GB2080696A - Upflow water filtration with buoyant filter media - Google Patents

Abstract

High rate upflow filtration is conducted using a bed of buoyant media particles 30 in a filter vessel 20 unobstructed by cross-sectional, media-confining screens. A horizontal trough structure 34 collects filter effluent and makes it possible to clean the bed by diffusing air into the liquid through perforated tubes 60 below the bed so that average fluid density in the bed is reduced. Alternative effluent collecting means are disclosed. The reduction in fluid density results in expansion of the bed as media particles descend by gravity. The water may pass through a settling tank prior to filtration. <IMAGE>

Description

SPECIFICATION Process and apparatus for high rate upflow water filtration with buoyant filter media The present invention relates to methods and apparatus for purification of water by filtration. More specifically, it concerns upflow filters which contain a buoyant filter media.

It has been known for some time that water can be filtered by passing it upwardly through a bed of filter media comprising grains or small pellets of a buoyant material such as polyethylene. Such a filtration method is shown in United Kingdom Patent Specification No. 833,327 to Smith. A related upflow filtration process is described in Example 4 of United States Patent No. 3,343,680 (Rice et al.).

While such filters may have shown promise in their time, they have never been successful since they heretofore have been contained within substantially closed tanks or in tanks internally divided by screens to prevent escape of the buoyant media. The filtration zones and media bed of such filters are inaccessible during filtration which limits control over the filtration process.

Furthermore, no effective methods have been found for cleaning the media in such filters. Backwashing of such filters has proved to be wasteful of energy, finished water and time since a very large downward flow of liquid is required before the media particles will separate. Mechanical agitation of the media, as described in the Smith specification, enhances cleaning, but requires a wasteful consumption of energy in orderto sufficiently agitate a packed media bed that impurities are released.

It has now been found that it is possible to provide trough-type liquid collectors to remove filtered water from the effluent side of an upflow filter. The use of such a collector makes it possible to improve the accessability to the filter apparatus by eliminating any top covering or screen when used in an appropriately shaped filter vessel. This arrangement allows for the incorporation of movable mechanisms which can extend from the exterior to the interior of the tank for free motion therethrough without the use of complicated seals or tortured screening arrangements.

The nature of buoyant media particles used in a filter bed has a substantial impact on the flow rate, solids capture rate and cleanability of the bed. Particularly significant are the shape and size of the media particles. Irregular, angularly shaped particles are found to have significant advantages.

It is also discovered that a relatively gentle cleaning procedure can be used if the media is sufficiently close in specific gravity to the specific gravity of water. Air bubbles dispersed throughout water flowing into the bed cause the overall specific gravity of the water and air mixture to fall to a level below that of the media in the bed so that the bed uniformly expands downwardly, thereby releasing trapped impurities without mechanical agitation.

Free standing filters according to this invention are particularly advantageous because they can be constructed in any desired exterior configuration or shape. The size of such a filter depends only on the amount of liquid to be filtered in a given amount of time.

It is therefore an object of this invention to provide a filter which is easily accessible for inspection, treatment by mechanical apparatus inserted from the exterior and for viewing and maintenance pur poses.

Also an object is to provide a filter which can operate at a high flow rate and which rapidly can be cleaned and returned to service after cleaning.

Another object is to provide an apparatus which can be cleaned easily and automatically.

A related object is to provide a plurality of such filters which operate in tandem so that filtering processes can proceed uninterrupted while one filter unit is being cleaned.

A further object is to provide a filter for use in virtually any location, the filter being a free standing unit which will operate with or independently of other associated equipment and which can be built in any size or shape.

These and other features, objects and advantages of the present invention will be apparent from the following detailed description thereof and from the attached drawings.

In the drawings: Fig. 1 is a perspective view of a filter apparatus according to the present invention shown in vertical section; Fig. 2 is a sectional side elevation of a collector trough shown in Fig. 1; Fig. 3 is a sectional side elevation of a first alternate embodiment of the trough shown in Fig. 2; Fig. 3a is a sectional side elevation of a second alternate embodiment of the trough shown in Fig. 2; Fig. 4 is a sectional side elevation of a third alternate embodiment of the trough shown in Fig. 2; Fig. 5 is a sectional side elevation of a fourth alternate embodiment of the trough shown in Fig. 2; Fig. 6 is a schematic sectional side elevation of a filter according to the present invention with its bed in the operating position; Fig. 7 is a schematic sectional side elevation of the apparatus shown in Fig. 6 the bed expanded during cleaning;; Fig. 8 is a perspective view of the filter apparatus of Fig. 1 incorporating another effluent collector and flow controller.

Fig. 9 is a perspective view of a horizontally oriented filter according to the present invention with a portion of the outer casing broken away to show interior detail; Fig. 10 is a perspective view of a filter according to the present invention incorporating suction sludge removal apparatus; Fig. 11 is a perspective view of a multicompartment filter apparatus according to the present invention shown in vertical section; Fig. 12 is a schematic diagram showing a water purification system according to the present invention including a clarifier tank, and both upflow and downflowfiltration units;; Fig. 13 is a sectional side elevation of a filter according to the present invention especially suited forfiltering water containing cellulosicfibers or other suspended solids which tend to form a mat on the influent diside of a filter bed.

Fig. 14 is a graph showing a wash waste profile of a filter according to the present invention; and Fig. 15 is a graph showing wash waste profiles of another filter according to the present invention.

As shown in Fig. 1, a filter vessel 20 provides an internal passageway 22 for water moving therethrough. An inlet 24 is provided nearthe base of the vessel 20 for supplying influent water into the passageway 22 and an outlet 26 is located near the top o, the vessel for removing filtered water from the passageway 22.

Located between the inlet 24 and outlet 26 is a bed of bouyant media particles 30 on which impurities collect as water to be filtered moves upwardly through the bed. This media must have a specific gravity less than that of water and, for the reasons described below, should have a specific gravity no less than 0.80. Most preferably, the media particles should have a specific gravity no less than 0.96.

To achieve effective filtration, a media of proper characteristics will be chosen depending upon the nature of the waterto be filtered. For example, the media should have an effective size between two and twenty millimeters; but optimal filtration of storm water or effluent from a biological treatment clarifier, requires a media having an effective size between about 2.0 and 10.0 millimeters in diameter.

The particles will have a uniformity coefficient no greater than 2.0 and sphericity of less than 0.7.

Water is filtered by passing it upwardly through the bed of such particles 30 and is collected in a horizontal collector apparatus 34 which delivers the filtered water to the outlet 26. To allow for access and ease of cleaning, the collector34 should extend horizontally into the filter vessel at a location near the top thereof without impeding flow through the entire cross-section of the vessel. However, the collector should be capable of receiving liquid at multiple locations in the cross section of the passageway 22 without loss of media particles. And yet the collector should be such that the filter can be established in an uncovered tank for ease of access.

