CA1171364A - Method of simultaneous air-water wash of multiple- media filters - Google Patents

Abstract

METHOD OF SIMULTANEOUS AIR-WATER WASH
OF MULTIPLE-MEDIA FILTERS
ABSTRACT

A method is described in which two or more granular filter media are used in combination in such a manner that the filter bed is backwashed using air and water simultaneously.
The media size gradation and density are selected so that, although the media may be mixed during backwash, the media are separated into layers according to their density and/or size following termination of the simultaneous air-water wash and prior to returning the filter to the filtration mode.
The simultaneous air-water wash step can be carried out without media loss by use of an appropriate media retaining procedure.

Description

L BACKGROU~ID AND COMMERCIAL PRIOR ART

The conventional method of backwashing or cleaning granular media filters, as commonly used for removing sus-pended solids from waters or wastewaters, is to pass water upward through the bed at sufficiently high velocities that the media are first fluidized, that is, the media grains are suspended or floated in the upward flowing backwash water, and secondly, expanded by as much as 50% over the settled bed height so tha~ particles of entrapped solids can be removed lo from between the media grains. After a backwash operation of this type, a single-medium filter bed is left stratified so that the smaller grains are in the upper part of the bed and the larger grains are in the lower part of the bed. When multiple-media filters - usually consisting of coal over sand, or coal over sand over garnet - are used, the sizes of the media are selected so that the lower density and usually larger grain size media overlie the higher density and usually smaller grain size media.

When-using multiple-media filter beds, a zone of ~20 intermixed media may occur in which the small sizes:of. the .:: higher density media are interspersed.with the:larger sîzed . . ~ , , .................... .. , . . . . ;.. . .
grains o~ the lower density media~; The.exten~ of~intermixing depends on the relative size and density of tke media~wit~in each layer and may be controlled to some extent b~ appropriate selection of size and gradation of each medium and by the rate and duration of back~ash~

. Conventional filter designs range from those having - .,.. -.:, ., ...- ~ , ~ essentially no intermixing to those`having intërm xing ". . ;

~7~3~4 1 throu~h most of the bed depth. Filters having as many as four distinct layers distinguished ~y media type and grain size or both have been used in full-scale water and waste-water filtration ~lants. The amount of intermixing is, to a large extent, a filter design option.

. :;
Conventional fluidization and expansion backwash methods are not always as effective as desired. The ineffec-tiveness of scouring of solids with water alone is well known, and is the reason for development of auxiliary media lo scouring tschniques such as air wash preceding water wash and media scouring with high velocity jets of water prior to and during water wash. Even these methods are not as effec-tive as desirable and examples of dirty media and ~Imudballs~
remaining after backwashing by such techniques also are well known.

An alternate backwash method is to use air and water simultaneously throughout much of the backwash cycle. Both research tests and field experience have shown this method to be much more effective in cleaning filter medla than 20 conventional backwash methods. Prior to the method described - , ~
by this invention, the simultaneous use of air and water at the same time water is flowing from the backwash colIector has been .. ~ . .
restricted in practice to single-medium beds that did not rèquire stratirication or separation into two or~more layers of dlfferent sizes or tyEesof media. The simultaneous use of air and water for durations longer than required to fill the volume between the lowest water level prevailing at the end o:the filtration sequence and the overflow weir of the backwash collector will re-. - ~ ... . . .- -sult in loss of significant amounts of media unless some ~ositive nYans`

~117~13~4 -1 is used to control media loss.

When simultaneous air-water wash has been used with multiple-media filters, its use has been restricted to a very short period during which water rises from a level slightly above the surface of the media to the edge of the waste backwash water collector. The air flow must then be terminated to prevent media loss, and the water wash is continued, usually at an increased rate, to wash entrapped solids from the bed and to separate the media into respective lo layers of different size or density. Using conventional media grain-size and density combinations requires that a relatively high rate water-only step follow the air wash or hydraulic scouring to fluidize and expand t-hese media so that solids washout and stratification occur. For example, a dual media mixture of 1.0 mm effective size (E.S.) coal over 0.5 mm E.S. silica sand can be backwashed using water at a rate as low as 8 gallons per minute per square foot (gpm/sq.ft.) simultaneously with air at rates as low as 2 standard cu.ft. per minute per square foot (scfm/sq/ft.), whereas washou~ of solids from these two media and subsequent restratification using~ water alone, as used in conventional backwash methods, require water rates as high as 20 gpm/sq.t.;
-:-3 and when using larger grain size combinations, even higher rates of water become necessary. However, it is desirable to be able to use a simultaneous air-water wash method with multiple-media filters that can be extended for a longer backwash duration than the rise time ~rom slightly above the surface of the media to the edge o the waste backwash water collector without the loss of media and subsequently will .

