JP2010029768A - Method and apparatus for treating organic wastewater - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To efficiently obtain treated water having excellent water quality by effectively improving the settleability, concentratability, and filterability of sludge in an activated sludge mixed liquor in a biological treatment tank by adding an iron salt. <P>SOLUTION: In biological treatment of organic wastewater after adding an iron salt, the organic wastewater is subjected to decarbonation treatment, the iron salt is added to and mixed with the decarbonated water, and the mixed water is mixed with activated sludge to be subjected to the biological treatment. The decarbonation treatment of the organic wastewater beforehand prevents generation of iron carbonate, and mixing the organic wastewater with the iron salt at around the optimum pH of ferric hydroxide prevents turbidity of treated water due to generation of iron oxide and iron carbonate. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an organic wastewater treatment method and treatment apparatus for biologically treating organic wastewater by the activated sludge method, and in particular, sludge sedimentation, concentration, and filtration when biologically treating organic wastewater by the activated sludge method. The present invention relates to a method and apparatus for improving the performance and efficiently obtaining treated water of good water quality.

  Biological treatment is known as a method for treating organic wastewater. Among biological treatment methods, an activated sludge method using a microbial community called activated sludge is widely used because it can be applied to organic matter-containing water having various properties and can provide treated water with good water quality.

  In the biological treatment tank that performs treatment by the activated sludge method, a liquid (active sludge mixed liquid) in which the organic wastewater introduced into the treatment tank and the activated sludge (microorganism) retained in the tank are mixed is retained. The For this reason, in order to obtain the clear treated water treated in the biological treatment tank, it is necessary to solid-liquid separate this activated sludge mixed solution.

  The solid-liquid separation means for the activated sludge mixed liquid includes sedimentation basins, membrane separation devices, and flotation separation devices. Among these, the membrane separation device has a higher solids separation ability than other solid-liquid separation devices. If a membrane separator is used, clear treated water can be obtained.

  Thus, when solid-liquid separation is performed on biologically treated water, the following techniques have been conventionally made in order to improve the quality of the obtained treated water and the treatment efficiency.

(1) When biologically treated water is solid-liquid separated in a sedimentation basin, a filter is further provided to improve the transparency of the treated water. Alternatively, the MLSS concentration in the biological treatment tank is optimized. Or enlarge the sedimentation basin.
(2) Adopt the two-stage activated sludge method to improve sludge sedimentation and concentration. Alternatively, a high specific gravity flocculant or the like (iron salt, calcium, etc.) is added. Alternatively, a polymer flocculant is added.
(3) In the membrane separation activated sludge method that separates the activated sludge mixture from the biological treatment tank, membrane chemical cleaning and intermittent treatment water to prevent clogging of the membrane and improve flux (permeation flux) Extraction, reverse cleaning of membrane, optimization of MLSS concentration in biological treatment tank, optimization of sludge residence time (SRT) in biological treatment tank, etc.

  For example, in Patent Document 1, a flocculant is added to a membrane-immersed biological treatment tank to agglomerate phosphorus to prevent elution of phosphorus into biologically treated water, and adhesion of slime in a reverse osmosis membrane separation device at the latter stage A method for preventing this problem has been proposed.

  In addition, in order to prevent clogging of the separation membrane in the membrane separation activated sludge method, the applicant firstly biologically treated organic wastewater in a biological treatment tank and separated the activated sludge mixed solution into a membrane. A method for adjusting the pH of the biological treatment tank to 5 to 6.5 while adding an iron salt was proposed (Patent Document 2).

In Patent Document 3, it is described that the calcium-containing wastewater is decarboxylated, then coagulated, and then subjected to membrane separation. Here, decarboxylation is a reduction in the amount of calcium carbonate sludge produced. It is not intended to improve the sedimentation, concentration, and filterability of sludge.
JP 2008-86849 A Japanese Patent Application No. 2007-41636 JP 2002-292399 A

  As described in Patent Document 2, while adding an iron salt to a biological treatment tank and adjusting the pH of the biological treatment tank to 5 to 6.5, an extremely strong and large floc can be formed, Sludge sedimentation, concentration, and filterability are improved, and the transparency of treated water is high. In particular, when this method is applied to the membrane separation activated sludge method, an excellent effect that the membrane flux can be kept high is exhibited.

