Filter underdrain block

A filter block having a plurality of exterior and interior walls defining a plurality of interior chambers, said chambers including at least a first and second conduits parallel to the longitudinal axis of said block, one disposed above the other, with a third conduit provided to supply gas under pressure to the upper conduit; a top exterior wall of the block having a plurality of apertures distributed thereover. The interior chambers comprise separate conduits for both gas and liquid backwash flows. The interior walls defining the conduits are disposed to provide even distribution of the backwashing gas and to provide bearing support for the top wall of the block.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention generally relates to filter bottoms for use in filters for 
liquids, and more particularly to the structure of a filter block for 
underdrains which when assembled form a filter bottom for supporting a bed 
of finely divided filtering media. The filter bottom provides liquid flow 
conduits below the bed of filtering media, which conduits make possible 
the collection of filtered liquid and the distribution of gas and fluid 
backwash. The present invention is especially directed to a filter bottom 
providing a virtually complete, uniform distribution of gas and fluid 
backwash media throughout the entire filter bed while requiring a minimum 
amount of energy to thoroughly and evenly backwash the filtering media. 
The invention features special and novel aspects which are primarily 
directed to the establishment and maintenance of an usually high degree of 
uniformity of gas and fluid backwash distribution at minimum energy 
expense while markedly broadening the range of allowable backwash flow 
rates. 
2. Description of the Prior Art 
Several assemblies for filter bottoms are known in the art, particularly, 
assemblies having individual units called filter blocks, which are 
assembled together and interconnected with the appropriate supplies and 
drains. The filter blocks, when assembled provide an upper surface for 
supporting a filter media. The filter bottom upper surface is provided 
with apertures to allow the flow of filtered liquid from the filter media 
to pass into the filter bottom where conduits carry the filtered liquid 
from the filter. The conduits also serve to provide backwashing fluids, 
either gas or liquid or both, to the filter media for cleaning. 
The filter bottom is covered with filtering media such as a bed of 
relatively coarse aggregate (the particles being too large to pass through 
the apertures in the top of the block), and several additional layers of 
graded material of larger to smaller and back to larger size farther above 
the filter bottom. 
Conventionally, the liquid to be filtered, typically water, enters from 
above, passing downwardly through the filtering media, through the various 
layers of coarser particles, then through the apertures in the tops of the 
blocks, through conduits in the blocks, and out through a take-off flume. 
Gravity flow moves the liquid to be treated through the filter. 
Periodically, the flow of liquid to be filtered is shut off and a washing 
medium is forced through the filter in reverse direction. 
The wash medium (typically water) flows from the flume into the conduits 
which distribute it laterally away from the flume and from the conduits up 
through the separate upper chambers, the beds of particulate filtering 
material and out at the top, thereby carrying off deposited particles 
dislodged from the filter media. The backwash procedure usually includes 
first air backwashing before water backwashing. The air backwashing step 
loosens and separates the particles of the filter bed and then the 
subsequent water backwashing step fluidizes the bed and carries the 
deposited particles upward and from the bed. In many instances, air and 
water are used simultaneously. In all steps, the air and water flow 
through the bed must be uniformly distributed over the area of the bed. If 
the backwashing flow is not uniformly distributed, then the filter areas 
of low backwash velocity provide little backwashing effect and in areas of 
high fluid velocity, the flow will cause filter media to be carried upward 
and lost to disposal. Moreover, when the filter media is present in layers 
of different particulate materials, or different particulate size, 
non-uniform backwashing can cause undesired mixing of the particulate 
layers. 
One type of the prior art filter blocks is shown in U.S. Pat. No. 3,110,667 
to Stuppy. Each filter block includes a pair of parallel upper and a pair 
of parallel lower conduits, shown in cross-section in FIG. 6. Water from 
the filter passes through apertures in the top of each block into the 
upper conduits, then through ports in the floors of the upper conduits 
into the lower conduits. The liquid then flows from block to block to a 
flume. The Stuppy patent also proposes a liquid backwash wherein liquid is 
supplied to the lower conduits, passes upwards through the ports to the 
upper conduits and out the apertures to the filter media. 
