JP2008086849A - Water treatment method and apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a water treatment method which enables the combination of membrane separation activated sludge treatment and reverse osmosis treatment by a reverse osmosis membrane, and effectively prevents a deterioration in permeability and separation performance of the reverse osmosis membrane caused by a growth and adhesion of microorganisms on the surface of the reverse osmosis membrane and adsorption of an organic matter. <P>SOLUTION: In the water treatment method comprising the process of treating water to be treated with activated sludge in a biological treatment tank 3 and subjecting the activated sludge-treated water to membrane separation treatment in the biological treatment tank, and a process 8 for treating the membrane separation-treated water with the reverse osmosis membrane, a coagulant is added into the biological treatment tank, and a reducing agent is added after the membrane separation treatment process and before the reverse osmosis treatment process, and then a chelating agent is further added before the reverse osmosis treatment process. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a water treatment method and a water treatment apparatus, 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 microfiltration membrane or an ultrafiltration membrane in place of the final sedimentation basin in a normal activated sludge method has been developed and is becoming popular. Membrane separation activated sludge method is a treatment method that uses a separation membrane instead of the final sedimentation basin of ordinary activated sludge method. (1) Biomass in biological reaction tank (generally MLSS (activity in mixed liquid in tank) Represented by the amount of suspended matter such as sludge) can be kept high and the installation area can be reduced, and (2) SS (substance suspended in water) does not flow into the treated water and is clear. There is an advantage that 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 a treatment method combining this membrane separation activated sludge method and a membrane separation treatment method using a reverse osmosis membrane, treated water treated using the membrane separation activated sludge method is used as the treated water of the reverse osmosis membrane. Since it is supplied, the so-called fouling problem that the organic and inorganic components contained in the water clog the reverse osmosis membrane occurs. In particular, chemical fouling caused by trace organic substances contained in the treated water that could not be treated by the membrane separation activated sludge method, and so-called biofouling that causes fouling by forming a biofilm on the reverse osmosis membrane This is the most troublesome problem 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, if the surface of the reverse osmosis membrane is in direct contact with the surface, the reverse osmosis membrane is oxidized and deteriorated due to the oxidizing power of the chlorine or the oxidizing substance (oxidant) generated by the reaction with chlorine. Therefore, it is necessary to dechlorinate. Recently, a film material having a relatively high oxidation resistance has been developed, but it cannot be said that its durability is sufficient.

  On the other hand, a sterilization method for sterilizing microorganisms by periodically lowering the pH of seawater supplied to a reverse osmosis membrane in a seawater desalination process has been proposed (for example, Patent Document 2). However, this method cannot completely prevent bio-fouling, and further, a large amount of alkali for neutralization is required, resulting in a problem that the chemical cost is greatly increased.

  On the other hand, in order to suppress biofouling of reverse osmosis membrane in seawater desalination process, sand filter for removing suspended matter in seawater and ultraviolet light for sterilizing microorganisms in seawater that passed through sand filter It has been proposed to provide a sterilizer (see, for example, Patent Document 3).

  In addition, in the seawater desalination process, the seawater is sterilized with sodium hypochlorite, then chlorine is reduced with a reducing agent such as sodium bisulfite, and a chelating agent is added to capture heavy metals. A method of enabling reverse osmosis membrane separation has been proposed (for example, Patent Document 4).

  However, there is no useful fouling suppression technology in the sewage purification and reuse process, especially the sewage treatment method that combines the membrane separation activated sludge method and the reverse osmosis membrane treatment, and a useful technology is desired. .

JP-A-4-305287 JP 2000-237555 A JP-A-2004-25018 JP 7-32891 A

  In the present invention, sewage such as sewage is treated with activated sludge in a biological treatment tank containing a flocculant, and then subjected to membrane separation treatment (hereinafter collectively referred to as “membrane separation activated sludge treatment”). In the water treatment method in which reverse osmosis treatment is performed using a reverse osmosis membrane, fouling on the reverse osmosis membrane surface caused by microorganisms and organic substances contained in the treated water treated with membrane separation activated sludge is suppressed. The purpose is to do.

