JP2587726B2 - Sewage treatment method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a single-tank type wastewater treatment method suitable for nitrification and denitrification of small-scale wastewater such as sewage and industrial wastewater. The present invention relates to an effective method for controlling aeration.

(Prior Art) As a method for nitrifying and denitrifying sewage, a batch method in which sewage is alternately flown into each reaction tank maintained in an aerobic state or an anaerobic state to perform nitrifying and denitrifying treatment, The reaction tank is partitioned by partition walls, one is aerobic, the other is anaerobic, and the water in the tank is circulated to perform a nitrification-denitrification process. By adjusting the amount of supplied oxygen, an aerobic state and an anaerobic state part are created along the direction of the reaction tank, and the nitriding and denitrification treatment is performed by circulating the inside of the reaction tank.
The law is widely known.

However, the batch method requires multiple reaction vessels,
Not only the installation area becomes large, but also if the inflow of sewage fluctuates, the treatment becomes unstable. Therefore, there is a drawback that an adjustment tank must be provided to adjust the flow rate. In addition, the modified circulation method requires circulating the water in the tank while separately managing the partitioned reaction tanks, and has a disadvantage that the control is complicated and expensive. In addition, the OD method produces an intermediate boundary in the reaction tank that is neither aerobic nor anaerobic, and the boundary does not contribute to nitrification or denitrification, resulting in wasted space and is uneconomical. There are drawbacks.

Therefore, the wastewater can be efficiently nitrified and denitrified in a single reaction tank without requiring a large installation area, and the operation is easy. As an effective sewage treatment method, a method described in JP-A-1-310798 has been proposed, and an advantage over other methods has been recognized by experiments. In this method, a stirrer and an aeration device are installed inside the reaction tank, and while the sewage in the reaction tank is continuously stirred by the stirrer, the aeration device is intermittently aerated, and the same reaction is performed every required time. By alternately switching the inside of the tank between an anaerobic state and an aerobic state, sewage is subjected to nitrification and denitrification.

In experiments using this method, anaerobic and aerobic have been performed cyclically by repeatedly stopping and operating the blower according to a time set in advance by a timer.

(Problems to be Solved by the Invention) However, in actual sewage treatment, the actual aerobic time and the anaerobic time may differ greatly from the set time of the timer due to load fluctuation. For example, at night, when the inflow is small and the load is small, the DO becomes too high, and even if the blower is stopped, it takes much time until the inside of the aeration tank becomes anoxic even if the blower is stopped. Thus, phenomena such as a decrease in the effect of removing nitrogen were observed. From such a situation, it has been found that there is a problem in the sewage treatment of the actual facility in how to control the load fluctuation of the inflow sewage.

An object of the present invention is to solve the problems of the related art and to provide a novel sewage treatment method capable of effectively treating sewage.

(Means for Solving the Problems) The present inventor has conducted intensive studies in order to solve the above problems, and as a result, has set the ratio of the anaerobic level time to the aerobic level time in one cycle of intermittent aeration to a predetermined range. By doing so, it was found that good processing results could be obtained, and the present invention was formed.

That is, in the sewage treatment method of the present invention, in a single reaction tank, the sewage is intermittently aerated while continuously stirring the sewage, and the sewage is alternately switched between an anaerobic state and an aerobic state in the tank. In one method of intermittent aeration, the anaerobic level time A when the DO of the mixed solution in the tank is 0.2 mg / l or less and the aerobic level time B when the DO is 0.5 mg / l or more in one cycle of intermittent aeration The operation is performed such that the A / B ratio, which is the ratio, is in the range of 0.6 to 1.0.

In the present invention, a device for performing mixing and agitation and oxygen supply, such as an underwater air radar, is installed in an intermittent aeration tank, and a DO meter and a timer that operates in conjunction with the DO meter are installed. An aerobic state can be formed, and when anaerobic, the supply of air from the compressor is stopped and only stirring is performed. In an aerobic state, oxygen supply can be performed simultaneously with mixing and stirring.

The necessary condition to promote effective nitrogen removal by the intermittent aeration method is to properly maintain the reaction time in the anoxic state and the aerobic state by the aeration cycle, and to repeat the reaction. The ratio of anaerobic level time to is important.

Above the anaerobic level time A DO of the mixture was as in the following 0.2 mg / l, as when the aerobic level time B DO of the mixed liquid is not less than 0.5 mg / l, the A / B ratio and K _je - N removal rate, in treated water
Many experiments have been performed on the relationship between the N _0x -N concentration and the TN removal rate, and the results shown in FIGS. 2 to 4 were obtained. According to this, the nitrification reaction is suppressed as the A / B ratio increases, that is, as the anaerobic level time increases,
As shown in the figure, the K _je -N removal rate decreases. On the other hand, when the A / B ratio is small, that is, when the anaerobic time is short, the denitrification reaction is inhibited, and the N _0x -N concentration in the treated water increases as shown in FIG. Also, from FIG. 5 showing the TN removal rate, when the A / B ratio is small, the denitrification is insufficient and the TN removal rate decreases, and when the A / B ratio is too high, Insufficient nitrification results in similarly reduced TN removal rates.

Comprehensively judging from the K _je -N removal rate, the residual N _0x -N concentration in the treated water and the TN removal rate, the ratio of the anaerobic level time to the aerobic level time, that is, the A / B ratio is 0.6.
The range of ~ 1.0 is appropriate, and better results are obtained when the aerobic level time and the anaerobic level time are equal or slightly longer.

