CA1313504C - Filter underdrain system - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Conventional filter underdrain apparatuses present many problems including non-uniform distribution of backwash water during cleaning of the filter media, inability to cope with explosive air release, designs that tend to cause channelling jetting through the media, designs that are prone to structural failure, designs that are expensive and difficult to install, designs that have no flexibility in terms of capability to operate with air scour assisted backwash or simultaneous air scour\water backwash, designs that require expensive, difficult to build, false bottom structures, or require tedious grouting procedures; designs not sufficiently corrosion resistant, designs that use relatively fragile materials; such as porous sintered plates or ceramics or clay tiles, designs that require gravel support layering as an inflexible requirement. A relatively simple underdrain apparatus includes plates defining inverted V-shaped arches, perforate grids extending across the troughs between or on top of adjacent arches near or at the top thereof for supporting filter media. Perforate grids are described so the underdrain can operate with gravel base layering or without such layering to suit the application. A row of air vent holes extending longitudinally of the arches provides spreading and harmless release of trapped air. Simple fabrication flexibility is described for the provision of air scour vents extending longitudinally of the arches under the grid, and inlets for introducing air into the arches for the air scouring of the media prior to water backwashing thereof. Fabrication flexibility is further described for operation of this underdrain in the mode of air scour simultaneous with backwash by providing air scour vents as described above and a partitioning plate inside the arch sections physically dividing those sections into an air section and a water section thus avoiding two-phase flow problems of wave action generation. The use of stainless steel as the preferred material of construction is reflected in the design which requires punching holes only with no drilling, tapping, or welding required.

Description

--" 1313504 This invention relates to a filter underdrain apparatus.
Filter underdrain apparatuses are by no means new.
Examples of such apparatuses are found in United States Patent Nos. 2,873,857, which issued to ~.L. Schied on February 17, 1959; 3,189,181, which issued to J.S. Couse on June 15, 1965;
3,247,971, which issued to R.E~. Kastler on April 16, 1966;
3,313,420, which issued to A.A. Hirsch on April 11, 1967;

2,615,019, which issued to F.J. Early, Jr., on October 26, 1972; 3,762,559, which issued to M.G. Knoy et al on October 2, 1973; and 3,968,038, which issued to D.H. Nilsson on July 6, 1976.
In general, underdrain apparatuses or systems of the type disclosed by the prior art possess serious flaws, including non-uniform or uneven backwash distribution which occurs because the momentum of the water passing through a perforated header or channel is an important but overlooked factor. Water at a high velocity across an orifice will not be discharged through the orifice as readily as it will when flowing at a lower velocity across another orifice. Other problems include structural failures because the underdrain system is not sufficiently strong or anchored strongly enough to resist the large upward thrust generated during a backwash operation.
Some underdrains are expensive to purchase and many are difficult and expensive to install, require tedious grouting procedures or cumbersome and expensive false bottom structures. Channelling and jetting and spouting bed action in .

