&EFA
United States
Environmental Protection
Agency
Office of Water
Washington, D.C.
EPA 832-F-00-053
September 2000
Biosolids
Technology Fact Sheet
Centrifuge Thickening and Dewatering
DESCRIPTION
Centrifugal thickening and dewatering is a high
speed process that uses the force from rapid rotation
of a cylindrical bowl to separate wastewater solids
from liquid (U.S. EPA, 1987). Centrifuges have
been used in wastewater treatment since the 1930s.
Thickening before digestion or dewatering reduces
the tankage needed for digestion and storage by
removing water. Dewatering removes more water
and produces a drier material referred to as "cake"
which varies in consistency from that of custard to
moist soil.
Dewatering offers the following advantages:
Reduces volume, saving money on storage
and transportation.
• Eliminates free liquids before landfill
disposal.
Reduces fuel requirements if the residuals
are to be incinerated or heat dried.
• Produces a material, which, when blended
with a bulking agent, will have sufficient
void space and volatile solids for
composting.
• Eliminates ponding and runoff, which can
be a problem when liquid is land applied on
the surface rather than injected.
Optimizes air drying and many stabilization
processes.
Centrifuges operate as continuous feed units which
remove solids by a scroll conveyor and discharge
liquid over the weir. The bowl is conical-shaped
which helps lift solids out of the liquid allowing
them to dry on an inclined surface before being
discharged (Kemp, 1997). Figure 1 shows a typical
centrifuge thickening and dewatering system.
Bowl drive
Scroll
Gear reducer
****'+~^fMHti jftR jf^f /ttR
Conveyor
Scroll
drive
Slurry
'Liquid
discharge
Solids discharge
Source: Ireland and Balchunas, 1998.
FIGURE 1 TYPICAL CENTRIFUGE THICKENING AND DEWATERING
SYSTEM
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Success in dewatering with high-solids centrifuges
makes this equipment worthy of consideration.
Although more expensive than other dewatering
systems, centrifuges generally achieve a higher
solids concentration. The most cost effective
method of biosolids dewatering depends on many
factors including plant size, the cost of further
processing, end-use and disposal, odor control
requirements, and space limitations.
APPLICABILITY
Thickening and dewatering systems can result in
significant savings in the cost of biosolids storage,
transportation, and end use or disposal. Thickening
liquid biosolids from three to six percent total
solids will reduce the volume by 50 percent.
Thickening is often used before anaerobic digestion
or lime stabilization to reduce the capital costs of
stabilization equipment. Thickening before storage
or transportation off site is also common but is not
usually performed before conventional aerobic
digestion because it is difficult to supply enough
oxygen when total solids are greater than 2 percent.
Centrifuges may be used to thicken or dewater
(U.S. EPA, 1987). The percent solids of the output
can be varied by changing operational parameters.
A wastewater treatment plant may want to recycle
biosolids in liquid form on sunny days when the
fields are open, while at other times they may wish
to dewater biosolids for storage or disposal.
Like all dewatering equipment, centrifuges require
a capital investment and labor to operate.
Mechanical dewatering equipment may not be the
most cost effective alternative for wastewater
treatment plants operating at less than four million
gallons per day (MGD). The selection of
dewatering equipment should be based on the
results of a site specific biosolids management plan
which identifies processing, end use alternatives,
and costs. It may be less expensive to haul liquids
to another facility for dewatering and processing or
disposal rather than installing dewatering
equipment. Smaller facilities should evaluate non-
mechanical dewatering methods, such as drying
beds or reed beds. Nonetheless, centrifugal
thickening can be cost effective for small plants.
Wastewater treatment plants that must landfill
wastewater solids may benefit from the use of a
centrifuge. Landfills require that biosolids contain
no free liquids during a paint filter test (material of
approximately 20 percent solids can usually pass
this test). Facilities with beneficial use sites more
than 30 minutes away may find it economical to
dewater before land application.
