GB2075853A - Liquid filter - Google Patents

Abstract

A band of filter medium is fed from a roll 15 and round a hollow cell 40, which has a perforate wall 41 and is submerged in liquid to be filtered, contaminated filter medium being periodically withdrawn from the cell by drive means 24,27 (Figure 3 not shown) to draw clean medium into place across wall 41. The drive means is controlled by a timer which also cuts off the suction applied by pump 13 to cell 40 to facilitate movement of the medium. The cell can be pivotally mounted and/or inclined (Figures 5,6 not shown). <IMAGE>

Description

SPECIFICATION Liquid filter system The present invention relates generally to liquid filter systems and, more particularly, to filter systems for removing contaminants from a liquid by passing the liquid through a continous web of filter media which can be automatically advanced so that it is not necessary to manually open the filtration chamber and change the filter media each time the media becomes clogged with contaminants removed from the liquid.

Heretofore, liquid filter systems utilizing continuous webs of filter media have generally involved either a pressurized liquid-tight housing which is purged and opened for each indexing movement of the media, or a secondary recovery system for removing the contaminants collected on the surface of the media. Thus, U.S. Patent No. 3,497,063 discloses a filtration system in which a continuous web of filter media is indexed through a pressurized housing formed by upper and lower shells which are opened during indexing movement of the media and then closed during filtration. This system necessitates purging of the housing before the shells are opened, and the effecting of a seal through the media when the shells are closed. U.S. Patent No.

3,305,094 discloses a vacuum filter which draws the liquid through a submerged horizontal web of filter media, with the filtered contaminants being collected on the underside of the submerged media.

The liquid flow through the media is periodically reversed to dislodge the filtered contaminants from the media to that the contaminants can settle to the bottom of the liquid being filtered, with the sunken contaminants then being removed from the liquid body by a partially submerged mechanical conveyor. This system not only requires a secondary removal system, e.g., the liquid flow reversal and the mechanical conveyor, but also permits reentrainment of the filtered contaminants in the liquid being filtered, thereby requiring the contaminants to be re-filtered from the liquid.

Vacuum "drum" filters have also been used heretofore. In these filters, a large rotating drum is partially submerged in the liquid to be filtered, with the drum carrying a closed loop or "belt" of filter media which is continuously rotated in and out of the liquid in which the drum is partially submerged.

The filtered contaminants are removed from the media surface each time it leaves the liquid so that it can be recycled through the liquid again. Suction is applied to the successively submerged portions of the media belt through the interior of the rotating drum, which has a series of sealed chambers around its periphery and connected through appropriate valving to a suction source. These drum filters are complex and costly to build, as will be appreciated from the examples described in U.S. Patents Nos.

3,143,502 and 3,347,389.

According to the invention, there is provided an automatic liquid filter system for removing contaminants from a body of liquid, said system comprising means for rotatably supporting a wound roll of a continuous web of filter media, guide means for guiding said continuous web of filter media downwardly into the body of liquid to be filtered and then upwardly out of said liquid body, a filter cell mounted for submersion in said liquid body and having at least one perforated wall for supporting a predetermined length of one side of the upward run of the media against a pressure differential across the media within the liquid body, said filter cell also forming an internal collection chamberforreceiving the liquid filtrate passing through said media and said perforated wall, a liquid pump having its suction side connected to said collection chamber for withdrawing the liquid filtrate from said collection chamber and thereby drawing liquid through said media and said perforated wall, whereby said contaminants contained in the liquid passing through said media are collected on the top surface of said upward run of said media, means for interrupting the suction action of said pump to reduce the pressure differential across the media on said perforated wall and thereby release the media for advancing movement across said perforated wall, and indexing means aligned with said filter cell but outside said liquid body for gripping the media carrying said contaminants and pulling said media across said filter cell to withdraw said media and the contaminants thereon from said liquid body, and to bring fresh areas of said web into register with said perforated wall.

