United States
                        Environmental Protection
                       Office of Water
                       Washington, D.C.
EPA 832-F-00-057
September 2000
Technology  Fact Sheet
Belt  Filter Press

Belt filter presses are used to remove water from
liquid   wastewater  residuals   and   produce  a
non-liquid   material   referred  to  as  "cake".
Dewatered residuals, or cake, vary in consistency
from that of custard to moist soil.   Dewatering
serves the following purposes:

      Reducing the volume, thus reducing storage
      and transportation costs.

•     Eliminating free  liquids  before landfill

      Reducing fuel requirements if residuals are
      to be incinerated or dried.

•     Producing  a material  which  will  have
      sufficient void space and volatile solids for
      composting when blended  with a bulking

      Avoiding the potential of biosolids pooling
      and  runoff associated with  liquid  land

      Optimizing subsequent processes such as
      thermal drying.

A belt filter dewaters by applying pressure to the
biosolids to squeeze  out  the water.  Biosolids
sandwiched between two tensioned porous belts are
passed over and under rollers of various diameters.
Increased pressure is created as the belt passes over
rollers which decrease in diameter. Many designs
of belt filtration processes are  available, but all
incorporate  the  following basic  features: polymer
                      conditioning zone, gravity  drainage zones, low
                      pressure  squeezing  zone,  and  high pressure
                      squeezing zones. Advanced designs provide a large
                      filtration area, additional rollers, and variable belt
                      speeds that can increase cake solids by five percent.

                      The general mechanical components of a belt filter
                      press include dewatering belts, rollers and bearings,
                      belt tracking and tensioning system, controls and
                      drives, and a belt washing system. Figure 1 depicts
                      a typical belt filter press.


                      Belt filter presses  can be used to dewater most
                      biosolids  generated  at  municipal  wastewater
                      treatment  plants and  are  a  common type  of
                      mechanical  dewatering   equipment.    Using
                      mechanical equipment to dewater solids may not be
                      the most cost effective alternative for wastewater
                      treatment plants operating at less than about 4 mgd.
                      The selection of dewatering equipment should be
                      based on the results of a site specific biosolids
                      management plan which identifies both processing
                      and end use alternatives and estimates costs. It may
                      be less expensive to haul liquid to an application
                      site or pay a processing facility to dewater and
                      process  or landfill the dewatered  cake.  Smaller
                      facilities  should  also  evaluate  non-mechanical
                      dewatering methods,  such as drying beds or reed


                      Advantages and disadvantages of belt filter presses
                      for dewatering wastewater solids are summarized

                                   Independent High
                                   Pressure Section
                   High Pressure
                   Shear Zone
                   Free Drainage
Source: U.S. EPA, 1987.
       Staffing requirements are low, especially if
       the equipment is large enough to process the
       solids in one shift (USEPA, 1987).

       Maintenance is relatively simple and can
       usually be  completed  by  a  wastewater
       treatment  plant  maintenance   crew.
       Replacing the belt is the major maintenance

       Belt presses can be started and shut down
       quickly  compared to  centrifuges, which
       require up to an hour to build up speed
       (Henderson and Schultz, 1999).

       There is less noise associated  with  belt
       presses compared to centrifuges (Henderson
       and Schultz, 1999).
       Odors  may  be a  problem,  but can  be
       controlled with good ventilation systems
       and  chemicals,   such  as   potassium
permanganate, to neutralize  odor-causing
compounds   (Rudolf,   1992).     Some
manufacturers   offer  fully   enclosed
equipment to minimize odors and reduce
vapors in the operating room air (Bain et al.,

Belt presses require more operator attention
if  the feed  solids  vary in their  solids
concentration or organic  matter.   This
should not be a problem if the belt presses
are   fed  from  well-mixed   digesters
(Henderson and Schultz, 1999).

Wastewater   solids   with   higher
concentrations of oil and grease can result
in  blinding the belt filter and lower solids
content cake.

Wastewater solids must be screened and/or
ground to minimize the risk of sharp objects
damaging the belt.

Belt washing at  the end of each shift, or
more frequently, can  be time consuming
and   require  large  amounts  of  water

       (Henderson  and  Schultz,  1999).    An
       automatic belt washing system and the use
       of effluent can minimize these costs.


Belt presses are sized on the basis of weight or
volume of solids to be dewatered rather than the
wastewater flow to the plant.  To determine how
many presses are needed, the wastewater treatment
plant must:

•      Determine the amount of primary solids that
       will flow through the plant per day.

