Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BIOFILTERING SYSTEM FOR TREATING
WASTEWATER EFFLUENT
The present invention pertains to improvements in the field of
wastewater treatment. More particularly, the invention relates to a
biofiltering
system for treating wastewater effluent leaving a septic tank.
Small wastewater treatment systems are typically designed so that raw
wastewater generated by a residence is discharged directly into a septic tank.
Once discharged into the septic tank, the effluent is allowed to partially
settle
and is then passed into a dosing chamber. From the dosing chamber, the
effluent
is fed into a filtering medium, e.g. a trickling bed filter, and then
collected in a
central drain pipe. Once the effluent is collected in the central drain pipe,
it is
then dispersed to a plurality of parallel drain pipes positioned generally
equidistant from one another across a specified area underneath the ground.
Each drain pipe has a plurality of orifices which allow the effluent to be
released into the surrounding environment.
Various filtering media and devices have been proposed for treating the
wastewater discharged from a septic tank. For example, US Patent
No. 5,206,206 discloses the use of pre-treated peat in biofilters for
wastewater
treatment. The pre-treated peat comprises a mixture of pre-sieved peat with a
Fe-containing compound and lime. A filter bed constituted of a layer of such a
pre-treated peat disposed between upper and lower layers of calcareous stones
is
arranged inside a filtration column. A rotary distribution system including a
perforated arm is used for distributing the wastewater to be treated on top of
the
filter bed. Since the pressure of the incoming wastewater serves to displace
the
perforated arm and the wastewater entering the distribution system contains
suspended particles of organic and inorganic materials, the suspended
particles
often clog the rotation mechanism, thus rendering the distribution system
inoperative.
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US Patent No. 5,618,414 also discloses a wastewater treatment system
utilizing peat as a filtering medium. Such a treatment system comprises a
container having an upper portion, a lower portion, at least one waster water
inlet in the upper portion of the container for receiving the wastewater, and
an
opening in the lower portion of the container for allowing the treated water
to
escape the container. At least one elongated hollow casing is mounted within
the lower portion of the container. The casing has an open bottom surface and
defines at least two treatment chambers within the container, each of the
treatment chambers containing a bed of peat for treating the wastewater. At
least one distribution means is also provided for distributing the wastewater
entering the container through the wastewater inlet into at least one of the
treatment chambers. Each distribution means comprises a water inlet pipe
connected to the wastewater inlet and leading above the casing and a trough
tiltably mounted on top of a corresponding casing. The trough extends along
the
casing and has two opposite sides. It defines at least one wastewater
receiving
means on one of the two sides and it is tiltable between a first position
whereat
the wastewater receiving means receives wastewater exiting the water inlet
pipe
and a second position whereat the wastewater received in the receiving means
flows out of the same. The trough also has counterweight means on its other
side for holding it in the first position while it is filled up and for
bringing it
back from the second position to the first position after the at least one
wastewater receiving means has been emptied. In addition, at least one
distribution plate is mounted above the bed of peat in one of the treatment
chambers defined by the corresponding hollow casing. This at least one
distribution plate comprises a plurality of channels projecting from the
trough,
each of the channels having an end for receiving wastewater flowing from the
trough so that the wastewater is divided into a plurality of flows each
flowing in
a corresponding channel, each of the channels also having at least one opening
for letting the wastewater drip into the corresponding treatment chamber.
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The use of the aforesaid tiltable trough in combination with the
distribution plate for distributing the wastewater over the bed of peat
presents
certain drawbacks. When the wastewater enters the container at a high flow
rate
and a large amount of wastewater is continuously received in the trough over a
period of time, the trough rapidly tilts between the aforesaid first and
second
positions so that the wastewater flowing therefrom floods the channels defined
in the distribution plate and flows over the edges of the distribution plate
and
onto the bed of peat. There is thus flooding of the bed of peat. Since any
given
peat has a predetermined biofiltering capacity over which the peat will not
act
as a biofiltering medium, flooding of the bed of peat will cause the
wastewater
to flow through the bed of peat at a flow rate greater than the biofiltering
capacity of the peat, resulting in a non-efficient removal of both organic and
inorganic materials.
