening.

For example, a well run gravity thickening

operation will save costs incurred in

downstream solids handlg steps.

In addition, facilities that land apply liquid

biosolids can benefit from thickening in several

ways, as follows:

• Truck traffic at the plant and the farm site

can be reduced;
• Trucking costs can be reduced;

• Existing storage facilities can hold more

days of biosolids production;

• Smaller storage facilities can be used;

• Less time will be required to transfer solids

to the applicator vehicle and to incorporate

or surface apply the thickened solids; and

• Crop nitrogen demand can be met with

fewer passes of the applicator vehicle,

reducing soil compaction.

Disadvantages

• Scum build-up can cause odors. This buildup, which can occur because of long

retention times, can also increase the torque

required in the thickener. Finally, scum

build-up is unsightly. 

• Grease may build up in the lines and cause

a blockage. This can be prevented by quick

disposal or a back flush. 

• Septic conditions will generate sulfur-based

odors. This can be mitigated by minimizing

detention times in the collection system and

at the plant, or by using oxidizing agents.

• Supernatant does not have solids

concentrations as low as those produced by

a DAF or centrifuge thickener. Belt

thickeners may produce supernatant with

lower solids concentrations depending on

the equipment and solids characteristics.

• More land area is needed for gravity

thickener equipment than for a DAF,

gravity belt, or centrifuge thickener.

• Solids concentrations in the thickened

solids are usually lower than for a DAF,

gravity belt, or centrifuge thickener.

DESIGN CRITERIA

Preliminary Bench-Scale Study

If representative samples of wastewater solids are

available, laboratory bench scale testing should be

conducted to determine design parameters. This

testing may be conducted by equipment

manufacturers or consulting engineers. The pilot

testing can help to determine:

• The optimum aspect ratio (depth and

diameter) of the tank.

• The maximum detention time before septic

conditions form gas bubbles and create

floatation, which causes odors and inhibits

gravity settling.

• The effectiveness of pickets (vertical bars

on the bottom collector) in helping to vent

gases from the biosolids, thus enhancing

settlement.

• The effectiveness of polymer or oxidizing

agents in improving settling and

consolidation.

• The efficiency of thickening primary and

secondary solids separately vs. thickening

them together.

• The anticipated solids concentration after

thickening and the design criteria for

subsequent biosolids processing.

Specific Design Criteria

Nearly static conditions are required in gravity

thickening tanks in order to induce settlement.

Therefore, the tank design must ensure a smooth,

continuous flow of liquid at the center well, the

weirs, and the solids removal point. Gravity

thickener designs usually include circular tanks, 3-4

m (10-13 ft) deep and up to 25 m (82 ft) in

diameter. The dimensions are based on prediction

of settling due to gravity alone. To assist solids

transport, gravity thickener bottoms are

constructed with a floor slope between 1:6 and 1:3.

 Rectangular units have been used as gravity

thickeners. For example, obsolete rectangular

clarifiers have been converted for use as gravity

thickeners. However, rectangular units generally

prove to be unsatisfactory. 
Thickened solids are removed from the tank by

pumping them from below the solids blanket. In

some systems, an infrared (light path) blanket

detector is used to determine when the blanket

starts to rise. In other systems, the pumps are on

timers, and thickened solids are drawn off at regular

intervals.

Gravity thickeners are usually operated at collector

tip speeds between 0.08-0.1 m/s (15 and 20 ft/min).

This gentle action is intended to break down any

matrix (flocs) that might form, while minimizing

agitation and resuspension of the solids. However,

moving the solids towards the discharge pipe at the

bottom of the tank can require a large amount of

torque. Therefore, gravity thickener mechanisms

tend to be heavier in construction than clarifiers to

compensate for these high torques. If high solids

concentrations or high viscosities are anticipated,

gravity thickeners may be supplied with a lifting

mechanism, which raises the thickener collector

arms above the solids blanket when torques are

extremely high. As solids are removed from the

tank and torque is reduced, the lift will lower the

mechanism back into the solids blanket until it

reaches its original position. 