.7~13~

1 restratify the filter bed in a manner suitable for filtration_ PATENT PRIOR ART

A number of multiple-media ilters and systems of filtration and backwashing have been patented. Rice et al patent 3,343,680 discloses a three-media filter wherein particles of all three media are intermixed. The filter is said to be designed so that in filtration use the number of particles per unit area continually increase in the direction of water flow through the bed. In preparing the filter, it is disclosed that ~he initially stratified layers of the beds are placed in the filter, and then "backwashing the bed until the particle distribution has reached a substantially constant orientation" (col. 2, lines 37-40). -Only backwashing with water alone is described. It is stated that thereafter both filtration and backwashing can be carried out without substantially changing the particle distribution.

Hsiung et al in patent 3,876,546 disclose an extension four-media hed of the Rice et aI filter-bed. As with the - ~ Rice et al bed, the particles of the differen~ media of the bed are said to be intermixed so that there is~a continually increasing ~umber of particles per unit area in the dlrection of water flow through the bed during filtration~ Theibed is backwashed with water at a rate su~ficient to adequately fluidize the bed, the backwashing being with water only except that air scourin~ or hydra~Ii~ scouring may precede water wash.

Multiple-media beds have been proposed which are designed to avoid or minimize intermixing of the stratified :, - `

, ~17~364 1 layers of the~different.media~ Hirsch 3,497,068 discloses a multiple-media bed in which the different media possess equal hydraulic uplift properties when subjected to backwashing.
Presumably, therefore, each layer of the bed expands to permit removal of the entrapped solids without intermixing .:.3 of the layers, thereby permitting the layers to settle back to their original stratified condition after backwashing is concluded. The backwashing is by water alone.

Hirs 3,925,202 discloses a dual-media filter bed lo where the layers are partially intermixed. The recommended method OL backwashing is to first scour the bed with air, and then to use a water wash without air. An alternate method .is mentioned in which air is introduced simultaneously with the backwash water (col. 4, lines 28-36), but this method was not related to media size, combination, preferred backwash rate, or desired media separation~

The problem~of reverse stratification during backwashing is discussed in Hirs patent 4,048,068. Bed designs nd a multiple-tank filter system are disclosed for avoiding such - 20 reverse stratification, which may require regr.ading of the beds to a more stratiied condition before filtration can be continued. Another system for avoiding the necessity of regrading layers of a multiple-media bed is described in Hirs ~ patent 3,814,247.

Simultaneous air-water backwashing of single-media filters has been proposed including the.desirablility of equipping such filters with baffled backwash troughs to reduce media loss. See Scholten et aI patent 4,076,625 and Row et al . - 6 - .

3~4 1 patent 2,453,345. Neither of these patents sugsests how such filters can be used with multiple-media beds.

SUMMARY OF THE INVENTION

The backwash method of this invention is applicable to the use of two or more layers of media differentiated in size or density or both and backwashed in such a manner that the bed is substantially mixed during backwash by simul-taneously passing air and water through the bed. The duration of this backwash operation is extended sufficiently that the air and water flow simultaneously while waste backwash water passes over the weir of the backwash collector. After the air application is discontinued, at a time determined by cleanliness of the backwash water o~ by the duration of back-wash or by other appropriate means, the flow of water is con-tinued to separate the media into respective layers distin guished hy ~ize and density or both. By properly selecting the backwash rate and the grain size and density of each individual mediu~, various types and sizes of media can be substantially separated without lncreasing the water rate 20 above that used during simultaneous air-water wash. For .. . ..
example, a filter bed consis~ing of 1.0 mm E.S.~anthracite coai.ha~ing a S~eCifiG .gravi~.~E.aoout:1.7 (~-~ght relativ.e.to equal volume of water) over 0.5 mm E.S, garnet sand or other granular m~terial having a specific gravity great~r than 2~60 can be washed .
effectively when using water at 10 to 12 gpm~sq~ft~ sîmul-taneously with air at 2 to 3 scfm/sq~ft~ and then can be stratified effectively without ha~ing to increase the ~ack-wash water rate. Th~s same filter media combination, wh.en backwashed by conventional methods~ typ.ically requires about 36~