In order to overcome the above drawbacks, Applicant has proposed in
co-pending Canadian Patent Application No. 2,304,935 filed on April 10, 2000
a biofiltering device comprising a housing having inlet means for receiving
the
wastewater to be treated and outlet means for discharging the treated
wastewater, a bed of peat disposed inside the housing between the inlet and
outlet means, the peat defining a biofiltering medium having a predetermined
biofiltering capacity, and means for aerating the peat. Such a device further
includes a fluid flow control system arranged over the bed of peat and in
fluid
flow communication with the inlet means, for distributing the wastewater
through the bed of peat in a manner such that when the wastewater enters the
system at a flow rate is greater than the biofiltering capacity of the peat,
the
wastewater discharged from the system flows through the bed of peat at a flow
rate no greater than the biofiltering capacity.
According to a preferred embodiment of the biofiltering device
described in the aforesaid application, the fluid flow control system
comprises
a fluid flow control unit having a chamber of variable volume in fluid flow
communication with the inlet means and a member provided with a plurality of
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spaced-apart discharge orifices in fluid flow communication with the chamber
and facing the bed of peat, the volume of the chamber varying in response to a
variation of the flow rate of the wastewater passing through the chamber and
increasing when the flow rate of the wastewater is greater than the
biofiltering
capacity. The discharge orifices each have a dimension selected so that the
wastewater discharged from the flow control unit through the orifices flows
through the bed of peat at a flow rate no greater than the biofiltering
capacity
when the flow rate of the wastewater passing through the chamber is greater
than the biofiltering capacity.
The above biofiltering device is typically capable of treating up to about
550 liters of wastewater per day. As described in Canadian Application
No. 2,304,935, a plurality of such devices can be connected by means of
conduits to a distributor box which in turn is in fluid flow communication
with
the septic tank, for handling larger quantities of wastewater. Generally, up
to
ten biofiltering devices can be so connected in order to treat up to about
5,500
liters of wastewater per day. When it desired to treat on a daily basis
quantities
larger than 6,000 liters, the installation of more than ten biofiltering
devices
becomes costly and complex.
It is therefore an object of the present invention to overcome the above
drawback and to provide an improved biofiltering system which utilizes peat as
biofiltering medium and which can efficiently treat large quantities of
wastewater irrespective of the flow rate thereof.
According to one aspect of the invention, there is provided a biofiltering
system for treating wastewater effluent for dissipation through an effluent
absorption area and into earth beneath the effluent absorption area. The
biofiltering system of the invention comprises a bed of peat overlying the
effluent absorption area, the peat defining a biofiltering medium having a
predetermined biofiltering capacity; a plurality of spaced-apart, elongated
distribution conduits arranged over the bed of peat for distributing the
wastewater effluent through the bed of peat, each distribution conduit having
a
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plurality of discharge orifices spaced longitudinally therealong; and fluid
flow
regulating means cooperating with the distribution conduits for supplying the
wastewater effluent to the bed of peat in a manner such that wastewater
discharged from the distribution conduits through the orifices flows through
the
bed of peat at a flow rate no greater than the biofiltering capacity of the
peat.
According to another aspect of the invention, there is provided a
biofiltering system for treating wastewater effluent for dissipation through
an
effluent absorption area and into earth beneath the effluent absorption area,
the
system comprising a bed of peat overlying the effluent absorption area, the
peat
defining a biofiltering medium having a predetermined biofiltering capacity,
and a fluid flow regulating unit for allowing the wastewater effluent to be
treated to flow at a flow rate greater than the biofiltering capacity of the
peat,
while not exceeding a predetermined threshold value. The biofiltering system
according to the invention further includes a predetermined number of spaced-
apart, elongated distribution conduits arranged over the bed of peat and in
fluid
flow communication with the flow regulating unit, each distribution conduit
having a plurality of discharge orifices spaced longitudinally therealong, for
distributing the wastewater effluent through the bed of peat in a manner such
that wastewater discharged from the distribution conduits through the orifices
flows through the bed of peat at a flow rate no greater than the biofiltering
capacity of the peat.