  However, when iron salt was added directly to the biological treatment tank, there was a phenomenon that the treated water became cloudy in some cases. As a result of the study by the present inventors, this phenomenon is caused by the fact that the iron salt added to the biological treatment tank becomes iron oxide or iron carbonate in the biological treatment tank and is not used for the formation of flocs, but leaks into the treated water as fine particles. It turned out to be because.

  The present invention solves this problem and, as a result, improves the sedimentation, concentration, and filterability of sludge in the activated sludge mixed liquid in the biological treatment tank by adding iron salt, and has improved water quality. It aims at providing the processing method and processing apparatus of the organic waste water which obtains efficiently.

  As a result of intensive studies to solve the above-mentioned problems, the inventors of the present invention preliminarily decarboxylate the organic wastewater, thereby preventing the production of iron carbonate and the organic wastewater near the optimum pH of ferric hydroxide. It has been found that the turbidity of treated water due to the production of iron oxide and iron carbonate can be prevented by previously mixing iron salt with iron salt.

  The present invention has been achieved on the basis of such findings, and the gist thereof is as follows.

[1] In a method of biological treatment by adding iron salt to organic wastewater, a decarboxylation step of decarboxylating organic wastewater, and a mixing step of adding iron salt to the treated water of the decarbonation step and mixing An organic wastewater treatment method comprising: a biological treatment step of biologically treating mixed water from the mixing step with activated sludge.

[2] The organic wastewater treatment method according to [1], wherein the mixing step has a pH of 4.5 to 6.5, and the biological treatment step has a pH of 5 to 6.5.

[3] In [1] or [2], the iron salt is added in the mixing step so that the iron content in the activated sludge in the biological treatment step is 10 to 45% by weight. Organic wastewater treatment method.

[4] The method for treating organic wastewater according to any one of [1] to [3], further comprising a membrane separation step of membrane separation treatment of the activated sludge mixed solution in the biological treatment step.

[5] In an apparatus for biological treatment by adding iron salt to organic wastewater, decarbonation means for decarboxylating organic wastewater, and a mixing tank for adding iron salt to treated water of the decarbonation means and mixing An organic wastewater treatment apparatus comprising: a biological treatment tank for biologically treating mixed water from a mixing tank with activated sludge.

[6] The organic wastewater treatment apparatus according to [5], wherein the pH of the mixing tank is 4.5 to 6.5 and the pH of the biological treatment tank is 5 to 6.5.

[7] In [5] or [6], the iron salt is added in the mixing tank so that the iron content in the activated sludge in the biological treatment tank is 10 to 45% by weight. Organic wastewater treatment equipment.

[8] The organic wastewater treatment apparatus according to any one of [5] to [7], further comprising membrane separation means for membrane separation treatment of the activated sludge mixed liquid in the biological treatment tank.

According to the present invention, the organic waste water is previously decarboxylated to remove the carbonic acid component, thereby preventing the production of iron carbonate, and the organic waste water and the iron salt near the optimum pH of ferric hydroxide. It is possible to effectively act the added iron salt as ferric hydroxide. This makes it possible to form extremely strong and large flocs in the biological treatment tank, effectively improving the sedimentation, concentration and filterability of sludge, reducing the outflow of iron to the treated water,
(1) In the sediment-type biological treatment in which the activated sludge mixed liquid is solid-liquid separated in a sedimentation basin, the SS of treated water can be lowered and the transparency can be improved.
(2) In the membrane separation activated sludge method for membrane separation of the activated sludge mixture, the membrane can be prevented from being clogged, the membrane flux can be increased, and the membrane flux can be stably maintained for a long time.
Such excellent effects are exhibited.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of an organic wastewater treatment method and treatment apparatus according to the present invention will be described in detail with reference to the drawings.

  1 and 2 are system diagrams showing an embodiment of the organic wastewater treatment apparatus of the present invention. 1 and 2, members having the same function are denoted by the same reference numerals.

  In the present invention, when raw water composed of organic wastewater is introduced into the biological treatment tank 3 and biologically treated with activated sludge, the raw water is first decarboxylated in the decarbonation tank 1 and then dehydrated in the iron salt mixing tank 2. Iron salt is added to and mixed with carbonated water, and the resulting mixed water is biologically treated in the biological treatment tank 3.

  Examples of the organic wastewater to be treated in the present invention include ground water, river water, natural water such as lake water (including dam lake) water, tap water, or recovered water obtained by treating waste water. Can be suitably used when these waters are treated as raw water, and the resulting treated water is used in the production of pure water.

