BACKGROUND OF THE INVENTION

Filters for the removal of solids and other contaminants contained in liquids, whether of the gravity, pressure or vacuum type, are generally comprised of granular filter media of various sizes. (Sometimes the filter media bed has only one filter medium, with all of the particles being of the same composition and substantially the same particle size, but by custom the material of which the bed is made is even in such a case referred to in the plural as the "filter media.") A liquid may be filtered by downward or upward flow through the filter media bed. In the case of a downward flow filter, the liquid being filtered flows downwardly through the filter media bed and through the media support bed (if any) , and exits the filtration tank through openings in an underdrain system. In the case of an upward flow filter, the unfiltered liquid enters the filtration tank through openings in an underdrain system, flows upwardly through the media support bed, if any, and through the media bed itself, with the filtered liquid exiting the filtration tank through collection troughs, or other collection devices, located above the media bed.

Whether a filter is of the downflow or upflow type, cleaning of the filter media (commonly referred to as

"backwashing") — either by water alone, by air and water separately, or by air and water simultaneously — always involves an upward direction of flow.

The filter media usually comprise layers of granules of various sizes. In the case of a downflow

filter the coarsest granules are at the top of the bed and the finest granules, commonly sand or garnet, are at the bottom. In an upflow filter the bed would normally consist of granules of one material, with the coarsest at the bottom and the finest at the top of the media bed.

Most often a media support bed, comprising graded layers of gravel or other suitable material, is provided to serve as a transition between the filter media bed and the underdrain system. The size of the gravel or other material comprising the support bed is larger than the size of the media granules at the interface between the support bed and the filter media bed. The main purpose of the media support bed is to provide a barrier against the possible migration of media particles into the underdrain system, and beyond.

A media support bed would not necessarily be required in a particular filter if the smallest filter media particles in the filter are larger than the openings in the underdrain system that permit the discharge and upward flow of fluid to clean the filter bed, or if the underdrain discharge openings are provided with some type of media retaining device such as, for example, a mesh screen. However, if no gravel support bed is employed and the filter media bed is directly supported on top of the underdrain system, the discharge openings for the exit of washing water or scouring air into the filter media bed (or the openings in the mesh screen, as the case may be) must be extremely small to keep the smallest particles of the filter media out of the underdrain system, and the extremely small size of these openings frequently leads to plugging of the underdrain system. Thus it is generally desirable to provide a support bed of gravel or other suitable material between the underdrain system and the filter media bed itself. Filters both of the downflow and upflow type have for many years been cleaned by backwashing. During this process, water or a combination of water and air (either

sequentially or simultaneously) is passed through the filter bed in an upward direction, which in the case of a downflow filter is opposite to the direction of fluid flow during filtration. During the filtering process, the underdrain system controls the flow of the liquid that is being filtered so that it is distributed as uniformly as possible over the entire horizontal cross-sectional area of the filter media bed. In a downflow filter, the underdrain system also provides uniform collection of the water after it has passed through the filter bed.

From time to time, cleaning of the filter media becomes necessary because of the increasing resistance to flow caused by the accumulation of suspended solids that have been captured by the filter media, and are attached to filter media particles or are lying in the interstices formed by adjoining filter media particles. During the cleaning of a downflow filter, the underdrain system controls the flow of water (and air as well, when it is used) in the reverse direction from the direction of flow that occurs during the filtering process.

The maximum rate of liquid flow through the filter media bed is as a rule greater during the relatively short periods of time that the filter media bed undergoes backwashing than the rate of flow is during the normal filtration mode. For this reason, the description in this specification of the operation of the present invention is confined to its operation during backwashing and scouring of the filter bed. When air is used in the scouring of a filter bed, the air bubbles up through the filter and provides a very thorough agitation of the particles in the filter media. The agitation dislodges accumulated dirt and/or gelatinous floe, which can then be removed easily by the liquid backwashing, whether carried out separately from or simultaneously with the air scouring. This thorough agitation of the filter media is particularly useful for

cleaning those filters in which heavy, sticky deposits are formed in the media during the filtering process.

