United States
                       Environmental Protection
                       Office of Water
                       Washington, D.C.
EPA 832-F-00-058
September 2000
Technology  Fact  Sheet
Recessed-Plate  Filter  Press

Recessed-plate filter presses are used to remove
water from liquid wastewater residuals and produce
a  non-liquid  material  referred  to as "cake".
Dewatered cake varies in consistency from that of
custard (12 to!5 percent solids) to moist soil (20 to
40 percent solids) and is used for the following

      To reduce volume, saving money on storage
       and transportation.

       To eliminate free liquids prior  to landfill

      To reduce fuel requirements if the residuals
       are to be incinerated or further dried.

       To produce a material with sufficient void
       space and  volatile solids for composting
       when blended with a bulking agent.

       To reduce pooling or runoff, which can be
       a problem  when liquid biosolids are land

       To optimize alkaline stabilization processes.

Recessed-plate filter presses are among the oldest
types of dewatering devices and can produce the
highest cake solids concentration of any mechanical
dewatering equipment (Kemp, 1997).  They are
more commonly used in industrial applications than
in municipal wastewater facilities.
                      concave depression and a hole in the middle.  Two
                      plates are joined to create a chamber to pressurize
                      solids and squeeze out liquid through a filter cloth
                      lining the chamber. Several plates (ranging from 12
                      to 80,  depending on the  capacity  required) are
                      suspended from a frame face to face.  A series of
                      chambers is formed when  the press is closed.
                      Conditioned solids are pumped into the center hole
                      to fill each chamber. As pressure increases, either
                      by adding more conditioned  solids (fixed volume
                      press)  or by  expanding a  membrane  (variable
                      volume press),  solids are retained on the filter cloth
                      while liquid passes through  and is  drained  away
                      from the machine.  Free  water is  released and
                      passes through the filter cloth during the filling
                      phase.  Pressure  builds as the chamber fills with
                      solids beginning the consolidation process (Kemp,
                      1997).  The feed pump must be able to develop the
                      required filtration pressure (lOOpsi to 225psi). To
                      perform effectively, the terminal pressure is reached
                      during  consolidation  and  filtrate flow  declines.
                      Cake is formed until a set-point of low filtrate flow
                      is reached to indicate the end of the cycle.

                      Filter press capacity is determined by the number
                      and size of plates and chambers in the press (Kemp,
                      1997).  The  plates are supported on a structural
                      frame with a shifting mechanism to  separate them
                      one at  a time.  Large presses have automatic
                      plate-shifting systems that press together the plates
                      and filter cloths with a hydraulic ram, sealing the
                      edges of the cloths on the plates and resisting the
                      filtration pressure developed by the filter feed pump
                      during the filtration process (Kemp, 1997). Figure
                      1 shows a typical recessed plate filter press.
The recessed-plate filters are polypropylene squares
which may be two to  four feet across, with a
                      Filter presses are normally mounted on the floor
                      above a conveying system or the containers that

receive the solids. The cake drops as each chamber
is opened (Kemp, 1997).
              Trays   gtcessegj
Source: Netzsch, Inc. 1999.
Biosolids managers should consider two types of
presses. The conventional press has a fixed volume
which removes moisture by adding more solids.
The diaphragm press is a variable volume press in
which   sturdy  hollow  rubber  diaphragm   or
membrane is  positioned behind each filter cloth.
Water is pumped to the interior of the diaphragms
when the maximum feed pump pressure is reached,
expanding the diaphragm reducing the volume of
cake solids.

The diaphragm press also uses  a feed pump to fill
the chambers and develop pressures of up to 690
kPA (100  psi) (WEF, 1992).  If the biosolids are
properly conditioned, the initial filling period will
remove considerable amounts of water or compress
air  at   substantially zero  headloss across  the
medium. The diaphragm filter press operates like
a  recessed  plate  press,  at  pressures  between
690-1,040 kPA  (100-225psi)  (USEPA,  1987).
However, higher pressure is achieved by expanding
the diaphragms,  reducing the  volume  of  the
chamber by squeezing out more water.

