Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
021 99 682
Title: WASTEWATER TREATMENT APPARATUS
FIELD OF THE INVENTION
This invention relates generally to municipal and industrial
wastewater treatment apparatus, and is concerned more particularly with
pre-treatment or so-called "headworks" apparatus used to treat raw
wastewater (sewage) as a preliminary to biological treatment for removal
of organic material.
BACKGROUND OF THE INVENTION
When typical raw wastewater arrives at the treatment plant it
may be septic (odorous) and contain dissolved gases, greases, oils, rags,
plastics of all sorts, grit, rocks, wood particles and a variety of other
undesirable materials. Therefore pre-conditioning, or pre-treatment, is
essential. Traditionally, preconditioning is accomplished by individual
screening and grit removal devices, arranged in series. Scum is removed
at primary or secondary clarifiers where it is collected and added to
biological sludge from the main treatment process, and is a constant source
of day-to-day operating problems. This technology also fails to prevent
rags, and other fibrous material from finding their way into the main
treatment units where they clog pumps and cause other problems.
Normally these individual units are placed close to the plant inlet, where,
in anticipation of future plant expansion, they occupy a large area. Also it
is often necessary to house some units, depending on the climatic
conditions and the particular equipment that has been selected.
This approach is costly in terms of initial plant design and
construction costs, and also in terms of on-going operating costs.
Operation of a plant of this type is fairly labor-intensive in that the
individual units often require personal supervision to ensure proper
operation. It is very difficult to automate or even partially automate a
sewage treatment plant designed on these principles.
Flow rate measurement normally is performed at a fairly
early stage in the treatment process in order to provide a means of
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monitoring the rate of sewage flow through the plant in comparison with
the design capacity of the plant. Typically, flow rate is measured
immediately after the fine screening, by passing the wastewater through a
partial flume having a restricted cross-section which causes the liquid
5 level to elevate. By measuring the liquid level within the restricted cross-
section, it is possible to measure the flow rate. The difficulty is that, at this
stage in the process, the wastewater still contains scum and other debris, as
well as bacteria that may tend to cause the flume become odorous.
SUMMARY OF THE INVENTION
An object of the present invention is to provide preliminary
treatment apparatus for wastewater which is designed to address the
problems of the prior art.
In one aspect, the invention provides a unitary wastewater
treatment apparatus having an inlet for raw wastewater and a treated
15 effluent outlet, and including a screen for receiving raw wastewater from
said inlet, the screen being designed to remove coarse particles from the
wastewater and having an outlet. The apparatus also includes a flume
having an inlet coupled to the fine screen outlet and a discharge. De-
ragging means is provided in the flume. The apparatus also includes
20 scum and grit removal means comprising a tank for receiving wastewater
from the flume and adapted to contain a body of water to a defined level.
The tank has an effluent outlet communicating with the treated effluent
outlet of the apparatus and a skimmer is provided for removing scum
from the surface of the body of the water in the tank. Pre-aeration means
25 and grit removal means are also provided in the tank. A chemical mixing
tank is located between the flume discharge and the effluent outlet from
the apparatus to permit chemical addition to the wastewater being treated.
For convenience, the term "headworks" will hereafter be
used to refer generally to preliminary treatment apparatus of the form
30 provided by the invention. Also, it should be noted that the term "de-
ragging" is intended to cover removal not only of rags as such, but also
other debris, fibrous or not, including plastic sheets, condoms and other
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items that might otherwise cause clogging, e.g. of pumps in the treatment
~ys~e~
A primary advantage of the headworks of the invention is
that it combines in one installation, a large number of functions that
traditionally have had to be provided by individual, custom-designed
treatment units. In its broadest aspect, the headworks combines the
functions of removal of grit and other debris, de-ragging, scum removal,
pre-aeration and chemical addition. In a particular embodiment that will
be described in more detail later, nine separate functions are in fact
combined in a single headworks installation, namely fine screening, de-
ragging, grit removal, scum removal, secondary de-ragging, flow
measurement, solids collection and disposal, pre-aeration and chemical
addition. Wastewater that has been treated in this way is a fairly clear
solution which primarily contains soluble organic matter (to be removed
in subsequent treatment steps).
