GB1582654A - Filter and method - Google Patents

Description

(54) FILTER AND METHOD (71) We, ECODYNE CORPORATION, a corporation organized under the laws of the State of Delaware, United States of America, of 90 Half Day Road, Lincolnshire, Illinois, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to liquid filtration, and more particularly to processes for gravity filtration and filters with a rotating hood for isolating discrete beds of filter metering during backwashing.

There are inherent economies and advantages in multiunit filters in which an individual filter unit is isolated from a common chamber of unfiltered influent for backwashing, while the remaining units continue to produce filtered effluent. Since the filter medium is segregated into several discrete beds, the backwash flow required by any single bed is so small that the beds remaining in service can provide sufficient filtered liquid. The relatively small volume of dirty backwash liquid can be returned directly to a clarafier or floculator combined with the filters or to waste without the danger of hydraulically upsetting the system. Since only one bed is backwashed at a time, the service flow from the multibed filter is substantially constant. Multiunit filter design has been extensively used for totally enclosed strainer or cartridge type pressure filters of relatively moderate or small size.

It has been necessary in such totally enclosed pressure filters to use mechanical implements to force the isolation device into sealing engagement with the unit being backwashed. However, multiunit design has not been employed on gravity filters. One reason for the lack of use of multiunit design on large gravity filters has been the unavailability at a reasonable cost of suitable prior art mechanical sealing mechanisms that are easily movable and operable over large surfaces and distances inherent in the common gravity filter designs.

According to the present invention there is provided a multibed liquid filter comprising: a container having an inlet for liquid to be filtered and an outlet for filtered liquid, means in said container supporting filter material and defining therebeneath a filtered liquid chamber communicating with said outlet; a wall of said container extending above said filter material so as to define a chamber for unfiltered liquid; partitions extending upwardly from said material supporting means so as to divide said filter material into a plurality of beds, the upper terminal edge of each partition extending above said beds into said unfiltered liquid chamber, said upper edges providing sealable surfaces; a backwash isolation hood for sealing with the said sealable surfaces provided by the partitions which define any of said beds; a backwash outlet connected to the underside of said hood, whereby during backwashing of a bed the hood is in sealing engagement with the partitions of that bed and liquid flows from the filtered liquid chamber through the bed, into the hood, and out through the backwash outlet; and means for increasing the fluid pressure in the hood at the termination of backwashing a bed to a value higher than the pressure which would prevail in the hood if, at the termination of backwashing, the backwash outlet was closed and the fluid pressure in the hood was allowed to rise to a value determined by the fluid pressure in the filtered liquid chamber, and thereby reduce the force required to lift said hood out of sealing engagement with said partitions.

According to the present invention there is furthermore provided a process for filtration of a liquid in a multiunit filter comprising: a container having an inlet for liquid to be filtered and an outlet for filtered liquid; means in said container supporting filter material and defining therebeneath a filtered liquid chamber communicating with said outlet; a wall of said container extending above said filter material so as to define a chamber for unfiltered liquid; partitions extending upwardly from said material supporting means so as to divide said filter material into a plurality of beds, the upper terminal edge of each partition extending above said beds into said unfiltered liquid chamber, said upper edges providing sealable surfaces; a backwash isolation hood for sealing with the said sealable surfaces provided by the partitions which define any of said beds; a backwash outlet connected to the underside of said hood, the process comprising: passing liquid through said beds until at least one bed requires backwashing, moving said hood into sealing engagement with the partitions of the bed in need of backwashing; backwashing said bed by flowing backwashing liquid therethrough into said hood and out through the backwash outlet; terminating the flow of backwashing liquid through the bed; increasing the fluid pressure in the hood to a value higher than the pressure which would prevail in the hood if, at the termination of backwashing, the backwash outlet was closed and the fluid pressure in the hood was allowed to rise to a value determined by the fluid pressure in the filtered liquid chamber, and thereby reducing the force required to lift said hood out of sealing engagement with said partitions; and moving the hood to another bed in need of backwashing.