The following is a more detailed description of some embodiments of the invention, by way of example, reference being made to the accompanying drawings, in which: Figure 1 is a perspective view of an automatic filter system embodying the present invention; Figure 2 is a vertical section taken generally along line 2-2 in Figure 1; Figure 3 is an enlarged section taken generally along line 3-3 at the top of the automatic filter as shown in Figure 1; Figure 4 is a schematic diagram of the control circuit utilized in the automatic filter of Figures 1-3; Figure 5 is a partial vertical section of a modified embodiment of the invention; and Figure 6 is a schematic diagram of another modified embodiment of the invention.

While the invention will be described in connection with certain preferred embodiments, it will be understood that it is not intended to limit the invention to these particular embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalent arrangements as may be included within the scope of the invention as defined in the appended claims.

Turning now to the drawings, there is shown a filter system 10 for removing particulate solid or semi-solid material from a body of liquid 11 containined in a tank 12. For example, the liquid 11 may be an electroplating solution, such as the sulfuric acid solution of nickel salts commonly used in nickel plating, which must be continually filtered to remove particulate material and thereby ensure a smooth bright finish on the articles being plated. The illustrative system is useful in removing not only solid particulate matter, but also flocculants, gels, pasty materials and the like. The filter system includes a pump 13 for passing liquid from the body 11 through a continuous web of filter media 14 which effects the removal of the undesired contaminants from the liquid being filtered.To prevent the filter from clogging, fresh lengths of the continuous web of filter media 14 are successively advanced through the filter from a supply roll 15 journaled on a pair of brackets 16 and 17.

Advancing movement of the filter media 14 is effected by a drive motor 18 coupled through a chain or drive belt 19 to a pair of nip rolls 20 and 21 which grip the media and pull it through the filter whenever the drive motor 18 is energized. More specifically, the motor 18 drives a sprocket 22 through a gear reduction box 23, and the sprocket 22 is coupled to an upper sprocket 24 by the toothed drive belt 19.

The sprocket 24, in turn, drives a pair of meshing gears 26 and 27 affixed to the ends of the nip rolls 20 and 21 on the outboard side of a side plate 28 which carries one end of each nip roll. The other ends of the nip rolls 20 and 21 are carried by a side plate 29.

The lower nip roll 20 has a knurled surface for engaging the relatively clean underside of the media, and rotates about a fixed axis defined by a pair of bearings 30 and 31 mounted within the side plates 28 and 29, respectively. The upper roll 21 comprises a relatively thin center shaft spaced above the knurled surface of the lower roll 20 to permit contaminants built up on the top surface of the media 14 to pass freely between the two rolls 20, 21.

At opposite ends of the thin center shaft, however, the roll 21 includes a pair of iarger diameter segments 21 a and 21b with knurled surfaces for gripping the longitudinal edge portions of the media 14 and thereby pulling the media between the two nip rolls. As illustrated in Figures 1 and 3, the segments 21 a and 21 b preferably grip only those edge portions of the media outside the central area on which the filtered contaminants are collected.

To permit the opposed surfaces of the rolls 20,21 to be biased toward each other (for gripping the media 14) while still permitting the rolls to be easily separated (for threading new media), the upper roll 21 rotates about an axis that can be moved about a pair of pivot points 32 and 33. More particularly, opposite ends of the roll 21 arejournaled in a pair of levers 34 and 35 whose lower ends are pivotally fastened to the side plates 28 and 29 at the pivot points 32 and 33. The lower ends of the levers 34 and 35 are biased in a counterclockwise direction, as viewed in Figures 1 and 2, by a pair of springs 36 and 37 anchored to the respective side plates 28 and 29.

This spring bias urges the nip roll 21 against the roll 20 to grip the media 14 firmly between the two rolls 20 and 21 while permitting the roll 20 to be retracted away from roll 21 forthethreading of a new web of media or in the event of a jam between the two nip rolls. This spring bias can also be adjusted to vary the pressure between the rolls 20 and 21 and/orto vary the pressures at opposite ends of the rolls to ensure that the media 14 follows a straight track. As can be seen in both Figures 1 and 3, the shaft of the movable nip roll 21 extends through a slot 38 in the side plate 28 for attachment to the drive gear 27, with the slot 38 providing the clearance needed to permit pivotal movement of the levers 34 and 35.

