JP2007069204A - Water treatment method, water treatment apparatus and method of manufacturing regenerated water - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing regenerated water by the combination of a membrane separation activated sludge method and a membrane separation treatment method using reverse osmosis membrane by which the deterioration of the permeation performace or the separation performance of the reverse osmosis membrane cuased by the propagation of microorganisms or the deposition of the microorganisms or the metabolite is prevented and an apparatus for the same. <P>SOLUTION: In the water treatment method having a step for treating water to be treated with activated sludge in a biological treatment vessel, membrane-separating the activated sludge-treated water in the biological treatment and reverse osmosis-treating the membrane separated water, ultraviolet treatment is carried out after the membrane separation treatment and before the reverse osmosis treatment. Further after heavy metal ions and a reducing agent are added, a reducing agent is added right before the reverse osmosis treatment. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a water treatment method, a water treatment apparatus, and a method for producing reclaimed water, in which sewage such as sewage is subjected to activated sludge treatment, membrane separation treatment, and reverse osmosis treatment using a reverse osmosis membrane.

  Since ancient times, sewage such as sewage has been treated with microorganisms mainly by the activated sludge process. In addition, when high water quality is required for treated water from the viewpoint of environmental conservation, so-called advanced treatments such as coagulation sedimentation, sand filtration, and ozone treatment are performed after biological treatment to remove components that cannot be treated by biological treatment alone. It has been broken.

  In recent years, a membrane separation activated sludge method using a precision membrane or an ultrafiltration membrane in place of the final sedimentation basin in the normal activated sludge method has been developed and is becoming popular. The membrane separation activated sludge method is a treatment method that uses a separation membrane in place of the final sedimentation basin of the normal activated sludge method. (1) Keeping the biomass (generally referred to as MLSS) in the biological reactor high and reducing the installation area (2) The turbid component (generally referred to as SS) does not flow out into the treated water, and there is an advantage that a clear treated water can be obtained.

  Furthermore, along with the recent increase in water demand, a proposal is made to treat the treated water treated using the membrane separation activated sludge method using a reverse osmosis membrane, etc., and use the obtained water as reclaimed water. (See, for example, Patent Document 1). By using this method, it is said that reclaimed water with extremely high water quality can be obtained, and its use is being studied in areas where water is insufficient.

  However, in the treatment method combining this membrane separation activated sludge method and the membrane separation treatment method using a reverse osmosis membrane, the water to be treated supplied to the reverse osmosis membrane was treated using the membrane separation activated sludge method. In the treated water, there arises a problem that organic components and inorganic components contained in the treated water clog the reverse osmosis membrane, that is, a so-called fouling problem. In particular, so-called biofouling, in which biofilm is formed on a reverse osmosis membrane and causes fouling, is regarded as one of the most troublesome problems in actual plant operation.

  As a method for suppressing biofouling, a method of adding a small amount of chlorine to water to be treated is generally used. When adding chlorine, the film deteriorates due to the oxidizing power of chlorine or an oxidizing substance (oxidant) generated by reaction with chlorine. Therefore, a reducing agent is further added to dechlorinate.

  On the other hand, in order to suppress biofouling of reverse osmosis membranes, when desalinating seawater, it is necessary to sterilize the sand filter for removing suspended matter in seawater and microorganisms in seawater that has passed through the sand filter. There has been proposed a seawater desalination apparatus including an ultraviolet sterilizer (see, for example, Patent Document 2).

  However, in the reuse of sewage, especially in the treatment method that combines membrane separation activated sludge process and reverse osmosis membrane treatment, organic components that cause biofilm formation in the treated water supplied to the reverse osmosis membrane treatment As a result, biofouling suppression technology useful for this treatment method has not yet been found, and a useful technology is desired.

JP-A-4-305287 JP-A-2004-25018

  In the present invention, sewage such as sewage is subjected to activated sludge treatment and then membrane separation treatment (hereinafter collectively referred to as “membrane separation activated sludge treatment”) and then reverse osmosis by a reverse osmosis membrane. In a water treatment method and a water treatment apparatus that performs treatment, biofouling that occurs on the surface of a reverse osmosis membrane caused by microorganisms and nutrients contained in treated water that has been subjected to membrane separation activated sludge treatment is suppressed. With the goal.

  The present invention for solving this problem has the following configuration.

