Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVEMENTS IN SEQUENCING BATCH REACTORS
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The present invention relates to waste water,,
treatment and more particularly to methods and apparatus for
use in a Sequencing Batch Reactor (SBR).
The Sequencing Batch Reactor (SBR) process is a
modification of the conventional activated sludge process
employing a fill, treat and draw sequence.
In the SBR process, raw waste water is transferred to
a reactor during a fill and react cycle. Air is introduced
into the tank during this cycle to aerobically oxydize carbon
and nitrogen in the form of biochemical oxygen demand (BOD)
and total Kjeldahl nitrogen (TKN). When the tank fills to a
predetermined high level, the raw sewage influent shuts off,
air may be shut off, and the tank contents begîn a react only
cycle to ensure that effluent discharge requirements are
obtained. Following the react cycle, the mixed liquor is
allowed to settle. Clarified supernatant (effluent) occupies
the upper half of the tank volume. After this settle cycle,
- a decant cycle begins, during which time a pump is used to
draw off and discharge the treated effluent. Following
effluent discharge, the apparatus enters an idle phase until
the fill and react cycle begins again to repeat the entire
process.
The present invention is concerned with a number of
improvements in a process of this general type. One of~these
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improvements involves controlling the process to provide a
predetermined average sludge retention time (SRT). This
provides control over the biological processes carried out
within the reactor so as to produce the desired carbon and
nitrogen oxydizing bacterial culture.
According to this aspect of the present invention
there is provided a method of treating waste water
comprisingo
a) supplying waste water to be treated to a reactor;
b) injecting air into the waste water in the reactor
for aerobic treatment of the waste water;
c) after step (b) allowing mixed liquor to settle in
the reactor;
d) after step (c) decanting supernatant from the
reactor;
e) withdrawing a predetermined quantity of mixed
liquor from the bottom of the reactor; and
f) repeating steps (a) to (e).
Preferably, the sludge containing mixed liquor is
withdrawn during the decanting of supernatant from the
reactor. It is also preferred that the quantity of mixed
liquor withdrawn is selected to provide an average sludge
retention time in the range of 10 to 15 days.
Another aspect of the invention provides a method of
storm control procedure which monitors the influent rate and
adjusts the processing of incoming waste to provide adequate
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treatment without overloadin~ the system.
According to this aspect of the present invention
there is provided a storm control procedure for use in a
process of treating influent waste water which treatment
~' process comprises:
:a fill and react cycle comprising supplying waste
water to be treated to a reactor and injecting air
~ into the waste water;
-a react cycle comprising aerobically treating the
-waste water in the reactor for a predetermined
react time after completion of the fill and react
cycle;
a settle cycle comprising maintaining quiescent
conditions in the reactor for a predetermined
settle time to allow sludge in the waste water to
settle, and
a decant cycle comprising withdrawing effluent from
the reactor,
the storm control procedure comprising:
monitoring the actual waste water influent rate:
comparing the actual waste water influent rate
with a selected waste water influent rate; and
reducing the duration of at least one of said
cycles when the actual waste water influent
rate exceeds the selected waste water influent
rate.
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The various cycles of the treatment process may be
reduced in sequence as the actual waste water in~luent rate
sequentially exceeds a sequence of selected waste water
influent ratesO The storm control procedure preferably
involves a first phase of reducing the decant cycle to one
half by turning on a back-up decant pumpO The second phase
involves reducing the duration of the settle cycle to one
half of its normal value. In the third phase of the storm
control procedure the duration of the react cycle is reduced
by one half. In the final phase, the react cycle duration is
reduced to zero.
A further aspect of the present invention involves
the provision of a method of controlling a waste water
treatment process in which hydraulic loads are controlled
according to liquid levels in the treatment apparatus and
biological processes are controlled by timed sequences.
According to this aspect of the present invention
there is provided a method of controlling a waste water
treatment process in an apparatus comprising a reactor, waste
water supply means for supplying waste water to be treated to
the reactor, air injector means for injecting air into waste
water in the reactor, decant means for withdrawing effluent
from the reactor, and sludge discharge means for withdrawing
settled sludge from the reactor, said method comprising:
monitoring the waste water level in the reactor;
initiating operation of the waste water supply means
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; and the air injector means when the waste water
:~ level is at or below a predetermined minimum level;
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`:~ stopping operation of the waste water supply means
when the waste water level reaches a predetermined
maximum level,
operating the air injector means for a predetermined
sequence of air injection times;
allowing waste water to settle under quiescent
conditions in the reactor for a predetermined
settle time after expiry of the sequence of air
. injection times:
:~ initiating operation of the decant means upon expiry
of the settle time;
stopping operation of the decant means in response to
the waste water level in the reactor reaching the
predetermined minimum level; and
operating the sludge discharge means for a
predetermined sludge discharge time.