Collectors can be constructed in a number of advantageous configurations, Figs. and 8 illustrating most preferred arrangements. In Fig. 2, a solid channel 38 is covered by a flat screen 40. While in Fig. 3, a similar channel 42 is covered by an arched screen 44. The arched screen is preferred since it is entirely self supporting and will not collapse even if, for some reason, a substantial amount of the filter media particles 30 climbs overthe trough 42.

A screen 44a, which flares upwardly from a trough 42a is shown in Fig. 3a. This arrangement is desirable because media and other solids tend to fall outwardly and downwardly from the sides of the screen 44a. It is almost impossible for media to climb over the screen; and the screen is less subject to plugging than in most other collector types.

Fig. 4 shows another alternate trough embodiment. This trough is substantially tubular in cross section, and includes a lower hemicylinder made of a solid material 46 and an upper hemicylinder arch made of screening 48. In Fig. 5, the trough constitutes a tubular screen. Another variation (not shown), is a perforated tube with screen covered openings.

Any of the troughs can include means for automatically cleaning the screen should it become clogged f either with media or debris. As illustrated in Fig. 4, a tube 52 can extend through the interior of the trough and include spray orifices at periodical intervals.

Should the screens become clog , pressurized water is delivered to the interior of. the tube 52 to form spray jets which backwash the screen as illustrated.

As an alternative, one or more exlemal tubes 53 can supply water for jets which spray onto the exterior of the screen to wash off adhering solids.

A somewhat different collector, as shown in Fig. 8, comprises a pipe manifold 54 having a plurality of collector heads 55 located in a common horizontal plane. The heads 55 are made of screen and surround the inlets to the manifold 54 so that filter effluent can flow into the manifold inlets while buoyant media particles 30 are held back by the heads.

All of the illustrated collector trough arrangements avoid horizontal, cross-sectional, media-retaining screens and thus allow easy access, through the top of the filter vessel, to bed and region therebelow.

Furthermore, the collector arrangement is compatible with effective methods and apparatus for cleaning upflow filters of the type illustrated.

As previously mentioned, cleaning the media particles can be a substantial problem in this type of filter. And, the availability of a practical cleaning method is essential to filter operability.

In the present invention, cleaning is accomplished by periodically dispersing gas bubblesthnoug,hout liquid in the filter bed. The amount of gas provided is selected to be just sufficient to reduce the density of the fluid in the passageway 22 to below the density of the particles 30 which make up the bediso that at least somaofthe media particles descend by gravity and the filter bed expands in volume. Asthe bed expands, media particles separatofrom one another, allowingtrapped impurities to mere oubrardly from the bed'to be collected and discarded To effectively accomplish the dispersion of gas inta liquid in the bed, the apparatus of Fig. 1 includes a gas injection mechanism 56 located inside the vessel 2(1 beneath the bed of particles 30. The injection mechanism is a manifold hailing a feeder pipe 58 connected to lateral delivery tubes 60 When air is introduced into the manifold through an air intake line 64, air bubbles are produced! at the numerous orifices located in the tubes 60 throughout the cross-section of the passageway 22 so that, during cleaning, the fluid withinthe passageway 22 is an air-water mixture. By dispersing bubbles of air throughout the fluid in the passageway 22, bed expansion is substantially uniform ateach horizontal cross-section of the passageway 22.

A different gas injection mechanism 56a is shown in Figs. 6 and 7. This mechanism includes a vertical feeder pipe 58a on which are mounted perforated delivery tubes 60a. This mechanism Is constructed so that the tubes 60a revolve horizontally to dispense air bubbles throughout the passageway 22 when pressurized air is supplied through the pipe 58a.

Regardless of the vessel or collector used, region R is provided between the bottom of the bed of particles 30 and the air distribution manifold. This region R must be unobstructed by screens or other structures which would limit downward movement of the media particles. It must also be of sufficient volume to allow the bed to expand downwardly during cleaning to the extent that trapped impurities in the bed are released.

A comparison of a bed before and during cleaning is found in Figs. 6 and 7. Fig. 6 shows a bed in the normal unexpanded condition which exists during normal filtration. During cleaning the bed is expanded as illustrated by Fig. 7, which shows a partially expanded bed.

When air is introduced through the manifold, the media particles 30 descend, usually to the extent that the lowest particles of the bed are located immediately above the manifold. Particles will typically fill the region R when the bed is expanded, but will not descend below the air manifold since liquid below that level remains at a greater density than the media.

To achieve effective cleaning with a minimum of procedural steps, it is advantageous to prevent all flow of liquid through the outlet 26 during cleaning of the bed. This is accomplished by closing valves 66,67 provided in lines extending from the outlet 26.

In most open vessels, however, the valves 66,67 can not be closed without simultaneously diverting the influentto prevent the open filter vessel from overflowing. Even in a closed vessel, merely closing the outlet 26 would not be acceptable because it would cause water to back up at some location upstream of the filter bed.

To divert the filter influent, a drain outlet 68, connected with a drain line 70 is provided to remove water from the passageway 22. During expansion of the bed, the impurities trapped in the bed are separated, descend by gravity and are carried out of the vessel through the open drain outlet 68.

Valves controlling the drain line 70 can be opened to match continued flow through the inlet 24.

Removal of liquid through a drain can, however, create problems since it is not only essential to keep liquid from flowing over the top of the vessel 22, but also important to prevent media particles 30 from being carried outwardly through the drain.

Screens can be placed overthe outlet 68; but these are prone to rapid clogging during draining. In the filters according to the present invention, therefore, the drain outlet 68 is located at a position at least one foot below the air delivery tubes 60 of the manifold 56. This positioning prevents media from decending to the level of the outlet 68 even when the bed is at its maximum expansion. Also, a mechanism is provided to automatically match the flow entering through the inlet 24 to the flow of liquid exiting through the drain line 70 so that the media bed remains at the surface.

In the apparatus of Fig.1, such automated flowmatching is accomplished by means of a flow meter 74 to measure the rate at which influent is entering the filter vessel 22. A combined flow meter and automatically actuated valve 76 are incorporated in the drain line 70. A processer 78 is connected between the entrance flow valve 74 and exit flow valve 76 to continuously monitor and compare the rate of inflow and outflow. If the rate of inflow becomes out of balance with the rate of outflow, the processor 78 signals the automatically actuated valve 76 to open or close an appropriate amount to balance the flow.

In this manner, the level of water in the vessel 22 is maintained at a substantially constant level, even during cleaning while the bed is expanded.

Fig. 8 shows another automatic control system for maintaining a substantially constant level of liquid in the vessel during cleaning with the effluent line closed. In this embodiment, a float valve chamber 82 is connected by a pipe 84 to the interior of the vessel 20 so that liquid in the chamber 82 is always at the same level as liquid inside the passageway 22. A float 88 inside the vessel 82 connects to an automatic valve controller 90.