U.S Pat. No. 4,065,391 to Farabaugh discloses another configuration of 
filter block which does not include upper and lower conduits, but instead 
has an arrangement of parallel primary and secondary conduits positioned 
horizontally adjacent each other and separated by inclined walls. The 
inclined walls contain relatively smaller gas metering orifices and 
relatively larger liquid metering orifices with the liquid metering 
orifices positioned below the gas metering orifices. Backwash gas or 
liquid is supplied through the primary conduits, passes through the 
metering orifices into the secondary conduits, and from the secondary 
conduits into the bed of filter media. The gas metering orifices control 
the rate at which a backwash gas passes from the primary to the secondary 
conduits. The liquid metering orifices, and to a lesser extent the gas 
metering orifices, control the flow rate of a liquid backwashing medium. 
The prior art filter block devices are deficient as to backwash operations. 
For example, the Stuppy device has relatively large liquid ports between 
the upper and lower conduits. If, although not disclosed in the Stuppy 
patent, a gas backwash were used with the device, the gas would be 
supplied to the lower conduits but then would pass easily through the 
first few of the relatively large liquid ports encountered to the upper 
conduits resulting in significantly unequal distribution of gas through 
the filter bottom. The uneven distribution of gas during such a backwash 
would serve to disrupt the filter media where too much gas flow occurs and 
to provide inadequate cleaning of the filter media where insufficient gas 
distribution occurs. 
The Farabaugh device depends on a gas/liquid interface to control gas 
distribution during gas backwash. When gas backwashing begins, since the 
entire block and the filtering media above it is under water at that time, 
each of the conduits is essentially filled with water. When backwash gas 
is supplied to the primary conduit, a gas/liquid interface is formed as 
shown in FIG. 6 of the Farabaugh patent, and gas is metered to the 
secondary conduits via the gas metering orifices in the upper portion of 
the wall separating the primary and secondary conduits. However, the 
Farabaugh system can tolerate only a limited range of backwashing flow. If 
that limit is exceeded, the gas/liquid interface level is forced down to a 
point at which gas escapes into the secondary conduits through the 
oversized liquid metering orifices which are also located in the wall 
separating the primary and secondary conduits. Because of the rapid escape 
of the gas through the oversized liquid metering orifices, unequal 
distribution of the backwash gas, with its consequent disadvantages, 
occurs. 
An additional problem with the Farabaugh device is that standing waves, 
created by a variety of phenomena during backwash of the filter such as a 
pressure shock from a sticking gas valve or other causes, can further 
limit the range of backwash flows. When such a standing wave is formed 
during backwash, it can reduce the level of the liquid/gas interface 
upstream of the wave so that the large liquid metering orifices are 
exposed to the gas flow, again creating unequal distribution of backwash 
gas. 
It has also been found in the operation of filter bottoms as taught by 
Farabaugh that waves which frequently occur on the surface of the liquid 
in the filter, by changing temporarily the liquid pressure head over 
portions of the filter bottom, can thereby change the level of the 
liquid/gas interface in the primary conduit. This fluctuation in the level 
of the interface cyclically exposes then covers the oversized liquid 
metering orifices in the primary conduit to the gas above the interface, 
and, consequently, results in maldistribution of gas backwash as the 
oversized orifice is exposed. 
The dependency of the Farabaugh design on a liquid/gas interface also 
limits the backwash rates which can be used during simultaneous gas and 
liquid backflush. The typical upper limit for simultaneous backwash in the 
Farabaugh device is approximately 5 standard cubic feet per minute 
("SCFM") gas per square foot of filter bottom upper surface and 5 gallons 
per square foot of filter bottom per minute ("GSFM"). It has been proposed 
that raising one or both of the backwash rates simultaneously would 
increase scouring and cleaning of the filter media. However, the Farabaugh 
device, with its limitation of the gas/liquid interface can only 
accommodate simultaneous backwash rates within a limited range. 