According to a preferred embodiment of the invention, the number of
distribution conduits, the inner dimension of each distribution conduit and
the
dimension of each discharge orifice are selected in accordance with the
predetermined threshold value so that the wastewater discharged from the
distribution conduits through the orifices flows through the bed of peat at a
flow rate no greater than the biofiltering capacity of the peat, and
preferably
equal thereto.
According to another preferred embodiment, the distribution conduits
are arranged in spaced-apart parallel relation to one another and are
connected
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downstream to at least one header conduit in fluid flow communication with
the flow regulating unit, the distribution conduits extending laterally of the
header conduit. Preferably, there are two header conduits arranged in spaced-
apart parallel relation to one another, the header conduits being connected
downstream to a supply conduit extending laterally thereof; the supply conduit
is connected downstream to a transport conduit in fluid flow communication
with the flow regulating unit.
In a particularly preferred embodiment, the distribution conduits, the
header conduits, the supply conduit and the transport conduit each have a
circular cross-section with a respective inner diameter. The inner diameter of
each distribution conduit is smaller than the inner diameter of each header
conduit and the inner diameters of the header conduits and supply conduit are
smaller than the inner diameter of the transport conduit. Such an arrangement
enables the wastewater discharged through the orifices to flow at a
substantially uniform flow rate.
According to yet another preferred embodiment, the fluid flow
regulating unit comprises a reservoir having inlet means for receiving the
wastewater effluent to be treated and outlet means coupled to a valve for
discharging the wastewater effluent at a flow rate greater than the
biofiltering
capacity of the peat, while not exceeding a predetermined threshold value; the
transport conduit is connected downstream to the valve. Preferably, the outlet
means include an outlet conduit connected upstream to the valve. Thus, when it
is desired to treat about 6,000 to about 30,000 liters of wastewater per day,
the
valve is operated so that the flow rate of the wastewater effluent discharged
from the reservoir will not exceed a threshold value within a range of about
250 to about 1250 ~ /hr. For a threshold value within such a range, the number
of distribution conduits generally ranges from about 6 to about 18, and the
inner diameter of each distribution conduit generally ranges from about 25 to
about 50 mm. The discharge orifices, on the other hand, are generally circular
and each have a diameter ranging preferably from about 2 to about 8 mm.
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When the peat used is sphagnum peat having a biofiltering capacity of about 20
~/hr, the orifices each have a diameter of about 6 mm. When using sphagnum
peat having a biofiltering capacity of about 22 ~/hr, the orifices each have a
diameter of about 5 mm. In the case of sphagnum peat having a biofiltering
capacity of about 24 ~/hr, the orifices each have a diameter of about 3 mm.
Such a selection of parameters ensures that the wastewater discharged from the
distribution conduits through the orifices will flow through the bed of peat
at a
flow rate no greater than the biofiltering capacity of the peat.
According to still another preferred embodiment, the biofiltering system
further includes fluid flow control means arranged between the bed of peat and
the distribution conduits for receiving the wastewater discharged therefrom
and
causing the wastewater to flow throughout substantially the entire bed of
peat.
Preferably, the fluid flow control means comprises an elongated, horizontally
extending porous membrane having upper and lower surfaces with the lower
I S surface contacting the peat. The membrane is capable of spreading the flow
of
the wastewater discharged from the distribution conduits as the wastewater
flows through the membrane from the upper surface to the lower surface, and
into the bed of peat.
In a particularly preferred embodiment, the membrane is a multilayered
membrane comprising upper and lower layers formed of non-woven fibers and
each having a predetermined density, and an intermediate layer disposed
between the upper and lower layers and formed of the non-woven fibers, the
intermediate layer having a density less than the predetermined density. The
upper and lower layers each have a plurality of spaced-apart apertures
extending therethrough.
