f/EPA
United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
Office of Municipal
Pollution Control
Washington DC 20460
Research and Development EPA/600/M-86/017
June 1986
Design Information
Report
Recessed Plate Filter Presses
The U.S. Environmental Protection Agency has undertaken a program to help municipalities and engineers
avoid problems in wastewater treatment facility design and operation. A series of Design Information
Reports is being produced that identifies frequently occurring process design and operational problems
and describes remedial measures and design approaches used to solve these problems. The intent is not to
establish new design practices, but to concisely document improved design and operational procedures
that have been developed and successfully demonstrated in field experiences.
With an increased emphasis being placed on environmental concerns associated with the disposal of
sludges from wastewater treatment facilities, there has been a growing awareness of the need for
improved efficiency and reliability in the performance of in-plant sludge treatment processes. The
dewatering of sludges is an important step in the total sludge processing train, and can have a negative
impact on the effectiveness and cost of subsequent sludge treatment processes. Recessed plate filter
presses are sometimes used when a high solids content sludge cake is desired, and they are often
considered as an alternative sludge dewatering device when upgrading an existing facility or planning a
new facility.
Introduction
This report presents current problems associated
with the selection, design and operation of recessed
plate filter press systems at municipal wastewater
treatment facilities. It discusses application of both
fixed volume and variable volume recessed plate
presses, as well as selection and application of
auxiliary equipment associated with these presses.
Major factors involved in successful application of
recessed plate filter presses are simplicity and
durability of the press; proper selection and applica-
tion of auxiliary equipment; and proper selection of
cloth filter media.
Recessed plate filter presses evolved from technology
developed for sugar manufacturing (11), and have
been successfully used in dewatering municipal
sludges since the late 1800's. Introduced in the
United States in the 1920's, there are currently 44
known recessed plate filter press installations in this
country. There are presently nine major suppliers of
this device in the United States. With this level of
competition, improvements to the equipment and its
operation are gradually being made and are expected
to continue.
Equipment Description
Both fixed volume and variable volume recessed plate
filter presses are available. The devices are used to
dewater chemically conditioned sludge on a batch
basis using mechanically-applied pressure to
achieve high sludge cake solids content. Simplified
cross sections of the devices are shown in Figure 1.
The fixed volume recessed plate filter press consists
of a series of parallel plates, each fitted with a filter
cloth and rigidly held together in a structural frame.
Sludge is pumped into a series of chambers formed by
recesses in the plates, which are held together by
hydraulic or electro-mechanical screw type mechan-
isms. As the sludge is pumped into the press, the
solids are captured within the chambers while excess
water (filtrate) passes through the filter cloth and
leaves the press through the filtrate collection system.
The accumulation of sludge solids within the cham-
bers causes the pressure in the sludge feed system
(filtration pressure) to increase. The sludge feed
pump continues to pump until reaching the terminal
filtration pressure of no more than 100 psi for low
pressure presses, or no more than 225 psi for high
pressure presses. Once this pressure is reached, the
pump will continue to maintain the terminal pressure
as the filtrate continues to drain from the sludge.
During the filtration cycle, the filtrate passes through
the filter cloth into collection ports located at the
corners of each plate and is usually discharged to a
filtrate weir box at the head end of the press. Filtrate
flow and quality can be monitored at this location.
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Figure 1. Recessed Plate Filter Press Cross Sections1
Cake
Slurry Inlet
Filtrate Outlets
Fixed Volume
Slurry Inlet
Top, Center or Bottom
Cake
Flexible
i j Membrane
Filtrate
Outlet
(Top or
Bottom)
Cloth
Cake Under
Compression
Flexible
Membrane
Shape of Filter Chamber
During Filtration
High Pressure
Water
Filtrate
Shape of Filter Chamber
During Cake Compression by Diaphragm
Variable Volume
The variable volume recessed plate filter press differs
from a fixed volume press in that it utilizes a flexible
membrane or diaphragm to provide a second squeez-
ing phase to the sludge within each chamber after
initial formation of the filter cake. At a predetermined
filter press feed pump pressure, the space between
the press plate and its flexible membrane will be filled
with water to provide additional compression of the
sludge.