The present invention for solving this problem has the following configuration.
(1) A process of subjecting treated water to an activated sludge treatment in a biological treatment tank, a membrane separation treatment of the activated sludge treated water in the biological treatment tank, and a reverse osmosis membrane for treating the water after the membrane separation treatment In the water treatment method comprising a step of using reverse osmosis treatment, a flocculant is added to the biological treatment tank, and after the membrane separation treatment step, the reverse osmosis treatment step The water treatment method characterized by adding a reducing agent before.
(2) The water treatment method according to (1) above, wherein a chelating agent is further added after the addition of the reducing agent and before the reverse osmosis treatment step.
(3) The water treatment method according to (1) or (2) above, wherein the flocculant added to the biological treatment tank is an iron-based flocculant.

(4) The reducing agent added before the reverse osmosis treatment step is selected from a sulfur-based reducing agent and an ascorbic acid-like compound, as described in any one of (1) to (3) above Water treatment method.
(5) The reverse osmosis membrane used in the reverse osmosis treatment is at least one selected from the group consisting of a cellulose acetate asymmetric membrane, a polyamide asymmetric membrane, and a polyamide composite membrane. The water treatment method according to any one of (4).
(6) A biological treatment / membrane separation apparatus including a biological treatment tank and a filtration membrane immersed in the biological treatment tank, and a water treatment apparatus in which a reverse osmosis apparatus using a reverse osmosis membrane is installed, A flocculant adding device for adding a flocculant into a biological treatment tank and a reducing agent adding device for adding a reducing agent after the filtration membrane and before the reverse osmosis membrane are installed. Water treatment equipment.

  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 becomes possible to effectively suppress fouling on the reverse osmosis membrane surface caused by microorganisms and organic substances contained in the treated water.

  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, followed by a step of performing reverse osmosis treatment using a reverse osmosis membrane, in a membrane separation activated sludge treatment step. A flocculant is added, and the reducing agent is added before the subsequent reverse osmosis treatment.

  FIG. 1 shows a water treatment for reusing treated water (raw water), which is a combination of a membrane separation activated sludge method and a membrane separation treatment method using a reverse osmosis membrane used in the water treatment method of the present invention. A schematic diagram of the apparatus is shown. A filtration membrane 2 for filtering the treated water treated with activated sludge in the biological treatment tank 3 to obtain filtered water, a biological treatment tank 3 for immersing the filtration membrane 2 in the treated water, and filtration A filtered water tank 4 for storing filtered water obtained by filtering the water to be treated by the membrane 2, a pump 5 for adding the flocculant to the biological treatment tank, and a flocculant tank 6 for storing the flocculant; A pump 7 that takes out and pressurizes filtered water from the filtered water tank 4, a reverse osmosis membrane 7 that performs reverse osmosis treatment of the pressurized filtered water, a pump 9 for injecting the reducing agent into the filtered water, and the reducing agent are stored. It consists of a reducing agent tank 10 to be kept.

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

  First, the treated water 1 is supplied into the biological treatment tank 3, and the treated water is treated with activated sludge in the biological treatment tank 3. The activated sludge introduced into the biological treatment tank 3 is generally used for wastewater treatment and the like, and the sludge extracted from other wastewater treatment facilities is usually used as seed sludge. 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 residence time in the biological treatment tank 3 of the water to be treated is usually 1 hour to 24 hours, but an optimum one may be selected according to the state of the water to be treated.

  Next, the water treated with activated sludge in the biological treatment tank 3 is filtered by the filtration membrane 2. The filtered water is stored in the filtered water tank 4.

  Here, in order to improve the handleability and physical durability of the filtration membrane, the filtration membrane 2 has, for example, a flat membrane element structure in which a filtration membrane is adhered to both sides of the frame and a filtration membrane is adhered thereto. It is desirable that 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 in which a flat membrane is wound in a spiral shape.

  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 membrane in which a rubbery polymer such as cross-linked silicone, polybutadiene, polyacrylonitrile butadiene, ethylene propylene rubber, or neoprene rubber is combined as a functional layer with these porous membranes can also be used as the filtration membrane 2.

  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 of parts, and a part of the plurality of divided tanks may be used as a tank for immersing the filtration membrane 2 and the other as a denitrification tank. Water may be circulated between the tanks divided from each other.

  The filtered water tank 4 is not particularly limited as long as the filtered water 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 filtration in the filtration water tank 4 is sufficient. The water level may be lower than the water level to be treated in the treatment tank 3.