According to a further preferred embodiment, the bed of peat, the
distribution conduits, the header conduits, the supply conduit, the transport
conduit and the valve are disposed inside a bottomless ventilated housing
resting on the effluent absorption area, the reservoir being disposed outside
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CA 02314689 2001-05-03
the housing with the outlet conduit extending through an opening formed in a
sidewall of the housing. In such an embodiment, the biofiltering system
further
includes means for aerating the peat. Preferably, the means for aerating the
peat
comprise a network of interconnected, open-ended aeration conduits each
having first and second end portions and an elongated intermediate portion
extending therebetween and horizontally through the bed of peat at a
predetermined depth, the first and second end portions of each aeration
conduit
extending upwardly from the intermediate portion and having respective free
ends disposed at a predetermined heights above the bed of peat with the free
end of the first end portion being disposed above the free end of the second
end
portion so as to permit air circulation in the aeration conduits. The
intermediate
portion of each aeration conduit is provided with a plurality of aeration
orifices
spaced therealong for discharging air into the bed of peat.
According to still a further preferred embodiment, the biofiltering
system further includes sampling means enabling a sample of treated
wastewater to be collected for analysis. Preferably, the sampling means
comprise a horizontal tray-like member disposed inside the housing adjacent an
other sidewall thereof and at a predetermined depth in the bed of peat, a
guide
member connected to the tray-like member and a drip conduit in fluid flow
communication with the guide member and extending through the other
sidewall, the drip conduit having a free end disposed exteriorly of the
housing.
The tray-like member has a main fluid-receiving surface extending along an
inclined plane for causing drops of treated wastewater received on the main
surface to flow in a direction towards the other sidewall and the guide member
has a guide channel arranged to receive the drops of treated wastewater from
the main surface for guiding the drops into the drip conduit. Preferably, the
tray-like member has two secondary fluid-receiving surfaces disposed opposite
one another and each extending along an inclined plane for causing drops of
treated wastewater received on the secondary surfaces to flow in a direction
towards the main surface.
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The biofiltering system according to the invention enables one to
efficiently treat large quantities of incoming wastewater effluent
irrespective of
the flow rate thereof.
Further features and advantages of the invention will become more
readily apparent from the following description of a preferred embodiment as
illustrated by way of example in the accompanying drawings, in which:
Figure 1 is a vertical cross-sectional view of a biofiltering system
according to a preferred embodiment of the invention, arranged downstream of
a septic tank (not shown), for treating the wastewater effluent leaving the
septic
tank;
Figure 2 is a horizontal cross-sectional view of the biofiltering system
shown in Fig. 1;
Figure 3 is an elongated vertical cross-sectional view of part of the
biofiltering system shown in Fig. l;
Figure 4 is an enlarged vertical cross-sectional view showing a detail of
the biofiltering system illustrated in Fig. 1;
Figure 5 is a partial sectional view of a fluid flow control membrane
used in the biofiltering system shown in Fig. 1;
Figure 6 is a partial sectional view taken along line 6-6 of Fig. 2;
Figure 7 is a top plan view of a sampling unit used in the biofiltering
system shown in Fig. 1 and enabling a sample of the treated wastewater
effluent to be collected for analysis;
Figure 8 is a side elevational view shown in Fig. 7; and
Figure 9 is an end elevational view of the sampling system shown in
Fig. 7, taken from the left-hand side of Fig. 7.
Refernng first to Fig. 1, there is illustrated a biofiltering system which is
generally designated by reference numeral 10 and serves to treat the
wastewater effluent leaving a septic tank (not shown) for dissipation through
an effluent absorption area 12 and into the earth 14, the effluent absorption
area
12 consisting of a layer of crushed stones. The biofiltering system 10
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comprises a bed of peat 16 overlying the effluent absorption area 14, a fluid
flow regulating unit 18 for allowing the wastewater effluent to be treated to
flow at a flow rate greater than the biofiltering capacity of the peat, while
not
exceeding a predetermined threshold value, and a fluid distribution unit 20 in
fluid flow communication with the flow regulating unit 18 for distributing the
wastewater effluent through the bed of peat 16. The bed of peat 16 and the
fluid distribution unit 20 are disposed inside a bottomless ventilated housing
22
resting on the effluent absorption area 12.
