JP2007167774A - Apparatus for treating activated sludge - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a membrane separation activated sludge treatment apparatus efficiently carrying out a denitrification reaction in a denitrification tank even if a dissolved oxygen amount in a nitrification tank is increased. <P>SOLUTION: This apparatus keeps the nitrification tank (4) immersing a membrane filtration unit (5) having a membrane module (9) and an air diffuser (15) in activated sludge arranged on the downstream side of the denitrification tank (3), and also a dissolved oxygen reducing tank (6) arranged adjacent to the upstream side of the denitrification tank (3). A part of the activated sludge is returned from the nitrification tank (4) to this dissolved oxygen reducing tank (6). By reducing the dissolved oxygen amount in raw water (drainage) containing a part of the activated sludge in the dissolved oxygen reducing tank (6), the dissolved oxygen amount in the raw water to be fed to the denitrification tank (3) can substantially be reduced to 0 mg/L, so that the denitrification reaction is activated. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a membrane separation activated sludge treatment apparatus for biologically continuously treating a large amount of wastewater containing organic matter and bacteria, and more particularly to an activated sludge treatment apparatus capable of activating a denitrification reaction in a denitrification tank. .

  According to the conventional membrane separation activated sludge system, the waste water (raw water) from which relatively small impurities are removed by the fine screen is introduced into the raw water adjustment tank. In this raw water adjustment tank, the liquid level is measured by a liquid level gauge, and the first liquid feed pump is intermittently operated to adjust the liquid level height in the tank to a predetermined range. Generally, the raw water delivered by the first liquid feed pump is introduced into the denitrification tank, and then the denitrified raw water overflowing from the denitrification tank is allowed to flow into the adjacent nitrification tank. A membrane filtration unit is immersed in this nitrification tank. In this nitrification tank, sludge is nitrified and phosphorus is ingested, and the membrane is separated into activated sludge and treated water by a membrane filtration unit. The treated water separated from the membrane is sent to the treated water tank by a suction pump. On the other hand, the solid content (suspended substance) of the sludge that has been aerated in the nitrification tank and propagated is sent to and stored in the sludge storage tank. Part of the sludge inside the nitrification tank is returned to the denitrification tank by the second liquid feed pump, and circulates between the denitrification tank and the nitrification tank.

  As an example of a suitable membrane filtration unit, for example, JP 2000-51672 A (Patent Document 1) can be cited. The membrane filtration unit includes a hollow fiber membrane module incorporating a plurality of sheet-like hollow fiber membrane elements in which a large number of hollow fiber membranes are arranged in parallel, and an air diffuser disposed below the hollow fiber membrane module. ing. The hollow fiber membrane module is composed of a plurality of sheets arranged with their membrane surfaces parallel to each other, and the overall shape thereof has a substantially rectangular parallelepiped shape. On the other hand, in the air diffuser, for example, a plurality of air diffuser pipes each having a hole formed in a pipe made of metal, resin or the like are arranged in parallel, and one end of each air diffuser pipe is connected to a blower. When treating air sewage such as domestic wastewater and factory effluent by generating air bubbles from the air diffuser, contact the organic matter in the sludge of the nitrification tank with the air generated from the air diffuser in the presence of aerobic microorganisms. As a result, the organic matter is adsorbed and metabolically decomposed by the aerobic microorganism, and biological treatment is performed.

  The hollow fiber membrane module and the air diffuser are surrounded by a shielding plate on all four sides. The shielding plate serves as a wall portion for generating a gas-liquid mixed flow by the rising of the bubbles generated from the diffuser and guiding the flow from the upward flow to the downward flow. The gas-liquid mixed flow generated from the diffuser does not scatter in an oblique direction, but rises straight and efficiently contacts the hollow fiber membrane module. At this time, the hollow fiber membrane module is uniformly washed by uniform dispersion of the gas-liquid mixed flow with respect to the membrane surface of the hollow fiber membrane module. In addition, the biological treatment is efficiently performed by this mixed liquid mixture, and solid-liquid separation is performed by the filtration function of the hollow fiber membrane. One end of an extraction pipe line is connected to the membrane separation unit, and a suction pump is connected to the other end, and treated water (filtered water) filtered by the membrane filtration unit is taken out through the extraction pipe line. It is transferred to the treated water tank.

