JP3376904B2 - Biological denitrification method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organism which decomposes and removes nitrogen in wastewater by a denitrification reaction of microorganisms, and separates solid and liquid into microorganisms and treated water by membrane separation. In connection with the denitrification method,
The present invention relates to a method for suppressing contamination of a separation membrane (reduction in membrane flux, increase in differential pressure) and performing stable and efficient treatment without chemical cleaning for a long time. 2. Description of the Related Art Conventionally, as a method for treating nitrogen-containing water, nitrogen-containing water is contacted with a microorganism under the supply of an electron donor to decompose and remove nitrogen by a biological denitrification reaction.
There is a method in which the denitrification-treated water is subjected to membrane separation treatment to be separated into a solid and a liquid to obtain permeated water as treated water. If the membrane separation of the denitrification treatment water is performed continuously for a long time by this method, the membrane flux may be reduced due to the contamination of the separation membrane, or a constant value may be maintained if a constant flow rate operation is performed. The differential pressure on the membrane flux increases. If the membrane flux is reduced or the differential pressure is increased due to the contamination of the membrane, the membrane is washed with a chemical or the membrane is replaced with a new one, so that the membrane flux is maintained at the predetermined membrane flux or the differential pressure again. Driving can be performed. Japanese Patent Publication No. 6-83838 proposes an apparatus for easily performing such chemical cleaning of a film. However, if the number of times of chemical cleaning of the film increases, the cleaning chemical cost and the maintenance cost increase. In addition, since the membrane is further deteriorated and the life of the membrane is shortened, the cost for replacing the membrane is increased. For this reason, a method is desired which not only devises a chemical cleaning means for the film but also suppresses the contamination of the film itself. [0006] On the other hand, Japanese Patent Application Laid-Open No. Hei 8-332495 discloses a method in which activated sludge treated water is subjected to membrane separation by measuring the respiration rate of the activated sludge and bringing the activated sludge closer to a state close to endogenous respiration. A method has been proposed in which membrane contamination is suppressed by performing membrane separation from water. Japanese Patent Application No. 8-302149 discloses a method of adding an electron donor for accelerating the denitrification reaction by performing intermittent aeration in an aeration tank and separating the treated water by membrane. It is described that, in order to accelerate the rise of the denitrification reaction at the start of the nitrification step (anaerobic step), an electron donor necessary for denitrification is rapidly added in the first 5 minutes after the start of the denitrification step. As described above, as a countermeasure against film contamination, as described in Japanese Patent Publication No. 6-83838, the equipment for chemical cleaning of the film is improved. Rather, it is desired to prevent the film itself from being contaminated, as described in JP-A-8-332495. However, when the activated sludge subjected to the denitrification reaction is subjected to membrane separation, according to the method described in JP-A-8-332495, the rate of contamination of the membrane can be suppressed to some extent by lowering the respiration rate of microorganisms, but it is not sufficient. There was no case. [0009] Japanese Patent Application No. 8-302149 describes a method of adding an electron donor, but it is not intended to prevent film contamination. The present invention solves the above-mentioned conventional problems. Under supply of an electron donor, nitrogen in wastewater is decomposed and removed by a denitrification reaction by microorganisms, and denitrification-treated water is subjected to membrane separation to form treated water. It is an object of the present invention to provide a biological denitrification method for solid-liquid separation into sludge, which can suppress membrane contamination and perform stable and efficient treatment for a long period of time. [0011] The biological denitrification method of the present invention is a denitrification method in which nitrogen-containing water is brought into contact with a microorganism under the supply of an electron donor to decompose nitrogen by a biological denitrification reaction. And a membrane separation step of membrane-separating the microorganism-containing mixed water of the denitrification step into treated water and sludge, wherein the denitrification step is performed during the supply of the electron donor. Supply rate of electron donor per microbial load
g-BOD / kg-VSS / day or less. According to the present invention, the supply rate of the electron donor in the denitrification reaction tank is set at 0.4 kg-BOD / per electron donor supply time and per microbial mass (VSS) in the tank.