  The flocculant addition apparatus includes a flocculant tank 6 for storing the flocculant and a pump 5 for feeding the flocculant to the biological treatment layer 3. By adding a flocculant to the activated sludge, the agglomeration property of the activated sludge itself is improved and it becomes easy to flock, and as a result, the filtration performance is improved. In addition, since the increase in the differential pressure in the filtration membrane 2 can be suppressed, the frequency of chemical cleaning of the filtration membrane can be reduced, leading to a reduction in cleaning chemical cost. Furthermore, since the ratio (removal rate) which can remove inclusions, such as phosphorus contained in to-be-processed water, can be improved, the water quality improvement of filtered water can be aimed at.

  The flocculant added to the biological treatment tank 3 is not particularly limited, and examples thereof include inorganic flocculants and organic flocculants. Examples of the inorganic flocculant include ferric chloride, sulfuric acid band, PAC (polyaluminum chloride), and examples of the organic flocculant include anionic, cationic, and nonionic flocculants. The concentration of the flocculant is about 1 mg / L to 1000 mg / L with respect to the water to be treated, and the optimum injection amount is preferably determined by the phosphorus concentration in the water to be treated and the sludge properties of the activated sludge. There is no particular limitation. Here, the flocculant is added to the biological treatment tank in the membrane separation activated sludge method, so that it is effective against microorganisms like iron-based flocculants (such as ferrous sulfate, ferric sulfate, and ferric chloride). Those that do not show inhibition are desirable. The injection rate of the flocculant may be calculated from the soluble total phosphorus concentration of the water to be treated, the flocculant addition molar ratio, and the design water amount. The number of moles of total phosphorus contained in the water to be treated is determined by the molybdenum blue absorptiometric method after decomposition with acidic potassium peroxodisulfate described in pages 163 to 166 of the “Sewage Test Method (1984)” edited by the Japan Sewerage Association. It can be quantified.

  The pump 7 is not particularly limited as long as the water pressure of the filtered water can be increased to a pressure required for the next reverse osmosis treatment, and is a centrifugal pump, a diffuser pump, a spiral mixed flow pump, a mixed flow pump, or a piston pump. , Plunger pumps, diaphragm pumps, gear pumps, screw pumps, vane pumps, cascade pumps, jet pumps, etc., but can be easily pressurized to the pressure required for reverse osmosis treatment. Pumps, piston pumps, plunger pumps, cascade pumps, jet pumps and the like are preferably used.

  In the reverse osmosis apparatus 8 using a reverse osmosis membrane, a semipermeable membrane (reverse osmosis membrane) that allows some components in the filtered water, for example, a solvent to permeate but not other components, is disposed as a filtration membrane. A nanofiltration membrane or a loose RO membrane is also included in the reverse osmosis membrane in a broad sense. As the material, polymer materials such as cellulose acetate polymer, polyamide, polyester, polyimide, and vinyl polymer are often used. Any of the reverse osmosis membranes used in the present invention can be used regardless of the membrane structure and membrane form, and both are effective, but the membrane structure of the reverse osmosis membrane is dense on at least one side of the membrane. An asymmetric membrane that has a layer and has fine pores with gradually increasing pore diameters from the dense layer toward the inside of the membrane or the other surface, and a very thin active layer formed of another material on the dense layer of the asymmetric membrane. A composite membrane having this is preferably used. Further, as the membrane form of the reverse osmosis membrane, a hollow fiber or a flat membrane is preferably used. The hollow fiber and the flat membrane preferably have a film thickness of 10 μm to 1 mm, and the hollow fiber has an outer diameter of 50 μm to 4 mm, and when the flat membrane is an asymmetric membrane or a composite membrane. The filtration functional layer is preferably supported by a substrate such as a woven fabric, a knitted fabric, or a nonwoven fabric.

  Typical reverse osmosis membranes that can be used in the present invention include, for example, a cellulose acetate asymmetric membrane, a polyamide asymmetric membrane, or a composite membrane having an active layer such as polyamide or polyurea. Among these, cellulose acetate asymmetric membrane, polyamide asymmetric membrane, and polyamide composite membrane are effective in the method of the present invention in terms of hydrolysis resistance, chemical resistance, and pressure resistance. In this respect, an aromatic polyamide composite membrane is particularly preferable.