The housing 22 comprises a foundation formed of four concrete
sidewalk 24, 26, 28 and 30 (sidewalk 26 and 30 being shown in Fig. 2) resting
on a footing 32 and on which are mounted four wooden sidewalk, only three
being shown and designated by reference numerals 34, 36 and 38, and a roof
40 having upper lower windbraces 42 and 44 and a plurality of trusses 46. The
roof 40 further includes a sheet metal roofing 48. The wooden sidewalk and
the lower windbraces are covered interiorly with STYROFOAM (trademark)
panels 50. The wooden sidewalk are also covered exteriorly with metal sheets
52. A pair of spaced-apart ventilation conduits 54 extend through the roof 40.
The fluid flow regulating unit 18 comprises a reservoir 56 having an
inlet conduit 58 connected to a dosing chamber (not shown) in fluid flow
communication with a septic tank (also not shown) for receiving the
wastewater effluent leaving the septic tank, and an outlet conduit 60
connected
to a valve 62 for discharging the wastewater effluent at a flow rate greater
than
the biofiltering capacity of the peat, while not exceeding a predetermined
threshold value. The reservoir 56 is disposed outside the housing 22 and
buried
in a mound of sand 68, the outlet conduit 60 extending through an opening 70
formed in the concrete sidewall 24. The valve 62 which is provided with a
flowmeter 64 is disposed inside the housing 22. The reservoir 56 typically has
a volume of about 8.5 cubic meters for handling up to about 6,000 liters/day
of
wastewater effluent, or a volume of about 43 cubic meters for handling up to
about 30,000 liters/day of wastewater effluent. If the wastewater effluent
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entering the reservoir 56 has a flow rate greater than the threshold value,
the
excess wastewater will simply accumulate in the reservoir 56. In such a case,
the valve 62 will partially close to compensate for the increased head of
wastewater in the reservoir 56, thereby ensuring that the wastewater effluent
discharged from the reservoir 56 through the valve 62 will flow at a flow rate
not exceeding the threshold value.
As best shown in Figs. 2-4, the fluid distribution unit 20 comprises a
predetermined number of elongated distribution conduits 72 of circular cross-
section arranged in spaced-apart parallel relation to one another over the bed
of
peat 16. Each distribution conduit 72 has a predetermined inner diameter and a
plurality of circular discharge orifices 74 (shown in Fig. 4) spaced
longitudinally therealong with each orifice 74 having a predetermined
diameter. The distribution conduits 72 are connected to a pair of spaced-apart
parallel manifold-type header conduits 76 of circular cross-section extending
laterally of the distribution conduits 72. The header conduits 76 are in turn
connected to a supply conduit 78 of circular cross-section extending laterally
of
the header conduits 76 and connected to a transport conduit 80 of circular
cross-section. The latter conduit is connected to the valve 62. The number of
distribution conduit 72, the inner diameter of each distribution conduit 72
and
the diameter of each discharge orifice 74 are selected in accordance with the
aforementioned threshold value so that the wastewater discharged from the
distribution conduits 72 through the orifices 74 flows through the bed of peat
16 at a flow rate no greater than the biofiltering capacity of the peat, and
preferably equal thereto. As shown in Fig. 4, each distribution conduit 72 is
provided at opposite ends thereof with vent orifices 82 for venting entrapped
air during initial filling of the distribution conduits 72 with wastewater.
The inner diameter of each distribution conduit 72 is smaller than the
inner diameter of each header conduit 76, and the diameters of the header
conduits 76 and supply conduit 78 are smaller than the inner diameter of the
transport conduit 80. Typically, the distribution conduit 72 each have an
inner
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diameter of about 2.5 cm, the header conduits 76 and supply conduit each have
an inner diameter of about 5 cm, and the transport conduit has an inner
diameter of about 10 cm. Such an arrangement of conduits causes the
wastewater discharged through the orifices 74 to flow at a substantially
uniform flow rate. In Figs. 1-3, the respective dimensions of the inlet
conduit
58, outlet conduit 60, distribution conduits 72, header conduits 76, supply
conduit 78 and transport conduit 80 are disproportionate the other components
of the system 10, for ease of illustration.