kg-VSS / day or less, preferably 0.3 kg-B
By setting the OD / kg-VSS / day or less, the rate of film contamination can be reduced. Although the details of the mechanism for reducing the film contamination by keeping the electron donor supply rate at a low value are not clear, the following may be considered. That is, in a region where the supply speed of the electron donor is high, it is presumed that a high-molecular viscous substance or the like is secreted from the microorganism, and this substance promotes membrane contamination. Usually, an electron donor is used for a microorganism to produce energy (ATP) by a nitrate respiration reaction. Therefore, a high addition rate of the electron donor means that the metabolic reaction of the microorganism becomes active. It is considered that the membrane contaminants are secreted with the active metabolic reaction. On the other hand, in the present invention, by suppressing the supply rate of the electron donor, the secretion of this membrane contaminant can be suppressed, and the contamination rate of the membrane can be reduced. By the way, as a membrane separation method for denitrification-treated water, cross-flow filtration is generally employed in order to prevent contaminants from adhering to the membrane surface at a membrane surface flow rate. In this cross-flow filtration, a method of giving a cross-flow flow rate to the membrane surface includes a method of using a circulation pump and a method of providing a cross-flow flow rate by providing a membrane module in a circulation flow path generated by aeration (immersion membrane method). They are roughly divided into
Compared to a membrane filtration device that secures the flow rate of the membrane surface by pump circulation and prevents the adhesion of contaminants, the immersion membrane separates the contaminants by the disturbance of the aerated water flow. Cheap. Comparing with the most effective value of the film surface velocity for removing contaminants, the pump circulation type is 1-3 m
/ Sec, while the immersion film has a flow rate of 0.2
Approximately 0.5 m / sec, and the immersion film is more likely to adhere to contaminants. In addition, in the case of the immersion membrane system, the sludge in the membrane immersion tank is over-aerated because of strong aeration for the purpose of applying a sufficient aeration water flow to the membrane surface, resulting in a remarkably oxidizing environment. On the other hand, during the denitrification reaction, the environment becomes oxygen deficient, and especially when the supply rate of the electron donor is high, the electron accepting reaction due to nitrate reduction cannot catch up, and the inside of the microorganism is considered to be a remarkably reducing environment. As described above, since a drastic change in the environment is a stress for microorganisms, it is presumed that mucilage and the like contaminating the membrane are more likely to be released. From the above, the method of the present invention is particularly effective when applied to a denitrification treatment employing a submerged membrane system, and the generation of pollutants in the membrane by not giving a remarkable reducing environment to the sludge. And film contamination can be effectively prevented. Embodiments of the present invention will be described below in detail. The biological denitrification method of the present invention basically comprises a denitrification step in which nitrogen-containing water is brought into contact with a microorganism while supplying an electron donor to denitrify the water, and a membrane separation treatment of the denitrification-treated water. And a membrane separation step. In the denitrification step, in the presence of an electron donor,
Under anaerobic conditions, nitrate nitrogen and nitrite nitrogen in water are decomposed into nitrogen gas by denitrifying bacteria. The denitrification treatment method is not particularly limited, and may be any of a flotation method, a carrier addition method, a continuous treatment method, a batch treatment method, and an intermittent aeration method for continuous supply of raw water. Normally, organic nitrogen and ammonia nitrogen are present as nitrogen in water, so the denitrification treatment is carried out by combining the denitrification step and the nitrification step. For example, a method of sequentially passing water through a nitrification step and a denitrification step, a method of adding a finishing denitrification step to the method, a circulation method of circulating the nitrification step treatment water to a denitrification step, and a method of supplying raw water to a plurality of nitrification and denitrification steps. There is a step denitrification method for dispensing and the like, and the present invention can be applied to any treatment method. On the other hand, the membrane separation means for subjecting the denitrification-treated water to membrane separation is not particularly limited, and any means can be applied. As the separation membrane, MF (microfiltration) membrane, U
Although an F (ultrafiltration) membrane is used, the operating pressure is particularly low,
An MF film having a small rise in differential pressure is preferable. Examples of the membrane type include a hollow fiber membrane, a tubular membrane, and a flat membrane, and examples of the apparatus type include an immersion membrane type, a spiral type, a plate type, and a rotating disk type. As the pressure forming system of the membrane separation apparatus, any of a system for supplying raw water under pressure by a pump and a suction system for reducing the pressure on the permeated water side may be used. As described above, the membrane installation type is as follows.
