Control method in double-tank-type intermittent aeration activated sludge process

In an intermittent aeration activated sludge process in which aeration and agitation is repeated alternately, a first aeration tank and a second aeration tank are connected to each other and an ORP meter is applied to each tank. In the first aeration tank, the sum of an aeration period and a denitrification period of an agitation step is controlled to a predetermined period T.sub.gs based on a time when a bending point appeared on an ORP curve in a previous cycle. In the second aeration tank, the sum of an aeration period and an agitation period is controlled to a predetermined period T.sub.ds longer than T.sub.gs based on a time when an ORP measured value reached a predetermined value in a previous cycle. Operations of the first and second aeration tanks are simultaneously transferred from the agitation to the aeration based on detection of the predetermined ORP value. As a result, a necessary anaerobic condition period is secured in the first aeration tank.

BACKGROUND OF THE INVENTION 
The present invention relates to a process of biologically treating sewage 
including domestic one. More specifically, the invention relates to a 
control method in an intermittent aeration activated sludge process which 
can eliminate nitrogen and phosphorus from sewage. 
Conventional processes for treating sewage, which are generally biological 
processes typically exemplified by the activated sludge process, have been 
mainly directed to the removal of organic matter. However, in order to 
cope with the recent serious problem of eutrophication in closed water 
areas such as lakes and marshes, it is important to eliminate nitrogen and 
phosphorus that may cause the eutrophication. To this end, as improvements 
of the activated sludge process, various treatment processes have been 
developed which can eliminate not only organic matter but also nitrogen 
and phosphorus. Typical examples of such new processes include the A.sub.2 
O process, sequencing batch reactor activated sludge process, and 
intermittent aeration activated sludge process (hereinafter abbreviated as 
"intermittent aeration process"). In these processes, bacteria are 
alternately placed under the aerobic and anaerobic conditions to eliminate 
organic matter, nitrogen and phosphorus. 
The principle of the sewage treatment for eliminating nitrogen and 
phosphorus is briefly described below. Organic matter in sewage is 
decomposed and eliminated by being eaten by bacteria that constitute the 
activated sludge. Nitrogen is eliminated such that NH.sub.4 -N (ammonia 
nitrogen) is oxidized under an aerobic condition to NO.sub.3 --N (nitrate 
nitrogen) by an activity of nitrifying bacteria, which is then reduced 
under an anaerobic condition to N.sub.2 (nitrogen gas) by an activity of 
denitrifying bacteria. The nitrification and denitrification are 
summarized below. 
______________________________________ 
Nitrogen form Reaction 
Reaction change condition Bacteria 
______________________________________ 
Nitrification 
NH.sub.4 --N .fwdarw. NO.sub.3 --N 
Aerobic Nitrifying 
(with DO) bacteria 
Denitrifica- 
NO.sub.3 --N .fwdarw. N.sub.2 
Anaerobic Denitrify- 
tion (without DO) 
ing bacteria 
______________________________________ 
Phosphorus is eliminated by utilizing activated sludge including bacteria 
capable of storing a large amount of phosphorus in their cells. This type 
of activated sludge is produced by alternately changing the operation 
condition of the aeration tank between aerobic and anaerobic conditions. 
Since this activated sludge releases phosphorus under the anaerobic 
condition and absorbs phosphorus under the aerobic condition, phosphorus 
can be eliminated by making the sludge absorb phosphorus under the aerobic 
condition and then removing the sludge that has absorbed a large amount of 
phosphorus from the treatment apparatus as excess sludge. The above 
process is summarized as follows. 
______________________________________ 
Phosphorus 
concentration 
Reaction Phosphorus 
Reaction in tank condition removal 
______________________________________ 
Phosphorus Increase Anaerobic -- 
release (without DO) 
Phosphorus Decrease Aerobic Removal of 
absorption (with DO) activated 
sludge 
______________________________________ 
As described above, the aerobic and anaerobic conditions are indispensable 
to the removal of nitrogen and phosphorus. Stated strictly, the anaerobic 
condition for the denitrification and that for the phosphorus release are 
different. In the intermittent aeration process, the phosphorus release 
from the activated sludge occurs after the denitrification is finished and 
oxygen molecules (originating from NO.sub.3 -N) become absent from the 
tank, and subsequently the phosphorus absorption is performed in the next 
aeration step. 
Much attention is now given to the intermittent aeration process because 
the ratio between the anaerobic condition steps can be set in terms of a 
time period and it can be applied to existing treatment facilities 
relatively easily. However, in order to efficiently eliminate nitrogen and 
phosphorus in the intermittent aeration process, it is necessary to 
properly control the aeration period and the agitation period (anaerobic 
condition) in accordance with the load. Several control methods have been 
proposed conventionally, two typical examples of which are disclosed in 
Japanese Patent Application Examined Publication No. Sho. 63-35317 and 
Japanese Patent Application Unexamined Publication No. Sho. 64-70198. In 
the control method disclosed in the former publication, an ORP meter 
(oxidation-reduction potential meter) is applied to an aeration tank. When 
the ORP value exceeds the range of +120 to +200 mV, the aeration is 
stopped to start the agitation. When it becomes smaller than the -250 to 
-350 mV range, the agitation is stopped to start the aeration. The 
publication Sho. 64-70198 describes a process for eliminating nitrogen 
from sewage in which the nitrification and denitrification in a tank is 
controlled on the basis of a detected ORP changing rate of the tank. More 
specifically, a bending point of the ORP variation is detected in an 
aeration step and the aeration step is stopped to transfer to an agitation 
step with the bending point regarded as a finishing point of the 
nitrification. In the agitation step for the denitrification, the 
agitation is stopped to start the aeration step when the ORP changing rate 
reaches a predetermined value (the denitrification is regarded as 
finished). 
However, since the above control methods are directed to the treatment 
process that has a single aeration tank and a settling tank, they are 
associated with a problem that the quality of effluent is not stabilized. 
To solve this problem, the present inventors have conceived an apparatus 
consisting of a first aeration tank into which sewage flows, a second 
aeration tank that is connected in series to the first aeration tank, and 
a final settling tank, and also have proposed a control method therefor. 
