JP2006142256A - Treating method of organic waste water - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a treating method of organic waste water by which dephosphorization efficiency can be raised by reducing solid content returning to a biological reaction tank or phosphorus content in flowing-in waste water in advance in a treating method by which amount of sludge produced in a water treating system is reduced by solubilization treatment and phosphorus is efficiently removed from the waste water. <P>SOLUTION: In the treating method of the organic waste water for biologically treating the organic waste water containing phosphorus by using activated sludge microorganisms, an acid is added to excess sludge D produced in waste water treating facilities so that pH may become 4 or less, physicochemical treatment is performed thereto to release phosphorus out of the activated sludge microorganisms into a solution, then solid-liquid separation is performed and solid content F is returned to the biological reaction tank 1 and supernatant water G is blended with inflow waste water. Further an alkali agent H is added thereto so that pH may become 5 to 10 and thereby phosphorus components in the mixture solution is deposited and is taken out of the system as a solid matter J. Thereafter the mixture solution I is returned to the biological reaction tank 1. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for treating organic wastewater, and in particular, biologically treats organic wastewater containing a large amount of phosphorus, solubilizes the generated microbial sludge, and again decomposes into microorganisms in the water treatment system. The present invention relates to a method for treating organic wastewater.

Currently, as a method for removing organic substances contained in organic wastewater, many methods of biological treatment using a group of microorganisms called activated sludge are incorporated. In this method, the activated sludge microorganisms can be mineralized or removed from the solution by using pollutants such as organic substances for their metabolism.
However, when contaminants exceeding the metabolic capacity of activated sludge microorganisms are included, or when it is desired to further increase the removal rate, it is necessary to adopt a treatment method that exceeds the treatment with general activated sludge microorganisms.

Among the pollutants, the phosphorus component includes a physicochemical method in which an inorganic or polymer flocculant is added and removed, and a biological treatment using microorganisms that specifically accumulate phosphorus in the body.
In the method of adding a flocculant, phosphoric acid in waste water and metal ions contained in the inorganic flocculant are combined to form an insoluble aggregate precipitate, which can be removed from the solution. Alternatively, when a polymer flocculant is used, the suspended phosphorus component in the wastewater becomes a floc and is separated into solid and liquid, or even when soluble, a floc composed mainly of other components is formed. It can be removed from the solution, for example by being incorporated into a floc.
On the other hand, in biological treatment, a microorganism called polyphosphate accumulating bacteria is used, but since this microorganism can accumulate a large amount of polyphosphate in the body, compared to the case of using a normal activated sludge microorganism group, Many phosphorus can be removed. That is, the amount of phosphorus contained in normal activated sludge is 1.5-2% (per dry weight), but 3-5% (per dry weight) in activated sludge where polyphosphate-accumulating bacteria are sufficiently active. )become.
Thus, phosphorus in wastewater can be removed from the solution by incorporating it into solid matter such as agglomerated sludge or activated sludge. In other words, since phosphorus has no inorganic gas state like nitrogen and carbon (nitrogen is nitrogen gas, carbon is carbon dioxide, etc.), it can be removed from the solution only as a solid.

By the way, sludge generated by water treatment needs to be properly treated and disposed of in the latter stage, but there is a problem that the landfill for final disposal is tight, and the treatment and disposal costs are high, There is also a problem that the operating cost of the wastewater treatment facility increases. In fact, the operating costs associated with these sludge treatments have reached nearly half of the total operating costs in wastewater treatment facilities.
The above-mentioned method for removing phosphorus from wastewater can also be removed only as a solid matter, that is, sludge. Therefore, when the concentration of phosphorus in the wastewater to be treated is high, the amount of generated sludge increases accordingly, and the above problem is further increased. It will be a thing.

In recent years, in response to these problems with sludge treatment, the generated surplus sludge is solubilized, that is, sterilized and liquefied activated sludge microorganisms, returned to the water treatment system and decomposed into activated sludge microorganisms as a whole facility. Technology to reduce the amount of generated sludge is being developed and put into practical use.
As a solubilization method, there are a method of adding ozone and utilizing its strong oxidizing power (see Patent Document 1) and a method of using thermophilic bacteria (see Patent Document 2). However, in these methods, phosphorus incorporated into activated sludge microorganisms including polyphosphate-accumulating bacteria in the water treatment system is also returned to the water treatment system together with the treated sludge after the solubilization treatment. Phosphorus cannot be removed from the water, and it flows out as treated water.

Accordingly, the present inventors have invented a technology that can remove and collect phosphorus at the same time as performing solubilization treatment for excess sludge generated in the water treatment system (see Patent Documents 3 to 6, hereinafter referred to as acid solubilization). The law).
In this treatment method, an excess sludge generated in a water treatment system is subjected to physicochemical treatment by adding an acid so that the pH is 5 or less (preferably 2 or less). Microorganisms in the sludge can be sterilized, and phosphorus incorporated in the microorganisms can be eluted to the solution side.
Next, the treated sludge is separated into a solid liquid and a supernatant water rich in phosphorus and a solid after the release of phosphorus. This solid is processed into a form that can be decomposed and treated by activated sludge microorganisms. Therefore, return to the water treatment system for treatment.
On the other hand, for supernatant water rich in phosphorus, adding alkali to the solution and setting the pH to 3 or more causes phosphorus to precipitate as phosphorus sludge mainly composed of inorganic components. -It can be removed. Therefore, in the treatment using this acid solubilization method, the amount of sludge generated can be reduced while reducing the phosphorus load on the water treatment system as much as possible.