There are an immersion membrane system and a non-immersion type pump circulation system. Among them, the immersion membrane system is more susceptible to membrane contamination than the pump circulation system, and the effect of preventing membrane contamination by applying the present invention is effectively exerted. Application to a separation device is preferred. [0025] The biological denitrification method of the present invention is a biological denitrification method in which the denitrification treatment water is subjected to membrane separation treatment in the biological denitrification method. (Electron donor supply rate per electron donor supply time per denitrification tank biomass; hereinafter simply referred to as “electron donor supply rate”) is 0.4 kg-BOD / The film contamination is prevented by controlling the pressure to not more than kg-VSS / day, preferably not more than 0.3 kg-BOD / kg-VSS / day. The electron donor added to promote the denitrification reaction may be any reducing substance that can be used for microorganisms (denitrifying bacteria), and any one can be used. In ordinary biological denitrification treatment, methanol is commonly used, but other lower organic substances such as ethanol and acetic acid may be used. In the present invention, the BOD of methanol is set to 1.0 k
g-BOD / kg-methanol. When BOD is contained in raw water, the BOD in the raw water is often effectively used as an electron donor in a denitrification reaction by introducing the raw water into a denitrification tank. In this case, if the BOD alone in the raw water does not provide sufficient reducing power to complete the denitrification reaction, an electron donor such as methanol is generally further supplied to the denitrification tank. According to the present invention, when all the electron donors such as methanol are added from the outside, B
It can be applied to the case where OD is used as an electron donor, or the case where BOD in raw water is used as an electron donor and the shortage is externally replenished. Also, during the period when the electron donor contributing to the denitrification is flowing into the denitrification tank, the total inflow of the electron donor into the denitrification tank is 0.4 kg-BOD / kg-VSS per VSS amount in the denitrification tank. / Day, preferably 0.3 kg-BOD / kg-VSS / day or less. If the supply rate of the electron donor is excessively reduced, the denitrification efficiency decreases and the nitrogen load needs to be excessively reduced. Therefore, the supply rate of the electron donor is 0.1 kg-BO.
D / kg-VSS / day or more is preferable. The method for adjusting the supply rate of the electron donor is as follows. The adjustment is made by directly increasing or decreasing the supply rate of the electron donor. It is adjusted by increasing or decreasing the amount of microorganisms in the denitrification tank. In the above method, the addition amount or the addition time of the electron donor and the inflow of raw water are adjusted so that the supply rate of the electron donor satisfies the above numerical conditions. In particular, when the denitrification reaction tank is an intermittent aeration tank, as described in Japanese Patent Application No. 8-302149, an electron donor is rapidly added during about the first 5 minutes of the denitrification step. When the method is adopted, the supply rate of the electron donor per the supply time of the electron donor (in this case, 5 minutes) becomes very large, and the rate of membrane contamination in the membrane separation step becomes high. Therefore, when the intermittent aeration method is used, 60 to 60
It is desirable to realize the invention by supplying the required amount of electron donor over 100% of the time. Other steps in intermittent aeration (nitrification step, phosphorus removal step, etc.)
If there is room for the reaction time, it is effective to shorten the time of these steps, take the denitrification step as long as possible, and reduce the supply rate of the electron donor as much as possible. In order to achieve the present invention, it is also effective to suppress load fluctuations and apply a load as uniform as possible. In other words, when a high load is applied, it is necessary to supply an appropriate amount of the electron donor, so that the supply speed of the electron donor is increased, and the contamination rate of the film is increased. On the other hand, by uniformly applying a load by using an adjustment tank or the like, the supply rate can be stably suppressed without changing the total supply amount of the electron donor, and the contamination of the film is suppressed. . The load may be adjusted by any conventionally known method as long as it satisfies the requirements of the present invention. However, it is usually preferable that the load at the peak time be 1.5 times or less the average load. The above method satisfies the supply rate of the electron donor of the present invention by maintaining the MLVSS in the denitrification tank at a high concentration. Usually, the MLSS concentration, MLV
SS concentration is 15000 mg / L or less, desirably 100
It was thought that the lower one, which is less than 00 mg / L, had the effect of reducing membrane contamination. However, in an apparatus for performing a denitrification reaction, if the MLVSS concentration was kept low, the electron donor supply rate per sludge would increase. Rather, it often results in increased membrane fouling. In such a case, the MLVSS concentration is kept high, and the supply rate of the electron donor per sludge is set to 0.4 kg-BOD / kg-VSS / da as in the present invention.