FIG. 11 schematically shows the main part of this apparatus including a 
control system. Referring to FIG. 11, an intermittent aeration process and 
a control method of this apparatus is summarized below. In FIG. 11, paths 
of water and air are indicated by solid lines and arrows, and control 
signal lines are indicated by dashed lines and arrows. This apparatus 
mainly consists of first and second aeration tanks 2a and 2b for 
eliminating organic matter, nitrogen and phosphorus from sewage 1 flowing 
thereto by means of activated sludge, a final settling tank 4 for 
separating the activated sludge by gravitational sedimentation to obtain 
effluent 3, and a pump 5 for returning the sedimented activated sludge to 
the first aeration tank 2a. The first and second aeration tanks 2a and 2b 
have a capacity ratio of 1:1, and the hydraulic retention time of the 
sewage 1 in the apparatus is 16-32 hours in total. The control system 
consists of a DO (dissolved oxygen) meter 10a for measuring a dissolved 
oxygen concentration of the first aeration tank 2a, an ORP meter 6b for 
measuring an oxidation-reduction potential of the second aeration tank 2b, 
a control panel 9 for providing, based on the values of the above 
measurements, control signals to an inverter 11a for controlling the DO 
concentration of the first aeration tank 2a, a first aeration blower 7a, a 
second aeration blower 7b, a first agitation pump 8a and a second 
agitation pump 8b. 
According to a typical control method of the above apparatus, the aeration 
period is set at one hour and the DO concentration of the first aeration 
tank 2a during the aeration period is controlled at 0.2 mg/1. During the 
agitation period, an OP changing rate of the second aeration tank 2b is 
measured and a bending point of the ORP is detected by performing a 
predetermined calculation. Upon detecting the bending point, the agitation 
is stopped to start the aeration. The aeration of the first aeration tank 
2a and that of the second aeration tank 2b are performed simultaneously, 
and the agitation of the tank 2a and that of the tank 2b are also done 
simultaneously. 
The treatment process proceeds in the following manner. In the first 
aeration tank 2a, the nitrification and the denitrification proceed 
simultaneously (aerobic denitrification) while the DO concentration is 
controlled to be kept low. In the second aeration tank 2b, the 
nitrification proceeds while the DO concentration is kept at about 2-3 
mg/1 and, at the same time, phosphorus is absorbed by the activated 
sludge. After a lapse of one hour, the process automatically transfers to 
the agitation step. In the agitation step, the denitrification finishes in 
a short period in the first aeration tank 2a because the aerobic 
denitrification has proceeded in the previous aeration step and therefore 
the NO.sub.3 -N concentration is low. Then, phosphorus is released from 
the activated sludge. The denitrification proceeds slowly in the second 
aeration tank 2b because the organic matter concentration is low and, at 
the same time, the ORP decreases. Since the ORP varies to produce a 
bending point when the denitrification is completed, the agitation is 
stopped when the bending point is detected and the process transfers to 
the aeration step. Therefore, almost no phosphorus release occurs in the 
second aeration tank 2b. That is, in the agitation step, the phosphorus 
release is performed mainly in the first aeration tank 2a and the 
denitrification is performed mainly in the second aeration tank 2b. By 
virtue of the existence of two aeration tanks, the above-described process 
has an advantage that the influent is more unlikely to be discharged 
without being subjected to the treatment than in the case of using only 
one aeration tank. 
However, the present inventors have thereafter found that the 
above-described control method has the following problem. That is, since 
the agitation period depends on the quality of the influent, the 
efficiency of eliminating nitrogen and phosphorus decreases depending on 
the concentrations of nitrogen and phosphorus in the influent. For 
example, where the nitrogen concentration in the influent is low and the 
phosphorus concentration is high, the agitation period of the second 
aeration tank 2b is short because the denitrification finishes in a short 
period. In this case, the short agitation period causes shortage of 
phosphorus release period in the first aeration tank 2a. As a result, the 
phosphorus absorption in the aeration step becomes insufficient to 
deteriorate the phosphorus removal efficiency. Further, since the low DO 
concentration operation of the first aeration tank 2a suppresses the 
growth of nitrifying bacteria, the nitrification capability of the entire 
apparatus becomes insufficient for a nitrogen load when the nitrogen 
concentration is high. In this case, the nitrification becomes incomplete 
and the nitrogen removal efficiency becomes low. 
SUMMARY OF THE INVENTION 
The present invention has been made in view of the above problems in the 
art, and has an object of providing a control method in an intermittent 
aeration process in which the quality of effluent is stabilized and highly 
efficient denitrification and phosphorus removal are obtained irrespective 
of nitrogen and phosphorus concentrations in influent. 
According to the control method of the invention, first and second aeration 
tanks are employed in an intermittent aeration process, and operations of 
the first and second aeration tanks are controlled using a DO meter and an 
ORP meter. 
According to first and second control methods of the invention, an ORP 
meter is applied to the second aeration tank. In the first aeration tank, 
after aeration is performed for a predetermined period T.sub.a, the 
aeration is stopped to start agitation. In the second aeration tank, the 
sum T.sub.d of an aeration period T.sub.b and an agitation period T.sub.c 
is controlled to a predetermined period T.sub.ds longer than T.sub.a based 
on a time when a bending point appeared on an ORP curve detected by the 
ORP meter in a previous cycle (first method) or on a time when an ORP 
value reached a predetermined value in a previous cycle (second method). 
Operations of the first and second aeration tanks are simultaneously 
transferred from the agitation to the aeration based on detection of the 
ORP bending point (first method) or the ORP value equal to the 
predetermined value (second method). 
More specifically, according to the first control method, nitrification and 
phosphorus absorption proceed in the first aeration tank for the 
predetermined period T.sub.a, for instance, 60 minutes. After a lapse of 
60 minutes, the aeration is stopped to start the agitation, i.e., to start 
denitrification and phosphorus release. In the second aeration tank, the 
sum T.sub.d of the aeration period T.sub.b for the nitrification and 
phosphorus absorption and the agitation period T.sub.c for the 
denitrification is controlled to assure that phosphorus has been released 
sufficiently in the first aeration tank when the denitrification of the 
agitation step is completed. That is, the sum period T.sub.d is controlled 
to the period T.sub.ds that is preset at a one-cycle period of the 
intermittent aeration determined by considering the phosphorus release 
period. For example, if the control is performed with T.sub.ds set at 120 
minutes, the phosphorus release period of about 50 minutes is secured in 
the first aeration tank. The agitation period T.sub.c is measured as a 
period from the start of the agitation to the appearance of the bending 
point B on the ORP curve detected by the ORP meter (the bending point B is 
associated with the completion of the denitrification). The sum period 
T.sub.d is obtained by adding T.sub.c to T.sub.b. The aeration period 
T.sub.b is adjusted based on the period T.sub.d in a previous cycle so 
that T.sub.d coincides with T.sub.ds. With the above control, even if the 
influent has a low nitrogen concentration, sufficient phosphorus release 
period can be secured in the first aeration tank. Therefore, the 
phosphorus absorption during the next aeration step is enhanced to provide 
a high phosphorus removal efficiency. Further, since the nitrification and 
the denitrification are performed in both of the first and second aeration 
tanks, it is apparent that this control method can provide a high nitrogen 
removal efficiency. 