Pre-aeration of the wastewater can be performed in the scum
removal tank by providing air diffusers in a lower region of the tank. It
has been found that pre-aeration can be performed in this way without
impairing the scum removal function. Scum rises naturally to the surface
of the body of liquid where it is removed by the scum skimmer. The air
diffusers can be arranged to cause controlled flow at a low level in the body
of water to encourage any grit that may remain to settle out.
Secondary de-ragging and flow measurement may be
performed at the downstream end of the scum removal tank.
Grit and other debris can be removed from the apparatus at
various points in the overall treatment process. While conventional grit
removal devices such as inclined screens can be used, according to another
aspect of the invention, a novel form of debris collection device is
provided. It is to be understood that a device of this form can be used not
only in a wastewater treatment apparatus of the form provided by the
present invention but also in other forms of wastewater treatment
apparatus in which debris removal is required. There is no limitation to
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use of this device in headworks.
According to this aspect of the invention, the debris collection
device comprises a tubular receptacle intended to be suspended from a
support in a generally upright orientation. The receptacle has a closed
lower end and an open upper end into which wet debris can be delivered.
The receptacle has a porosity selected to allow water to pass through while
retaining the debris within the receptacle. The receptacle is capable of
removal from the support for disposal of the debris contained therein.
In a specific aspect, the receptacle is a bag or sock, for example
of woven fabric having an appropriate mesh size selected to allow water to
pass through while retaining the particular debris that is to be collected.
For convenience, the term "sock" will be used in the following description
to refer to the receptacle. Alternatively the sock may be a rigid receptacle
constructed of metal or plastic or some other such material.
The support from which the sock is suspended can be a short
collar or tube arranged with its axis vertical. The upper end or mouth of
the sock is then simply drawn around the collar or tube and tied or
otherwise clamped in place. When the sock is full, it can be removed from
the support and either emptied or discarded as a whole.
The inlet to the sock can be through a second, similar type of
enclosure, or sock, of a smaller diameter, or cross-section, and which
contains larger perforations such that larger particles will be removed
prior to the liquid entering the sock proper. It will be possible to remove
this enclosure, for cleaning, without disturbing the contents of the sock
proper.
A debris collection device of the form provided by the
invention represents a marked improvement as compared with prior art
devices such as inclined screen filters, in which the debris simply slides off
the inclined screen (while the water passes through the screen) and
accumulates as a sludge. Odor problems often develop from such sludges
and in any event the sludge represents a disposal problem in itself. In
contrast, the disposal sock provided by the invention confines the debris in
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an receptacle which is easily manageable and disposable as a unit. If an
odor problem should develop, it is a simple matter to remove the sock and
dispose of the debris. However, odor problems are likely to be less
prevalent because the debris can dry out while it hangs suspended in the
5 sock.
While the material discharged from the fine screen and the
grit from the grit separator are de-watered in a sock, the scum that is
skimmed off of the top of the scum removal tank is directed to a scum
concentrator, from which it may be treated and disposed of separately.
10 Conventionally, the scum is added to other sludges for final disposal. This
procedure is most unsatisfactory because scum is not compatible with
conventional sludge digestion processes; the scum therefore is a source of
on-going day-to-day operating problems.
BRIEF DESCRIPTION OF DRAWINGS
In order that the invention may be more clearly understood,
reference will now be made to the accompanying drawings which
illustrate a particularly preferred embodiment of the invention by way of
example, and in which:
Fig. 1 is a schematic illustration of a conventional multiple
unit wastewater pre-treatment plant;
Fig. 2 is a perspective view of a unitary pre-treatment
apparatus of the form provided by the invention;
Fig. 3 is a schematic plan view of the apparatus of Fig. 2, with
the individual components of the apparatus shown in "exploded"
positions;
Fig. 4 is a perspective view from one side and above of a
screen forming part of the apparatus of Figs. 2 and 3;
Fig. 4a is a partial schematic plan view of a de-ragger shown
in Fig. 4;
Fig. 5 is a perspective view from the opposite side and to the
right in Fig. 4;
Fig. 6 is a perspective view showing the chemical mixing tank
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and the scum and grit removal tank of the apparatus, again in "exploded"
positions; and,
Fig. 6a is a schematic partial vertical sectional view in the
direction of arrow A in Fig. 6.