Within the body of liquid being filtered, the media 14 passes around a submerged hollow filter cell 40 that includes a perforated wall 41 for supporting a predetermined length of the filter media 14 while permitting liquid to pass therethrough. Liquid filtrate which passes through the filter media 14 and the perforated wall 41 of the filter cell 40 enters a liquid collection chamber 42 formed inside the filter cell.

From the collection chamber 42 the filtrate is discharged from the filter cell through a pair of exit ports 43 and 44 leading to a manifold tube 45 which in turn is connected to a suction line 46 leading to the suction side of the pump 13. Thus, it can be seen that the pump 13 draws the liquid being filtered through the media 14 and the perforated wall 41 into the collection chamber 42, and then on through the exit ports 43,44 and the manifold 45 into the suction line 46.

The collection chamber 42 formed by the filter cell 40 must be sealed from the liquid body 11 sufficientlyto permitthe pump 13 to draw liquid into the receiving chamber 42 primarily through the perforated wall 41, thereby also drawing the filter media 14 tightly against the outer surface of the perforated wall 41 so that any liquid that passes through the perforated wall must first pass through the filter media. However, one of the advantages of the illustrative arrangement is that it is not necessary for the collection chamber 42 formed by the filter cell 40 to be perfectly liquid-tig ht in the joint regions because the entire filter block is submerged in the liquid being filtered, and thus a minor amount of leakage through the joints of the filter block is insignificant.Any minor leaks that develop also tend to be self-sealing as they become plugged with the contaminants being filtered.

Even more important is the fact that no liquid seal is required around the periphery of the particular area of the filter media 14 that is being utilized at any given time, because the effect of the pump 13 is to draw, rather than force, the liquid through the filter media. That is, the illustrative system utilizes a negative pressure drop across the media to cause liquid to pass through the media, and it is not necessary to provide any means for containing liquid under pressure on the feed side of the filter media. Furthermore, the system is extremely safe because of the absence of any high pressures in the filter system. Indeed, the maximum pressure in the system is that produced by the head of liquid in the tank 12. Thus, the combination of the submerged filter cell and the use of the suction side of the pump to produce a negative pressure drop across the media and thereby draw liquid through the media, permits an extremely simple, economical, and safe structural arrangement while still permitting the attainment of relatively high liquid throughput rates.

For example, it has been found that liquid throughput rates as high as 1200 gallons per hour can be achieved with the illustrative filter utilizing a filter cell with a perforated wall that is only ten inches square. Thus, an extremely compact structure can provide relatively high throughput rates.

The particular filter cell 40 shown in the drawings is formed from three flat plates 50, 51 and 52, with the plate 50 forming the perforated wall 41, the plate 51 forming the cavity of the collection chamber 42, and the plate 52 forming the rear wall of the block and the collection chamber therein. These three plates are stacked against each other and held together by a plurality of screws 53 disposed around the periphery of the cell, with the shanks of the screws 53 being threaded into the rear plate 52 and the heads of the screws being countersunk into the face plate 50. The plates 50-52 may be formed of any desired material, depending upon the corrosion properties of the particular liquid being filtered.For example, when the filter is used for the filtering of the acids which are commonly use in electroplating, the filter cell 40 is preferably formed of polypropylene or a similar polymeric material which is resistant to the highly corrosive properties of the electroplating solutions.

It is particularly advantageous to suspend the filter cell 40 vertically within the body of liquid being filtered because this permits the submerged portion of the filter to be accommodated in a small space along the vertical side wall of the tank 12. For example, when the filter is used in an electroplating tank which receives racks containing hundreds or thousands of work pieces to be plated, tank space is at a premium. However, the submerged portion of the filter of the present invention can be accommodated in the narrow clearance space, typically only two or three inches wide, provided along the walls of the electroplating tank.If there is not sufficient clearance within the electroplating tank to accommodate the filter, it may be mounted on a small auxiliary tank placed alongside the main tank, with the liquid being siphoned from the main tank into the auxiliary tank so that there are still no pressures greater than those produced by the liquid heads within the tanks. In this latter arrangement, the filtrate discharged from the pump 13 is normally conducted back into the main electroplating tank.

