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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