the filter media occurs in many strainer type designs. Many underdrains have no ability to cope with trapped air which on explosive release is very disruptive in a filter particularly those with gravel bedding. Many underdrains lack the flexibility to operate in the air scour assisted backwash mode or air scour simultaneous with backwash. Some types of underdrains, for example those of tile or porous tile, are quite fragile and much breakage during installation results.
Inadequate corrosion resistance is a factor with some underdrains. Many types of underdrains require gravel layering as an inflexible requirement. Thus, in spite of the large number of different apparatuses or systems presently available, there is still much room for improvement in the filter underdrain field.
The object of the present invention is to meet the above need by providing a relatively simple filter underdrain apparatus, which substantially reduces the likelihood of most or any of the above mentioned problems being encountered.
While the term filter underdrain is used throughout for brevity, the application of the invention is not restricted by any means to filters only. There are various types of water\waste and process equipment that are not filters at all but where improved flow collection and backwash distribution would be most desirable. Examples of such equipment, and this list is by no means intended to ~e all encompassing are:
- upflow or down flow contact clarifiers or filters - activated carbon contractors - ion exchange units - iron removal units - greensand/catalyzed sand/birm - catalyst bed contactors, e.g. desilicizers - neutralizing media contac-tors Thus rather than writing underdrains or flow distributors for all of the various types of equipment to which this invention is applicable (filters, carbon contactors, process contactors, ion exchangers, catalyst beds, etc.) every time, the term filter underdrains i5 used and understood to encompass units other than true filters.
In some process equipment vessels (upflow mode filters and contact clarifiers as examples) the underdrain serves a somewhat different function than in downflow, i.e. it serves to distribute incoming service flow as well as backwash.
Backwash in filters is clearly defined as a periodic reverse flow through the media to flush out trapped impurities.
The term is used in ion exchange and carbon contactors as well, but means a somewhat different thing. In filters dirt is flushed from the bed by backwash. In ion exchange and carbon contactors, and the like, water is typically filtered in advance so backwash serves to "fluff up the bed" to eliminate packing and flow channelling so that contact is improved and short circuiting averted in carbon contactor units. In ion exchangers a "backwash" is required to wash any dirt from the bed, but more to "fluff the bed" 50 that regenerant contact is maximized, i.e. regenerant short circuiting avoided.
Accordingly, the present invention relates to a filter underdrain apparatus comprising plate means of inverted V-shaped cross section, whereby a plurality of said plate means can be assembled in juxtaposed relationship to define alternatlng V-shaped troughs and inverted V-shaped arches;
perforate grid means for extending aGross a trough proximate top thereof between ad~acent arches or across such arches for supporting filter media; a first row of air vent holes extending longitudinally of said plate means on the top or near the top of said arches; and a row of water holes extending longitudinally of said plate means beneath said first grid means proximate the bottom of each side of each said arch.
The invention will be described in greater detail with reference to the accompanying drawings, which illustrate preferred embodlments of the invention, and wherein:
Figure l i8 a schematic, perspectlve, partly sectioned view of a common t~pe of filter tank or basin incorporating an apparatus in accordance with the present invention;
Figure 2 is a plan view of a panel or plate used to form a filter element for use in the apparatus of the pre~ent invention;
Figure 3 i5 a schematic, cross-sectional view of a portion of a filter element in accordance wlth the present invention;
Figure 4 i5 a perspective view of one end of a portion of the filter element of Figure 3;
Figure 5 i9 a schematic, side elevational view of a ~oint between two sections of the filter element;

Figure 6 is a cross section taken generally along line VI-VI of Figure 5;
Figure ~ is a side elevational view of one end of the filter element of Figures 3 and 4;
Figure 8 is a cross section of a trough in the filter elements of Figures 3 and 4;
Figures 9, 10, and 11 are perspective views of sections of grids used in the apparatus of Figure l;
Figure 12 is a cross-sectional view of an alternate form of a grid used in the apparatus of the present invention;
Figure 13 is a cross section of one side of a filter element in accordance with the present invention;
Figures 14 and 15 are plan views of two additional forms of panels or plates used to form filter elements in the present invention;
Figure 16 is a perspective view of a section of another embodiment of the filter apparatus of the present invention;
Figure l~ is a cross section of a portion of the filter apparatus of Figure 16;
Figure 18 is a perspective view from above of yet another embodiment of the filter apparatus of the present invention;
Figure 19 i5 a partly sectioned, side elevational view of a portion of the apparatus of Figure 18;
Figure 20 is a side elevational view of one end of a filter element incorporating a different form of air inlet;

