JPS6258799B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an improvement in a method for removing nitrogen from wastewater by an endorespiration type biological denitrification method. The biological nitrification and denitrification method uses nitrifying bacteria to convert nitrogen (hereinafter abbreviated as NH ₃ ) in wastewater into NOx.
(NO ₂ and/or NO ₃ ), and then removes NOx by reducing it to N ₂ gas under anaerobic conditions using denitrifying bacteria (denitrification). As a NOx reducing agent, a nitrogen-free organic carbon source such as methanol is usually used. However, since methanol is a valuable industrial chemical, in order to reduce running costs, endogenous respiration denitrification is used, which uses the reducing power of activated sludge itself without adding methanol. Additive denitrification (called exogenous respiration denitrification) is also used. This endogenous respiration type denitrification has the disadvantage that the denitrification rate is low, so the capacity is larger than the denitrification process using exogenous respiration type denitrification, but it is a resource-saving method that does not use methanol. Therefore, there are many examples of its adoption. Like other biological reactions, the reaction rate of endogenous respiration-type denitrification increases as water temperature increases, and the amount of denitrification is proportional to the microbial concentration, so increasing the sludge concentration increases the rate of denitrification. The amount of denitrification per unit reaction volume can be increased. The present inventors conducted a continuous test of denitrification by endogenous respiration of human waste at high water temperature and high MLSS (approximately 20,000 mg/approx.) in order to overcome the disadvantage of endogenous respiration denitrification that the process volume is large. However, it was learned that high concentrations of BOD and NH ₃ remained in the treated water. In order to investigate the causes of this BOD and NH ₃ , sludge was pulled out from the test equipment and a batch denitrification test was conducted, and the results shown in Figures 1 to 4 were obtained. The conditions for these experiments are: (1) Figure 1...Water temperature 20℃, MLSS concentration 20000mg/
(2) Figure 2... Water temperature 30℃, MLSS concentration Same as above (3) Figure 3... Water temperature 40℃, MLSS concentration Same as above (4) Figure 4... Water temperature Same as above, MLSS concentration 10000mg/
It is. From Figures 1 to 3, NH ₃ -N increases regardless of the presence or absence of NO ₃ -N, but BOD increases with NO ₃ -N.
NH _{3 −N and BOD increase rapidly after disappearing, and both NH 3} −N and BOD increase as the water temperature increases.
The figure shows that BOD and NH ₃ -N increase as the MLSS concentration increases. After examining the data from continuous tests after these batch tests, we found that when MLSS concentrations were high, they remained in the summer when the water temperature was higher than in the spring.
It was found that the BOD and NH ₃ -N concentrations were high and matched well with the results of the batch test. Therefore, in continuous tests, when the air supply amount to the aeration tank installed after the denitrification tank was significantly increased, the residual concentration of BOD decreased, but as NH ₃ was nitrified to NO ₃ , the nitrogen concentration decreased. I didn't. In addition, BOD and NH ₃ -N increased in the denitrification tank, but the reason for this is that sludge decomposition produces BOD components and NH ₃ , and while NO ₃ exists, this BOD component cannot be used for NO ₃ denitrification. NH ₃ − because it is used and disappears.
Only N increases, but after NO ₃ disappears, BOD
It is thought that BOD and NH ₃ increased simultaneously because the components were not consumed. Water temperature 40℃, MLSS20000
After 24 hours of the batch test (Figure 3), which was set at mg/mg/ml, the liquid emitted a foul odor of organic acids.
The sludge probably underwent acidic fermentation. As mentioned above, the denitrification rate decreases as the water temperature decreases, so in order to stably perform a high rate of denitrification throughout the year, the minimum water temperature in winter is used to determine the volume of the denitrification tank. The volume becomes too large and BOD and NH ₃ -N remain at high concentrations. That is, as is clear from the comparison of Figures 1, 2, and 3, the time required for the residual concentration of NO ₃ -N to reach its minimum value is 9 hours in each of Figures 1 to 3. , 6 hours, 4.3 hours,
At the same MLSS concentration, the higher the water temperature, the faster the rate of disappearance of NO ₃ -N, and the lower the residual concentrations of BOD and NH ₃ -N. Therefore, it is desirable to set the denitrification treatment time depending on the water temperature.
This kind of operation is difficult in actual equipment, and the MLSS concentration is kept almost the same regardless of water temperature fluctuations in winter and summer, so as mentioned above, the denitrification treatment time is too long in summer. This leads to an increase in BOD and NH ₃ -N due to the decomposition of the sludge, and the quality of the treated water deteriorates. On the other hand, in exogenous respiration type denitrification that adds a carbon source such as methanol, such BOD, NH ₃ −N
No increase was observed. The present invention relates to the endogenous respiration type denitrification tank described above.
The purpose of this invention is to provide a method that can prevent an increase in BOD and reduce the amount of residual nitrogen in denitrified water. That is, in the biological denitrification process of wastewater, the present invention includes a denitrification process in which the wastewater and the nitrification liquid circulated from the nitrification process come into contact with each other before the nitrification process.
An endogenous respiration type denitrification process is provided after the nitrification process, and an aeration process is provided from the endogenous respiration type denitrification process to the nitrification process and/or after the endogenous respiration type denitrification process. This is a biological denitrification method for wastewater, characterized in that the liquid is circulated from the nitrification process to the nitrification process. Next, one embodiment of the present invention will be explained based on FIG. 5. The wastewater 1 containing NH ₃ and BOD, together with the returned sludge 2 and the circulating nitrification liquid 12 from the nitrification process 4, is sent to the first desorber under anaerobic conditions. Flowing into the nitrogen process 3, NOx (NO ₂ and/or NO ₃ ) in the circulating nitrification liquid 12 is reduced and decomposed into N ₂ gas by denitrifying bacteria using the BOD component in the wastewater 1 as a carbon source, and then NH ₃ and residual BOD