In either type of press, the dewatering cycle is
complete when filtrate is reduced to a minimum flow
at the filtrate weir box and/or cycle time (determined
by experience) expires. Before the filter plates are
separated, the sludge feed pump is shut down and
liquid sludge is cleared from the sludge feed port by
the application of compressed air (core blowing). Core
blowing keeps unprocessed wet sludge in the press
core from running over the face of the filter plates
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when the plates are separated. A plate shifting
mechanism controls the sludge cake discharging
operation by allowing only one plate to be separated
at a time. When the plates on a fixed volume press are
separated, the weight of the sludge cake allows it to
drop from the plates onto sludge handling facilities
located beneath the press. On a variable volume
press, cake release is enhanced by a mechanical
system that shifts the filter cloth around the bottom of
each plate and then back into position.
Typical performance data for recessed plate filter
presses for various types of sludge are shown in Table
1. The sludge cake produced on recessed plate
presses has the highest cake solids content of the
currently available dewatering devices. For this
reason, it is amenable to all dry sludge disposal
methods. When recessed plate dewatering is followed
by incineration, process integration considerations
should recognize trade-offs between low moisture
content of the sludge cake and the inert solids
concentration resulting from the conditioning step.
Conventionally conditioned sludge cake has a low
moisture content but it can have a high inert solids
level, while polymer conditioned sludge cake may
have a higher moisture content but also a higher level
of volatiles. Either conditioning step produces a cake
of somewhat different characteristics, but both cakes
are amenable to incineration with trade-offs between
moisture content and inert solids.
In addition to the actual dewatering equipment
discussed above, there is an interrelated system of
auxiliary components that supports the operation of
the recessed plate filter press. This auxiliary equip-
ment includes:
1. Sludge Feed Pumps: These positive displace-
ment pumps must be capable of delivering a
wide range of pressures and flows. At the
beginning of the filtration cycle the pumps
deliver a maximum flow against a very low back
pressure. As filtration progresses and the back
pressure increases due to solids accumulation
in the press, the flow rate drops to a very low
rate at the terminal pressure. The pumping
system includes specially designed flow control
devices which automatically adjust flow rate
with increasing pressure. Piston-membrane,
hydraulic ram, and progressing cavity pumps
have all been used for this service.
2. Chemical Conditioning. Conditioning of munic-
ipal sewage sludge can be achieved either by
adding lime and ferric chloride, lime only, alum,
or polymers or by adding ash or other granular
materials. Lime and ferric chloride are most
commonly used although polymer has been
recently shown to be cost effective at some
facilities.
3. Filter Cloth/Plate Washing System. High pres-
sure washwater (1500 psi) is required to clean
accumulated sludge from the filter cloths and
plates.
4. Acid Washing. When lime is used to condition
sludges, a hydrochloric acid washing system is
normally provided to eliminate the build-up of
lime scale on the equipment and in the asso-
ciated piping.
5. Core Blowing. This system uses compressed air
to blow liquid sludge out of the sludge feed ports
Table 1. Typical Dewatering Performance of Recessed Plate Filter Presses*
Type of Sludge
Raw Primary
Raw Primary with
less than 50% WAS
Raw Primary with
more than 50% WAS
Raw Primary with
Trickling Filter
Raw WAS
Digested Primary
Digested Primary
with WAS:
—less than 50% WAS
(% Solids)
5-10
3-8
1-4
5-6
4-5
8
3-10
Conditioner
(% Lime) (?
10
10
12
20
15
30
10
6 FeCI3)
5
5
6
6
7.5
6
5
Filter Cake
(% Solids)
45
50
40-45
50
45
50
38
35-45
50
40
35-45
Cycle Time (Min)
Fixed Volume Variable Volume
120
90
150
120
150
120
120
150
120
120
120
20
--
-.
--
--
--
--
--
--
--
—more than 50% WAS ' 2-6
Heat Treated Primary 12-16
with WAS
15
7.5
50
40-50
50
50-60
90
150
90
20
"Summary of data provided in references 5 and 10.
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before the press is opened to help minimize the
frequency of filter cloth and plate washing.