  Furthermore, as reverse osmosis membranes, chemical fouling (chemical fouling) in which dissolved organic matter adheres to the membrane surface, and biofouling (biological fouling) in which microorganisms grow and adhere to the membrane surface using dissolved organic matter as a nutrient source It is preferable to use a low fouling reverse osmosis membrane that is less likely to occur. 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) And HFCs manufactured by Hydranautic LFC1, LFC3, Dow BW30-365FR, 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 reducing agent addition device for adding a reducing agent to filtered water has a tank 12 for storing the reducing agent and a pump 11 for adding the reducing agent to the filtered water tank 4. Here, the reducing agent refers to a compound that can be oxidized and reduce the partner. For example, when sodium hypochlorite is reduced with sodium hydrogen sulfite, sulfite ions are oxidized into sulfate ions according to the following reaction formula, and hypochlorite ions are reduced to chloride ions with it.
2ClO ⁻ + 2SO ₃ ²⁻ → 2Cl− + 2SO4 ²⁻

  Here, the reducing agent to be used may be water-soluble, highly reducing and having no influence on the reverse osmosis membrane. Furthermore, a sulfur-based reducing agent or an ascorbic acid-like compound can be used because it is inexpensive and easy to handle. Examples of the sulfur-based reducing agent include sodium sulfite, sodium hydrogen sulfite, and sodium thiosulfate. Ascorbic acid analogs include ascorbic acid, erythorbic acid and salts thereof, and optical isomers thereof.

  Most of the iron-based flocculants such as ferric chloride added to the liquid to be treated in the biological treatment tank 3 contribute to improving the sludge aggregation effect, but some of the dissolved components permeate the filtration membrane 2. Therefore, it dissolves in the filtered water and flows out into the filtered water tank 4.

  When a heavy metal ion such as iron reacts with a reducing agent such as sodium hydrogen sulfite, a strong oxidant is generated in the system, further enhancing the sterilization of microorganisms remaining in the filtered membrane treated water and the decomposition of trace organic substances. Can do. That is, when iron ions and a reducing agent such as sodium bisulfite or ascorbic acid are present in the system, iron ions serve as catalysts to produce strong oxidizing agents such as hydrogen peroxide and hydroxy radicals, It is thought to have an organic matter decomposition effect.

  Although the water quality standard value of iron ions in drinking water in Japan is set to 0.3 mg / l or less, an iron ion concentration of about 10 to 50 ppb is sufficient to generate an oxidizing agent by the above reaction. is there. If the concentration of iron ions dissolved in the filtered water is less than 10 ppb, it can be further added to the treated water tank.

  The amount of reducing agent added to the filtered water tank 4 is appropriately selected between 0.01 ppm and 1.0%. If the amount of the reducing agent added is less than 0.01 ppm, a sufficient amount of oxidizing agent for sterilizing microorganisms or decomposing organic substances will not be produced.

  Since the oxidant produced in the filtered water tank 4 can increase the sterilizing effect by retaining for a certain period of time, it is preferable that the filtered water stays in the filtered water tank for at least 10 minutes. If this 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.

  Moreover, in this invention, in order to stop the production | generation of the oxidizing agent by an iron ion and a reducing agent, it is preferable to add a chelating agent to the filtered water before introduce | transducing into a reverse osmosis membrane. In order to add the chelating agent, a chelating agent adding device having a tank 14 for storing the chelating agent and a pump 13 for feeding the chelating agent to the filtered water tank 4 may be installed.

  The chelating agent used here is not particularly limited, and polymer-based, organic-based and inorganic-based chelating agents can be used. Examples of the polymer chelating agent include polyacrylic acid, polystyrene sulfonic acid, maleic anhydride (co) polymer, lignin sulfonic acid and the like. Examples of the organic chelating agent include amino trimethylene phosphonic acid, phosphonobutane tricarboxylic acid, polyaminocarboxylic acid, oxycarboxylic acid, and condensed phosphoric acid. Among these, polyaminocarboxylic acid, oxycarboxylic acid, and condensed phosphoric acid are preferable. As the polyaminocarboxylic acid, nitrilotriacetic acid, ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid and the like are preferable. As the oxycarboxylic acid, citric acid, malic acid and the like are preferable. Examples of these salt forms include alkali metal salts such as sodium, potassium and lithium, ammonium salts and alkanolamine salts. As the condensed phosphoric acid, pyrophosphoric acid, tripolyphosphoric acid, tetrametaphosphoric acid, hexametaphosphoric acid, trimetaphosphoric acid and the like are preferable. Examples of the salt include alkali metal salts such as sodium, potassium and lithium, ammonium salts and the like.