5 DESCRIPTION OF PREFERRED EMBODIMENT
Referring first to Fig. 2, a unitary headworks (pretreatment)
apparatus provided by the invention is generally denoted by reference
numeral 20. An inlet for raw wastewater is indicated at 22 and a treated
effluent outlet is represented by a pipe 24 which leads to downstream
10 treatment units indicated respectively at 26, 28 and 30 in Fig. 3. In a typical
sewage tertiary treatment plant, unit 26 may be a biological treatment
installation, unit 28 an effluent filter and unit 30 a sludge digestion and de-
watering unit. Since units of this type are conventionally used in sewage
treatment plants, they will be familiar to a person skilled in the art. For
15 this reason, and since the particular forms of these units are irrelevant to
the invention, the units will not be described in detail. A treated "clean"
water outlet from the overall treatment plant is indicated at 32.
In Fig. 3, the components of the headworks are shown in
exploded positions for clarity of illustration; Fig. 2 on the other hand
20 shows a practical example of a unitary headworks plant incorporating
those components. Referring primarily to Fig. 3, reference numeral 34
denotes a fine screen which receives raw wastewater from the raw
wastewater inlet 22 and which is designed to remove coarse particles from
the wastewater. An outlet from filter 34 is indicated at 36 and
25 communicates with a flume 38 having an inlet which is coupled to outlet
36, and a discharge 40. The flume incorporates a de-ragger which is not
specifically illustrated in Fig. 2 but which will be shown and described in
detail later.
In the preferred embodiment illustrated, flume 38 discharges
30 into a chemical mixing tank 42 which has an outlet pipe 44
communicating with a scum and grit removal tank 46. As will be
described in more detail later, tank 46 is provided with scum skimmers, air
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diffusers for pre-aeration of the wastewater, grit removal means, secondary
de-ragging means and a partial flume for flow measurement purposes.
These components are generally indicated in Fig. 3 and are shown in more
detail in later views. In Fig. 3, an air diffuser pipe is indicated at 48, a grit
5 collection sock at 50, a secondary de-ragger at 52 and a flow measuring
flume at 54. Also to be described in more detail later but not specifically
shown in Fig. 3 is a skimmer for removing scum from the surface of a
body of water in tank 46.
In the embodiment illustrated, the chemical mixing tank 42 is
10 shown located upstream of the scum removal tank 46. However, the
chemical mixing tank could be located downstream of the scum removal
tank if required.
Referring now more particularly to Fig. 2, it will be seen that
the components referred to previously are assembled into a unitary
15 pretreatment plant. Specifically, the fine screen 34 is supported by a grating
55 that is mounted on the top edges of the walls of the scum and grit
removal tank 46. Similarly, the chemical mixing tank 42 is supported by
brackets 56 from an end wall of tank 46. It should be noted that, in Fig. 6,
the tank walls are shown as being of reduced height, for convenience of
20 illustration.
By comparing Fig. 2 with the schematic illustration of a
conventional wastewater pretreatment plant as shown in Fig. 1, it will be
apparent that the invention provides a compact, unitary assembly that
offers numerous advantages as compared with the prior art. In the prior
25 art, multiple individual treatment units must be provided and, typically,
perform the functions indicated in Fig. 1. These units are often installed
in widely spaced locations, sometimes several hundred feet apart. As
indicated previously, this approach is costly both in terms of initial
planned design, construction costs, land use cost and in terms of on-going
30 operating costs. The headworks of the form provided by the invention, on
the other hand, can be assembled in a relatively small space. The
individual components can be factory prefabricated and shipped to the job
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site as a "kit". Typically, the tank 46 would be poured on site in concrete
and the remaining components would then be assembled onto the tank.
Overall, this technique should account for significant capital cost savings.
Further, the particular headworks apparatus provided by this
5 embodiment of the invention provides for nine separate treatment
functions that are integrated into the unitary assembly shown in Fig. 2.
These functions will be described in more detail later.
Referring now to Figs. 4 and 5, the fine screen 34 is shown in
some detail and in Fig. 4 part of flume 38 is shown associated with the fine
10 screen.
