Deep bed filtration system

A deep bed filter system comprising a plurality of filter media beds, each having a respective media regeneration sub-system that includes some components associated with an individual filter bed and other components that are shared with all filter beds in the system, thereby improving the utilization of available space and lowering the materials and labor costs relative to known systems. A central column extends upwardly along the axis through the upper and lower portions of the vessel, thereby defining an annular volume extending substantially from the bottom of the vessel through the upper and lower portions. A plurality of distinct tubes extend through the central column, with each tube having a lower, intake end situated at the bottom for drawing dirtied filter media from the lower portion of the vessel into the column and transporting the drawn media upwardly through the column to a second discharge end at an elevation above the filtrate level of the filtrate region. A plurality of media washing devices are each situated in an azimuthally distinct location above the region of filter media and are fluidly connected to at least one of the transport tubes such that each tube delivers a substantially continuous flow of dirtied media to only one washing device. A plurality of media distribution devices associated respectively with the plurality of washing devices, direct the dropping clean media to a respective plurality of locations on the region of filter media.

BACKGROUND OF THE INVENTION 
The present invention relates to so-called deep bed media filters, 
particularly of the type that include a plurality of filter units arranged 
or connected as a filter system or installation. 
U.S. Pat. No. 5,277,829, which issued on Jan. 14, 1994 to the assignee of 
the present invention, discloses a deep bed filter unit in which the 
infilt flows upwardly through a sand bed such that the filtrate 
accumulates above the sand bed while the dirty sand is continuously 
regenerated. A plurality of such units can be connected by a common inlet 
manifold and a common outlet manifold, to form a system or installation, 
e.g., at a wastewater treatment plant. Another type of regenerative deep 
bed filter unit in which the infilt flows upwardly, is disclosed in U.S. 
Pat. Nos. 4,123,456 and 4,126,546. These patents also shows a plurality of 
modular units clustered to form a system or installation. 
In northern areas of the United States (and other regions of the world that 
experience frozen ground during winter or other ground instabilities such 
as flooding or earthquakes), the filter systems are preferably supported 
above ground, on stilts or the like. In areas of more stable temperature 
and other ground conditions, it may be desirable to wholly or partially 
embed the filter systems in the ground. A major advantage of the latter 
arrangement, is the lower cost associated with pouring large concrete 
basins into a ground excavation, rather than forming and shipping vessels 
made of steel to be supported by stilts. One drawback of the poured 
concrete basin, however, is the difficulty some contractors face in 
properly contouring the basin, especially where the basin is constituted 
by a plurality of modular vessel units each of which requires its own 
cylindrical funnel-shaped bottom, as disclosed in said U.S. Pat. No. 
4,126,546. These units require tight tolerances for the components to fit 
properly and to seal against the basin walls. 
SUMMARY OF THE INVENTION 
It is, accordingly, an object of the present invention, to provide a deep 
bed filter system or installation that accommodates a plurality of filter 
units within a simplified basin that can be readily fabricated, preferably 
by poured concrete in a ground excavation. 
It is another object to provide a deep bed filter system having no seals 
between the concrete basin walls and the internal components. 
It is a further object of the invention, to provide a deep bed filter 
system comprising a plurality of filter media beds, each having a 
respective media regeneration sub-system that includes some components 
associated with an individual filter bed and other components that are 
shared with all filter beds in the system, thereby improving the 
utilization of available space and lowering the materials and labor costs 
relative to known systems. 
It should be appreciated that many of the inventive features described 
herein can readily be incorporated into a deep bed filter system that is 
not constructed from a poured concrete basin, but rather has the form of 
an above ground, upright vessel, such as shown in U.S. Pat. No. 5,227,829. 