If the level of liquid inside the chamber 82 falls below a predetermined level during cleaning of the filter, the controller 90 automatically closes a valve 92 in the drain line 70 which causes the level of liquid to rise in both the vessel 20 and chamber 82. The control may also be constructed so that if the float 88 exceeds a predetermined maximum level in the chamber 82, the controller 90 signals the valve 92 to open, thereby draining liquid from the vessel until an acceptable reduced level is reached.

A filter according to the present invention can be constructed in a variety of configurations otherthan the open-topped, vertical, circular cylinder shown in Figs. 1 and 8. For instance, Fig. 9 shows a vessel 100 which is a substantially horizontally extending circular cylinder closed at both ends. A passageway 102 is defined between the level of the inlet 104 and outlet 106 with media particles 110 forming a bed through which liquid must travel from the inlet 104 to the outlet 106. A collector trough 114 is a forminous tube which extends longitudinally along the top of the container and connects to the outlet 106. A gas injection manifold 116 including a plurality of lateral tubes 120 connects to an intake line 124 and is located below the level of the particles 110 which form the bed. Liquid is removed through a drain outlet (not visible) which connects to a drain line 130.

The operating controls and mode of operation for the apparatus shown in Fig. 9 are similartothose discussed above in relation to Figs. 1 and 8.

Yet another embodiment is shown in Fig. 10 wherein a filter apparatus 136 includes a vessel 138 having a wall 140 in the shape of a rectangular cylinder. Adjacent to the vessel 138 is a headwater compartment 142 which receives water to build up a head sufficient for moving the water upwardly through the filter bed. The bottoms of the headwater chamber 142 and the vessel 138 are interconnected to provide an inlet 144 for water to enter the vessel.

An outlet 146 is provided near the top of the vessel 140 with a bed of buoyant media particles 150 being located between the inlet 144 and outlet 146. A trough-type collector 154 lies parrellel to a wall of the vessel 138. Air injection apparatus 156 are provided below the bed along with a drain outlet 158 and drain line 160.

Fig. 10 illustrates some of the specific advantages of the present invention. Water delivered to the filter ing system shown collects in the headwater chamber 142 and is free to rise or fall independently of the source of the water. The chamber 142 is constructed so that water can not flow back from the chamber 142 into upstream apparatus to cause back ups and overflows.

The open top of the apparatus shown in Fig. 10 accommodates a movable bridge mechanism 164 which can travel across the entire surface of the filter and which supports apparatus that depends into the water inside the vessel 140. In the illustrated embodiment a siphon-type sludge removal device of a type described in U.S. patent No. 4,094,785 to Booty, is carried by the bridge 164. The use of such a device would be awkward or impossible if the filter vessel were closed at the top or if the bed of media particles 150 was retained above or below a horizontal screen.

Fig. 11 shows an apparatus similar to the one illustrated in Fig. 10 and corresponding components are marked with similar reference numerals. The apparatus of Fig. 11, however, includes multiple filtervessels 138a-l38dwhich share common walls.

These multiple cells are connected in parallel and can operate together. Advantageously, flow through any individual filter can be halted during cleaning or maintenance of a particular cell while uninterrupted flow continues through the others.

The present invention further comprises other multi-unit systems for the purification of water containing suspended impurities. Such systems are illustrated by Fig. 12. A clarifier tank 160 has a wall 162 which defines a settling zone 164. Inlet means 166 deliver a stream ofwaterwith suspended solids into the settling zone 164 wherein the water is partially clarified by settling. Partially clarified water leaves the clarifier tank 160 by means of an outlet, illustrated in Fig. 12 as a weir 168.

Water passing over the weir 168 is delivered to a headwatercontainersuch asa pipe 170 wherein its level can, to some extent, vary independently of the level of the liquid in the clarifier. The pipe 170 delivers partially clarified water to the inlet 174 of an upflow filter vessel 176 which defines a vertical passageway for water between the inlet 174 and an outlet 178 located above the inlet Buoyant media of the type previously described is located in the vessel in an amount sufficient to form a floating filtration bed. Gravity urgeswatertoflowfromthe pipe 170 into the vessel 176 and through the bed so long as the level of water in the pipe 170 is greater than the level of water in the filter vessel 176.

Another advantageous system illustrated by Fig.

12 has side-by-side upflow and downflow filter vessels 176, 180. These may or may not share a common wall. Water which leaves the filter vessel 176 through the outlet 178 flows into the top of the downflow filter where the water is finished by flow ing downwardly through sand or some other non- buoyant media. The arrangement is ideal to minim ize energy consumption because the driving force which raises liquid through the filter vessel 176 is all that is necessary to supply influent to the downflow filter 180, even if the filters 176,180 are built to the same elevation and located alongside each other on, level ground. Chemical compounds to aid filtration, particularly in the downflow filter 180, can be added either upstream or downstream of the upflow filter.

Thus, in Fig. 12, coagulants, filter conditioners, pH adjustment chemicals and the like can be added through either an upstream line 182 or a downstream line 183.

It can further be seen from Fig. 12 that a comprehensive water purification system comprises a clarifier, followed by an upflow and then a downflow filter to make maximum use of the available head and minmize or avoid the cost of Incorporating and operating pumping mechanisms. A very fine quality water is produced by such a three stage system.

And, the system is very compact since only a small difference in height is required between the units to accomplish operation entirely by gravity.

it is also a discovery that certain upflow filters can be used to solve specific, difficult filtration problems.

Fig. 13 shows an apparatus according to the invention specifically adapted for use in filtering liquids containing cellulosic fibers or other material which forms a mat as the suspending liquid flows through bedded filter media. The apparatus of Fig. 13 is very similartothatshown in Fig. 1 so that both figures contain a number of corresponding reference numerals. The apparatus of Fig. 13 can em ploy filter media having an effective size of up to twenty millimeters.Media of larger effective size clogs less rapidly so the use of media of an effective size between ten and twenty millimeters is advantageous if the liquid to be filtered contains fibers or other solids which form a mat A water injection means 184 is located about six to twelve inches below the bed of media particles 30 for directing jets of water into the bed to break up the mats of fiber which form on the bottom of the bed. Pressurized water is supplied to a distribution pipe 186 having a numberainozzles 188, preferably directed horizontally or upwardly.

The pipe 188 can be fixed or, as an alternative, may comprise a rotor having an interior cavity which connects to the nozzles. In the illustrated embodiment, water is supplied to the cavity through a central vertical shaft and means are provided to turn the rotor so that the jets move along a horizontal path to break up any mat which forms. The procedure for operation using this embodiment is more fully explained below.

While it is believed the general operation of the invention will be understood from the previous description of the apparatus, various aspects of the, operation, particularly the filter cleaning procedures, are explained as follows.