(1) The treated water is treated with activated sludge in a biological treatment tank, the activated sludge-treated water is subjected to membrane separation treatment in the biological treatment tank, and then the membrane-separated water is subjected to reverse osmosis treatment. The water treatment method which has a process WHEREIN: An ultraviolet treatment is performed to the to-be-processed water after the said membrane separation process and before the said reverse osmosis process.
(2) After adding a heavy metal ion and a reducing agent to the water to be treated after the membrane separation treatment and before the reverse osmosis treatment, a reducing agent is added immediately before the reverse osmosis membrane treatment. The water treatment method according to (1) above, which is characterized.

(3) At least one additional sterilization selected from the group consisting of ozone treatment, hydrogen peroxide treatment, chlorine treatment, and photooxidation catalyst treatment after the membrane separation treatment and before the reverse osmosis treatment The water treatment method according to (1) or (2) above, wherein treatment is performed.
(4) The above (1) to (1), wherein after the membrane separation treatment and before the reverse osmosis treatment, additional purification treatment of at least one of activated carbon treatment and sand filtration treatment is further performed. The water treatment method according to any one of (3).
(5) The water treatment method according to (2), wherein heavy metal ions and a reducing agent are added in a tank provided in the middle of a water channel between the membrane separation treatment and the reverse osmosis treatment.

(6) The water treatment method according to any one of (1) to (5) above, wherein a flocculant is added to the water to be treated containing activated sludge stored in the biological treatment tank.
(7) Any one of the above (1) to (6), wherein the water to be treated between the membrane separation treatment and the reverse osmosis treatment in the biological treatment tank is always placed in a light-shielded environment. The water treatment method as described in any one of.

(8) A biological treatment tank, a filtration membrane immersed in the biological treatment tank, and a water treatment device provided with a reverse osmosis membrane, wherein the water treatment device is subsequent to the filtration membrane, A water treatment device, wherein an ultraviolet treatment device is installed in front of the reverse osmosis membrane.
(9) The above (8), characterized in that a heavy metal ion addition device and a reducing agent addition device are connected to a tank or a pipe located after the filtration membrane and before the reverse osmosis membrane. The water treatment method as described in any one of.

(10) An activated carbon treatment device, an ozone treatment device, a sand filtration treatment device, a hydrogen peroxide treatment device, a chlorination treatment device, and a photooxidation catalyst treatment device, which are subsequent to the filtration membrane and before the reverse osmosis membrane. The water treatment device according to (8) or (9), wherein at least one device selected from the group consisting of:
(11) The water treatment apparatus according to (9), wherein a tank is provided after the filtration membrane and before the reverse osmosis membrane.

(12) The apparatus according to any one of (8) to (11) above, wherein a device for adding a flocculant to water to be treated containing activated sludge stored in the biological treatment tank is installed. Water treatment equipment.
(13) Treating sewage containing a dirt substance with the water treatment method according to any one of (1) to (7) above, or water according to any one of (8) to (12) above A method for producing reclaimed water, characterized in that, when reclaimed water is produced by water treatment using a treatment device, the permeate obtained by reverse osmosis treatment using the reverse osmosis membrane is reclaimed water.

  According to the present invention, in a water treatment method and a water treatment apparatus for performing reverse osmosis treatment with a reverse osmosis membrane after subjecting sewage such as sewage to activated sludge treatment and membrane separation treatment, membrane separation activated sludge treatment was performed. It is possible to effectively suppress biofouling that occurs due to microorganisms and nutrients contained in the treated water and is formed on the reverse osmosis membrane surface.

  The water treatment method of the present invention is a water treatment method comprising a step of subjecting water to be treated to membrane separation activated sludge treatment and then a step of performing reverse osmosis treatment using a reverse osmosis membrane. The obtained treated water is subjected to ultraviolet treatment and then subjected to reverse osmosis treatment.

  By performing the ultraviolet treatment, sterilization of microorganisms and soluble organic substances contained in the treated water obtained by the membrane separation activated sludge treatment are decomposed. In the present invention, since the ultraviolet ray treatment is performed on the water having almost no turbidity after the membrane separation activated sludge treatment, the efficiency of the ultraviolet ray treatment can be increased. Further, according to the present invention, it is possible to avoid many problems existing in the case of chlorination, including that the injection of the dechlorinating agent can be omitted as compared with the case of chlorination.