Gravity thickeners may be supplied with or without

pickets, vertical pipes, or angles attached to the

thickener arm. These devices can aid in releasing

gas from the solids blanket, which reduces the

potential for floating solids. This may be useful if

the biosolids tend to gasify. However, there is

concern that pickets may create eddy currents and

disturb the static conditions needed for settling.

Thickened solids removal is best accomplished

with positive displacement pumps, which should be

located as close as possible to the solids draw-off

point. The pumps should be positioned so as to

create a positive head on the suction side

(sometimes under the gravity thickener).

Progressive cavity pumps may be used for drawoff, but they should only be used if there is efficient

grit removal at the head of the plant, because grit

can cause excessive wear on the pump’s rotor and

stator.

Ideally, solids removal will be done continuously.

If solids removal is automated, the pump speed

should be adjusted to maximize running time while

maintaining good supernatant quality and avoiding

“ratholing.” Ratholing occurs if thickened solids

are drawn off too quickly, and results in a cone of

depression at the draw-off point. This phenomenon

short-circuits the draw-off and results in poorly

consolidated solids being removed, while

concentrated solids remain in the tank.

There are two methods for handling in-house scum.

The first option is pumping the skimmings to a

digester or incinerator immediately after removal

from the thickeners. The second option is pumping

the skimmings to a thickener and consolidating

them for subsequent handling. This can be difficult

because gravity thickeners are rarely equipped

with skimming equipment that is able to handle

large amounts of scum - particularly accumulated

scum that may have congealed and hardened.

PERFORMANCE

The operator can optimize thickening by

monitoring the quality of the supernatant and the

total solids concentration in the thickened solids,

while also tracking feed rate, pumping times and

rates, and solids blanket depth, and inspecting the

tank surface for potential problems, including gas

bubbles and odors. Gas bubbles cling to solid

particles, increasing the solids’ buoyancy and

reducing their settling. Odors may be the result of

septic conditions, which can also lead to the

formation of gas bubbles. The operator should also

check for turbulence in the tank. Turbulence can

cause the solids to either miss the weirs, or to

become unsettled. 

Table 1 summarizes conventional gravity

thickening performance for various types of

wastewater solids. As shown in the table, the

addition of chemicals may increase thickening.

However, the effects on both solids and supernatant

downstream processes must be considered. Metal

salts of bacteriostatic agents may have inhibitory

effects on the digestion process. The use of

polymer in the thickener may have positive or

negative effects on downstream dewatering.
Operators of gravity thickeners observe that

operating with a high solids blanket results in better

thickening. However, if operating at a higher

blanket depth creates longer detention times, gases

may begin to form. A reasonable detention time for

primary solids is 24 to 48 hours. Combinations of

primary and secondary solids may be retained

between 18 and 30 hours. 

Operators also observe that the highest solids

concentrations are achieved during colder

wastewater temperatures. Again, the reduction in

biological activity prolongs the period before

formation of gas bubbles. 

Table 2 lists several other factors that may affect

gravity thickener performance.

OPERATION AND MAINTENANCE

Operators should monitor operating parameters in

the gravity thickener to ensure optimum

performance. Routine monitoring should include

the following:

• Sample the feed solids and the thickened

solids once per day and analyze for TS. If

thickened solids are not thick enough, it

could be due to a high overflow rate, a high

thickened solids pumping rate, or short

circuiting of solids flow through the tank.

• Sample the supernatant once per day and

analyze for SS.

• Sample the supernatant two times per week

and analyze for BOD.

• Record the flow into the gravity thickener

and the volume of solids removed.

• Record the type and amount of chemicals

used.

• Measure the temperature in the thickener

and the influent to the thickener once per

shift.

• Record the depth of the solids blanket and

note the transition in the solids liquid

interface once per shift. 