In order that the invention may be well understood there will now be described some embodiments thereof, given by way of example only, reference being had to the accompanying drawings in which: Figure 1 is a partially broken away, top plan, schematic view of one embodiment of the invention; Figure 2 is a cross-sectional view of the embodiment of the invention shown in Figure 1; Figure 3 is a cross-sectional view taken along the line 3-3 in Figure 1; Figure 4 is an enlarged, partially broken away, cross-sectional view of a drive coupling for the hood; Figure 5 is a top plan view on a reduced scale of the coupling shown in Figure 4; Figure 6 is a broken-away cross-sectional view corresponding to Figure 2 and including pressure equalizing and leak detecting means; Figure 7 is a top plan schematic view of another embodiment of the invention; Figure 8 is a cross-sectional view of the embodiment shown in Figure 7; Figure 9 is a cross-sectional view taken along the line 9-9 in Figure 7; Figure 10 is an enlarged, partially brokenaway, cross-sectional view of the hood moving mechanism of the embodiment of Figures 7 and 8; and Figure 11 is a cross-sectional view taken along the line 11-11 in Figure 10.

Figs. 1-5 show an embodiment of a gravity liquid filter 1 comprising numerous discrete filter beds 2 occupying an open-topped cylindrical container tank 3. Beds 2 are preferably granular filter material 4, such as sand, gravel, coal, activated carbon, or a combination of such materials, supported on a perforated plate 6 that spans the interior of tank 3. Strainers 7 surround each perforation in plate 6 and prevent escape of filter material 4. Upstanding radial partitions 8 separate beds 2. The upper, perpendicular, terminal flanged edge 9 of each partition 8 extends above the beds 2, and flanged edges 9 all lie in the same horizontal plane. A peripheral flange 10 is connected to and in the same plane as flanged edges 9.

Liquid to be treated enters filter 1 through an inlet pipe 11 and exits through a filtered effluent outlet pipe 12. A baffle 13 dissipates the energy of the incoming liquid. The level 14 of the liquid in tank 1 is kept at a predetermined height above edges 9 by an outlet valve 16 which has the size of its port controlled in a conventional manner by a liquid level sensing float mechanism 17.

The portion 18 of the wall of tank 3 extending above beds 2 defines an unfiltered liquid or floculation chamber 19 which collects an atmospheric pool of liquid of sufficient depth standing on beds 2 to cause the unfiltered liquid to percolate under the influence of gravity through material 4. Plate 6 defines therebeneath a filtered liquid chamber 21 in communication with outlet 16. Conventional accessories, such as a guard rail 22 and catwalk 23 should be provided when required.

When tank 3 is filled, liquid accumulates in chamber 19 until there is sufficient static head to cause it to pass through filter material 4, leaving most or all of the undesired contaminants on or in beds 2. When the amount of filtered contaminants on or in beds 2 reaches some predetermined quantity, it is necessary to begin backwashing, one bed at a time.

A hollow backwash hood 25 is mounted for rotation around the center of filter 1.

The flat undersurface 26 of hood 25 is perforated to permit liquid to flow into hood 25, and a pliable gasket 27 extends around the periphery of undersurface 26 for sealing against flanged surfaces 9 and 10. A protruding cylindrical collar 28 extends perpendicularly from the inner end of hood 25 into center pipe 29. Collar 28 acts as a guide and a bearing for hood 25 and also enables liquid to flow out of hood 25 into pipe 29. The space between pipe 29 and collar 28 is sealed by a suitable gasket that permits collar 28 to rotate and to move up and down in pipe 29. A backwash outlet pipe 30 is connected to center pipe 29, and a backwash valve 31 opens and closes outlet pipe 30.

Hood 25 is turned by a gear-motor 32 that is controlled by a conventional timer, pressure, or liquid-level actuated electrical circuit. Gear-motor 32 rotates a first hollow, open-ended shaft 33 that is connected in a gas-tight manner to a second hollow, openended shaft 34 by a coupling 35. The upper end of shaft 34 slides vertically up and down in sleeve 36 of coupling 35. The lower end of shaft 34 is attached to hood 25, and the inside of shaft 34 communicates with the inside of hood 25 through a port 37. A slotted indexing disc 38 attached to pipe 34 and a positioning switch 39 shut off gearmotor 32 when hood 25 is moved to the correct position over the next bed 2 in need of backwashing. Limit switch 40 mounted on support 41 detects the vertical position of disc 38 as it moves vertically up and down with hood 25 and provides a signal that gasket 27 is disengaged from the surfaces 9 when switch 40 is actuated by contact with disc 38. The upper end of shaft 33 extends above gear-motor 32 into an airtight housing 42. An electrically operated vent valve 43 is connected to the inside of housing 42 through a T-connection 44. An electrically operated valve 45 connects the inside of housing 42 to a source of pressured air or other gas through T-connection 44.