There is also an advantage in having the exit ports 43 and 44 located at the top of the filter cell 40. This permits any entrained gases that separate from the liquid in the collection chamber to be continuously purged from the filter cell 40 through the exit ports 43 and 44. Because this gas is lighter than the liquid, the gas passes on through the suction line 46 and the pump 13 and is eventually vented to the atmosphere after being discharged from the pump along with the filtrate, so that the pump does not become "air bound".

In order to guide the web of filter media 14 from the supply roll 15 to the submerged filter cell 40, and then on to the nip rolls 20 and 21, with as little friction as possible, a plurality of guide members are disposed along the media path. Thus, as the media leaves the supply roll 15 it passes under a guide rod 60 and then upwardly along the outer surface of the tank side wall to a guide roller 61. From roller 61, the media passes horizontally over the top edge of the tank wall and then downwardly over a guide roller 62 to the filter cell 40. The central portion of the lower end of the cell 40 is recessed and rounded, as at 63, so as to form a smooth channel that guides the media as it is drawn downwardly along the rear wall of the filter cell and then upwardly over the perforated front wall 41.At the top of the filter assembly the media passes over a final guide roller 64 before it enters the nip of the two driven rolls 20 and 21. As the media 14 exits from the rolls 20 and 21, it drops onto a discharge chute 65 and slides downwardly thereof to a waste container 66. A plurality of raised ribs 67 are preferably provided on the surface of the chute 65 to preventthe media 14from "hanging up" on the chute.

It will be noted that all the functional elements of the filter system are mounted on a saddle-type frame which straddles the tank wall with one pair of depending legs 70 and 71 extending downwardly along the inside surface of the tank wall and a second pair of depending legs 72 and 73 extending downwardly along the outside surface of the tank wall. The inside legs carry the filter cell 40 and the manifold tube 45 connected thereto. The outside legs carry the media supply roll 15, the media drive motor 18, and the pump 13. The two pairs of depending legs 70,71 and 72, 73 are interconnected at the top of the saddle-type frame by a horizontal channel member 74 which rests on the top of the tank wall. The upper ends of the legs 70-73 are also fastened to the side plates 28 and 29 which carry the guide rolls 61, 62 and 64 and the driven nip rolls 20 and 21.For stability purposes, a plurality of spacer bolts 75 project inwardly from the outside legs 72 and 73 against the outer surface of the tank wall, and a plurality of spacer pads 76 on the inside legs 70 and 71 engage the inner surface of the tank wall.

This saddle mounting arrangement for the filter system provides a compact assembly on both sides of the tank wall. Thus, the elements mounted on the outside legs of the saddle frame generally extend less than about two feet beyond the outside surface of the tank wall, thereby avoiding any significant obstruction of the space adjacent the tank wall, which is often used as a walkway between adjacent tanks. Similarly, the side plates mounted on the top of the saddle frame extend only a few inches above the top of the tank, thereby avoiding any significant obstruction of the space above the tank wall. Furthermore, the entire filter system occupies only a relatively short horizontal length of the tank wall, typically less than a two-foot length of the tank in a conventional electroplating facility.

To minimize the power requirements for advancing the filter media and to avoid tearing of the media, control means are provided for interrupting the operation of the pump each time the media drive motor 18 is energized to advance a fresh length of the filter media 14 into register with the perforated wall 41 of the filter cell 40. This permits the filter media 14to be advanced with a minimum amount of power by interrupting the suction which holds the filter media tightly against the perforated wall of the filter cell whenever the pump 13 is operating.

Consequently, a relatively small drive motor can be used to advance the filter media, consuming only a small amount of power each time it is turned on.

Moreover, the drive motor is never working against the suction force that holds the media against the filter cell 40 when the pump 13 is operating, so that there is less likelihood of tearing the media in the nip of the rolls 20,21 when the drive motor 18 is energized. As a further refinement, a time delay relay may be provided in the energization circuit for the media drive motor 18 to further ensure that there is no suction holding the media to the filter cell when the drive motor is energized.