  Originally, these waters have a low BOD concentration of about 0.1 to 100 mg / L. When these waters are used for producing pure water, microorganisms called oligotrophic bacteria such as Pseudomonas are mainly used. After the biological treatment, solid-liquid separation is performed with an ultrafiltration (UF) membrane or a membrane having a pore size of about 0.2 μM or less. The membrane used for the treatment of pure water production water is likely to be clogged because of its small pore size. In particular, natural water contains humic substances and urea that tend to clog the membrane, and the concentration of insoluble suspension (SS) may be high. According to the present invention, since a high fouling prevention effect is obtained, the raw water may contain one or both of high-concentration humic substances and urea exceeding 1 mg / L. It may be contained in the range of about 30 mg / L.

  By setting the MLSS concentration in the biological treatment tank to a high concentration of 2,000 to 50,000 mg / L, particularly 5,000 to 20,000 mg / L, the biological treatment efficiency can be increased.

  Here, the ratio of the amount of organic substances in the MLSS, specifically, the activated sludge organic suspended solid MLVSS (Mixed Liquor Volatile Suspended Solids) / MLSS ratio is about 0.1 to 0.8, particularly 0.2 to 0.6. It is better to be in the range. When the organic matter concentration of water containing organic matter introduced into the biological treatment tank is extremely low (for example, an assimilable organic carbon, which is a biodegradable organic substance, hereinafter, “AOC” concentration is less than about 100 ng / L), the activity in the biological treatment tank In some cases, the sludge growth is reduced and the MLVSS / MLSS ratio is outside the above range. In such a case, a small amount of organic substance may be added to the biological treatment tank, or other organic substance-containing water having a high organic substance concentration may be mixed.

  The carrier may be suspended in the biological treatment tank. Examples of such floating carriers include sponges and gels. The BOD load of the biological treatment tank may be the same as that of a normal activated sludge method, for example, 0.5 to 5.0 kg-BOD / day, particularly 0.5 to 2.0 kg-BOD / day is preferable, but lower. Even if it is a load, sludge is not dispersed by the effect of the iron salt, and a sufficiently large floc can be generated to perform good treatment.

  In the present invention, prior to biological treatment of raw water in such a biological treatment tank, first, raw water is decarboxylated. 1 and 2, a decarbonation tank 1 is provided, raw water is introduced into the decarbonation tank 1, and a pH adjuster is added as necessary from the pH adjuster addition means 1C linked to the pH meter 1B in the tank. The pH is adjusted to 4.0 to 5.0 by adding, and air is aerated from the diffuser 1A under such pH acidic conditions. Thereby, the carbonate ion in raw | natural water is converted into a carbon dioxide gas, and is removed.

  The decarboxylation means is not limited to such a decarboxylation tank, and a membrane deaeration device, nitrogen aeration under a neutral pH, or the like can be used.

  In the present invention, it is preferable to obtain decarboxylated water having a carbonic acid concentration of about 0 to 20 mg / L by decarboxylating raw water having a normal carbonic acid concentration of about 20 to 200.

  The decarboxylated water obtained by decarboxylation in this way is then fed to the iron salt mixing tank 2 and, if necessary, a pH adjuster from the pH adjuster adding means 2C linked to the pH meter 2B. Is added to adjust the pH to 4.5 to 6.5, and the iron salt is added under this pH condition and mixed with stirring.

  The iron salt is not particularly limited, and iron salts such as ferric chloride, ferrous chloride, polyiron sulfate, and ferric sulfate can be used. These may be used alone or in combination of two or more.

  The amount of iron salt added is preferably such that the iron content (Fe content) in the activated sludge MLSS in the biological treatment tank 3 is 10 to 45% by weight, particularly 10 to 35% by weight. . If the addition amount of the iron salt is too small, a sufficient addition effect cannot be obtained, and if it is too much, the amount of activated sludge increases and the floc strength decreases.

  The iron salt addition amount is preferably controlled by analyzing the Fe content in the activated sludge MLSS, but simply, it may be controlled by the BOD of the raw water, for example, iron per 1 mg / L of the raw water BOD. It is preferable that the addition amount of salt in terms of Fe is about 0.03 to 0.3 mg / L. It is preferable to finely adjust the iron salt addition amount by analyzing the MLSS Fe content of the sludge while adding the iron salt in this addition amount range.