In many of the known underdrain systems for filter beds, gases and liquids flow through common passages to common points of discharge. The fact that in these systems physically very disparate fluids — a liquid and a gas — are simultaneously conveyed and discharged through the same channel has in practice often led to very serious problems. Two of these are that (1) because of the conflict between the flow rates of the two fluids, strict upper limits are imposed on the liquid and gas flow rates, and (2) the intermixing of liquid and gas caused by the common passages and discharge openings results in coalescence of the air bubbles into larger bubbles, and in unwanted turbulence.

If such coalescence occurs inside a conduit used for both water and air, the larger bubbles thus formed can act like valves and impede the flow of water through a number of discharge openings, which may produce damage to the system as the water backs up. In some prior art underdrain systems, the intermixing of water and air and the resulting coalescence into larger air bubbles take place outside the conduits for the liquid and gas, in the gravel support bed above the underdrain, before the two fluids enter the filter bed itself. The resulting turbulence can produce an undesirable movement and expansion of the gravel support bed.

The goal of backwashing and scouring is to loosen and agitate the filter media itself as much as feasible. In fact, in the case of a filter bed that has more than one filter media layer, the materials comprising the bed are preferably selected so that their size and specific gravity will mean that the entire bed will fluidize at the same backwash flow rate. In contrast to this, any loosening and expansion of the support bed directly under the filter media bed is not desired, because it may disrupt the physical integrity of the filter bed lying above it. Thus

it is important that intermixing of the streams of water and air and the resulting turbulence be avoided, to the extent possible, at all times prior to the introduction of the two fluids into the filter media itself. Still another reason for avoiding intermixing of the washing liquid and scouring air as long as possible, until the two fluids are actually introduced into the filter media bed, is that the likelihood of maintaining uniform distribution of the water and air within the filter media bed is greater if the intermixing first occurs in the media bed itself. Lack of uniformity of distribution of backwash water and scouring air within the filter bed can seriously impair the filtering action of the filter because various portions of the filter bed may remain contaminated even after backwash. In addition, non-uniform distribution can disrupt the bed. Furthermore, non-uniform distribution of wash water within the filter bed tends to result in the formation of "mud balls," which are balls of. contaminant that form in small portions of the bed through which minimum backwash water flows. Non-uniform distribution may also cause "sand boils," and even shifting of the media and/or piling up of the media particles in some portions of the bed. It is then often necessary to remove the filter media and place a completely fresh filter media bed. Backwashing a filter bed with water and scouring with air, carried out either separately or simultaneously, have been known for a very long time. The use of water and air in this way is referred to, for example, in U.S. patent No. 801,810 issued to Parmelee on October 10, 1905, as being already well known at that time. Over the intervening decades, a very large number of underdrain systems for cleaning beds of filter media in this way have been devised. However, there has been no recognition in these systems of the desirability of avoiding intermixing of the washing water and scouring air at all times prior to introduction into the filter media bed itself. In fact, most of these systems have permitted, or even deliberately

brought about, mixing of the water stream and air bubbles at some point before they are introduced into the filter media.

The advantages of an integrally constructed, completely self-contained module that can be assembled side-by-side with other modules to form an underdrain system for a filter bed have also been long recognized. (Typical of such prior devices is the device disclosed in U.S. patent No. 2,378,239 issued to Myron on June 12, 1945.) However, the only other underdrain system known to the applicant that entirely precludes the intermixing of water and air within an integrally constructed underdrain module (U.S. Patent No. 4,995,990 issued to Weston on February 26, 1991) does not suggest any of the following: (1) An underdrain system the top surface of which is substantially flat, covering substantially the entire filter surface area, thereby providing virtually unlimited flexibility for the spacing of water and air discharge openings. (2) Horizontal separation of air and water openings.

(3) The avoidance, or even the minimizing, of intermixing of water and air within the support bed of gravel or other material before the rising streams of water and air bubbles reach the filter media bed itself.

SUMMARY OF THE INVENTION The underdrain module of this invention, which in use is positioned below a filter bed that removes particulate matter and other contaminants from a liquid that passes through the filter bed, is an elongated, integrally constructed, self-contained unit having an enclosed interior and a substantially flat, substantially rectangular exterior top wall. It is preferred that it have a substantially rectangular exterior cross section. The module has separate discharge openings in the top exterior wall through which washing water and scouring air, respectively, can pass separately from said interior

during the cleaning of the filter which is carried out from time to time. The module contains two enclosed conduits, one for water and one for air, for separately conveying water and air (which are introduced into the interior of the module under pressure for use in cleaning the filter bed) , to the respective discharge openings for the two fluids. The water and air are introduced into the interior of the module through separate inlet openings for introducing water at a first rate of flow and air at a second rate of flow.