ADVANTAGES AND DISADVANTAGES
Advantages
• Centrifuges may offer lower overall
operation and maintenance costs and can
outperform conventional belt filter presses.
Centrifuges require a small amount of floor
space relative to their capacity.
• Centrifuges require minimal operator
attention when operations are stable .
Operators have low exposure to pathogens,
aerosols, hydrogen sulfide or other odors.
• Centrifuges are easy to clean.
• Centrifuges can handle higher than design
loadings and the percent solids recovery can
usually be maintained with the addition of a
higher polymer dosage.
Major maintenance items can be easily
removed and replaced. Repair work is
usually performed by the manufacturer.
Disadvantages
Centrifuges have high power consumption
and are fairly noisy.
• Experience operating the equipment is
required to optimize performance.
Performance is difficult to monitor because
the operator's view of centrate and feed is
obstructed.
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• Special structural considerations must be
taken into account. As with any piece of
high speed rotary equipment, the base must
be stationary and level due to dynamic
loading.
• Spare parts are expensive and internal parts
are subject to abrasive wear.
Start-up and shut down may take an hour to
gradually bring the centrifuge up to speed
and slow it down for clean out prior to shut
down.
DESIGN CRITERIA
Centrifuges are sized on the basis of the weight or
volume of biosolids to be dewatered. To determine
the number and size of centrifuges needed, the
following information is required:
• Amount of primary solids flowing through
the plant per day.
• Amount of waste activated sludge produced
per day.
Seasonal variation in the quantity of solids
produced.
• Volume of thickened solids to be
dewatered per day.
Estimate of the range of solids
concentration in the feed solids.
• The hours per day and number of days per
week of operation.
Estimate of future increases in biosolids.
Anticipation of changes in sewer discharges
or operation that could alter biosolids
quality, such as the organic matter content.
In addition to the above information, an effective
biosolids management plan should provide excess
capacity to ensure that all incoming biosolids can be
dewatered during operating hours. Allowing for
excess capacity also ensures that the plant will not
experience a build-up of biosolids if a unit is out of
service. If only one unit is planned, the plant
should have an alternate program to remove
biosolids in liquid form and haul them to an
alternate processing site.
Automation can reduce the number and size of
centrifuges required. A wastewater treatment plant
staffed only one shift per day can operate an
automated centrifuge 24 hours per day (Brady and
Torpey, 1998; Matheson and Brady, 1998).
A poorly designed polymer system can result in
high polymer costs and reduced centrifuge
efficiency. Polymer systems should have sufficient
mixing and aging capacity. It is also important to
introduce and blend the polymer with the solids
feed to provide sufficient contact and laminar (not
turbulent) mixing.
Feed pumps that have been used successfully
include adjustable speed progressive cavity pumps
and rotary lobe pumps. These semi-positive
displacement pumps supply consistent feed without
destroying the effectiveness of the polymer (low
sheer forces).
A high solids centrifuge incorporates wear resistant
materials, faster bowl speed, deeper ponds, higher
torque rating, and better controls to hold solids in
the machine longer (Ireland and Balchunas, 1998).
The choice of dewatering technique and chemical
polymer or salts will affect dewaterability and the
potential for odor during further processing or
recycling. The designer must relate plant
processing to the potential for odor production
during further processing and recycling. This factor
has only recently been recognized as an important
consideration during the design stage.
Odor complaints at wastewater treatment plants and
end use sites can interfere with implementation of
the most cost effective biosolids management
options, so control measures should be included in
design of dewatering facilities. Odor control is
addressed in more detail in another fact sheet, but
briefly, the methods include:
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• Adding potassium permanganate or other
oxidizing agent.
• Minimizing liquid storage prior to
dewatering to less than 24 hours. The
longer the biosolids are stored, the lower the
pH, the higher the liquid ammonia
concentration, and the higher the organic
sulfide emissions.