Diaphragm filter presses often result in a cake with
a higher solids content.  Bench testing should be
performed on a  representative sample  of each
wastewater treatment plant's solids  to determine
whether a membrane filter offers advantages over a
conventional,   fixed  volume  filter  press.    An
economic  analysis  should be  also  conducted  to
determine whether the additional capital cost of a
diaphragm filter press will result in long-term cost


Recessed-plate filter presses can be used to dewater
most biosolids generated at municipal wastewater
treatment plants.  Like all dewatering equipment,
these filter presses require a capital investment and
labor to operate  and 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
that identifies processing and end use alternatives
and estimates costs.  It may be less expensive to
haul liquid and 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

Wastewater plants  faced with high end  use  or
disposal costs will  benefit from the ability  of a
recessed-plate filter press to produce the driest cake

Plants that want to produce  a  lime stabilized
product for agricultural use can use a recessed-plate
filter press with lime as  a conditioner.  The end
product will meet the 40 CFR Part 503, Standards
for  Use and Disposal of Sewage Sludge  with
respect to vector attraction reduction and Class A or
B pathogen reduction. The metals concentration in
the final product will be lower and in a less mobile
form due to the high pH.

If the wastewater treatment plant wants to process
the cake further, there may be economic advantages
to producing the driest cake possible.  Incineration
or heat drying requires less fuel and may operate at
a higher capacity because there is less water to
evaporate while alkaline stabilization technologies
require less additive, saving on the cost of the
additive, storage requirements, and transportation of
the final product.



Recessed-plate  filter  presses  offer  several
advantages   compared   to  other  mechanical
dewatering methods, as follows:

      High   cake  solids  concentration   with
       associated low biosolids  storage,  hauling,
       and disposal costs (WEF, 1992).

      Little   or  no  operator  attention  during
       dewatering phase of cycle (one  to  three
       hours) (WEF, 1992).

      Cake   solids concentration  is  relatively
       independent of feed solids  concentration
       (WEF, 1992).

      Use of lime as a conditioner stabilizes and
       disinfects the final product.

The  advantages of a diaphragm filter press over
conventional  recessed-plate filter presses include
the following:

       Usually produces a drier cake.

       Substantially greater uniformity of  solids
       concentration in the cake.

      Easier to dose polymers as an alternative to
       ferric  salts  and   lime   conditioning

      The use of high pressure without having to
       introduce more liquids reduces the tendency
       to squeeze biosolids into the filter  cloths
       because substantial quantities of water are
       eliminated  before   starting  the   pressing

      Removes  water uniformly  because the
       pumping cycle is only the first part  of the
       overall cycle.

       The cycle time for  a selected cake  solids
       concentration is usually lower (USEPA,
      Higher cake solids content improves release
       of the cake from the filter cloths.

       Wastewater solids only need to be pumped
       into the diaphragm filter press at pressures
       up  to  865  kPA  (100   psi)  reducing
       maintenance costs (USEPA, 1987).


There  are  also several  disadvantages  to using
recessed-plate filter presses compared with other
mechanical dewatering  methods, as follows:

      Batch   operation   produces   more
       heterogeneous influent (WEF, 1992).

      Process is mechanically complex.

       Capital  costs are  relatively high (WEF,

      Requires special support structure (WEF,

       Requires relatively large area (WEF, 1992).

       Filter cloth preparation, cleaning, and cake
       removal may be operator intensive (WEF,

       Cannot  be  totally   enclosed,  leaving
       operators exposed to  odors, gaseous  and
       vaporous sulfur compounds, and ammonia
       during the cake  release phase.

      When lime and ferric  chloride  are used in
       conditioning,  then account for a significant
       portion (15 to 40 percent) of the cake solids
       offsetting the weight  reduction  of high
       water removal efficiency.

      May  require  polymers   for   optimum


Recessed-plate filter presses are sized based on the
volume of solids to be dewatered.  To determine the

number and  size  of presses  for  a project, the
following information must be determined:

      Amount  of primary solids that will  be
       flowing through the plant per day.

      Amount  of  waste  activated  solids  or
       trickling filter solids produced per day.

       Amount of tertiary solids produced per day.

       Volume of thickened solids to be dewatered
       per day.

      Seasonal  variation in solids production.

      Range of solids  concentration in the feed

       Future increases in biosolids.

       Changes  in solids quality or quantity from
       industrial  sewer  users or in-plant process

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

Pilot testing by the vendor offers the best way to
obtain data  on  the  important  design  aspects
(USEPA, 1987).