The discharge openings for a given fluid are preferably all substantially the same shape and size, and preferably substantially uniformly spaced along the length of the underdrain module. They are in addition preferably located so that when a plurality of these modules are assembled side-by-side in a filter underdrain system with each module separated from the adjacent module on either side by substantially the same predetermined distance, the discharge openings for a given fluid are substantially uniformly spaced laterally across the assembled elongated modules. The discharge openings for the two fluids are all located in the same exterior wall, the top wall of the module.

One or both of the conduits for water and air within the underdrain module may be provided with at least one internal partition to form a plurality of ducts for each fluid. Any such internal partition contains at least one opening through which the fluid in question can pass from one of these ducts for a given fluid to the other duct for the same fluid. As used in this specification and the accompanying claims, the term "conduit" is used for the total passageway for a particular fluid (such as water or air) , and the term "duct" is used for a portion of a given conduit. Satisfactory and preferred values are disclosed for the ratio of the cross-sectional area of the interior of the conduit in the underdrain module and the cross-

sectional area of the interior of the conduit in the module for conveying scouring air. Similar figures are disclosed for the ratio of the total cross-sectional area of all the discharge openings for washing water and the total cross- sectional area of all the discharge openings for scouring air.

The filter underdrain system of this invention is comprised of at least one row of a plurality of the underdrain modules of the invention. The modules in each of the rows are operatively connected in series, with the downstream end of the last module in the row being closed, and are operatively connected to sources that supply water and air, respectively, both under pressures appropriate to the fluid in question. The rows in a system having a plurality of rows are arranged side-by-side.

Among other things, satisfactory, improved and further improved spacing of the underdrain modules from each other in the assembled underdrain system are disclosed. The assembled, side-by-side modules are preferably in substantial contact with each other.

ADVANTAGES OF THE INVENTION Two of the advantages of using the filter underdrain module and underdrain system of this invention are: (1) The ease of installing a filter underdrain system using such modules.

(2) The low cost of such a system.

The most important advantages stemming from use of the module and system of this invention, however, are the facts that:

(3) Although the flow channels for both the liquid and gas used in cleaning a granular media filter are combined in a single, integrally constructed, self-contained module, the channels are arranged so that the liquid and gas are separately conveyed to entirely separate discharge openings in the same module wall for each of the two fluids.

These separate conduits and discharge openings avoid altogether intermixing of the two fluids within the interior of the module, and thus avoid the serious problems resulting from such intermixing that are discussed above.

The two main problems avoided by the present invention are (a) unwanted coalescence of the air bubbles and the resulting unwanted turbulence, and (b) undesirable restrictions on the respective flow rates for the liquid and gas. To avoid the second problem, with the present invention the respective discharge openings for air and water can be specifically sized to provide the highest degree of uniformity of distribution at the lowest headloss over the anticipated range of flows. This has never before been thought to be possible with an integral, self- contained underdrain module, but only with an entirely separate set of water underdrains retrofitted with various types of air headers, grids or manifolds. (4) The discharge openings for water are preferably uniformly spaced longitudinally of the elongated underdrain module and, when the module is incorporated in an underdrain system with other modules of the same kind, are preferably uniformly spaced laterally as well. The same is true of the discharge openings for air. This uniform distribution of the water and air discharge openings optimizes the uniformity of the distribution of washing water and scouring air throughout the filter media bed. (5) The substantially flat, substantially rectangular exterior cross-section of the underdrain module, with all the modules in a given underdrain system being of the same height, produces a flat upper surface when the modules are assembled in the system. This provides substantially complete flexibility in the selection of the locations of the discharge openings for the washing water and scouring air, thus

making possible a greater degree of uniformity in the distribution of those openings.

