In situ filter cleaning

A filter which can be cleaned in situ consists of a filter membrane (33), and fluid permeable electrodes (34, 35). The membrane may be one of the electrodes, or may be electrically non-conducting but integral with or in contact with one of the electrodes (34). The electrodes (34, 35) are separated from each other only by a fluid permeable electrically insulating sheet. This sheet may be less than 1 mm thick, and may be the filter membrane (33). The filter is cleaned by periodic brief applications of electric current between the electrodes (34, 35) so that gas is generated electrolytically, and the close spacing of the electrodes (34, 35) reduces the electric power required.

This invention relates to a filter, and in particular to a filter 
incorporating means by which it can be cleaned in situ, and to a method of 
cleaning such a filter. 
A method for cleaning an electrically-conducting filter is described in GB 
2 160 545 B which enables fouling to be removed from the filter in situ. 
In this method an electrochemical cell is established comprising the 
filter as a first electrode, a counter electrode, and the process liquid 
as electrolyte. At intervals during the filtration process a potential 
difference is applied briefly between the two electrodes, so as to 
generate at the filter a gaseous product which may be in the form of 
microbubbles, and which cleans the filter. For example a current of 
500-3000 A m.sup.-2 of membrane might be applied for 1-5 seconds 
periodically between 4-15 times an hour. The potential difference is 
typically applied such that the filter itself is cathodic, to minimise its 
corrosion, and the filter may be a metallic microporous membrane, or a 
conducting ceramic membrane. A similar process is described in EP 0 380 
266 A, in which the filter may be a porous layer for example of sintered 
zirconia incorporating a metal mesh, or may be a porous layer superimposed 
on such a metal mesh. The counter electrode may be of platinised titanium, 
or as described in EP 0 474 365 A it may be of low chromium stainless 
steel. It will be appreciated that during the applications of the cleaning 
potential difference there is considerable electrical power consumption. 
According to the present invention there is provided a filter including a 
filter membrane, a fluid-permeable first electrode and a fluid-permeable 
second electrode, electric supply means for causing an electric current to 
flow periodically and briefly between the first and the second electrodes 
through a process liquid as electrolyte with a current density of at least 
500 A m.sup.-2 so as to generate a gaseous product, at least 75% of the 
brief pulses of current having the same polarity, wherein the filter 
membrane either is one of the electrodes or is not electrically conducting 
but is integral with or in contact with one of the electrodes, and wherein 
the first and the second electrodes are separated from each other only by 
a fluid-permeable electrically insulating sheet. 
The insulating sheet ensures that the two electrodes cannot touch each 
other, and so ensures that electrolysis of the process liquid occurs. The 
sheet therefore enables the electrodes to be arranged much closer 
together, and so considerably decreases the electrical power consumption. 
Furthermore the sheet ensures that the separation of the electrodes is 
uniform, so ensuring uniform generation of the gaseous product and hence 
uniform cleaning of the membrane. The sheet may be a single layer of 
permeable material, for example a nylon mesh, or may be a stack of such 
layers for example two or three layers of a polyamide cloth with pore 
sizes up to 18 .mu.m, each of thickness 0.3 mm. The thickness of the 
sheet, and hence the separation of the electrodes, is preferably between 
0.2 mm and 3.0 mm, more preferably less than 1 mm, for example 0.6 mm. 
Because both electrodes are in such close proximity to the filter 
membrane, the gas bubbles generated at both electrodes help to remove 
foulant from the filter membrane. 
Where the filter membrane is not electrically conducting and is not 
integral with an electrode, then the filter membrane may be the insulating 
sheet, or be one layer of it. The electrodes should be much more readily 
fluid permeable than the filter membrane, and preferably define pores or 
perforations at least 0.5 mm across, more preferably at least 1 mm, for 
example 2 mm or 4 mm across. The insulating sheet, if it isn't the filter 
membrane, should also have larger pores than the filter membrane, 
preferably at least five times larger pores, and may have pores or 
perforations as large as those in electrodes that is to say up to 1 mm, 2 
mm or even 4 mm. It is desirable to bond the electrodes to the opposite 
surfaces of the insulating sheet, and this prevents them separating as a 
result of gas bubble generation. 