As perhaps best shown in Fig. 5, fine screen 34 is essentially
an inclined parabolic screen which includes a screen 58 of a shape that
tapers from an elevated "input" end 60 to a lower discharge end 62. The
input end 60 of the screen in effect forms a weir at one side of a tank 64
15 which receives raw wastewater from inlet 22 (Fig. 4). Thus, as the tank 64
fills, the raw wastewater will spill over the weir and flow down the
inclined screen 58. The screen has a mesh size selected to allow water to
pass through the screen while coarse particles are held back and flow down
the screen towards the discharge end 62. Water that passes through the
20 screen flows down an inclined surface 66 to an outlet 36 (Fig. 4).
In a conventional inclined screen, the debris that is held back
on screen 58 would simply be allowed to accumulate as sludge. However,
in accordance with the present invention, the debris is directed into a
novel debris collection device generally indicated by re~rence numeral 68.
25 In this embodiment, device 68 is a fabric sock which is suspended from a
tubular support or collar 70 mounted at the discharge end 62 of the
inclined screen 58. A band or clamp which holds the sock on support 70 is
indicated at 72. It will be seen that the sock is suspended in a generally
upright orientation and has a closed lower end 74 and an open upper end
30 76 into which wet debris falls from screen 58. In this embodiment, the
sock can be but is not limited to woven fabric and may be made, for
example, from DACRONTM or other appropriate material. The weave is
9 ~ ~ 1 g9 682
selected to provide the sock with a porosity which allows water to pass
therethrough while retaining grit and other debris within the sock. A tank
78 is provided below the sock to collect water that may drain from the sock
(see Fig. 4).
As was discussed previously, this form of debris collection
device has significant advantages over the prior art. The debris that is
flushed from screen 58 is collected and retained within the sock in a form
in which it can conveniently be handled for disposal. For example, when
the sock is full of debris and becomes odoriferous, the clamp 72 can be
removed and the sock taken off the support 70. The sock can then be
emptied, or the entire sock and its contents can be disposed of together. A
replacement sock can quickly be placed on collar 70 with minimum screen
"down time".
While the sock is suspended from collar 70, liquid can
continuously drain from the debris within the sock and the debris can at
least partially dry, minimizing odor problems.
It should be understood at this stage that the particular
application of the debris collection device 68 shown in Figs. 4 and 5 is an
example only and that the device may be used in other situations in
wastewater treatment plants. Another example is shown in Fig. 6 (to be
described).
It should also be understood that the particular form of debris
collection device may vary. For example, as noted previously, a rigid
receptacle may be used. The receptacle may take the form of a wire cage.
Conversely, a wire cage may be provided inside a sock of the form shown
in the drawings (flexible or rigid) in order to trap large solid particles before
they enter the sock proper. In one embodiment, the receptacle may
comprise a coarse wire mesh sock surrounded by an enclosure of finer
wire mesh or fabric.
Referring specifically to Fig. 4, it will be seen that flume 38
comprises a channel or trough that slopes downwardly from the screen
outlet 36 to a discharge end 40 of the flume at the chemical mixing tank 42
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(Fig- 3).
As mentioned previously, the flume 38 is provided with de-
ragging means, which are generally indicated at 80 in Fig. 4. The de-
ragging means comprises a series of cross members 82 which extend
5 transversely of the flume, generally between the tops of its sidewalls.
Suspended from each cross member are a series of fine bars 84 which
extend over substantially the entire height of the flume and are free at
their lower ends. In other words, the bars 84 simply hang from the cross
members 82. As such, the bars represent obstructions to water flowing
10 down the flume and rags, condoms, plastic pieces and other like debris
tend to be trapped by the bars. In another embodiment, the bars could be
supported at the bottom, e.g. by a plate that lies transversely on the bottom
wall of the flume, or the bars could extend horizontally across the flume
from a support at the side.
As best seen in Fig. 4a, the bars on adjacent cross members are
offset laterally with respect to one another so that the bars on a
downstream cross member tend to catch debris that might flow between
the bars that are immediately upstream.
The sidewalls of the flume are provided with seats 38a or
20 retainers for the ends of the cross members 82 so that the cross members
(together with the bars 84 that are suspended from them) can simply be
lifted out of the flume and any trapped rags or other debris manually
removed.