Figure 21 is a schematic, cross-3ectional view of a portion of another embodiment of the filter element of the present invention incorporating gravel or fine media res~raining grlds;
Figures 22 and 23 are perspective view~ of two types of grids;
Figure 24 is a perspective view of portlons of two grids and a seal;
Figure 25 is a perspective view of another embodiment of the grid using strainers as media retainers;
Figure 26 is a cross section taken generally along line XXVI-XXVI of Fig. 25;
Figure 27 is a view similar to Fig. 1 showing an inlet flume maldistribution corrector plate; and Figures 28 and 29 which appear on the ~econd last sheet of drawings, are sections of the plate of Fig. 2~, showing a palr of anti-turbulence, flow director devices.
Wlth reference to Figure 1, the filter underdrain apparatus of the present invention is shown with a bed 1 of filt~r media of the type which includes a top layer 2 of anthraclte coal followed by a layer 3 of sand, then several layers of progressively coarser gravel, down to a base layer of coarse gravel. The mode illustrated is water backwash only.
It should be clearly understood that the configuration and type of filter media shown in Figure 1 are for illustration only. The underdrain ha~ embodiments where for example no support gravel layering is required, and is applicable to the wide variety of materials which are used as filter media.
Commcnly, filters do incorporate a top layer of anthracite over a layer of fine sand as shown in Fi.gure 1.
Many filters operate with no anthracite layer. Many filters incorporate a layer of fine heavy material such as garnet or ilmenite under the filter sand. On occasion filters operate with other materials entirely for example coke, magnesium oxide, activated carbon, etc.
The underdrain apparatus is not restricted by any means to the layering shown for illustration in Figure 1.
The apparatus and the bed 1 are located in a concrete, open top tank or basin 6, which is defined by a bottom slab 8, side walls 9 and end walls 10. A partition 11 parallel to one side wall 9 defines an overflow trough or gullet 13 for receiving backwash water from semicylindrical metal/concrete/fiberglass troughs 14, which extend transversely of the basin 6 above the bed 1.
A transversely extending trough or flume 15 is provided in the bottom of the basin 6 at one end thereof for receiving filtered and backwash water. Filtered water is discharged from the flume 15 via a pipe 16, which is also used to introduce backwash water into the basin ~. A shoulder or ledge 17 is provided at the outer end of the flume 15 for 25 supporting one end of filter elements generally indicated at 18.
It should be again clearly understood that Figure 1 is for illustration only of one type of filter. Other configurations are common and the underdrain apparatus is readily adaptable to these. Examples of varied configurations:
-transversely extending trough or flume or embedded pipe across the centre width, with filter outflow and backwash inlet at the side.
-trough or f lume or embedded pipe running the length of the filter down the center line, or along one side, or externally down one side. In these cases the troughs and channels of the underdrain apparatus run transversely.
-circular filters with cross diameter inlet/outlet f lume or trough or embedded pipe with the underdrain apparatus running transversely to such flume or trough or embedded pipe.
Frequently, the side gullet 13 of Figure 1 is across the end of the filter with backwash troughs 14 of E'igure 1 then running down the length of the f ilter. The plethora of drawings required to illustrate such variations is not considered essential to this application.
The filter elements 18 include a first set of elongated, rectangular plates 20 (Figure 2), which are bent to an inverted V-shape to define arches 21 (Figures 1, 3 and 4).
Starting with a flat sheet of, for example 4" x 10" x 1/8"
thick 304 stainless steel, holes 22 for receiving anchor bolts 24, water inlet and drain holes 25, and air vent holes 26 are punched through the sheet. The sheet is then bent along lines 2~ and 28 to form the sloping sides 30 of the arches 21 and the bases 31 of the troughs therebetween. While two arches 21 are formed in the same sheet, a narrower sheet can be used to form a single arch. The bases at the outer edges of the sheet are defined by flanges 33, which overlap similar flanges on adjacent sheets (Figures 3 and 4) to form a plurality of parallel arches alternating with troughs. It is normally not necessary to provide a seal between overlapping flanges 33.
However, when conditions warrant such a seal, the seal can be a strip of rubber or an elastomeric material. The filter elements are ins~alled on the bottom slab 8 of the basin 6 before the bed 1 of the filter media, and are secured to such bottom wall 8 by the bolts 24.
An extremely important feature of the invention relates to the sizing of the water inlet/outlet orifices in the inverted V-shaped arches. Such orifices are shown as number 25 in various of the accompanying figures. ln the embodiments shown, the orifices vary in diameter along the length of the inverted V-shaped arch conduits to compensate for the velocity and momentum changes in the backwash flow. Water flowing at high velocity across an orifice will not be discharged through such orifice as readily as when flowing at a lower velocity across another orifice of the same diameter. What occurs then, in a conduit having uniformly sized orifices or lateral connections along the length is a maldistribution of the backwash flow so that the far end of the conduit passes more flow than the inlet end. Such maldistribution is common in many types of underdrains and is very disruptive. The key to the design of this filter underdrain apparatus is to vary the size of the orifices 25 so that the coefficient of flow discharge through each orifice is the same. The variation in orifice diameter is calculated for each filter on a custom design basis, such calculation taking into account the variables of flow velocity entering and along the conduit, fluid viscosity, allowable pressure drops through the orifices and along the conduit, orifice spacing center-line to center-line and desired maximum maldistribution required. The properorifice size spacing and variation in diameter thus calculated is then used in the fabrication of plates shown in Figure 2, 14, 15.
For the air scour mode the orifices 2S are compensated for in the calculation for the water which passes through the air distribution holes on backwash. For the simultaneous backwash air scour mode of operation where the conduit is divided by plate 6~ (Figs. 16 and 17) into an upper air passage 71 and a lower water passage 70, orifices 25 are calculated based on actual flows pertaining in the passage 70.
Side conduit orifices are made slightly larger than all the others to compensate for the somewhat greater area of filter media subtended.
Thus, the inverted V-shaped arched conduits or passages are fabricated with varying orifice sizes to insure essentially no maldistribution along the conduit length.
The size and distribution of the air and water holes makes it expensive and difficult to produce the holes by drilling, particularly by stacked plate drilling. The holes are readily made by a computerized punch press. While 1/8"
stainless steel is considered to be standard, in many filters the underdrain can be made of thinner guage stainless steel and still be strong enough for the intended purpose.

