Filtration apparatus

A filtration apparatus for filtering water has a filter cloth which is supported on one side by support means against the pressure applied by water being filtered. During cleaning by reverse flow of water the cloth billows away from said support and cleaning means contact the billowed out filter cloth to clean it. To improve the cleaning effect, the cleaning means comprise a movable plate which is contacted by a region of said billowed out filter cloth during the reverse flow. The plate has at least one aperture at which said reverse flow occurs, and the aperture is moved relative to the filter cloth by movement of the plate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to filtration apparatus having a filter cloth, 
particularly for filtering water containing solid material which collects 
as a sludge on the cloth. The apparatus may comprise a plurality of filter 
elements. 
2. Description of the Prior Art 
NL-A-8802292 describes a filtration system comprising at least one filter 
element which at least on one side is provided with a filter cloth through 
which water to be filtered is sucked. The filter cloth is provided on its 
downstream side in the normal filtration direction with means for 
supporting it against the pressure difference. The filter cloth is 
provided on its upstream side with means for removing the sludge collected 
on the filter cloth. The sludge collected on the filter cloth is removed 
in the course of time by reversal of the direction of flow of water so 
that the filter cloth billows out from the support means, whereupon a 
scraper is moved along the billowed out filter cloth to remove the sludge. 
A problem with this known apparatus is that the reverse flow through the 
filter cloth only leads to local loosening of the sludge layer. This is 
believed to occur because the reverse flow finds the path of least 
resistance, so that if locally some sludge is loosened, there is 
insufficient pressure remaining for removing the sludge from the filter 
cloth elsewhere as well. 
SUMMARY OF THE INVENTION 
It is an object of the invention to solve this problem. At the same time 
other advantages which will be described below may be achieved. 
The filtration apparatus in accordance with the invention is characterised 
in that the means for removing sludge collected on the filter cloth 
comprises a movable member preferably plate-shaped, which is provided with 
at least one aperture. The member contacts the filter cloth which billows 
away from its support means during the reverse flow. The aperture in the 
movable member defines a position on the filter cloth at which the 
loosening of the sludge is forced to occur. The sludge may be removed from 
the entire filter cloth by moving the member over the filter cloth so that 
the aperture or apertures in the plate pass over the entire filter cloth. 
After the sludge has been loosened, it may for example sink down into a 
settling tank in which the filtration apparatus is suspended or it may be 
sucked away. After cleaning the filtration system may be reset to normal 
operation in which the contamination to be removed from the influent water 
adheres to the filter cloth and in the course of time again forms a sludge 
layer for removal. 
It is mentioned that the idea of applying suction via a rotating member to 
achieve reverse cleaning flow of a filter is known, e.g. from DE-A-1554798 
and U.S. Pat. No. A-4486304 which apply it to the filtration of plastics 
material being processed, and also from GB-A-2088232 where the filtered 
liquid is unspecified. In all these disclosures, the filter is a rigid 
element. The present invention in contrast applies a movable apertured 
cleaning member to a billowed out filter cloth. 
It has been found to be an advantage to make the filtration system in 
accordance with the invention in such a way that the means for sludge 
removal further comprise suction means for discharging the sludge, which 
is connected to the movable member in such a way that the suction is 
applied to the aperture or apertures. This prevents the sludge held in 
suspension in the liquid from being sucked back onto the filter cloth 
after re-setting to normal operation. In this way, with this preferred 
embodiment of the filtration system in accordance with the invention, the 
normal operating time is extended compared with the filtration system in 
accordance with the current state of the art. 
Preferably, during sludge removal, the suction means can apply a difference 
in pressure across the filter cloth which is in the range 1-10 m water 
column. This improves the speed with which the filter cloth is cleaned and 
is possible because, during the reverse flow of the water through the 
filter cloth, the billowed out filter cloth finds support against the 
movable member which prevents damage to the filter cloth. 
A simple embodiment is that in which the filtration apparatus is made 
rotationally symmetrical and the movable member can be rotated and the 
aperture is slot-shaped. Preferably the plate is also at least part of a 
circle in shape and the slot is directed radially. In that case the 
movable member may be driven by means of an electric motor or another 
suitable rotating drive and the entire filter cloth may be cleaned in one 
turn of the movable member by the radially directed slot. Since no 
mechanism is required for converting movement from rotary to linear, there 
is less susceptibility to defects of the filtration apparatus than with an 
embodiment with rectangular filter cloths in which a plate is used which 
has to be moved linearly along the filter cloth. 