       Determine the amount of waste-activated or
       trickling filter solids produced per day.

•      Determine the volume of thickened solids to
       be dewatered per day.

       Estimate  the   range   of   dry   solids
       concentration in the feed.

•      Estimate  future   increases  in   solids

       Anticipate changes in sewer discharges or
       operation that could change solids quality or
       organic matter content.

An  effective  biosolids  management  plan will
include the above information.  It is important to
design for excess capacity  so that the anticipated
amount of incoming solids can be easily dewatered
during operating hours.   Allowing for  excess
capacity  also  ensures  that  the  plant  will  not
experience a build-up of solids if a unit is  out of
service.  If only one unit is  required,  the plant
should have an alternate program to  remove solids
in liquid form.

The polymer conditioning zone can be a small tank,
approximately 265 to 379 liters (70 to 100 gallons)
located 0.6 to 1.8 meters (2 to  6 feet) from the
press, a rotating drum attached to the top of the
press,  or  an  in-line  injector.     The  press
manufacturer usually supplies this zone along with
the belt filter press (USEPA, 1986).
The  gravity drainage zone is  a flat or slightly
inclined belt unique to  each model.   Solids are
dewatered by the gravity drainage of the free water.
A 5 to 10 percent increase in solids concentration
from the original biosolids should occur in this zone
(USEPA,  1987).   The  free water  drainage is a
function  of  wastewater   solids  type,  quality,
conditioning, screen  mesh,  and  design  of the
drainage zone.

The low-pressure zone is the area where the upper
and lower belts  come together with the wastewater
solids in between.   This is sometimes called the
"wedge  zone," because   the  feed  solids  are
sandwiched between the upper  and lower belts.
The low-pressure zone  prepares the biosolids by
forming a firm cake which can withstand the forces
of the high pressure zone.

In the high-pressure zone, forces are exerted on the
solids by  the movement of the upper  and lower
belts as they move over and under a series of rollers
of decreasing  diameter.  Some belt filter press
models separate from the rest of the unit to increase
pressure on the biosolids.  This produces a drier
cake,  an important factor for plants that incinerate
the final product or face high end use or disposal
costs.    A  biosolids management  plan  should
evaluate the advantages and disadvantages of a high
performance belt filter press.

An  additional  design feature is a self-enclosed
facility to reduce odors and protect worker health
(Bain et al., 1999).  Workers in the belt press areas
are exposed to  aerosols  from wash spray nozzles
and  pathogens and hazardous  gasses such as
hydrogen sulfide.   Enclosing the press reduces
visibility to the operators and produces a corrosive
environment  for  the rollers  and bearings,  but
automating the system can alleviate these problems.

The automation of belt presses is the subject of a
Water Environment Research Foundation project.
Benefits of automation include optimization of non-
linear variables which  was rarely  possible with
manual or semi-automated operation, and the ability
to produce dewatered  cake  at  a constant  rate.
Automation generally increases capital costs by 10
percent.    Manufacturers  claim  that  this  extra
expense is worthwhile because it lowers labor costs,

reduces polymer use, and  maximizes the solids
content of the cake, reducing disposal and end use
costs (Gillette et al., 2000).

The choice of dewatering technique and chemical
polymer or salts impacts dewaterability as well as
the potential for odor during further processing or
recycling to land.

Ancillary equipment for efficient operation of a belt
press includes:

•      Polymer.

•      Mixing,  aging, feed, liquid feed day tank.

•      Liquid residuals feed pump.

•      Odor control and ventilation.

•      Conveyor and/or  pump to move dewatered

       An  enclosed  area  to  load  trucks  or


Manufacturers should be consulted  for design and
performance data early in the planning stage. Data
should  be  confirmed   with  other  operating
installations and/or thrash pilot testing.  Evaluation
of equipment should consider capital and operating
costs, including polymer, electricity, wash water,
solids capture, and ventilation  and  odor control
during  dewatering and  further  processing or
recycling.   The  operator  can  ensure  system
integration by requiring that the self-enclosed belt
press, ventilation, and polymer system  is supplied
by a single provider.  Since  solids characteristics
and quantity vary from plant to plant, it is important
to evaluate different  weaves, permeability, and
solids retention  abilities  of dewatering belts to
ensure optimum performance.  Surveys of similar
plants or testing of wastewater solids can be helpful
in the decision-making process.