When a bed 2 has been sufficiently backwashed to render it suitable for return to filtering service, the conventional control circuit closes valves 31 and 43 and opens valve 45. Gas under pressure passes into housing 42 and enters pipe 33. The gas pressure is transmitted into pipe 34 and expells liquid therefrom. Pressurized gas enters hood 25 through port 37 and displaces backwash liquid through the filter material 4 in bed 2 into chamber 21, thus breaking the seal between surfaces 9 and 10 and gasket 27.

When sufficient gas has entered hood 25 the hood floats up a short distance out of contact with surfaces 9 and 10. This vertical movement of hood 25 carries pipe 34 and attached disc 38 upwardly until limit switch 40 contacts and is actuated. Tripping of limit switch 40 causes the control circuit to actuate gear-motor 32, which rotates shafts 33 and 34 until movement of indexing disc 38 trips positioning switch 39. Actuation of switch 39 stops gear-motor 32 when hood 25 is properly positioned over the next bed 2 in need of backwashing and also causes the control circuit to close pressure valve 45 and open vent valve 43. When valve 43 is opened, the interior of hood 25 is vented to the atmosphere. The static head from the liquid in chamber 19 forces liquid into hood 25 and displaces gas up pipe 34 and out through valve 43. When all or almost all of the gas has been expelled from hood 25, it will sink under the influence of gravity until gasket 27 rests on the flanged surfaces 9 and 10 surrounding the next bed in need of backwashing. This lowers disc 38 and thereby actuates switch 40, thus providing a signal which opens valve 31. The lower static head at valve 31 draws filtered liquid from chamber 21 up through the strainers 7 into the bed 2 being backwashed. The filtered liquid flushes contaminates from such bed into hood 25, down center pipe 29, and out through pipe 30. The greater static pressure in chamber 19 forces hood 25 against flanged surfaces 9 with sufficient force to seal gasket 27. However, no harm results if gasket 27 does not seal perfectly, because the unfiltered liquid which would then leak into hood 25 mixes with the dirty liquid resulting from backwashing of the filter bed and is expelled from filter 1 through pipe 30. If desired, as for example when the liquid being filtered is sewage or when solids are being floculated in chamber 19, the dirty liquid leaving through pipe 30 can be pumped directly back into chamber 19 for continued treatment, in which case valve 31 can be replaced by or used with a pump. Solids or floc which accumulate in chamber 19 can be removed by an air lift pump or other conventional means.

Fig. 6 shows how the force required to lift hood 25 can be reduced, and also how the degree, if any, of leakage between hood 25 and flanges 9 can be detected. The force required to lift hood 25 is reduced by equalizing the pressure on opposite sides of hood 25 at the same time as, or just before, valves 31 and 43 are closed and valve 45 is opened. This is accomplished by connecting electrically operable valve means 50 to backwash pipe 30 by conduit 51 and to chamber 19 by conduit 52, and by opening valve means 50 before starting to raise hood 25. This immediately overcomes the effect on hood 25 of the static head in chamber 19, and reduces the force required to lift hood 25 to an amount equal to its weight plus the friction and inertia of the moving parts. This reduces the amount of air which must be pumped into hood 25 and hence the time needed to index the hood to the next filter to be backwashed. Valve 50 is closed before backwashing of the next unit is started.

An indicating pressure differential switch 53 is connected by a conduit 54 to filtered liquid chamber 21, and to conduit 51 between valve means 50 and pipe 30 by a conduit 55. When a particular bed 2 is in service, the pressure at the upper surface of the bed will be higher than the pressure in the chamber 21 because of the pressure drop across the beds. However, if the hood 25 effectively seals against edges 9 the pressure above the bed in question falls to the pressure in the chamber 21. When the valve 31 is closed the pressure in the pipe 30 is equal to the pressure above the bed which; if the sealing is effective, is the same as the pressure in chamber 21. Accordingly, when valve 50 is closed and hood 25 is seated on edges 9, switch 53 will indicate zero pressure differential when there is no leakage under hood 25. The greater the leakage the greater will be the pressure differential indicated by switch 53. Switch 53 can be connected in a conventional circuit which can sound an alarm, or prevent the start of the backwash cycle if the pressure differential, and hence the leakage, is above a predetermined amount. An indicating pressure differential switch suitable for use as described herein is obtainable from Ellison Instrumental Company of Boulder, Colorado, as their Eagleeye Model 77c.

Figs. 7 and 8 show another embodiment of the invention to be a combined gravity liquid filter and floculator 60 having numerous discrete filter beds 61 occupying an open-topped cylindrical container tank 62. Beds 61 are made of the same materials as in the embodiment of Figs. 1 through S, and are supported on a perforated plate 63 that spans the interior of tank 62.