  FIG. 1 shows an example of a water treatment apparatus for reusing water to be treated, which combines a membrane separation activated sludge method and a membrane separation treatment method using a reverse osmosis membrane, which are used in the water treatment method of the present invention. The schematic of is shown. Filtration membrane 2 for filtering treated water 1 to obtain filtered water, biological treatment tank 3 for immersing the filtration membrane 2 in treated water 1, and filtering treated water 1 with filtration membrane 2 The filtered water tank 4 for storing the filtered water obtained in this manner, the ultraviolet ray processing device 5 for ultraviolet-treating the filtered water, the pump 6 for taking out and pressurizing the filtered water from the filtered water tank 4, and the pressure It comprises a reverse osmosis membrane 7 for reverse osmosis treatment of filtered water.

  Here, the filtration membrane 2 has, for example, a flat membrane element structure in which the filtration membrane is bonded to both sides of the frame with the filtration water channel material sandwiched between them in order to improve the handleability and physical durability of the filtration membrane. It is desirable. The structure of the filtration membrane 2 is not particularly limited and may be an element using a hollow fiber membrane, but the flat membrane element structure is soiled by shearing force when a flow velocity parallel to the membrane surface is applied. This is suitable for the present invention. The flat membrane element structure includes a rotating flat membrane structure.

  Examples of the membrane structure of the filtration membrane 2 include a porous membrane and a composite membrane in which a functional layer is combined with the porous membrane, but is not particularly limited. Specific examples of these membranes include polyacrylonitrile porous membrane, polyimide porous membrane, polyethersulfone porous membrane, polyphenylene sulfide sulfone porous membrane, polytetrafluoroethylene porous membrane, polyvinylidene fluoride porous membrane, polypropylene Examples of the porous film include a porous film and a porous film such as a polyethylene porous film, and a polyvinylidene fluoride porous film and a polytetrafluoroethylene porous film are particularly preferable because of high chemical resistance. Furthermore, a composite film in which a rubbery polymer such as cross-linked silicone, polybutadiene, polyacrylonitrile butadiene, ethylene propylene rubber, or neoprene rubber is compounded as a functional layer can be given as a functional layer.

  The biological treatment tank 3 is not particularly limited as long as the treated water can be stored and the filtration membrane 2 can be immersed in the treated water, and a concrete tank, a fiber reinforced plastic tank, or the like is preferably used. Moreover, the inside of the processing tank 3 may be divided into a plurality. When the inside of the processing tank 3 is divided into a plurality of parts, as shown in FIG. 2, a part of the processing tank is used as a tank (membrane separation activated sludge tank 32) in which the filtration membrane 2 is immersed, and the other is removed. It may be used as the nitriding tank 31 so that the water to be treated is circulated between the divided tanks.

  The activated sludge introduced into the biological treatment tank 3 is generally used for wastewater treatment and the like, and as the seed sludge, drawn sludge from other wastewater treatment facilities is usually used. In the membrane separation activated sludge method, the operation is performed at a sludge concentration of about 2,000 mg / L to 20,000 mg / L. The activated sludge method makes it possible to purify water by using microorganisms as contaminants in the wastewater.

  The filtered water tank 4 is not particularly limited as long as the filtered water filtered by the filtration membrane 2 can be stored, and a concrete tank, a fiber reinforced plastic tank, and the like are preferably used. Moreover, in order to filter to-be-processed water with the filtration membrane 2, you may provide a pump etc. between the filtration membrane 2 and the filtration water tank 4, and in order to apply a head pressure difference, the filtered water in the filtration water tank 4 The liquid level may be lower than the surface of the water to be treated in the treatment tank 3.

  The ultraviolet treatment device 5 is intended for the sterilization of microorganisms in the treated water (filtered water) obtained by the membrane separation activated sludge treatment and the decomposition of the soluble organic matter. Optimum ones such as an irradiation type for irradiating, an immersion type for immersing a lamp in water, an external illuminating type for irradiating running water from the outside, an internal illuminating type for running from the inside, and the like can be adopted. Ultraviolet rays have the strongest bactericidal effect at 250 to 260 nm, and light of about 185 nm is effective for oxidizing organic substances. It is thought that the bactericidal effect by ultraviolet rays is exhibited by damaging microbial DNA and RNA by vibration.

  In the present invention, examples of the ultraviolet treatment apparatus include those using a low-pressure mercury lamp that irradiates ultraviolet rays having a wavelength of about 185 nm and ultraviolet rays of a wavelength of about 254 nm. Absent.