(6) The preferred rectangular exterior cross- section of the modules makes it possible for the upper surface of the underdrain system to extend uninterrupted from wall to wall throughout the entire filter tank. This also provides greater flexibility in the placement of the discharge openings for the two fluids. (7) The resulting increased degree of uniformity in the distribution of discharge openings helps to avoid, insofar as possible, any intermixing of the liquid and gas in any support bed of gravel or other material beneath the filter media, before the rising stream of water and the rising air bubbles reach the actual filter media bed itself. This minimizes, if it does not avoid altogether, undesirable coalescence of the air bubbles, and increased turbulence, that such premature intermixing would otherwise cause. (8) The rectangular shape of the conduits within the filter modules provides the greatest efficiency, for a filter tank of a given width, in the flow of water and air at given flow rates through the underdrain system. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by reference to the accompanying drawings, in which:

FIG. l is a plan view of one embodiment of the underdrain system of this invention, partly broken away, including a plurality of the underdrain modules of this invention;

FIGS. 2, 3 and 4 are partial cross-sectional views taken along the lines 2-2, 3-3, and 4-4, respectively, in FIG. 1, showing alternative constructions for introduction of water and air under pressure into the underdrain system during cleaning of the filter bed;

FIG. 5 is an enlarged, fragmentary plan view of

one of the modules of the underdrain system of FIG. 1;

FIG. 6 is a cross-sectional view taken along the line 6-6 in FIG. 5 but, for purposes of illustration, showing another internal conduit structure, with a gravel support bed above the module;

FIG. 7 is a cross-sectional view taken along the line 7-7 in FIG. 5, again showing for purposes of illustration another internal conduit structure;

FIG. 8 is a cross-sectional view of an underdrain system for this invention in which each of the three underdrain modules shown has a different combination of ducts for the water and air, with a gravel support bed above all the modules; and

FIG. 9 is a similar view illustrating three other embodiments of underdrain modules according to this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Various preferred embodiments of the underdrain module and underdrain system of this invention will now be described.

General Construction

FIG. 1 provides a plan view of one embodiment of the filter bed underdrain system of this invention. Filter tank 20 encloses within its walls 22, 24, 26 and 28 a filter bed comprised of granular filter media of various sizes (not shown) . If conditions require it, a support bed of gravel or other suitable material (not shown in this

Figure) may be provided between the filter media bed and the underdrain system. Underdrain system 30 includes a plurality of underdrain modules 32, which in an actual installation are preferably all identical in configuration and size. In this embodiment, each underdrain module extends from end wall 22 to end wall 26 of the filter tank. If desired, each underdrain module can be shorter. In any such case the modules in a row that contains more than one module are operatively connected in series. Underdrain system 30

shown in FIG. 1 includes eight rows of underdrain modules 32 assembled side-by-side. These modules substantially cover the entire floor of filter bed 20.

As will be described below, each underdrain module 32 contains at least two enclosed conduits for separately conveying water and air under pressure that are introduced into the interior of the module for cleaning of the media filter bed. Each of said conduits is operatively connected to sources (not shown) of water and air, respectively.

In the embodiment of FIG. 1, wash water for cleaning the filter bed is supplied under pressure through enclosed flumes 34, which for illustrative purposes are shown in three different forms. Air is also shown in this Figure as being supplied under pressure to conduit 40 in each of the underdrain modules in three different ways, two of them through air duct 36 and one of them through air duct 38.

FIG. 2, a cross section taken along the line 2-2 in the lower right hand corner of FIG. 1, illustrates the first of these. As seen in FIGS. 1 and 2, scouring air is supplied by main duct 36 under pressure to branch ducts 39, and from there to separate conduits 40 in each module 32. These separate ducts are formed by horizontal partition 42. On the other side of partition 42, water flows under pressure from enclosed flume 34a through opening 43 into separate conduits 44. (As will be seen, the discharge openings for air and water in conduits 40 and 44, respectively, are omitted from this Figure, and from FIGS. 3 and 4 as well.)

FIG. 3, which is a sectional view taken along line 3-3 in the lower, central portion of FIG. 1, shows a second construction for introducing water under pressure into conduit 44 of underdrain module 32. In this construction, water is supplied to conduit 44 from enclosed flume 34b through opening 46. Water flume 34b underlies the confines of the filter cell, and water is allowed to

enter each row of underdrain modules through the opening at the bottom of each water duct.