In each embodiment, the bed is established by providing the desired amount of particulate filter media inside a vessel and then filling the vessel with water so that a floating bed is formed. Next water to be filtered is introduced into the inlet of the vessel and flowed upwardly through the bed to the outlet where filtered water is collected.

Periodically, when flow through the bed drops off or suspended solids break through, it is necessary to clean trapped impurities from the bed. To accomplish the cleaning, a stream of gas bubbles is distributed uniformly into water at a location upstream of the bed. As the air bubbles move into the bed, the average density of fluid in the bed is reduced to the point where particles in the bed descend. As particles descend, the bed expands and impurities trapped during filtration are released so that they can be washed out of the filter vessel.

There are two ways to discharge separated impurities from the filter vessel. One way, described in reference to the apparatus of Fig. 12, is to close the valve 66 which connects to the finished water line and to open the valve 67 which connects to a drain line. While air expands the bed, water is allowed to continue moving upwardly therethrough, carrying with it separated impurities. The valve 76 remains closed so that water laden with separated solids flows into the collector 34 and through the valve 67 to the drain. Normal filtration is resumed by stopping the air and allowing any residual solids to flow through the drain valve 67. As soon as water of sufficient clarity is being collected, the drain valve 67 is closed and valve 66 is reopened.

Another method for discharging separated impurities from the vessel is to drain water from the bottom of the vessel at a rate substantially equal to the rate of flow through the inlet while gas bubbles are being introduced into the water. This halts the flow of water through the outlet, without closing a valve on the outlet or substantially lowering the level of liquid in the vessel. Preferably, the draining rate and influent rate are automatically sensed and matched so that the height of liquid inside the vessel is maintained without a predetermined range.

If the liquid being filtered contains fiber or other materials which tend to form a mat on the filter media, the cleaning process will further include the step of directing jets of water into the mat suffi cientlyto disruptthe mat, preferably while draining water from below the bed sothatthe mat is moved downwardly and disintegrated as it passes through the level of the water jets. Such draining can be accomplished by opening the valve 76 in the case of the apparatus of Fig. so thatthe bed of media particles 30 push the mat downwardly past the distribution pipe 186. The disintegrated mat pieces fall to the bottom of the vessel and are carried out through the drain line 70.

The operation of the present invention will further be understood from the following examples.

EXAMPLE 1 Tests were conducted to determine the suitability of apparatus and process of the present invention or use for the filtration of effluent from activated sludge sewage treatment plants. The apparatus tested was generally as shown in Fig. 1. Certain of the tests were conducted in a filter column three inches in diameter and ten feet high. Other vessels having a square cross-section sixteen inches on a side and eight feet high were also used. During the tests, filtered influent was fed to the bottom of each column and effluent collected at the top. The effluent collector was a trough covered with No. 16 wire screen to prevent the loss of media. A drain line was provided at the base of the columns and air for cleaning was supplied through 1/4 inch tubing located at a distance below the bed.

Separate tests were conducted using polyethylene pellets and polypropylene pellets for filter media particles: a. The polyethylene pellets were the more spherical in shape and had a very smooth surface. Sieve analysis showed this test media to have an effective size of 2.9 millimeters and a uniformity coefficient of about 1.2. Specific gravity of the beads was 0.96.

b. The polypropylene media had a rough surface and were angularly shaped. Specific gravity of the polypropylene was 0.09. Pellets had an effective size of about 3.5 millimeters and a uniformity coefficient of about 1.8.

In comparing the two types of media, it was found that the polyethylene media was more sensitive to flow rate. Specifically, it was inefficient to operate a polyethylene media bed at a rate of higher than 10 gpm per square foot. At that rate, no more than about 30% of suspended solids could be removed. A 50% removal of solids required reducing the flow rate to 6 gpm per square foot or less. The results were much better when the polypropylene media was used, most likely due to the irregular shape of the particles which increased the interstitial volume of the bed. Fifty percent suspended solids removal could be achieved even at flow rates of twenty gpm per foot square.

Beds ranging in depth from three to seven feet were tested. Suspended solid removal was signifc- antly lower when a bed of three feet was used. A substantial increase in efficiency was used when the bed depth was increased to five feet. But, operation at a depth of seven feet produced little or no improvement over filters containing five feet of media.

The above test results and efficiency percentages were determined when the influent contained less than 10 mg/l of suspended solids, typical for the sewage treatment plants where testing occurred.

However, when the suspended solids content was increased to 56 mg/l for a short period, the effluent from a filter containing the polypropylene media was essentially unchanged, displaying a removal efficiency of about 94%. It thus appears that removal efficiency will increase with increasing influent solids content so the above figures concerning removal efficiency are for comparison purposes only.

Tests further demonstrated that the media beds could be cleaned effectively with only a small addition of air properly applied at the bottom of the filter.

Air was injected into the liquid below the bed at a sufficient distance that when the upwardly travelling mixture of air and water entered the bed, particles in the lower part of the bed would descend, apparently due to a reduction in fluid density, to about the level of the air inlet. The result was an expansion of the bed and release of trapped impurities due to the increase in the distance between media particles and enhanced fluid flow therethrough.

To achieve such bed expansion, it is necessary to reduce the density of the liquid by an amount sufficient to overcome the buoyancy of the media. The volume ratio of airto water needed to match the specific gravity of the liquid to the media is about 0.1 to specific gravity of the media. For example, if the media has a specific gravity of 0.9, the ratio of airto water needed to counteract the force of buoyancy is about .1 to .9 or1/9. This is calculated on the following basis: Pm = (Pw/P) P + (Pa(P)Pa Pm@ P and Pa are specific gravities of mixture, water and air respectively, P=Pa+Pw, Pa is the air fraction in the pore, and pw is the water fraction in the pore.

Since Pa is much less than p, the above equation may be approximated to: Pm = [Pw/(Pw + Pa) ] P For a buoyant media having a specific gravity of 0.9 Pm < 0.9, for bed expansion.

Therefore, a limiting value of pw/(pw + pa) =0.90, for p = or pw = 0.90(Pw+Pa) (1 -0.90) pw = 0.90 Pa P2Pa = 0.90/0.10 = 9 Thus the maximum water-air ration for bed expansion is about 9 to 1.

Tests showed that for a seven foot column of water containing a five foot buoyant media bed, expansion would occur in about one minute when air was added atthe base of the column at a rate of about 1 cubic foot per minute per square foot of filter area regardless of media size. Thus,the energy expeditu re for operating air pumps to clean buoyant media by this method is minimal.

This is in sharp contrast to energy expenditure requirements to clean a conventional rapid sand or like heavy media filter by air scouring during backwashing. For example, to obtain equivalent cleaning of a heavy media bed using air scouring, itwould be necessary to supply about three cubic feet of air per minute per square foot of filter media while backwashing with water at between about fifteen and for tyfive gallons per minute depending on the size and type of media.