The amount of ultraviolet rays necessary for sterilization varies greatly depending on the type of microorganisms to be sterilized, but by targeting 30,000 μW · sec / cm ² or more, most bacteria can be sterilized by 99% or more. is there. The amount of ultraviolet rays (30,000 μW · sec / cm ² ) is expressed by ultraviolet irradiation intensity (30,000 μW / cm ² ) × ultraviolet irradiation time (sec).

  The pump 6 is not particularly limited as long as the filtered water can be pressurized, and is a centrifugal pump, diffuser pump, spiral mixed flow pump, mixed flow pump, piston pump, plunger pump, diaphragm pump, gear pump, screw pump, Vane pumps, cascade pumps, jet pumps, etc. can be used, but since they can be easily pressurized to the pressure required for reverse osmosis treatment, centrifugal pumps, diffuser pumps, piston pumps, plunger pumps, cascade pumps, jet pumps Etc. are preferably used.

  The reverse osmosis membrane 7 has no particular problem as long as it has a function of reducing solutes and suspended substances in filtered water to a concentration that can be used as reclaimed water, but dissolved organic substances adhere to the membrane surface. A low-fouling reverse osmosis membrane that is resistant to chemical fouling (chemical fouling) and biofouling (biological fouling) in which microorganisms grow and adhere to the membrane surface using dissolved organic matter as nutrients is preferable. . Examples of low-fouling reverse osmosis membranes include TML20 manufactured by Toray Industries, Inc. and LF10 manufactured by Nitto Denko Corporation (the surface of the membrane is neutral, a hydrophilic group is introduced, adsorption of charged substances and heavy metals such as iron colloids) MFC), LFC1, LFC3 manufactured by Hydranautics, BW30-365FR manufactured by Dow, and the like. In addition, if the concentration of solutes and suspended substances in filtered water is low, there is a particular problem even if a nanofiltration membrane that is a liquid separation membrane that blocks particles and polymers smaller than about 2 nm is used as a reverse osmosis membrane. Absent.

  The pipe connected from the filtration membrane 2 to the filtration water tank 4 and the pipe connected from the filtration water tank 4 to the reverse osmosis membrane 7 are shielded from light so as to prevent the growth of microorganisms in the pipe. Furthermore, it is preferable that the structure does not directly touch the outside air. Similarly, the filtered water tank 4 is preferably structured to be shielded from light and prevent the growth of microorganisms. In this way, it is preferable to keep the water to be treated between the membrane separation process and the reverse osmosis process in the biological treatment tank 3 in an environment where light is always shielded in order to further reduce the growth of microorganisms during this period. .

  Below, the processing flow which shows the embodiment of the water treatment method of this invention is outlined.

  In FIG. 1, first, the treated water 1 is treated with activated sludge in the biological treatment tank 3. The activated sludge concentration is about 2,000 mg / L to 20,000 mg / L and the retention time of the water to be treated is usually 1 hour to 24 hours, but the optimum one is selected according to the state of the water to be treated. Good. Moreover, it is also possible to install a device for adding a flocculant and add the flocculant to the water to be treated containing activated sludge stored in the biological treatment tank 3, which is a cause of fouling of the reverse osmosis membrane. And soluble organic substances can be reduced from the membrane-separated activated sludge treated water.

  Next, the water treated with activated sludge in the biological treatment tank 3 is filtered by the filtration membrane 2. The filtered water is transferred by piping and stored in the filtered water tank 4. The treatment time for ultraviolet treatment by the ultraviolet treatment device 5 installed in the filtered water tank 4 is generally designed from the strength of the lamp used, the amount of water, the water quality, etc., but is usually about 5 seconds to 10 minutes. is there.

  Next, the treated water subjected to the ultraviolet treatment is supplied through a pipe to the reverse osmosis membrane 7 through the pump 6. The water that has permeated through the reverse osmosis membrane 7 is obtained as permeated treated water 8 and used for applications such as reclaimed water. On the other hand, the concentrated water 9 coming out of the reverse osmosis membrane is discharged out of the system.

  In the present invention, it is essential to perform ultraviolet treatment after membrane separation activated sludge treatment, that is, separation treatment with a membrane before ultraviolet treatment, so that suspended substances are surely removed before ultraviolet treatment. Has been. Therefore, the sterilization efficiency by the ultraviolet treatment is remarkably superior even when compared with the conventional water treatment method combining sand filtration and ultraviolet treatment.