  If the pH of the iron salt mixing tank 2 in which iron salt is added to decarboxylated water of raw water is less than 4.5, extremely fine particles of iron hydroxide are generated and the sedimentation property of sludge deteriorates, exceeding 6.5 Again, carbonic acid in the air dissolves and iron carbonate is generated, and aggregation of humic acid and the like worsens. Therefore, the pH in the iron salt mixing tank 2 is preferably 4.5 to 6.5, particularly 4.5 to 5.5.

  In the conventional activated sludge method, the addition of iron salt to the biological treatment tank is generally performed for the purpose of preventing bulking, removing phosphorus, and the like. However, in this case, the amount of iron salt added is only a very small amount for phosphorus removal or addition that can prevent bulking. Furthermore, when pH control is not performed or when pH control is performed, the pH is usually 6.5 or more for phosphorus removal or nitrification.

  In the present invention, as will be described later, the pH of the biological treatment tank is preferably 5 to 6.5, more preferably 5.5 to 6.0, and after satisfying this condition, In the mixing tank 2 provided separately, an iron salt is added, and the pH of this tank is preferably 4.5 to 6.5.

  By such an operation, the SS of biologically treated water is always 5 mg / L or less, usually 2 mg / L or less, and the transparency reaches 3 m or more. Further, when applied to a membrane separation activated sludge method for membrane separation of biologically treated water, the membrane flux can be improved from the usual 0.5 m / day to about 1 m / day.

  In this iron salt mixing tank 2, it is preferable to stir and mix with a residence time of about 3 to 20 minutes in order to sufficiently mix the decarboxylated treated water with the iron salt.

  The water added and mixed with the iron salt in the iron salt mixing tank 2 is then fed to the biological treatment tank 3 for biological treatment.

  In this biological treatment tank 3, a pH adjuster is added as necessary by the pH adjuster adding means 3C interlocked with the pH meter 3B, preferably pH 5 to 6.5, more preferably pH 5.5 to 6.0. Thus, the biological treatment is performed under aeration by the diffusing tube 3A.

  In addition, in the decarboxylation tank 1, the iron salt mixing tank 2, and the biological treatment tank 3, an acid or alkali such as hydrochloric acid is used as a pH adjuster added as necessary, and as the alkali, scale formation is prevented. Therefore, it is preferable to use a soda-based alkali such as caustic soda rather than slaked lime.

  When the biologically treated water is solid-liquid separated by a separation membrane, the separation membrane may be any of an MF (microfiltration) membrane, a UF (ultrafiltration) membrane, an NF (nanofiltration) membrane, and the like. The form of the membrane may be any of a flat membrane, a tubular membrane, a hollow fiber and the like. Examples of the material of the film include, but are not limited to, PVDF (polyvinylidene fluoride), PE (polyethylene), PP (polypropylene), and the like. The separation membrane may be immersed in the biological treatment tank 3 as shown in FIG. 1, or may be installed as a pressurized membrane separation device separate from the biological treatment tank 3 as shown in FIG. However, the immersion film is preferable because the floc is less likely to be destroyed.

  In the biological treatment tank of FIG. 1, the mixed water from the iron salt mixing tank 2 is introduced into the biological treatment tank 3 and mixed with the activated sludge, and aerated by the air diffuser 3 </ b> A provided at the bottom of the biological treatment tank 3. Below, biological treatment is performed.

  In the biological treatment tank 3, a pH adjusting agent such as an acid or an alkali is added from the adding means 3C so that the pH detected by the pH meter 3B is within a predetermined range. The biologically treated water passes through the separation membrane 4 and is taken out as treated water. In FIG. 1, the permeated water is taken out by the pump 5, but the permeated water may be taken out by gravity.

  Excess sludge in the biological treatment tank 3 is taken out by the take-out pipe 3D. A part of the extracted sludge may be solubilized with ozone or the like and then returned to the biological treatment tank 3.

  In FIG. 1, the separation membrane 4 is immersed in the biological treatment tank 3. However, as shown in FIG. 2, the biological treatment water in the biological treatment tank 3 is supplied to the pressurized membrane separation device 7 by the pump 6 and permeated. The water may be taken out as treated water, and a part (or all) of the concentrated water may be returned to the biological treatment tank 3.

  Examples of the type of membrane used in the membrane separator 7 include MF membranes and UF membranes, and examples of the membrane module format include spiral membranes other than hollow fiber membranes and flat membranes, but are not limited thereto.

  Also in the case of FIG. 2, a part of the concentrated water of the membrane separation device 7 may be guided to a sludge solubilization tank and solubilized with ozone or the like, and then returned to the biological treatment tank 3.