While the solids removal efficiency of tested buoyant media filters would vary depending upon bed depth, flow rate and influent characteristics, it was found that the average solids retaining capacity for polypropylene media was about 0.11 pounds per square foot per inch of headloss increase, which was about ten fold higher than for a conventional heavy filter media. Using the polypropylene media, an exceptional 50% solids removal could be achieved at a rate of 20 gallons per minute per square foot with a five foot bed.

EXAMPLE 2 In another set of experiments filter columns 18 inches in diameter and ten feet high were arranged generally asthe apparatus shown in Fig.1. Submers- ible pumps installed in the final clarifier of the sewage treatment plant at Philomath, Oregon were used to provide secondary clarifier effluent for filtration.

Comparison tests were conducted to determine the size of screen suitable for use on the effluent collector of the filter. The tests showed that No. 8 or No. 10 screen is the optimum chose for use with a filter in this situation. If the filter is upwasted for cleaning so that washing waste is discharged by way of the effluent collector, the screens would become clogged. But, clogging is cleared by spv#vlng the col- lector screen with water as shown In Fig. 4. If clean ing was accomplished using balanced influent and drain flows, no cleaning of the collector screen was required except for the occasional removal isiine.

A single quarter inch air orifice supplied LAcfm of air (1.36 cfm per square foot) during cleaning, which was sufficient to cause bed expansion e the teen inch circular column. The minimum air requirement was about 1 dm per square foot. Using the single air injector, the whole fourfoot high bed could be expanded within a minute The air distnbu- tion in a larger filter will be less ideal; but 2 cim per square foot would be adequate for most largerfil- ten Using the filters, tests were conduced to compare methods for cleaning buoyant media beds: a. Bottom draining through screen.

A wash cycle was started by closing the influent valve 74 and lowering the water level In the pews.

sageway 22 to about six inches below the effluent collector 34. Then air was introduced at a rate of about 1 cfm per square foot for oneto one and a half minutes After that, the drain valve 76 was opened to allow approximately four feet of water drain from the column through a No.4 screen placed m'erthe drain outlet 68 to prevent media loss. Then the coF umn was refilled with influent water and air applied followed by a repetUlon of the draining.

As shown in Table 1, the total amount of solids in the wash out of one filter drain was ro low.

@.@. only 0.06 pounds per squrefootwere drained whereas 0.48 pounds per square tot were collected during filtration en alter a sewnd draining signiF iaant amounts of solids remained in the filter medium. The inadequacy of this cleaning method was further evidenc@d by the relatively rapid increase of headloss in filter runs conducted alter cleaning according to this method.

TABLE 1 WASH WASTE SOLIDS Bottom discharge through drain screen at 76 gpm/sq. fL nilh influent and air shut off.

Time (min.) SuspendedSollds (nwlld 0-1 268 1-2 114 23 102 3-4 91 45 76 56 67 50 743 49 49 Total Total solids drained = 0.06 IbW fL Total solids filtered =0.48 lb/sq. ft.

b. Dischargethrouth effluent collector.

In this method,the washing cycle was started by closing the influent valve 74 and lowering the water level to about six inches below the effluent collector 34. Air was introduced for about a minute and a half or until the bed was fully expanded. Then, influent valve 74 was opened while the flow of air remained on so that washing waste was collected through the effluent collector 34 and passed to a waste line via the valve 67.

When the filter bed was washed according to this process, more solids were washed out within the same time interval as compared to a washing with process "a." Table II shows the wash out solids profile for filter cleaning according to this method.

TABLE 11 WASH WASTE SOLIDS Discharge through effluent collector at 11.3 gpm/sq.

ft. with both influent and air on.

Time (mien.) Suspended Solids (mglLJ 0-1 1,035 1-2 364 2-3 255 3-4 205 4-5 144 Total 2,003 Total solids discharged = 0.19 Total solids filtered = 0.47 Iblsq. ft.

Although good solids removal was achieved by this method, significant clogging of the effluent screen was experienced due to the presence of fibrous material in the wash water. The clogging was alleviated by spray washing as illustrated in Fig. 4.

It is a possible drawback of this cleaning method that some portion of media near the top of the bed might be restrained by the effluent collector 34 from free expansion when the collector 34 is used for collecting and washing waste. But, such a problem is likely to occur only when the influent flow rate is high.

c. Bottom draining without screen.

The same procedure was followed as mentioned in paragraph "a" above. However, no screen was present over the drain and the drain rate was increased substantially. It took about one half minute to drain out three feet of water which was equivalentto an average rate of 25 gallons per minute per square foot. For each draining, the maximum amount of water to be drained out was limited by the depth of the tank and the media.

With the screen absent, the drain valve 76 was closed before any media reached the drain line 70.

After draining, it took about two and one half minutes to refill the tank with influent at a rate of about eight and one half gallons per minute per square foot so that each drain-refill cycle took about three minutes. About four cycles were required for complete washing. The typical wash out profile shown in Fig. 14 illustrates that removal of the drain screen produced some improvement in media cleaning. This drain-refill method, however, would require complicated control apparatus or very diligent operators.

d. Drain the wash waste with both influent and air on without drain screen.

In this procedure waste was effectively drained through the drain line 70 while influent continued to flow through the inlet 24 and air was applied continuously. For successful operation, the piping was arranged substantially as illustrated in Fig. 1 with the air distributor at an elevation at least twelve inches, and preferably fifteen inches, above the invert elevation of the drain pipe. In other words, the dimension v in Fig. 13 is at least twelve inches. The region R should be large enough to provide for a least 50% expansion of the bed to achieve optimum cleaning.

In other words, the dimensicn of w shown in Fig. 13 is at least 50% of dimension x for a vertically cylindrical filter vessel. Satisfactory results could be obtained if the space provided for expansion was at least 40% of the bed volume, i.e. w = 40% x in Fig.

13. To achieve any meaningful bed expansion, the unobstructed expansion space must be at least 10% of the volume of the bed, i.e. w = 10% x in Fig. 13.

Most preferably, the influent should be applied to the filter at a distance y about two feet below the bottom of the media with the influent inlet 24 located at a vertical distance z above the air distributor 56, preferably of at least one foot.

The wash procedure was started by adjusting the drain valve 76 to maintain a draining rate equal to the influent rate through inlet 24 as previously described. Automatic controls would maintain a constant water level in the filter during washing.

Once balanced flow is established, the air valve is opened to allow about one cubic foot per square foot of air to enter the bed to cause expansion. Operation is continued in this mode until water passing through the drain line 70 is substantially clear. Since the inlet and drain flows are balanced, operation is very simple; and no screens need be positioned over the inlet 24 or drain outlet 56. Thus, clogging problems are entirely eliminated.

In some instances, a very small amount of media could escape to the drain at the start of the wash cycle if some aglomerates of solids and media happened to fall belowthe level of the air distributor 56 before breaking up. To prevent such loss of media, the wash cycle may be started with the influent valve 74 and drain valve 76 closed. The air is applied for about one minute to expand the bed and then the valves 74 and 76 opened.

