AU2011203015B2

AU2011203015B2 - Wastewater treatment system and wastewater treatment method

Info

Publication number: AU2011203015B2
Application number: AU2011203015A
Authority: AU
Inventors: Makoto Onishi; Kiyokazu Takemura; Shinichi Yoshikawa
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 2010-07-15
Filing date: 2011-06-21
Publication date: 2012-12-20
Anticipated expiration: 2031-06-21
Also published as: JP5627322B2; CN102336499B; CN102336499A; JP2012020236A; AU2011203015A1

Abstract

WASTEWATER TREATMENT SYSTEM AND WASTEWATER TREATMENT METHOD Abstract 5 A supernatant water withdrawing pump (150) is driven, and supernatant water is withdrawn from to-be-treated water in an oxygen-free reactor tank (10). The supernatant water flows through a pipe (154). After flocculating agent is added from an inlet port (156) of the pipe (154), the flocculating agent is mixed by a line mixer (90). Further, the supernatant water flows through the pipe (154), and is returned back to a first 1o sedimentation basin (70).

Description

S&F Ref: 996260 AUSTRALIA PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT Name and Address Hitachi Plant Technologies, Ltd., of 5-2, Higashi of Applicant : Ikebukuro 4-chome, Toshima-ku, Tokyo, Japan Actual Inventor(s): Shinichi Yoshikawa Kiyokazu Takemura Makoto Onishi Address for Service: Spruson & Ferguson St Martins Tower Level 35 31 Market Street Sydney NSW 2000 (CCN 3710000177) Invention Title: Wastewater treatment system and wastewater treatment method The following statement is a full description of this invention, including the best method of performing it known to me/us: 5845c(5363256_1) 1 WASTEWATER TREATMENT SYSTEM AND WASTEWATER TREATMENT METHOD Field of the Invention 5 The present invention relates to a wastewater treatment system and a wastewater treatment method, and more particularly to a wastewater treatment system and a wastewater treatment method that uses a membrane bioreactor method obtaining treated water by filtering water through a membrane. 10 Background of the Invention In the past, membrane bioreactor methods are used in treatment systems for industrial drainage, wastewater, and the like. In the membrane bioreactor method, to-be treated water is treated by bioreaction processing using activated sludge in a biological reactor tank, and the to-be-treated water is filtered through a membrane element is immersed in the biological reactor tank, whereby treated water (membrane-filtered water) is obtained. In this method, the membrane filtration is employed as a method of solid liquid separation, and this can prevent outflow of suspended matter component to the treated water and can completely remove Escherichia coil. Therefore, the method is characterized by being capable of stably obtaining sanitary treated water with a high 20 degree of transparency. In addition, the activated sludge can be held at a high concentration, and this enables reducing the processing time and reducing the size of the processing facilities. As described above, the membrane bioreactor method is a processing method having extremely many advantages, but has a major drawback in that the membrane is 25 clogged. In particular, the clogging is caused by microscopic sludge not forming any flock and organic polymers such as metabolites of organisms. This clogging is the cause of disturbing stable operation of the membrane, and limiting the amount of filtered water per unit area (Flux). In order to solve this problem, there is a method for precipitating activated 30 sludge in a biological reactor tank intermittently or periodically, and removing supernatant water including microscopic sludge and the like, whereby the filtering processing is improved, and the stability of membrane operation is maintained. Furthermore, the amount of filtered water per unit area of the membrane (Flux) is increased or is prevented from decreasing (for example, see patent document 1).