1 20 ypm/sq.ft. to expand the bed during backwash sufficiently to remove entrapped solids and to separate the media into its respective layers. Although -the backwash water rate used in the method of the present invention can be held substantially constant throughout the entire backwash .... :1 ~ .
operation, and this is a preferable procedure, the method of backwashing cf this invention can also be used with a low rate of water during simultaneous air-water-wash, which is followed by a higher rate of water alone to separate 10 the media after the air flow is terminated~

The method also can be usPd for backwashing filter bed combinations having media sizes and densities other than those referred to above, providing the backwas-h air and water rates are adjusted to those appropriate for the different combination.

Features of the present invention include:

(1~ providing a method of back~Jashing granular-media filtars comprised of two or more media sizes or densities or both by using air and water simultaneously w~île waste back-.., :.
20 wash water is passing into a bac~wash water collector,

(2) controlling media loss during simultaneous air-water wash of multiple-media filters by using a system of baffles surrounding the backwash collector or other suitable means to separate the air from the was~e backwash water preceding its exit over the backwash water collector; ànd ~ 3~ providing a method of washing multiple-media filters with the simultaneous use of air and water but without 1 requiring an increase in the backwash water rate to separate the media into size or density layers after the air flow is terminated.

The improvements in backwashing achieved by the method of this invention provide an increased abllity to scour entrapped solids from the surface of the media grains of multiple-media filters and from the interstitial spaces between grains. Further, these solids can be removed effectively at lower backwash rates than used in conventional lo backwash techniques. The method also permits effective cleaning of multiple-media filter beds containing larger grain sizes than are used in conventional filter designs.
This feature permits significant increase in run time between backwashing and produces savings in filter operating costs and costs for treatment of waste backwash water. Conventional multiple-media filters usually are restricted to the use of media that will expand 20 to 50% at backwash rates of 15 to 20 gpm/sq.ft. This means that the use of anthracite coal media having a specific gravity of 1.6 to 1.7 is limited to effective sizes of about 1.0 mm; silica sand (S.G. - 2.6 to - 2.7) effective sizes are limited to about 0.6 mm; and garnet ~S.G. about 4.1~ or ilmenite (S.G. abou~ 4.6) sizes are limited to about 0.3 mm E.S. Larger sizes of these media are commer-cially available but when used in multiple-media ilters, require such high backwash rates as to be impractical. For example, the use of 1.5 mm coal over Q~8 mm sand~re~u~res about 30 gpm/sq.ft. backwash rate to expand the bed ~0 to 30%, and although the use of such rates is possible, the size of physical appurtenances such as v~lves, piping, pumps, etc, ~17~3~4 j 1 cause filter operating costs to increase appreciably.

BRIEF DESCRIPTION OF THE DRAWINGS
, The accompanying drawings and diagrams are illustrative of preferred embodiments of the method of this invention.

~ IG. 1 is an elevational sectional view of a water filtration apparatus, including a baffle and wash trough assembly in the upper portion thereof, and a multiple-media filter bed in the lower position;

FIG. 2A is a fr~ment~ry enla~ged view of a portion oof the multiple-media bed of FIG~ l illustrating the appearance of the bed during normal filtration; -FIG. 2B is an enlarged fragmentary view of a cross-section of the multiple-media bed simllar to FIG. 2A but illustrating the appearance of the bed during combined air-water backwashing;

FIG~ 3 is a diagram illustra~ing t~e preferred size relation between the coal medium and the silica sand medium for use as adjacent layers in a multiple~media bed~

FIG. 4 is a diagram showing the preferred size relation -~ 20 between a coal medium and garnet sand or ilmenite medium for use as adjacent layers of multiple-media beds; and - FIG. 5 is a diagram illustrating the preferred size relation between a silica sand medium and garnet sand or ilmenite medium for use as adjacent lâyers in multiple-media beds.

.

~17~364 DESCRIPTION OF PREFERRED EMBODIMENTS

Looking first at FIG. l thereis shown a vertically extending tank 10, which as shown is formed of metal tviZ.
steel), but it can be constructed of other ma~erials, such as concrete. It will also be understood that although the tank, as shown, is circular in horizontal cross-section, other shapes may be used.