Pressure is determined by filter feed-pump output
(Kemp, 1997).  Presses are usually designed to
operate at 689.5 kPa  (100 psi) or  1551  kPa (225
psi)  terminal  pressure.   Progressive-cavity  and
piston-membrane  pumps  have  been  used  as
filter-feed   pumps   in  these   systems.     A
progressive-cavity pump must have variable-speed
and high turndown-ratio capabilities to  meet low
flow requirements at the end of the cycle (Kemp,
1997).  A piston-membrane  pump automatically
compensates for increasing pressure by bypassing
hydraulic  fluid and reducing  the  pump's  stroke

Recessed-plate  filter   press   installations  are
mechanically complex (Kemp,  1997).   System
components  may  include conditioning  tanks,
mixers,   multiple  chemical-feed  systems,  feed
pumps  and a  filtrate  removal  system.    Ferric
chloride requires corrosion-resistant facilities and
extreme  caution  in handling.    The press  has a
hydraulic power-pack system and other mechanical
accessories for plate shifting and washing, as well
as drip trays (Kemp 1997). System components are
generally  reliable, but  require routine  inspection
and lubrication.

Most buildings  must be custom  designed  to
accommodate  a plate and frame filter press. The
cake is released to fall into a bin below the floor of
the press and must be moved to a truck or roll-off

Because recessed-plate filter presses operate in a
batch mode, the system may require a liquid storage
tank.  The operator may want to remove solids from
a digester or settling tank in small quantities every
15 minutes rather than in large quantities every
several hours.

A wide  variety of filter cloth material is available.
The manufacturer should test to determine the best
cloth for each  facility.  Selecting the correct filter
cloth will  improve  release  of cake, minimize
cleaning requirements, and maximize service life.


Recessed-plate filter presses result in the highest
cake solids  content and the highest rate of solids
capture compared to belt presses and centrifuges
(Kemp, 1997).

Recessed-plate  filter  presses  provide  a  good
alternative  for   processing  solids  with   poor
dewatering characteristics (WEF, 1992).  Pressure
filtration  allows  many  types  of  solids  to  be
dewatered to a solids content above 30  percent.
Table 1  shows the performance of various types of

domestic wastewater solids, with 10 to 30 percent
lime and 5 to 7.5 percent ferric chloride on a dry
weight basis added for conditioning.

Type of
Primary + WAS
Primary + WAS +
Trickling Filter
Primary + WAS +
Primary + WAS +
FeCI3 - digested
Tertiary with lime
Tertiary with
TS (%)
Cycle Time
TS (%)
  Source: WEF.1992 and conversations with
  manufacturers and operators.
Septic sludges  are difficult to dewater.  Adding
potassium permanganate to the thickener or prior to
pressing will reduce odors and improve dewatering.

Tests with polymer instead of lime  and  ferric
chloride resulted in a cake of 45 percent total solids
(TS) for primary solids and a cake of 35 percent TS
for a mixture of primary solids and waste activated
sludge (WAS).


Unless the inorganic content of the feed solids is
high,  conditioning  chemicals  are required for
successful filter press dewatering (Kemp 1997). In
the past, filter  presses relied on lime and ferric
chloride for conditioning.  While these chemicals
typically produced a dewatered cake with more than
40 percent solids content, they increased the mass
to be stored, transported, used or disposed. Lime is
also associated with ammonia releases which must
be considered in overall facility  design, including
ventilation and odor control requirements.
Operating a filter press manually in a small plant is
simple (Kemp,  1997).   Batches of  solids are
preconditioned and fed to the press. Monitoring is
not required  during the filtration cycle.   If cake
release is good, it will drop cleanly from the cloth
when the plates  are shifted.  In larger units, plate
shifting  is   automatic;  smaller  units   use  a
power-assisted plate shifter.

Facilities  with  multiple  presses  need  a  fully
automatic system for efficient operation  (Kemp,
1997).  Maintain proper chemical dosages, open
and close the press, and blow out the core at the end
of the cycle.   Even in an automated system, the
plate-shifting  step must be initiated manually so the
facility can prepare to receive the cake drop.