2 The supernatant water withdrawn from the biological reactor tank includes microscopic sludge with a low sedimentation velocity, a microorganism and/or microorganism groups forming no flock, and organic polymers of microorganism metabolites that are not taken into the sludge. The above things flow into the biological 5 reactor tank again to reach the membrane, thereby clogging the filtering membrane. Accordingly, in the method disclosed in patent document 1, the supernatant water withdrawn from the biological reactor tank is discharged out of the wastewater treatment system. [Patent document 1] Japanese Patent Application Laid-Open No. 2004-141874 10 However, when a portion of the amount of raw water is withdrawn as the supernatant water and discharged out of the wastewater treatment system, the amount of water filtered by the membrane is decreased by the amount of water withdrawn as the supernatant water. Therefore, when the membrane bioreactor method is applied in order to reuse the treated water, there is a problem in that the amount of water that can be is reused is reduced. In contrast, the membrane filtration in the membrane bioreactor method is performed in order to only clean wastewater, and the treated water is simply discharged into environment such as river and ocean and is not particularly used for any other purpose other than that, the decrease of the amount of water through the membrane 20 filtration does not cause any particular problem. The withdrawn supernatant water is supernatant water to which the biological treatment is applied. Therefore, discharging the withdrawn supernatant water into environment does not cause any problem in terms of water quality. However, when the treated water filtered by the membrane is reused since the membrane bioreactor method has the feature that the membrane bioreactor method 25 provides water of an extremely higher quality than the quality of treated water provided by ordinary activated sludge method particularly when, an NF membrane (Nanofiltration Membrane) and an RO membrane (Reverse Osmosis Membrane) are employed for the membrane filtration in order to reuse the treated water, a decrease of the amount of membrane-filtered water directly results in a decrease of the amount of reused water. 30 If the withdrawn supernatant water is not discharged into the environment outside of the system but is returned back into the wastewater treatment system, the reduction of the amount of membrane-filtered water can be prevented. However, the microscopic sludge with a low sedimentation velocity, the microorganism and/or microorganism groups forming no flock, and the organic polymers of microorganism 35 metabolites that are not taken into the sludge, which are discharged with the supernatant 3 water, may flow into the biological reactor tank again and reach the membrane. This causes an adverse effect in that the filtering membrane is clogged. Object of the Invention 5 It is the object of the present invention to substantially overcome or at least ameliorate one or more of the foregoing disadvantages. Summary A wastewater treatment system according to an aspect of the present invention io includes a biological reactor tank for filtering to-be-treated water through a membrane using activated sludge and separating treated water therefrom and one or a plurality of first sedimentation basins provided at a stage prior to the biological reactor tank to directly or indirectly provide the to-be-treated water to the biological reactor tank. The wastewater treatment system includes supernatant water withdrawing means for 15 withdrawing supernatant water from the biological reactor tank, flocculating agent adding means for adding a flocculating agent to the supernatant water withdrawn by the supernatant water withdrawing means, mixing means for mixing the supernatant water and the flocculating agent added by the flocculating agent adding means, and supernatant water returning means for returning the supernatant water mixed by the mixing means 20 back to at least one of the plurality of first sedimentation basins and/or a raw water supply tank for supplying raw water to the first sedimentation basin. According to an embodiment of the present invention, in the wastewater treatment system, the supernatant water is withdrawn from the biological reactor tank, and the flocculating agent is added and mixed to the withdrawn supernatant water. Then, 25 the mixed supernatant water is returned back to at least one of the plurality of first sedimentation basins provided at a stage prior to the biological reactor tank and/or a raw water supply tank for supplying raw water to the first sedimentation basin. Therefore, even when the supernatant water is withdrawn from the to-be-treated water, the amount of to-be-treated water filtered through the membrane is not reduced, and clogging of the 30 membrane can be prevented. In the wastewater treatment system, the supernatant water returning means may include a pipe for connecting the biological reactor tank with the first sedimentation basin to which the supernatant water is returned back, and the flocculating agent adding means and the flocculating agent mixing means may be provided in the pipe.