It should of course be understood that de-ragging means 80
25 can be located at any convenient point along the length of the flume and
may comprise any number of de-ragging assemblies each comprising a
cross member 82 and the bars 84 suspended therefrom.
Referring now to Fig. 6, it will be seen that the discharge end
40 of the flume 38 is located in a recess in a sidewall of the chemical
30 mixing tank 42. Tank 42 is simply a plain receptacle to which the flume 38
represents an inlet and pipe 44 an outlet. A body of water within the tank
is indicated at 86 and will normally extend to a level above the inlet of
021 99 682
11 -
pipe 44, depending on the flow rate of wastewater into the mixing tank. A
mechanical mixer 88 is provided in tank 42 for mixing of chemicals into
the wastewater. Alternatively, the tank 42 and the mechanical mixer 88
can be replaced by an inline static mixer. Means for delivering chemicals
5 into tank 42 have not been shown but may comprise any appropriate form
of chemical dispenser -- or the chemicals may be manually introduced
into tank 42.
As noted previously, chemical addition is preferably
performed upstream of the scum removal tank 46. Typically, the
10 chemicals are selected to cause agglomeration of particles within the
wastewater or to remove phosphorous. Sludge that may arise from this
chemical addition will often appear as a scum on the water or will settle
with the grit. By adding the chemicals upstream of tank 46, any such scum
or settled particles can be removed in tank 46, i.e. soon after it has been
15 formed. This contrasts with conventional practice where chemicals are
added in an aeration basin at a subsequent stage in the treatment process.
In this case, the chemicals often have an adverse effect on the biological
solids within the wastewater.
Scum removal tank 46 receives wastewater from the flume 38
20 via the chemical mixing tank 42, through pipe 44. In this embodiment,
the tank has an inlet box 90 into which pipe 44 discharges and the box has a
discharge pipe 92 that delivers incoming water to a relatively low level in
tank 46. Normally, box 90 will be at least partially full and pipe 92 will be
sized to provide an appropriate rate of flow of water into tank 46. An
25 effluent outlet from the tank is indicated at 94 and communicates with the
treated effluent outlet pipe 24 shown in Fig. 3.
Tank 46 is designed to contain a body of water to a defined
level denoted L (liquid level) in Fig. 6. A skimmer for removing scum
from the surface of the body of water is indicated at 96. Skimmers are in
30 fact provided at both ends of the tank, though only one skimmer has been
shown. The skimmer essentially comprises an endless belt 98 which
extends around rollers 100 and 102 disposed transversely with respect to
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the length of the tank. Belt 98 has a series of transverse flights 104 which
are designed to move across the surface of the body of water in tank 46 (to
level L) to move any scum on that surface into a scum collector 106
disposed generally at the center of the tank via a ramp 106a. Suitable drive
5 arrangements (not shown) are provided for rotating the belt 98 at a
relatively slow speed so that the lower run of the belt moves towards the
scum ramp 106a and collector 106. The other skimmer (not shown) moves
in the opposite direction and also delivers scum into collector 106, via a
second ramp 106a.
Collector 106 is mounted at a slight downward inclination to
the right as shown in Fig. 6 so that collected scum will tend to move in
that direction. A pipe 108 at the right hand end of the collector delivers
the scum liquid into a scum concentrator 110 comprising a cone bottom
tank in which the scum can accumulate at an upper level while liquid can
15 be withdrawn from below the scum through an outlet 110a at the bottom
of the tank. The liquid will be returned to inlet box 90. The concentrated
scum will be removed from the tank and disposed of separately and
therefore will not be a problem to the main treatment process.
The wall 46a of tank 46 at the extreme right hand (discharge)
20 end as seen in Fig. 6 is provided in its upper margin with a recess which
defines a weir 112 over which treated wastewater flows from tank 46. A
baffle 114 is provided between weir 112 and the scum skimmer 96 to
prevent scum from being drawn over the weir.