6. Precoating System. When sludge cake regularly
adheres to the plates after the press in opened, a
precoating system is used to improve cake
release. This system pumps a slurry of ash or
other similar substance to the filter to coat the
filter cloths before the sludge introduced. A
precoating system is not necessary on a variable
volume press because cake release is assisted
by a mechanical system that pulls the filter cloth
down between the plates.
7. PrefillingSystem. Priortopumpingsludgetothe
press, it should be filled with effluent water to
purge any trapped air from the press. Elimi-
nating this step can sometimes result in the
press being only partially filled during a cycle.
Design Considerations
The major components of the recessed plate filter
pressare its frame, plates, filter cloth, hydraulic plate
closing mechanism, and plate shifting mechanism.
The design and materials of construction vary among
manufacturers.
The structural frame of the filter press consists of a
fixed head, a moveable head, and a plate support
system. The two most common types of plate support
systems are the side bar and overhead types, as
illustrated in Figure 2. The side bar type supports each
side of the filter plate at a point slightly above the
center of the plate. In the overhead type, the plates
are hung from a support beam by a carriage assembly
attached to the top center of each plate. Problems
have been reported with the side bar type support
related to frequent jamming of the plate shifting
mechanism. The side bar design also prevents easy
access to the plates during the cake discharging
operations.
Presses are available with plates ranging in size from
one-half meter square to two meters by three meters.
They can be grouped to form as few as four to as many
as 175 chambers. The materials most commonly
available for filter plates of fixed volume presses are
gray cast iron, ductile iron, rubber covered steel,
epoxy coated steel, polypropylene and polyester.
Materials used for filter plates of variable volume
presses include polypropylene, ductile iron and steel.
All of these plates are equipped with an elastomeric
diaphragm. Ductile iron plates have proven to be the
most durable on both types of press. Polypropylene
plates less than 1.2 meters square have also been
shown to have a relatively long life. The rubber
covering on steel plates can crack and result in
corrosion problems if the covering is not replaced
when cracks are identified.
Filter cloth for sewage sludges is available with
different permeabilities and is made of polypropylene,
polyester, or nylon. Polypropylene is the most com-
monly used due to its resistence to both acid from the
washing system and alkaline conditioning chemicals
such as ferric chloride and lime. Polyester is more
durable than the other materials since very little
stretching will occur over the life of the cloth;
however, it is the most expensive. Nylon should only
be used where conditioning and/or cleaning chem-
icals are shown not to deteriorate the cloth. The
permeability of filter cloth ranges from 30 to 100
cubic feet per minute (cfm). A permeability of 60 cfm
is normally used. Filter cloth having monofilament
construction and a satin weave has been shown to
reduce cloth blinding and provide better cake release
than other construction methods (i.e. multifilament,
basket weave, and quill weave). Calendering, an
optional process in cloth manufacturing that presses
or irons the fibers in the cloth to produce a smooth
finish, has improved cake release. A good quality
polypropylene filter cloth will have a life between
4,000 and 10,000 cycles, depending on sludge type
and press operation and maintenance.
Fixed volume presses are normally designed to
operate at either 100 psi or 225 psi terminal pressure.
These levels correspond to the pressure rating of
commonly available piping. The operating terminal
pressure of the press can be set below the maximum
design pressure and is dependent on the cake solids
concentration desired. Fixed volume press systems
designed for a terminal pressure of 100 psi are
normally used for lower dry solids applications where
30 to 35 percent dry solids are required. Press
systems designed for 225 psi are normally used for
higher dry solids applications where 40 to 50 percent
dry solids are required. Variable volume presses are
generally designed for 100-125 psi during the initial
stage of the dewatering cycle followed by a final
compression of 200-250 psi.
When designing a recessed plate filtration system
together with its auxiliary equipment, other important
features that should be considered include the
following:
• To remove solids from the washwater and reduce
spray bar orifice plugging, strainers should be pro-
vided on the high pressure washwater system.
Mist suppression or control devices should be
installed on spray bars.
• When a press has more than 75 filter chambers,
sufficient piping and pumping flexibility should be
provided in order to feed the press from both ends.
This will decrease the time to fill the press, and
reduce the chance of operating problems.