The tank 10 has an open top, circular side walls 11, and a closed bottom 12. It also will be understood that lo enclosed tanks for filtration under pressure may be used.
Spaced upwardly ~rom bottom 12 there is provided an under-drain plate 13, supported by bracing 14 and connected to the tank sides 11 by means to form a water-tight seal therebetween.
Extending through plate 13 are a plurality of nozzle tubes 15 having strainer caps 16 on the top thereof above plate 13.
During downflow filtration the water passes downwardly through the strainers 16 and the nozzles 15 into the underdrain chamber 17 and is removed through pipe connection 18 to the filtered water outlet pipe 19. The same arrangement can be used for upflow filtration the water to be filtered entering chamber 17, which now serves as a water introduction chamber, rather than as an underdrain chamber, the water then passing ~ upwardly through the nozzle tubes 15 and the stainer cap 16.

During washing, water is supplied under pressure through pipe 20 to pipe 18 and chamber 17 for passing upwardly through nozzle tubes 15 and strainers 15~ Al~o during washing r air is supplied under pressure through pipe 21 which connects with underdrain chamber 17. ~ir enters the nozzles through holes .:. ` ;!

~7~364 1 in the upper portions thereof, while water enters through the lower ends of the nozzle tubes. Further details of this filtration apparatus are described in Scholten and Young patent 4,076,625. Similar apparatus is available commercially from General Filter Company, Ames, Iowa. Alternate means of ....~
` ` adding the air to the filter can be used, for example, as through a distribution grid placed within or immediately below the filter media bed. The pipes 19, 20 and 21 are usually provided with separate shut-off valves as is drainpipe 22.

lo A multiple-media filter bed is provided above plate 13.
Where the fine granular material, such as sand, extends to the drain plate 13, the nozzles 15 may be equipped with strainer caps 16 having a series of narrow annular slots through which the water flows while retaining the granular medium above plate 13. (See U.S~ patent 4,076,525) The filter medium comprising the multiple-media bed will contain two or more different filtering materials, such as granular materials of different average size, different denisty, etc.

.
- For example, the multiple media bed may comprise a 20 two-media, three-media, or four-media bed, the media being selected in accordance with media previously used for such multiple-media beds. Two or more layers of media will be ~3 ~ provided which are differentiated in size or density or both.
Commonly,- there will be a dist~nct upper layer of a medium of lower densi~y and larger ef~ective size and at least one distinct layer of a medium of higher density and smaller effecti~e size. For example, in a two-media bed, the upper layer may be comprised of coal particles and the lower layer particles of silica sand. AIternatively, in a two-media bed, `the upp~r layer may be o~p~ised;o ooal particles, and the lower layer of .;

1 either garnet or ilmenite sand. As a further example, in a three-media bed, the uppermost layer mayib~ o~rised of ~x~ pa~ticles, the intermediate layer particles of silica sand, and the lower layer particles of garnet or ilmenite sand. For other useable multiple-media beds, reference may be had to the media dis-closed in the United States patents 3,343,680 and 3,876,546.

Returning to FIG. 1, the wash trough and baffle assembly used for preventing media loss during simultaneous air and water backwash will now be described. A wash water lo collection trough 27 is supported to extend horizontally across the upper portion of tank 10. Trough 27 has an upper edge on at least one side thereof functioning as an overflow weir. As shown in FIG. 1, trough 27 provides overflow weirs on both sides thereof, being respectively designated by the numbers 28 and 29. Trough 27 as shown in FIG. 1 can vary in design and construction detail without affecting its function.

Baffles 33 and 35 are supported adjacent to both sides of the trough in spaced relation to the trough and extending horizontally along the weir edge 28. Baffle 33, referred to as the long baffle lies adjacent to side 27a of trough 27 and includes a portion 33a extendlng below the trough bottom - 30 and a portion 33b extending to a level above weir edge 28, .~
thereby defining a restricted flow channel 35 ~or the passage of backwash water into trough 27. Baffle 35, referred to herein as the short baffle, is supported adjacent the other side of the trough (side 27b3 and extends horizontally along weir edge 29. Baffle 35 also incIudes a portion 35b extending to a level above weir edge 2g, thereby defining a second restricted flow channel 36 for the passage of backwash 1~7~ ;4 1 water into the trough 27. The lower end of portion 35a of baffle 35 terminates above and is spaced from the lower end portion 33a of the long baffle 33. For further details, reference may be had to patent 4,076,625.

