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Michael Price, Edwards &Jones, UK, and David Shaw, US&

USA, chronicle the developments in the design and operation of

thefilter press that have turned it into a high-tech system

suitable for a variety of complex filtration applications.

How the Filter Press is Meeting Today’s

Demands

T hc filter press, and its well

proven technology as a

dcwatcriy machine, is c used throughout a number

of industries. Its appli-

cations arc not restricted or limited to the

dewatering of binsolids. It can operate

effectively in many other arcas, particularly

in the chemical and pharmaceutical fields

where complex filtration, cake washing and

air drying techniques arc required. In such

casts, the filtcrctl product and or the

filtrate can bc recovered efficirntly and

economically. In such applications the filter

press has been used succcssfullv for manv

scars, where it has provctl to lx reliable

and consistent in its operation.

Whilst the basic filter prcs5 and its

process principles arc gcncrally well

known, the modern day developments and

lcvcls of automation now available arc not.

This article reviexvs the process and

mechanical developments

of a modern day filter

press.

Filter prcssc5 prc- 1980

were always looked upon

as being robust hcavv i pieces of cquipmcnt. The)

wcrc gcncrallv viewed as

old fashioned looking, but

at the same time

recognised as txing

reliable and consistent in

their operation. They were

certainly well known and

instantly recognisablc in

the market place, their

appearance having changctl

little in over 100 years.

However, during the late

1970s and early 1980s the

market place demanded

larger, more cfficicnt and

certainly more

aesthetically dcsigncd

machinch.

It xvas during this time

that the rrcognisrd filter

press manufacturers made

a conscious decision to

review the design of their

cquipmcnt. This entailed rcvicwing the

materials of construction and manu-

facturing techniques, reappraising the

aesthetic appearance of the filter press

\vhilst, at the same time, reviewing the

potential for automation and larger

machines. This rcvcalcd that several radical

changes \vcrc necessary to bring the filter

press in lint with modern day thoughts

and technology.

Main Areas of Change (i) Fabrication of the main components

from steel (Figure l), replacing the more

accepted material of manufacture, i.e. cast

iron. This also led to considcrablc

manufacturing improvcmcnts: c l iz significant saving in Lvcight.

l Grcatcr flcxihility in manufacturing

techniques.

l Acsthctics and fabrications gave the

design engineers considcrabl~ more

scope and flcxibilit\ to rcvicw and

implement design changes in this

important arca.

(ii) The drive end, or ‘nut a5acmblv’ as it

\vas known, consisted of a motor driven

epicyclic gear box, driving either a single

or double scrc\v, depending on the size of

press. This type of drive, \vhile still used,

\vas essentially replaced by an hydraulic

system giving the following advantages:

l Reduced manufacturing time & costs.

l Easier access for maintenance purposes,

l Most components could bc obtained

‘off the shelf’ as standard hvdraulic

componcnta.

(iii) Filter plates: in terms of process

pcrformancc the plates and filter cloths arc

Filtration+Separation

fundamental to successful dewatering. Up

to and including the 196Os, cast iron was

predominately used for the manufacture of

filter plates, other materials of

construction were either not available,

uneconomical to produce or not suitable

for withstanding the high differential

pressures/temperatures involved during

the filtration process. Throughout the

197Os, however, plate manufacturers

invested heavily in the development of

new materials and the design and

construction of the plates. During this

time two materials were clearlv identified

as being suitable for the application of

pressure dcwatering, they were rubber and

polvpropvlene (Figure 2) I , Numerous other materials have also

been assessed over a period of time but, as

yet, none have been found suitable or

capable of withstanding the high

differential pressures involved.

Both rubber and polypropylene filter

plates are now used extensively to devvater

biosolids and other materials because they

also offer considerable advantages when

compared with cast iron plates, The

introduction of these two materials led to

the instigation of a major process initiative,

the introduction of the ‘diaphragm’ or

‘membrane filter plate’ (Figure 3). This

was achieved through considerable research

and development work being undertaken to

successfully develop this new innovation. It

is a rccogniscd scientific fact that ‘the

filtration time is proportional to the cake

thickness’ [2]; therefore, by effective use of

the membrane plate the filter press cvcle

time could be optimised. This opened up

substantial advantages to the press

manufacturers.

(iv) Mechanised plate movement and the

introduction of automatic cloth washing

machines. The earlv 1970s savv the

introduction of mechanisation to the

movement of the filter plates, early designs

incorporated a square threaded screw

mounted on the side of each side bar. A

carriage then traversed backwards and

forwards along each screw moving each

plate individually to facilitate cake

discharge. This mechanism, while effective,

was not practical in its operation due, in

part, to the length and stability- of the

screws during plate movement. The design

was subsequently revised to incorporate

reciprocating slide bars (Figure 4) for the

side bar machines which were more robust

and operated more effectively. However,

Filtration+Separation

the introduction of

hydraulic systems

to the filter press

saw, in many cases,

the replacement of

the reciprocating

mechanism with a

more adaptable

hydraulic system,

which in general

could be used on

either the side bar

or ovcrhcad beam

machines.