• Conducting bench-scale and full-scale
testing of liquid sludge to determine if
combined storage of primary and waste
activated sludges accelerates the
deterioration of biosolids.
• Specifying polymers that are stable at
elevated temperatures and pH. This is
especially important at facilities using lime
stabilization or high temperature processing
such as heat drying, thermophyllic
digestion, or composting.
Design specifications should include requirements
for ancillary equipment for efficient operation of a
centrifuge, including:
Polymer mixing, aging, and feed systems.
Liquid feed day tank.
Liquid residuals feed pump.
Odor control and ventilation.
Conveyor and/or pump to move dewatered
cake.
• An enclosed area to load trucks or
containers.
PERFORMANCE
Manufacturers should be consulted for design and
performance data early in the planning stage. These
data should be confirmed with other operating
installations and/or through pilot testing.
Evaluation of equipment should consider capital
and operating costs, including polymer, electricity,
wash water, ventilation, and odor control. The
operator can ensure system integration by requiring
that the centrifuge feed pump and the polymer
system be provided by a single supplier.
Feed rate, polymer dosage, and differential scroll
speed can be adjusted during operation for optimum
performance (Kemp, 1997). The use of polymers
improves centrate clarity, increases capacity,
improves the conveying characteristics of the
discharged solids, and increases cake dryness (U.S.
EPA, 1987).
Table 1 shows the range of performance of a
centrifuge with various types of wastewater solids.
The solids content for a blend of primary and waste
activated sludge (WAS) will vary depending on the
percentage of each type of solid. Wastewater solids
with a higher percentage of primary can be
dewatered to the higher end of the range of total
solids cake. Wastewater solids with a higher
percentage of WAS will probably dewater to the
lower end of the range and require polymer to reach
the higher end of the range.
Biosolids must be conditioned with polymers to
ensure optimum performance. Polymer feed pumps
should be designed to inject polymer at several
locations to ensure flexibility, optimum
performance, and biosolids/polymer effectiveness.
The biosolids/polymer mixture should be gently
mixed because turbulent conditions can sheer the
floe, minimizing polymer effectiveness. Polymer
dilution and aging systems should be large enough
to optimize polymer usage.
The operator should be able to add potassium
permanganate or other oxidizing agents to the
system, for the following reasons:
To destroy sulfides which cause odors at the
dewatering facility and in the end product.
• To reduce polymer usage and increase
biosolids cake solids.
To reduce odors associated with biosolids
cake.
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TABLE 1 RANGE OF EXPECTED CENTRIFUGE PERFORMANCE
Type of Wastewater Solids
Primary Undigested
WAS Undigested
Primary + WAS undigested
Primary + WAS aerobic digested
Primary + WAS anaerobic
digested
Primary anaerobic digested
WAS aerobic digested
Hi-Temp Aerobic
Hi-Temp Anaerobic
Lime Stabilized
Feed %TS
4-8
1-4
2-4
1.5-3
2-4
2-4
1-4
4-6
3-6
4-6
Polymer Ib/DTS
5-30
15-30
5-16
15-30
15-30
8-12
20
20-40
20-30
15-25
Cake %TS
25-40
16-25
25-35
16-25
22-32
25-35
18-21
20-25
22-28
20-28
Source: Various centrifuge manufacturers; Ireland and Balchunas, 1998; Henderson and Schultz, 1999; Leber and
Garvey, 2000.
Facilities of many different sizes are achieving
some or all of the above benefits.
OPERATION AND MAINTENANCE
It is important to monitor operating parameters to
achieve optimum performance. The operator
should ensure that the biosolids are properly
conditioned and observe the conditioned biosolids
using jar tests. Operation and maintenance training
should also be provided.
Centrifuge operations can be fully automated, but
starting the bowl and putting feed into the machine
are usually performed manually. Routine
maintenance is relatively simple because it is
usually performed by the manufacturer. A good grit
removal system should be incorporated into the
plant design in order to reduce abrasive wear.