A variety of materials could be used to fabricate this underdrain. For example: steel (painted or galvanized), aluminum, fiberglass, various types of plastics and fiber reinforced plastics, concrete, etc. The material preferred for the great majority of installations is 304 or other grade st~inless steel because of its corrosion resistance properties and great strength. However, stainless steel is difficult to drill, machine and weld so the underdain was designed so that fabrication would be by punching and bending and assembly by bolting, with the use of seal strips, thus eliminating any requirement for drilling, tapping, machining, welding, etc.
Referring to Figures 5 and 6, the junctions between the ends 35 of aligned sections 36 of filter elements are sealed by flexible strips 38 of generally H-shaped cross section. The ends of the arches 21 are closed by rectangular end plates 39 with seals 88 (Figure 7) of generally C-shaped cross section, of neoprene or other elastomer, between the sections and the plates. Corner brackets 40 of L-shaped cross sections are used to connect the sections 36 to the end plates 39 by bolting. Referring to Figures 1, 3, and 8 to 11, the top end of each trough between adjacent arches 21 is closed by an elongated grid 41 defined by a rectangular plate with a plurality of openings 42 therein, and inclined side edges 44 for connecting the grid to the sloping sides 30 of the arches 21.
As shown in Figure 8, anchor bolts 24 extend through the grid 41, nuts 45, washers 46 and the bases 31 or flanges 33 of the elements 18 into the floor 8 of the basin 6. When sections of grid 41 are to be joined end-to-end, a strip 38 of H-shaped cross section is used (Figure 10). When the grid 41 is to be used with fine sand or another fine filter media, screens 47 are provided over the openings 42 and a cover plate 48 is mounted on the grid to sandwich the screens 4~ in position. Bolt holes 49 are provided near the side edges of the grid 41 and of the cover plate 48 for securely connecting the elements to each other. Openings 50 similar in size to the grid openings 42 are provided in the cover plate 48. The openings 50 are aligned with the openings 42.
The half troughs formed by the sides of the filter elements 18 and the side 9 of the basin 6 or the partition 11, are closed by a grid 51 (Figures 1 and 13). The grid 51 has one inclined side edge 52 for engaging the side 30 of the element 18. The other edge 53 is supported in the horizontal position by a generally C-shaped channel member 55. An anchor bolt 56 extends through the member 55, and through nuts 57, washers 58 and the flange 33 of the element 18 into the bottom slab 8 of the basin 6.
In cases where the filter media 1 i9 to be cleaned by water backwash only, i.e. without a preliminary air scour, or air addition simultaneous with water backwash, the grids 41 and 51 are mounted at approximately the middle of the sides 30 of each arch 21 or trough (Figures 8 and 13). As shown in Figure 12, when air scour is to be used prior to or simultaneous with backwashing of the filter media 1, a wider grid 60 is mounted near the top of the arches and troughs so that the air holes 61 are under the grid. Of course a wider half grid (not shown) similar to the grid 51 is provided at the sides of the elements 18.
As shown in Figure 14, additional air distribution openings 61 can be provided in a plate 62 used to form a filter element. Such openings 61 are in parallel longitudinal rows above the water inlet and outlet openings 25 on each side 30 of the arches 21 formed using the plate 62. This type of structure is preferred in the air scour mode.
Another embodiment of the invention (Figures 15 and 17) for use when the media îs subjected to simultaneous air and water backflushing includes a sheet 64 which incorporates the lower water openings 25, air outlet openings 26 and 61, and the bolt holes 65 for receiving bolts 67 (Figures 16 and 17) which are used to mount a horizontal partition 68 i beneath each arch when the sheet 64 is bent to form two adjacent arches with a trough therebetween. Neoprene or other elastomeric sealing strips 69 are provided between the ends of the partition 68 and the sides 30 of the arches. The partition 68 divides the interior of the arch into a lower water passage 70 and an upper air passage 71. Air is introduced into the passage 71 via an inlet tube 72, a header 73 (Figure 16) in the flume 15 and a pipe 75 extending through the wall of the basin 6 to a source of air under pressure air (not shown). The tube 72 is surrounded by a sleeve 76, which extends downwardly from the partition 68 to receive the top end of the tube 72.
Figure 18 shows the type of air introduction system used when air scour not simultaneous with backwash is used. In this case there are no division plates such as plate 68 of Figure 1~ or downwardly extending sleeves such as sleeve 76 of Figure 1~-~6.
~he lnlet tubes ~2, closed ended header 73, air inlet ~5 are identical in both Figures 16 and 18. Such lnlet tubes 72 are open at thelr bottom ends and extend downwardly into the header ~3. Openings 83 of Figure 190 for controlled metering of air are provided in the sides of each tube 72.