• In order to minimize the impact of falling sludge
cake on the collection equipment, cake-breaking
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Figure 2. Recessed Plate Filter Press Support Systems1
Moveable Head
J I
Overhead Support System
Fixed Head-
• Plates
.— Moveable
J Head
Opening/Closing System
Side Bar Support System
Side View
bars should be installed under each filter to break-
up the filter cake. In those installations where
sludge cake drops onto a conveyor, the conveyor
should be specified with additional rollers at this
location for added durability.
In order to facilitate filter plate removal, an
overhead crane, monorail, or other device should
be provided.
Where feed sludge solids concentrations are likely
to vary, sludge blending/thickening tanks are
recommended prior to pumping sludge to the
presses. Without these tanks sludge conditioning
is more difficult to control and the time required for
dewatering is harder to predict.
In those installations where lime conditioning will
be practiced, the ventilation system in the dewater-
ing room should be adequately sized to handle
ammonia odors. Ammonia odor can be objection-
able, cause unhealthy side effects and impact
operators' performance. A minimum ventilation
rate of 6 air changes per hour for summer
ventilation and 3 air changes per hour for winter
ventilation should be considered. Other problems
and remedies associated with lime handling
systems are discussed in a recently published
brochure by EPA. (12)
• When incineration follows dewatering, a shredder
must be incorporated into the filter cake handling
system. Filter cake pieces having any dimension
larger than one inch will be only partially burned,
forming clinkers. Clinker build-up can lead to
jamming of the incinerator ash removal system.
Comparison of Press Types
In general, both types of presses can achieve the
same range of filter cake dry solids. However, the
variable volume press has a shorter cycle time and
lower sludge throughput per cycle due to the second
stage "squeezing" action that this press provides. In
comparable applications, a fixed volume press of the
same size and number of plates can produce two to
four times the cake volume per cycle than can be
produced on a variable volume basis. This volumetric
limitation means that the variable volume press has
to operate several more cycles per day than a fixed
volume press in order to dewater the same volume of
sludge.
In those applications where sludge is anticipated to
be difficult to dewater, a variable volume press should
be considered. Underthese circumstances, a variable
volume press may be more cost effective due to the
cost savings obtained in the reduction of conditioning
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chemicals (due to the second squeezing phase)
required over that necessary for a fixed volume press.
Another important consideration when choosing
between the variable and fixed volume press is the
mechanical complexity and estimated operation and
maintenance requirements of each press. The vari-
able volume press, which differs in design complexity
among the different manufacturers, is a far more
mechanically complex machine than the fixed volume
press and it requires more operator attention and
greater maintenance. Table 2 presents a summary of
the advantages and disadvantages of fixed and
variable volume presses that should be considered in
the selection and application of this equipment.
Problem Areas and Potential Solutions
The major problems associated with the design and
operation of recessed plate filter presses can be
categorized within the following areas:
• Equipment quality
• Operational concerns
• Process integration
• Auxilliary system selection
Equipment Quality Problems
Equipment problems involve filter plate deflection,
rapid filter cloth wearing, deteriorating stay-bosses,
and defective plate shifter mechanisms.
Plate Deflection—Plate deflection problems gener-
ally result in plate breakage. These problems are most
common with polypropylene and gray cast iron plates.
In general, the life of a polypropylene plate that is
larger than 1.2m square has been shown to be
approximately 5 to 7 years. This is a relatively short
life when compared with fixed-volume recessed plate
installations using ductile iron plates for more than
35 years without any plate breakage. The major
causes of plate deflection and associated remedial
measures are shown in Table 3.