Strainers 64 surround each perforation in plate 63. Upstanding radial partitions 65 separate beds 61, and the upper terminal edges 66 of each partition 65 extends above beds 61 and lie in the same horizontal plane.

The upper terminal edge 67 of a cylindrical wall 68 and an outer peripheral rim 69 are connected to, and terminate in the same plane as, edges 66. Rim 69 is spaced from side wall 70 of tank 62 to provide clearance for the hood seal.

Liquid to be treated enters through inlet pip 71, and filtered effluent exits through outlet pipe 72. The level 73 of the liquid in tank 62 may be kept at a predetermined height by an outlet valve 74, which has the size of its port controlled in the same manner as described above with reference to valve 16. The portion of wall 70 extending above beds 61 defines an unfiltered liquid and floculation chamber 75 which collects an atmospheric pool of liquid of sufficient depth standing on beds 61 to cause the unfiltered liquid to percolate under the influence of gravity through the filter medium. Plate 63 defines therebeneath a filtered liquid chamber 76 in communication with outlet 72.

Tncoming liquid from pipe 71 passes into header pipe 77 and then into distribution pipes 78 and 79. A generally cylindrical vertical floculation column 80 is centered in tank 62, and its top edge 81 terminates below liquid level 73. Cables 83 support column 80 from beams 84 which span tank 62. Pipe 78 enters a header 85 on the outside of column 80. Liquid flows from pipe 78 into header 85 and then through holes 86 into adjustable nozzles 87. Liquid and solids expelled from nozzles 87 cause a turbulent upward swirling and mixing of the contents of column 80, which promotes floculation.

Pipe 79 enters a header 89 on the outside of an outwardly flaring end portion 90 of column 80. Liquid and solids flow through holes 91 and continue to promote floculation as the liquid and solids leave column 80 and enter floculation chamber 75, where the solids continue to floculate as they settle on the top of filters 61. Sedimentation also occurs in column 80, and solids fall through a quiescent zone 92 defined by an outwardly flaring bottom portion of the column 80.

The solids settle into a collection chamber 94 defined by cylindrical wall 68, which serves as an inner end wall of filters 61 and of their common filtered chamber 76. Liquid and solids are drawn under the flaring bottom portion of the column 80 from chamber 75 by the upward swirling flow from nozzles 86; some of such solids settle into collection chamber 94. An inverted conical wall 96 guides the settled solids toward the center where they are withdrawn in conventional manner through a sludge blowdown pipe 97. Chemicals which promote floculation may be added to the liquid in pipe 71, or in column 80, or in chamber 75.

After liquid has filled chamber 75, there is sufficient static head to cause it to pass downwardly through filter beds 61 into chamber 76 and out through pipe 72, leaving most or all of the undesired contaminants on or in the beds. Solids in floculation chamber 75 also sedimentate on to beds 61, and a peripheral, tilted baffle 99 attached to wall 70 directs the settling solids towards the center of tank 62. When the amount of contaminants on or in beds 61 reaches some predetermined quantity, it is necessary to begin backwashing, one bed at a time.

A hollow backwash hood 100 is mounted for rotation around the center of tank 62 underneath the column 80. The underside 101 of the hood is perforated to permit entry of backwash liquid. An inverted U-shaped flange 102 is sector-shaped like the top of each filter 61, as defined by edges 66 and portions of rims 67 and 69, and extends around the underside of hood 100. A pliable gasket 103 held within flange 102 seals against the upper terminal edges of the filters. A protruding cylindrical collar 105 extends perpendicularly from the inner end of hood 100 into a center pipe 106. Collar 105 acts as a guide and a bearing for hood 100 and also directs backwash liquid flowing out of hood 100 into pipe 106. The space between pipe 106 and collar 105 is sealed by suitable gasket means that permits collar 105 to rotate and also to move up and down in pipe 106. A backwash outlet pipe 107 is connected to pipe 106, and a three-way backwash valve 108 opens and closes pipe 107. Opening of valve 108 causes filtered liquid to flow upwardly from chamber 76 through plate 63 and thus to backwash a bed 61. A recirculation pump 109 may be used to recirculate all or some of the dirty backwash liquid and solids through return pipe 110 to header pipe 77, from which the backwash liquid and solids mix with incoming liquid and are distributed through pipes 78 and 79 into floculation column 80. When all of the backwash liquid and solids are recirculated into chamber 80, the only effluents from combined filterfloculator 60 are treated liquid from pipe 72 and sludge from pipe 97.