Finally, another problem with the Farabaugh device is that, because of the 
presence of the large liquid metering orifices in the primary conduit, it 
is particularly susceptible to problems stemming from non-level 
installation of the filter blocks. At column 5, line 14, Farabaugh states 
that the liquid metering orifices are preferably placed about 31/2 inches 
below the gas metering orifices. Accordingly, even slight errors in 
installation of the filter blocks can markedly reduce the vertical 
distance between the lowest gas orifice and the highest liquid orifice 
which share the same gas/liquid interface. This non-level installation 
significantly reduces the safe operating range for gas backwashing to 
avoid escape of the gas through the liquid orifices. Although 
modifications of the Farabaugh device have been proposed, extending the 
vertical distance between the gas and liquid metering orifices to almost 9 
inches, the same problems still occur. 
The previously known filter block arrangements suffer from sensitivity to 
non-level alignment of the blocks. Even small divergences from level 
alignment of the blocks leads to significantly non-uniform backwashing 
performance, particularly when the backwash medium is a gas. Accordingly, 
it is an object of this invention to provide a filter underdrain block 
structure which when assembled and arranged to form a filter bottom, 
maximizes uniform distribution of backwashing gas and backwashing liquid, 
fluidizes the filtration media over the underdrain block, dislodges dirt 
and debris entrapped in the filter media, and thoroughly cleans the media. 
It is another object of the present invention to provide a filter block 
underdrain with a reduced sensitivity to non-level block alignment, 
particularly with regard to gas backwashing. A further object of the 
invention is to provide a filter block weighing less than conventional 
filter blocks yet having a good structural integrity thereby being easier 
to handle and easier to install than conventional clay filter blocks. 
Other objects and advantages of the present invention will be apparent from 
the following detailed description and from the appended drawings. 
SUMMARY OF THE INVENTION 
The present invention, generally described, provides a filter block having 
both upper and lower conduits or chambers and having conduits for 
distribution of liquid backwash which are separate from a conduit for 
distribution of gas backwash. 
In a preferred embodiment, the upper chambers comprise a primary gas 
conduit and two secondary conduits. The lower chambers comprise two 
primary liquid conduits. The primary gas conduit is in communication with 
the secondary conduits via appropriately sized gas orifices to allow the 
flow of gas from the primary gas conduit to the two secondary conduits. 
Liquid orifices are provided between the secondary conduits and the 
primary liquid conduits to allow the flow of liquids between the secondary 
conduits and the primary liquid conduits. 
Filtered liquid passes through apertures in the top of the filter block, 
into the secondary conduits, through orifices into the primary liquid 
conduits and from block to block to a flume. The primary gas conduit 
essentially does not contribute to the flow of liquids during filtering 
operations. 
During backwash operations, gas or liquid backwash can be carried on 
simultaneously or independently. Gas backwash flow is supplied to the 
primary gas conduits and is passed from block to block across the filter 
bottom. Backwash gas passes from the primary gas conduit through the gas 
orifices to the secondary conduits. Liquid backwash flow is supplied to 
the primary liquid conduits and is distributed thereby evenly across the 
filter bottom. Backwash liquid passes from the primary liquid conduits 
through liquid orifices into the secondary conduits, through the apertures 
in the top surface of the filter bottom to the filter media. 
Having separate primary conduits for the gas backwash and the liquid 
backwash, with the primary gas conduit specially designed with all 
orifices therefrom constituting gas orifices, (as compared to having only 
one primary conduit designed to accomplish both liquid and gas backwash), 
provides significant advantages over the prior art. The present invention 
allows for a greatly increased range of gas backwash flow rates, increased 
simultaneous gas/liquid backwash rates, the minimization of problems 
inherent in systems dependent on a gas/liquid interface during backwash, 
and numerous other advantages.