An elongated, horizontally extending porous membrane 84 is arranged
between the bed of peat 16 and the distribution conduits 72 for receiving the
wastewater discharged therefrom and causing the wastewater to flow
throughout substantially the entire bed of peat 16. As shown in Figs. 4 and 5,
the membrane 84 has upper and lower surfaces 86,88 with the lower surface 88
contacting the peat. The distribution conduits 72 contact the upper surface 86
of
the membrane 84 with the discharge orifices 74 facing the membrane. The
membrane 84 is a multilayered membrane comprising upper and lower layers
90,92 formed of non-woven polypropylene fibers and having a density of about
0.1 g/cm3, and an intermediate layer 94 also formed of non-woven
polypropylene fibers, but having a density of about 0.05 g/cm3. The upper and
lower layers 90,92 each have a plurality of spaced-apart apertures 96
extending
therethrough and formed by piercing the layers 90,92 with needles. The
membrane 84 is capable of spreading the flow of wastewater discharged from
the distribution conduits 72 as the wastewater flows through the membrane
from the upper surface 90 to the lower surface 92, and into the bed of peat
16.
Thus, the membrane 84 constitutes a fluid flow control unit adapted to receive
the wastewater discharged from the distribution conduits 72 and cause the
wastewater to flow throughout substantially the entire bed of peat 16.
The treated wastewater effluent discharged from the system 10 is
dissipated through the effluent absorption area 12 and into the earth 14. The
peat filters suspended particles of organic and inorganic materials present in
the
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wastewater effluent. The anti-microbial properties of the peat combined with
those of fungi and actinomycetes present in the peat contribute to eliminating
fecal coliforms.
As shown in Figs. l, 2, 3 and 6, a network 98 of interconnected, open-
ended aeration conduits 100 is provided for aerating the peat. Each aeration
conduit 100 is generally U-shaped with two upwardly extending portions
102,104 and an intermediate portion 106 extending therebetween and
horizontally through the bed of peat 16 at a predetermined depth. The conduit
portions 102 and 104 extend through openings 108 (only one shown in Fig. 6)
formed in the membrane 84, and have respective free ends 110 and 112
disposed at predetermined heights above the bed of peat 16, the free end 110
of
conduit portion 102 being disposed above the free end 112 of conduit portion
104 so as to permit air circulation in each aeration conduit 100. Typically,
the
free end 110 of conduit portion 102 is disposed at a height of about 2.5
meters
above the bed of peat 16, whereas the free end 112 of conduit portion 104 is
disposed at a height of about 0.3 meter above the bed of peat 16. As shown in
Fig. 6, the intermediate portion 106 of each aeration conduit 100 is provided
with a plurality of longitudinally spaced-apart aeration orifices 114 for
discharging air into the bed of peat. Each aeration orifice 114 is oriented
downwardly.
As shown in Figs. 1, 2 and 3, the biofiltering system 10 includes a
sampling unit 116 enabling a sample of the treated wastewater to be collected
for analysis. The concrete sidewall 28 of the housing 22 being provided with
an
integral column 118 of concrete having a sampling hole 120. The sampling unit
116 which is illustrated in more detail in Figs. 7-9 comprises a horizontal
tray-
like member 122 disposed adjacent the sidewall 28 and at a predetermined
depth in the bed of peat 16, a guide member 124 connected to the tray-like
member 122, and a drip conduit 126 in fluid flow communication with the
guide member 124 and extending through the sidewall 28 and into the sampling
hole 120. As shown in Figures 7-9, the tray-like member 122 has a main fluid-
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receiving surface 128 extending along an inclined plane for causing drops of
treated effluent received on the main surface 128 to flow in a direction
towards
the sidewall 28 (shown in Figs. 1-3). The guide member 124, on the other hand,
has a guide channel 130 arranged to receive the drops of treated wastewater
from the main surface 128 for guiding the drops into the drip conduit 126. The
tray-like member 122 further has two secondary fluid-receiving surfaces
132,134 disposed opposite one another and each extending along an inclined
plane for causing drops of treated wastewater received on the secondary
surfaces 132,134 to flow in a direction towards the main surface 128.
The sampling hole 120 is closed with a removable cover 136. Removal
of the cover 136 enables one to gain access to the sampling hole 120 and, by
using an elongated spoon-shaped sampling instrument, one can take a sample of
the treated wastewater dripping into the hole 120 for analysis of the sample.
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