This aspect of the invention also provides an
apparatus for treating waste water comprising:
a reactor;
: waste water supply for supplying waste water to be
treated to the reactor;
air injector means for injecting the air into waste
water in the reactor;
decant means for withdrawing afluent from the
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reactor;
sludge discharge means for withdrawing settled sludge
from the reactor; and
control means including:
waste water level responsive means for controlling
the waste water supply means, initiating
operation of the air injector means and stopping
operation of the decant means;
timer means for operating the air injector means
or a predetermined air injection time, for
timing a predetermined settled time in response
to expiry of the air injection time, for
initiating operation o~ the decant means in
response to expiry of the settled time and for
operating the sludge discharge means for a
predetermined sludge discharge time.
Another aspect of the present invention relates to a
system for trouble free drawing off of effluent from the
reactor. In the past, the drawing of effluent without taking
sludge with it has proven a problem.
According to this aspect of the present invention
there is provided a waste water reactor for aerobic treatment
of waste water, and effluent withdrawal means comprising an
inlet at a fixed position in the reactor, the inlet opening
downwardly, and a check valve associated with the inlet to
prevent flow of effluent out of the withdrawal means inlet
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into the reactor, and permitting the substantially free flow
of effluent into the inlet~
In the accompanying drawings, which illustrate an
exemplary embodiment of the present invention:
Figure 1 is a schematic plan view of a two-reactor
SBR system;
Figure 2 is an elevation of a two-reactor tank; and
Figures 3A and 3B together constitute a flow chart
showing the treatment process.
Referring to the accompanying drawings, and espe-
cially to Figure 1, there is illustrated a sequencing batch
reactor installation 10 with two reactors 12 and 14, a
transfer tank 16 and a trash and scum separator 18. The trash
and scum separator 18 has an inlet 20 and an outlet 22 that
leads to the transfer tank 16. Raw sewage is discharged from
the transfer tank to the reactors using the respective
transfer pumps 24 and 26.
The reactors 12 and 14 are equipped with blowers 28
and 30 for injecting air into the reactor. The reactor 12 has
two effluent pumps 32 for drawing effluent from the reactor,
while the reactor 14 has two similar effluent pumps 34.
Sludge is withdrawn from the reactors 12 and 14 using
sludge withdrawal pumps 36 and 38 respectively. These pump
the sludge through a pipe 40 to the trash and scum separator
18 which, in this embodiment, serves as a sludge digester.
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A more detailed view of the reactors is shown in
elevation in Figure 2. As shown in the drawing, the reactor
12 has two effluent withdrawl lines 42 leading to the
respective pumps 32 while the reactor 14 has two effluent
withdrawl lines 44 for the pumps 34. The effluent withdrawl
lines 42 and 44 are vertically oriented pipes extending into
the reactor and equipped with gravity closed check valves 46
and 48 respectively at their lower ends.
The reactors 12 and 14 are also equipped with high
level sensors 50 and 52 respectively and low level sensors 54
and 56 respectively. These sensors are positioned to detect
the presence of liquid at the maximum and minimum liquid
levels 62 and 64 respectively in the reactor.
Figure 2 also illustrates the air diffusers 58 and 60
that extend across the bottom of the reactors 12 and 14 and
are coupled to the blowers 20 and 30 to inject air into the
reactors.