In a general way, the invention as implemented in a vessel, includes an 
upright vessel having a central vertical axis, an upper portion of 
substantially uniform cross section, and a lower portion that tapers 
downwardly from the upper portion towards the axis to a closed bottom. A 
central column extends upwardly along the axis through the upper and lower 
portions of the vessel, thereby defining an annular volume extending 
substantially from the bottom of the vessel through the upper and lower 
portions. A region of filter media fills the annular volume in a lower 
portion of the vessel, and extends upwardly into the annular volume of the 
upper portion of the vessel. The media in the lower portion of the basin 
is in fluid communication with the bottom. Inlet means are provided for 
receiving a continuous flow of infilt from a source outside the vessel and 
distributing the infilt at a plurality of azimuthally spaced locations in 
the region of filter media. The hydraulic pressures within the vessel are 
such that the infilt flows upwardly through the media in the upper portion 
of the vessel and accumulates therein as a filtrate region above the 
region of filter media while dirt in the infilt is trapped in the region 
of filter media. Transport means including a plurality of distinct tubes 
extend through the central column, with each tube having a lower, intake 
end situated at the bottom for drawing dirtied filter media from the lower 
portion of the vessel into the column and transporting the drawn media 
upwardly through the column to a second discharge end at an elevation 
above the filtrate level of the filtrate region. This transport induces a 
downward movement of the media in the region of filter media. A plurality 
of media washing devices are each situated in an azimuthally distinct 
location above the region of filter media and are fluidly connected to at 
least one of the transport tubes such that each tube delivers a 
substantially continuous flow of dirtied media to only one washing device. 
The washing device includes means for contacting the delivered dirty media 
with a flow of wash water such that the wash water carries the dirt in the 
dirty media out of the vessel as cleaned filter media drops toward the 
region of filter media. A plurality of media distribution devices 
associated respectively with the plurality of washing devices, direct the 
dropping clean media to a respective plurality of locations on the region 
of filter media, each of said locations defining an apex of a respective 
plurality of pyramidal media beds. 
The embodiments described herein can be characterized as having a plurality 
of e.g., either two or four cells, with each cell defined by its own 
filter media wash and cleaned media distribution devices, and associated 
dirty media transport tubes. Each cell also has its own media bed 
associated therewith, i.e., the region of filter media has an upper 
surface which appears as a plurality of pyramidal mounds, each having an 
apex in contact with a respective media distribution device associated 
with a respective cell. At the option of the user of the invention, the 
filter beds associated with each cell can be either completely or 
partially isolated from each other. 
The hybrid nature of the present invention, wherein distinct cells have 
some features which are associated uniquely therewith, and yet have other 
features which are shared with other cells within the system, provides 
several advantages relative to known systems. The present invention can be 
fabricated at less cost and installed more quickly in the field. The 
system permits a redundant transport subsystem, preferably an airlift 
sub-system, such that a problem in any one cell does not disable the 
entire system. The system can be installed without the cost associated 
with maintaining tight tolerances on the concrete basin, because the 
concrete does not provide any sealing interfaces that could leak and 
contaminate the filtrate or otherwise degrade the efficiency of operation. 
With the present invention, there is virtually no possibility of structural 
failure of the system. The operation of each cell maintains the flow and 
cleaning of the dirty sand in a manner that does not contaminate the 
filtrate or otherwise filtrate collection volumes.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIGS. 1-6 show various views associated with a first embodiment of the 
present invention, directed to a deep bed filter system 100 having two 
filter media beds 101,102 and two media washing devices 103,104 for 
providing substantially continuous regenerative operation for the 
secondary treatment of, for example, water in a waste water treatment 
plant. As will be described in greater detail below, a plurality of filter 
systems of the type described with respect to FIGS. 1-6, can be situated 
side-by-side to form a filter installation. 
In the plan view of FIG. 1, the system 100 has a substantially rectangular 
perimeter, which, as can be seen in the two section views of FIGS. 2 and 
3, is preferably in the form of upright walls 105, 106, 107, 108 of poured 
concrete. The poured concrete forms a basin or vessel which is preferably 
embedded in a ground excavation, so that most of the vertical extent of 
the basin is below ground level 109. The basin has a central vertical axis 
110, an upper portion 111 having vertical perimeter walls, 105, 106, 107, 
108 and a lower portion 112 having side walls 113, 114, 115, 116, angled 
or tapered obliquely downwardly from the perimeter walls towards the axis 
and a bottom 117 wall which closes the side walls. The upper portion 
preferably has a substantially uniform cross section. An, e.g., 
cylindrical central column 118 extends upwardly along the axis, thereby 
defining, e.g., a cylindrical outer surface that is spaced from the basin 
walls 105-108 and 113-116 so as to define a substantially annular side 
volume 119 therebetween. The lower end of the column 118 is spaced from 
the bottom wall 117. A skirt or similar structure 120 can optionally 
extend from the lower end 121 of the central column toward the bottom 
wall, thereby encapsulating a bottom volume 122 which is in fluid 
communication with the side volume 119. 