The second control method is different from the first control method only 
in the method of detecting the completion of the denitrification in the 
second aeration tank. That is, since the ORP value corresponding to the 
completion of the denitrification is almost fixed, this ORP value is 
preset, and as soon as the ORP value detected by the ORP meter reaches the 
preset ORP value, the denitrification is stopped to transfer to the 
aeration step judging that the denitrification has been completed. 
According to third and fourth control methods, a DO meter and a first ORP 
meter are applied to the first aeration tank and a second ORP meter is 
applied to the second aeration tank. In the first aeration tank, a DO 
concentration is controlled so that the sum T.sub.g of an aeration period 
T.sub.e and a denitrification period T.sub.f of an agitation step becomes 
equal to a predetermined period T.sub.gs based on a time when a bending 
point appeared on an ORP curve detected by the first ORP meter in a 
previous cycle. In the second aeration tank, the sum T.sub.d of an 
aeration period T.sub.b and an agitation period T.sub.c is controlled to a 
predetermined period T.sub.ds longer than T.sub.gs based on a time when a 
bending point appeared on an ORP curve detected by the second ORP meter in 
a previous cycle (third method) or on a time when an ORP value reached a 
predetermined value in a previous cycle (fourth method). Operations of the 
first and second aeration tanks are simultaneously transferred from the 
agitation to the aeration based on detection of the ORP bending point 
(third method) or of the ORP value equal to the predetermined value 
(fourth method). 
More specifically, according to the third control method, the aeration 
period T.sub.e is set at a predetermined period, and the sum period 
T.sub.g being T.sub.e plus the denitrification period T.sub.f is 
controlled to the predetermined period T.sub.gs. For example, 
nitrification and phosphorus absorption proceed during aeration period 
T.sub.e that is set at 30 minutes. After a lapse of 30 minutes, the 
aeration is stopped to start the agitation, i.e., to start 
denitrification. The denitrification period T.sub.f is measured as a 
period from the start of the agitation to the appearance of the bending 
point A on the ORP curve detected by the first ORP meter (the bending 
point A is associated with the completion of the denitrification). The sum 
period T.sub.g is obtained by adding T.sub.f to T.sub.e. Although the 
agitation continues thereafter, after the lapse of T.sub.g phosphorus is 
released from the activated sludge in the first aeration tank. Since a 
sufficient phosphorus release period is required for phosphorus removal, 
the sum period T.sub.g is controlled to be a predetermined period in the 
first aeration tank so that T.sub.g does not become unduly long. For 
example, if the period T.sub.gs is set at 60 minutes, since the aeration 
period T.sub.e is fixed at 30 minutes the denitrification period T.sub.f 
needs to be controlled to 30 minutes. Since the denitrification period 
T.sub.f is proportional to the concentration of NO.sub.3 -N that has been 
generated by the nitrification during the aeration period T.sub.e, the 
denitrification period T.sub.f can be controlled by controlling the 
nitrification during the aeration period T.sub.e and, as a result, T.sub.g 
can be controlled to T.sub.gs. The DO concentration control is performed 
during the aeration period T.sub.e because the activity of nitrifying 
bacteria depends on the Do concentration and therefore the nitrification 
can be controlled by the DO concentration control. The DO concentration is 
set based on the sum period T.sub.g in a previous cycle. A specific 
control method with respect to the second aeration tank is not described 
here because it is similar to that of the first control method. Since the 
sum period T.sub.g can be controlled stably to 60 minutes in the first 
aeration tank, the third control method can almost certainly assure the 
phosphorus release period of about 60 minutes to provide a high phosphorus 
removal efficiency. Further, since the DO concentration in the first 
aeration tank does not become too low, the growth of nitrifying bacteria 
is scarcely suppressed and therefore the apparatus can maintain, as a 
whole, a high nitrification ability. As a result, even if the influent has 
a high nitrogen concentration, a high nitrogen removal efficiency can be 
obtained. 
The fourth control method is different from the third control method only 
in the method of detecting the completion of the denitrification in the 
second aeration tank. That is, since the ORP value corresponding to the 
completion of the denitrification is almost fixed, this ORP value is 
preset, and as soon as the ORP value detected by the second ORP meter 
reaches the preset ORP value, the denitrification is stopped to transfer 
to the aeration step judging that the denitrification has been completed. 
According to fifth and sixth control methods, a first ORP meter is applied 
to the first aeration tank and a second ORP meter is applied to the second 
aeration tank. In the first aeration tank, the sum T.sub.g of an aeration 
period T.sub.e and a denitrification period T.sub.f of an agitation step 
becomes equal to a predetermined period T.sub.gs based on a time when a 
bending point appeared on an ORP curve detected by the first ORP meter in 
a previous cycle. In the second aeration tank, the sum T.sub.d of an 
aeration period T.sub.b and an agitation period T.sub.c is controlled to a 
predetermined period T.sub.ds longer than T.sub.gs based on a time when a 
bending point appeared on an ORP curve detected by the second ORP meter in 
a previous cycle (fifth method) or on a time when an ORP value reached a 
predetermined value in a previous cycle (sixth method). Operations of the 
first and second aeration tanks are simultaneously transferred from the 
agitation to the aeration based on detection of the ORP bending point 
(fifth method) or of the ORP value equal to the predetermined value (sixth 
method). 