DETAILED DESCRIPTION OF THE INVENTION 
With reference now to the drawings wherein like numerals designate 
corresponding parts throughout the several views, there is shown in FIG. 1 
a preferred embodiment of a filter block 10 of the present invention and 
which has a top wall 12 provided with a plurality of apertures, one of 
which is indicated at 14, distributed in a substantially even pattern over 
the top wall 12. The block may be integrally formed by injection molding 
with a pair of side walls 16 and a bottom wall 18 with a plurality of 
reinforcing ribs 20, 21 formed on the exterior surface of the walls 12, 16 
and 18. Additionally, a plurality of reinforcing ribs 23 may be formed on 
the interior surface of walls 16 and 18. Ribs 23 may or may not extend the 
full length of block 10. 
At one end, a receiving collar 22 is provided having dimensions to snugly 
interfit with an adjacent block which will have an end configuration to 
closely interfit within the collar 22. This close fit substantially 
prevents either gas or liquid from entering or exiting the interior of the 
assembled blocks during use. Grouting and/or an adhesive is employed to 
provide a fluid-tight joint between the ends of the blocks. It will be 
understood that an opposite end 25 of each block 10 may be molded to the 
desired size and shape to facilitate the assembly of an interlocking 
relationship along an axis of the filter bed. 
With reference now to FIG. 2, there is shown a cross-sectional view along 
lines 2--2 of FIG. 1 wherein a transverse wall 24 extends from one side 
wall 16 to the opposite side wall 16 to divide the interior of the block 
10 into an upper and a lower portion. The lower portion is further divided 
by a partition 26 into a pair of primary liquid conduits or chambers 28 
and 30 which are substantially rectangular in cross-section and which 
extend parallel to the longitudinal axis of the block 10 and to each other 
along the length of the block 10. The lower portion may be still further 
divided by another partition (not shown) to enhance the structural 
integrity of block 10. The upper portion is further divided by a pair of 
angularly extending walls 32 and 34 into three chambers, secondary 
conduits 36 and 40, and a primary gas conduit 38, all of which also extend 
parallel to the longitudinal axis of the block and extend along the length 
of each block 10. Secondary conduits 36 and 40 receive water or liquid 
passing down through the filter bed and through the apertures 14 provided 
in wall 12. Primary gas conduit 38 serves to distribute backwash gas 
axially along the length of the filter block 10 and through apertures in 
walls 32 and 34 into secondary conduits 36 and 40. As is also apparent in 
FIG. 1, an end portion 27 of wall 26 is recessed from the end of filter 
block 10 to allow the communication of fluids between primary liquid 
conduits 28 and 30. 
Secondary conduit 36 is in communication through a plurality of liquid 
orifices 48 with primary liquid conduit 28 while a plurality of liquid 
orifices 50 in wall 24 provide flow communication between conduits 40 and 
30. Liquid orifices 48 and 50 are provided in evenly spaced relation along 
wall 24. 
Walls 32 and 34 adjacent wall 24 are each provided with a plurality of gas 
orifices along the length of the block as at 33 and 35. These gas orifices 
are sized so that during backwash operations when gas is supplied to 
primary gas conduit 38, the injected gas forces the water out of primary 
gas conduit 38 and an even distribution of gas is accomplished along the 
length of primary gas conduit 38 through gas orifices 33 and 35. 
In a preferred embodiment of the present invention, each filter block is 
designed to provide the width of 1 foot (measured in the direction of wall 
24) of filter bottom. Secondary conduits 36 and 40 each have approximately 
16.9 square inches of cross-sectional area. Primary gas conduit 38 has 
approximately 17.4 square inches of cross-sectional area. Primary liquid 
conduits 28 and 30 each provide approximately 22.6 square inches of 
cross-sectional area. In the preferred embodiment, upper gas orifices 35 
have a diameter of 3/32 inch and are spaced at four orifices per axial 
foot of filter block. Lower gas orifices 33 have a diameter of 1/8 inch 
and are also spaced at four orifices per axial foot of filter block. 