A region of particulate filter media, such as sand, fills the side volume 
119 in the lower portion 112 of the basin and extends upwardly into the 
side volume of the upper portion 111 of the basin. For reasons that are 
explained more fully below, the media region can be understood as being 
constituted by two media beds 101,102. The media in the side volume in the 
lower portion of the basin is in fluid communication with the bottom 
volume 122 through openings 123 or the like in the skirt 120. A 
frustoconical apron or similar structure 124 is situated in the lower 
portion 112 of the basin, preferably surrounding the central column 118 
and extending obliquely toward the basin walls, thereby defining an 
annulus 125 of relatively small cross sectional area through which media 
above the apron 124 can flow toward the bottom volume 122. 
An inlet conduit 126 receives a substantially continuous flow of liquid to 
be filtered, i.e., infilt, from a source outside the basin. In the 
illustrated embodiment, the inlet includes a pipe section 127 which 
receives infilt at the upper portion of the basin for delivery through two 
vertical conduits 128, 129 to an elevation above the apron 124, where the 
infilt is distributed at a plurality of azimuthally spaced locations in 
the media in the side volume 119. Distribution members can preferably take 
the form of a plurality of distribution spokes 130,131, that extend 
radially from respective conduits 128,129 and are perforated along their 
radial extension such that infilt can be distributed substantially 
uniformly in the respective media beds 101,102 at an elevation 
substantially between the upper and lower portions 111, 112. The infilt 
then flows upwardly through the media in the side volume 119 of the upper 
portion of the basin and accumulates as a region 132 of filtrate. During 
this upward flow, the dirt and other solids are trapped in the media such 
that, upon emerging from the upper surfaces 133,134 of the media beds 
101,102, clean liquid accumulates in the side volume above the media beds 
in the upper portion of the basin, while the dirt in the infilt remains 
trapped in the media in the side volume. 
The basic operating principle of the present invention, is somewhat similar 
to that described in U.S. Pat. No. 5,277,829, the disclosure of which is 
hereby incorporated by reference. The infilt moves upwardly through the 
region of filter media, while the filter media moves downwardly, carrying 
the accumulated dirt therewith, whereupon, at the bottom volume 122 of the 
basin, the dirty media is transported to a media washing device 103 or 
104. The washing device provides a flow of washing fluid, preferably in 
counter current to the flow of dirty media, such that the dirt is flushed 
from the media and withdrawn from the filter system as reject flow via 
line 135 or 136, while the cleaned sand is deposited at the upper surface 
133,134 of the filter media beds. In other words, the filter media is 
substantially continuously regenerated. 
In the present invention, the dirty filter media is transported through a 
plurality of distinct tubes 137a, 137b and 138a, 138b extending through 
the central column 118, each tube having a lower, intake end 139 situated 
in the bottom volume 122 for drawing dirtied filter media through the 
skirt openings 123 from the side volume in the lower portion of the basin, 
into the column 118 and transporting the drawn media upwardly through the 
column to a discharge end 140 at an elevation above the filtrate level 141 
of the filtrate region 132. This transport of the media upwardly through 
the tubes, induces the downward movement of the filter media through the 
region of filter media. 
A plurality of the washing devices 103,104 are situated at least partly in 
the basin, each washing device located in an azimuthally distinct position 
above the region of filter media and fluidly connected to at least one of 
the transport tubes 137,138. Preferably, each tube delivers a 
substantially continuous flow of dirtied media to only one washing device. 
The washing device induces contact of the delivered dirtied media with a 
flow of wash water such that the wash water carries the dirt in the dirty 
media out of the basin and the clean filter media drops toward the region 
of filter media where a plurality of media distribution devices 142,143 
associated respectively with the plurality of washing devices 103,104, 
direct the dropping cleaned media to a respective plurality of distinct 
locations on the region of filter media. 
An outlet line 144, in fluid communication with the filtrate region 132, 
draws a flow of filtrate from the basin. 