Filter Cloth Wear—The wearing of filter cloth in the
area of the stay bosses is a frequently reported
problem. The cylindrical stay bosses are interior
supports in the recessed area of the plate. These
supports are located directly opposing each other on
either side of the chamber that is formed by the
plates. As the plates undergo increasing pressure the
stay bosses butt up against each other and prevent
excessive plate deflection, ensuring a uniform cake
thickness. The minor plate deflection that does occur
produces a rubbing action at the stay bosses causing
the filter cloth to wear. Remedial measures include
either:
(1) Use of sewn-in reinforcement patches on the
filter cloth in the area of each stay boss, or
Table 2. Advantages and Disadvantages of Fixed Volume
and Variable Volume Recessed Plate Filter Presses
Type of
Filter Press
Advantages
Disadvantages
Fixed • Higher volumetric
Volume capacity requires
fewer dewatering
cycles per day
• Less complex
instrumentation
• Fewer moving parts
• Longer plate life
• Lower maintenance
Variable • Dewaters marginally
Volume conditioned sludges
• Fewer chemicals
required for
conditioning
• Shorter cycle time
• Precoating system is
not required
• Dewaters only well
conditioned sludges
• More chemicals
required for
conditioning
• Longer cycle time
• Limited volumetric
capacity, requires
more cycles per day
• Mechanically complex
• Complex
instrumentation
• Labor intensive
filter cloth
replacement
• Shorter plate life
• Higher maintenance
Table 3.
Causes of and Remedial Measures for Plate
Deflection
Causes
Remedial Measures
High differential pressure
across plates
2. Use of a center feed
system on variable-volume
presses with polypropylene
plates.
3. Residual studge build-up
on plates.
Reduce operating
pressure of high
pressure presses to 150 psi.
Use polypropylene plates
1 2 m square and smaller
which have not exhibited
this problem.
Avoid the selection of
center feed systems m new
plant design. In existing
plants, use good sludge
feed techniques and
maintain good
housekeeping practices
3. Increase frequency of
plate washing.
4. Uneven sludge distribution 4. Improve sludge feed
and cake formation. distribution as summarized
in Table 4.
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(2) Use of stainless steel protective covers which fit
directly over the filter cloth and protect the filter
cloth from wear, help keep the filter cloth in
place, and minimize filter cloth stretching. The
covers conform to the shape of the stay boss and
are fastened at their centers with a machine
screw, threaded into the stay boss.
Stay Boss Deterioration—The deterioration of rub-
ber stay bosses is associated with deflection of the
steel plates. Once the stay bosses have deteriorated,
increased flexing of plates will ultimately lead to
breakage. Consequently, it is advisable and econom-
ical to repair deteriorated stay bosses as soon as
possible. Replacement of stay bosses and the entire
rubber covering of the filter plate can be performed at
a fraction of the cost of an entirely new plate.
Deterioration of stay bosses can be minimized by
following the remedial measures discussed under
Plate Deflection, or by use of stainless steel covers.
Defective Plate Shifter Mechanism—On some
filters, a defective plate shifter mechanism may move
several plates at once. When this occurs the operator
must manually separate the plates to remove sludge
cake and clear the plates, thus increasing the cycle
time of the dewatering operation. This problem is a
design deficiency that requires corrective modifica-
tion to the plate shifter mechanism. In some cases,
field modifications have been made by plant person-
nel. These modifications include braising a catch onto
the plate shifter which prevents it from attaching to
more than one plate at a time. However, because of
the equipment stresses involved during this part of
the dewatering operation, such a modification should
not be made without the consultation and evaluation
of the equipment manufacturer.
Operational Concerns
Problems related to operation of recessed plate filter
presses include nonuniform feed sludge distribution,
improper sludge conditioning, poor cake release,
inoperable or disconnected safety curtains, inability
to estimate cycle completion, and lime scaling.
Although many of these problems can be related to
other problem areas, particularly equipment design, it
is believed that they are most easily solved in existing
facilities by improved operating practices.
Sludge Feed Distribution—Unequal distribution of
feed sludge can impart a pressure differential be-
tween adjacent filter press plates, causing plate
deflection. Excessive deflection will ultimately cause
the plates to fail, stay bosses to deteriorate, and filter
cloth to wear out. In addition, unequal feed sludge
distribution results in a low cake solids concentration
due to incomplete cake formation. The causes of this
problem and the remedial measures taken to solve
unequal distribution of feed sludge are shown in
Table 4. Prefiltration, stated as one cause in this table.
Table 4.