Hood 100 is raised, lowered and rotated by the mechanism shown in Figs. 10 and 11. A hollow, open-ended shaft 112 is attached perpendicularly at its lower end to hood 100, and the inside of shaft 112 communicates with the inside of hood 100 through a port 113. A hole 114 in shaft 112 may connect the inside of hood 100 to a source of pressurized gas that is controlled by valves (not shown) corresponding to valves 43 and 45, so that hood 100 may also be raised by air pressure in the manner described above with regard to the embodiment of Figs. 1-5. The upper end 115 of shaft 112 is attached to and movable with a first cylinder 116. A first piston 117 in cylinder 116 is secured to one end of a rod 118, and the other end of rod 118 is connected to plate means 120. First piston 117 and cylinder 116 and shaft 112 are axially aligned at the center of tank 62, but the inside of shaft 112 does not communicate with the inside of cylinder 16. A pair of identical, parallel guide rods 122 are spaced on opposite sides of, and are parallel to, shaft 112. Rods 122 are attached to plate 120 at their upper ends, and their lower ends are reduced to define indexing pins 123 which fit into holes 126 in plate 127. There are two holes in plate 127 corresponding to each filter bed 61, and holes 126 are uniformly spaced around the circumference of a circle which is centered at the center of shaft 112. Plate 127 is journaled for partial rotation around shaft 112, and shaft 112 slides through holes 128 and 129. Plate 127 is supported by a bearing 130, which rests on plate means 131, which in turn is supported over tank 62 by beams 84. A pair of identical, spaced, parallel plates 132 are secured to shaft 112 and slidably receive guide rods 122, which pass through bushings 133 that connect plates 132 to each other.

A limit switch actuating rod 135 is also attached to, and spans the space between, plates 132.

A projecting arm 136 integral with plate 127 is connected by a pin 137 to an end of a rod 138. The other end of rod 138 is connected to a second piston 139 in a second cylinder 140, which is attached by a bracket 141 to plate means 131. A pair of threaded, adjustable rotation limit stops 142 and 143 attached to plate means 131 accurately set the extent of travel of arm 136, and hence the amount that indexing plate 127 can rotate. Conventional air pressure lines 144 and 145 are connected to opposite ends of first cylinder 116, and conventional air pressure lines 146 and 147 are connected to opposite ends of second cylinder 140.

A valve 148 connected by conduit 149 to backwash pipe 107 and by conduit 150 to chamber 75, equalizes the pressure on opposite sides of hood 100 when it is opened in the same manner described above with regard to valve 50. A pressure differential indicating switch 151 connected between chamber 76 and conduit 149 indicates the amount of leakage under hood 100 when valve 148 is closed in the same manner described above with regard to switch 53.

Air is forced under pressure into the opposite ends of first cylinder 116 in a first predetermined sequence that raises and lowers hood 100, and air is forced under pressure into the opposite ends of second cylinder 140 in a second predetermined sequence coordinated with the first sequence that causes hood 100 to rotate or index from one filter 61 to another. These sequences are controlled by a conventional electrical control circuit. Assuming that hood 100 is seated on a filter 61 and backwashing is taking place, the parts of the mechanism would be in the position shown in Figs. 10 and 11. Air pressure applied through line 145 moves first piston 117 upwardly and this raises plate 120 until it contacts limit switch 153, which shuts off the air; raising of plate 120 lifts indexing pins 123 at the ends of guide rods 122 out of the holes 126 which correspond to the filter 61 then being backwashed. Air pressure applied through line 146 moves second piston 139 away from the end of cylinder 140 attached to bracket 141, and this pushes rod 138 out of cylinder 140; this moves arm 136 until it hits limit stop 143, as shown in phantom in Fig. 11, and indexes the next pair of holes 126 under ends 123. Air pressure is then applied through line 144 and moves piston 117 downwardly until plate 120 contacts limit switch 155; this lowers pins 123 into the just positioned holes 126.

When the filter bed 61 being backwashed is sufficiently clean to be returned to service, as determined by a conventional timer or pressure actuated control circuit, valve 148 is opened and thus equalizes the pressure on opposite sides of hood 100; valve 108 is now closed. If air is also being used to raise hood 100, it would now be pumped through hole 114 into shaft 112 and thus into the hood through port 113; at the same time, air pressure applied through line 144 would raise cylinder 116 with respect to piston 117 until the upper plate 132 contacts limit switch 156, and thus would raise or unseat hood 100. Although plates 132 slide upwardly with cylinder 116 and shaft 112, pins 123 remain seated in holes 126.