FIG. 4 is a cross-sectional view taken along line 4-4 in FIG. 1. In this construction, air is introduced under pressure into conduit 40 of underdrain module 32 from main duct 38. Water is supplied under pressure to duct 44 from enclosed flume 34c through opening 48 at the bottom of the duct. This method of air and water introduction is useful with longer underdrain modules, in order to enhance the uniformity of fluid distribution, as well as to permit the use of underdrain modules of smaller cross-sectional area.

In FIG. 1, top walls 35a, 35b and 35c of enclosed flumes 34a, 34b, and 34c, respectively, are broken away to expose the other elements of the underdrain system illustrated in the Figure. For clarity, in FIGS. 2 through

4, top walls 35a, 35b and 35c are omitted altogether.

Pressure Within Single Row of _ Conduits, and Between Rows of Conduits, for Given Fluid

As will be recognized, the pressure differential along the entire length of a single row of similar water conduits 44 (or a single conduit, as the case may be) is in most cases virtually insignificant compared to the pressure differential, or headloss, generated across the discharge openings for that row of conduits (or single conduit) , since the ratio of the total cross-sectional area of all the water conduits 44 to the total cross-sectional area of all the water discharge openings 50 is relatively high. Combining this with the fact that water under pressure is typically introduced into conduits 44 from a common source (through enclosed flumes 34a, 34b, or 34c) means that the water pressure in each row of conduits 44 will tend to be substantially equalized with the pressure in the other conduits 44 in the underdrain system. This is important, of course, in order to achieve the highest possible degree of uniformity in the distribution of the washing water in the filter media.

Similarly, in the case of air introduced from a common source (main air duct 36) , with a high ratio of total air conduit cross-sectional area to total air discharge opening cross-sectional area, the air pressure in each conduit 40 will tend to be substantially equalized with the pressure in the other conduits 40 throughout the system. This is, again, important in order to achieve the highest possible degree of uniformity in the distribution of the scouring air in the filter media. The higher the ratio between the total cross- sectional area of the conduits for a given fluid — water or air — to the total cross-sectional area of the discharge openings for that fluid, the better the degree of uniformity of fluid distribution will be. Naturally there is a practical upper limit for this ratio, usually dictated by energy management requirements or other limitations.

It will be understood that, if desired, additional means may be included in the underdrain modules that will provide — especially in any case in which the various water conduits do not have a common source — increased fluid communication between the washing water conduits in adjacent modules. Such increased fluid communication may further help to equalize the water pressure throughout all the assembled modules of the underdrain system.

Separate Discharge Openings And Conduits FIG. 5 is an enlarged, fragmentary plan view of a particular underdrain module 52 that can seen on the left-hand side of FIG. 1. It illustrates the same distribution of water through discharge openings 50 as has been described. As already pointed out, conduits 44 and 40 of underdrain module 32 (and thus of particular module 52 as well) provide two entirely separate passageways for water and air, respectively. Openings 50 in top wall 53 of module 52 are provided for discharge of streams of water from the module into the filter media bed or the support bed, as the case

may be. These discharge openings are preferably all substantially the same shape and size and preferably substantially uniformly spaced along the length of the module. Openings 54, usually smaller than water discharge openings 50, are provided for discharge of streams of scouring air in the same way. (If for some reason a fewer than usual number of air discharge openings is used, it may be that they will not be smaller than the water discharge openings.) These openings for air are also preferably all substantially the same shape and size, and preferably substantially uniformly spaced along the length of underdrain module 52.

In the embodiment of FIG. 1, discharge openings 50 for water are located in top wall 53 of module 52 in positions such that in a plurality of elongated modules of this type assembled side-by-side in a filter underdrain system, the discharge openings for water are substantially uniformly spaced laterally across the top walls of the assembled modules. In this embodiment, there are two parallel rows of water discharge openings 50 in the top wall of each module 52, and each module is in full contact with adjacent modules of the same kind on both sides

(except for the modules on the far left- and right-hand sides of FIG. 1, which have such contact on only one side) .