  In general, the sheet-like membrane module is not limited to a hollow fiber membrane, and may be, for example, a flat membrane type, a tubular membrane type, a bag-like membrane type, or the like as long as it has a filtration membrane having a plurality of fine holes. Various known separation membranes can be applied. Examples of the material include cellulose, polyolefin, polysulfone, PVDF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene), and ceramics.

  The average pore size of the micropores formed in this sheet-like membrane module is 0.001 to 0.1 μm in the average pore size in a membrane generally called an ultrafiltration membrane, 0.1 to 0.1 μm in the membrane generally called a microfiltration membrane. 1 μm. For example, when used for solid-liquid separation of activated sludge, the pore diameter is preferably 0.5 μm or less, and when sterilization is required as in the case of filtration of purified water, the pore diameter is preferably 0.1 μm or less.

  The membrane separation activated sludge treatment system biologically purifies raw water with activated sludge in a denitrification tank and a nitrification tank (aerobic tank). Nitrogen is removed by a so-called nitrification denitrification reaction by circulating sludge between the denitrification tank and the nitrification tank. The organic matter converted into BOD is aerobically oxidized and decomposed by the air discharged from the air discharge part of the membrane filtration unit, which is mainly an aeration apparatus disposed in the nitrification tank. The removal of phosphorus is performed by being taken into the microorganism as polyphosphoric acid by the action of microorganisms (phosphorus accumulating bacteria) in the sludge. This microorganism takes up phosphorus in an aerobic state and releases phosphorus stored in the body in an anaerobic state. When repeatedly exposed to anaerobic and aerobic conditions, phosphorus-accumulating bacteria absorb more phosphorus in an aerobic state than the amount of phosphorus released in the anaerobic state.

  Some of the nitrogen compounds such as biological excrement and carcasses are assimilated into plants and bacteria as fertilizers. Some of these nitrogen compounds are oxidized to nitrous acid and nitric acid by autotrophic ammonia oxidizing bacteria and independent nitrite oxidizing bacteria under aerobic conditions with a lot of oxygen. On the other hand, under anaerobic conditions without oxygen, microorganisms called denitrifying bacteria produce nitrous acid from nitric acid instead of oxygen, and further reduce to dinitrogen monoxide and nitrogen gas to release nitrogen. This reduction reaction is referred to as the nitrification denitrification reaction.

  A membrane separation activated sludge treatment apparatus based on such a nitrification denitrification reaction is disclosed, for example, in International Publication No. 03/101896 (Patent Document 2). According to Patent Document 2, when sending sludge as a circulating liquid from a nitrification tank to a denitrification tank, the sludge is taken out from the vicinity of the bottom of the bottom of the diffuser at the lowest position arranged in the nitrification tank. Thus, nitrogen and phosphorus are removed using only two treatment tanks, a nitrification tank and a nitrification tank, without using a flocculant or the like.

In the membrane separation activated sludge treatment apparatus, the position for taking out the sludge as the circulating liquid is further separated by 20 cm or more downward from the air diffuser disposed at the lowest position. By doing so, the dissolved oxygen concentration is 0.2 mg / L or less at the position where the sludge fed from the nitrification tank is introduced into the denitrification tank, and the dissolved oxygen concentration is 0.5 mg / L at the position where the sludge is taken out from the nitrification tank. L or less.
JP 2000-51672 A WO03 / 101896 pamphlet

  By the way, according to the description in Patent Document 2, organic substances are decomposed anaerobically by denitrifying bacteria when dissolved oxygen, nitrate ions, and nitrite ions are not substantially present in the anoxic tank (denitrification tank). The polyphosphoric acid accumulated in the nitrifying bacteria is released out of the cells as phosphoric acid. According to the invention described in Patent Document 1, the dissolved oxygen concentration at the site where the circulating fluid (sludge) from the aeration tank enters the anoxic tank needs to be 0.2 mg / L or less, and 0.1 mg / L If it is L or less, it is preferable because phosphorus removability is more stable, and if it is 0.05 mg / L or less, it is more preferable. However, there is a limit even if it is attempted to reduce the dissolved oxygen concentration of the circulating fluid by the method according to Patent Document 1. Further, in recent years, advanced nitrogen removal has been demanded, and in order to increase the removal rate in the above-described circulation type nitrification denitrification method, it is often necessary to increase the circulation ratio for feeding liquid from the nitrification tank to the denitrification tank. I came. When the circulation rate is increased, the amount of dissolved oxygen brought into the denitrification tank increases, the denitrification tank is less likely to become anaerobic, and nitrate ions may not be completely denitrified in the denitrification tank.