Air pressure applied through line 147 moves piston 139 so as to retract rod 138 into cylinder 140 and thus rotate or index plate 127 until rod 135 actuates limit switch 154 and arm 136 hits limit stop 142; this rotation of plate 127 also rotates rods 122 and plates 132 which are secured to shaft 112.

Thus, hood 100 is indexed to a position over the next filter 61 in need of backwashing. If air pressure was also being used to raise hood 100, it is no longer supplied through hole 114 so hood 100 will begin to sink under the influence of gravity. To speed the seating of hood 100, air pressure is applied through line 145 which forces cylinder 116 downwardly until the hood and gasket 103 are firmly seated. Valve 148 is now closed, valve 108 is opened, and backwashing begins and continues until the control circuit causes the above sequences to be repeated.

It has thus been shown that by the practice of this invention a multibed liquid filter can be backwashed in a process that uses gas to float a backwash isolation hood out of sealing engagement with a filter bed without requiring mechanical implements that directly move the hood vertically. The process also causes the hood to sink into sealing engagement under influence of gravity and hydraulic pressure without the use of devices that exert mechanical force directly on the hood. The hood can also be raised and lowered by a mechanism that employs two pistons in cylinders and a perforated indexing plate, either with or without gas pressure. This makes it practical to use rotating backwash isolation hoods with relatively large (e.g. over 20 feet in diameter) opentopped, gravity filters utilizing granular filter media. Filters can be constructed in accord with this invention which have relatively low overall height, and do not require separate tanks for storage of backwash liquid. The amount of backwash liquid used is low because minimum free board above the sector-shaped, discrete filter beds is achieved by uniform liquid collection across the top of the beds by the sector-shaped, perforated backwash hood. The embodiments shown in the drawing include features that lower cost by making efficient use of materials and space. For example, rods are not needed to support perforated plates 6 and 63, because they are attached to the undersides of partitions 8 and 65, which function as rigid plate girders. When the unfiltered liquid chambers 19 and 75 also function as floculation chambers, the dirty backwash liquid can be recycled so that the only waste from the system is sludge.

While the present invention has been described with reference to particular embodiments, it is not intended to illustrate or describe herein all of the equivalent forms or ramifications thereof. For example, the flange 102 and edge 66 sealing arrangement shown for the embodiment of Figs. 7-11 could be used with the embodiment of Figs.

1-6, and the hood undersurface 26 and flanges 9 of the Fig. 1-6 embodiment could be used with the embodiment of Figs. 7-11.

WHAT WE CLAIM IS: 1. A multibed liquid filter comprising: a container having an inlet for liquid to be filtered and an outlet for filtered liquid; means in said container supporting filter material and defining therebeneath a filtered liquid chamber communicating with said outlet; a wall of said container extending above said filter material so as to define a chamber for unfiltered liquid; partitions extending upwardly from said material supporting means so as to divide said filter material into a plurality of beds, the upper terminal

Claims (29)