Weekly, monthly, and yearly maintenance should

also be performed on the unit. Summaries of the

recommended maintenance procedures are

provided below.

Weekly Maintenance

• Check all oil levels and ensure the oil fill

cap vent is open.

• Check condensation drains and remove any

accumulated moisture.

• Examine drive control limit switches.

TABLE 1 PERFORMANCE OF

CONVENTIONAL GRAVITY

THICKENING

Type of Solids Feed

(%TS)

Thickened

Solids (%TS)

Primary (PRI) 2-7 5-10

Trickling Filter (TF) 1-4 3-6

Rotating Biological

Contactor (RBC)

1-3.5 2-5

Waste Activated Solids

(WAS)

0.2-1 2-3

PRI + WAS 0.5-4 4-7

PRI + TF 2-6 5-9

PRI + RBC 2-6 5-8

PRI + Lime 3-4.5 10-15

PRI + (WAS+ Iron) 1.5 3

PRI + (WAS+

Aluminum salts)

0.2-0.4 4.5-6.5

Anaerobically digested

PRI + WAS

4 8

Source: WEF,1987.
• Visually examine the skimmer to ensure

that it is in proper contact with the scum

baffle and the scum box.

• Visually examine instrumentation and clean

probes.

Monthly Maintenance

• Inspect skimmer wipers for wear. 

• Adjust drive chains or belts.

Yearly Maintenance

• Disassemble the drive and examine all

gears, oil seals, and bearings.

• Check oil for the presence of metals, which

may be a warning sign of future problems.

• Replace any part with an expected life of

less than one year.

TABLE 2 FACTORS AFFECTING GRAVITY THICKENING PERFORMANCE

Factor Effect

Nature of the solids feed Affects the thickening process because some solids thicken more easily than

others.

Freshness of feed solids High solids age can result in septic conditions.

High volatile solids concentrations Hamper gravity settling due to reduced particle specific gravity.

High hydraulic loading rates Increase velocity and cause turbulence that will disrupt settling and carry the

lighter solids past the weirs.

Solids loading rate If rates are high, there will be insufficient detention time for settling. If rates are

too low, septic conditions may arise.

Temperature and variation in

temperature of thickener contents 

High temperatures will result in septic conditions. Extremely low temperatures

will result in lower settling velocities. If temperature varies, settling decreases

due to stratification. 

High solids blanket depth Increases the performance of the settling by causing compaction of the lower

layers, but it may result in solids being carried over the weir.

Solids residence time An increase may result in septic conditions. A decrease may result in only

partial settling.

Mechanism and rate of solids

withdrawal 

Must be maintained to produce a smooth and continuous flow. Otherwise,

turbulence, septic conditions, altered settling, and other anomalies may occur.

Chemical treatment Chemicals - such as potassium permanganate, polymers, or ferric chloride - may

improve settling and/or supernatant quality.

Presence of bacteriostatic agents or

oxidizing agents 

Allows for longer detention times before anaerobic conditions create gas bubbles

and floating solids.

Cationic polymer addition Helps thicken waste-activated solids and clarify the supernatant.

Use of metal salt coagulants Improves overflow clarity but may have little impact on underflow concentration.

Source: Parsons, 2003.
Chemical Addition

Chemical additives may be used to enhance

performance and improve solids recovery and

supernatant quality. Table 3 shows typical

chemical dosages used in thickening. Polymer

dosage will vary between 2.5-6 g/kg (5-12 lbs/ton)

of dry solids.

Odor Control

Odors are frequently caused by long retention times

or by attempts to store solids in the thickener. The

principle producers of odors are hydrogen sulfide

and ammonia, but odorous organics such as

skatoles and indoles may be present. One

mechanism for controlling odors is to increase the

solids retention time in the activated solids system

to produce less biologically active solids that

generate fewer odors. Make-up water should be

chlorinated to obtain a 1 mg/L residual in the

supernatant for combating the formation of

hydrogen sulfide. The presence of some dissolved

oxygen in the thickener will reduce anaerobic

activity that produces the most serious odor

producing compounds. Other odor-reducing

chemicals, such as ozone, potassium permanganate,

chlorine, and hydrogen peroxide, may be effective,

whereas efforts to chemically mask odors lead to

unpredictable results. In worst cases, gravity

thickeners may be covered with domes to contain

the odorous atmosphere. The foul air can then be

scrubbed using a commercially available odor

control system.