Liquid orifices 48 and 50 have a diameter of 3/4 inch and are spaced at 
two orifices per axial foot of filter block. 
In the preferred embodiment, it has been found that the slightly larger 
diameter of lower gas orifices 33 over the diameter of upper gas orifices 
35 is beneficial to a rapid evacuation of liquid from primary gas conduit 
38 during gas backwashing. It has been found that a 50 foot length of 
primary gas conduit 38 in a filter block assembly can be evacuated of 
liquid at normal gas backwash pressures within ten seconds from the start 
of gas backwash operations. Also, primary gas conduit 38 is essentially 
fully evacuated of liquid at the extremely low gas backwash rate of 1 
SCFM. 
Since primary gas conduit 38 is essentially completely evacuated of liquid 
during gas backwashing, a number of problems inherent in prior art designs 
are overcome. The otherwise severe effects of non-level installation of 
the filter blocks is almost completely overcome. So also, are the effects 
of surface waves on the filter liquid overcome. Additionally, since there 
is no significant gas/liquid interface in primary gas conduit 38 during 
gas backwash, the problem of standing waves is avoided. 
Having separate conduits for gas and liquid backwashing allows independent 
adjustment of either or both of the gas or liquid flow rates during 
backwash operations, an option not possible with the prior art devices. It 
has been found that the desired flow rates of 5 SCFM gas and simultaneous 
10 GSFM can be accomplished in the design of the preferred embodiment. It 
has been found that gas backwash rates of 1-10 SCFM can be realized in the 
present invention. 
It will also be apparent from FIG. 1 that the ribs 21 along the upper 
portion of the block 10 are in staggered relationship relative to the ribs 
20 surrounding the side wall and bottom wall 18 along the lower portion of 
the block. In addition, a plurality of spaced perpendicularly extending 
flange members 52 are provided along each side wall 16 immediately above 
the position where the lower ribs 20 terminate. These will not only assist 
in handling the individual blocks but in placement and positioning of the 
blocks along the bottom of the filter bed. In addition, the side wall 16 
may be provided with a plurality of indentations 39 as shown in FIG. 1 to 
facilitate intimate contact with grouting material when the block is 
assembled in a filter bed bottom. 
With reference now to FIG. 3, there is shown a sectional view along lines 
3--3 of FIG. 1, illustrating the cooperation of an air inlet tube 56, 
which may be provided with a temporary cap 58, with an opening 60. The 
lower end of the tube 56 is secured about the opening 60 provided through 
the upper wall 12 whereby communication with the interior of primary gas 
conduit 38 is effected. The upper end of tube 56 will be connected to a 
pressurized air or gas supply. As an alternative, in this embodiment as 
well as the embodiment described below, gas may be introduced into primary 
gas conduit 38 through a pipe that is provided with spaced openings with 
the pipe extending in primary gas conduit 38 parallel to the wall 24. It 
will be understood, as shown in FIG. 4, that each section of filter block 
in the filter bottom does not need to include an air inlet tube 56. 
The filter block of the present invention is further characterized by the 
ratio of the sum of the cross-sectional areas, of the secondary conduits 
36 and 40 relative to the primary liquid conduits 28 and 30. This unique 
ratio of cross-sectional areas allows for significant reduction in head 
loss during the backwash cycle. Specifically, the ratio of the combined 
cross-sectional areas of primary conduits 28 and 30 to the combined 
cross-sectional areas of the secondary liquid conduits 36 and 40 ranges 
from about 1 to 5:1 and, preferably, the ratio ranges from about 
1.5-3.5:1. 
A filter bottom must provide uniform distribution of backwash gas and 
liquid, for example air and water, over the entire area of the filter. As 
is well known, lack of uniformity can seriously impair the effectiveness 
of the filter because various portions of the bed may retain deposited 
particulate even after a backwash cycle. The useful life of a filter is 
directly proportional to the uniformity of distribution of the backwash 
medium. Localized variations in distribution of the backwash flow will 
disrupt the filtration support media layers, necessitating frequent 
replacement and/or regrading. A uniform distribution of backwash gas and 
liquid is dependent upon uniform distribution of the backwash liquid from 
primary liquid conduits 28 and 30 into secondary conduits 36 and 40. 