In the above-mentioned aeration cycle, experiments were conducted for various types of general raw sewage for various lengths of time, but from the viewpoint of treatment stability and removal performance, about 120 minutes is considered appropriate. For example, when the aeration cycle is 120 minutes (aeration 60 minutes, aeration stop 60 minutes), if the air volume is adjusted so that the maximum DO (immediately before the stop of aeration) becomes 2-3 mg / l, the oxygen consumption rate at this time is Anoxic,
Clear driving in an aerobic state is possible.

(Operation) In the method of the present invention having the above-described configuration, the sewage continuously flowing into the reaction tank is intermittently aerated while being continuously agitated by the aeration and agitating devices. Is repeated, nitrification, denitrification and dephosphorization are effectively performed.

(Example) Next, an example of the present invention will be described with reference to the attached drawings.

FIG. 1 shows a flow sheet of the apparatus, wherein 1 is an intermittent aeration tank used as a reaction tank, and a submersible aerator 2 is installed in the lower center of the tank to supply air from a compressor C. Then, the inside of the tank 1 is in an aerobic state, and when stopped, only stirring is performed, and the inside of the tank 1 is in an anaerobic state. The supply and stop of air are performed by a predetermined value of a DO meter (not shown). It is performed by turning on and off the operating timer. And this reaction tank 1
A sedimentation tank 3 is provided adjacent to and receives treated sewage discharged from the reaction tank 1 and performs solid-liquid separation. A part of the sludge 4 settled at the bottom of the settling tank 3 is returned to the reaction tank 1 by the sludge pump P through the return pipe 4a, and the remaining sludge is discharged 4b as surplus sludge. Then, the treated water is discharged 5 from the upper part of the settling tank 3. The inflow sewage enters the raw water storage tank 6, from which it is continuously supplied to the reaction tank 1 by the raw water supply pump P.

The experiment was performed mainly for nitrogen removal, and the experimental conditions in the reaction tank (intermittent aeration tank) used in the experiment are as shown in Table 1.

Table 2 shows the quality of raw test water and treated water in this experiment.

FIG. 2 shows an example of changes over time of DO and ORP in the reaction tank 1 in this experiment. This is an aeration cycle 120 minutes (aeration 60
Minute, stop 60 minutes). After the start of aeration in the aeration cycle, DO in the tank starts to rise sharply, and decreases due to oxygen consumption accompanying the respiration of sludge as the aeration stops. In this case, the time when DO becomes zero depends on the maximum value of DO and the oxygen consumption rate of the mixture. As shown in this figure, in the intermittent aeration process, the anaerobic and aerobic states are alternately repeated temporally. ORP also reflects changes in the oxidation-reduction state in conjunction with the supply of oxygen (air) being stopped.

In this experiment, all of the BOD of the treated water was 8 mg / l or less, and 90% of the BOD was 6 mg / l or less, and good treated water quality was obtained. BOD-VSS load 0.05 ~ 0.30Kg / VSS-
BOD removal rate of 90% or more within the range of Kg / day, treated water BOD 6mg / l
The following was obtained: All of the treated water TN is 7 mg / l
And the frequency of heterogeneity with T-N5mg / l or less is as high as 80%
Stable nitrogen removal performance was obtained. In addition, the K _je -N removal rate was high, and particularly, when the BOD-VSS load was 0.15 kg / VSS-Kg · day or less, the K _je -N removal rate of about 85 to 95% was stably obtained.

According to the above experimental results, it is understood that in the intermittent aeration process, aeration conditions are an important operating factor for nitrogen removal.

(Effects of the Invention) As described above, according to the method of the present invention, the following excellent effects can be obtained.

(1) By setting the aeration level time and the aerobic level time in one cycle of intermittent aeration, the reaction time in the anoxic state and the aerobic state is properly maintained, and by repeating the cycle, the anaerobic with the swelling・ Aerobic operation is possible, and nitrogen removal can be performed with high efficiency and stability.

(2) Organic and SS removal rates are also good.

(3) Since anaerobic and aerobic treatments are repeatedly performed in the same reaction tank, a large installation area is not required, facility costs are low, and the method is particularly suitable for small-scale sewage treatment.

[Brief description of the drawings]

FIG. 1 is a flow sheet diagram of an apparatus according to one embodiment of the present invention, FIG. 2 is a diagram showing a change over time of DO and ORP in a reaction tank,
FIG. 3 shows the relationship between the ratio of the anaerobic level time to the aerobic level time in one cycle and the K _je -N removal rate, and FIG. 4 shows the relationship between the treated water N _0x -N concentration. FIG. 5 is a diagram showing the relationship with the TN removal rate. 1… Reaction tank, 2… Aerator underwater, 3… Final sedimentation basin

 ────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-1-310798 (JP, A) JP-A-61-54296 (JP, A)

Claims (1)

(57) [Claims] A method for treating sewage by intermittently aerating sewage in a single reaction tank while continuously stirring the sewage and alternately switching the tank between an anaerobic state and an aerobic state. In one cycle of intermittent aeration, the anaerobic level time A at which the DO of the mixed solution in the tank becomes 0.2 mg / l or less, and D
It is the ratio with the aerobic level time B when O becomes 0.5 mg / l or more.
A sewage treatment method characterized by operating so that the A / B ratio is in the range of 0.6 to 1.0.