  As described above, it is preferable to use the immersion type separation membrane 4 shown in FIG. 1 rather than the pressure type membrane separation device 7 as shown in FIG.

  According to the present invention, as shown in FIGS. 1 and 2, in an organic wastewater biological treatment method in which biologically treated water is directly solid-liquid separated by membrane separation, the biological treatment is performed by an immersion membrane module immersed in a biological treatment tank. In the organic wastewater biological treatment method for separating water into membranes, it is possible to obtain treated water with good water quality while preventing clogging of the membrane and effectively preventing a decrease in membrane flux.

  However, in the present invention, the solid-liquid separation of biologically treated water may use a separation tank, a precipitation tank, a cyclone, etc., and if a precipitation tank is used, the sludge settling in the precipitation tank, concentrated water While improving, SS of separation water (process water) can be reduced and transparency can be improved.

  Regardless of which solid-liquid separation means is used, liquid and separated solids (separated sludge) are partly returned to the biological treatment tank as return sludge as necessary, and the sludge in the biological treatment tank is removed. It is preferable to extract the sludge so that the residence time is about 2 to 50 days, particularly about 10 to 30 days. Or when an immersion type separation membrane is used, it is preferable to draw out sludge so that it may become such sludge residence time. The extracted sludge may be discharged as surplus sludge or may be reduced in volume by a volume reduction means such as an ozone reaction tank or a digestion tank.

  Hereinafter, examples and comparative examples will be described.

  The raw water used in the following examples and comparative examples is organic wastewater (carbonic acid concentration 82 mg / L, pH 7.6) having a BOD concentration of 50 mg / L.

  As the apparatus, an apparatus provided with the immersion type separation membrane 4 shown in FIG. 1 was used. However, in Comparative Examples 1 and 2, the decarboxylation tank 1 and the iron salt mixing tank 2 were not used, and in the Comparative Example 3, the decarboxylation tank 1 was not used.

The volume of the biological treatment tank 3 is 0.5 m ³ . As the immersion type separation membrane 4, three 3m ² / piece hollow fiber MF membranes (manufactured by Mitsubishi Rayon Co., Ltd., pore diameter: 0.4 μm) were used. The capacity of the iron salt mixing tank 2 is 15L. The decarboxylation tank 3 is a tank for decarboxylation treatment by lowering the organic wastewater to pH 4.5 and aeration.

  For convenience of explanation, a comparative example is given first.

[Comparative Example 1]
The raw water is directly introduced into the biological treatment tank at a raw water flow rate of 10 m ³ / day, treated under the conditions of BOD load 1.0 kg-BOD / m ³ / day, SRT 25 days, and in the middle of the treated water pipe connected to the submerged separation membrane. The treated water (permeated water) was taken out from the treated water pipe by reducing the pressure with the provided vacuum pump.

  As a result, the membrane was clogged in one day from the start of the experiment, and the treated water could not be drawn. The TOC concentration of the treated water at this time was 3.5 mg / L, and the properties of the activated sludge mixed solution in the tank were as follows.

MLSS concentration: 7000 mg / L (Fe content is 4.7% by weight of MLSS)
MLVSS concentration: 4900 mg / L
pH: 6.8

[Comparative Example 2]
The biological treatment tank in which the treated water could not be pulled out in Comparative Example 1 was emptied, and activated sludge was added to the biological treatment tank so as to have an MLSS concentration of 5000 mg / L. An aqueous iron solution was added at a rate of 1000 mg-Fe / L in terms of Fe. In addition, the pH was adjusted with sodium hydroxide in conjunction with the pH meter in the biological treatment tank, and the pH in the tank was maintained at 5.5.

In this state, raw water is directly supplied to the biological treatment tank at a flow rate of 10 m ³ / day, and 0.5 wt% ferric chloride aqueous solution is 25 mg-Fe / L with respect to the inflow raw water amount (Fe with respect to the BOD load). As a result, the increase in the differential pressure of the submerged separation membrane was reduced from 3 days after the start of water flow. The TOC concentration of the treated water at this time was 2.3 mg / L, and the properties of the activated sludge mixed solution in the biological treatment tank were as follows.