Troubleshooting

Table 4 provides a listing of typical operating

problems, their causes, and possible solutions.

COSTS

Any type of thickening equipment will have a

significant capital cost. A biosolids management

plan should be developed as part of a

comprehensive economic analysis. It should

identify both processing and end use or disposal

alternatives. Odors at the processing facility and

the end use locations should also be considered. If

further processing, end use, and/or disposal costs

can be reduced by thickening, the cost of

construction, operation, and maintenance may be

justified. Many smaller facilities find it more

economical to haul thickened liquid to a larger

wastewater treatment plant for dewatering than to

install and operate their own dewatering

equipment.

Capital costs for gravity thickeners vary based on

the site conditions and the characteristics of

wastewater solids. If the solids have poor settling

characteristics, a larger tank and collector will be

required. If pilot testing shows that solids are well

suited to consolidation and compaction, the

required tank may be smaller, but additional costs

may be incurred for instrumentation and

automation. Table 5 estimates construction costs

based on data collected by the U.S. EPA during the

construction grants program. Costs are adjusted for

inflation. 

Gravity thickening is a relatively inexpensive

operation. Unit costs range from $0.30 to $3.00 per

dry ton. Larger facilities (>100 MGD) will incur

costs at the lower end of this range due to

economies of scale. Operation and maintenance

costs include power usage and mechanical

maintenance. The addition of chemicals may add

additional costs. Operator time can be reduced if

instrumentation and automation are used. Table 6

estimates annual staff hours based on conventional

thickener area.

TABLE 3 CHEMICAL DOSE RATES

Type of

Solids

FeCl3

(mg/L)

CaO

(mg/L)

K2MnO5

(mg/L)

Primary 1-2 6-8 10-40

Primary +

tricking

filter

2-3 6-8 10-40

Primary +

WAS

1.5-2.5 7-9 10-40

Source: WPCF, 1987, WEF, 1996.
REFERENCES

Other Related Fact Sheets

Centrifugal Dewatering/Thickening

EPA 832-F-00-053

September 2000

Filter Belt Press Dewatering

EPA 832-F-00-057

September 2000

Land Application of Biosolids

EPA 832-F-00-064

September 2000

Odor Control at Biosolids Management Facilities

EPA 832-F-00-067

September 2000

TABLE 4 GRAVITY THICKENING TROUBLESHOOTING GUIDE

Indicators Probable Cause Check or Monitor Solution

Septic odor, rising solids Thickened solids pumping

rate is too slow, thickener

overflow rate is too low

Check thickened solids

pumping system for

proper operation, check

thickener collection

mechanism for proper

operation

Increase pumping rate of

thickened solids, increase

influent flow to thickener-a

portion of the secondary effluent

may be pumped to thickener to

bring overflow rate to 400-600

GPD/sq ft., chlorinate influent

solids

Thickened solids not

thick enough

Overflow rate is too high,

thickened solids pumping

rate is too high, short

circuiting of flow through tank

Check overflow rate, use

dye or other tracer to

check for circulation

Decrease influent solids flow

rate, decrease pumping rate of

thickened solids, check effluent

weirs: repair or re-level, check

influent baffles: repair or relocate

Torque overload of

solids collecting

mechanism

Heavy accumulation of

solids, foreign object jammed

in mechanism, improper

alignment of mechanism

Probe along front of

collector arms

Agitate solids blanket in front of

collector arms with water jets,

increase solids removal rate,

attempt to remove foreign object

with grappling device, if problem

persists drain thickener and

check mechanism for free

operation

Surging flow Poor influent pump

programming

Pump cycling Modify pump cycling, reduce flow

and increase time

Excessive biological

growths on surfaces and

weirs (slimes, etc.)