4 According to an embodiment of the present invention, the flocculating agent can be easily added and mixed to the supernatant water within the pipe. A wastewater treatment method according to another aspect of the present invention is a wastewater treatment method performed by a wastewater treatment system. 5 The wastewater treatment system includes a biological reactor tank for filtering to-be treated water through a membrane using activated sludge and separating treated water therefrom and one or a plurality of first sedimentation basins provided at a stage prior to the biological reactor tank to directly or indirectly provide the to-be-treated water to the biological reactor tank. The wastewater treatment method includes a supernatant water 10 withdrawing step for withdrawing supernatant water from the biological reactor tank, a flocculating agent adding step for adding a flocculating agent to the supernatant water withdrawn in the supernatant water withdrawing step, a mixing step for mixing the supernatant water and the flocculating agent added in the flocculating agent adding step, and a supernatant water returning step for returning the supernatant water mixed in the 15 mixing step back to at least one of the plurality of first sedimentation basins and/or an raw water supply tank for supplying raw water to the first sedimentation basin. In the wastewater treatment method according to an embodiment of the present invention, in the supernatant water returning step, the supernatant water in the biological reactor tank may be returned back via the pipe for connecting the biological reactor tank 20 with the first sedimentation basin to which the supernatant water is returned back, and in the flocculating agent adding step, the flocculating agent may be injected through an inlet port provided in the middle of the pipe, and in the flocculating agent mixing step, a mixer provided downstream of the inlet port of the pipe may mix the injected flocculating agent and the supernatant water. 25 According to an embodiment of the present invention, in the wastewater treatment system, the supernatant water is withdrawn from the biological reactor tank, and the flocculating agent is added and mixed to the withdrawn supernatant water. Then, the mixed supernatant water is returned back to at least one of the plurality of first sedimentation basins provided at a stage prior to the biological reactor tank and/or a raw 30 water supply tank for supplying raw water to the first sedimentation basin. Therefore, even when the supernatant water is withdrawn from the raw water, the amount of treated water filtered through the membrane to be reused is not reduced, and clogging of the membrane can be prevented.