Causes and Remedial Measures for Unequal
Distribution of Feed Sludge
Causes
Remedial Measures
Prefiltration of sludge
at feed end of press
2 Cloth Blinding
1 Prefill press with plant
effluent followed by rapid
filling with feed sludge
2 a. Modify sludge feed rate
b Experiment with
different types of filter
cloth (see section on
Design Considerations )
c. Optimize uniformity of
sludge feed by proper
storage and blending
3. Poorly conditioned sludge 3. Perform capillary suction
time (CST) test or Buchner
funnel test on conditioned
sludge prior to feeding
sludge to filter
generally occurs on presses with more than 75 plates
and/or when air is trapped in the press. When
prefiltration occurs, sludge in the initial chambers
begins to filter before downstream chambers are
filled.
Sludge Conditioning Tests—A capillary suction time
(CST) test is a rapid, easy, inexpensive and reproduc-
ible method for determining whether a sludge is
properly conditioned. This test measures the time it
takes to wet a given area of filter paper with filtrate
that is withdrawn from a sludge sample by the
capillary forces of the filter paper. The shorter the
measured time the better is the filterability of the
sludge. If the approximate CST time is known for
optimally conditioned sludge, the operator can test
sludge samples prior to dewatering to determine
whether they have been properly conditioned. The
Buchner funnel test also measures sludge filterability.
Although this test requires some additional work to
get the desired results, it can be used effectively in
avoiding problems due to poorly conditioned sludge.
Poor Filter Cake Release—The causes and solutions
associated with poor filter cake release are shown in
Table 5. Although operators can manually remove
adhering sludge from the press (generally with a
paddle), this is not an optimum solution. This problem
should be avoided because it results in increased
cycle time that is required, increased frequency of
plate and filter cloth washing that is needed, and the
possibility of damaging the filter cloth.
Safety Curtain Reliability—The safety curtain is
designed to protect the operator from injury when the
plate shifter mechanism is operating. Often referred
to as a "light curtain", it consists of a series of narrow
beamed lights at one end of the press, focused on a
series of matching photocells at the opposite end of
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Table 5.
Causes of and Remedial Measures for Poor Cake
Release
Causes
Remedial Measures
1. Worn or improper filter
cloth
1. a. Replace worn filter cloth
b Experiment with filter
cloths of different
materials, permeability
ratings, and surface
finish and select
more applicable cloth
2. Poorly conditioned sludge 2. Optimize sludge
conditioning by
performing CST or
Buchner funnel test on
feed sludge.
3. Lack of precoating of
filter cloth
Utilize or incorporate
a precoating system
the press. If the light curtain is interrupted by the
operator reaching into the press, the plate shifting
operation stops automatically. Due to equipment
corrosion, electrical failure, faulty alignment of the
transmitter and receiver, or simply inactivation by
operators, many light curtains are often not in use.
This equipment performs an important safety func-
tion and its disabling should not be permitted.
Additional training, where necessary, should be
provided to operators on proper press operation with
this safety system functioning.
Determination of Dewatering Cycle Completion—
Since the formation of filter cake in the filter press is
not observable, the operator relies on either one or
both of the following parameters to determine the
completion of a cycle:
1. Elapsed Time. Most manufacturers of this
equipment provide a time clock in their control
panel that car\be set based on expected length
of cycle. After the set period of time has expired,
an alarm will sound to notify the operator the
cycle is complete.
2. Filtrate Flow. This method entails the operator
monitoring the quantity of filtrate flow through
the weir box after the press has attained
operating pressure. Once filtrate flow has been
reduced to a minimum (determined by exper-
ience) the cycle is considered complete.
The successful use of either timing method requires
considerable operator experience. The control of
press operation relies on interrelationships among
process variables and the type and performance of
equipment, and a lack of experience can result in
poorly dewatered sludge. Consequently, the operator
must be aware of the normal range of operating
parameters discussed above as well as (a) time
required to fill the press, (b) degree of sludge
conditioning based on CST or Buchner funnel test
results, and (c) filtrate quality. Because raw sludge
characteristics affect press performance, the operator
should also monitor such data as mix of primary and
secondary sludge, and sludge age. If these parameters
vary from the normal range, the operator should make
adjustments to conditioning chemicals or cycle time
during the cycle and determine potential problems
and solutions before the next cycle.