Immediately downstream of weir 112 is a wastewater launder
25 120 into which the de-scummed and de-gritted wastewater will flow. This
liquid will then flow through opening 118 into a second de-ragging flume
52 and then into partial flume 54 (both mentioned previously in
connection with Fig. 3), leaving via outlet 94. The secondary de-ragger has
not been shown in detail but essentially is the same as the de-ragger means
30 80 described in connection with Fig. 4. Again, the number of individual
de-ragger assemblies may vary, as may the spacing between the assemblies
and the gaps between the individual bars 84.
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In addition to the features already described, scum removal
tank 46 is provided with an air diffuser pipe 124 for pre-aeration of the
wastewater within tank 46, and grit removal means generally indicated at
126.
It will be seen that the bottom wall of the tank is contoured to
provide a longitudinally extending trough or sump 128 which is offset
towards one of the longitudinal sidewalls of the tank (the rear sidewall as
seen in Fig. 6). Fig. 6a shows the profile of the bottom wall of the tank in
some detail and illustrates this offset. Air diffuser pipe 124 extends
generally longitudinally of the sump 128 but again offset somewhat
towards the rear wall of the tank. Pipe 124 has diffuser outlets 130 that are
directed upwardly in the tank so as to induce in the wastewater a circular
"rolling" action as generally represented by the arrow denoted 132 in Fig.
6a.
To the right of sump 128 as seen in Fig. 6a, a portion 134 of
the bottom wall of the tank slopes towards the sump. The location and
direction of the air diffusers 130 are selected so that the rolling action
imparted to the wastewater tends to cause the water to flow downwardly
towards surface portion 134 again as shown in Fig. 6a. It has been found
that this rolling action tends to cause any grit that remains within the
wastewater to settle out onto surface 134 and flow into sump 128.
It can be seen from Figs. 6 and 6a that the bottom wall of
sump 128 is also contoured to slope inwardly from opposite ends of the
sump towards a low point in the center, indicated at 136 in Fig. 6. The grit
collection means 126 includes a pump 138 the suction side of which is
connected to a pipe 140 having an inlet that is positioned at this low point
136 in the bottom wall of the trough. Accordingly, grit that tends to
accumulate at this low point can be sucked out of the trough by pump 138.
The pump discharges upwardly into a highly efficient cyclone separator
142 which has a discharge 142a at its lower end fitted with a sock 144.
Again, the sock is essentially of the same form as the sock 68 described
previously in connection with Figs. 4 and 5. Accordingly, grit that is
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separated out by the cyclone can be collected in similar fashion to the
screening that is removed by the primary fine screen of Figs. 4 and 5. The
cyclone 142 may be a conventional unit of the type that is available from
Derrick Equipment Company of Houston, Texas under the trade mark
5 HYDROCYCLONE.
To summarize, the headworks provided by the invention, in
its preferred embodiment, is essentially a unitary treatment apparatus that
provides nine treatment functions that would normally be performed by
individual and separate treatment units in a conventional sewage
10 treatment plant. The first is the initial screening provided by fine screen
34 for removing relatively coarse debris. This is followed by de-ragging in
flume 38 and then by chemical addition in the chemical mixing tank 42.
In tank 46 there is an opportunity for grit and scum removal and a
secondary de-ragger. Scum is also removed and the wastewater is
15 subjected to pre-aeration from air line 124. Finally, flow measurement is
accomplished in flume 54.
The debris collection device provided by the invention, a
specific embodiment of which is shown in detail in Figs. 4 and 5 is also
believed to represent a significant advance in the art. For example, in the
20 field of aerated grit separators, conventional practice is to design the uniton the basis of separating out the grit only and leaving organic material in
suspension to be removed at the next treatment stage. The removed grit is
washed to ensure that it contains little or not organic matter. This is done
to eliminate the possibility of odors that may develop in the collected grit.
25 With the debris collection device of the invention, a completely enclosed
container for the grit is provided and the danger of odors is removed.
There is no longer a need for grit washing and an aerated grit separator can
be designed to remove settled organic particles as well as grit. For example,
the grit and scum removal tank 46 is in essence a combination grit
30 separator and primary clarifier. This greatly reduces the organic load on
downstream stages of the treatment process.
It will of course be appreciated that the preceding description
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relates to a particular preferred embodiment of the invention and that
many modifications are possible within the broad scope of the invention.
Some of those modifications have been described and others will be
apparent to a person skilled in the art.