As is evident from FIGS. 1 and 3, the source of infilt can be an elongated 
trough 145 which runs alongside a plurality of systems 100 situated 
side-by-side to form a large filter installation or plant. Similarly, 
another trough 146 can run along the installation, for receiving and 
carrying away the filtrate contributed by the outlet line 144 of each 
system. As shown in FIG. 3, the horizontal run of inlet conduit 127 from 
the infilt trough 145 includes branches which each deliver the infilt to 
the vertical conduit portion 128,129 which passes respectively through 
each wash device 103,104. The filtrate level 141 in the filtrate region is 
established by the elevation of a weir 147 which forms the inside of the 
filtrate trough 146 such that, during operation, the filtrate level in the 
filtrate region is at or above the liquid level in the horizontal run 127 
of the inlet conduit. 
The reject from each of the two wash devices 103,104 in the system 100, 
flows first horizontally and then downwardly via lines 135,136 to a common 
removal pipe 148 that also receives contributions from each of the systems 
in the installation. 
As shown in FIGS. 1 and 3, the two wash devices 103,104 are preferably 
situated equidistantly from the central column 118, and preferably 
situated midway between the column 118 and the perimeter walls 106,108. 
The vertical run 128, 129 of each distribution conduit, passes through a 
respective media distribution device 142,143 associated with a wash 
device, and is approximately equidistantly situated between the central 
column and the perimeter walls in the upper portion of the basin. Below 
the radial distribution spokes 130,131, however, the basin walls taper 
inwardly to form a natural funnel which, in cooperation with the apron 124 
on the central column, limits the downward flow of dirty media. 
It can be appreciated that the basin as shown in FIGS. 1-3, can be 
constructed from a wooden form for receiving poured concrete. If the wall 
deviations are angular, the construction contractor need not dig a 
cylindrical hole in the ground, nor fabricate cylindrical or frustoconical 
wall surfaces. 
FIG. 10 shows the preferred way in which a basin can be constructed for 
implementing the present invention with a substantially conical lower 
basin. The basin 200 has four rectilinear perimeter walls that are first 
poured into a rectilinear excavation, in a conventional manner. These 
walls include four side walls 201 and a bottom wall 202. When the concrete 
has dried, a tapered liner, preferably of thin stainless steel, is 
inserted such that all or a portion of the upper edge of the liner 
contacts the perimeter walls 201, thereby leaving a tapered volume between 
the lower portion of the side walls and the lower surface of the liner. 
This tapered volume 204 is then backfilled with, for example grout, or 
other material which upon hardening, provides a rigid foundation for the 
tapered liner. The tapered liner 203 can be substantially frustoconical in 
the sense that a small diameter opening 205 is provided on the axis so to 
rest against bottom wall 202 and form a boundary for the lower volume 122 
(as shown in FIG. 3). 
The liner 200 as placed within basin walls 201 against wall 202, has 
downwardly arched side edges 206 which abut walls 201 and high points 207 
that fit into the corners 208. The shape of the liner is preferably 
similar to that resulting from making four mutually perpendicular, 
non-intersecting vertical cuts through a frustoconical steel member. 
Precise fitting of the liner 203 against the walls 201,202 is not 
necessary, in that the grout or other backfill material will, upon drying, 
provide sufficient barrier to the loss of sand or water. 
FIG. 4 is a section view along line 4--4 of FIG. 2, showing the skirt 120, 
the inside surface of the central column 118, and four transport tubes 
137,138 which are part of an air lift sub-system, to be described in 
detail below. For present purposes it should be understood that two of the 
transport tubes 137a, 137b service one of the wash devices 103, whereas 
the other two transport tubes 138a, 138b service the other wash device 
104. Thus, the central column 118 includes a sub-system which services a 
plurality of wash devices within the same basin. 
Divider structure (shown in phantom at 149) can be provided to maintain 
isolation between the portion of the filter media region associated with 
each wash device, i.e., to maintain separation of the multiple filter beds 
101,102. Such separating wall could extend to an elevation above the 
filtrate level. Under such circumstances, separate channels leading to the 
filtrate trough 146 should be provided. It is preferred, however, that no 
dividing wall will be provided within the basin. Some co-mingling of dirty 
sand may thus be possible in the lower volume 122, and a co-mingling of 
the filtrate would occur in the region of filtrate 132, before the 
filtrate passes over the weir 147. 
Importantly, the dirty media transport tubes 137,138 do not pass through 
the filter media region, but rather are isolated therefrom as a result of 
their vertical traverse through the central column 118. The central column 
as shown in FIG. 4, provides sufficient space for service operations to be 
performed on the transport subsystem, without draining the media from the 
basin. 
