Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1140Z78
The present invention relates to a method and ap-
paratus for purifying sewaye by means of activated sludge.
In co-pending Canadian application serial number 318,258,
filed December 20, 1978, now Canadian patent 1,099,037,
issued April 7, 1981, sewage to be purified is treated in
a reaction tank, an activation or aeration tank, a sedimen-
tation tank and a sludge-return unit which picks up the
returning activated sludge from the sedimentation tank.
Sewage-treatment installations of this kind are intended to
considerably reduce the chemical and biological 2 demand,
and to eliminate nitrogen.
It is known that when sewage containing nitrates
is mixed with activated sludge the 2 is consumed after a
short time, while the released nitrogen from the resulting
nitrates escapes into the atmosphere. In the above noted co-
pending application a large part of the nitrogen is elimina-
ted, however, this cannot be controlled by a particular de-
sign or method of operation of the installation.
According to an aspect of the invention there is
provided a process for sewage purification by means of ac-
tivated sludge, wherein raw sewage to be purified is fed
successively to a reaction tank for aeration, to an aeration
tank, also for aeration, and to a sedimentation tank, a portion
of the raw sewage being fed directly to the aeration tank,
wherein settled and concentrated activated sludge is returned
from the sedimentation tank partly to the reaction tank
and partly to the aeration tank, and wherein, dependent upon
the oxygen content, the mixture in the reaction tank and the
mixture in the aeration tank are caused to rotate, and de-
pendent upon the oxygen content, are aerated and circulated,
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and wherein the activated sludge in the lcwer parts of the reaction
tank is circulated and thickened, characterized in that there are pro-
vided in parallel at least two treatment units, each comprising a
reaction tank and an aeration tank, and that Lhe treatment units are
aerated intermittently in an operation which, in other respects, is in
the parallel mcde.
According to a f~rther aspect of the invention there is pro-
vided a sewage purification apparatus for sewage purification which
includes a raw-sewage influent pipe, a reaction tank provided with
means for aeration dependent upon the oxygen content of the reaction
tank, an aeration tank provided with means for aeration dependent upon
the oxygen content of the aeration tank, a sedimentation tank, an
effluent pipe for re~oving clarified sewage from the sedimentation
tank, means including branch conduits for conveying settled and concen-
trated activated sludge in the sedimentation tank to the reaction and
aeration tanks, means including branch conduits for supplying raw sewage
to the reaction and aeration tanks and conveyor means located between
the reaction tank and the aeration tank, characterized in that there are
provided in parallel at least two treatment units, each comprising a
reaction tank and an aeration tank, including associated raw-sewage
supply means, activated-sludge-return means and aeration means, and
that an aeration-air distributor means feeds aeration-air intermit-
tently to the treatment units in a controllable ratio between periods
of aeration/non-aeration which periods are particularly dependent upon
the oxygen content of the aeration tanks.
Thus, according to the invention, nitrogen separation is
assured; and since nitrification of nitrogen-containing contaminants
in the sewage is dependent upon concentration, temperature and other
factors, the periods of residence for nitrification and denitrification
may differ in length. This improves purification, reduces power and plant
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costs and provides increased reliability.
According to the invention, the raw sewage is
purified as disclosed in the co-pending Canadian application,
but aeration of the respective tanks is not carried out
continuously. Instead, the aeration, activated-sludge and
reaction tanks are, preferably, in the form of at least two treat-
ment units of the same size arranged side by side and paral-
lel with each other, the treatment units being aerated con-
trollably and intermittently for a predetermined time until
nitrification is achieved. The necessary air-supply is
switched, by suitable means, after nitrification has been
achieved in a first unit from that unit to a second unit,
in which further biological purification and nitrification
is carried out; during this time, no air is supplied to the
first unit, for which reason the nitrates, developed in the
mixture of activated sludge and water, are consumed by
micro-organisms because the 2 content of the sludge mix-
ture is rapidly reduced; thus the released nitrogen can es-
cape into the atmosphere. Continuous repetition of this pro-
cedure eliminates the nitrogen which is normally unwanted
in the discharge.
Before being introduced into the aeration tank,
`` the incoming raw sewage is passed into the reaction tank
and is mixed with the return-sludge from the sedimentation
tank. This removes some of the nitrogen from the nitrate-
containing return-sludge, which means that the incoming
raw sewage is preliminarily cleaned, to a certain extent,
particularly as regards contamination and nitrogen-content
and only then passes to the aeration or activation tank.
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Further contaminants are removed from the sewage in the
aeration tank and, for this reason, draining the purified
sewage into a public sewer has fewer disadvantages than in
the case of conventional biological sewage treatment.
The purification process then proceeds as set forth
in the noted co-pending application; the necessary supply-
lines to the centre of the reaction tanks are provided with
branch lines, thus providing a selective supply, as ex-
plained in the co-pending application.
Aeration is also carried out, in the manner ex-
plained in the co-pending application, by means of a rotating,
continuous aeration bridge having aerators extending to the
bottom of the tanks. This not only produces aeration but
also keeps the activated sludge in suspension, both in the
aeration tank and in the reaction tank.