According to the present invention, this is more readily achievable by 
maintaining air or gas under pressure in primary gas conduit 38 and the 
distribution of that gas through gas orifices 33 and 35 into secondary 
conduits 36 and 40. Also, by maintaining primary gas conduit 38 under 
adequate gas pressure slight variations in the levelness of adjacent 
blocks will not cause significant variations in the distribution of the 
backwash flow. In addition, it has been found that whereas prior block 
structures could only tolerate a feed rate of 3-5 SCFM per square foot 
over the filter bed, the present invention can accommodate gas backwash 
rates of 1-10 SCFM per square foot of the filter bed without significant 
disruption of the filter layers or unacceptably high energy losses in gas 
distribution. 
The filter block lo of the present invention may be made of fired clay or a 
light weight, high density, injection molded plastic such as polyethylene 
of high molecular weight. The polyethylene is more easy to handle and more 
durable during transportation and installation. Alternatively, the 
exterior and interior walls of block 10 may be extruded to form continuous 
lengths of filter block. 
With reference now to FIG. 4, there are shown two blocks 10 of the present 
invention connected end-to-end with flange 23 inserted into collar 22 of 
an adjacent block 10. A bracket member 60 may be employed to secure the 
ends of the block in abutting relationship as shown. The ribs 20 extending 
around the lower portion of each block will rest on a previously 
constructed floor 62 in a tank 68. Rows of blocks assembled as shown in 
FIG. 4 will extend across the floor 62 of the tank 68 with the ends of the 
rows connected to a common header 70 which in turn is connected through a 
duct 72 to a pump 74. It will be understood that this arrangement is 
illustrative in that other designs may be employed. 
Conventionally, a plurality of layers of particulate material 76 are 
deposited over the top walls of all the rows of blocks 10 to a level 
deemed sufficient to effect the degree of cleansing required for the 
liquid to be treated. The block 10 located adjacent one of the walls of 
the tank will be connected through its tube 56 to a source of gas such as 
air under pressure by a tube 57 provided for each row of blocks 10. 
Suitable valving controls would, of course, be employed and since these 
are of conventional construction, they need not be further described 
herein. Alternatively, the tube 56 may be omitted and the gas supplied to 
primary gas conduit 38 by a flume and sleeve arrangement in tank 68 or by 
separate air blocks positioned between tank 68 and the end of each row of 
filter blocks 10. 
In the embodiment described above, the apex of the primary gas conduit 38 
is formed integrally with the top wall 12 and thereby provides support for 
the wall 12 which carries the weight of the filter media thereon. Thus, 
the blocks 10 may be constructed of lighter material without sacrificing 
structural stability. 
The present invention, having primary liquid conduits on the lower level of 
the filter block and the primary gas conduit substantially in the upper 
level of the filter block, also provides the advantage that portions of 
side walls 16 of the primary liquid conduits can be cut away between 
adjacent filter blocks to allow further equalization (by flow across the 
rows of filter blocks) of liquid backwash flow during backwash operations. 
In alternate embodiments of the present invention, the primary gas conduit 
can be fashioned in cross-section so that its apex does not extend to the 
top wall of the filter block. In such case, the secondary conduits can 
comprise either one or two conduits. Additionally, the primary gas conduit 
can be of numerous different cross-sectional designs, and it may in some 
cases consist of a cylindrical conduit centrally disposed within the 
secondary conduit. It is preferred, although not necessary, that the 
primary gas conduit be symmetrically disposed about a vertical plane 
extending through the axis of the filter block. 
In other embodiments of the present invention, the primary liquid conduits 
can comprise a single conduit as can the secondary conduits. 
Having described the invention, it will be apparent to those skilled in 
this art that various modifications may be made thereto without departing 
from the spirit and scope of this invention as defined in the appended 
claims.