  Furthermore, it is preferable that a tank is provided after the filtration membrane and before the reverse osmosis membrane treatment to add heavy metal ions and a reducing agent.

  When heavy metal ions such as manganese, copper, and iron and reducing agents such as sodium bisulfite are added, a strong oxidizing agent is generated in the system, which can further enhance the sterilizing effect of microorganisms remaining in the filtered membrane treated water. it can. That is, when heavy metal ions and sodium hydrogen sulfite as a reducing agent are present in the system, heavy metal ions serve as catalysts, and sulfite ions become sulfite radicals, from which persulfuric acid having strong oxidizing properties, and further in the system It reacts with chloride ions to generate perchlorate ions and chlorine, and has a bactericidal effect.

  Water quality standard values of heavy metal ions in drinking water in Japan are set to 1.0 mg / l or less for copper, 0.3 mg / l or less for iron, and 0.05 mg / l or less for manganese. For this, a concentration of about 10 to 50 ppb is sufficient, and heavy metal ions can be added within the range of water quality standards. In addition, the addition of heavy metal ions into the tank is performed by measuring the concentration of heavy metal ions contained in the membrane filtered water. If a concentration of about 10 to 50 ppb is included, the reducing agent is added without adding heavy metal ions. Only the addition of may be added. The heavy metal ions may be added continuously or intermittently in the tank.

  As the reducing agent to be added together with the heavy metal ions, those that are water-soluble, highly reducible, and have no influence on the reverse osmosis membrane can be used. Further, sodium sulfite, sodium hydrogen sulfite and the like are preferable from the viewpoints of low price and easy handling. The amount of the reducing agent added is appropriately selected between 0.01 and 50 ppm. If the amount of the reducing agent added is less than 0.01 ppm, a sufficient amount of oxidizing agent to sterilize the microorganisms will not be produced. If the amount added exceeds 50 ppm, reoxidation with a large amount of reducing agent will result in no generation of oxidizing agent. It will be enough.

  The oxidant produced by the heavy metal ions added in the tank can increase the sterilizing effect by retaining it for a certain period of time. The residence time of the filtered water in the tank is preferably at least 10 minutes. If the residence time is too short, it is insufficient to sterilize microorganisms contained in the filtered water. The longer the residence time, the better. However, the residence time may be set as appropriate in consideration of the tank capacity, the installation area, the required amount of production water, and the like.

  If the oxidizing agent produced by heavy metal ions and reducing agent comes into direct contact with the reverse osmosis membrane, the functional layer on the surface of the reverse osmosis membrane is subject to oxidative degradation, so it is reduced before entering the reverse osmosis membrane. Need to be removed. The reducing agent is preferably added immediately before the high-pressure pump that sends filtered water to the reverse osmosis membrane. The reduction process can be efficiently performed by the mixing effect of the high-pressure pump. As this reducing agent, the same reducing agent as that added for producing the oxidizing agent can be used.

  In order to monitor the generated oxidant so as not to enter the reverse osmosis membrane, an ORP meter can be installed immediately before the reverse osmosis membrane. If the value of the ORP meter becomes high and shows an abnormality, the inflow of filtered water to the reverse osmosis membrane can be stopped, and oxidative deterioration of the reverse osmosis membrane due to contact with the oxidizing agent can be prevented.

  As shown in FIG. 3, the tank for adding the above heavy metal ions and the reducing agent may be disposed after the ultraviolet treatment or may be disposed before the ultraviolet treatment (not shown). ).

  Furthermore, as shown in FIG. 4, an apparatus for performing at least one additional sterilization treatment among ozone treatment, hydrogen peroxide treatment, chlorination treatment, and photooxidation catalyst treatment, and addition such as activated carbon treatment and sand filtration treatment. It is also possible to perform an at least one treatment by installing a device for performing an automatic purification treatment in the subsequent stage of the filtration membrane and in the previous stage of the reverse osmosis membrane. By applying one of these additional sterilization treatments and additional purification treatments in combination with UV treatment, the load on the reverse osmosis membrane can be further reduced, and biofouling can be effectively suppressed. It becomes possible.