Raw sewage arriving at the reactor installation is
received by the trash and scum separator, where trash is
removed before the raw waste is transferred to the transfer
tanks 16. When the transfer tank is full and one of the
reactors 12 and 14 is empty, as detected by a level sensor 66
in the transfer tank and sensors 54 and 56 in the reactors,
one of the transfer pumps 24 and 26 and one of the blower 28
and 30 are turned on to start a fill and react treatment
cycle in one of the reactors. Mixed liquor microorganisms
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remaining from the previous cycle begin the new treatment
cycle. The raw waste is aerated to oxydize the carbon (BOD)
and nitrogen (TKN). The inherent flow equalizaton
characteristic of the partially full tank minimizes the shock
of erratic hydraulic and/or organic loads. The fill and
react time is dependant on the incoming flow rate it may be
in the order of 12 hours. The high level sensor in the
reactor or a high level override clock 68 ~Figure 3) provided
in the control system ultimately determines the end of the
fill and react period. During this period, designated "A" in
Figure 3, the blower is turned on and off at preprogrammed
intervals as determined by field operational data for the
particular installation. This will save energy during lower
than design flow conditions~ It also maximizes the blower
operation energy cost and oxygen transferring efficiency
inasmuch as alternating aerobic-anoxic periods enhance floc
settleability.
When the reactor has filled or the high level
override clock has timed out, the flow is automatically
switched to the second reactor which enters a fill and react
cycle.
During the react only phase following the fill and
react phase, aeration may continue to ensure that organic
oxydation is completed to the required level. If
denitrification is required, an air off mix period during
the react phase will reduce total nitrogen levels in the
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effluent. Mixers may be included in the reactor for thispurpose. The on/off cycling of ~he blower is programmable
according to the treatment conditions encountered.
Following the react only cycle, the reactor contents
are allowed to settle. The settle period is necessary to
separate the bacteria (mixed liquor) from the liquid
supernatant. In this process the reactor vessel becomes a
clarifier. The design settling period is a preset period
that may be in the order of one hour. During this period
solids settle to occupy approximately the bottom 30~ of the
reactor volume, leaving the treated, clarified effluent at
the top.
Upon completion of the settle cycle, the reactor
enters the decant cycle that is used to remove the treated
effluent from the reactor. One of the decant pumps is turned
on to draw liquid through the associated vertical check valve
equipped effluent withdrawl line. The pipe intake is
normally located at the middepth of the tank, approximately 2
feet above the design level of the mixed liquor interphase.
The check valve ensures pump prime and precludes the entry of
mixed liquor solids into the intake pipe during aeration.
The decant period ends when the liquid level in the reactor
reaches the low level sensor.
The waste sludge cycle convenientely takes place
simultaneously with the decant cycle. The advantage of this
is that the mixed liquor sludge has compacted after settling.
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This decreases the amount of sludge that must be handled.
The sludge pump operates for a preselected time in each
cycle, with the time of operation selected volumetrically to
maintain the desired sludge retention time in the system.
After the reactor has been drawn down to the design
low level, the reactor is once again available to accept more
raw waste water so that the process sequence can be repeated.
This occurs automatically if the second reactor is full. If
` it is not, the reactor will enter an idle phase waiting for
the second reactor to fill.
- During the idle period, the blower may be operated
intermittently as in the fill and react period to maintain
microbiological viability in preparation for the next cycle.
! Although sludge may be wasted at any point in the
process, it is advantageous to waste the sludge after
settling and during the decant phase.
~; Sludge wasting controls the desired sludge retention
time of 10 to 15 days. This retention time is required to
provide a residence time that will produce a carbon and
nitrogen oxydizing bacterial culture with good settling
properties.
The sludge waste period is convenientely programmed
to occur after settling so that the compaction of the mixed
liquor will minimize the volume of sludge that must be pumped
and ultimately handled in sludge digestion facilities.
The reactor system may be equipped with a storm cycle
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procedure control that accommodates excessively high
hydraulic loads on the system. According to this control,
the waste water influent rate to the system is monitored and
if it exceeds a predetermined phase one influent rate, as
recorded in the control system, the back-up decant pump is
turned on to halve the decant time. A further increase of
the influent rate beyond a recorded second predetermined
level results in the program control reducing the duration of
the settle cycle by one half. A further increase in the
influent rate past a pre-recorded third predetermined level
causes a reduction of the react time to one half. The final
phase of the storm procedure eliminates the react time
entirely, so that the reactor is operating at approximately
three times its design capacity. This control is acceptable
since the larqe volume of material entering the reactor is
primarily storm water and not highly contaminated, so that
minimal treatement is required.
While one embodiment of the invention has been
described in the foregoing, it is to be understood that other
embodiments are possible within the scope of the invention.
The invention is to be considered limited solely by the scope
of the appended claims.
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