As an alternative, the media drive motor 18 may be operated continuously if the particular liquid being filtered does not require a high suction pressure, or if the contaminants collected on the outside surface of the filter media and/or the texture of the media do not cause the media to be held against the filter cell 40 too tightly while the pump 13 is operating. In applications which permit this continuous mode of operation, the continuously advancing filter media is utilized more efficiently because the entire media surface passes over the staggered array of apertures in the perforate wall of the filter cell while the pump is operating and, therefore, while filtering is taking place.

As still another alternative, the filter media 14 can be indexed in successive lengths which are shorter than the height of the filter cell 40 to optimize efficient utilization of the media. For example, if the media is advanced by only one third of the height of the filter cell in each cycle, then at any given time when the pump is operating the perforated wall of the filter cell is covered with media that, in effect, provides a gradation of filtering characteristics.

More particularly, one-third of the media will be fresh, one-third will be in its second filtering cycle and therefore carry a relatively light "cake" of contaminants that effectively reduces the porosity of the media, and one-third will be in its third filtering cycle and therefore carry a heavier "cake" of contaminants that reduces the filtering porosity still further. This pattern of media advancement also provides more efficient utilization of the media because a larger percentage of the media surface area will come into register with the apertures in the perforated wall of the filter cell 40 due to the fact that each segment of the media is used at three different locations along the surface of the perforated wall.

That is, each indexing movement of the filter media is generally in random increments rather than being precisely synchronized with the center-to-center spacing of the apertures, so that the particular areas of the media that register with the apertures in one cycle normally do not register with the apertures in the next cycle, so the filtered contaminants are distributed rather evenly over the media surface. If desired, the indexing movements of the media can be precisely controlled to ensure that successive indexing steps bring successively different areas of the media into register with the apertures of the filter cell.

A preferred electrical control circuit for the illustrative filter system is illustrated in Figure 4. In this circuit, a-c. power is supplied to the pump 13 from a source 80 via contact 81 a of a single-pole, doublethrow switch 81 and a normally closed manual pushbutton switch 82. At preselected intervals determined by a timing motor 83, a cam 84 driven by the timing motor 83 throws the switch 81 to the contact 81 b, thereby de-energizing the pump 13 and energizing a time-delay relay 85. After a predetermined delay interval built into the relay 85, the relay closes contacts 85a, thereby supplying power to the media advance motor 18. As explained previously, the time delay ensures that the motor 18 is not energized until the media has been released from the suction effect of the pump 13.Advancement of the media continues for an interval determined by the speed of the timing motor 83 and the shape of the cam 84. At the end of this interval, the switch 81 is returned to the contact 81a, which de-energizes the relay 84 and the, motor 18 and re-energizes the pump 13. This completes one operating cycle. The duration of each cycle, as well as the duration of each step of each cycle (i.e., the pumping step, the time-delay interval, and the media-advance step) can be readily adjusted to suit different filtering applications. For example, a number of different commercial time-controlled switches are available with a variety of adjustment mechanisms for changing the time periods during which the switch remains at its different positions.

Similarly, commercially available time-delay relays are capable of providing virtually any desired delay interval. If it is desired to override the action of the switch 81, the pushbutton switch 82 can be manually depressed to close contacts 82a, which has exactly the same effect as throwing the switch 81 to the contact 81 b.

In Figure 5 there is illustrated a modified embodiment of the invention which permits adjustment of the angle at which the filter cell 40 is disposed relative to the liquid surface level. Thus, the support legs 70 and 71 for the filter cell 40 are pivoted to the frame plates 28 and 29, as at 90, rather than being rigidly fastened to those plates or the channel 74.

This permits the entire portion of the filter system carried by the legs 70,71 to be pivoted about a horizontal axis defined by the pivotal connections of the legs 70r 71 to the plates 28, 29, which axis is preferably aligned with the axis of the media guide roller 64.