**WARNING** start of CLMS field may overlap end of DESC **. When the filter bed 61 being backwashed is sufficiently clean to be returned to service, as determined by a conventional timer or pressure actuated control circuit, valve 148 is opened and thus equalizes the pressure on opposite sides of hood 100; valve 108 is now closed. If air is also being used to raise hood 100, it would now be pumped through hole 114 into shaft 112 and thus into the hood through port 113; at the same time, air pressure applied through line 144 would raise cylinder 116 with respect to piston 117 until the upper plate 132 contacts limit switch 156, and thus would raise or unseat hood 100. Although plates 132 slide upwardly with cylinder 116 and shaft 112, pins 123 remain seated in holes 126. Air pressure applied through line 147 moves piston 139 so as to retract rod 138 into cylinder 140 and thus rotate or index plate 127 until rod 135 actuates limit switch 154 and arm 136 hits limit stop 142; this rotation of plate 127 also rotates rods 122 and plates 132 which are secured to shaft 112. Thus, hood 100 is indexed to a position over the next filter 61 in need of backwashing. If air pressure was also being used to raise hood 100, it is no longer supplied through hole 114 so hood 100 will begin to sink under the influence of gravity. To speed the seating of hood 100, air pressure is applied through line 145 which forces cylinder 116 downwardly until the hood and gasket 103 are firmly seated. Valve 148 is now closed, valve 108 is opened, and backwashing begins and continues until the control circuit causes the above sequences to be repeated. It has thus been shown that by the practice of this invention a multibed liquid filter can be backwashed in a process that uses gas to float a backwash isolation hood out of sealing engagement with a filter bed without requiring mechanical implements that directly move the hood vertically. The process also causes the hood to sink into sealing engagement under influence of gravity and hydraulic pressure without the use of devices that exert mechanical force directly on the hood. The hood can also be raised and lowered by a mechanism that employs two pistons in cylinders and a perforated indexing plate, either with or without gas pressure. This makes it practical to use rotating backwash isolation hoods with relatively large (e.g. over 20 feet in diameter) opentopped, gravity filters utilizing granular filter media. Filters can be constructed in accord with this invention which have relatively low overall height, and do not require separate tanks for storage of backwash liquid. The amount of backwash liquid used is low because minimum free board above the sector-shaped, discrete filter beds is achieved by uniform liquid collection across the top of the beds by the sector-shaped, perforated backwash hood. The embodiments shown in the drawing include features that lower cost by making efficient use of materials and space. For example, rods are not needed to support perforated plates 6 and 63, because they are attached to the undersides of partitions 8 and 65, which function as rigid plate girders. When the unfiltered liquid chambers 19 and 75 also function as floculation chambers, the dirty backwash liquid can be recycled so that the only waste from the system is sludge. While the present invention has been described with reference to particular embodiments, it is not intended to illustrate or describe herein all of the equivalent forms or ramifications thereof. For example, the flange 102 and edge 66 sealing arrangement shown for the embodiment of Figs. 7-11 could be used with the embodiment of Figs.

1-6, and the hood undersurface 26 and flanges 9 of the Fig. 1-6 embodiment could be used with the embodiment of Figs. 7-11.

WHAT WE CLAIM IS: 1. A multibed liquid filter comprising: a container having an inlet for liquid to be filtered and an outlet for filtered liquid; means in said container supporting filter material and defining therebeneath a filtered liquid chamber communicating with said outlet; a wall of said container extending above said filter material so as to define a chamber for unfiltered liquid; partitions extending upwardly from said material supporting means so as to divide said filter material into a plurality of beds, the upper terminal edge of each partition extending above said beds into said unfiltered liquid chamber, said upper edges providing sealable surfaces; a backwash isolation hood for sealing with the said sealable surfaces provided by the partitions which define any of said beds; a backwash outlet connected to the underside of said hood, whereby during backwashing of a bed the hood is in sealing engagement with the partitions of that bed and liquid flows from the filtered liquid chamber through the bed, into the hood, and out through the backwash outlet; and means for increasing the fluid pressure in the hood at the termination of backwashing a bed to a value higher than the pressure which would prevail in the hood if, at the termination of backwashing, the backwash outlet was closed and the fluid pressure in the hood was allowed to rise to a value determined by the fluid pressure in the filtered liquid chamber, and thereby reduce the force required to lift said hood out of sealing engagement with said partitions.

2. A filter according to claim 1 wherein the pressure increasing means is capable of equalising the pressures on the inside and

outside of the hood.

3. A filter according to claim 2 wherein the pressure increasing means comprises a liquid conduit connecting the backwash outlet to the unfiltered liquid chamber, and valve means for selectively opening and closing said conduit.

4. A filter according to claim 3 further comprising pressure measuring means for detecting leakage between said hood and said sealable surfaces.

5. A filter according to claim 4 wherein said means for detecting leakage comprises a pressure differential indicating switch connected to said filtered liquid chamber and to said liquid conduit between said valve means and said backwash outlet.

6. A filter according to any preceding claim comprising means connecting the underside of said hood to a source of pressurised gas for floating said hood out of sealing engagement with said partitions, and means for venting the underside of said hood so as to release gas pressure and cause said hood to move into sealing engagement with said sealable surfaces.

7. A filter according to claim 1 wherein said pressure increasing means comprises means for forcing pressurised gas into the space beneath the hood.

8. A filter according to any preceding claim including means for moving said hood from one bed to another comprising: shaft means attached to said hood, a guide rod; means connecting said guide rod to said shaft means; indexing plate means having at least one hole therein corresponding to each filter unit, the holes each being sized to receive an end of said guide rod; a first piston and cylinder one of which is connected to said shaft means and the other of which is connected to said guide rod, a second piston and cylinder one of which is connected to said indexing plate means, a source of hydraulic fluid connected to each end of each of said cylinders on opposite sides of their respective pistons, flow of said fluid to opposite ends of said first cylinder in a first predetermined sequence causing said shaft means and attached hood to be raised and lowered, and flow of said fluid to opposite ends of said second cylinder in a second predetermined sequence coordinated with said first predetermined sequence causing said indexing plate means to move said hood from one filter bed to another.