Lime Scaling—When lime is used for sludge condi-
tioning lime scaling has been reported to occur in the
chemical and sludge feed piping, on the filter cloth,
and on the filter plates. If the scale is allowed to
accumulate, cycle time may be increased, sludge
throughput can be reduced, cake release problems
can occur, cake dryness can be affected, and/or the
filter cloth can blind with sludge. It is often cost-
effective in those facilities where lime scaling is a
problem to add an acid wash system to periodically
remove the lime scale buildup.
The feasibility and cost-effectiveness of other condi-
tioning chemicals such as polymers could be con-
sidered to eliminate or reduce lime use. Some
installations have been successful in converting from
lime and ferric chloride to a highly charged cationic
polymer for conditioning sludge prior to dewatering.
However, despite some successes and a possible cost
savings (one facility reported a 68 percent condi-
tioning chemical cost savings), other facilities at-
tempting to switch to polymers have not achieved
adequate dewatering and have had to revert to other
conditioning chemicals.
Process Integration
Because recessed plate presses are capable of
handling nearly all types and mixtures of sludges, and
because their batch operation allows modification of
operating criteria with changes in sludge feed
characteristics, few problems are related to process
integration. The major problems that do occur include
improper removal or grinding of rags, and nonuniform
sludge feed characteristics.
Rag Problems—Rags carried through the waste-
water treatment process to the dewatering equipment
will adversely impact sludge conditioning and even-
tually plug the m-line mixers in the sludge feed lines.
To clear the feed lines, the dewatering process must
be temporarily shut down. Rags can also interfere
with plate closure and cause deflection. Conse-
quently, it is essential that upstream screening
and/or grinding equipment is properly operated and
maintained. Where problems persist, operators
should consider the addition of more effective screen-
ing and/or disintegration equipment (i.e., mechanical
bar screens in the headworks and/or grinders on
sludge feed pumps).
8
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Sludge Feed Continuity—Although recessed plate
filter press operation can be modified to accommodate
sludges of varied characteristics, press performance
can be most efficiently maintained if the sludge feed
characteristics do not vary abruptly. Sludges should
be blended prior to the addition of conditioning
chemicals and, where possible, small (one to two
hour detention time) holding tanks should be provided
prior to dewatering to provide a representative sample
for testing. This determines adjustments needed in
the conditioning process to provide an optimum feed
to the filter.
Auxiliary Systems
The problems associated with the auxiliary systems
for recessed plate presses include corrosion and
failure of pneumatic cylinders, and excessive misting
by filter media washing systems.
Pneumatic Cylinder Corrosion—High moisture con-
tent of the compressed air used in pneumatic
cylinders for recessed plate presses can cause
corrosion of the cylinders and ultimately lead to
failure. This problem can easily be solved by the
addition of air drying equipment to the compressed air
system.
Excessive Misting—Excessive misting during the
plate washing operation has caused corrosion and
subsequent failure of nearby mechanical devices,
instrumentation, and electrical devices. Some plate
wash systems are provided with brush assemblies
around the spray bar which effectively contain the
mist. Where brushes have not been used, spray
curtains have been installed and have successfully
contained the spray.
Summary
The design of a recessed plate filter press system for
municipal sludge requires careful consideration of
the need to attain the degree of dryness that can be
achieved with such devices. When this dewatering
alternative is selected, the designer and operator
must keep in mind that successful operation is
dependent upon a number of key factors, including
system integration, correct equipment design and
operation, and proper sludge conditioning.
The following considerations and recommendations
should be included in the design and operation of
recessed plate filter press facilities:
1. Equipment
• Fixed volume presses are less complex and
more easily maintainedthan variable volume
presses.
e Variable volume presses have features that
provide good results particularly when used
on sludges that are difficult to dewater.
• Filter precoating and prefilling systems
should be provided.
• Plate deflection problems can be reduced by
proper selection of plate material.
• Filter cloth wear can be reduced by reinforcing
the cloth and/or utilizing protective cups at
stay bosses.
• Reliable plate shifting mechanismsshouldbe
provided.
• Sludge blending and storage tanks should be
included prior to dewatering to assure sludge
feed continuity.