Inadequate cleaning program Frequent and thorough cleaning

of surfaces, apply chlorination

Oil Leak Oil seal failure Oil seal Replace seal

Noisy or hot bearing or

universal joint

Excessive wear, improper

alignment, lack of lubrication

Alignment, lubrication Replace, lubricate, or align joint

or bearing as required

Pump overload Improper adjustment of

packing, clogged pump

Check packing, check for

trash in pump

Adjust packing, clean pump

Fine solids particles in

effluent

Waste activated solids Portion of WAS in

thickener effluent

Better conditioning of the WAS

portion of the solids, thicken

WAS in a flotation thickener

Source: WEF, 1987.
Other EPA Fact Sheets can be found at the

following web address:

http://www.epa.gov/owm/mtb/mtbfact.htm.

1. Gabb, D., D.R. Williams, B.K. Horenstein,

P.J. Suto, A.L Chacon, and E.H.

McCormick, 1998. Waste Activated Sludge

Thickening: A Custom Fit for East Bay

Municipal Utility District. Water

Environment & Technology. 

2. Hentz, L. H., A.F. Cassel, and S. Conley,

2000. The Effects of Liquid Sludge Storage

on Biosolids Odor Emissions. WEF Annual

WEFTEC, 2000, Session 14. 

3. Nowak, M. J., 1996. Fast-Track Odor

Control. Operations Forum, Vol. 13, No. 8.

4. U.S. EPA, 1979. Process Design Manual:

Sludge Treatment and Disposal.

Environmental Protection Agency.

5. U.S. EPA, 1987. Design Manual:

Dewatering Municipal Wastewater Sludges.

Environmental Protection Agency.

6. Water Pollution Control Federation, 1987.

Operation and Maintenance of Sludge

Dewatering Systems: Manual of Practice

No. OM-8. Water Pollution Control

Federation, Water Task Force on Sludge

Thickening, Conditioning, and Dewatering.

7. WEF, 1996. Operation of Municipal

Wastewater Treatment Plants. WEF 5th ed.,

Vol. 3.

ADDITIONAL INFORMATION

Pennridge Wastewater Treatment Authority

Kevin Franks

180 Maple Avenue, P.O. Box 31

Sellersville, PA 18960-0031

East Bay Municipal Utility District

Bennett K Horenstein 

P.O. Box 24055 

375 11th Street, MS 702

Oakland, CA 94604

Warminster Wastewater Treatment Plant

George Pfeiffer 

Log College Drive 

Warminster, PA 18974

Middlesex County Utilities Authority

Victor Santamarino

P.O. Box B-1 

Sayreville, NJ 08872

TABLE 5 ESTIMATED CAPITAL

COSTS FOR GRAVITY THICKENERS

(ADJUSTED TO THE YEAR 2000)

Plant

Capacity

(MGD)

Construction

Costs ($)

Construction+

Eng.&Adm.* ($)

2 162,000 199,000

4 348,000 426,000

8 745,000 914,000

15 1,488,000 1,825,000

20 2,042,000 2,504,000

25 2,610,000 3,200,000

Source: U.S. EPA, 1979

Eng.&Adm. = Engineering and Administration

TABLE 6 GRAVITY THICKENER LABOR

REQUIREMENTS

Thickener

Surface

Area, sq. ft

Operation

Labor,

hr/yr

Maintenance

Labor, hr/yr

Total

>500 310 180 490

>1000 350 200 550

>2000 420 240 660

>5000 680 370 1050

Source: WPCF, 1987.
For more information contact:

Municipal Technology Branch

U.S. EPA

1200 Pennsylvania Avenue, NW

Mail Code 4204M

Washington, D.C. 20460

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-03-022

September 2003