Table 1 displays the range of performance of a high
pressure belt press  on various types of wastewater

Odor Control

Odor complaints at wastewater treatment plants and
biosolids  end  use   sites   can  interfere   with
implementation of the most cost effective biosolids
management  options.   Odor control  measures
should be  included when designing  dewatering
facilities. Odor control is addressed in more detail
in another fact  sheet, but briefly,  the  methods
                                      FILTER PRESSES
Type of Wastewater Sludge
Raw Primary
Raw Primary + WAS
Anaerobically Digested Primary
Anaerobically Digested WAS
Anaerobically Digested Primary + WAS
Aerobically Digested Primary + WAS
Oxygen Activated WAS
Thermally Conditioned Primary + WAS
Total Feed Solids
3 to 10
0.5 to 4
3 to 6
3 to 10
3 to 4
3 to 9
1 to 3
1 to 3
4 to 8
1 to 5
1 to 10
1 to 10
1 to 5
2 to 10
2 to 8
2 to 8
4 to 10
Total Cake
Solids (percent)
28 to 44
20 to 35
20 to 35
25 to 36
12 to 22
18 to 44
12 to 20
15 to 23
25 to 50
Source: U.S. EPA, 1987.

•      Using a self enclosed belt press.

•      Adding potassium permanganate or other
       oxidizing agent to minimize  odors in the

•      Minimizing liquid  storage prior to belt
       pressing to less than 24 hours.  The longer
       the solids are stored, the lower the pH, the
       higher the liquid ammonia concentration,
       and the higher the organic sulfide emissions
       (Hentz et al., 2000).

       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  (Hentz et al.,

•      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.

Self-Enclosed Belt Presses

The main purpose of a self-enclosed system is to
minimize the amount of foul air needing treatment
in an odor control system.  An induced draft fan
provides a slight negative pressure (typically 100
cubic feet per meter per meter of belt width.) The
system design should:

•      Minimize gaps in the enclosure.

•      Minimize enclosure volume.

•      Locate   mechanical   and   electrical
       components requiring maintenance outside
       the  enclosed  area  for easy  access  and
       reduced maintenance.

       Include   automation   to   optimize
       performance of the belt press.

•      Use stainless steel materials.
•      Provide  multiple access  hatches to allow
       operator viewing and clean up.

•      Provide  for easy removal of the belt for
       replacement (Bain et al., 1999).

Chemical Addition

Solids must be conditioned with polymer to ensure
optimum performance. Polymer feed points should
be designed at several locations to ensure flexibility
and  optimum  performance.  The  solids/polymer
mixture should be subject  to  gentle  mixing as
turbulent conditions can sheer the floe, minimizing
polymer effectiveness. Polymer dilution and aging
systems should be  large  enough  to optimize
polymer usage.

Potassium permanganate or other oxidizing agents
are often added to solids prior to dewatering. These
have been shown to  reduce  odors  caused by
sulfides, reduce the amount of polymer needed, and
increase  cake  solids  content  (Rudolf,  1992).
Figure  2 shows  dewatered solids from a belt filter
press after processing.
Source: Dr. Peter Wright, Cornell University, 1996.



It is important to monitor operating parameters to
achieve  optimum performance  and ensure that
solids are properly conditioned and  that good
gravity drainage occurs.  The manufacturer should

provide operation and maintenance training after
installation as well as ongoing training to maintain

Dewatering  belts should  be designed  for easy
replacement  with  minimum  downtime.  Belt
washing  should occur  daily after  the cake  is

Replacement  of  filter  belts   is  a   common
maintenance requirement. Belt life averages about
2,700 running hours, with a range of 400 to 12,000

A belt press operator is responsible for polymer
mixing,  dosing  and  monitoring  usage,  and
observing the feed and cake several times per day,
making adjustments  as  necessary.  Rollers and
bearings require frequent lubrication.

It is important for the operator to keep records of all
press  performance  parameters,  including  the
volume of biosolids fed to  the press,  polymer
dosage, and potassium permanganate  or  other
chemical usage.  A sample of the biosolids to the
press, cake discharge, and filtrate should be taken at
least once per shift and analyzed for total solids.  At
the end of each shift, the belt should be cleaned
with high-pressure wash water. Labor is relative to
plant size.  A plant with a single belt press needs
four to eight staff hours per day (including lab
testing),  whereas six to  eight  presses can  be
operated with eight  to ten  staff hours  per day.
Large plants use less operating effort per belt press.
Highly  automated  systems   reduce  labor
requirements,  but  require   an  instrumentation
specialist to maintain the system.