5 Brief Description of the Drawings A preferred form of the present invention will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 is a figure illustrating a wastewater treatment system according to a first 5 embodiment of the present invention; Fig. 2 is a flow diagram illustrating steps of withdrawing supernatant water by membrane bioreactor method performed by the wastewater treatment system according to the first embodiment; Fig. 3 is a figure illustrating a wastewater treatment system according to a second io embodiment of the present invention; Fig. 4 is a flow diagram illustrating steps of withdrawing supernatant water that are performed by the wastewater treatment system according to the second embodiment; Fig. 5 is a figure illustrating a wastewater treatment system according to a third embodiment of the present invention; is Fig. 6 is a flow diagram illustrating steps of withdrawing supernatant water that are performed by the wastewater treatment system according to the third embodiment; Fig. 7 is a figure illustrating a wastewater treatment system according to a fourth embodiment of the present invention; Fig. 8 is a figure illustrating a wastewater treatment system according to a fifth 20 embodiment of the present invention; and Fig. 9 is a figure illustrating a wastewater treatment system according to a sixth embodiment of the present invention. Description of the Preferred Embodiments 25 Embodiment of the present invention will be hereinafter described with reference to the attached drawings. (First embodiment) Fig. I is a figure illustrating a wastewater treatment system according to the first embodiment of the present invention. As shown in the figure, the wastewater treatment 30 system includes a flow amount adjustment tank 80, a first sedimentation basin 70, and a biological reactor tank 100. Industrial drainage and wastewater discharged from a factory and a house are passed through the above in this order as raw organic water. The biological reactor tank 100 includes an oxygen-free reactor tank 10, an aerobic reactor tank 20, and a membrane separation reactor tank 30. The oxygen-free 35 reactor tank 10 performs denitrification processing by denitrifying bacteria in activated 6 sludge included in raw water under oxygen-free condition, thus decomposing nitric acid and nitrous acid into nitrogen gas and water. An oxygen-free reactor tank agitation device 120 is provided on the bottom of the oxygen-free reactor tank 10 in order to accelerate biological reaction by agitating the raw water. 5 In the aerobic reactor tank 20, phosphorus is taken into the activated sludge under aerobic condition, and nitrification processing is performed with nitrification bacteria in the activated sludge. An aerobic reactor tank aeration device 170 is provided on the bottom of the aerobic reactor tank 20 in order to generate air bubbles to provide air to the to-be-treated water. 1o In the membrane separation reactor tank 30, a membrane 40 is installed for solid liquid separation. The membrane 40 separates membrane-filtered water (treated water) and sludge, and recovers the separated membrane-filtered water to reuse the membrane filtered water. The shape of the membrane 40 may be in any one of flat membrane, hollow fiber, tubular, and monolith. The material of the membrane 40 may also be an is organic material such as PVDF, PAN, and CA or may be an inorganic material such as ceramics and metal. The bottom portion of the membrane separation reactor tank 30 is arranged with a membrane cleaning diffuser device 110 for cleaning the membrane 40 by generating bubbles and pipes such as opening/closing devices such as valves, not shown, for 20 removing remaining sludge 180. The membrane separation reactor tank 30 and the oxygen-free reactor tank 10 are connected via a pipe 130 for circulating sludge, and the pipe 130 is arranged with a sludge circulation pump 145. The to-be-treated water in the membrane separation reactor tank 30 is returned from the membrane separation reactor tank 30 via the pipe 130 to the oxygen 25 free reactor tank 10, and is reprocessed there. The oxygen-free reactor tank 10 and the first sedimentation basin 70 are connected via a pipe 154. The pipe 154 is connected to an upper position of the oxygen free reactor tank 10. At the end portion of the pipe 154 protruding to the inside of the reactor tank, a port 152 is provided to suck the supernatant water. A supernatant water 30 removing valve 160, a supernatant water withdrawing pump 150, a line mixer 90, an inlet port 156 for injecting a flocculating agent 60 into the pipe 154 are arranged at an outside portion of the oxygen-free reactor tank 10 of the pipe 154. The supernatant water withdrawing pump 150 sucks the supernatant water in the oxygen-free reactor tank 10 via the port 152 and the pipe 154, and pumps the supernatant water into the first 35 sedimentation basin 70. The supernatant water removing valve 160 is opened and closed, 7 and this controls start and stop of flow of the supernatant water from the oxygen-free reactor tank 10 to the first sedimentation basin 70. The inlet port 156 is an opening through which the flocculating agent 60 is injected when the supernatant water flows through the pipe 154. The line mixer 90 is provided downstream of the inlet port 156, the 5 line mixer 90 mixes the supernatant water flowing in the pipe 154 from the oxygen-free reactor tank 10 and the flocculating agent 60 injected from the inlet port 156. Further, the wastewater treatment system includes a sludge circulation pump 145, a supernatant water withdrawing pump 150, a line mixer 90, an oxygen-free reactor tank agitation device 120, an aerobic reactor tank aeration device 170, and a control unit 10 200 for controlling opening/closing of a valve and operation of the membrane cleaning diffuser device 110 and the like. In this case, supernatant water withdrawing means is constituted by the pipe 154, the port 152 at the end of the pipe, the supernatant water withdrawing pump 150, and the supernatant water removing valve 160. Alternatively, the supernatant water withdrawing is means may be constituted only by the supernatant water removing valve 160 without installing or using the supernatant water withdrawing pump 150. In this case, the supernatant water is withdrawn by opening/closing the