  For example, when the treated water obtained by membrane separation activated sludge treatment is treated with activated carbon and then subjected to ultraviolet treatment, soluble organic substances contained in the treated water obtained by membrane separation activated sludge treatment are first removed by activated carbon treatment. Thereafter, the remaining organic matter is decomposed by sterilization of microorganisms and activated carbon treatment by ultraviolet treatment. Thereby, soluble organic matter removal and sterilization of microorganisms are efficiently performed, and as a result, fouling of the reverse osmosis membrane that continues to the subsequent stage is suppressed.

  In addition, so-called accelerated oxidation treatment can be performed by applying at least one of ozone treatment, hydrogen peroxide treatment, and photooxidation catalyst treatment in combination with ultraviolet treatment. In the accelerated oxidation treatment, hydroxyl radicals having extremely strong ability to decompose organic matter and sterilization are generated by using a combination of ultraviolet rays and other oxidizing agents. When such a method is applied, the decomposition of organic substances contained in the membrane-separated activated sludge treated water and the sterilization of microorganisms are performed at the same time. As a result, fouling of the reverse osmosis membrane that continues to the subsequent stage can be suppressed.

  Multi-stage sterilization with UV treatment and oxidant generation sterilization enables effective sterilization even in treated water where microorganisms are likely to propagate, and prevents biofouling in reverse osmosis. . Furthermore, it can accelerate the degradation of persistent organic substances and high molecular weight organic compounds, not only to prevent biofouling, but also to prevent chemical fouling that causes organic substances to adsorb on the reverse osmosis membrane surface and cause performance degradation. Can be useful.

  It is also possible to sterilize the reverse osmosis membrane surface directly by adding a bactericidal agent into the reverse osmosis membrane. In general, for sterilization of reverse osmosis membranes, chemicals that have little influence on the reverse osmosis membrane and the human body, are inexpensive and easy to handle, and sulfuric acid, which is a strong acid, is preferably used.

  Below, the water treatment method thru | or water treatment apparatus of this invention are demonstrated more concretely using an Example. In addition, this invention is not limited to the aspect as described in an Example.

[Example 1]
In this embodiment, a water treatment method using a membrane separation activated sludge method and a membrane separation treatment method using a low-pressure reverse osmosis membrane is shown, and a water treatment apparatus used for carrying out the water treatment method is shown in FIG. As treated water 1, sewage flowing into an agricultural settlement wastewater treatment plant was used. Table 1 shows the average water quality of the treated water used in this example. Table 2 shows the operating conditions of the membrane separation activated sludge tank part of the biological treatment tank, Table 3 shows the conditions of the UV treatment apparatus following the subsequent stage, and Table 4 shows the operating conditions of the reverse osmosis membrane. The flow of the water treatment process in the present embodiment will be described below.

The treated water 1 is first introduced into a denitrification tank 31 in a biological treatment tank 3 consisting of oxygen-free and aerobic. The biological treatment process is advanced by a nitrification process (aerobic) and a denitrification process (oxygen-free) for nitrogen removal. Nitrification of ammonia nitrogen (NH ₄ -N) is advanced in the membrane separation activated sludge tank (aerobic tank) 32 in the subsequent stage, and the nitrification liquid is circulated from the membrane separation activated sludge tank 32 to the denitrification tank 31 in the preceding stage, where denitration is performed. Nitrogen is used to remove nitrogen. The biological nitrogen removal is briefly described below.

1) Nitrification Step In the aerobic tank (membrane separation activated sludge tank 32) in the biological treatment tank 3, NH ₄ -N is converted to nitrate by the action of nitrous acid-producing bacteria and nitrate-producing bacteria that are autotrophic bacteria in an aerobic state. is oxidized to a sexual nitrogen _(NO 3 -N). This is called a nitrification reaction, and nitrite-producing bacteria and nitrate-producing bacteria are collectively called nitrifying bacteria. The reaction by nitrifying bacteria can be generally expressed as follows.

NH ₄ ⁺ + 2O ₂ → NO ₃ ⁻ + H ₂ O + 2H ⁺
This reaction provides energy for the growth of nitrifying bacteria.