In the embodiment of Figure 19, optional shoulder 85 is shown to extend into the flume 80 from the inner side thereof. In underdrain systems of the type disclosed herein, it i5 desirable to be able to alter the flume size to limit the flow velocity of backwash water in such flumes.
With reference to Figure 20, another form of air inlet includes a top inlet tube 86, which is connected to the top end of the arch 21 by an internally threaded connector 8~, which is welded or bolted to the top of the arch near one end 39 thereof.
Figures 21 to 26 show embodiments of grids 90 intended to replace the grids 41, 60 and 51. The grids 90 of Figs. 21 to 26 arre not mounted in the trough areas of the apparatus but span the arches transversely. Larger grid sections can be used with this embodimènt with co~t savings in larger filters in fabrication and installation. The problem of protecting the air vent holes 26 from fine media ingress by individual air vent hole screening ~s eliminated, because the vent holes are beneath the grids. The air vent holes 26 are punched slightly spaced from the arch 30 apex so the holes are not blocked by the top grid.
Figure 21 shows the transverse spanning position of the grids 90 in this embodiment of the invention. The grids 90 are held in position by extended anchor bolts 24. At the sides, the grid ends are fastened to the side edge channel member 55 by anchor bolts 56. The member 55 and the anchor bolts 56 are lengthened to conform with the horizontal spanning position of the grid. The grid ends are overlapped over an arch apex and fastened at that point with self tapping screws 91. The anchor bolts 24 support the grid 90. However, self tapping screws at the apex of each or some of the arches spanned is workable, to eliminate anchor bolt e~tensions.
As best shown in Fig. 22, a perforated grid 90 for supporting graded gravel with fine media above such gravel is provided in lengths and widths to suit. Slots 92 are punched longitudinally with slots 93 (Fig. 24) of the overlapping adjoining grid punched transversely for ease of matching and installing self tapp~ng screws. Circular openings 94 similar 20 to the openings 42 (Fig. 9) are provided in the grid 90.
A laminated perforated structure similar to that of Fig. 11 includes stainless steel mesh 96 sandwiched between grids 90 and 97 to act as a retainer for fine media with no necessity for gravel barrier layering.
There are numerous ways of constructing grids including the use of flat profile wire (wedge wire) sheets.
Referring to E'ig. 24, the edges of adjacent grids 90 are interconnected by I-shaped seal strips 38 of neoprene or other elastomer. Sealing the grid 90 where it rests on side channel members 55 or where ends overlap is not normally necessary, al~hough such sealing could readily be accomplished using strips of neoprene or another elastomer.
As best shown in Figs 25 and 26, strainers 9~ can be used on the grid 90 to act as fine media retainers. each strainer 98 includes a frusto-conical body 99 with slots 100 therein, and a threaded bottom end 102 for mounting the strainer in the grid 90.
During normal use of the basic apparatus practicing water backwash only (Figures 1 to 4), water is filtered through the media 1, the aches 21 and the grids 41, flowing into the flume 15 for discharge through the pipe 16.
Periodically, the flow of water is reversed to backwash the media. Backwash water and impurities dislodged from the media 1 overflow into the troughs 14 for discharge via the gullet 13.
When backwashing operations involving a preliminary air scour and water backflush or a combination air scour and water backflush are desirable modes it is necessary to use the apparatuses of Figures 14 to 20 for such procedures. During air scouring, water is drained to beneath the trough level 14, and air is introduced through the pipe 75, the header 73 and the tubes 72 into the arches 21. The use of the header 73 and metering tube ~2 ensures uniform distribution of the air (which may be another gas; however, for the sake of simplicity the term "air" is used throughout this case) to the individual arch shaped conduits. By varying the number and size of the air openings 26 for approximately 3"-4" of water head loss uniform distribution of air from conduits 21 to all parts of the media 1 is assured.
In the air scour mode, air is introduced to bubble through the media 1 to cause vigorous agitation of the media to S loosen material adhering thereto. Air scour is performed for a variety of periods of time, typically three-five minutes.
The airflow is then stopped and, after a delay of a few minutes to permit air release from the media l, backwash flow is initiated as in an ordinary hydraulic backwash.