supernatant water removing valve 160 using difference of water levels. Flocculating agent adding means is constituted by the inlet port 156 of the flocculating agent 60. Mixing means is consttuted by the line 20 mixer 90. The added flocculating agent 60 is not particularly limited. The added flocculating agent 60 may be selected according to substances included in the supernatant water of the facility from among inorganic flocculating agents such as ferric chloride, poly aluminum chloride, aluminium sulfate, and iron (II) polysulfate, organic polymer 25 flocculating agents such as polyacrylamides, and biodegradable flocculating agents using, e.g., chitosan and polyglutamic acid. Before or at the time when the flocculating agent 60 is added, an acid or alkaline agent may be added in order to adjust pH. Characteristics of sludge such as viscosity, particle size distribution, filtration ratio resistance, supernatant water transparency, supernatant water turbidity, color of 30 treated water, and TOC may be monitored using a sludge characteristic measuring device, and the supernatant water may be withdrawn when the values of the sludge characteristics attain certain values, or the supernatant water may be withdrawn periodically with a timer setting. When the wastewater treatment system having the above configuration is 35 operated normally, raw water, i.e., organic to-be-treated water of industrial drainage and 8 wastewater discharged from a factory and a house, flows into the flow amount adjustment tank 80. In the flow amount adjustment tank 80, the amount of flow of the raw water into the first sedimentation basin 70 is adjusted, and the raw water flows from the flow amount adjustment tank 80 to the first sedimentation basin 70. The to-be-treated water 5 140 overflowing out of the first sedimentation basin 70 flows into the oxygen-free reactor tank 10. In the oxygen-free reactor tank 10, denitrification processing is performed under oxygen-free condition. The to-be-treated water flows from the oxygen-free reactor tank 10 into the aerobic reactor tank 20, and in the aerobic reactor tank 20, nitrification io processing is performed under aerobic condition. The to-be-treated water flows from the aerobic reactor tank 20 to the membrane separation reactor tank 30, and in the membrane separation reactor tank 30, solid-liquid separation is performed with the membrane 40. The separated membrane-filtered water is recovered and reused. On the other hand, the separated to-be-treated water is returned back to the oxygen-free reactor tank 10 via the is pipe 130 as circulation sludge, and is circulated in the wastewater treatment system. Further, remaining sludge 180 is withdrawn. Fig. 2 is a flow diagram illustrating steps of withdrawing supernatant water by membrane bioreactor method performed by the wastewater treatment system according to the embodiment. When the steps for withdrawing the supernatant water are started during 20 normal filtration operation described above (step S 101), the control unit 200 stops flow of the to-be-treated water 140, stops the oxygen-free reactor tank agitation device 120 in the oxygen-free reactor tank 10, and stops sludge circulation (step S 102), and the to-be treated water is kept still in the oxygen-free reactor tank 10 to precipitate the sludge (step S103). 25 After the oxygen-free reactor tank 10 is kept still for a certain period of time to precipitate the sludge, the supernatant water withdrawing pump 150 is driven to withdraw the supernatant water to the pipe 154 via the port 152 (step S104). The entire supernatant water may not be always withdrawn. In some cases, sludge at an upper portion of the partially precipitated sludge may be included. 30 Subsequently, the flocculating agent 60 is added from the inlet port 156 of the pipe 154 to the withdrawn supernatant water flowing through the pipe 154 (step S 105), the supernatant water added with the flocculating agent 60 is mixed by the line mixer 90 (step S 106), and this is returned back to the first sedimentation basin 70 (step S107). After the supernatant water is returned, the to-be-treated water 140 starts to flow, 35 and the oxygen-free reactor tank agitation device 120 agitates the water and resume the 9 sludge circulation (step S 108), whereby the steps for withdrawing the supernatant water are finished, and normal operation is resumed (step S109). As described above, the supernatant water withdrawn from the oxygen-free reactor tank 10 is not discharged out of the wastewater treatment system but is returned 5 back to the first sedimentation basin 70, so that the withdrawn supernatant water can be filtered through the membrane in the membrane separation reactor tank 30. Therefore, the supernatant water can be withdrawn without reducing the amount of membrane-filtered water recovered from the wastewater treatment system. The supernatant water added with the flocculating agent 60 includes microscopic sludge with a low sedimentation velocity, io a microorganism and/or microorganism groups forming no flock, and organic polymers of microorganism metabolites that are not taken into the sludge, which deteriorate the stability of the membrane filtration. These are coarsed and precipitated in the first sedimentation basin 70, so that they are removed in the first sedimentation basin 70. Those not removed in the first sedimentation basin 70 are taken into the activated sludge is in the oxygen-free reactor tank 10, and they do not clog the membrane 40 by coming into contact with the membrane 40. Therefore, stable operation can be maintained. It should be noted that a flocculating agent mixing tank having an agitation function may be used as the flocculating agent adding means and the mixing means. However, flocks may not be completely precipitated in the first sedimentation basin 70. 