The components flow into the next nitrification step 4, where NH ₃ is nitrified to NOx, residual BOD components are oxidized and removed, and then part of the NOx is recycled to the first denitrification step 3, and the remaining NOx is internally removed. It flows into the second denitrification step 5 where denitrification is performed by living respiration and is denitrified. However, as mentioned above, it is impossible to operate the second denitrification process 5 under exactly the same conditions throughout the year, both in terms of management and economics (costs of heavy oil or electricity used to control water temperature). During the summer season when the NH 3 −N concentration increases, the BOD increases significantly in the second denitrification step 5, and an increase in the NH ₃ −N concentration is unavoidable. Therefore, in the present invention, as a method for preventing increases in BOD and NH ₃ -N concentrations, the liquid from the second denitrification step 5 is circulated to the nitrification step 4 via the valves 10 and 9.
As a result, the returned BOD components are oxidized and removed, and at the same time, the returned NH ₃ is oxidized to NOx together with the residual NH ₃ from the first denitrification step 3, and these NOx are removed in the second denitrification step 3. As the amount of NOx in this process increases because it flows into process 5, a large amount of BOD components generated from sludge in this process are consumed in the reduction of this NOx (N ₂ gasification), and NOx and BOD are treated at the same time. proceed. In this case, the effect that the liquid in the second denitrification process 5 is diluted by the inflow liquid from the nitrification process 4 can also be obtained. The NH ₃ and BOD components remaining in the second denitrification step 5 are treated in the aeration step 6, and the BOD components disappear through oxidative decomposition, and NH ₃ changes to NOx. If it exceeds the nitrification process, the liquid from the aeration process 6 may be returned to the nitrification process 4 via the valves 11 and 9, and the same effect as described above can be obtained. In addition, if necessary, the second denitrification step 5
Of course, the liquid may be returned from the aeration process 6 to the nitrification process 4 at the same time. 8 in the figure is a circulation pump. The effluent from the second denitrification process 5 whose BOD and NH ₃ -N have been reduced by the above method flows into the aeration process 6, but since the BOD has decreased, the capacity of the aeration process 6 is reduced. can do. The mixed liquid from which residual BOD has been removed in the aeration step 6 flows into the solid-liquid separation step 7 to obtain separated water 13, which may be discharged as treated water or subjected to further advanced treatment. good. On the other hand, the separated sludge is returned to the first denitrification step 3 as return sludge 2. Therefore, in the present invention, the flow rate of the circulating nitrifying liquid 12, that is, the amount of circulating liquid is determined as follows.
This circulating nitrification liquid 12 was generated in the nitrification process 4.
This is to denitrify NOx using the BOD component of wastewater 1, and the circulating fluid volume C is the same as that of nitrification process 4.
Since it is determined by the NOx concentration and the BOD amount of inflowing wastewater, assuming that all NH ₃ is nitrified, it can be expressed by the following equation. n・Q・BOD=C・NH ₃ −N・Q/C+R+Q ... where, Q: Wastewater inflow (m ³ /day) BOD: Wastewater BOD concentration (mg/) C: Circulating fluid amount (m ³ /day) NH ₃ -N: Wastewater NH ₃ -N concentration (mg/) R: Returned sludge amount (m ³ /day) n: BOD required to remove NOx-N (Kg/
Kg), that is, (BOD/NOx-N). Note that n is 2.5 for BOD/NO ₂ -N, and BOD/
For NO ₃ -N, it is sufficient to look at about 4. Next, the amount of liquid circulating from the second denitrification process 5 to the nitrification process 4 will be explained with reference to FIGS. 1 and 3. Winter water temperature 20℃, MLSS 20000mg/, inflow NO ₃
- If the N concentration is 120mg/, the second denitrification step 5
Since the volume V _B of the inflow water residence time must be 9 hours from Figure 1, V _B /Q'=9/24... ∴V _B ≒0.38Q'...' Here, V _B : 2 Volume of denitrification process 5 (m ³ ) Q′: Amount of water flowing into the same process (m ³ /day) The number 24 in the equation is the number of hours in a day, and from the equation ′, for example, Q′ is 100 m ³ /day, V _B
is approximately ^38m3 . On the other hand, if the water temperature reaches 40°C in the summer, the second denitrification step 5 will occur at the above residence time, as shown in Figure 3.
BOD and NH ₃ -N would rise to 100 mg/ and 43 mg/, respectively, so to avoid this, the liquid from the second denitrification step 5 is circulated to the nitrification step 4 to increase the residence time.
4.3 hours (NO ₃ -N reaches _its lowest value) (assumed to remain unchanged). In this case, the amount of circulating liquid from the second denitrification process 5 to the nitrification process 4
C′ (m ³ /day) can be expressed by the following formula. V _B /Q'+C'=4.3/24... ∴V _B ≒0.18 (Q'+C')...' Therefore, if Q' is 100m ³ /day and V _B is 38m ³ ' From the formula, C′ is approximately 111 m ³ /day. In addition,
The meanings of V _B and Q' are as described above. Thus, by circulating the liquid in the second denitrification process 5, the BOD of the effluent from this process is 18 mg/NH ₃
-N is 23 mg/×Q'/Q'+C'≒11 mg/. Next, an example of the present invention (implemented according to the flow sheet shown in FIG. 5) will be described. Implementation conditions ● Type of wastewater... Human waste ● Processing amount...... 10Kg/day ● Nitrification fluid circulation amount...... 200m ³ / day ● Returned sludge amount...... 5m ³ / day ● Nitrification tank MLSS...... 20000mg / ●First denitrification process capacity: 20m ³ ●Second denitrification process capacity: 18m ³ ●Nitrification process capacity: 20m ³ ●Aeration process capacity: 10m ³ ●Water temperature: 40 ℃ Implementation results