Filter cloths   require  periodic washing   (Kemp,
1997). Larger presses have automatic washers that
require a high-pressure pumping system to supply
spray water. In some installations, the press can be
filled with an acid cleaning solution to remove scale
deposits when lime is  used for  conditioning.  Acid
washing may reduce filter cloth life and replacing
filter  cloths is labor-intensive.  Filter cloth life
depends on the material, solids type, conditioning,
and washing frequency.

The  use  of polymer in variable speed plate and
frame presses requires automation to control the
dose as polymer dose is related to the volume  of
filtrate exiting the press.  Polymer conditioning has
been used with some success in recessed-plate filter
press dewatering (Kemp, 1997).  High cake solids
are possible, but cycle times  are long  and the cake
often sticks to the cloth, requiring assistance for its
removal.  Residual solids often remain on the cloth,
reducing solids capture and requiring more frequent
cleaning. Polymer conditioning is most successful
in diaphragm  systems because dosing is not  as
complicated.  Filtration cycle times with lime and
ferric chloride conditioning range between one and
three hours. With polymer conditioning, filtration
cycles may exceed three hours and tend to dewater
the core,  making core blowout ineffective.  An
advantage  of polymer conditioning  is  that  it
produces  cake with fewer inert  solids, enabling
more solids to be processed per  cycle than with
lime and ferric chloride.  The cake contains more
volatile solids than when processed with inorganic

conditioning and therefore can be disposed of by
incineration  or other type of thermal  processing
(Kemp, 1997).

The degree of operator activity associated with filter
presses is similar to that of belt presses. Although
the press operates unattended during filtration, the
system uses  a batch process that requires regular
operator attention to fill and unload the press
(Kemp, 1997).   When  filtration is complete,
compressed air should be used to blow out the core
since it is filled with partially dewatered cake at the
end of the filtration cycle.

It is not possible to  take grab samples of solids
during the operation with  a diaphragm  filter press,
requiring the  creation of  a  directed sampling
program to  obtain a full description  of machine
operation (USEPA, 1987).

Record-keeping is essential for all operations that
require conditioning (USEPA, 1987). The operator
must keep a log of the lime and ferric  or polymer
dosage required to reach a given degree of cake
solids with  a particular blend or type of solids
(USEPA, 1987).  It is also helpful to keep track of
pressing  time  and  filtrate quality  to  gauge
performance of the filter cloths.


Recessed-plate filter  presses carry relatively  high
capital costs compared  with other   mechanical
dewatering methods due to equipment and the need
for standby capability for cake handling. Operation
and  maintenance  (O&M)   costs may  also be
relatively  high.   O&M  cost elements  include
chemicals (for sludge conditioning and precoating),
cloth  washing  and  replacement,  and  operator
activity.   Operation costs  for  the  filter press
facilities  at  Williamsburg,   Virginia  total
approximately $40/dry ton and maintenance costs
amount to $12/dry ton (1987 costs) (WEF 1992).

Other Related Fact Sheets

Alkaline Stabilization of Biosolids
September 2000

In-Vessel Composting
September 2000

Land Application of Biosolids
September 2000

Odor Management in Biosolids Management
September 2000

Centrifugal Dewatering and Thickening
September 2000

Belt Filter Press
September 2000

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

1.     Baker, D. R. and Johnston, T., 1999. "Lime
      Addition   Does More   Than   Stabilize
      Biosolids," Biosolids  Technical  Bulletin
       Septemebr   1999,   Water  Environment
2.     Garvey, D.D. and Ferrero, T., 1999.  "Lime
      Conditioning Produces Exceptional Quality
      Biosolids"  Biosolids Technical  Bulletin
       September   1999,   Water  Environment

3.     Kemp, Jay S.,  1997.  "Just the Facts on
      Dewatering  Systems:  A Review  of the
      Features of Three Mechanical Dewatering
      Technologies".   Water Environment &
      Technology, December 1997.

4.     Netzsch,     Inc.,     1999.

5.     USEPA, 1987. Design Manual: Dewatering
      Municipal Wastewater Sludges.

6.     Water Environment  Federation,  1992.
      "Operation  and Maintenance  of Sludge
      Dewatering Systems: Manual of Practice
      No. OM-8."


John Schon
Butler Area Sewer Authority
Butler, PA 16001-3256

Alex Yaz
Netzsch, Inc.
119 Pickering Way
Exton, PA 19341-1393

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 Ave., NW
                                                       Washington, D.C. 20460
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