y or less, desirably 0.3 kg-BOD / kg-VSS
/ Day or less. According to the invention, the ML
Even in a high concentration region of a VSS concentration of 10,000 to 30,000 mg / L, solid-liquid separation can be performed with a low membrane contamination rate. However, MLVSS concentration 20000-30
When the concentration is set to 000 mg / L, the supply rate of the electron donor is further reduced to 0.025 kg-BOD / kg-VSS /
It is desirable to keep it below day. As described above, in order to increase the amount of sludge in the denitrification tank, the amount of sludge withdrawn from the denitrification tank or the amount of sludge returned from the membrane separation step may be adjusted. That is, compared to the suspended sludge concentration in the denitrification tank (denitrification tank or intermittent aeration tank) on the upstream side of the membrane separation step, for example, the suspended sludge concentration in the membrane separation tank is concentrated as shown by the following equation. If concentration is prevented by increasing the amount of returned sludge, the concentration of suspended sludge on the upstream side can be increased. ## EQU1 ## For example, when α = 1, the concentration of the suspended sludge on the upstream side is の of the concentration of the suspended sludge in the membrane separation tank. It can be increased to 3/4 of the concentration in the separation tank. Therefore, by setting α to 2 or more, preferably 3 or more, the supply rate of the electron donor of the present invention can be easily realized. In the biological denitrification method of the present invention, nitrogen which can be removed by a biological nitrification or denitrification reaction, such as organic nitrogen, ammonia nitrogen, nitrate nitrogen, nitrite nitrogen, etc. It can be applied to the treatment of all wastewater containing water. The present invention will be described more specifically below with reference to examples and comparative examples. Examples 1 and 2 and Comparative Examples 1 and 2 Landfill leachate was continuously removed by calcium coagulation and precipitation with sodium carbonate in the apparatus shown in FIG.
Biological denitrification treatment was performed by continuously passing water neutralized to 6.0 as raw water at 800 L / day. The capacity of each tank was as follows, and the water quality was adjusted as shown in Table 1 for each RUN. The BOD concentration was adjusted by adding a mixture of lower fatty acids mainly containing acetic acid, and the TN was adjusted by adding ammonium sulfate. Intermittent aeration tank 1: 800 L finishing nitric acid tank 2: 67 L finishing denitrification tank 3: 133 L Membrane separation tank 4: 322 L In the intermittent aeration tank 1, the cycle of the aerobic step (nitrification step) for 30 minutes and the anaerobic step (denitrification step) for 30 minutes is repeated, and the intermittent aeration tank 1 is filled with caustic soda (NaOH) or The pH was adjusted to around pH 7.2 by adding sulfuric acid (H ₂ SO ₄ ). In addition, methanol was used as an electron donor for denitrification and added in an anaerobic step. However, in RUN1, the required amount was added in 5 minutes immediately after the start of the anaerobic process, and in RUN2 and RUN3, the required amount was added continuously for 30 minutes in the anaerobic process. Sludge is extracted from the intermittent aeration tank 1 at a rate of 22.3 L / day, and the sludge retention time (SRT) per intermittent aeration tank 1 is set to 3
It was constant in 6 days. The intermittent aeration tank 1 was operated under the conditions described above, and the effluent from the intermittent aeration tank 1 was transferred to the finishing nitrification tank 2 and the finishing denitrification tank 3 sequentially, and further introduced into the membrane separation tank 4. In the finishing nitrification tank 2, the DO value of the liquid in the tank is 2 m
Aeration was performed so as to be at least g / L, and in the finishing denitrification tank 3, methanol was added in conjunction with the ORP meter and stirred so that the ORP became -100 to -150 mV. At this time, the methanol addition rate in the finishing denitrification tank 3 is 0 to 0.25 kg-methanol / kg-VSS / day.
Range. As the immersion membrane 5 of the membrane separation tank 4, a hollow fiber MF membrane “STERApore L (UMF424S) manufactured by Mitsubishi Rayon Co., Ltd.