Preferably the suction means mentioned above is coupled to the movable 
member via a conduit in a central drive member for rotating the movable 
member. 
Besides the advantages described above, the filtration apparatus in 
accordance with the invention is capable of providing long-lasting, 
trouble-free operation. Particularly, it is possible to avoid silting up 
and bacterial growth in the sludge layer on the filter cloth, which 
necessitates regular stripping down and thorough cleaning of a filtration 
system.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The filtration apparatus shown in FIG. 1 comprises a central discharge and 
drive tube 1, a plurality of filter elements 2 equipped with filter cloths 
9, a discharge pipe 3 and rotating cleaning plates 4. These cleaning 
plates 4 may have the shape of for example, a complete circle of the size 
of a filter cloth 9, or part of such a circle. The filter elements are 
arranged in a tank filled with water to be filtered. This water, the 
influent, is sucked through the filter cloths 9 fitted on the filter 
elements 2 and from the inside of the filter elements 2 is discharged as 
cleaned effluent via coupling pieces 7 towards discharge pipe 3. To this 
end an extraction pump (not shown in drawing) is connected to the 
discharge pipe 3. The filter cloth 9 is supported against the pressure 
difference across it by ribs 8 which are provided on each filter element 
2, seen also in FIGS. 2A and 2B. 
Each plate 4 is semi-circular in plan view and has flat opposite faces 
which provide support surfaces for the filter cloths 9 above and below 
when the cloths billow out during reverse water flow for cleaning. During 
filtration, the cloths 9 are spaced from the plates 4, thereby allowing 
the water being filtered to reach the cloths. In the opposite faces of the 
plate 4 there are elongate radial grooves or recesses (not shown) which 
form slots or apertures in these surfaces at which the sludge is sucked 
from the cloth during cleaning. The grooves or recesses are connected by 
passages in the plate 4 to connections 10 leading into the tube 1. The 
size of the plate can be altered Preferably it is at least 40% of the area 
of the cloth 9. At its periphery the plate 4 is supported by a sliding 
shoe. 
On the central discharge and drive tube 1 a motor is connected on the drive 
end 6 for rotating the tube 1, and on a branch connection 5 a suction unit 
(not shown in the drawing) is connected, the function of which is now 
explained. 
After a sludge layer has accumulated on the filter cloths 9 it has to be 
removed. To this end the pumping direction of the pump unit which is 
connected to discharge pipe 3 is reversed, so that cleaned effluent is 
forced in the opposite direction through the filter cloth 9. This makes 
the filter cloth 9 billow out away from the ribs 8 and the sludge layer 
present on it breaks. 
On the central discharge and drive tube 1 are mounted the rotating plates 4 
which are rotated by turning the tube 1 and thus move along the billowed 
out cloths 9 of the filter elements 2. The cloths thus contact and are 
supported by the plates 4 during this reverse flow. The rotating plates 4 
are each provided with a radially directed slot on each side as described 
in the drawing. This slot in each plate 4 is connected inside of the plate 
4 and thus to the central discharge and drive tube 1 via the connections 
10 in the extraction tube 1. As the discharge tube 1 rotates the suction 
unit coupled to branch connection 5 of the discharge tube 1 is switched 
on, so that liquid and the sludge broken loose from the filter cloths 9 
via the openings in the respective rotating plates 4 is extracted via 
connections 10 and the discharge tube 1, in order to be removed from the 
tank. Consequently there is no settling of the sludge. 
The rotating plates 4 may also be provided with a scraper to encourage the 
sludge to loosen from the filter cloths 9. However, this is not essential. 
Because the rotating plates 4 support the filter cloths 9 as they billow 
out, the suction pressure imposed by the suction unit coupled to the 
branch connection 5, may be set at a very high value, for example between 
1 and 10 m water column. As a result of this, the time needed for cleaning 
the filter cloths 9 and for removing the sludge caked on them is short and 
only slightly dependent on the period for which the filtration system was 
running in normal operation. For example, in the case of a continous 
filtration operation time of approx. 50-100 hours it has been found that 
sludge removal is sufficient for making the filtration system suitable for 
a fresh period of operation of 50-100 hours. The effectiveness of this 
filtration system in accordance with the invention is therefore very high 
and may be improved by a factor of at least five compared with a known 
filtration system in accordance with the current state of the art.