In order to achieve reliable elimination of nitro-
gen, 02-poor, 02-less and 02-rich zones are created tempor-
arily in different parts of the apparatus, by controlled
switching of the aeration air supply. This makes it pos-
sible to allow for changes in sewage volume, contamination,
temperature and the like, thus achieving optimal nitri-
fication and denitrification times and promoting the best
possible purification process.
The necessary aeration, the duration of which is
controlled, as already explained, by means comprising a
timed control-unit, is according to sewage volume; by means
of a compressor unit containing either one multi-stage com-
presser or a plurality of single-stage compressors adapted
to be operated simultaneously. These compressors are acti-
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vated by the t~ed control-unit, in conjunction with a probe
located in the aeration tank. Since the aeration pipes
within the tanks are supplied with air during active aera-
tion periods only, it is desirable for the air outlet aper-
tures in the pipes to be designed in such a manner that they
do not fill with water when the air-supply is shut off, but
remain filled with air. This may be achieved, with advan-
tage, by supplying a small amount of air to the inoperative
aerators, in order to prevent them from becoming blocked.
This air may be taken, through a bypass line and a control-
lable slide-valve, from a compressed-air equalizing container
used to damp-out surges of air.
The invention is explained hereinafter in conjunc-
tion with the examplary embodiment illustrated in the accom-
panying drawing, in which the single figure is a plan view of
a sewage-purification apparatus according to the invention,
comprising a two unit apparatus.
The drawing shows two reaction tanks 1 and 1.1,
two aeration tanks 2 and 2.1, respectively, associated with
the reaction tanks, a sedimentation tank 3 associated with
aeration tanks 2 and 2.1 and a return conveyor-device 4 for
transporting the activated sludge from sedimentation tank 3.
Reaction tanks 1 and 1.1 have surrounding walls 5 and 5.1,
respectively, separating them from aeration tanks 2 and 2.1
associated therewith. Each of the latter has a surrounding
wall 6 and 6.1, respectively. Reaction tank 1 and aeration
tank 2 constitute one unit 10 of the apparatus, while reaction
tank 1.1 and aeration tank 2.1 form the second unit 10.1
thereof. Each of these units comprises an aeration bridge
18 and 18.1, respectively, rotating in the direction of arrow
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7, the bridges being equipped with aerators 1.6 and 1.61,
respectively. Probes 9 and 9.1, are provided in units 10
and 10.1, respectively, of the apparatus, more particularly
in aeration tanks 2 and 2.1.
Raw sewage is fed, through an inlet 8, to the
sewage-treatment apparatus entering first i.nto a distribution
duct 1.9, and then passing, through lines 1.2 and 1.21, to
the centre of reaction tanks 1 and 1.1, respectively. The
mixture of sludge and sewage passes from the reaction tanks
around the periphery of walls 5 and 5.1 surrounding them and
into aeration tanks 2 and 2.1, where it is aerated and flows,
through outlets (not shown) and lines 10.2 and 10.21, respectively,
to the centre of sedimentation tank 3. The purified sewage
then passes, through an outlet 10.4 and a drain-line 10.5,
into a drainage ditch (not shown).
Concentrated activated sludge flows, through a line
3.1, into return device 4, passing thence, through supply-
: lines 2.4 and 2.41, provided with slide-valves (not shown),
to the centre of reaction tanks 1 and 1.1, respectively.
As explained in the noted co-pending application,
a part of the sewage is also fed to respective aeration tanks
2 and 2.1, through branch-lines 1.3 and 1.31 fitted with
slide-valves, while activated sludge is supplied from return
device 4 through branch-lines 2.3 and 2.31.
In this connection, raw sewage fed through line 8
may be passed, through a projecting structure 19, to distri-
buting duct 1.9. Concentrated activated sludge, taken from
the bottom of sedimentation tank 3, is lifted, in return
device 4, by means of a worm-pump or the like, so that it
may be fed, respectively, to units 10 and 10.1 of the apparatus.
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Moreover, as described in the co-pending application,
and not shown here in detail, part of the mixture emerging
from aeration tanks 2 and 2.1, and to be passed to the sedi-
mentation tank, is fed directly to return device 4.
Assuming that units 10 and 10.1 of the apparatus are
of the same size, it is desirable to feed them with equal
amounts of raw sewage through lines 1.2 and 1.21, respectively.
The air required for aeration is drawn in through
a compressor unit consisting, in the examplary embodiment
illustrated, of a compressor 15 for minimal loading (during the
night, for example), a compressor 16 for average loading
and a compressor 17 for maximal loading. Obviously it is
also possible to use a single compressor adapted to be
operated in several modes, or to use a plurality of compres-
sors of the same or different capacities. The air drawn in
passes to a compressed-air shock-absorber 14. The compres-
sed air passes thence, through supply-lines 2.8 and 2.81,
to bridges 18 and 18.1, respectively,preferably to a point
above the centres of rotation thereof. It then passes,
through aerators 1.6 and 1.61, into reaction tanks 1 and 1.1,
respectively, and through further aerators 2.5 and 2.51, into
aeration tanks 2 and 2.1, respectively. Agitators 1.4 and
1.41 may also be fitted to the bridges, more particularly
in reaction tanks 1 and 1.1, respectively.