LI) ". This membrane has a hollow fiber MF membrane made of hydrophilic polyethylene having a nominal pore diameter of 0.1 μm, an outer diameter of 410 μm, and an inner diameter of 270 μm arranged in a flat plate shape, and has a membrane area of 4 m ² . The membrane separation tank had a swirling flow structure in which a baffle plate 6 was installed, and air was aerated from below the membrane at 160 L / min (100 m ³ / m ² / hour per bottom area of the swirling flow rising portion). The film flux is 0.3 m ³ / m ² / day
The operation was performed for 8 minutes, and the intermittent operation was performed for a stop of 2 minutes. The pH of the membrane separation tank 4 was adjusted to about 7.2 by adding sulfuric acid, and the separated sludge was returned to the intermittent aeration tank 1. This returned sludge amount was set to be three times the raw water flow rate. FIG. 2 shows the change in transmembrane pressure in RUN1.
FIG. 3 shows changes in the transmembrane pressure in RUN2 to RUN4. The following is clear from FIGS. That is, in RUN1 (Comparative Example 1), methanol was rapidly added in the first 5 minutes of the anaerobic step.
As shown in Table 1, the feed rate of the electron donor was 0.88 k
The value was as high as g-BOD / kg-VSS / day.
At this time, the differential pressure of the membrane increased at an average of 0.87 kPa / day. Under these conditions, as shown in FIG. 2, the membrane differential pressure rises to about 26 kPa in one month, so that it is necessary to perform chemical cleaning almost every month. In addition, the standard of chemical cleaning is the time when the membrane differential pressure rises to 25 to 30 kPa or more in the operation with the membrane flux of 0.3 m ³ / m ² / day. On the other hand, in RUN2 (Example 1), methanol was continuously added during the anaerobic step, and the supply rate of the electron donor was 0.32 kg-BOD / kg-VSS / da.
It is kept at a value of about y, which is lower than that of RUN1. At this time, the rising speed of the transmembrane pressure is 0.1 kPa / day on average.
Thus, the level could be suppressed to a very low level as compared with RUN1. With this differential pressure increase rate, chemical cleaning once every six months is sufficient, and the frequency of chemical cleaning can be suppressed to 1/6 of that in RUN1. RUN3 (Comparative Example 2) was prepared by adding methanol in the same manner as in RUN2 and increasing the nitrogen load and the amount of methanol added. In this RUN3, the supply rate of the electron donor is 0.64 kg-BOD / kg-VSS.
/ Day, and the rate of rise of the transmembrane pressure during this time is 0.38 kPa / day, which is about four times as high as RUN2. This rate of increase in the membrane differential pressure is a rate that requires chemical cleaning once every two months, and it is necessary to perform chemical cleaning three times as frequently as RUN2. After the RUN3, the membrane was washed with a chemical, and the same conditions as those of the RUN2 were returned again to examine the increase in the differential pressure (RUN4 in FIG. 3: Example 2). Was. This also indicates that the supply rate of the electron donor is closely related to film contamination. From the above results, it can be seen that according to the present invention, by controlling the supply rate of the electron donor, it is possible to suppress an increase in the differential pressure of the film and to significantly reduce the frequency of chemical cleaning. As described above in detail, according to the biological denitrification method of the present invention, in the biological denitrification method in which denitrification-treated water is subjected to membrane separation treatment, an increase in the membrane differential pressure or a decrease in the membrane flux is achieved. Can be prevented, and the frequency of chemical cleaning can be greatly reduced.
As a result, it is possible to reduce costs such as the amount of medicine used and maintenance costs. It is also effective in preventing deterioration of the membrane, extending the life of the membrane and greatly reducing the cost of replacing the membrane.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a system diagram showing a biological denitrification apparatus used in an embodiment. FIG. 2 is a graph showing a change with time of a transmembrane pressure in RUN1 of Example 1. FIG. 3 is a graph showing a change with time of a transmembrane pressure in RUNs 2 to 4 of Example 1. [Description of Signs] 1 intermittent aeration tank 2 finishing nitrification tank 3 finishing denitrification tank 4 membrane separation tank 5 immersion membrane 6 baffle plate

Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C02F 3/12 C02F 3/28-3/34

Claims (1)

(57) [Claim 1] A denitrification step in which nitrogen-containing water is brought into contact with a microorganism under the supply of an electron donor to decompose nitrogen by a biological denitrification reaction, and the denitrification step. A membrane separation step of membrane-separating the microorganism-containing mixed water into treated water and sludge, wherein during the supply of the electron donor, electrons are supplied per amount of microorganisms in the denitrification step. 0.4kg-BOD
/ Kg-VSS / day or less.