The pivotal mounting of the legs 70,71 permits the filter cell 40 to be easily pivoted out of the liquid body 11 to facilitate the threading of a new web of media 14 without removing the entire filter assembly from the tank 12. In addition, the pivotal mounting permits the perforated surface of the filter cell 40 to be oriented at any desired angle to help retain certain types of filtered contaminants on the media 14 as it is withdrawn from the liquid body 11. For example, certain types of contaminants tend to be slippery and therefore tend to slide off the filter media during advancement of the media when the suction effect of the pump 13 is no longer present to hold the contaminants on the media. By raising the angle of inclination of the filter cell 40 toward horizontal, as indicated by the dashed-line position in Figure 5, for example, certain types of "slippery" contaminants can be held on the media and therefore removed from the liquid body 11.

When the filter system of Figures 1-5 is operated at high liquid throughput rates, the pump used therein can have a relatively short life, particularly when the liquid being filtered is at an elevated temperature. It has been found that the pump life can be extended considerably in such applications by using the modified system illustrated in Figure 6. In this system, the pump 100 is mounted beneath the filter cell 14 and is connected to the lower end of the filter cell 14 by a suction line 101 passing through the bottom wall of the tank 12. The entire length of the suction line 101 is either horizontal or extends downwardly toward the intake of the pump 100, so that the pump can be primed by gravity flow of liquid from the cell 14 down into the pump.

In order to prolong the life of the motor 100a which drives the pump 100, it is preferred to operate the pump 100 continuously, and thus a valve 102 is provided in the discharge line 103 from the pump.

When the filter media on the cell 14 becomes clogged and is ready to be indexed, the valve 102 is closed to remove the suction from the filter media so that it can be advanced to bring a fresh length of media into register with the cell 14. The pump 100 continues to run, but does not produce any suction on the filter media as long as the valve 102 remains closed. When the filter media has been indexed, the valve 102 is re-opened to permit liquid to once again be discharged from the pump 100 and pass through a return line 104 back to the tank 12 or any other desired receptacle.

Even with the suction line 101 illustrated in Figure 6, which permits liquid to flow by gravity into the intake of the pump 100, vapor pockets can still be developed between the filter cell 14 and the pump 100 when the filter media becomes clogged. For example, the reduced pressures produced by the pump in the suction line 101 may cause flashing of the liquid therein, particularly when the liquid is at an elevated temperature. These vapor pockets can starve the pump and cause it to fail. To avoid this problem, a second valve 105 connects the return line 104 to a bypass line 106 which allows liquid to drain from the return line 104 back into the suction line 101 when the valve 102 is closed, thereby purging any vapor pockets from the suction line 101 through the filter cell 14.Thus, when the valve 102 is re-opened, the pump 101 will receive a steady supply of liquid without any pockets of vapor or air which might cause the pump to become starved and fail. The valve 105 is closed simultaneously with the opening of valve 102, and may also be opened simultaneously with the closing of valve 102.

In the particular arrangement shown in Figure 6, the valves 102 and 105 are controlled by a vacuum switch 107 and the same timer 108 which controls the media indexing motor 18. More particularly, the vacuum switch 107 closes the valve 102 and opens the valve 105 when the pressure within the suction line 101 drops to a level which indicates that the filter media is sufficiently clogged that the media should be indexed. The timing 108 also receives a signal from the vacuum switch 107 to initiate indexing movement of the filter media via the motor 18. After a predetermined time interval, the timer 108 times out and sends a signal to the valves 102 and 105, re-opening valve 102 and closing valve 105. The timer also de-energizes the drive motor 18 at the end of the predetermined time interval. If desired, the two valves 102 and 105 may be replaced with a single three-way valve.

As can be seen from the foregoing detailed description, the embodiments of the invention described above with reference to the drawings provide an improved automatic filter system which efficiently extracts contaminants from a liquid body on the filter media itself, without the use of any secondary extraction medium, and at a rapid rate.

This system is efficient and economical to manufacture and operate because it does not require any submerged moving parts otherthan the media itself.