During the late

1960s and early

1970s cloth

washing was

traditionallv carried

out by either in-situ

vvashing, vvith a

portable washing

machine and a hand

lance, or removal of

the cloths from the

plates and washing in an industrial type

washing machine. Both methods were

effective and cleaned the cloths efficiently,

however, both methods were labour

intensive and created operational

difficulties.

This problem was addressed by the filter

press manufacturers and numerous designs

of automatic cloth washing machines vvere

introduced depending on the type of press

used. Such was the success of the machines

it is now accepted practice that the large

filter presses are fitted with a cloth washing

machine as standard.

Larger Machines During the early 1970s a considerable

number of filter presses were installed for

the dewatering of biosolids, some of which

were large, to deal with considerable

volumes of sludge. Edwards &Joncs

(Vivendi Water Systems) for their part

provided filter presses and ancillary

equipment for three of these large

installations, in total, sixty two 1300 mm x

1300 mm machines. Whilst the 1300 mm

machine was the largest available at that

time, its suitability for the larger

installations was questionable. This led to

the introduction of a 6 foot x 4 foot

machine, the first of which w-as installed at

a chemical plant in Switzerland, 16 of these

machines were provided. Because of its

success this size of filter press was

subsequently replaced with its metric

equivalent, the 2 m x 1.5 m machine.

Substantial numbers of 2 m x 1.5 m filter

prcsscs have been installed particularly in

the mining industry. Its success vvas

recognised and, as such, this size of

machine is still offcrcd by the manu-

facturcrs as part of their product range

todav.

During the late 1970s and early 1980s

it vI;as universally recogniscd that the

larger machines were here to stay, which

led to the development and introduction

of the 2 m x 2 m machines of today,

sophisticated PC controlled machines

fitted with their own integral automatic

washing machine.

Automation & Computers The modern dav filter press and its

ancillary equipment generally incorporate

the USC of automation and computer

control. Manual input from an operator is

kept to a minimum, where all mechanical

operations such as press opening &

closing, pump operation and valve opening

& closing are fully automat& and con-

trolled by a I’LC.Thc process variables

such as sludge and polymer flow-rate,

filtration prcssurc, pump speeds and time

can all be monitored and incorporated into

the ‘Floctronic’ computer svstcm, which

controls the filtration cvclc. The Floctronic

system achieves the following:

l Automatically controls the filtration

cvclc.

June 2001 27

Maintaina the required polymer dose by

keeping the speeds of the pumps in

proportion.

Monitors the quality of flocculation

using an optical system and optimises

the polymer dose.

Ensures that the correct mixing of sludge

& tlocculant is created through either a

mixing valv? or in-lint rotating mixer,

Logs details of all process variables at one

minute intervals which can be down-

loaded to a PC later for examination.

The USC of a PLC has

significantly improved

the operation of the

filter press and its

ancillary equipment,

Improvement is such

that total control of the

overall svstcm is now

feasible involving littlc

or no intcrvcntion by

the operator.

Dcvclopment work is

continuing through the

introduction of cake

shakers and other

options to achieve total

automation.

Environmental

Impact Legislation in the

USA through the US

Environmental

Protection Agency

(EPA), has Icd to the

introduction of the 503

Regulations. The UK

Water Industry, main])

through pressurisation

from the BRC and other reputable

organisations, has also had to rethink its

‘sludge strategy’. Perception is a major

force in cvcrything that we do, the UK

Water industry must recognise that the

public, and in particular, the BRC will

not accept biosolids to land without

fully understanding and, more

importantly, accepting that its treatment

is safe.

USF rcvicwcd the US EPA 503

regulations, together with other relevant

28 June 2001

legislation, with a view to offering

suitable equipment capable of achieving

pasteurisation and a 90% tlrv solids

sludge cake in a single operation. Under

normal circumstances this would require

a combination of two processes,

dewatering followed by thermal drying.

This combined process rrquircs

additional materials handling and buffer

storage with associated increased energy

costs. Howcvcr, cxtcnsive research and

dcvclopmcnt work carried out by USF

revealed that the process could be

achicvcd in a single stage unit, thus

reducing the number of process. stages

required with the consequential savings

in capital and running costs.

It is well known that by reducing the

prcssurc under which a liquid is held the

boiling point can be reduced. Combining

this \vith the abilitv to transfer heat

cffcctivclv into a cake formed within a I filter press chamber led to the develop-

mcnt of the ‘J-VAP’ process. Using

polypropqlenc plates and heating the

chambers using the now well established

filter press membrane technology, a

combined system of dewatcring and drying

was developed.

The plant is normally operated at

bctwccn 80-95 “C and because the cake is

contained within the filter chamber the

time/temperature can be controlled to

satisfy the US EPA 503 rule and the UK’s

S+ Slu+ Matrix jbr Atlvanced Treutment.

Short circuiting of solids cannot occur,

thus guaranteeing the selected

timc/tcmpcrature profile is met at all

times.

Over 50 systems arc now currently in

operation throughout North America, and

the resulting product is demonstrating the

ability of the system to significantly reduce

pathogens.

The above article is based on a

presentation given at the 8th World

Filtration Congress, Brighton, UK, 2000,

and a paper subsequently published in The

Filtration Society’s The Journal,

Issue 1, October 2000.

Filtration+Separation