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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