[Table] From the above table, it can be seen that by performing the liquid circulation treatment from the second denitrification step and/or aeration step, significantly better quality treated water can be obtained than in the conventional method. As described above, according to the present invention, high-quality treated water can be obtained stably throughout the year, and treatment can be carried out that effectively utilizes the advantages of endogenous respiration type denitrification treatment.

[Brief explanation of the drawing]

1 to 4 are graphs showing the results of basic experiments for completing the present invention, and FIG. 5 is a flow sheet showing an embodiment of the present invention. 1... Wastewater, 2... Returned sludge, 3... First denitrification process, 4... Nitrification process, 5... Second denitrification process, 6... Aeration process, 7... Solid-liquid separation process, 8
...Circulation pump, 9-11...Valve, 12...
Circulating nitrification liquid, 13...separated water.

Claims (1)

[Claims] 1 A first denitrification process into which raw water flows, a nitrification process, a second denitrification process of endogenous respiration type, and an aeration process are arranged in series in this order, and at least a part of the nitrified liquid from the nitrification process is transferred to the first denitrification process. Biological denitrification of wastewater, characterized in that at least a portion of the liquid from the second denitrification step and/or at least a portion of the liquid from the aeration step are returned to the nitrification step as well as the liquid from the second denitrification step. Nitrogen method.