Centrifuges are normally operated to obtain
maximum solids concentrations, while maintaining
a 95 percent solids capture. Operators can adjust
the solids feed rate, polymer dosage, and
differential scroll speed to optimize performance
operators can judge performance by sampling the
centrate stream and cake solids. As with all
mechanical dewatering equipment, feeding from
well-mixed digesters or day tanks is important for
optimum operation.
Responsibilities of the centrifuge operator include
polymer mixing, dosing and monitoring usage;
observation of the feed and cake several times per
day and adjustments, if necessary; and operation
and lubrication of ancillary equipment, including
feed pump and cake conveyor or pump.
It is important to keep records of all performance
parameters including volume of biosolids fed to the
centrifuge and chemicals used. A sample of the
feed biosolids to the centrifuge, cake discharge, and
centrate should be taken each shift and analyzed for
total solids. Prior to shut down, the centrifuges
should be emptied and the speed gradually reduced.
The amount of labor is relative to plant size. A
plant with a single centrifuge requires four to eight
staff hours per day (including lab testing) whereas
six to eight centrifuges could be operated with eight
to ten staff hours per day. Large plants use less
operating effort for dewatering (per ton of solids
processed). Highly automated systems reduce labor
requirements, but operators must have access to an
instrumentation specialist to maintain the
automation system.
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COSTS
REFERENCES
In the past, engineers believed that centrifuges had
higher capital and operating costs than belt presses.
However, recent innovations in equipment design
and polymers, combined with increased concerns
over odors and worker health, have changed the
economics. The choice of the most economical
equipment should be based on an economic
analysis, bench testing, and the method of end use
or disposal.
A centrifuge sized to process 750 pounds of solids
per day in an eight hour shift will cost about
$215,000. To install the equipment in an existing
building with a polymer feed system and odor
control will run about $650,000. Construction of a
building, conveyor, and truck loading area are
additional costs.
Polymer costs are lowest when the machinery is
running at reduced capacity. Typical polymer
conditioning costs for centrifugal dewatering range
from $2.65 to $91.15 per million gallons of
biosolids processed, with an average of $24 per
million gallons. Permanganate adds about $1 per
million gallons to the cost of dewatering. Costs
vary widely depending on the source of the
residuals. The polymer costs for raw primary solids
may cost $12 per dry ton solids (DTS) whereas
residuals that are difficult to dewater may cost
$80/DTS. Overall operation and maintenance costs
range from $65/DT to $209/DT (Bain, et al., 1999;
Leber and Garvey, 2000; Rudolf, 1992).
Capital costs of a centrifuge are more than a belt
press but operation and maintenance can be less
expensive depending on size of plant, cost of
polymer, cost of labor, local electric rates, whether
wash water is plant water or potable water, as well
as other factors. The range of $65 to $209 does not
include debt service and is based on documented
actual operating costs.
Other Related Fact Sheets
Alkaline Stabilization for Biosolids
EPA 832-F-00-052
September 2000
In-Vessel Composting
EPA 832-F-00-061
September 2000
Land Application of Biosolids
EPA 832-F-00-064
September 2000
Odor Management in Biosolids Management
EPA 832-F-00-067
September 2000
Belt Filter Press
EPA 832-F-00-057
September 2000
Filter Press, Recessed Plate
EPA 832-F-00-058
September 2000
Other EPA Fact Sheets can be found at the
following web address:
http://www.epa.gov/owmitnet/mtbfact.htm
1. Bain, Robin E. et al. "Regional Approach
Turns Reno-Stead Water Reclamation
Facility's Solids Disposal into Biosolids
Reuse." 1999 WEF/AWWA Joint Residuals
and Biosolids Management Conference:
Strategic Networking for the 21st Century.
Charlotte.