2) Denitrification step Next, the nitrification liquid containing nitrate nitrogen NO ₃ --N and nitrite nitrogen NO ₂ --N generated by the above nitrification reaction overflows and flows into the denitrification tank 31. There is no dissolved oxygen (DO) in the denitrification tank 31, and the reduction of oxidized nitrogen by nitrate respiration or nitrite respiration by denitrifying bacteria that are facultative anaerobic bacteria occurs, and NO ₃ --N or NO ₂ --N is reduced to the nitrogen gas _{(N 2).} That is, a denitrification reaction occurs. The amount of water to be treated is about 2 to 3 times the amount of water to be treated of 12 m ³ / day, and in this example, 36 m ³ / day, which is 3 times the amount, was circulated.

The denitrification reaction can be generally expressed as follows.
2NO ₃ ⁻ +5 (H ₂ ) → N ₂ + 2OH ⁻ + 4H ₂ O

(H ₂ ) in this reaction is given from a hydrogen donor (organic substance). In general, heterotrophic bacteria using organic matter as a hydrogen donor are involved in the denitrification reaction.

  The sludge mixed liquid in the denitrification tank 31 is circulated by a sludge circulation pump and flows into the membrane separation activated sludge tank 32. In the membrane separation activated sludge tank 32, the air blown by the air supply device 14 is aerated through a diffuser. By this aeration, activated sludge is maintained in an aerobic state, and nitrification reaction and BOD oxidation are performed. Furthermore, this air aeration can prevent the adhesion and accumulation of sludge adhering to the membrane separation device 2.

  Membrane separation activated sludge treated water treated with membrane separation activated sludge is stored in the filtration water tank 4. An ultraviolet treatment device 5 is immersed in the filtered water tank 4, and sterilization of the membrane separation activated sludge treated water is performed in this tank. Table 3 shows the specifications of the ultraviolet lamp. As the ultraviolet ray, an immersion type low-pressure ultraviolet lamp was used. The contact time is 5 minutes.

  Next, the processing flow of the reverse osmosis membrane is shown. The ultraviolet-treated water is pressurized by the pump 6 and then supplied to the reverse osmosis membrane 7. On the other hand, the sulfuric acid stored in the bactericide storage tank 10 is supplied by the bactericide pump 11 through the RO supply line. In this example, sulfuric acid (5%) was supplied once a day for 1 hour so that the pH of the water supplied to the reverse osmosis membrane was 2.5. By this sulfuric acid sterilization, it is possible to sterilize a biofilm or the like adhering to the membrane surface and suppress biofouling. However, in the operation of the membrane separation activated sludge method and the reverse osmosis membrane, it has been found that it is difficult to efficiently suppress biofouling only by sulfuric acid sterilization.

  Thereafter, the reverse osmosis membrane treated water is used as reclaimed water, and part of the circulating water is discharged out of the system as waste water.

  In this example, as the pretreatment of the reverse osmosis membrane, 1) both sulfuric acid sterilization and ultraviolet sterilization 2) only sulfuric acid sterilization 3) both sulfuric acid sterilization and ultraviolet sterilization were carried out and compared. The change in flux for these three cases is shown in FIG.

  From this result, suppression of fouling was in the order of 3) both sulfuric acid sterilization and ultraviolet sterilization, 2) only sulfuric acid sterilization, and 1) neither sulfuric acid sterilization and ultraviolet sterilization. From this, it became clear that ultraviolet sterilization effectively sterilized membrane-separated activated sludge treated water and suppressed biofouling.

[Example 2]
It was performed on the same conditions as Example 1 except having performed water treatment with the water treatment apparatus shown in FIG.
That is, another tank 15 is installed as shown in FIG. 3 after the ultraviolet irradiation devices 4 and 5 with respect to the apparatus of FIG. 2 used in Example 1, and the residence time of water in the tank 15 is 10 minutes. Was set to be. Thereto, manganese ions and sodium hydrogen sulfite were added so that manganese ions were 30 ppb and sodium hydrogen sulfite was 1 ppm. Further, 5 ppm of sodium bisulfite was added immediately before the pump 6 in the front stage of the reverse osmosis membrane.

  The change in flux with the progress of water treatment was as shown in FIG. UV irradiation and heavy metal addition suppressed the degradation of reverse osmosis membrane performance due to fouling and enabled stable operation.

  According to the present invention, in a method for producing reclaimed water combining a membrane separation activated sludge and a reverse osmosis membrane, microbial growth or adhesion of microorganisms and their metabolites to the membrane surface (so-called biofouling) occurs on the reverse osmosis membrane surface. Thus, it is possible to efficiently prevent troubles that cause deterioration in permeation performance and separation performance of the reverse osmosis membrane.