Water discharge openings 50 on the right-hand side of the leftmost module 52 in FIG. 1 are located at distance 49 from the right-hand edge of the module, and water discharge openings 50 on the left-hand side of the module are spaced a substantially equal distance 49 from the left-hand edge of the module. In addition, the two rows are spaced from each other a distance 51 which is substantially twice distance 49.

As will be seen from this example, with the module of this invention the lateral spacing and location of the rows of water discharge openings 50 in top wall 53 of a given underdrain module 52 can readily be selected so

that substantially equal lateral spacing with all the rows of discharge openings 50 on other modules 52 of the total underdrain system can be achieved. The selection will be affected, of course, by the number of longitudinal rows of water discharge openings that are present in the top wall of each module.

In the embodiment of FIGS. 1 and 5, there are two rows of water discharge openings 50 in top wall 53 of module 52. There are also two rows of air discharge openings 54 in wall 53. This is advantageous because underdrain modules are ordinarily fairly wide, and if there is only one row of either water or air discharge openings in each module of conventional width, there would be an undesirable separation of each such row from the similar row in the adjacent module when the two modules are installed in an underdrain system. Too large a separation between adjacent rows of discharge openings of the same type (especially the air discharge openings) will result in undesirably wide areas in which there is little or no distribution of the fluid in question. One row of discharge openings for water and one row for air are, however, satisfactory if the individual modules are significantly narrower in width than is conventional.

As will be noted, in the embodiment of FIG. 5 there are also two rows of air discharge openings 54 in the top wall of each underdrain module. As with the water discharge openings, it is important to have at least two rows of air discharge openings in each module of conventional width. For if there is only one row of air discharge openings in each such underdrain module, there will again be such a wide area without air discharge openings in the top wall of the module on both sides of that single row of openings that when two or more modules are placed side-by-side in an assembly of modules in an underdrain system, there will be much too large an area between the rows of openings in adjacent modules, which will adversely affect the degree of uniformity of

distribution of the streams of scouring air across the filter bed as a whole.

In addition to achieving uniform lateral spacing of air discharge openings and water discharge openings, the underdrain module and system of FIG. 1 provide uniform longitudinal spacing of the discharge openings for each fluid in relation to the discharge openings in the two adjacent longitudinal rows of discharge openings for the same fluid. As will be seen from the embodiment of FIG. 1, successive air discharge openings 54 in a given longitudinal row are separated by a distance "d." In turn, a given air discharge opening 54* is spaced by a distance substantially equal "d/2" (measured in the longitudinal direction) from the two closest air discharge openings 54 in the next adjacent rows of such openings. In other words, the given air discharge opening is spaced in this way from the closest openings in the next adjacent rows of the same kind of opening that lie upstream and downstream, respectively, of the given opening. The result is that the air discharge openings in three adjacent longitudinal rows together form a parallelogram, which helps to ensure uniform distribution in each of the lateral and longitudinal directions for the air discharge openings across the entire system of assembled underdrain modules. As shown in FIG. 1, the same relationship pertains between water discharge openings 50 and 50' in adjacent longitudinal rows of water discharge openings.

Still another aspect of the positioning of the discharge openings in the embodiment of FIG. 1 is that each water discharge opening lies in the same longitudinal position as the air discharge openings in the next adjacent row of openings. This, in turn, helps to ensure uniform distribution of the combination of water and air discharge openings in the lateral direction and in the longitudinal direction.

FIGS. 6 and 7 are cross-sectional views that are

taken at the locations indicated by lines 6-6 and 7-7 in FIG. 5 but, for purposes of illustration, they show two other internal conduit arrangements from what is shown in FIGS. 2-4. In FIG. 6, horizontal partition 55 divides underdrain module 56 into separate passageways for water and for air. The separate passageway for water is further divided by horizontal internal partition 58. This latter partition divides the conduit for the passage of water into two separate ducts 60 and 62. (As used in this specification and the accompanying claims, the term "conduit" is used for the total passageway for a particular fluid (such as water or air) , and the term "duct" is used for a portion of a given conduit.) These separate ducts are in this embodiment connected by aperture 64, through which water can pass from one of the ducts to the other. Duct 62 may be considered to be a feeder duct, and duct 60 may be considered to be a distributor duct. In the operation of this underdrain module during the cleaning process, water under pressure passes through pipe 66 (which extends through air duct 68) and out through discharge opening 70, to flow upward through graded gravel support bed 72.