It will be understood that the above detailed physical description of a granular medium filter and its appurtenances such as the backwash water collection trough and air-water separator baffles are given here to aid in~.the description of the backwash method of the invention and that tank size lo and configuration and trough and baffle assembly designs can vary without affecting the purpose and execution of the subject backwash method.

With the foregoing background, the multiple-media washing method of the present invention can ~e understcod.
It is carried out in a filter including a tank ha~ing a ~ratifi~d`nL~tip~.~media bed therein including a distinct upper layer of a medium of lower density ~and preferably also a larger ef~ective particle size) and at least one distinct lower Iayer of a medium of higher densi~y (and - 20~.~preferably also sm~ller effective particle siæej.~ An upflow air-water wash means- is provided which permits simultaneous washing of the bed with a mixture of air and water followed .. ~ .
: by washing with water alone, and there is provided water withdrawal means in the uppe~ portion of the tank above the bed including means for diverting media suspended in the wash water from exiting therewith~ As the first step of the washing method, there is passed a wash mixture of air and water upwardly through the bed at comblned air and water flow rates sufficient to cause partial intermixing o the distinct , .
- 14 ~

~.~.713~4 l layers. The combined air-water backwash flow is continued while simultaneously removing water from the tank through the wash water withdrawal means in the upper portion of the tank. Advantageously, from 25 to 75% or more of the water used for the wash c~cle can be passed through the bed and exit through the water withdrawal means during the simultane-our air-water wash. The combined air-water wash is then interrupted. For example, the air flow is discontinued while the water flow is continued to complete the washing lo and to obtain restratification of the layers. The water flow may be continued at the same rate as in the combined air-water washing, or at a higher rate if re~uired to obtain the desiréd regrading of the layers. In either case, the water rate should be sufficient to cause the intermixed portions of the media to at least properly separate, and the water wash flow should be continued until the distinct layers of the bed have reformed to substantially the sa~e extent of separation as before the start of the wash cycle.

In a specific embodiment, the filter apparatus is 20 similar to the one described with respect to FIG. l. It may be used as a downflow filter with the washing performed in an upflow direction, that is, the multiple-media bed will be subjected to "backwashing." Furthex, baffle means of the kind described with respect to FIG. l may be interposed in the path of the backwash water flow from the tank into the .

~17~4 , 1 backwash water removal trough for diverting media suspended in the backwash water from exiting therewith.

It will be understood that similar procedures can be employed in connection with upflow filters in which the filtering and washing 10ws are in the same direction, and that other means besides baffles may be employed to divert media suspended in the wash water, such as occurs particularly in the simultaneous air-water wash, from exiting with the waste backwash water.

Returning to FIG. 1, a backwash procedure in accordance with the method of this invention. After a number of backwashings, the multiple-media filter bed will achieve a xelatively fixed orientation with the higher density, and usually smaller grain size medium, such as garnet or silica sand, on the bottom, and the lower density and usually larger diameter, medium such as anthracite coal on top. As illustrated more clearly in FIG. 2A, an intermediate zone of intermixed media may occur in which the smaller sizes of the higher 20 density media are interspersed with the larger size grains of the lower density media. The extent of intermixing depends on the relative size and density of the media layers and may be ~ontrolled by appropriate selection and gradation of each medium and by the rate and duration of bac~wash.

.

As the backwash operation is initiated, water is passed from pipe ~0 through pipe 18, into plenum 17, through media retaining nozzle assemblies 15 and 16 and upward throùgh ~~- the filter bed. Air is supplied within a few seconds of ~7~3~

1 turning on the backwash water--and in fact can precede the turning on of the water--through pipe 21 and is metered into the filter bed 22 through properly-sized orifices located in the tailpipes 15 of the media-retaining nozzle assemblies.

-' The simultaneous passage of air and water through .:~
the multiple-media bed causes substantial mixing of the media layers except that with some combinations of media sizes and backwash rates, a predominance of the high density medium may remain in the lower levels of the bed and a pre-lo dominance of the lower density medium may remain in the upper reaches of the bed. Substantially complete mixing is the preferred effect, but the method hereof is also applicable to partial intermixing of the adjacent layers, that is, the intermixing during washing, as shown in FIG. 2B, is sub-stantially greater than during filtration, as illustra~ed in FIG. 2A.