2. Brady, Peter and Pat Torpey. Spring 1998.
"Better, Safer Dewatering: Automated
Control System Eliminates the Guesswork
Associated with Solids Dewatering and
Allows Containment of Dewatering
Equipment." WEF Wastewater Technology
Showcase. Volume 1. Number 1.
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9.
10.
11.
Gabb, Donald M.D.,David R. Williams
David R. et al., October 1998."Waste
Activated Sludge Thickening: A custom Fit
for East Bay Municipal Utility District."
Water Environment & Technology.
Henderson, R. Todd, Stephen T. Schultz,
and Michael Itnyre. "Centrifuges Versus
Belt Presses in San Bernardino, California."
1999 WEF/AWWA Joint Residuals and
Biosolids Management Conference:
Strategic Networking for the 21st Century.
Charlotte, NC.
Ireland, James S. and Brian M. Balchunas,
October 1998. "High-Speed, High-Solids
Centrifuges: Sorting Through Mechanical
Features, Manufacturers' Claims, and
Owners' Opinions." Water Environment &
Technology.
Isett, Barry, February, 2000. "What
Smells?" Water Environment &
Technology.
Kemp, Jay S. December 1997. "Just the
Facts on Dewatering Systems: A Review of
the Features of Three Mechanical
Dewatering Technologies." Water
Environment & Technology.
Leber, Robert S. and Diane Garvey. March
2000. "Centrifuge or Belt Press? Odor
Control May Be a Pivotal Factor." WEF
14th Annual Residuals and Biosolids
Management Conference.
Matheson, R. and P. Brady. "Vallejo SFCD
Experience with Dewatering Automation."
1998 WEF Residuals Conference.
Rudolph, Donald J., P.E. 1992. "Solution
to Odor Problem Gives Unexpected
Savings."
Sharpies (R) Field Report 19: DS-706
Solves Ocean Dumping Dilemma.
12. Sharpies (R) Field Report 23: 33% Cake at
Little River.
13. Sharpies (R) Field Report 8: 60% Dryer
Cake, Lower Costs.
14. Sharpies (R) Field Report 13: 33.6% Cake
on Anaerobically Digested Sludge with
99.8% Recovery.
15. Sharpies (R) Field Report 14: 32% Cake
Daily at Ashbridges Bay.
16. Sharpies (R) Field Report 22: Operator-free
performance with 32% cake.
17. U.S. EPA. September 1987. "Design
Manual: Dewatering Municipal Wastewater
Sludges."
18. Yonkers Joint WWTP. 1997. Process
Compatibility Testing D. Odor. In
Specifications for Furnishing and
Delivering Liquid Emulsion Type Polymer
(40-50 percent active) for Centrifuge
Dewatering of Sludge. Yonkers Joint
WWTP, LudlowDock, South Yonkers, NY.
19. Zenz, David R. et al. "Mechanical
Dewatering of the Biosolids from the
Metropolitan Water Reclamation District of
Greater Chicago's Stickney Water
Reclamation Plant." 1999 WEF/AWWA
Joint Residuals and Biosolids Management
Conference: Strategic Networking for the
21st Century. Charlotte.
ADDITIONAL INFORMATION
Abington Wastewater Treatment Plant
Robert Leber
1100 Fitzwatertown Road
Roslyn, PA 19001
East Bay Municipal Utilities District
Bennett K. Horenstein
P.O. Box 24055
Oakland, CA 94623
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Philadelphia Water Department
James Golumbeski
7800 Pennrose Ferry Road
Philadelphia, PA 19153
San Bernadino Municipal Water Department
Stephen Schultz
399 Chandler Place
San Bernadino, CA, 92408
The mention of trade names or commercial
products does not constitute endorsement or
recommendation for use by the U.S. Environmental
Protection Agency.
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
1200 Pennsylvania Avenue, NW
Washington, D.C., 20460
MTB
Excelence fh tompfance through optfhial tethnltal solutfons
MUNICIPAL TECHNOLOGY BRANCH
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