More specifically, according to the fifth control method, the sum period 
T.sub.g being the aeration period T.sub.e plus the denitrification period 
T.sub.f is controlled to the predetermined period T.sub.gs. Nitrification 
and phosphorus absorption are performed during the aeration period 
T.sub.e, and then the aeration is stopped to start the agitation, i.e., to 
start denitrification. The denitrification period T.sub.f is measured as a 
period from the start of the agitation to the appearance of the bending 
point A on the ORP curve detected by the first ORP meter (the bending 
point A is associated with the completion of the denitrification). The sum 
period T.sub.g is obtained by adding T.sub.f to T.sub.e. Although the 
agitation continues thereafter, after the lapse of T.sub.g phosphorus is 
released from the activated sludge in the first aeration tank. Since a 
sufficient phosphorus release period is required for phosphorus removal, 
the sum period T.sub.g is controlled to the predetermined period T.sub.gs, 
for instance, 60 minutes in the first aeration tank so that T.sub.g does 
not become unduly long. In this control method, there is no limitations on 
the proportion between T.sub.e and T.sub.f and it is just necessary that 
the sum period T.sub.g be controlled to T.sub.gs, i.e., 60 minutes. To 
this end, the aeration period T.sub.e is adjusted based on the sum period 
T.sub.g in a previous cycle. A specific control method with respect to the 
second aeration tank is not described here because it is similar to that 
of the first control method. Since the sum period T.sub.g can be 
controlled stably to 60 minutes in the first aeration tank, the fifth 
control method can almost certainly assure the phosphorus release period 
of about 60 minutes to provide a high phosphorus removal efficiency. 
Further, since the DO concentration in the first aeration tank can be kept 
in a range where the growth of nitrifying bacteria is not suppressed and 
therefore the apparatus can maintain, as a whole, a high nitrification 
ability. As a result, even if the influent has a high nitrogen 
concentration, a high nitrogen removal efficiency can be obtained. 
The sixth control method is different from the fifth control method only in 
the method of detecting the completion of the denitrification in the 
second aeration tank. That is, since the ORP value corresponding to the 
completion of the denitrification is almost fixed, this ORP value is 
preset, and as soon as the ORP value detected by the second ORP meter 
reaches the preset ORP value, the denitrification is stopped to transfer 
to the aeration step judging that the denitrification has been completed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The fundamental concept of the present invention is described first. In 
conducting experiments of controlling the double-tank intermittent 
aeration process, the present inventors have found the following control 
method very effective to eliminate nitrogen and phosphorus simultaneously 
and efficiently irrespective of their concentrations. That is, in the 
first tank, a nitrification and denitrification period is controlled to a 
fixed period to secure a sufficient phosphorus release period positively. 
In the second tank, while nitrification and denitrification are performed, 
a one-cycle period of the control is maintained at a predetermined period. 
In other words, while the one-cycle period is controlled in the second 
tank, in the first tank a sufficient phosphorus release period that is 
necessary for phosphorus removal is secured in the one-cycle period. 
Further, the nitrification, denitrification and phosphorus removal are 
performed in the process that is subjected to the above control. 
Embodiments of control methods of the invention are described hereinafter 
with reference to the accompanying drawings. 
FIG. 1 schematically shows the main part of an intermittent aeration 
apparatus, including a control system, to which a first control method of 
the invention is applied. The parts common to those in FIG. 11 are 
represented by the same symbols, and the meaning of the lines with arrows 
is the same as in FIG. 11. The apparatus of FIG. 1 is basically the same 
as that of FIG. 11 except that the FIG. 1 apparatus does not have the DO 
meter 10a and the inverter 11a of the FIG. 11 apparatus. 
The first control method of the invention to be applied to the apparatus of 
FIG. 1 is described below also with reference to FIGS. 2(a)-2(b) and 
3(a)-3(b). FIGS. 2(a) and 2(b) are graphs respectively showing variations 
of the DO concentration and ORP of the first aeration tank 2a with respect 
to the elapsed time, and the operation status of the first aeration tank 
2a is indicated in FIG. 2(a). Similarly, FIGS. 3(a) and 3(b) are graphs 
respectively showing variations of the DO concentration and ORP of the 
second aeration tank 2b with respect to the elapsed time, and the 
operation status of the second aeration tank 2b is indicated in FIG. 3(a). 
The variations of the DO concentration and ORP shown in FIGS. 2(a)-2(b) 
and 3(a)-3(b) are ones taken at an arbitrary time point (the origin of the 
graphs is the aeration start time) while the first control method of the 
invention is being practiced. 
In the first aeration tank 2a, the DO concentration is set at a low value 
of about 0.2 mg/1. This may be done by adjusting the air flow rate of the 
first aeration blower 7a in proportion to the flow rate of the influent 1. 
The aeration period is preset at 60 minutes. That is, the aeration is 
stopped upon the lapse of 60 minutes. In the first aeration tank 2a, in 
addition to the removal of organic matter, the nitrification and 
denitrification simultaneously proceed because of the low DO 
concentration. Further, phosphorus is absorbed by the activated sludge. 
After the transfer to the agitation step, the DO concentration immediately 
drops to zero and the ORP decreases to have a bending point A (see FIG. 
2(b)) about 10 minutes after the start of the agitation. The bending point 
A appears at a time point when the denitrification is completed, and the 
phosphorus release from the activated sludge proceeds during the remaining 
agitation step. This release period needs to be long enough to assure a 
high efficiency of phosphorus removal. This is done by properly 
controlling the second aeration tank 2b. 
Control on the second aeration tank 2b and resultant alteration of water 
quality are described below. A period T.sub.ds that corresponds to the 
one-cycle period of the control, i.e., the sum of the aeration period and 
the agitation period is set at 120 minutes. The aeration of the second 
tank 2b is started at the same time as the first aeration tank 2a, and 
during an aeration period T.sub.b the ordinary aeration is performed with 
the DO concentration set at 2-3 mg/l (suitable for the nitrification), to 
allow the nitrification and phosphorus absorption to proceed 
simultaneously. The nitrification is finished in the period T.sub.b, and 
NH.sub.4 -N that has not been nitrified in the first aeration tank 2a is 
converted to NO.sub.3 -N. After the lapse of T.sub.b, the process 
transfers to the agitation step to start the denitrification. Since the 
denitrification in the second aeration tank 2b is one due to endogenous 
respiration, the denitrification rate is relatively low and therefore the 
ORP decreases slowly. Upon completion of the denitrification, a bending 
point B (see FIG. 3(b)) appears on an ORP curve obtained by the ORP meter 
6b. The detection of the bending point B gives an agitation period Tc, and 
a period T.sub.d is obtained by adding T.sub.c to T.sub.b. As shown in 
FIG. 3(a), T.sub.b, T.sub. c and T.sub.d are 80 minutes, 35 minutes and 
115 minutes, respectively. The one-cycle period of the control is finished 
upon the detection of the bending point B, and the first and second 
aeration tanks 2a and 2b return to the aeration step at the same time. As 
a result of the above control, the phosphorus release is performed in the 
first aeration tank 2a for 45 minutes, which means that a sufficient 
phosphorus release period is secured positively without being influenced 
by the quality of influent. The secured sufficiently long period of the 
phosphorus release leads to satisfactory phosphorus absorption in the 
aeration step to thereby provide a high phosphorus removal efficiency. 