As water and air flow continue simultaneously, the water begins to flow through restricted flow channels 34 and 3~ located between air-water separator baffles 33 and 35 and trough 27 and over the edges~ 28 and 2~ of t~e backwash water collection trough 27 carryin~ with it the sol~ds washed from the filter media~ This simultaneous flow o~ air and water is pe~nitted to continue until the filter bed IS
substantially clean as determined by clean}~nes$.~f t~e backwash water or by timed duration or by other suitable means. In the method of th.is inVention, the backwash water (or wash water) must flow into the was~ water collector while air and water are applied simultaneously.

, _ .

3~

1 After the media bed is cleaned of entrapped solids, the air flow is turned off by closing valve 21a and the water flow is continued until the media bed is restratified sufficiently for subsequent filter operation. In the preferred application, the same water rate is used for re-stratification as is used during simultaneous air-water wash, but a higher water rate can be used to restratify the bed if required. The exact rates of air and water to be used in the method of the invention will necessarily vary for different lO combinations of media size and density.

While the method of this invention as described --above: is nat limited to speciic media combinations, media sizes, or media densities, there are certain embodiments which are presently believed to be the mostidesirable fox commercial use. Further, with respect to these media com-binations, there are preferred size ratios between the adja-cent media layers. These combinations and the size ratios are illustrated in FIGS. 3, 4, and 5. ~ith reference to these figures and as used herein, the term "effective size" is 20defined as the media grain dLameter at which 10%~of the total grains by weight are smaller and 90~- are larger.
- (ASTM Committee E-ll, 1969.) ...... ~
~ FIG~ 3 illustrates the wa~ ~n which useable or preferred size ratios may b~ determined ~etween ~ coal medium us~d as the next stratified layer ov~r a silica sand layer, that is, the coal is the larger particle size, lower density medium, while the silica sand is a smalier particle size, higher density medium. Such a com~ination may be used as - the two-media of a dual-media filter~ or as the upper and .. . .
~ ~8 ,, , ~.

1 intermediate media la~ers of a three-media filter, using garnet or ilmenite sand as the lowermost layer, which has the smallest size particles of highest density.

As will be noted with reference to FIG. 3, the effective size of the coal medium in millimeters (mm) is plotted on the vertical axis, while the effective size of the silica sand medium in millimeters (mm) is plotted on the horizontal axis. In this lllustration the coal has a specific gravity of from 1.5 to 1.7. The shaded region en-lo closed by the tetrahedran A-B-C-D including the boundary line therearound represents the desirable size ratios of the coal particles to the silica sand particles. In the interest of precise illustration, the coordinates for the points A, B, C, and D are shown on the diagram. For example/ point A
represents a ratio of 0.67 mm coal to 0.30 mm silica sand, etc.
The size ratios defined by the diagram of FIG. 3 are pre-~erably used in a coal and silica sand dual-media bed.

FIG. 4 illustrates preferred size ratios for a dual-media bed using coal particles as the larger lower density 20 medium over garnet or ilmenite sand as the smaller higher densi~y medium. In this illustration, the coal has a specific gra~ity of from 1.5 to 1.7. The useable or preferred ratios of :~ the coal ~o the garnet or ilmenit~ sand are defined by the region enclosed by the ~etrahedran E-F-&-H including the boundary line therearound. The coordinates for the points E, F, G, and H are shown on the diagram. For ~xample, point E represents a ratio of 1.2 mm coal to 0.3 mm garnet or ilmeni'ce sand.

1~7~64 1 I FIG. 5 illustrates Ihe preferred ratios for a silica sand medium used over a garnet or ilmenite sand medium. These ratios may be used in a dual-media:bed employing silica sand as the upper media and garnet or ilmenite sand as the lower media, or in a three-media bed using coal as the upper media, silica sand as the intermediate media, and garnet or ilmenite sand as the lowermost media. In the latter case, the ef~ective size ratios of the uppermost coal layer to the intermediate lo silica sand layer will be defined by the ratios of FIG. 3.