When operating with long arches 21, it may be necessary to stagger the air openings 61, or to provide additional holes. Custom designs of the air openings compensates for filter floors which are not dead level, and assists in proper air distribution in long conduits where wave action results due to the velocity of the air over water surface. Concerning the surface of the bottom wall 8 of the basin 6, normal construction tolerance on concrete filter floors is l/8" for ten feet of length. When operating in an air scour mode, the floor level is much more critical than when using a water backflush only. When normal construction tolerance has not been achieved, a concrete floor or bottom wall 8 can be made dead level using self-levelling grout. It is preferable when using a preliminary air scour to provide a dead level floor in the basin 6 by grouting, rather than compensating by changing the openings 61.
During a simultaneous air scour/water backwashing operation, air or another gas is bubbled through the media 1 while water flows therethrough. The water flow rate is too low ~ 1313504 to cause fluidization of the media 1, but is sufficiently high to sweep out dirt loosened by the air agitation of the media.
In dual media or multi-media filters, it is necessary to increase the backwash rate sufficient to cause fluidization, on completion of backwashing, to restratify the media. When effecting a simul~aneous air scour/water backwash operation in long arches 21, there is a marked tendency to generate severe wave action. The wave action can be so severe that some of the water openings 25 pass air, and some of the air openings 26 and/or 61 pass water. Thus, the distribution of air and water is very poor. If water velocities are kept low, i.e. less than l fps, the tendency to wave action is minimal. The preferred solution to the problem of wave action is to provide the horizontal partition 68 (Figures 16 and 17), which divides each arch 21 into a lower water passage 70 and an upper air passage 71.
Further embodiments of the invention relating to inlet flumes (see Figure 1, flume 15) will now be considered.
Figure 1 shows a front flume design, though other configurations are common, for example:
-trough flume or pipe-in-trough or embedded pipe running the length of the filter down the centre line or along the inner side. The troughs and conduits of the underdrain apparatus run across the width of the filter.
-trough flume or pipe-in-trough or embedded pipe running across the center width of the filter with such trough flume receiving backwash flow or discharging filtered water via an embedded pipe through the side or via an embedded pipe through the end of the filter. The troughs and conduits of the underdrain apparatus run the length of the filter.
-on occasion the conduits of the apparatus may be fed via a side or end pipe or conduit through wall sleeves connecting to the ends of the conduits.
-circular filters with across the diameter inlet/outlet trough flume or pipe-in-trough or embedded pipe, with the underdrain apparatus conduits running transversely to such trough flume or pipe.
The underdrain apparatus is compatible with and readily adaptable to any of these (and other) modes of backwash introduction/filtered water outlet.
It is readily apparent, however, from previous discussion of velocity/momentum considerations in the conduits of the apparatus that maldistribution from the flume or pipes introducing backwash water into the conduits of the apparatus is important to consider.
On any of the sunken concrete flumes in any configuration the provision, as part of the underdrain, of stainless steel flume cover plates, with orifices sized using the same hydraulic calculation method as for orifices 25 in the conduits of the underdrain ensures an evenly distributed flow of backwash water into each conduit section. Figure 2~ shows such a cover plate 104 having such sized orifices 105 therein to introduce backwash water into each conduit section.
Similar cover plates for any flume configuration are proposed.
A further embodiment would be the addition of a flow director apparatus to eliminate turbulence from the orifices ~.
105. As shown in Figure 28, one form of flow director includes a 90 elbow 10~ where the introduction of backwash water i8 at the bottom end of the conduits, or a tee 108 (Fig.
29) where ~uch introduction is mid way of a conduit.
On pipe-in-trough type of backwash-in/filtered water out systems orlfices would be drilled in the top of the pipe, such orlfices located under each conduit section. These orifices are sized using the same hydraulic calculation method as for orifices 25 in the conduits of the underdrain, to ensure even flow dlstribution from the pipe into each conduit section.
Again flow director devices 10~ and 108 of Figs. 28 and 29 may be desirable to eliminate turbulence.
On embedded pipe much the ~ame procedure would be used. Varied orifices would be provided in the pipe with welded-on riser pipes upward to the top of the concrete bottom slab. These risers would all be the same diameter, l.e.
slightly larger than the largest orifice hole. Flow director devices 107 and 108 may be desirable on the risers to elimlnate turbulence.
Where the underdrain condults are fed through their end via an external conduit or pipe with wall sleeves, the same hydraulic treatment would be used to vary orifice sizes in such pipe with the sleeves all the same size, or to calculate plate orifice si~e variation for wall sleeves from a concrete flume.
In this case, the term "proximate" is intended to mean at or near.