20 Instead, the flocks may be taken into the activated sludge in the oxygen-free reactor tank 10, the aerobic reactor tank 20, or the membrane separation reactor tank 30. Therefore, it is not necessary to coarse the flocks to such an extent that the flocks are completely precipitated in the first sedimentation basin 70. Therefore, it is only sufficient to inject the flocculating agent 60 into the pipe 154 and sufficiently mix it in the line mixer 90, and it 25 is not necessary to make large flocks. Further, since the flocculating agent 60 is added only to the supernatant water, the target that should be flocculated is a very small amount of, i.e., several dozens to several hundred mg/L of, the microscopic sludge with a low sedimentation velocity, the microorganism and/or microorganism groups forming no flock, and the organic polymers of microorganism metabolites that are not taken into the 30 sludge, which are included in the supernatant water. Therefore, the amount of the added flocculating agent 60 is greatly less than the amount added when it is directly added to the biological reactor tank 100. It should be noted that the supernatant water may be returned back to the flow amount adjustment tank 80 serving as a raw water supply tank depending on the capacity 35 of the first sedimentation basin 70. The flow amount adjustment tank 80 may be used 10 instead of the sedimentation basin. Alternatively, as shown in Fig. 1, the supernatant water may be returned back to both of the first sedimentation basin 70 and the flow amount adjustment tank 80. (Second embodiment) 5 Fig. 3 is a figure illustrating a wastewater treatment system according to the second embodiment of the present invention. The present embodiment is different from the first embodiment in that the supernatant water is not withdrawn from the oxygen-free reactor tank 10 but is withdrawn from the aerobic reactor tank 20. Therefore, in the present embodiment, the pipe 154 and the port 152 for withdrawing the supernatant water 10 are provided not in the oxygen-free reactor tank 10 but in the aerobic reactor tank 20. Fig. 4 is a flow diagram illustrating steps of withdrawing supernatant water that are performed by the wastewater treatment system according to the present embodiment. When the steps for withdrawing the supernatant water are started during normal operation (step S101), a control unit 200 stops flow of to-be-treated water 140 and stops sludge is circulation and an aerobic reactor tank aeration device 170 in the aerobic reactor tank 20 (step S202), and the to-be-treated water is kept still in the aerobic reactor tank 20 to precipitate the sludge (step S203). The processings from step S104 to step S107 are the same as those of the first embodiment. In step S208, the to-be-treated water 140 starts to flow, and the sludge circulation and the aeration by the aerobic reactor tank aeration 20 device 170 are restarted, whereby the steps for withdrawing the supernatant water are finished, and normal operation is resumed (step S 109). (Third embodiment) Fig. 5 is a figure illustrating a wastewater treatment system according to the third embodiment of the present invention. The present embodiment is different from the first 25 embodiment in that the supernatant water is not withdrawn from the oxygen-free reactor tank 10 but is withdrawn from the membrane separation reactor tank 30. Therefore, in the present embodiment, the pipe 154 and the port 152 for withdrawing the supernatant water are provided in the membrane separation reactor tank 30. Fig. 6 is a flow diagram illustrating steps of withdrawing supernatant water that 30 are performed by the wastewater treatment system according to the embodiment. When the steps for withdrawing the supernatant water are started during normal operation (step S 101), a control unit 200 stops flow of to-be-treated water 140, stops sludge circulation, stops withdrawing remaining sludge 180, and stops the membrane cleaning diffuser device 110 of the membrane separation reactor tank 30 and filtration through the 35 membrane 40 in the membrane separation reactor tank 30 (step S302), and the to-be- 11 treated water are kept still in the membrane separation reactor tank 30 to precipitate the sludge (step S303). The processings from step S104 to step S107 are the same as those of the first embodiment. In step S308, the to-be-treated water 140 starts to flow, the sludge circulation is resumed, the withdrawing of the remaining sludge 180 is resumed, the 5 membrane cleaning diffuser device 110 in the membrane separation reactor tank 30 is resumed, and the filtration through the membrane 40 in the membrane separation reactor tank 30 is resumed, whereby the steps for withdrawing the supernatant water are finished, and normal operation is resumed (step S109). As described in the second embodiment and the third embodiment shown above, io the pipe 154 and the port 152 constituting the supernatant water withdrawing means can be provided in any one of the reactor tanks including the oxygen-free reactor tank 10, the aerobic reactor tank 20, and the membrane separation reactor tank 30 constituting the biological reactor tank 100. (Fourth embodiment) is Fig. 7 is a figure illustrating a wastewater treatment system according to the fourth embodiment. The present embodiment is different from the first embodiment in that the aerobic reactor tank 20 is not included as a constituent element, and a membrane separation reactor tank 30 according to the present embodiment also serves as the aerobic reactor tank 20 and the membrane separation reactor tank 30 according to the first and 20 third embodiments. The structure of the apparatus and the steps for withdrawing the supernatant water other than the above are the same as those of the first embodiment. (Fifth embodiment) Fig. 8 is a figure illustrating a wastewater treatment system according to the fifth embodiment. The present embodiment is different from the first embodiment in that the 25 membrane separation reactor tank 30 is not included as a constituent element, and a membrane 40 is housed in a casing, so that a side stream-type is formed in which the membrane 40 is installed outside of the aerobic reactor tank 20. The structure of the apparatus and the steps for withdrawing the supernatant water other than the above are the same as those of the first and third embodiments. 30 (Sixth embodiment) Fig. 9 is a figure illustrating a wastewater treatment system according to the sixth embodiment. The present embodiment is different from the first embodiment in that the supernatant water withdrawing means is constituted by a float-type withdrawing pump 158 floating on water. The structure of the apparatus and the steps for withdrawing the 35 supernatant water other than the above are the same as those of the first embodiment.