Since the denitrification period of the second aeration tank 2b is 
determined by the quality of influent and the state of the activated 
sludge, a 5-minute difference from the preset period is corrected in the 
next cycle by adjusting the aeration period T.sub.b to equalize T.sub.d to 
T.sub.ds. 
Specifically, the aeration period T.sub.b is adjusted according to equation 
(1): 
EQU T.sub.bn =T.sub.bn-1 +K.sub.1 (T.sub.ds -T.sub.d) . . . (1) 
where 
T.sub.bn : aeration period of the second aeration tank in the next cycle, 
T.sub.bn-1 : aeration period of the second aeration tank in the current 
cycle, 
K.sub.1 : constant, 
T.sub.ds : preset value of the sum of the aeration period and the agitation 
period, and 
T.sub.d : sum of the aeration period and the agitation period in the 
current cycle. 
The sum period T.sub.d may be an average (moving average) over several 
cycles before the current cycle of the sums of the aeration period and the 
agitation period of the second aeration tank 2b. 
In the first aeration tank 2a, the growth of nitrifying bacteria is 
suppressed because of the low DO concentration. However, since the first 
control method of the invention is directed to the case of the low 
nitrogen concentration, there does not occur such a case that the 
nitrification and denitrification rates become insufficient to deteriorate 
the nitrogen removal efficiency. The phosphorus removal is completed by 
wasting, as excess sludge, the activated sludge that includes a large 
amount of phosphorus from the settling tank 4 (This step is not shown in 
FIG. 1). As described above, according to the first control method of the 
invention, the denitrification and phosphorus removal proceed 
satisfactorily. It is noted that the point at which the slope of the ORP 
curve obtained by the ORP meter 6b for the second aeration tank 2b changes 
suddenly is employed as the bending point B. 
The bending point B is detected in the following manner. The slope of the 
ORP curve is calculated with a time increment .DELTA.t. Then, D.sub.n 
/D.sub.n-1 is calculated, where D.sub.n and D.sub.n-1 represent slopes at 
a certain time point and a time point .DELTA.t before it, respectively. 
The ratio D.sub.n /D.sub.n-1 is about unity while the slope of the ORP 
curve does not vary, and takes a larger value ranging from 1.5 to 4 when 
the slope suddenly changes at the bending point B. Therefore, employing a 
threshold of, for instance, 1.5, the bending point B can be detected when 
a condition D.sub.n /D.sub.n-1 .gtoreq.1.5 is satisfied. 
A second control method according to the invention is described below. 
Since the second control method is basically the same as the first control 
method except for the method of detecting the completion of the agitation 
step of the second aeration tank 2b, the following description is directed 
only to that difference. In the agitation step of the second aeration tank 
2b as shown in FIG. 3(b), the ORP curve has the bending point B (ORP value 
is about -50 mV) when the denitrification is completed. According to the 
studies of the present inventors, in many cases the ORP value 
corresponding to the completion of the denitrification is within the range 
of +50 to -150 mV, and once the quality of influent and the operation 
conditions are determined the ORP at the bending point B takes an almost 
fixed value. Therefore, by experimentally predetermining the ORP value at 
the bending point B for a subject sewage treatment apparatus, the 
completion of the denitrification can be detected without detecting the 
bending point B. Based on this fact, according to the second control 
method of the invention, an ORP threshold value of the ORP meter 6b is set 
at a value that has been predetermined experimentally, and the agitation 
is stopped to transfer to the aeration step as soon as the ORP is 
decreased down to the threshold value. Since the remaining control is the 
same as the first control method, descriptions therefor are omitted here. 
A third control method of the invention is described below with reference 
to the drawings. FIG. 4 schematically shows the main part of an 
intermittent aeration apparatus, including a control system, to which the 
third control method of the invention is applied. The parts common to 
those in FIG. 11 are represented by the same symbols, and descriptions 
therefor are omitted here. The apparatus of FIG. 4 is different from that 
of FIG. 11 in that a first ORP meter 6a for detecting an ORP bending point 
is applied to the first aeration tank 2a. 
The third control method of the invention to be applied to the apparatus of 
FIG. 4 is described below also with reference to FIGS. 5(a)-5(c). FIGS. 
5(a) and 5(b) are graphs respectively showing variations of the DO 
concentration and ORP of the first aeration tank 2a with respect to the 
elapsed time, and the operation status of the first aeration tank 2a is 
indicated in FIG. 5(b). Similarly, FIG. 5(c) is a graph showing a 
variation of the ORP of the second aeration tank 2b with respect to the 
elapsed time, and the operation status of the second aeration tank 2b is 
indicated in FIG. 5(c). The variations of the DO concentration and ORP 
shown in FIGS. 5(a)-5(c) are ones taken at an arbitrary time point (the 
origin of the graphs is the aeration start time) while the third control 
method of the invention is being practiced. 
In the first aeration tank 2a, an aeration period T.sub.e is set at 30 
minutes and a preset period T.sub.gs of the sum of the aeration period and 
the denitrification period is 60 minutes. During the aeration period 
T.sub.e, the DO control is performed with a preset DO value of 1.2 mg/l, 
and the nitrification and the phosphorus absorption proceed. After a lapse 
of 30 minutes, the process transfers to the agitation step to start the 
denitrification. On an ORP curve detected by the first ORP meter 6a, a 
bending point A appears 25 minutes after the start of the agitation (see 
FIG. 5(b)). A denitrification period T.sub.f is measured by detecting the 
bending point A, and a period T.sub.g amounts to (30+25)=55 minutes. The 
measured period T.sub.g being 5-minutes shorter than the preset period 
T.sub.gs is due to a shortage of the denitrification period T.sub.f, i.e., 
insufficient nitrification during the aeration period T.sub.e. This means 
that the DO concentration which has been determined from the period 
T.sub.g of the preceding cycle is too low and the nitrification 
suppression by the DO control is excessive. Therefore, the DO 
concentration is controlled to increase in the next cycle to accelerate 
the nitrification so that T.sub.g will coincide with T.sub.gs. 