  An object of the present invention is a membrane separation activated sludge treatment method using a circulatory nitrification denitrification method, which further reduces the dissolved oxygen concentration in the sludge returned to the denitrification tank and improves the phosphorus removal performance. To provide a processing device.

  The purpose is to sequentially arrange a denitrification tank and a nitrification tank, which are the basic configuration of the present invention, to circulate sludge between the denitrification tank and the nitrification tank to biologically treat the wastewater, and to the nitrification tank. Is an activated sludge treatment apparatus in which one or more membrane filtration units are immersed, and the wastewater is separated into activated sludge and treated water, and dissolved to reduce the amount of dissolved oxygen in the treated wastewater upstream of the denitrification tank. An oxygen reduction tank is arranged, and is effectively achieved by an activated sludge treatment apparatus characterized by having a sludge circulation path for circulating the activated sludge of the nitrification tank to the dissolved oxygen reduction tank or its upstream part. .

  According to a preferred embodiment, upstream of the dissolved oxygen reduction tank, raw water is intermittently sent to the dissolved oxygen reduction tank via a liquid supply pipe by a liquid supply means, and the denitrification tank or the nitrification tank Adjust the liquid level. And the liquid feeding from a raw | natural water adjustment tank may be distributed to a dissolved oxygen reduction tank and a denitrification tank, and the liquid feeding distribution means for the liquid feeding can also be provided further.

Effect

  Raw water is stored in the dissolved oxygen reduction tank for a required time. The raw water of the stored dissolved oxygen reduction tank originally contains dissolved oxygen. In addition, according to this type of membrane separation activated sludge treatment method, sludge having been aerated from a nitrification tank and having a high dissolved oxygen concentration is returned to the denitrification tank for circulation. In the above-mentioned Patent Document 2, a region having a low dissolved oxygen concentration in the nitrification tank is selected and sludge having a relatively low dissolved oxygen concentration is returned to the denitrification tank, but the amount of dissolved oxygen cannot be reduced to a concentration close to 0. .

  In that respect, according to the present invention, as described above, the dissolved oxygen reduction tank is arranged on the upstream side of the denitrification tank, and the raw water is fed to the BOD component in the raw water so that the nitrification tank is consumed. The dissolved oxygen brought in from is consumed rapidly. Since there is almost no dissolved oxygen in the sludge overflowing from the dissolved oxygen reduction tank, the denitrification tank is stably maintained in an oxygen-free state and can be denitrified at high speed. Since a BOD component is required for the denitrification reaction, in some cases, raw water is distributed not only to the dissolved oxygen reduction tank but also to the denitrification tank. Since this leads to an increase in the anaerobic state of the denitrification tank, it is possible to release sufficient phosphorus.

  When the raw water from the raw water adjustment tank is distributed to the dissolved oxygen reduction tank and the denitrification tank, it is necessary to be able to adjust the distribution ratio. When distributing from a branch liquid supply line with an open / close valve, it is difficult to adjust the distribution ratio. Usually, a variable capacity pump is provided in each liquid supply path, and a suitable distribution ratio is determined by each pump. It is desirable that the liquid is fed under the condition of the above or distributed via a distribution tank.

  Hereinafter, preferred embodiments of the present invention will be described in detail, but the present invention can be modified in various ways within the scope of the claims and should not be construed as being limited to the following embodiments.

  FIG. 1 is a schematic configuration diagram of an activated sludge treatment apparatus showing an example of an embodiment of the present invention. In the figure, reference numeral 1 is a fine screen, reference numeral 2 is a raw water adjustment tank, reference numeral 3 is a denitrification tank, reference numeral 4 is a nitrification tank, reference numeral 5 is a membrane filtration unit, reference numeral 7 is a sludge storage tank, reference numeral 8 is a treated water tank. Show. Reference numeral 6 denotes a dissolved oxygen reduction tank which is the most characteristic configuration of the present invention.

  In the present embodiment, the dissolved oxygen reduction tank 6 is disposed between the raw water adjustment tank 2 and the denitrification tank 3. The dissolved oxygen reduction tank 6 is fed with raw water continuously or intermittently and returns the circulating sludge from the nitrification tank. The average residence time of the dissolved oxygen reduction tank 6 varies depending on the circulation ratio, sludge concentration, BOD of raw water, etc., but about 30 minutes to 3 hours is appropriate. If it is too short, the dissolved oxygen will not be reduced sufficiently. In the dissolved oxygen reduction tank, the dissolved oxygen brought in from the nitrification tank is consumed by using the raw water BOD component of the microorganism.