FIG. 5 shows the preferred relationship of each wash device 103' and 
associated distribution member 142, to the vertical run 128' of the inlet 
conduits and to the media transport tubes 137. Although certain details 
shown in FIG. 5 differ somewhat from the arrangement shown in FIGS. 1-4, 
the wash and distribution functionality thereof is substantially 
equivalent so that the primed numeric identifiers denote functionally 
similar structure to that shown in FIGS. 1-4. The media transport tube 
137' is isolated from the region of filtrate 132' as it passes upwardly to 
an elevation above that of the level 141' of filtrate in the filtrate 
region 132. The dirty media is deposited into a wash chamber 150, which 
includes a baffled lower portion 151 which has a substantially zig-zag 
shape. The lower end 152 of the baffled portion enters the upper, covered 
portion 153 of the media distribution member 142' such that, the discrete 
particles of media fall downwardly through the wash chamber, baffles, and 
into the distribution member. The chamber, baffle, and distribution member 
are isolated from the filtrate in the filtrate region 132' above the 
surface 133' of the media region. The distribution member 142' has an open 
bottom 154 in contact with the upper surface 133' of the media. As cleaned 
media falls into the distribution member, the media accumulates therein to 
form a reservoir 155 of clean media. This assures that a sufficient supply 
of clean media is available to pass out of the distribution member at 154 
to the apex of the filter bed 101', without creating a gap between the 
lower, open end 154 of the distribution member and the filter media bed 
101'. 
The flow of wash liquid passes upwardly from the filter bed 101' into the 
distribution member 142', and continues through the baffle portion 151 of 
the wash device where the dirt experiences a counterflow which carries the 
dirt upwardly and out through the reject conduit 135'. 
The distribution member 142' preferably has a frustoconical shape at the 
lower portion 156, with the smaller diameter defining the opening 154 and 
the larger diameter 157 defining the maximum width of the distribution 
member. The upper portion 158 of the distribution member could be 
cylindrical, or frustoconical in the opposite orientation to the lower 
portion, but with the difference that the upper surface 153 of the 
distribution member is closed except for the penetration by the baffle 
portion 152 of the wash device. 
All of the wash device 103' is isolated from the filtrate in the filtrate 
region 132'. Because the level 141' of filtrate in the filtrate region is 
above the level 159 at a which the reject flow passes out of the wash 
device, there is a net hydraulic pressure differential which maintains the 
flow of filtrate from the filter bed 101' directly into the open lower end 
154 of the media distribution member 142'. 
The wash and distribution sub-system described immediately above, provides 
the important advantage in that none of the dirt present in the wash and 
distribution system has any path for entering into the filtrate region 
132'. The shape of the distribution member 142' maximizes the reservoir 
155 of clean sand available for flow into the filter bed 101', while 
minimizing the extent of the surface area 133' of the top of the filter 
bed that has been removed as a source of filtrate flow into the filtrate 
region 132'. 
It can be appreciated from FIG. 5, that the horizontal portion of the inlet 
line 127' can be at an elevation above the basin, rather than at an 
elevation corresponding to the filtrate level 141' in the filtrate region. 
FIG. 6 schematically shows the preferred air lift media transport 
sub-system as implemented in the system shown in FIG. 3. Like numerals 
refer to like components in the two views. 
The central column 118 is preferably cylindrical with an open bottom 121 
and an airtight, selectively removable top 160. Sealed penetrations 161 
are associated with the air lift sub-system. The air lift sub-system 
provides a plurality of air lift tubes 137, as mentioned above, with each 
tube 137a having a lower end 139 that is at or slightly spaced from the 
bottom wall 117 in the bottom volume 122 of the basin, and an opposite end 
140 which discharges into one wash device. As shown in FIGS. 1 and 3, the 
portion of each transport tube between penetration 161 and discharge end 
140, can be a replaceable hose. A typical transport tube would have an ID 
of about 11/4 inch (3.5 cm). All of the transport tubes (only one is 
shown) would be supplied from a common source of compressed air, such as 
the manifold 162, via an individual supply line 163 which extends 
downwardly through the column 118 to an injector collar 164 near the open 
end of the column. It is expected that, due to the slurry-like consistency 
of the dirty sand at the lower portion of the basin, some water will 
separate from the slurry and rise in the central column to a level such as 
165. To prevent this water level from rising excessively, a branch 166 
from the compressed air manifold 162 delivers air into the column 118, 
thereby establishing a super-atmospheric pressure therein, which can be 
regulated to maintain the water at a desired level 165. The injection 
collar 164 for each transport tube is preferably below the water level in 
the column. 