12 It should be noted that the supernatant water withdrawing means may be constituted by a combination of one or a plurality of ports 152 and one or a plurality of float-type withdrawing pumps 158. In the embodiments described above, the flow amount adjustment tank 80 and 5 the first sedimentation basin 70 are provided at a stage prior to the biological reactor tank 100. However, the structure is not limited thereto. At a stage prior to the biological reactor tank 100, two or more sedimentation basins 70 may be provided. In this case, the supernatant water may be returned back to at least one of the two or more sedimentation basins 70. 10 Industrial Applicability The wastewater treatment system and wastewater treatment method described above can be used to clean industrial drainage, wastewater, and the like.

Claims

1. A wastewater treatment system comprising: a biological reactor tank for filtering to-be-treated water through a membrane 5 using activated sludge and separating treated water therefrom; and one or a plurality of first sedimentation basins provided at a stage prior to the biological reactor tank to directly or indirectly provide the to-be-treated water to the biological reactor tank; supernatant water withdrawing means for withdrawing supernatant water from io the biological reactor tank; flocculating agent adding means for adding a flocculating agent to the supernatant water withdrawn by the supernatant water withdrawing means; mixing means for mixing the supernatant water and the flocculating agent added by the flocculating agent adding means; and 15 supernatant water returning means for returning the supernatant water mixed by the mixing means back to at least one of the plurality of first sedimentation basins and/or an raw water supply tank for supplying raw water to the first sedimentation basin.

2. The wastewater treatment system according to claim 1, wherein the supernatant water returning means includes a pipe for connecting the 20 biological reactor tank with the first sedimentation basin to which the supernatant water is returned back, and wherein the flocculating agent adding means and the flocculating agent mixing means are provided in the pipe.

3. A wastewater treatment method performed by a wastewater treatment 25 system, the wastewater treatment system including: a biological reactor tank for filtering to-be-treated water through a membrane using activated sludge and separating treated water therefrom; and one or a plurality of first sedimentation basins provided at a stage prior to the 30 biological reactor tank to directly or indirectly provide the to-be-treated water to the biological reactor tank, the wastewater treatment method comprising: a supernatant water withdrawing step for withdrawing supernatant water from the biological reactor tank; 14 a flocculating agent adding step for adding a flocculating agent to the supernatant water withdrawn in the supernatant water withdrawing step; a mixing step for mixing the supernatant water and the flocculating agent added in the flocculating agent adding step; and 5 a supernatant water returning step for returning the supernatant water mixed in the mixing step back to at least one of the plurality of first sedimentation basins and/or an raw water supply tank for supplying raw water to the first sedimentation basin.

4. The wastewater treatment method according to claim 3, wherein in the supernatant water returning step, the supernatant water in the 10 biological reactor tank is returned back via the pipe for connecting the biological reactor tank with the first sedimentation basin to which the supernatant water is returned back, in the flocculating agent adding step, the flocculating agent is injected through an inlet port provided in the pipe, and in the flocculating agent mixing step, a mixer provided downstream of the inlet is port of the pipe mixes the injected flocculating agent and the supernatant water.

5. A wastewater treatment system substantially as hereinbefore described with reference to any one of the embodiments of the system as that embodiment is shown in Figs. I and 2, 3 and 4, 5 and 6, 7, 8 or 9 of the accompanying drawings.

6. A wastewater treatment method substantially as hereinbefore described 20 with reference to any one of the embodiments of the method as that embodiment is shown in Figs. I and 2, 3 and 4, 5 and 6, 7, 8 or 9 of the accompanying drawings. Dated 15 June 2011 Hitachi Plant Technologies, Ltd. 25 Patent Attorneys for the Applicant/Nominated Person SPRUSON & FERGUSON