2. Operations
• Positive methods of rag removal and/or
disintegration should be provided.
• Proper sludge conditioning should be
achieved through use of CST or Buchner
funnel tests.
• Safety devices such as light curtains should
be installed, used, and maintained.
• Unequal distribution of sludge within the
presses causes equipment and performance
problems and should be avoided.
• The required dewatering time should be
estimated and monitored to assure that ade-
quate time is allowed.
• Start-up and on-going operator training
should be required.
3. Auxiliary Systems
• A mist suppression system should be provided
on spray bars
• A core blowing system to back-flush the
sludge feed lines should be provided to
minimize plate washing and ease housekeep-
ing.
• Where lime is utilized, an acid wash system
for scale removal is recommended.
• An automatic high pressure, filter cloth/plate
washing system should be included.
• Washwater systems should include strainers
to remove solids.
• To minimize safety and corrosion concerns,
ventilation should be adequate and odor
control should be practiced.
Acknowledgements
This report was prepared for the U.S. Environmental
Protection Agency by Metcalf & Eddy, Inc., Wakef ield,
Massachusetts under contract no. 68-03-3208.
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Mr. Francis L Evans, III, EPA Project Officer, was
responsible for overall project direction. Other EPA
staff who contributed to this work included:
Dr. Harry E. Bostian, Technical Project Monitor,
Water Engineering Research Laboratory
Mr. Walter Gilbert, Office of Municipal Pollution
Control
Dr. Joseph B. Farrell, Water Engineering Research
Laboratory
Metcalf & Eddy staff participating in this project
included:
Allan F. Goulart, Project Director
Thomas K. Walsh, Project Manager
Thomas C. McMonagle, Project Engineer
Kenneth D. Klint, Mechanical Engineer
Centerfor Environmental Research Information,
Cincinnati, Ohio, October 1978.
11. U.S Environmental Protection Agency, Process
Design Manual for Sludge Treatment and
Disposal. EPA-625/1 -79-011, Center for Envi-
ronmental Research Information, Cincinnati,
Ohio, September 1979.
12. U.S Environmental Protection Agency, Lime
Handling Systems-Problems and Remedies,
Office of Municipal Pollution Control, Washing-
ton, D.C., August 1984.
References
1. Greenwood, Stephen J. and Maier, Walter,
Computer Simulations and Process Studies of
Pressure Filtration for Sludge Dewatering.
Department of Civil and Mineral Engineering,
University of Minnesota, June, 1982.
2. Moir, Douglas N., Selecting Batch Pressure
Filters. Chemical Engineering, July 26, 1982.
3. Nelson, 0. Fred., Operational Expertise with
Filter Pressing, Kenosha, Wisconsin. Deeds &
Data, Water Pollution Control Federation, March
1978.
4. Sligar, Michael J., Chemical Selection and
Operational Considerations for Filter Press
Dewatering. Journal Water Pollution Control
Federation, Vol 56:4, April 1984.
5. Sludge Dewatering Manual of Practice No. 20.,
Water Pollution Control Federation, Washing-
ton, DC, 1983.
6. Thomas, C.M., The Use of Filter Presses for the
Dewatering of Sludges. Journal Water Pollution
Control Federation, Vol 43:1, January, 1971.
7. U.S Environmental Protection Agency, Process
Design Manual for Dewatering Municipal
Wastewater Sludges. EPA-625/1-82-014,
Centerfor Environmental Research Information,
Cincinnati, Ohio, October 1982.
8. U.S Environmental Protection Agency, Evalua-
tion of Dewatering Devices for Producing High-
Solids Sludge Cake, Report prepared under EPA
Contract No. 68-03-2455, Municipal Environ-
mental Research Laboratory, Cincinnati, Ohio,
1979.
9. U.S Environmental Protection Agency, Opera-
tions Manual Sludge Handling and Conditioning,
EPA-430/9-78-002, Washington, D.C., Febru-
ary 1978.
10. Harrison, J.R., Developments in Dewatering
Wastewater Sludges. EPA-625/4-78-01 2,
w
•&(). S. GOVERNMENT PRINTING OFFICE:1986/646-l 16/20850
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