With reference to the foregoing examples as illustrative of the method of this invention, the combined air-water backwash flow is continued for at least two minutes or more and preferably for.at least five minutes.
During the combined backwash., usually 25% or more of.the water used for the wash cycle passes through the mixed media bed and into the water removal means. For example, from 40 to 75% of the water used for the wash cycle can be used in this manner.

While the foregoing disclosure assumes.that the invention will be applied primarily to multiple-media beds where a low-density, large grain size medium overlies a high-density, small grain size medium,~it will be apparent to those skilled in the filtration arts that the invention may .~
be applied in an equivalent embodiment wherein the multiple-media bed comprises a low-density, small grain size medium over a high-density, large grain size medium.

Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. The method of washing a multiple-media filter for removal of filtered solids therefrom, said filter including a tank having a stratified multiple-media bed therein including a distinct upper layer of a medium of lower density and larger effective size and at least one distinct lower layer of a medium of higher density and smaller effective size, upflow air-water washing means permitting simultaneous washing of said bed with a mixture of air and water followed by washing with water alone, and water withdrawal means in the upper portion of said tank above said bed including means for diverting media suspended in the wash water from exiting therewith, comprising the steps in a wash cycle of:
(a) passing a wash mixture of air and water upwardly through said bed simultaneously at air and water flow rates sufficient to cause at least partial intermixing of said distinct layers, (b) continuing said simultaneous air-water wash flow while simultaneously removing water from said tank by said withdrawal means;

(c) thereafter passing wash water without said air upwardly through said bed at a rate at least as great as the rate used for said simultaneous air-water washing, said water rate being sufficient to cause said intermixed media to at least partially separate; and (d) continuing said water wash flow until the distinct layers of said bed have reformed to substantially the same extent of separation as before the start of said wash cycle.

2. The method of claim 1 in which said water flow rate is the same in said water wash as in said air-water wash.

3. The method of claim 1 in which said water flow rate in said water wash is greater than in said air-water wash.

4. The method of washing a multiple-media filter for removal of filtered solids therefrom, said filter including a tank having a stratified multiple-media bed therein including a distinct upper layer of a medium of lower density and larger effective particle size and at least one distinct lower layer of a medium of higher density and smaller effective particle size, upflow air-water wash means permitting simultaneous washing of said bed with a mixture of air and water followed by washing with water alone, and wash water collection trough means in the upper portion of said tank above said bed including means interposed in the path of water flow from said tank into said trough for diverting media suspended in the wash water from exiting therewith, comprising the steps in a wash cycle of:

(a) passing a wash mixture of air and water upwardly through said bed simultaneously at air and water flow rates sufficient to cause at least partial intermixing of said distinct layers;

(b) continuing said simultaneous air-water wash flows while simultaneously removing water from said tank by said withdrawal means and until at least 25% of the water used for the entire wash cycle has exited from said tank;

(c) thereafter passing wash water without said air upwardly through said bed at a rate at least as great as the rate used for simultaneous air-water washing, said water rate being sufficient to cause said intermixed media to at least partially separate; and (d) continuing said water wash flow until the distinct layers of said bed have reformed to substantially the same extent of separation as before the start of said wash cycle.

5. The method of claim 4 in which said bed is a dual-media bed containing coal particles as the lower density medium and silica sand particles as the higher density medium.

6. The method of claim 4 in which said bed is a dual-media bed having particles of coal as the lower density medium and particles of a higher density medium selected from the class consisting of garnet and ilmenite sand.

7. The method of claim 4 in which said bed is a tri-media bed containing particles of coal as the upper lower density medium, particles of silica sand as an intermediate medium, and particles of a higher density medium beneath said silica sand selected from the class consisting of garnet and ilmenite sand.

8. The method of claim 5 in which the effective size of said coal particles has a ratio to the effective size of said silica sand particles defined by the points falling within the Region A-B-C-D of FIG, 3 including the boundary line therearound.

9. The method of claim 6 in which said particles of coal have an effective size ratio to said particles of higher density medium defined by the points falling within the Region E-F-G-H of FIG, 4 including the boundary line therearound.

10. The method of claim 7 in which said particles of coal have an effective size ratio to said particles of silica sand defined by the points falling within the Region A-B-C-D of FIG. 3 including the boundary line therearound, and said particles of silica sand have an effective size ratio to said particles of said higher density medium are defined by the points falling within the Region I-J-L-K
of FIG. 5 including the boundary line therearound.