  In the present embodiment, as shown by the solid line in FIG. 1, the raw water in the raw water adjustment tank 2 is intermittently sent to the dissolved oxygen reduction tank 6 through the liquid supply path L1 by the first liquid feed pump P1. As indicated by a virtual line in the figure, the first liquid feeding path L1 may be branched and the branch path L1 ′ may be extended to the denitrification tank 3. In that case, it is preferable to install an opening / closing valve 25 in the branch path L1 ′. When the opening / closing valve 25 is closed, the raw water in the raw water adjustment tank 2 is sent exclusively to the dissolved oxygen reduction tank 6, 25 is opened, and the raw water in the raw water adjustment tank 2 is distributed and sent to the dissolved oxygen reduction tank 6 and the denitrification tank 3. The distribution ratio of the raw water at this time can be arbitrarily determined by adjusting the opening degree of the opening / closing valve 25. However, it is difficult to adjust the distribution ratio of raw water only by adjusting the opening degree of the opening / closing valve 25. Therefore, normally, variable capacity pumps (not shown) are arranged in the respective liquid supply passages, and the liquids are supplied under a suitable distribution ratio by the respective pumps, or are distributed through distribution tanks (not shown). To.

  The reason why a part of the raw water is sent to the denitrification tank 3 is to secure a BOD component for denitrification in the denitrification tank 3. If the BOD is excessively processed in the dissolved oxygen reduction tank 6, the BOD component for denitrification is insufficient, and the denitrification rate may be reduced. In the dissolved oxygen reduction tank 6, there may be a BOD component necessary for the dissolved oxygen to be consumed. The distribution ratio is appropriately determined based on the dissolved oxygen concentration in the dissolved oxygen reduction tank, the quality of the treated water, and the like.

  Furthermore, in this embodiment, as shown in FIG. 1, the denitrified sludge from the denitrification tank 3 is actively sent to the nitrification tank 4 through the second liquid feed path L2 by the second liquid feed pump P2. Part of the activated sludge in the nitrification tank 4 is continuously returned to the dissolved oxygen reduction tank 6 through the third liquid feed path L3 by the third liquid feed pump P3. Therefore, a part of the activated sludge in the nitrification tank 4 circulates between the dissolved oxygen reduction tank 6 and the denitrification tank 3 to be subjected to nitrification denitrification treatment.

  Thus, by disposing the dissolved oxygen reduction tank 6 adjacent to the upstream side of the denitrification tank 3, the waste water (raw water) is separated into fine impurities through the fine screen 1 and sent to the raw water adjustment tank 2. It is. In the raw water adjustment tank 2, the first liquid feed pump P <b> 1 is driven to feed the raw water into the dissolved oxygen reduction tank 6 where oxygen dissolved in the raw water and a part of the activated sludge returned from the nitrification tank 4 is supplied. The amount of dissolved oxygen is reduced to almost zero. The wastewater whose dissolved oxygen concentration is close to 0 flows into the denitrification tank 3 due to overflow from the dissolved oxygen reduction tank 6, and is efficiently denitrified and released of phosphorus. The drainage from which the denitrification and the release of phosphorus have been performed is actively sent from the denitrification tank 3 to the nitrification tank 4 by the first liquid feed pump P1 through the second liquid feed path L2. The wastewater from which the denitrification and phosphorus are released is subjected to oxidative decomposition of organic matter in the nitrification tank 4, nitrification by aerobic bacteria, and intake of phosphorus to grow activated sludge. In the nitrification tank 4, membrane separation is performed by the membrane filtration unit 5 into activated sludge solids and treated water, and the treated water is sucked by the suction pump Pv and sent to the treated water tank 8. A part of the excess sludge generated at this time is returned to the denitrification tank 3 via the third liquid feeding pump P3 as described above, and the remainder of the excess sludge is discharged to the sludge storage tank 7.

  In the present embodiment, one or more membrane filtration units 5 are immersed in parallel in the nitrification tank 4. According to the illustrated example, two membrane filtration units 5 are shown for easy understanding. However, a single unit may be used, or a plurality of two or more groups may be arranged. When a large amount of wastewater treatment is performed, it is preferable to arrange 4 or more and 50 or less membrane filtration units 5 in parallel. The membrane filtration unit 5 includes a hollow fiber membrane module 9 and an air diffuser 15 as shown in FIG.