Specifically, the DO preset value is adjusted according to equation (2): 
EQU DO.sub.n =DO.sub.n-1 +K.sub.2 (T.sub.gs -T.sub.g) . . . (2) 
where 
DO.sub.n : DO preset value of the first aeration tank in the next cycle, 
DO.sub.n-1 : DO preset value of the first aeration tank in the current 
cycle, 
K.sub.2 : constant, 
T.sub.gs : preset value of the sum of the aeration period and the 
denitrification period, and 
T.sub.g : sum of the aeration period and the denitrification period in the 
current cycle. 
Since the growth of nitrifying bacteria is suppressed only when the DO 
concentration is not more than 2 mg/l, the DO concentration is adjusted 
within this range. If the denitrification period is still short even with 
the DO concentration of 2 mg/l, it is judged that the nitrification 
ability of the activated sludge is lowered and a known measure for 
accelerating the nitrification ability is taken, for example, the 
activated sludge concentration is increased in the entire treatment 
apparatus. 
The period T.sub.g may be an average (moving average) over several cycles 
before the current step of the sums of the aeration period and the 
denitrification period of the first aeration tank 2a. 
Control on the second aeration tank 2b and resultant alteration of water 
quality are described below. A period T.sub.ds that corresponds to the 
one-cycle period of the control, i.e., the sum of the aeration period and 
the agitation period is set at 120 minutes. The aeration of the second 
tank 2b is started at the same time as the first aeration tank 2a, and 
during the aeration period T.sub.b the ordinary aeration is performed with 
the DO concentration set at 2-3 mg/l (suitable for the nitrification), to 
allow the nitrification and phosphorus absorption to proceed 
simultaneously. The nitrification is finished in the period T.sub.b, and 
NH.sub.4 -N that has not been nitrified in the first aeration tank 2a is 
converted to NO.sub.3 -N. After the lapse of T.sub.b, the process 
transfers to the agitation step to start the denitrification. Since the 
denitrification in the second aeration tank 2b is one due to endogenous 
respiration, the denitrification rate is relatively low and therefore the 
ORP decreases slowly. Upon completion of the denitrification, a bending 
point B (see FIG. 5(c)) appears on an ORP curve obtained by the second ORP 
meter 6b. The detection of the bending point B gives an agitation period 
T.sub.c, and a period T.sub.d is obtained by adding T.sub.c to T.sub.b. As 
shown in FIG. 5(c), T.sub.b, T.sub.c and T.sub.d are 53 minutes, 60 
minutes and 113 minutes, respectively. The one-cycle period of the control 
is finished upon the detection of the bending point B, and the first and 
second aeration tanks 2a and 2b return to the aeration step at the same 
time. As a result of the above control, the phosphorus release is 
performed in the first aeration tank 2a for 58 minutes, which means that a 
sufficient phosphorus release period is secured positively without being 
influenced by the quality of influent. The secured sufficiently long 
period of the phosphorus release leads to satisfactory phosphorus 
absorption in the aeration step to thereby provide a high phosphorus 
removal efficiency. Since the denitrification period of the second 
aeration tank 2b is determined by the quality of influent and the state of 
the activated sludge, a 7-minute difference from the preset period is 
corrected in the next cycle by adjusting the aeration period T.sub.b to 
equalize T.sub.d to T.sub.ds. Specifically, the aeration period T.sub.b is 
adjusted according to equation (1) described above. 
Since the DO concentration control range of the first aeration tank 2a is 
1-2 mg/l, the suppression of the growth of nitrifying bacteria by the DO 
control is weak and there is no possibility that the nitrification rate 
becomes insufficient. While in the first aeration tank 2a the 
denitrification proceeds with organic matter being supplied from the 
influent, the denitrification also proceeds in the second aeration tank 
2b. Therefore, the third control method can provide a high nitrogen 
removal efficiency not only when the nitrogen concentration is low but 
also when it is high. As in the case of the first control method, the 
phosphorus removal is completed by wasting, as excess sludge, the 
activated sludge that includes a large amount of phosphorus from the 
settling tank 4 (This step is not shown in FIG. 4). As described above, in 
the third control method the denitrification and phosphorus removal 
proceed satisfactorily. 
A more specific example of the third method, which is based on an 
experiment, is described below. According to the third control method, the 
present inventors conducted a long-term control experiment for the 
simultaneous removal of nitrogen and phosphorus on a sewage sample 
prepared by mixing human feces, waste water from a restaurant, soapy 
water, tap water, sodium acetate, etc. using an apparatus having functions 
equivalent to those of the FIG. 4 apparatus. Table 1 shows the main 
specification of the experiment apparatus and experimental conditions. 
TABLE 1 
______________________________________ 
Item Unit Value 
______________________________________ 
Aeration Water temperature .degree.C. 
20 .+-. 2 
tanks Hydraulic retention time 
hour 16.0 
First aeration tank 
Capacity 1 76 
Aeration period (T.sub.e) 
minute 30 
Sum of aeration period and 
minute 60 
denitrification period (T.sub.gs) 
Second aeration tank 
Capacity 1 74 
Sum of aeration period and 
minute 120 
agitation period (T.sub.ds) 
MLSS mg/l 4,730 
SRT day 24.0 
Settling Hydraulic retention time 
hour 3.9 
tank Surface-loading rate 
m/day 4.5 
Recirculation ratio 
% 100 
______________________________________ 
Experimental results are shown in FIGS. 6(a)-6(d), 7(a)-7(d) and 8(a)-8(c) 
and Table 2. FIGS. 6(a)-6(d) show variations of water quality parameters 
in the first aeration tank 2a during a one-cycle period, more 
specifically, relationships with the elapsed time of the NH.sub.4 -N and 
(NO.sub.2 -N+-NO.sub.3 -N) concentrations (NO.sub.2 -N means nitrite 
nitrogen and is generated during the nitrification), PO.sub.4 -P 
(orthophosphate phosphorus) concentration, ORP, and DO concentration, 
respectively. The operation status of the first aeration tank 2a is 
indicated in FIG. 6(a). Similarly, FIGS. 7(a)-7(d) show variations of 
water quality parameters in the second aeration tank 2b during a one-cycle 
period, more specifically, relationships with the elapsed time of the 
NH.sub.4 -N and (NO.sub.2 -N+NO.sub.3 -N) concentrations, PO.sub.4 -P 
concentration, ORP, and DO concentration, respectively. The operation 
status of the second aeration tank 2b is indicated in FIG. 7(a). 