A high liquid throughput rate is achieved without requiring a liquid seal around the particular area of the filter media being utilized in any given time, and without requiring a liquid-tight housing for the filter media; in this connection, there is also no need for a secondary energy source to purge the filter housing prior to each media advancement, so that the operating cycle is quite simple. This improved automatic filter system is extremely reliable in operation and can be left unattended for automatic operation over long periods of time. It is safe to operate, with no fail-safe problems, and can be operated with no super-atmospheric pressures and no danger of spillage of corrosive liquids in the event of a power failure or other malfunction.The apparatus is also extremely compact so that it can be accommodated in a very small space, thereby permitting it to be utilized in even the most cramped plant conditions, while still providing desirable high liquid throughput rates. The system can be easily fabricated from material which can be safely used to filter even the most corrosive liquids, including strong acids, while still providing a long operating life, and it is particularly useful in the filtration of corrosive electroplating solutions. Efficient utilization of the filter media is also provided, with little or no wasted area on the continuous media web.

Virtually any desired type of filter media can be employed, with the media being advanced either continuously or intermittently at any desired rate, and with the use of minimal drive power. Moreover, this system continuously purges itself of any gases entrained in the liquid being filtered.

Claims (9)

1. An automatic liquid filter system for removing contaminants from a body of liquid, said system comprising means for rotatably supporting a wound roll of a continuous web of filter media, guide means for guiding said continuous web of filter media downwardly into the body of liquid to be filtered and then upwardly out of said liquid body, a filter cell mounted for submersion in said liquid body and having at least one perforated wall for supporting a predetermined length of one side of the upward run of the media against a pressure differential across the media within the liquid body, said filter cell also forming an internal collection chamber for receiving the liquid filtrate passing through said media and said perforated wall, a liquid pump having its suction side connected to said collection chamber for withdrawing the liquid filtrate from said collection chamber and thereby drawing liquid through said media and said perforated wall, whereby said contaminants contained in the liquid passing through said media are collected on the top surface of said upward run of said media, means for interrupting the suction action of said pump to reduce the pressure differential across the media on said perforated wall and thereby release the media for advancing movement across said perforated wall, and indexing means aligned with said filter cell but outside said liquid body for gripping the media carrying said contaminants and pulling said media across said filter cell to withdraw said media and the contaminants thereon from said liquid body, and to bring fresh areas of said web into register with said perforated wall.

2. An automatic filter system as set forth in clam 1 wherein the perforated portion of said filter cell is narrower than the width of said web of filter media so that the longitudinal margins along both edges of said web remain free of said contaminants, and said indexing means comprises a driven roll engaging the underside of the media carrying said contaminants, and a pair of pressure rolls engaging only said longitudinal margins of the top side of said media and pressing the media against said driven roll.

3. An automaticfiltersystem as set forth in claim 2 wherein said driven roll is a knurled roll extending across the entire width of the underside of said media.

4. An automatic filter system assetforth in claim 2 or claim 3 wherein said pressure rolls comprise a pair of idler rolls which are spring-loaded to press said longitudinal margins of said media ag#ainst said driven roll.

5. An automatic filter system as set forth in any one of claims 1 to 4wherein said guide means comprise at least one pair of flanges extending outwardly from said filter cell in a direction perpen dicularto the path of said web of media and adjacent the longitudinal edges of said media for maintaining said media in the desired alignment with the perforated wall of said cell.

6. An automatic liquid filter system as set forth in any one of claims 1 to 5 which includes a primary tank containing the primary body of liquid to be filtered, a secondary tank containing said filter cell for submersion in a secondary body of the liquid to be filtered, and siphon means for transferring the liquid from the primary tank to the secondary tank so that the liquid level in the secondary tank does not rise above the liquid level in the primary tank.

7. An automatic liquid filter system as set forth in any one of claims 1 to 6 which includes support means above said liquid body and said filter cell depends from said support means into the liquid body, said filter media being guided downwardly along one side of said filter cell and upwardly along said perforated wall on the opposite side of said filter cell, and means for adjusting the angle at which said filter cell depends from said support means into said liquid body.

8. An automatic liquid filter system as set forth in claim 7 wherein said filter cell is pivotally suspended from said support means for pivotal movement about a horizontal axis to permit said angle to be adjusted.

9. An automatic liquid filter substantially as hereinbefore described with reference to Figures 1 to 4 orto Figures 1 to 4 as modified by Figure 5 or Figure 6 of the accompanying drawings.