The compressed air can be delivered in any known manner through the intake 
slots 167 of the tube 137 within the collar, thereby creating a vacuum at 
the lower end 139 of the tube, which draws the slurry of dirty media into 
the tube and transports it to the wash device. 
The supply line 163 for the compressed air is preferably a pipe having an 
ID of about 1/4 inch (0.6 cm), with air introduced into the injection 
collar at a pressure of about 20 psi (140 kp). 
The upflow of air in the tubes can be augmented by providing a second 
collar 168, in the form of a sleeve or the like which has perforations 169 
through which the superatmospheric air in the column flows radially 
inwardly such that the secondary air flow joins the primary air flow to 
reinforce the upward lift. The air pressure in the column 118, which 
induces the flow of secondary air, is preferably maintained at about 15 
psi (100 kp). 
In the embodiment of FIGS. 1-5, at least two transport tubes deliver dirty 
media to each washing device. By using at least two tubes per cell, rather 
than a single large tube to service all cells, higher velocities can be 
achieved, providing an improved scrubbing effect which tends to better 
separate the dirt from any adherence to the filtrate particulates, thereby 
facilitating rapid separation in the wash device. Furthermore, if a 
problem should arise with one transport tube, the other can maintain 
operation of the affected cell, albeit perhaps at a reduced efficiency. In 
any event, multiple transport tubes are provided within the central column 
118. 
FIGS. 7 and 8 illustrate an alternative embodiment 300 of a deep bed filter 
system, in which four wash and distribution members 201,202,203,204 are 
situated within a single basin. The basin has a substantially square 
perimeter 205, and each wash and distribution device is located in one 
quadrant along a diagonal between the central column 206 and the corners 
of the square. In the illustrated embodiment, the two inlet conduits 
207,208 penetrate one of the upright walls 209 and have branches 
210,211,212,213 that extend toward a respective distribution member 
214,215,216,217 and pass centrally therethrough. The reject lines 
218,219,220,221 from the four wash devices empty into a common reject pipe 
222 which penetrates the opposite side wall 223 of the basin. The filtrate 
region 224 above the filter media 225, has a level 226 above the inlet 
conduits 207,208, which spills over a weir 227 into a collection trough 
228. 
As in the previously described system 100, the infilt is distributed 
throughout the region of filtrate, via spoke-like members 229 which extend 
radially from a vertical run of inlet pipe in each quadrant. 
FIG. 9 schematically illustrates a third embodiment 400, in which the 
central column 401 contains not only the dirty sand transport tubes 402, 
but also the vertical run of a common inlet conduit 403. In this 
embodiment, the infilt distribution spokes 404 extend radially 
substantially from the axis 405 of the basin to a multiplicity of 
azimuthal locations in the various filter media beds. A large basin having 
a perimeter in plan view in the shape of, for example, an octagon, (with 
eight contiguous, inwardly angled lower walls) could easily accommodate 
four sets of wash and distribution members 406,407, i.e., one in each 
quadrant. Two of these are shown in FIG. 9, with their associated dirty 
media transport tube, wash chamber and baffle portion, and clean media 
distribution member. As in the previously described embodiments, the 
distribution member 407 has a closed top except for the penetration of the 
baffle portion or other fluid connection to the wash device, and an open 
bottom portion through which a continuity of media extends through the 
filter bed up into the distribution member. The lower portion of the 
distribution member is frustoconical, to provide the desired reservoir 
while minimizing the area of the filter bed that does not contribute to 
the flow of filtrate into the region of filtrate. 
As in the other embodiments, an apron 408 is provided from the central 
column 401 for shaping the multiple filter beds in the basin, and a skirt 
or similar structure 409 is provided at the bottom, for defining a bottom 
volume 410 where the lower ends 411 of the transport tubes 402 draw up the 
dirty media. The filtrate from the filtrate region 412 passes over a weir 
413 into a filtrate trough 414, and the reject lines 415 pass out of the 
basin wall. In this embodiment, the air transport tubes 402 would pass 
through the central column, on the outside of the single vertical run 403 
of inlet pipe. The spokes 404 extent radially through the central column 
401 and apron 408 into the media beds substantially between the upper and 
lower side volumes.