Specific cleaning tests were conducted with filters that had previously been operated at 8.5 gpm per square foot for about 20 hours. Total solids filtered were estimated to be about 1.5 pounds per square foot.

As shown by the draining waste profiles of Fig. 15, such a filter was cleaned adequately within 10 minutes at the highest washing rate, although the wash out solids had not fully reached the turbidity level of the influent. A small amount of residual solids left in the system was tolerable and sometimes could be beneficial for ripening of the filter bed. The required wash time was shortest at the highest wash rate of 22.6 gallons per minute per square foot.

The required time for washing also varied as a function of the total amount of solids removed during the filtration.

In addition to the washing tests, a brief test of the filtration capability was made at a filter rate of 36 gallons per minute per square foot. At this rate, 50% removal efficiency was achieved for suspended sol ids of good filterability. Thus, a filter rate higher than 20 gpm per square foot is feasible under certain cir cumstances.

However, for polishing secondary clarifier effluent, the filtration rate should not exceed 20 gpm per square foot at peak flow. At that rate, a bed compris ing five feet of polypropylene media appears to be adequate; but, a shallower bed could be used if the filtration rate were lower.

it was generally observed that, when the influent had a solids concentration higher than 200 or turbid ity higher than 100, the filter could not be cleaned adequately by using influent as wash water. The filter effluent or other water sources should be used instead.

EXAMPLE 3 To determine the suitability of filters according to this invention for the filtration of water containing cellulose fiber or other materials which tend to mat and clog a filter bed, tests were made using upflow filters with buoyant media on paper mill effluent, brown suspension from pulp plants, suspensions of recycled short fibers and final effluent from aeration lagoons. The test apparatus comprised filter columns three inches in diameter and ten feet high, generally arranged as shown in Fig. 13. The media depth varied from eighteen inches to 84 inches among the test runs. Influent was provided to the filter vessel by a submersible pump.

a. A substantial amount of testing was done on paper mill effluent since this liquid has the greatest potential for reuse. A major portion of the paper mill effluent comes from the recirculated water of paper machines. The main constituent of the waste solids is fibrous materials, particularly cellulose fibers. The particular paper mill effluent filtered in the tests was at a temperature of about 105% and contained fibers of various lengths which were able to pass through 3/16 inch holes. Suspended solids concentration of the influentvaried from 108 to 1,617 milligrams per liter, the average being about 690 milligrams per liter. Settlable solids varied from 32 to 320 mgil and averaged at around 140 mg/l after thirty minutes settling.

Using polypropylene media of the type previously described, the removal efficiency of filters tested was over 98%. However, the fibers accumulated very rapidlyatthe lower surface of the media and formed a mat about one half inch thick every fifteen to thirty minutes. Operating at constant pressure of 3.2 psi, the flow rate would initially be as high as twenty gpm per square foot, but would decline to about two gpm per square foot within thirty minutes. During that thirty minute period, the total volume of water filtered was equivalent to a run at a constant rate of about six gpm per square foot. The headloss would reach about seven feet in thirty minutes.

In order to provide a practical filter, various methods were attempted to extend the length of the run by periodically breaking up the mat which formed in the bottom of the media bed. Mechanical raking, water jetting and air bumping all proved to be less than satisfactory. Because the media parti cles were tightly packed; mechanical raking was dif ficu It to accomplish.Water jetting would disrupt the mat but could not release it from the media surface; and air bumping could expand or agitate the media but had little effect on the mat it was discovered, however, that cleaning could be effectively accomplished by partially draining the vessel 20 through the drain outlet 66 every fifteen to thirty minutes and simultaneously directing water jets produced by the nozzles 188 into the mat Forthis purpose, the surface wash system 184 should be located at about six to twelve inches below the bottom of the unexpanded bed. ring the simultaneousdrainings and jettings, prebrabGy regulated by automatic controli,the water newel is lowered by about twelve inches,Or until the loser bed boundary has passed to below the level olive nozzles The water jetting commences urktle the mat and media lxd are still located about sis inches above the nozzles 188. With continued draining, the mat passes downwardly through the levei of the water distributor 184 where the water jets break it up. Still further draining would lower media particles 30 into the jets which would cause agitation and facilitate the release of solids from lower portions of the bed.

This would make the eventual filter washing atthe end of a run easier to accomplish Using this mat disintegrating technique, the filter can run for approximately three hours At the end of that time, the filter is washed by slowing one of the procedures described in Example 2.

b. Brown suspension from pulpplarrtwas also tested. This water source contained about 1700 mgn of usable fibrous material. Operating at six gpm per square foot, the test filter was able to reduce the suspended solids to 50 mali. The settlable solids in the influent water were 120 mg/I after thirty minutes of settling. Effective cleaning required the same procedure previously described for operation with pepermill effluent c. The tested effluent containing short fibers had a relatively low solids concentration of about 300 mill.

The fibers inthe test samples were shorter than those which were present in the paper mill effluent and readily penetrated the filter.

Operating at five gpm per square fool, about 4757% removal of suspended solids was achieved without chemical addition. When 30 mgn of alum and 0.3 mgil of 985N were added, the removal eS ciency was increased to 60 -86% for a filter empltry- ing a five foot bed.

d. Tests were conducted on aerated lagoon effluentwhich comprised effluent from the paper and pup plant after settling in a clarifier and liiologC cal treatment in aerated lagoons.

The lagoon effluent had a temperature of around 80 F. and suspended solids of about 2040 mg/l.

About 40% removaLwas achieved at a flow of 15 gpm per square foot. This was comparade to the : removal experienced with domestic waste water a#d described in previous examples While I have shown and described the preferred embodiments of my invention, it will be apparent to those skilled in the art that changes and modifica- tions may be made withoutispartling from my invention in its broaderaspect#itherefore intend the appended claims to cover all such changes and modifications as follow the true spirit and scope of my invention.

Claims (57)

1. A freestanding filter for upflow filtration of water comprising: a filter vessel which is open at the top and is constructed to provide an unobstructed passageway for water moving therethrough; an amount of buoyant particulate media in the vessel sufficient to form a floating filtration bed suitable to collect and retain impurities suspended in water moving through the bed; water outlet means located a sufficient distance below the top of the vessel that the level of the media is maintained below the top of the vessel; means for periodically treating the bed to cause the release of impurities trapped in the bed during filtration; and diverter means to cause a flow of liquid from the passageway at a location below the bed and thereby halttheflowofwaterthroughthe water outlet means during those periods of time when the bed is being treated to release impurities trapped during filtration.

2. A filter for filtration of water comprising: a filter vessel adapted to permit an upward flow of watertherethrough; a buoyant filter bed in the vessel, the bed having particles of a density less than the density of water and greater than the density of an admixture of water and minute air bubbles; gas injection means located upstream of the bed, the injection means being adopted periodically to uniformly dispense minute air bubbles throughout water flowing upwardly into the bed in such a mannerthat at least some of the particles will descend to expand the bed and, due to such bed expansion, release impurities trapped therein during filtration.