In the first aeration tank 2a, the nitrification proceeds during the 
aeration period as shown in FIG. 6(a), and the denitrification takes about 
30 minutes during the agitation period. The denitrification finishes when 
a bending point A appears as shown in FIG. 6(c). FIG. 6(b) shows that the 
phosphorus absorption occurs during the aeration period to lower the 
PO.sub.4 -P concentration, and that during the agitation period the 
phosphorus release proceeds after completion of the denitrification. 
In the second aeration tank 2b, there proceed the nitrification during the 
aeration period and the denitrification during the agitation period as 
clearly seen from FIG. 7(a). As shown in FIG. 7(c), a bending point B 
appears when the denitrification is completed. As shown in FIG. 7(b), the 
PO.sub.4 -P concentration is very low, and the phosphorus release does not 
occur even during the agitation period. 
As is understood from FIGS. 6(a)-6(d) and 7(a)-7(d), the nitrogen and 
phosphorus removal mechanism of the third control method works 
satisfactorily. On the other hand, Table 2 shows water quality that was 
obtained after continuous control of about 2 months. 
TABLE 2 
______________________________________ 
Removal 
ratio 
Unit Influent Effluent (%) 
______________________________________ 
COD mg/l 97.2 10.6 89.1 
TOC mg/l 126.7 6.5 94.9 
SS mg/l 145.1 3.8 97.4 
T-N (total nitrogen) 
mg/l 37.8 4.0 89.4 
T-P (total phosphorus) 
mg/l 3.97 0.18 95.5 
______________________________________ 
As shown in Table 2, there were obtained satisfactory control results of a 
T-N removal ratio of 89.4 and a T-P removal ratio of 95.5. Further, the 
phosphorus concentration in the activated sludge was 3.1%, which indicates 
the existence of activated sludge having a high phosphorus removal 
capability. 
FIGS. 8(a)-8(c) show water quality of the continuous control experiment. 
More specifically, FIGS. 8(a)-8(c) show relationships with the elapsed 
time of the concentrations of TOC, T-N and T-P in the influent and the 
effluent, respectively. It is understood from FIGS. 8(a)-8(c) that the 
quality of the effluent changes only slightly even if the quality of the 
influent somewhat changes. 
Next, a fourth control method of the invention is described. Since the 
fourth control method is basically the same as the third control method 
except for the method of detecting the completion of the agitation step of 
the second aeration tank 2b, a description is made only of that point here 
with reference to FIGS. 5(a)-5(c). In the agitation step of the second 
agitation tank 2b as shown in FIG. 5(c), the ORP curve has the bending 
point B (the ORP value is about -50 mV) when the denitrification is 
completed. According to the studies of the present inventors, in many 
cases the ORP value corresponding to the completion of the denitrification 
is within the range of +50 to -150 mV, and once the quality of influent 
and the operation conditions are determined the ORP at the bending point B 
takes an almost fixed value. Therefore, by experimentally predetermining 
the ORP value at the bending point B for a subject sewage treatment 
apparatus, the completion of the denitrification can be detected without 
detecting the bending point B. The fourth control method has the same 
relationship with the third control method as the second control method 
has with the first control method. Therefore, according to the fourth 
control method of the invention, an ORP threshold value of the second ORP 
meter 6b is set at a value that has been predetermined experimentally, and 
the agitation is stopped to transfer to the aeration step as soon as the 
ORP is decreased down to the threshold value. 
A fifth control method of the invention is described below with reference 
to the drawings. FIG. 9 schematically shows the main part of an 
intermittent aeration apparatus, including a control system, to which a 
fifth control method of the invention is applied. The parts common to 
those in FIG. 11 are represented by the same symbols, and descriptions 
therefor are omitted here. The apparatus of FIG. 9 is different from that 
of FIG. 11 in that the FIG. 9 apparatus does not have the DO meter 10a and 
the inverter 11a of the FIG. 11 apparatus but has a first ORP meter 6a 
applied to the first aeration tank 2a to detect an ORP bending point. 
The fifth control method of the invention to be applied to the apparatus of 
FIG. 9 is described below also with reference to FIGS. 10(a) and 10(b). 
FIG. 10(a) is a graph showing an ORP variation of the first aeration tank 
2a with respect to the elapsed time, and the operation status of the first 
aeration tank 2a is also indicated in FIG. 10(a). Similarly, FIG. 10(b) is 
a graph showing an ORP variation of the second aeration rank 2b with 
respect to the elapsed time, and the operation status of the second 
aeration tank 2b is also indicated in FIG. 10(b). The ORP variations shown 
in FIGS. 10(a) and 10(b) are ones taken at an arbitrary time point (the 
origin of the graphs is the aeration start time) while the fifth control 
method of the invention is being practiced. While the fifth control method 
is common to the third control method in many aspects, the former is 
different from the latter in that the period T.sub.g which is the sum of 
the aeration period T.sub.e and the denitrification period T.sub.f in the 
agitation step is controlled by adjusting the aeration period T.sub.e. 
As shown in FIG. 10(a), the period T.sub.gs is set at 60 minutes. During 
the aeration period T.sub.e of 25 minutes, the ordinary aeration is 
performed to have the nitrification and the phosphorus absorption proceed 
with the DO concentration controlled within the range of 2-3 mg/l. After 
the lapse of T.sub.e, the process transfers to the agitation step to start 
the denitrification. In an ORP curve detected by the first ORP meter 6a, a 
bending point A appears 30 minutes after the start of the agitation. The 
denitrification period T.sub.f is measured by detecting the bending point 
A, and therefore the period T.sub.g is (25+30)=55 minutes. The period 
T.sub.g being 5-minutes shorter than the preset value T.sub.gs means that 
the aeration period T.sub.e that has been set based on the periods T.sub.g 
of the preceding cycle is insufficient. Since the denitrification period 
is determined by the water quality and the state of the activated sludge, 
the difference from the preset period T.sub.gs is corrected by adjusting 
the period T.sub.e so that T.sub.g coincides with T.sub.gs. 