  As shown in FIG. 2, the hollow fiber membrane module 9 has a substantially cubic shape in which a large number of sheet-like hollow fiber membrane elements 10 are arranged in parallel at a predetermined interval. In this embodiment, the hollow fiber membrane element 10 is formed into a sheet by arranging a number of porous hollow fibers 10a in parallel as shown in FIG. 3, and one end side of each porous hollow fiber 10a is fixed. The resin 11 is closed and fixed, and the other end is fixed with the fixing resin 11 by opening the hollow portion of each porous hollow fiber 10a. Examples of the material of the porous hollow fiber 10a include cellulose, polyolefin, polysulfone, PVDF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene), and ceramics. Further, the sheet-like membrane element is not limited to a hollow fiber membrane, and may be, for example, a flat membrane type, a tubular membrane type, a bag-like membrane type, or the like as long as it has a filtration membrane having a plurality of fine holes. Various known separation membranes can be applied.

  The average pore size of the micropores formed in the sheet-like hollow fiber membrane element 10 is 0.001 to 0.1 μm in the average pore size in a membrane generally called an ultrafiltration membrane, and 0 in the membrane generally called a microfiltration membrane. .1 to 1 μm. For example, when used for solid-liquid separation of activated sludge, the pore diameter is preferably 0.5 μm or less, and when sterilization is required as in the case of filtration of purified water, the pore diameter is preferably 0.1 μm or less.

The membrane area of the hollow fiber membrane element 10 according to the present embodiment is 25 m ² per sheet, the membrane area of the hollow fiber membrane module 9 is 500 m ² , and one hollow fiber membrane element 10 is used for one day. 400t of waste water can be treated. In this connection, 20 pieces, 40 pieces, and 60 pieces of hollow fiber membrane elements 10 can be incorporated in one membrane filtration unit 5, and the total membrane area per unit of the membrane filtration unit 5 is 500 m ² or 1000 m ^2. The processing amount per unit is 1500 m ² and 400 t, 800 t, and 1200 t per day.

  Therefore, in order to carry out a treatment of 10,000 t or more per day, it is necessary to immerse 10 or more membrane filtration units side by side in a single treatment tank (nitrification tank) even if the largest membrane filtration unit is used. Usually, the plurality of membrane filtration units 5 immersed in these single tanks are each sucked by a single suction pump Pv through the same suction header via a branch pipe and are collectively processed as treated water. When processing by this single pump is made into one series, in order to further increase the processing amount, naturally the pump capacity must be increased and the processing tank must be enlarged, but the number of membrane filtration units is further increased, and in some cases the series is increased. You must also increase the number.

  As shown in FIG. 2, the hollow fiber membrane module 9 has an upper opening end of a sheet-like hollow fiber membrane element 10 in which a large number of porous hollow fibers 10 a are arranged in parallel via a fixing resin 11. The lower end 13 is closed and fixedly supported by the lower frame 13 through the fixing resin 11, and both ends of the filtered water outlet 12 and the lower frame 13 are connected to a pair of vertical rods. 14 is configured to be supported. A large number of hollow fiber membrane elements 10 are accommodated in substantially the entire volume of a rectangular cylindrical upper casing 20 in which the upper and lower end surfaces are opened with the porous hollow fiber 10a being vertical, and are supported in parallel in a sheet form.

The porous hollow fiber 10a according to the present embodiment uses a hollow fiber made of PVDF (polyvinylidene fluoride) that is hollow in the longitudinal direction along the center, and the pore diameter of the filtration hole is 0.4 μm. It is. The effective membrane area per sheet is 25 m ² . Twenty sheets of the sheet-like hollow fiber membrane element 10 are used per membrane filtration unit 5, and the size is 30 mm in depth, 1250 mm in width, and 2000 mm in height. The size of the single membrane filtration unit 5 including the air diffuser 8 is 1552.5 mm in depth, 1447 mm in width, and 3043.5 mm in height. The filtered water outlet 12 is made of ABS resin, and the vertical rod 14 is made of SUS304. However, the material such as the porous hollow fiber 10a, the filtered water take-out pipe 12 and the vertical rod 14, the size of the hollow fiber membrane element 10, the size of one membrane filtration unit, and the number of hollow fiber membrane elements 10 per unit The number of sheets can be variously changed according to the application. For example, the number of hollow fiber membrane elements 10 can be arbitrarily set to 20, 40, 60,... According to the processing amount.