Claims (10)

1. A filter underdrain apparatus comprising plate means of inverted V-shaped cross section, whereby a plurality of said plate means can be assembled in juxtaposed relationship to define alternating V-shaped troughs and inverted V-shaped arches; perforate grid means for extending across each trough near or at the top thereof between adjacent arches for supporting filter media; a first row of air scour vent holes extending longitudinally of said plate means on the top of said projections; a row of water inlet/outlet means on the top of said projections; a row of water inlet/outlet holes extending longitudinally of said plate means beneath said first grid means proximate the bottom of each side of each said arch.

2. A filter underdrain apparatus according to claim 1, including a second row of air scour vent holes extending longitudinally of said plate means between said first row of holes and said grid means on each side of said arch.

3. An apparatus according to claim 1, including partition means in each said arch between said air scour vent holes and said water holes, said partition means dividing the interior of the arch into a lower water passage and an upper air passage.

4. An apparatus according to claim 1, including header means for receiving air from a source of air under pressure; and inlet tube means for introducing and metering air from said header means into each of said arches.

.

5. An apparatus according to claim 3, including header means for receiving air from a source of air under pressure; and inlet tube means for introducing air from said header means into said upper air passage.

6. An apparatus according to claim 5, wherein said inlet tube means includes an inlet tube extending upwardly from said header means, and a sleeve coaxial with said inlet tube and extending downwardly from said partition means into overlapping relationship with said inlet tube for receiving air therefrom.

7. An apparatus according to claim 3, including inlet tube means in the top of each said arch for introducing air into said upper air passage.

8. An apparatus according to claim 1, including basin means for receiving said plate means and said grid means, said basin means including side wall means, end wall means, and bottom wall means.

9. An apparatus according to claim 8, including flume means extending transversely of said basin means at one end of said bottom wall means, said plate means extending longitudinally of said basin means, whereby one end of each said arch and each said grid means overlies said flume means.

10. An apparatus according to claim 9, including header means in said flume means for receiving air from a source of air under pressure; an inlet tube means connected to said header means for introducing air from said header means into each of said arches.