The separate, interconnected plurality of ducts 60 and 62 just described provide greater flexibility in the sizing and spacing of the discharge openings for the washing water. Without this flexibility, substantially greater limitations are imposed on the flow rate ranges of the two fluids. In this embodiment, conduit 68 for the passage of air is a single passageway. The scouring air that flows under pressure through separate conduit 68 of underdrain module 56 flows from discharge openings 74 in the form of air bubbles through gravel support bed 72, and from there into the filter media bed.

In the embodiment of FIG. 7, horizontal partition 80 divides underdrain module 82 into separate passageways

for water and for air. The separate passageway for water is further divided by horizontal internal partition 84, and the separate passageway for air is divided by horizontal internal partition 86. Partition 84 divides the conduit for the passage of water into two separate ducts 88 and 90. These separate ducts are in this embodiment connected by aperture 96, through which water can pass from one of the ducts to the other.

Ducts 88 and 90 may be considered to be a distributor duct and a feeder duct, respectively. In the operation of this underdrain module, washing water under pressure passes through pipe 98 (which extends through air ducts 92 and 94) and out through discharge opening 100, to flow upward through graded gravel support bed 72. Separate air ducts 92 and 94 are connected by aperture 102, through which air can pass from one of the ducts to the other. Ducts 92 and 94 may be considered to be a distributor duct and a feeder duct, respectively. The separate, interconnected plurality of ducts 92 and 94 (formed by horizontal partition 86) provide greater flexibility in the sizing and spacing of the discharge openings for the scouring air. During the cleaning process, air flows under pressure out of discharge opening 104 in the form of air bubbles through a gravel support bed 72, and from there into the filter media bed. Partition 86 divides the conduit for the passage for air into two separate ducts 92 and 94.

Additional Embodiments FIG. 8 shows cross-sectional views of three underdrain modules according to this invention, which are divided in various ways by horizontal partitions. The three modules rest on concrete floor 106 at the bottom of the filter cell. They are separated from each other by a relatively short distance, with the spaces 108 between them filled with a suitable grout.

Underdrain module 110 on the left-hand side of FIG. 8 contains single separate ducts 112 and 114, formed

by horizontal partition 116, for water and air, respectively. In the operation of this embodiment during the cleaning process, water under pressure passes through pipe 118 (which extends through air duct 114) and out through discharge opening 120, to flow upward through graded gravel support bed 72. In this embodiment, the water and air discharge openings are longitudinally spaced from each other along the underdrain module, so the discharge openings for air are not seen in the cross- sectional view of FIG. 8.

In the embodiment illustrated in the central portion of FIG. 8, underdrain module 122 has dual ducts 124 and 126, formed by horizontal partition 128, for water. Passage of water from one of these ducts to the other is provided through aperture 130 in the partition. Water under pressure exits from duct 126 through pipe 132 and out discharge opening 134. Pipe 132 extends through single air duct 136.

In the embodiment shown on the right-hand portion of FIG. 8, underdrain module 138 has dual ducts for both water and air. Horizontal partition 140 forms ducts 142 and 144 for water, with intercommunication between these two ducts provided by aperture 146 in partition 140. During the cleaning operation, water under pressure exits from distributor duct 144 through pipe 148 (extending through air ducts 150 and 152) , and then out through discharge opening 154.

In this embodiment, downwardly extending stem 156 is indicated as being longitudinally spaced from the plane of the cross section shown in FIG. 8. Scouring air entering underdrain module 138 flows first into feeder duct 152, and from there through an aperture (not shown) into distributor duct 150. It then flows out through stem 156 and into gravel support bed 72 through air discharge opening 162. Because the bottom end of stem 156 is positioned near the bottom of distributor duct 150, if any water has entered either air duct 150 or 152, it will be

expelled by the pressure of the air, first from the feeder duct and then the distributor duct, so that air can pass upward through stem 156 and opening 162, as described.

