vvEPA
United States
Environmental Protection
Agency
Office of Water
Washington, D.C.
EPA 832-F-00-058
September 2000
Biosolids
Technology Fact Sheet
Recessed-Plate Filter Press
DESCRIPTION
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
purposes:
• To reduce volume, saving money on storage
and transportation.
To eliminate free liquids prior to landfill
disposal.
• 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
applied.
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
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receive the solids. The cake drops as each chamber
is opened (Kemp, 1997).
Trays gtcessegj
flstts
Source: Netzsch, Inc. 1999.
FIGURE 1 RECESSED-PLATE FILTER
PRESS
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
savings.
APPLICABILITY
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
beds.
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
possible.
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.
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ADVANTAGES AND DISADVANTAGES
Advantages
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
techniques.
• 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
operation.
• 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,
1987).
• 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).
Disadvantages
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,
1992).
• Requires special support structure (WEF,
1992).
Requires relatively large area (WEF, 1992).
Filter cloth preparation, cleaning, and cake
removal may be operator intensive (WEF,
1992).
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
performance.
DESIGN CRITERIA
Recessed-plate filter presses are sized based on the
volume of solids to be dewatered. To determine the
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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
solids.
Future increases in biosolids.
Changes in solids quality or quantity from
industrial sewer users or in-plant process
changes.
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
volume.
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
container.
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.
PERFORMANCE
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
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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.
TABLE 1 PERFORMANCE FOR
VARIOUS TYPES OF DOMESTIC
WASTEWATER SOLIDS
Type of
Wastewater
Solids
Primary + WAS
Primary + WAS +
Trickling Filter
Primary + WAS +
FeCI3
Primary + WAS +
FeCI3 - digested
Tertiary with lime
Tertiary with
aluminum
Feed
TS (%)
3-8
6-8
5-8
6-8
8
4-6
Typical
Cycle Time
(hours)
2-2.5
1.5-3
3-4
3
1.5
6
Cake
TS (%)
45-50
35-50
40-45
40
55
36
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).
OPERATION AND MAINTENANCE
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
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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.
COSTS
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).
REFERENCES
Other Related Fact Sheets
Alkaline Stabilization of Biosolids
Management
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
Centrifugal Dewatering and Thickening
EPA-832-F-00-053
September 2000
Belt Filter Press
EPA-832-F-00-057
September 2000
Other EPA Fact Sheets can be found at the
following web address:
http://www.epa.gov/owmitnet/mtbfact.htm
1. Baker, D. R. and Johnston, T., 1999. "Lime
Addition Does More Than Stabilize
Biosolids," Biosolids Technical Bulletin
Septemebr 1999, Water Environment
Federation.
2. Garvey, D.D. and Ferrero, T., 1999. "Lime
Conditioning Produces Exceptional Quality
Biosolids" Biosolids Technical Bulletin
September 1999, Water Environment
Federation.
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.
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4. Netzsch, Inc., 1999.
http://www.netzschusa.com/FilterPress/fil
terhome2.htm.
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."
ADDITIONAL INFORMATION
John Schon
Butler Area Sewer Authority
lOOLitmanRoad
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
!MTB
Excelence fh tompfance through optfhial tethnltal solutfons
MUNICIPAL TECHNOLOGY BRANCH
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