  The amount of these chelating agents to be added is sufficient if at least iron ions in the water (filtered water) supplied to the reverse osmosis device 8 can be taken in. It is 01 to 100 ppm, and depends on the iron ion concentration contained in the filtered water, and is preferably 50 to 10,000 times the iron ion concentration contained. If the addition amount is less than 50 times, the chelating action may not be sufficiently exhibited, and if it is 10,000 times or more, the chelating agent itself is adsorbed on the membrane surface or the water quality is deteriorated. On the other hand, the chelating agent in the present invention can obtain a sufficient effect at a concentration lower than that generally used, and the chemical cost required for operating the apparatus can be reduced.

  Next, the filtered water subjected to such treatment is supplied through a pipe to the reverse osmosis device 8 via the pump 7. The water that has permeated through the reverse osmosis device 8 is obtained as permeated treated water 9 and used for applications such as reclaimed water. On the other hand, the concentrated water 10 exiting from the reverse osmosis device 8 is discharged out of the system.

  The oxidizing agent generated by the iron ions and the reducing agent exerts a function of sterilizing the filtered water that has permeated through the filtration membrane 2 in the course of the route to the reverse osmosis device 8. However, if this generated oxidant 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 that the generated oxidant does not enter the reverse osmosis membrane. Therefore, as described above, the chelating agent is added in front of the reverse osmosis device 8 to prevent the generation of the oxidizing agent. Furthermore, in order to monitor the generated oxidant so as not to enter the reverse osmosis device 8, an ORP meter can be installed immediately before the reverse osmosis device 8. If the value of the ORP meter becomes high and shows an abnormality, the inflow of filtered water to the reverse osmosis membrane is stopped, or the oxidizing agent is eliminated by increasing the amount of reducing agent added in front of the reverse osmosis device 8. What is necessary is just to prevent the oxidative deterioration of the reverse osmosis membrane due to contact with the oxidizing agent.

  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.

  In this example, a water treatment method for membrane separation activated sludge treatment and then reverse osmosis treatment using a low-pressure reverse osmosis membrane was used, and a water treatment apparatus used for carrying out such a 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 for the membrane separation activated sludge treatment in the biological treatment tank, and Table 3 shows the operating conditions for the reverse osmosis treatment performed in the subsequent stage.

  The flow of the water treatment process in the present embodiment will be described below.

The treated water 1 supplied to the biological treatment tank 3 is first introduced into a denitrification tank 31 made of oxygen-free. The biological treatment tank 3 is divided into a denitrification tank 31 and an aerobic tank 32, and the filtration membrane 2 and a diffuser are installed in the aerobic tank 32. The biological treatment in the tank is advanced through a nitrification step (aerobic) and a denitrification step (oxygen-free) in order to remove nitrogen. Nitrogen of ammonia nitrogen (NH ₄ -N) is advanced in the aerobic tank 32 in the subsequent stage, and the nitrification liquid is circulated from the aerobic tank 32 to the denitrification tank 31 in the previous stage, where it is denitrified to remove nitrogen. is there. The biological nitrogen removal is briefly described below.

1) Nitrification Step In the aerobic tank 32 in the biological treatment tank 3, NH ₄ -N is converted to nitrate nitrogen (NO ₃ -N) by the action of nitrite-producing bacteria and nitrate-producing bacteria that are autotrophic bacteria in an aerobic state. ) Is oxidized. 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 nitrous acid 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 circulation rate is 3 times twice the treated water ^12m 3 / day, the present embodiment has been circulated ^36m 3 / day, which is three times.

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 (not shown) and flows into the aerobic tank 32. In the aerobic tank 32, the air blown by the air supply device 15 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 adhesion and accumulation of sludge adhering to the filtration membrane 2.

  Here, the flocculant is added by the pump 5 to the aerobic tank in which the filtration membrane is installed. Ferric chloride was used as the flocculant, and was added so that the concentration in the aerobic tank 32 was 25 mg / l.