  As shown in FIG. 3, an outlet 12a for high quality treated water filtered by each porous hollow fiber 10a is formed at one end of the filtrate outlet pipe 12 of each hollow fiber membrane element 10. In this embodiment, as shown in FIG. 2 and FIG. 3, L-shaped joints 12 b are attached to the respective outlets 12 a in a liquid-tight manner via sealing materials. Further, a water collection header pipe 21 is provided horizontally along the upper end edge where the outlet 12a at the upper end of the upper casing 20 is formed. A water collection port 21a is formed in each of the water collection header pipes 21 at a position corresponding to the plurality of outlets 12a provided in the filtrate water extraction pipe 12, and each of the water collection ports 21a has the above-described outlets. An L-shaped joint 21b similar to 12a is liquid-tightly attached via a sealing material. The treated water outlet 12a of the filtered water outlet 12 and the water outlet 21a of the water header 21 are connected to each other by connecting the L-shaped joints 12b and 21b attached to each other. . At one end of the water collection header pipe 21, a water suction port 21 c connected through the suction pump Pv and the suction pipe line 22 is formed. As shown in FIG. 1, the water inlet 21 c formed for each water collecting header pipe 21 and the suction pipe line 22 are open / close valves interposed in branch pipe lines 22 a branched from the suction pipe line 22. 23 are connected.

  As shown in FIG. 4, one air diffuser 15 is formed of a rectangular cylindrical body that is open at the top and bottom coupled to the lower end of the upper casing 20, and includes four support columns 24 a that extend downward from the lower ends of the four corners. It is accommodated and fixed at the bottom of the lower casing 24 provided. The air diffuser 15 includes a plurality of pipe-shaped air diffusers 17 made of an elongated metal or a synthetic resin whose both ends are fixedly supported by a rectangular frame. The air diffuser 17 is usually formed with an air ejection slit or a plurality of air ejection holes extending in the length direction on the lower surface side. One end of the air diffuser 17 is closed, and the other end communicates with an air introduction pipe 16 branched from an air main pipe 18 extending from the aeration blower B via an open / close valve 19.

  According to the illustrated example, the main body of the diffuser tube 17 is composed of a rubber tube with a slit, and a slit (not shown) communicating inward and outward along the length direction is formed on the lower surface of the horizontally disposed rubber tube. Is formed. The air diffuser 15 is preferably disposed at a distance of 45 cm downward from the lower surface of the hollow fiber membrane module 9, and the column 24 a protrudes downward from the lower casing 24 to be opened to the outside. Desirable for smooth sludge flow.

  In addition, the air diffuser 15 according to the present embodiment is arranged for each of the plurality of membrane filtration units 5, in order to divert the air sent from a single aeration blower B to each air diffuser 15. The air main pipe 18 is directly connected to the aeration blower B, and the air main pipe 18 is connected to the air introduction pipe 16 of each air diffuser 15. The end of the air introduction pipe 16 on the side of the air main pipe is disposed outside the tank. Actually, the opening / closing valve 19 is provided at the end of the air introduction pipe 16 outside the tank. It is desirable to be able to do it outside the tank.

  In the present invention, a plurality of membrane filtration units 5 having the above-described configuration illustrated are immersed in the same nitrification tank 4 and juxtaposed between the denitrification tank 3 and the nitrification tank (aerobic tank) 4. As described above, the biochemical activated sludge treatment as described above is performed.

  By the way, when performing such activated sludge treatment, if the amount of dissolved oxygen is large in the denitrification tank 3, the denitrification bacteria will preferentially proliferate and proliferate, thereby inhibiting the denitrification reaction. Therefore, in the denitrification tank 3, the amount of dissolved oxygen is as small as possible, preferably 0.05 mg / L or less. However, as well as the conventional general activated sludge treatment, the sludge in the region where the dissolved oxygen amount dissolved in the activated sludge in the nitrification tank 4 is the least is denitrified as disclosed in Patent Document 2 above. Even if it is made to return, the amount of dissolved oxygen in the sludge introduction region of the denitrification tank 3 is 0.2 mg / L at most, and it is difficult to reduce the amount of dissolved oxygen beyond that. On the contrary, in the nitrification tank 4, the aerobic environmental condition required by nitrifying bacteria requires about 1 to 2 mg of dissolved oxygen. Therefore, an air diffuser 15 is attached to the membrane filtration unit 5 as described above, and air is released into the sludge to generate a swirling flow so that oxygen is dissolved in the sludge simultaneously with the stirring of the sludge. .

