xvEPA
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 v-
notch 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
HANDRAILING
1"GROUT
INFLUENT
PIPE
BAFFLE
SUPPORTS
TURNTABLE
EFFLUENT
WEIR
MAX. WATER SURFACE
1'3"MIN.
114" BLADE
CLEARANCE
ADJUSTABLE
SCRAPER SQUEEGEES
BLADES
SLUDGE
HOPPER
1 ft = 0.305 m
1 in = 2.54 cm
HOPPER
SCRAPERS
FIGURE 1 GRAVITY THICKENER
Source: U.S. EPA, 1987.
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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;
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• 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 build-
up, 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.
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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 (floes) 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 draw-
off, 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.
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TABLE 1 PERFORMANCE OF
CONVENTIONAL GRAVITY
THICKENING
Type of Solids
Primary (PRI)
Trickling Filter (TF)
Rotating Biological
Feed
(%TS)
2-7
1-4
1-3.5
Thickened
Solids (%TS)
5-10
3-6
2-5
Contactor (RBC)
Waste Activated Solids
(WAS)
PRI + WAS
PRI + TF
PRI + RBC
PRI + Lime
PRI + (WAS+ Iron)
PRI + (WAS+
Aluminum salts)
Anaerobically digested
PRI + WAS
0.2-1
0.5-4
2-6
2-6
3-4.5
1.5
0.2-0.4
4
2-3
4-7
5-9
5-8
10-15
3
4.5-6.5
8
Source: WEF, 1987.
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.
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Factor
TABLE 2 FACTORS AFFECTING GRAVITY THICKENING PERFORMANCE
Effect
Nature of the solids feed
Freshness of feed solids
High volatile solids concentrations
High hydraulic loading rates
Solids loading rate
Temperature and variation in
temperature of thickener contents
High solids blanket depth
Solids residence time
Mechanism and rate of solids
withdrawal
Chemical treatment
Presence of bacteriostatic agents or
oxidizing agents
Cationic polymer addition
Use of metal salt coagulants
Affects the thickening process because some solids thicken more easily than
others.
High solids age can result in septic conditions.
Hamper gravity settling due to reduced particle specific gravity.
Increase velocity and cause turbulence that will disrupt settling and carry the
lighter solids past the weirs.
If rates are high, there will be insufficient detention time for settling. If rates are
too low, septic conditions may arise.
High temperatures will result in septic conditions. Extremely low temperatures
will result in lower settling velocities. If temperature varies, settling decreases
due to stratification.
Increases the performance of the settling by causing compaction of the lower
layers, but it may result in solids being carried over the weir.
An increase may result in septic conditions. A decrease may result in only
partial settling.
Must be maintained to produce a smooth and continuous flow. Otherwise,
turbulence, septic conditions, altered settling, and other anomalies may occur.
Chemicals - such as potassium permanganate, polymers, or ferric chloride - may
improve settling and/or supernatant quality.
Allows for longer detention times before anaerobic conditions create gas bubbles
and floating solids.
Helps thicken waste-activated solids and clarify the supernatant.
Improves overflow clarity but may have little impact on underflow concentration.
Source: Parsons, 2003.
• 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.
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TABLE 3 CHEMICAL DOSE RATES
Type of
Solids
Primary
Primary +
tricking
filter
Primary +
WAS
FeCI3
(mg/L)
1-2
2-3
1.5-2.5
CaO
(mg/L)
6-8
6-8
7-9
K2Mn05
(mg/L)
10-40
10-40
10-40
Source: WPCF, 1987, WEF, 1996.
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 Ibs/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.
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TABLE 4 GRAVITY THICKENING TROUBLESHOOTING GUIDE
Indicators
Probable Cause
Check or Monitor
Solution
Septic odor, rising solids
Thickened solids not
thick enough
Torque overload of
solids collecting
mechanism
Surging flow
Excessive biological
growths on surfaces and
weirs (slimes, etc.)
Oil Leak
Noisy or hot bearing or
universal joint
Pump overload
Fine solids particles in
effluent
Thickened solids pumping
rate is too slow, thickener
overflow rate is too low
Overflow rate is too high,
thickened solids pumping
rate is too high, short
circuiting of flow through tank
Heavy accumulation of
solids, foreign object jammed
in mechanism, improper
alignment of mechanism
Poor influent pump
programming
Inadequate cleaning program
Oil seal failure
Excessive wear, improper
alignment, lack of lubrication
Improper adjustment of
packing, clogged pump
Waste activated solids
Check thickened solids
pumping system for
proper operation, check
thickener collection
mechanism for proper
operation
Check overflow rate, use
dye or other tracer to
check for circulation
Probe along front of
collector arms
Pump cycling
Oil seal
Alignment, lubrication
Check packing, check for
trash in pump
Portion of WAS in
thickener effluent
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
Decrease influent solids flow
rate, decrease pumping rate of
thickened solids, check effluent
weirs: repair or re-level, check
influent baffles: repair or relocate
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
Modify pump cycling, reduce flow
and increase time
Frequent and thorough cleaning
of surfaces, apply chlorination
Replace seal
Replace, lubricate, or align joint
or bearing as required
Adjust packing, clean pump
Better conditioning of the WAS
portion of the solids, thicken
WAS in a flotation thickener
Source: WEF, 1987.
REFERENCES
Other Related Fact Sheets
Filter Belt Press Dewatering
EPA 832-F-00-057
September 2000
Centrifugal Dewatering/Thickening
EPA 832-F-00-053
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
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TABLE 5 ESTIMATED CAPITAL
COSTS FOR GRAVITY THICKENERS
(ADJUSTED TO THE YEAR 2000)
TABLE 6 GRAVITY THICKENER LABOR
REQUIREMENTS
Plant
Capacity
(MGD)
2
4
8
15
20
25
Construction
Costs ($)
162,000
348,000
745,000
1,488,000
2,042,000
2,610,000
Construction*
Eng.&Adm.* ($)
199,000
426,000
914,000
1,825,000
2,504,000
3,200,000
Source: U.S. EPA, 1979
Eng.&Adm. = Engineering and Administration
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
onBiosolids 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
Thickener
Surface
Area, sq. ft
>500
>1000
>2000
>5000
Operation
Labor,
hr/yr
310
350
420
680
Maintenance
Labor, hr/yr
180
200
240
370
Total
490
550
660
1050
Source: WPCF, 1987.
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. BoxB-1
Sayreville, NJ 08872
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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
For more information contact:
Municipal Technology Branch
U.S. EPA
1200 Pennsylvania Avenue, NW
Mail Code 4204M
Washington, D.C. 20460
1MTB
Excellence in compliance through optimal technical solutions
MUNICIPAL TECHNOLOGY BRANCH
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