FIG. 9 shows cross sections of three underdrain modules according to this invention that have vertically disposed internal partitions separating the water and air conduits in each case. In two of the embodiments, other vertical partitions form dual ducts in one or both of the fluid conduits. These configurations tend to enhance the structural integrity of the module, especially in deeper ones, by the additional support provided by the vertical partitions. Nevertheless, in most applications the underdrain module with horizontally disposed internal partitions is preferred, because of the virtually unlimited flexibility it provides in locating the discharge openings for the cleaning water and scouring air.

In underdrain module 164 (on the left-hand side of FIG. 9) , vertical partition 166 divides the module into separate ducts 168 and 170 for water and air, respectively. During the cleaning process, water exits through discharge opening 172 in the top wall of the module, and air exits through discharge opening 174.

In the embodiment shown in the central portion of FIG. 9, partition 176 separates dual ducts 178 and 180 for water from single duct 182 for air. The dual ducts for water are formed by vertical partition 184, which contains aperture 186 for communication between the dual ducts. Water exits from this module 175 through discharge opening 188 at the top of the module. In this embodiment, air exits from the module through discharge opening 190.

In underdrain module 192 shown on the right-hand side of FIG. 9, dual water ducts 194 and 196 are formed by vertical partition 198. Communication between these two ducts is provided by aperture 200 in partition 198. During cleaning of the filter bed, water under pressure exits through discharge opening 202 at the top of the module. Dual air ducts 204 and 206 are formed by vertical partition

208. Communication between these two ducts is provided by aperture 210 located in vertical partition 208. Air exits from the module through discharge opening 212 at the top of the module. Important Ratios

As mentioned above, the water discharge openings of the underdrain module of this invention are usually larger than the air discharge openings (for example, openings 50 and 54 in FIGS. 1 and 5.) For best results when using a granular media filter bed in the filtration of water or in the filtration of waste water of industrial or municipal or domestic origin, the ratio of the total cross- sectional area of all the discharge openings for backwash water and the total cross-sectional area of all the discharge openings for scouring air should fall between certain limits. This ratio will produce satisfactory results if it falls in the range from about 10 to about 40. It is preferred that the ratio fall in the range from about 15 to about 30. The particular ratio for a given installation will be determined by the respective flow rates of the water and air, and by the probable maximum variation in each of these flow rates and the degree of uniformity desired. In other filtration systems in which either the backwashing liquid or the scouring gas (or both) , is other than water or air, the indicated satisfactory and preferred ratios are somewhat different from the figures just given.

In the embodiment shown in FIG. 6, the total cross-sectional area of water ducts 60 and 62 is a little more than twice the cross-sectional area of air duct 68. For best results when using a granular media filter bed in the filtration of water or in the filtration of waste water of industrial or municipal or domestic origin, the ratio of the cross-sectional area of the conduit for backwash water and the cross-sectional area of the conduit for scouring air should fall between certain limits. This ratio will produce satisfactory results if it falls in the range from

about 1.5 to about 25. It is preferred that the ratio fall in the range from about 4 to about 15.

Again, the particular ratio for a given installation will be determined by the respective flow rates of the water and air, and by the probable maximum variation in each of these flow rates. In other filtration systems in which either the backwashing or the scouring gas (or both) , is other than water or air, the indicated satisfactory and preferred ratios are somewhat different from the figures just given.

Another important ratio in the underdrain system of this invention involves the positioning of the underdrain modules on the bottom floor of the filter bed. The closer the underdrain modules forming the underdrain system of this invention are positioned to each other, the greater is the flexibility in positioning the water and gas discharge openings on the upper surface of the side-by-side modules. Some flexibility is provided when the distance between adjacent underdrain modules is no greater than a minor fraction of the width of one of the elongated modules. Greater flexibility is provided if that distance is no more than about 1/5 of the width of a module, and still more flexibility is provided when the distance in question is no greater than about 1/10 of the width of one of the elongated modules. For the greatest flexibility in the positioning of the water and gas discharge openings, it is preferred that the adjacent modules be in substantial contact with each other.

While this invention has been described in connection with the best mode presently contemplated by the inventor for carrying out his invention, the preferred embodiments described and shown are for purposes of illustration only, and are not to be construed as constituting any limitation of the invention. Modifications will be obvious to those skilled in the art, and all modifications that do not depart from the spirit of the invention are intended to be included within the scope of the appended claims.