Capital costs for belt filter presses vary with the
size of the equipment.  Vendor estimates vary from
$47,500 (0.5 meter belt, approximate capacity  of
500 dry pounds per hour) to $115,000 (1.5 meter
belt with approximate capacity of 1,625 dry pounds
per hour).  These estimates  are based on  a feed
material which is 5 percent solids. These prices do
not include  the  cost of installation, shipping,  or
ancillary  equipment,  such  as flow control  and
centrate management.
Overall operation and maintenance costs range from
$80 per dry ton of solids (DTS) to $200 per DTS.
Typical  polymer conditioning costs for belt filter
press  dewatering range from  $2.65  per million
gallons to $91.15 per million gallons,  and average
$24.38  per million  gallons.   Permanganate adds
about $1 per  million  gallons  to  the  cost  of
dewatering the biosolids. These costs vary widely,
depending on  the source of the residuals.   The
polymer costs  for raw primary may cost $12 per
DTS,  but may be as high as $80 per  DTS  for
residuals that are difficult to dewater.


Other Related Fact Sheets

Odor Management in Biosolids Management
EPA 832-F-00-067
September 2000

Alkaline Stabilization of Biosolids
EPA 832-F-00-052
September 2000

Land Application of Biosolids
EPA 832-F-00-064
September 2000

In-Vessel Composting
EPA 832-F-00-061
September 2000

Other EPA Fact Sheets  can be  found at the
following web  address:

1.     Bain, R.E., Brady, P., and Torpey, P.,  1999.
      "Experience  With 70+  Self-Enclosed Belt
      Presses and Thickeners." In Proceedings of
      the  WEF/AWWA  Joint  Residuals  and
      Biosolids  Management  Conference:
      Strategic Networking for the 21st Century.
      Arlington,   VA.   Water  Environment

Gillette, R.A., Joslyn,  S.D., Palmer,  T.,
2000.  "Optimization of Thickening and
Dewatering  Through  Automation."   In
Proceedings of the 14th Annual Residuals
and Biosolids Management Conference.
Arlington,   VA.   Water   Environment

Hamilton,  K.,  and  Millard,  R.,  1999.
"Innovative  Mechanical Dewatering and
Thermal  Drying  Technologies."   In
Proceedings  of  WEF/AWWA   Joint
Residuals  and  Biosolids  Management
Conference:  Strategic Networking for the
21st  Century.  Arlington,  VA.  Water
Environment Federation.

Henderson, R.T. and Schultz, S.T.  1999.
"Centrifuges Versus Belt Presses in San
Bernardino, California." In Proceedings of
WEF/AWWA Joint Residuals and Biosolids
Management   Conference:   Strategic
Networking for the 21st Century. Arlington,
VA. Water Environment Federation.

Hentz, L.  H. Jr.,  P.E.,  Cassel, A.F., and
Conley, S. 2000.  "The Effects of Liquid
Sludge Storage   on  Biosolids  Odor
Emissions." In Proceedings of 14th Annual
Residuals  and  Biosolids  Management
Conference.     Arlington,  VA.  Water
Environment Federation.

Kemp,  I,  1997. "Just the  Facts   on
Dewatering Systems." Water Environment
and Technology, December 1997.

Rudolf, D.J.  1992,  "Solution  to  Odor
Problem   Gives   Unexpected  Savings."
WATER/Engineering & Management.

U.S.  EPA,  1987. "Design  Manual  for
Dewatering  Municipal   Wastewater
Sludges." U.S. EPA, Washington, D.C.

Wright, Peter, 1996. Cornell University.

Jodi Levi
Middlesex County Utilities Authority
P.O. Box 159, Main Street Extension.
Sayreville, NJ 08872

Kevin Franks
Pennridge Wastewater Treatment Authority
P.O. Box 31
Sellersville, PA 18960

Stephen Schultz
San Bernadino Municipal Water Department
399 Chandler Place
San Bernadino, CA 92408

Peter Brady
Alpine Technology
1250 Capitol of Texas Highway
Austin, TX 78746

The  mention  of trade  names  or  commercial
products does  not  constitute endorsement  or
recommendation for use by the U.S. Environmental
Protection Agency (EPA).
          For more information contact:

          Municipal Technology Branch
          U.S. EPA
          Mail Code 4204
          1200 Pennsylvania Avenue, NW
          Washington, D.C. 20460
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