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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