  Next, the filtered water that has permeated through the filtration membrane 2 installed in the aerobic tank 32 is sent to the filtered water tank 4 and temporarily stored in the filtered water tank 4. The iron ion concentration in the filtrate in the filtration water tank 4 was 0.2 mg / l. A reducing agent is added to the filtered water tank 4 by a pump 11. As the reducing agent, sodium ascorbate was used and added so that the concentration in the filtered water tank 4 was 10 mg / l. Moreover, the residence time of the filtered water in the filtered water tank 4 was 30 minutes.

  Water from the filtered water tank 4 is added before the pump 7 and a chelating agent is added by the pump 13. As the chelating agent, ethylenediaminetetraacetic acid tetrasodium was used and added so as to have a concentration of 10 ppm.

  The water to which the chelating agent was added was pressurized by the pump 7 and then supplied to the reverse osmosis device 8 having a reverse osmosis membrane. The treated water 9 taken out through the reverse osmosis membrane was used as reclaimed water, and the concentrated water 10 was discharged out of the system as waste water.

  When the water treatment operation according to this example was continued for 1000 hours, the pressure at the reverse osmosis membrane hardly changed compared to the initial stage. By adding a flocculant, a reducing agent and a chelating agent under the conditions of the method of the present invention, a decrease in reverse osmosis membrane performance due to fouling was suppressed, and a stable operation could be achieved.

[Comparative Example 1]
Water treatment was performed under the same conditions as in Example 1 except that the reducing agent and the chelating agent were not added. By operating for 1000 hours, the pressure in the reverse osmosis membrane decreased by about 40% compared to the initial value, and the reverse osmosis membrane performance was reduced by fouling.

  According to the present invention, in a water treatment method that combines membrane separation activated sludge treatment and reverse osmosis treatment, it occurs due to growth and adhesion of microorganisms on the reverse osmosis membrane surface, adsorption of organic matter on the reverse osmosis membrane surface, and the like. Since deterioration of the permeation performance and separation performance of the reverse osmosis membrane (so-called fouling) can be suppressed, the present invention is a process for producing reclaimed water by treating the sewage with membrane separation activated sludge and treating it with a reverse osmosis membrane. It is suitable and greatly implied in order to suppress reverse osmosis membrane fouling, which is one of the major obstacles in operating the apparatus.

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.

Explanation of symbols

1: treated water 2: filtration membrane 3: biological treatment tank 31: denitrification tank 32: aerobic tank (membrane separation activated sludge tank)
4: Filtration water tank 5: Pump 6: Coagulant tank 7: Pump 8: Reverse osmosis device 9 by reverse osmosis membrane 9: Treated water 10: Concentrated water 11: Pump 12: Reductant tank 13: Pump 14: Chelating agent tank 15: Air supply device

Claims (6)

Treating the water to be treated with an activated sludge in a biological treatment tank, subjecting the activated sludge treated water to a membrane separation treatment in the biological treatment tank, and inverting the water after the membrane separation treatment using a reverse osmosis membrane In the water treatment method comprising the step of osmosis treatment, a flocculant is added to the biological treatment tank, and after the step of membrane separation treatment and before the step of reverse osmosis treatment, A water treatment method comprising adding a reducing agent. The water treatment method according to claim 1, wherein a chelating agent is further added after the addition of the reducing agent and before the reverse osmosis treatment step. The water treatment method according to claim 1 or 2, wherein the flocculant added to the biological treatment tank is an iron-based flocculant. The water treatment method according to claim 1, wherein the reducing agent added before the reverse osmosis treatment is selected from a sulfur-based reducing agent and an ascorbic acid-like compound. The reverse osmosis membrane used in the reverse osmosis treatment is at least one selected from the group consisting of a cellulose acetate asymmetric membrane, a polyamide asymmetric membrane, and a polyamide composite membrane. The water treatment method as described in any one of. A biological treatment / membrane separation apparatus including a biological treatment tank and a filtration membrane immersed in the biological treatment tank, and a water treatment apparatus provided with a reverse osmosis apparatus using a reverse osmosis membrane, wherein the biological treatment tank A water treatment device comprising: a flocculant adding device for adding a flocculant therein; and a reducing agent adding device for adding a reducing agent downstream of the filtration membrane and upstream of the reverse osmosis membrane .

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