  The release of air into the sludge not only promotes the agitation of the sludge and the dissolution of oxygen, but is also used for cleaning the membrane surface of the membrane filtration unit 5. Therefore, the hole diameter of the air discharge hole formed in the air diffuser 17 in the normal air diffuser 15 is formed to be relatively large. As a result, the dissolution of air into the sludge often decreases, and in addition to the air diffuser 15, the fine air discharge holes are provided and fine bubbles that can efficiently dissolve oxygen are released. An auxiliary diffuser is also arranged in the nitrification tank 4 in a region where the dissolved oxygen amount is small.

  Thus, since the conventional activated sludge treatment requires a high dissolved oxygen concentration in the nitrification tank 4, the activated sludge returned from the nitrification tank 4 to the denitrification tank must also have a dissolved oxygen amount dissolved therein. The amount of dissolved oxygen cannot be reduced below a certain value.

  On the other hand, by disposing the dissolved oxygen reduction tank 6 adjacent to the upstream side of the denitrification tank 3 as in the present embodiment, the dissolved oxygen reduction tank 6 is supplied before the raw water containing the circulating sludge is sent to the denitrification tank 3. Even if the dissolved oxygen amount required for the activated sludge in the nitrification tank 4 is satisfied, the dissolved oxygen amount can be reduced to almost 0 mg / L in the dissolved oxygen reduction tank 6, The denitrification reaction in the denitrification tank 3 is efficiently performed.

It is a schematic block diagram of the membrane separation activated sludge processing apparatus which shows typical embodiment of this invention. It is the whole solid figure which cuts off an example of a part of membrane filtration unit applied to the same membrane separation activated sludge processing apparatus. It is a three-dimensional view showing the connection relationship between the separation membrane element applied to the membrane filtration unit and the water collection header pipe. It is a schematic block diagram which shows an example of the diffuser applied to the said membrane filtration unit.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Fine screen 2 Raw water adjustment tank 3 Denitrification tank 4 Nitrification tank 5 Membrane filtration unit 6 Dissolved oxygen reduction tank 7 Sludge storage tank 8 Treated water tank 9 Hollow fiber membrane module 10 (sheet-like) hollow fiber membrane element 10a Porous hollow fiber DESCRIPTION OF SYMBOLS 11 Resin for fixing 12 Filtrated water extraction pipe 12a Filtrated water outlet 12b L-type joint 13 Lower frame 14 Vertical gutter 15 Air diffuser
16 Air introduction pipe (branch pipe)
17 Aeration pipe 18 Air main pipe 19 Open / close valve 20 Upper casing 21 Water collecting header pipe 21a Water collecting port 21b L-shaped joint 21c Water inlet 22 Suction pipe 22a Branch pipe 23 Opening / closing valve 24 Lower casing 24a Post 25 Opening / closing valve P1 First feed Liquid pump P2 Second liquid pump P3 Third liquid pump L1 First liquid path L1 'Branch path L2 Second liquid path L3 Third liquid path Pv Suction pump B Aeration blower

Claims (3)

A denitrification tank and a nitrification tank are sequentially arranged, and sludge is circulated between the denitrification tank and the nitrification tank to biologically treat the waste water, and at least one membrane filtration unit is immersed in the nitrification tank, In an activated sludge treatment device that separates wastewater into activated sludge and treated water,
A dissolved oxygen reduction tank that reduces the amount of dissolved oxygen in the treated waste water is disposed upstream of the denitrification tank, and has a sludge circulation path that circulates the activated sludge of the nitrification tank to the dissolved oxygen reduction tank or its upstream part. An activated sludge treatment apparatus characterized by comprising   On the upstream side of the dissolved oxygen reduction tank, raw water is intermittently sent to the dissolved oxygen reduction tank via a liquid supply line by liquid supply means, and the liquid level of the denitrification tank or nitrification tank is adjusted. The activated sludge treatment apparatus according to claim 1, wherein a raw water adjustment tank is arranged.   The activated sludge treatment apparatus according to claim 2, further comprising a liquid feed distribution means for distributing the liquid feed from the raw water adjustment tank to the dissolved oxygen reduction tank and the denitrification tank.

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