MLSS concentration: 7500 mg / L (Fe content is 35% by weight of MLSS)
MLVSS concentration: 3000 mg / L
pH; 5.5

However, when the operation was continued, the differential pressure of the immersion type separation membrane increased, and chemical cleaning was required in 2 weeks. When the activated sludge mixed liquid in the biological treatment tank was taken out and the sedimentation property was confirmed, the supernatant after standing for 30 minutes was turbid in brown, and when SS was measured, it was 70 mg / L.
[Comparative Example 3]
In Comparative Example 2, the raw water was introduced not into the biological treatment tank but into the iron salt mixing tank in the preceding stage of the biological treatment tank, and an aqueous ferric chloride solution was added to the iron salt mixing tank, and the organic waste water and the second chloride chloride were added. After stirring and mixing with iron for 5 minutes, the treatment was performed under the same conditions except that it was supplied to the biological treatment tank. The pH of this iron salt mixing tank was 5.5.

  As a result, there was almost no increase in the differential pressure of the submerged separation membrane, and stable operation was possible for one month, but after one month, there was a differential pressure increase of 30 kPa. The TOC concentration of treated water was 2.1 mg / L, and the properties of the activated sludge mixed liquid in the biological treatment tank were as follows.

MLSS concentration: 7200 mg / L (Fe content is 31% by weight of MLSS)
MLVSS concentration: 2800 mg / L
pH; 5.5
[Example 1]
In Comparative Example 3, the raw water was first decarboxylated in the decarboxylation tank 1 in the preceding stage of the iron salt mixing tank 2 to adjust the carbonic acid concentration to 5 mg / L, and then fed to the iron salt mixing tank 2 to pH 5.5. The treatment was performed under the same conditions except that it was mixed with the iron salt.

  As a result, there was almost no increase in the differential pressure of the submerged separation membrane, and stable operation could be performed for 1 month, and the increase in differential pressure after 1 month was 15 kPa. The TOC concentration of treated water was 1.9 mg / L, and the properties of the activated sludge mixed liquid in the biological treatment tank were as follows.

MLSS concentration: 7500 mg / L (Fe content is 35% by weight of MLSS)
MLVSS concentration: 2500 mg / L
pH; 5.5

  From the above results, after decarboxylation of organic wastewater, it is mixed with iron salt at a predetermined pH condition, and this mixed solution is biologically treated, thereby improving the sedimentation, concentration and filterability of activated sludge. It can be seen that treated water with good water quality can be obtained stably over a long period of time.

It is a systematic diagram which shows embodiment of the processing apparatus of the organic waste_water | drain of this invention. It is a systematic diagram which shows other embodiment of the processing apparatus of the organic waste_water | drain of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Decarbonation tank 2 Iron salt mixing tank 3 Biological treatment tank 4 Immersion type separation membrane 7 Membrane separation device

Claims (8)

In the method of biological treatment by adding iron salt to organic wastewater,
A decarboxylation step of decarboxylating organic wastewater, a mixing step of adding and mixing iron salt to the treated water of the decarboxylation step,
An organic wastewater treatment method comprising: a biological treatment step in which the mixed water from the mixing step is mixed with activated sludge to biologically treat.   In Claim 1, pH of the said mixing process is 4.5-6.5, pH of the said biological treatment process is 5-6.5, The processing method of the organic waste water characterized by the above-mentioned.   3. The organic waste water according to claim 1, wherein in the mixing step, an iron salt is added so that an iron content in the activated sludge in the biological treatment step is 10 to 45% by weight. Processing method.   The organic wastewater treatment method according to any one of claims 1 to 3, further comprising a membrane separation step of membrane separation treatment of the activated sludge mixed liquid in the biological treatment step. In a device for biological treatment by adding iron salt to organic wastewater,
Decarbonation means for decarboxylation of organic waste water, mixing tank for adding iron salt to the treated water of the decarbonation means and mixing, and biological treatment for mixing the mixed water from the mixing tank with activated sludge for biological treatment An organic wastewater treatment apparatus comprising a tank.   6. The organic wastewater treatment apparatus according to claim 5, wherein the pH of the mixing tank is 4.5 to 6.5, and the pH of the biological treatment tank is 5 to 6.5.   In Claim 5 or 6, in the said mixing tank, an iron salt is added so that the iron content in the said activated sludge in the said biological treatment tank may be 10 to 45 weight%, Organic drainage characterized by the above-mentioned. Processing equipment.   The organic wastewater treatment apparatus according to any one of claims 5 to 7, further comprising membrane separation means for membrane separation treatment of the activated sludge mixed liquid in the biological treatment tank.

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