The filter membrane may be an electrode or integral with an electrode, and 
so may comprise a porous carbon structure, such as a tube, with a zirconia 
surface coating (suitable for fine microfiltration or ultrafiltration), or 
a sintered stainless steel microfibre layer (with pore size about 3 
.mu.m), or a titania coated stainless steel filter (with pore size about 
0.2 .mu.m). Alternatively the filter membrane may be non-conducting, and 
in contact with an electrode; in this arrangement a wide variety of 
different types of membrane can be used. For example the filter membrane 
might be of polypropylene, nylon, PVdF (polyvinylidenefluoride), 
polycarbonate, zirconia cloth or other known filter membranes or micro- or 
ultrafiltration membranes. In these cases the electrode at the filtrate 
side of the filter membrane is desirably arranged to be cathodic during 
cleaning pulses, as twice as much gas will be generated at it. 
The invention also provides a method of cleaning such a filter in situ, by 
periodically and briefly causing an electric current to flow between the 
electrodes through the process liquid. Preferably while the current is 
caused to flow the pressure difference across the filter membrane is 
reduced to zero or to negative values, for example by preventing outflow 
of filtrate and/or by ceasing to supply process liquid to the upstream 
side of the filter membrane, as this ensures that foulant material is 
dislodged by bubbles even if those bubbles are generated at the filtrate 
side of the membrane, and any resulting backflow of filtrate aids the 
cleaning process. 
The electric current that brings about the cleaning of the filter membrane 
is only applied periodically and briefly, but when it is applied it must 
be of large enough current density to produce a gaseous product by 
electrolysis. Thus the current density must be at least 500 A m.sup.-2. 
The frequency and duration of the pulses of cleaning current clearly 
depend on how rapidly the membrane becomes fouled, and so on the nature of 
the liquid being treated. Pulses of current of duration between 1 second 
and 5 seconds are usually found to be effective, although the duration 
might be as much as 10 or even 15 seconds. Similarly the frequency might 
typically be between 4 and 15 times per hour, i.e. at intervals of between 
15 and 4 minutes, but if the process liquid contains very little foulant 
material then cleaning might only be necessary once every hour or two. It 
should be appreciated that, as described in GB 2 290 086 A, the polarity 
of the cleaning pulses may be occasionally reversed for example for one 
pulse in every eight, or that a considerably smaller current may be caused 
to flow for prolonged periods between the electrodes in the opposite 
direction to that of the cleaning pulses, this reverse current being at a 
density no more than 200 A m.sup.-2. 
The invention will now be further described by way of example only, and 
with reference to the accompanying drawings in which:

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1 there is shown a filter apparatus 10 comprising a 
treatment chamber 12 through which a liquid or a slurry to be treated is 
caused to flow, the liquid (or slurry) entering as indicated by arrow A 
and leaving the chamber 12 as indicated by arrow B. A filtrate chamber 14 
is separated from the chamber 12 by a filter assembly 16 through which a 
filtrate liquid permeates, and the filtrate liquid can leave the filtrate 
chamber 14 as indicated by arrow C through an outlet duct 15. 
The filter assembly 16 includes a stainless steel microfibre filter layer 
18 which has pore sizes about 3 .mu.m. The surface of the layer 18 facing 
the treatment chamber 12 is covered by a coarse woven nylon mesh 20 about 
0.5 mm thick and with holes about 2 mm wide, which is itself covered by 
perforated counter electrode 22 of 9% chromium stainless steel with 2 mm 
wide diamond-shaped apertures. The filter layer 18 and the 
counter-electrode 22 are connected to a source 24 of electric current and 
a switch 25. 