Specifically, the aeration period T.sub.e is adjusted according to equation 
(3): 
EQU T.sub.en =T.sub.en-1 +K.sub.3 (T.sub.gs -T.sub.g) . . . (3) 
where 
T.sub.en : aeration period of the first aeration tank in the next cycle, 
T.sub.en-1 : aeration period of the first aeration tank in the current 
cycle, 
K.sub.3 : constant, 
T.sub.gs : preset value of the sum of the aeration period and the 
denitrification period, and 
T.sub.g : sum of the aeration period and the denitrification period in the 
current cycle. 
The period T.sub.g may be an average (moving average) over several cycles 
before the current step of the sums of the aeration period and the 
denitrification period of the first aeration tank 2a. 
Since the control on the second aeration tank 2b is the same as in the 
third control method of the invention, a description therefor is omitted 
here. In the fifth control method, since the DO concentration range of the 
first aeration tank 2a is 2-3 mg/l, there is no suppression of the growth 
of nitrifying bacteria by the DO control and there is no possibility that 
the nitrification rate becomes insufficient. While in the first aeration 
tank 2a the denitrification proceeds with organic matter being supplied 
from the influent, the denitrification also proceeds in the second 
aeration tank 2b. Therefore, the fifth control method can provide a high 
nitrogen removal efficiency not only when the nitrogen concentration is 
low but also when it is high. The phosphorus removal is completed by 
wasting, as excess sludge, the activated sludge that includes a large 
amount of phosphorus from the settling tank 4 (This step is not shown in 
FIG. 9). As described above, in the fifth control method the 
denitrification and phosphorus removal proceed satisfactorily. 
Next, a specific example of the fifth control method is described based on 
experimental results. According to the fifth control method, the present 
inventors conducted a control experiment for about one month using the 
apparatus used for the above-described experiment according to the third 
control method. Table 3 shows the main specification of the apparatus and 
experimental conditions. 
TABLE 3 
______________________________________ 
Item Unit Value 
______________________________________ 
Aeration Water temperature .degree.C. 
20 .+-. 2 
tanks Hydraulic retention time 
hour 16.0 
First aeration tank 
Capacity 1 76 
Sum of aeration period and 
minute 60 
denitrification period (T.sub.gs) 
Second aeration tank 
Capacity 1 74 
Sum of aeration period and 
minute 120 
agitation period (T.sub.ds) 
MLSS mg/l 3,780 
SRT day 21.0 
Settling Hydraulic retention time 
hour 3.9 
tank Surface-loading rate 
m/day 4.5 
Recirculation ratio 
% 100 
______________________________________ 
An example of experimental results is shown in Table 4. 
TABLE 4 
______________________________________ 
Removal 
ratio 
Unit Influent Effluent (%) 
______________________________________ 
COD mg/l 81.5 10.3 87.4 
TOC mg/l 116.2 7.8 93.3 
SS mg/l 121.0 6.0 95.0 
T-N (total nitrogen) 
mg/l 37.0 2.4 93.5 
T-P (total phosphorus) 
mg/l 6.0 0.49 91.8 
______________________________________ 
As shown in Table 4, there were obtained satisfactory control results of a 
T-N removal ratio of 93.5% and a T-P removal ratio of 91.8%. The fifth 
control method of the invention can provide high nitrogen and phosphorus 
removal ratios. 
Next, a sixth control method of the invention is described. Since the sixth 
control method is basically the same as the fifth control method except 
for the method of detecting the completion of the agitation step of the 
second aeration tank 2b, a description is made only of that point here. In 
the agitation step of the second agitation tank 2b as shown in FIG. 10(b), 
the ORP curve has the bending point B (the ORP value is about -50 mV) when 
the denitrification is completed. According to the studies of the present 
inventors, in many cases the ORP value corresponding to the completion of 
the denitrification is within the range of +50 to -150 mV, and once the 
quality of influent and the operation conditions are determined the ORP at 
the bending point B takes an almost fixed value. Therefore, by 
experimentally predetermining the ORP value at the bending point B for a 
subject sewage treatment apparatus, the completion of the denitrification 
can be detected without detecting the bending point B. Therefore, 
according to the sixth control method of the invention, an ORP threshold 
value of the second ORP meter 6b is set at a value that has been 
predetermined experimentally, and the agitation is stopped to transfer to 
the aeration step as soon as the ORP is decreased down to the threshold 
value. Since the remaining control is the same as of the fifth control 
method, a description therefor is omitted here. 
In practicing the control methods of the invention, the capacity ratio of 
the first aeration tank 2a to the second aeration tank 2b need not be 
fixed to 1:1, but may be, for instance, 2:1. Even with the latter ratio, 
the control methods of the invention can be practiced satisfactorily to 
provide a good quality of the effluent. Further, the control methods of 
the invention can be applied to both of a case where the sewage 
continuously flows into the first aeration tank 2a and a case where it 
flows into the first aeration tank 2b intermittently. For example, the 
sewage may be input to the first aeration tank 2a only while it is in the 
agitation step. In this case, since the quantity of organic matter added 
under the anaerobic condition is increased, the phosphorus release 
quantity is increased and the phosphorus removal efficiency may be 
improved for influent having a certain type of water quality. Since the 
control methods of the invention can provide the anaerobic condition in 
the sewage input portion (i.e., first aeration tank), it is apparent that 
the filamentous bulking is unlikely to happen. 
As described above, according to the first and second control methods of 
the invention, even if the influent has a low nitrogen concentration, 
sufficient phosphorus release period is secured in the first aeration tank 
and phosphorus can be eliminated efficiently. Since the nitrogen 
concentration is low, there is no possibility that the nitrification 
ability becomes insufficient and nitrogen can also be eliminated 
satisfactorily. 
According to the third to sixth control methods, a sufficient phosphorus 
removal period is secured positively manner and phosphorus can be 
eliminated efficiently irrespective of the nitrogen concentration of the 
influent. That is, since the growth of nitrifying bacteria is scarcely 
suppressed in the first aeration tank, a high nitrification rate is 
maintained and nitrogen can be eliminated satisfactorily even if the 
nitrogen concentration of the influent is high. In this manner, the 
invention can improve the nitrogen and phosphorus removal rates 
irrespective of the nitrogen and phosphorus concentrations in the 
influent.