Also provided is a timed control-unit 11 which,
according to signals received from probes 9 and 9.1 in units
10 and 10.1, respectively, of the apparatus, more particul-
arly from aeration tanks 2 and 2.1 therein, controls the
opening of valves or slides 12 and 12.1 at the outlet from
compressed-air shock-absorber 14, thus introducing air either
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- into line 2.8 or line 2.81. In this connection, slide-valve
12 may be bypassed by a line 13 with a slide-valve (not shown),
while slide-valve 12.1 may be bypassed by a line 13.1 with a
slide-valve (not shown), in such a manner that, with slide-
valves 12 and 12.1 closed, and bypass lines 13 and 13.1 open,
a small amount of air may be fed to air-supply lines 2.8 and
2.81.
Finally, as in the co-pending application, radial
partitions 2.7 and 2.71 may be provided in each aeration tank
2 and 2.1, respectively, as explained therein and for the
purpose set forth therein.
The sewage-treatment apparatus according to the
invention is operated in such a manner that the raw sewage, to
be purified, is distributed as uniformly as possible to both
units 10 and 10.1 of the apparatus. Aeration of tanks 2 and
2.1 is carried out in such a manner that tank 2 is supplied
; with air for a predetermined length of time through open slide-
valve 12, and with slide-valve 12.1 closed, after which tank
2.1 is supplied with air through open slide-valve 12.1 and with
slide-valve 12 closed. Thus units 10 and 10.1 of the apparatus
are aerated consecutively and, whereas slide-valve 12 is
opened while air is supplied to unit 10, slide-valve 12.1 is
opened while air is supplied to unit 10.1, i.e., tank 2.1 is
aerated for a predetermined length of time while air is sup-
plied to unit 10, slide-valve 12 being opened and slide-valve
12.1 being closed. Aeration times for units 10 and 10.1 are
selected by e~periment.
As already indicated, several compressors 15, 16
and 17, possibly with reserves, are provided for the supply of
air, the compressors being switched on and off, according to
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the volume of incoming sewage, by signals from electrical
probes 9 and 9.1. In the case of a multi-stage compressor,
suitable change-overs may be carried out. During the night,
when the flow of water and contaminants is at its lowest,
timed control-unit 11, without altering the aeration times,
ensures that the 2 content in the tanks is neither above nor
below a predetermined value. Moreover, the aeration times are
such that the treated sewage which drains away is sufficiently
nitrified or denitrified.
For example, during the night, the time of minimal
flow, the minimal-capacity compressor 15 is switched on, and
the sewage is aerated thereby for a certain length of time,
namely until the selected and necessary 2 content in the
tanks and corresponding units 10 and 10.1 of the apparatus is
reached. This takes a certain time, as determined empirically
when the sewage-treatment apparatus is started up. If the
selected 2 content of the mixture of sewage and activated
sludge, to be aerated, as a result of uninterrupted input,
drops to below 1 mg/l of 2~ signals from probe 9, for example,
cause slide-valve 12 for unit 10 to open and, simultaneously,
slide-valve 12.1 for unit 10.1 to close and vice-versa. This
continues intermittently as long as there is no change in the
volume, contamination, temperature or other condition of the
incoming sewage. In the event of sewage concentration or tem-
perature change, or some other unfavourable condition, a cor-
rection is made in the aeration time to allow optimal puri-
fication action to be achieved. This is effected by timed control-
unit 11. If the 2 content drops below the set level, another
compressor, compressor 16 for example, is switched on. This
increases the volume of air supplied, restoringthe 2 content
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of units 10 and 10.1 of the apparatus to the predetermined
level, and ensuring nitrification of the mixture of actuated
sludge and water in units 10 and 10.1 of the apparatus.
Further increases in the volume of incoming water, contaminants,
etc. switches on a third compressor 17. Through slide-valves
12 and 12.1, these three compressors supply units 10 and 10.1
of the apparatus with sufficient air so that, in spite of
large volumes and high degrees of contamination, the 2 con-
tent of the mixture of activated sludge and water remains at
the desired level selected.
However, if there is a drop in volume and/or conta-
mination in inlet 8, signals from probes 9 and 9.1 cause one
; of the compressors, compressor 17 for example, to be switched
off, thus making it possible to avoid any increase in 2 con-
tent. In the examplary embodiment illustrated, comprising
three compressors, under normal conditions only two compres-
; sors 15 and 16 are in operation until either an increase in
- volume causes compressor 17 tc be swit~ed on, or at night
only compressor 15 remains in operation. In this way, the
installation achieves nitrification and denitrification with
the same 2 content.
` According to the invention, therefore, timed control-
- unit 11 provides adjustable, predetermined aeration times.
This assures the lowest permissible 2 content in the mixture
of activated sludge and water. ~oreover, in spite of the
increased elimination of nitrogen, the sewage-treatment
apparatus achieves substantial purification at a very low
power cost.
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