In operation of the filter apparatus 10 a liquid to be treated is pumped 
through the chamber 14 and as a result of the pressure drop across the 
filter assembly 16 a filtrate liquid permeates into the chamber 14 to 
emerge through the duct 15. The filtrate flux gradually decreases due to 
build-up of foulant on the surface of the filter layer 18. At periodic 
intervals, for example once every 20 minutes, the power supply 24 is 
connected briefly to the filter layer 18 and the counter electrode 22 for 
say 5 seconds, so the current density at the surface of the layer 18 is 
600 A m.sup.-2. Electrolysis of the liquid results in the generation of 
microbubbles of gas at both the filter layer 18 and the counter electrode 
22 which disrupt the foulant and dislodge it. This considerably improves 
the filtrate flux. During the application of the current pulse it is 
desirable to also decrease the pressure drop across the filter assembly 
16, which may be achieved by closing or obstructing the outlet duct 15. 
Referring now to FIG. 2 there is shown an alternative filter apparatus 30 
which has many features in common with the apparatus 10 of FIG. 1, 
comprising a treatment chamber 12 and a filtrate chamber 14 which are 
separated by a filter assembly 32. The filter assembly 32 comprises a 
woven polypropylene cloth filter membrane 33 sandwiched between and 
thermally bonded to a stainless steel wire gauze 34 on the filtrate side, 
and a 9% chromium stainless steel perforated plate 35 on the treatment 
chamber 12 side. The membrane 33 has pore sizes of about 3 .mu.m, while 
the gauze 34 has 1 mm apertures, and the perforated plate 35 has 2.5 mm 
apertures. The gauze 34 and the plate 35 are connected to a switch 25 and 
an electric power supply 24 such that the gauze 34 is the cathode. 
In operation of the filter apparatus 30 the liquid or slurry to be treated 
is pumped through the chamber 12, and filtrate emerges through the filter 
assembly 32 into the chamber 14. A valve 36 enables the outflow C of 
filtrate from the apparatus 30 to be controlled. Foulant will gradually 
build up on the filter membrane 33, and at periodic intervals the power 
supply 24 is switched on for say 5 or 10 seconds, at a current density at 
the gauze 34 and the plate 35 of 600 A m.sup.-2. At the same time the 
valve 36 is closed. Microbubbles are generated by electrolysis at both the 
gauze 34 and the plate 35, and so gas bubbles pass through the membrane 33 
towards the chamber 12 (because the valve 36 is closed), dislodging and 
removing the foulant. This considerably improves the filtrate flux when 
the valve 36 is reopened and the power supply 24 switched off. 
It will be appreciated that a wide variety of different filter membranes 
may be used in place of the woven polypropylene cloth membrane 33 in the 
apparatus 30, being chosen in accordance with the nature of the liquid to 
be treated, and the type of filtration required. For example the membrane 
33 might be replaced by a PVdF membrane with pore sizes about 0.45 .mu.m 
if microfiltration is required. The membrane 33 might be replaced by a 
fine woven zirconia cloth, with pore sizes less than 1 .mu.m, if 
chemically aggressive (e.g. alkaline) or high temperature liquids are to 
be filtered; in this case the electrodes 34, 35 would be spot-glued to the 
membrane. Thus the membrane may be one intended for ultrafiltration (with 
pore sizes typically less than about 0.03 .mu.m), or one intended for 
microfiltration (with pore sizes between about 0.1 .mu.m and 5 .mu.m), or 
for conventional filtration (with pore sizes larger than about 5 .mu.m). 
Another suitable material for the filter membrane comprises glass fibres 
coated with PTFE (polytetrafluoroethylene) and woven to form a cloth. 
It will also be appreciated that the shape of the filter membrane may 
differ from the flat sheet shown in the drawings, and that the filter 
membrane might instead be a cylindrical tube, or might be a spiral. For 
example a polycarbonate filter membrane sandwiched between a flexible 
stainless steel wire gauze and a flexible platinised titanium wire gauze 
may be sandwiched between coarse nylon mesh spacer meshes and then wound 
into a spiral filter assembly along with an impermeable plastic sheet.