3. The filter of claim 2 wherein the gas injection means comprises means located in the vessel below the bed for producing bubbles at numerous locations throughout the stream of water flowing upwardly into the bed.

4. The filter of claim 3 wherein the gas injection means comprises: a hollow rotor having at least one vent for supplying gas into the water from the interior of the rotor; and means to rotate the rotor to move the vent through the water.

5. The filter of claim 2 wherein there is an unobstructed region of sufficient volume in the vessel below the bed to allow the bed to expand to the extent that trapped impurities can be released without substantial agitation of particles in the bed.

6. The filter of claim 5 wherein the unobstructed region has at least ten percent of the volume of the bed.

7. The filter of claim 5 wherein the unobstructed region has at least forty percent of the volume of the bed.

8. The filter of claim 7 wherein the unobstructed region has at least fifty percent of the volume of the bed.

9. The filter of claim 7 wherein the vessel is substantially a vertical cylinder and the unobstructed region extends to a depth below the bed of at least forty percent of the bed depth.

10. The filter of claim 9 wherein the region extends to a depth belowthe bed of at leastfifty percent of the bed depth.

11. The filter of claim 5 wherein the gas injection means is located upstream of the unobstructed region.

12. The filter of claim 5 wherein the gas injection means is located in the vessel below the unobstructed region.

13. the filter of claim 12 wherein the gas injection means comprises a plurality of gas inlet vents, the vents being spaced transversely to the flow of water through the vessel so that there is at least one vent for each two square feet of transverse vessel crosssectional area.

14. The filter of claim 2 wherein the particles have a specific gravity no less than 0.80.

15. The filter of claim 14 wherein the particles have a specific gravity no less than about 0.96.

16. The filter of claim 2 wherein the particles have an effective size between about 2.0 and 10.0 millimeters.

17. A filter for upflow filtration of water com pris- ing: a filter vessel adapted to permit an upward flow of water therethrough; a bed in the vessel, the bed being made of particles having a specific gravity no less than 0.80, an effective size between about 2.0 and 10.0 millimeters, a uniformity coefficient no greater than 2.0 and a sphericity of less than 0.7; and means for preventing the particles from being carried out of the vessel with water flowing upwardly therethrough.

18. A filter for upflow filtration of a continuous liquid stream comprising: a filter vessel having an inlet for admitting a continuous flow of influent water, an effluent outlet located above the inlet and a drain outlet located below the inlet; an amount of buoyant particulate media in the vessel sufficient to form a floating filtration bed in the vessel between the inlet and the effluent outlet when the vessel is filled with water to the level of the effluent outlet; and means for periodically withdrawing water from the vessel through the drain outlet at substantially the same rate as water is admitted through the inlet so that flow through the effluent outlet stops and the elevation of the bed is maintained to allow cleaning of the bed in situ without interruption of flow through the inlet.

19. The filter of claim 18 further comprising automatic control means for adjusting the flow of water through the drain outlet to match the flow of water through the inlet.

20. The filter of claim 18 wherein the drain is located at least one foot below the bottom of the bed so that media is not carried out of the vessel during periods when water is being withdrawn from the vessel through the drain outlet.

21. A filter for upflow filtration comprising: a filter vessel having an inlet and an outlet located above the inlet, the vessel being constructed to provide a vertical passageway for water moving therethrough; an amount of buoyant particulate media in the vessel sufficient to form a floating filtration bed in the passageway; horizontal trough means connected to the outlet and extending into the passageway to receive water rising above the level of said trough means and deliver such water to the outlet; and screen means located in the flow path of water from the passageway to the trough means to prevent particles of the media from being carried into the trough means.

22. The filter of claim 21 wherein: the trough means comprises a watertight channel having an upwardly opening mouth; and the screen means covers the mouth.

23. The filter of claim 22 wherein the screen means is in the shape of an arch extending overthe mouth.

24. The filter of claim 21 wherein the trough means is constructed entirely of a foraminous material which admits water and excludes particles of the media.

25. The filter of claim 21 further comprising means periodically to deliver a stream of pressurized backwash water into the region defined by the trough means and the screen means and to direct the backwash water outwardly in jets to clean the screen means.

26. A filter for upflow filtration comprising: a filter vessel including a substantially cylindrical wall which defines a vertical passageway for water moving upwardly therethrough; an amount of buoyant particulate media in the passageway sufficient to form a floating filtration bed in the passageway; a horizontal trough extending through the passageway, between two locations on the wall, to receive water rising above the level of the trough and carry such water from the vessel; and means to prevent particles of the media from being carried into the trough.

27. The filter of claim 26 wherein said wall is in the shape of a rectangular cylinder.

28. A filter for upflow filtration comprising: an elongated, horizontally extending filter vessel which defines a closed compartment greater in length than in height and has an inlet and an outlet adapted to permit the upward flow of liquid through the compartment; an amount of buoyant particulate media in said vessel sufficient to form a floating bed in the vessel; a liquid collector positioned within the vessel at a location near the top thereof where said collector extends into the bed to receive liquid which flows upwardly through the bed.

29. The filter of claim 28 wherein the vessel comprises a horizontally extending circular cylinder which is closed at both ends.

30. The filter of claim 29 wherein the collector comprises a foraminous tube which extends lon gitudinally along the top of the compartment and connects to the outlet.

31. The filter of claims 1, 3, 15, 16, 19, 24 and 26 further comprising mechanical pump means for moving water upwardly through said vessel.

32. A system for the purification ofwatercontain ing suspended impurities comprising: a clarifier tank having a wall which defines settling zone and inlet means for delivering a stream of water with suspended solids into the settling zone wherein the water is partially clarified by settling, and clarifier outlet means for removing a stream of partially clarified water from the settling zone; a headwater container defining an unobstructed chamber wherein the level of water can vary inde pendently of the level of liquid in the clarifier, the container being connected to the clarifier outlet means such that the stream of partially clarified water is delivered into the chamber;; a filter vessel defining a vertical passageway for water and having a filter inlet and a filter outlet located above the filter inlet, the filter inlet being connected to the headwater container such that the water is urged by gravity to flow from the chamber thereof into the vessel passageway wherein the level of water in the container is greater than the level of water in the filter vessel; and an amount of buoyant particulate filter media in the vessel sufficient to form a floating filtration bed in the passageway.

33. The system of claim 32 wherein the headwater container is connected to the clarifier outlet in such a manner that water can not flow from the con tainerto the clarifier outlet so that water in the vessel can be maintained at a predetermined minimum level regardless of the level of liquid in the clarifier tank.

34. The system of claim 33 wherein the relative elevations of the container and clarifier are such that water can only flow downward from the clarifier outlet to the container.