9. A filter according to claim 8, wherein said first cylinder is attached to said shaft means, and said first piston is attached to said means connecting said guide rod to said shaft means.

10. A filter according to claim 8 or claim 9, wherein said first piston and cylinder and said shaft means are axially aligned, and said shaft means is perpendicular to said hood.

11. A filter according to any of claims 8 to 10, wherein a pair of guide rods are parallel to said shaft means and are spaced on opposite sides of said shaft means.

12. A filter according to any of claims 8 and 11, wherein said holes in said indexing plate means are equally spaced around the circumference of a circle, and said plate means is mounted for partial rotation around an axis passing through the centre of said circle.

13. A filter according to claim 12, wherein said shaft means passes through the centre of said circle.

14. A filter according to the invention defined in any of claims 8 to 13 when appendant to claim 7 wherein said shaft means is hollow and connects a source of pressurized gas to the underside of said hood.

15. A filter according to any of the preceding claims, wherein said upper edges of said partitions terminate in the same plane.

16. A filter according to claim 15, wherein said upper edges are flanges extending at right angles to said partitions.

17. A filter according to claim 15 or claim 16, wherein the undersurface of said hood includes a gasket and an inverted Ushaped flange which captures said upper edges which seal against the gasket.

18. A filter according to any of the preceding claims, wherein said hood is hollow and has a perforated undersurface.

19. A filter according to claim 18, wherein said container is circular and includes a central pipe connected to the backwash outlet, and wherein a protruding cylindrical collar extends perpendicularly from said hollow hood into the central pipe, said collar being vertically movable in said central pipe.

20. A filter according to any of the preceding claims, including a floculator comprising a generally cylindrical vertical column centered in said container and terminating below the top of said container.

21. A filter according to claim 19, including means for pumping backwash liquid from the hood through a series of nozzles in said column to cause turbulence in the liquid in said column.

22. A filter according to any of the preceding claims, in which the filter material is granular.

23. A multibed liquid filter substantially as herein described with reference to Figures 1 to 5 or Figure 6 or Figures 7 to 11 of the accompanying drawings.

24. A process for filtration of a liquid in a multibed filter comprising: a container having an inlet for liquid to be filtered and an outlet for filtered liquid; means in said container supporting filter material and defining therebeneath a filtered liquid cham ber communicating with said outlet; a wall of said container extending above said filter material so as to define a chamber for unfiltered liquid; partitions extending upwardly from said material supporting means so as to divide said filter material into a plurality of beds, the upper terminal edge of each partition extending above said beds into said unfiltered liquid chamber, said upper edges providing sealable surfaces; a backwash isolation hood for sealing with the said sealable surfaces provided by the partitions which define any of said beds; a backwash outlet connected to the underside of said hood, the process comprising: passing liquid through said beds until at least one bed requires backwashing, moving said hood into sealing engagement with the partitions of the bed in need of backwashing; backwashing said bed by flowing backwashing liquid therethrough into said hood and out through the backwash outlet; terminating the flow of backwashing liquid through the bed; increasing the fluid pressure in the hood to a value higher than the pressure which would prevail in the hood if, at the termination of backwashing, the backwash outlet was closed and the fluid pressure in the hood was allowed to rise to a value determined by the fluid pressure in the filtered liquid chamber, and thereby reducing the force required to lift said hood out of sealing engagement with said partitions; and moving the hood to another bed in need of backwashing.

25. A process according to claim 24 wherein the fluid pressure in the hood is increased by forcing gas under pressure into the space beneath the hood.

26. A process according to claim 25 wherein gas is forced into the space beneath the hood until the hood floats out of sealing engagement with the partitions.

27. A process according to claim 24 wherein fluid pressure in the hood is increased by hydraulically connecting the inside and outside of the hood.

28. A process according to claim 27 comprising forcing gas under pressure into the space beneath the hood after hydraulically connecting the inside and the outside of the hood until the hood floats out of sealing engagement with the partitions.

29. A process for filtration of a liquid, substantially as hereinbefore described with reference to Figures 1 to 5 or Figure 6 or Figures 7 to 11 of the accompanying drawings.