  That is, the present invention prevents biofouling of a reverse osmosis membrane, which is one of the major obstacles in operating the apparatus when the sewage is treated with a membrane separation activated sludge method and a reverse osmosis membrane to produce reclaimed water. It is a big surprise.

It is the schematic which shows an example of the water treatment apparatus used for the water treatment method of this invention. It is the schematic which shows another example of the water treatment apparatus used for the water treatment method of this invention. It is the schematic which shows another example of the water treatment apparatus used for the water treatment method of this invention. It is the schematic which shows another example of the water treatment apparatus used for the water treatment method of this invention. It is a graph which shows the change of the flux in Example 1 (a comparative example is included). 6 is a graph showing changes in flux in Example 2.

Explanation of symbols

1: treated water 2: filtration membrane 3: biological treatment tank 31: denitrification tank 32: membrane separation activated sludge tank 4: filtration water tank 5: UV treatment device 6: pump 7: reverse osmosis membrane 8: treated water 9: concentrated water 10: Disinfectant storage tank 11: Disinfectant pump 12: At least one device selected from activated carbon treatment, ozone treatment, sand filtration treatment, hydrogen peroxide treatment, chlorination treatment and photooxidation catalyst treatment 13: Heavy metal ion addition device 14: Reducing agent addition device 15: Tank

Claims (13)

It comprises a step of treating the water to be treated with activated sludge in a biological treatment tank, subjecting the water treated with activated sludge to membrane separation treatment within the biological treatment tank, and then subjecting the water subjected to membrane separation treatment to reverse osmosis treatment. In the water treatment method, the water to be treated is subjected to ultraviolet treatment after the membrane separation treatment and before the reverse osmosis treatment. A heavy metal ion and a reducing agent are further added to the water to be treated after the membrane separation treatment and before the reverse osmosis treatment, and then a reducing agent is added immediately before the reverse osmosis membrane treatment. The water treatment method according to claim 1. After the membrane separation treatment and before the reverse osmosis treatment, at least one additional sterilization treatment selected from the group consisting of ozone treatment, hydrogen peroxide treatment, chlorine treatment, and photooxidation catalyst treatment is further performed. The water treatment method according to claim 1 or 2, characterized in that. The additional purification treatment of at least one of activated carbon treatment and sand filtration treatment is further performed after the membrane separation treatment and before the reverse osmosis treatment. The water treatment method as described. The water treatment method according to claim 2, wherein heavy metal ions and a reducing agent are added to a tank provided in the middle of a water channel between the membrane separation treatment and the reverse osmosis treatment. The water treatment method according to claim 1, wherein a flocculant is added to water to be treated containing activated sludge stored in the biological treatment tank. The water treatment method according to any one of claims 1 to 6, wherein the water to be treated between the membrane separation treatment and the reverse osmosis treatment in the biological treatment tank is always placed in a light-shielded environment. . A biological treatment tank, a filtration membrane immersed in the biological treatment tank, and a water treatment device provided with a reverse osmosis membrane, wherein the water treatment device is a subsequent stage of the filtration membrane and the reverse osmosis A water treatment apparatus, wherein an ultraviolet treatment apparatus is installed in front of the membrane. The heavy metal ion addition device and the reducing agent addition device are connected to a tank or a pipe located after the filtration membrane and before the reverse osmosis membrane, according to claim 8. Water treatment equipment. A group consisting of an activated carbon treatment device, an ozone treatment device, a sand filtration treatment device, a hydrogen peroxide treatment device, a chlorination treatment device, and a photo-oxidation catalyst treatment device after the filtration membrane and before the reverse osmosis membrane. The water treatment device according to claim 8 or 9, wherein at least one device selected from the following is installed. The water treatment apparatus according to claim 9, wherein a tank is provided after the filtration membrane and before the reverse osmosis membrane. The water treatment device according to any one of claims 8 to 11, wherein a device for adding a flocculant to water to be treated containing activated sludge stored in the biological treatment tank is installed. Wastewater containing a soil substance is treated with the water treatment method according to any one of claims 1 to 7, or treated with the water treatment device according to any one of claims 8 to 12. A method for producing reclaimed water, characterized in that when the reclaimed water is produced by the method, the permeated water obtained by the reverse osmosis treatment using the reverse osmosis membrane is reclaimed water.

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