35. The system of claim 33 further comprising means for delivering the stream of partially clarified water into the container at a level above the max imum surface level of water in the chamber.

36. Asystem forthe purification ofwatercontain- ing suspended impurities comprising: a clarifier tank having a wall which defines a settling zone, clarifier inlet means for delivering a stream of water with suspended solids into the settling zone wherein the water is partially clarified by settling, and clarifier outlet means for removing a stream of partially clarified liquid from the settling zone; a headwater container connected to the clarifier outlet means such that the stream of partially clarified water is received in the container, the container being isolated from the clarifier in such a manner that water can not flow back into the clarifier from the container if the level of water in the clarifier drops; ; a filter vessel connected to the headwater con tainer such that water flows from the container, into the vessel and upwardly therethroùgh; and a bed of buoyant particulate filter media in the )path of water flowing through the vessel.

37. A filter for filtration of water containing fibrous materials comprising: a filter vessel adapted to permit an upward flow of water therethrough; a buoyant filter bed in the vessel, the bed being made of particles having a specific gravity no less than 0.80, an effective size between two and twenty millimeters; and means for preventing particles from being carried out of the vessel with water flowing upwardly therethrough.

38. A filter for filtration of water containing fibrous materials comprising: a filter vessel adapted to permit an upward flow of water therethrough; a buoyant filter bed in the vessel; and water injection means located below the bed for directing at least one jet of water upwardly into the bed to break up a mat of fibers which forms on the bottom of the bed.

39. The filter of claim 38 wherein the water injection means comprises a rotor having an interior cavity connected with at least one upwardly oriented nozzle opening so that when the cavity is connected to a source of pressurized water, a jet of water, which extends upwardly into the bed, is produced through the opening; and means to rotate the rotor to move the jet relative to the bed.

40. A filter for upflow filtration of water comprising: a filter vessel which is constructed to provide an unobstructed interior passageway for water moving therethrough; an inlet for supplying influent water to the passageway; an outlet, located above the inlet, for removing filtered water from the passageway; a buoyant filter bed in the vessel between the levels of the inlet and the outlet, the bed being made of particles having a specific gravity less than the specific gravity of water and no less than 0.80; horizontal trough means connected to the outlet and extending into the passageway to receive rising water and deliver filtered water to the outlet; screen means located in the flow path of water from the passageway to the trough means to prevent particles from being carried into the trough means;; a drain outlet located at least one foot below the bed so that water can be drained from the vessel without loss of media particles; means to halt the flow of water through the outlet during cleaning of the bed; gas injection means located in the vessel below the bed for producing bubbles at numerous locations throughout the stream of water flowing upwardly into the bed such that at least some of the particles will descend due to the reduction in water density which occurs between the levels of the inlet and the outlet; an unobstructed region in the vessel between the bed and the gas injection means, the regions being of sufficient volume to allow the bed to expand downwardly to the extent that trapped impurities are released from the bed with no more than minimal agitation thereof.

41. A method for cleaning an upflow filter having a bed of buoyant particulate media without substantially agitating the bed, comprising lowering the density of liquid flowing upwardly through said bed to a density below the density of the media so that at least some media particles descend to expand the bed and release impurities trapped therein during filtration.

42. A method of filtration of water flowing through a filter vessel having an inlet and an outlet located above the inlet comprising: floating, in the vessel, a bed of particles having a density less than the density of water; flowing water upwardly into the bed as the water moves from the inlet to the outlet; and periodically disbursing minute gas bubbles into water at a location upstream of the bed to reduce the density of the water and cause at least some of the particles to descend to expand the bed and release impurities trapped in the bed during filtration.

43. The method of claim 42 further comprising during the disbursing of gas bubbles, draining water from the vessel upstream of the bed at a rate substantially equal to the rate of flow through the inlet to halt the flows of water through the outlet without substantially disturbing the bed.

44. The method of claim 43 wherein the draining comprises allowing water to flow through a drain outlet located at least one foot below the location where gas bubbles are disbursed into the water.

45. The method of claim 43 further comprising matching the draining rate and rate of flow through the inlet by sensing, measuring and comparing both rates and by automatically adjusting the draining rate to equal to rate of flow through the inlet.

46. The method of claim 42 further comprising: establishing the bed in the vessel such that there is a volume of water in the vessel below the bed sufficient that the bed can expand by at least forty percent in volume; and periodically injecting air into water in the vessel at a location not higher than the bottom of the volume of water below the bed.

47. The method of claim 46 further comprising establishing the bed such that the volume of water below the bed is sufficient that the bed can expand by at least fifty percent.

48. The method of claim 42 wherein the disbursing comprises injecting air into water in the vessel, below the bed, at a plurality of sites spaced transversely to the flow of liquid through the vessel, there being at least one site for each two square feet of transverse vessel cross-sectional area.

49. The method of claim 42 further comprising: continuing to collect water through the outlet during the dispensing; and diverting the collected water to a disposal site.

50. A method for the purification of water comprising: continuously flowing a stream of water containing suspended impurities through a clarifier tank wherein the water is partially clarified by gravitational separation; delivering the stream of partially clarified water produced in the clarifier into a headwater container which is separated from the clarifier in such a mannerthat it is not possible for water to flow from the container into the clarifier so that the level of water in the container can vary independently of the level of liquid in the clarifier; and flowing water upwardly through a bed of buoyant filter media using the pressure provided by the head in the container.

51. A method for filtering water containing fibers or other materials which tend to form a mat on filter media comprising: floating, in a filter vessel, a bed of particles having a density less than the density of water; flowing water to be filtered upwardly into and through the bed until a mat is formed on the bottom of the bed; at least one jet of water into the mat sufficient to disrupt the mat; and draining water and the disrupted mat from the bottom of the vessel.

52. The method of claim 51 wherein the draining is commenced substantially concurrently with the directing.

53. After system comprising: an upflow filter vessel defining a vertical passageway for water and having an upflowfilterinlet and an upflow filter outlet located above the upflow filter inlet; an amount of buoyant particulate filter media in the vessel sufficient to form a floating filtration bed in the passageway; a downflow filter vessel having a downflowfilter inlet which is connected to the upflow filter outlet and a downflow filter outlet located below the upflow filter inlet; and an amount of non-buoyant particulate filter media in the downflow filter vessel sufficient to form a filtration bed at the bottom thereof.

54. The system of claim 53 wherein: the upflow filter outlet and downflow filter inlet are at about the same elevation; and the upflow and downflow filter vessels share a common wall.

55. A filter for upflow filtration, substantially as hereinbefore described with reference to, and as shown in, any of Figures 1 to 13 of the accompanying drawings.

56. A method of filtration, substantially as hereinbefore described with reference to any one of Figures 1 to 15 of the accompanying drawings.

57. Any novel feature or combination offeatures described herein.