However, phosphorus that can be removed by the acid solubilization method is discharged from the water treatment system as surplus sludge, and is only precipitated as phosphorus sludge in the solubilization treatment system. Therefore, in order to reduce the amount of sludge generated and to actively remove phosphorus from the wastewater itself, in the water treatment system, more phosphorus is separated as solids, extracted as excess sludge outside the system, and solubilized. Need to be processed. In particular, when the phosphorus concentration in the wastewater is high, a large amount of solid matter is generated in the water treatment system.
Thus, when the concentration of phosphorus in the wastewater is high, excess sludge generated in large quantities in the water treatment system must be solubilized in order to remove phosphorus, so that sludge treatment and disposal costs can be reduced. The solubilization process, on the other hand, increases the processing cost and can be very inefficient.
Japanese Patent No. 3397096 Japanese Patent No. 3048889 JP 2002-361278 A Japanese Patent Laid-Open No. 2003-001300 Japanese Patent Laid-Open No. 2003-211178 JP 2003-190999 A

  In view of the problems of the above-mentioned conventional organic wastewater treatment methods, the present invention reduces the amount of sludge generated from the water treatment system by solubilization treatment and efficiently removes phosphorus from wastewater. It aims at providing the processing method of the organic wastewater which can improve the dephosphorization efficiency by reducing beforehand the phosphorus content of the solid content which flows back into a biological reaction tank, or the inflow wastewater.

  In order to achieve the above object, a method for treating organic wastewater according to the first aspect of the present invention is an organic wastewater treatment method in which organic wastewater containing phosphorus is biologically treated using activated sludge microorganisms. In addition to adding acid so that the pH is 4 or less with respect to the excess sludge generated in step 3, the phosphorus is released into the solution from the activated sludge microorganisms by performing physicochemical treatment, followed by solid-liquid separation, The solid content is returned to the biological reaction tank, and the supernatant water is mixed with the inflow wastewater, and an alkaline agent is added so that the pH is 5 to 10, thereby precipitating the phosphorus component in the mixed solution to form a solid. It is characterized in that it is taken out from the system as a product and then returned to the biological reaction tank.

  In order to achieve the same object, the organic wastewater treatment method of the second invention is an organic wastewater treatment method in which organic wastewater containing phosphorus is biologically treated using activated sludge microorganisms. After adding acid to the surplus sludge generated at the treatment facility so that the pH is 4 or less and releasing physicochemical treatment from the activated sludge microorganisms into the solution, the treated sludge and The influent wastewater is mixed, then solid-liquid separation is performed, and the obtained solid content is returned to the biological reaction tank to be decomposed and treated by activated sludge microorganisms, so that the supernatant has a pH of 5 to 10. An alkaline agent is added, the phosphorus component in the supernatant water is precipitated and taken out of the system as a solid, and then the supernatant water is returned to the biological reaction tank.

  In this case, an acid can be added so that the pH is 2 or less, and an alkaline agent can be added so that the pH is about 7.

  Moreover, a coagulant | flocculant can be added with an alkali agent.

  Further, as the physicochemical treatment, excess sludge can be heated to 30 to 300 ° C.

  Further, as the physicochemical treatment, hypochlorous acid or hypochlorite can be added to the excess sludge.

  In this case, hypochlorous acid or hypochlorite ions can be generated by adding a chloride salt such as sodium chloride to the excess sludge and performing electrolysis.

  In addition, as the physicochemical treatment, at least one treatment of adding an oxidizing agent such as ozone or hydrogen peroxide, crushing by ultrasonic waves, crushing by cavitation, or crushing by microwave irradiation can be performed.

According to the organic wastewater treatment method of the first and second inventions, the treated sludge obtained by subjecting the excess sludge generated in the water treatment system to the acid solubilization method or the supernatant water of the sludge is introduced into the influent water. In order to remove the phosphorus from the wastewater by solid-liquid separation in the latter stage, the solids and inflow wastewater returned to the biological reaction tank The phosphorus content can be reduced in advance, and the dephosphorization efficiency can be improved.
It is possible to remove the portion of the wastewater from which phosphorus has been partially removed without being taken up by activated sludge microorganisms or by generating flocculent sludge by adding a flocculant. Conventionally, phosphorus is removed. Therefore, the amount of excess sludge is inevitably increased. However, in the present invention, it can be directly removed from the wastewater, so that it does not increase more than necessary.
Moreover, by treating the excess sludge with acid solubilization, the treated sludge can be decomposed by the activated sludge microorganisms held in the water treatment system, and the amount of generated sludge can be further reduced.
Thus, in the present invention, the amount of sludge generated as a whole wastewater treatment facility can be reduced and phosphorus can be efficiently removed, and is particularly effective when the concentration of phosphorus in the wastewater is high.

  In this case, it is possible to effectively solubilize and precipitate phosphorus by adding an acid so that the pH is 2 or less and adding an alkali agent so that the pH is about 7.

  Moreover, phosphorus can be precipitated more effectively by adding an aggregating agent together with an alkali agent.

  Moreover, as the said physicochemical process, solubilization of sludge microorganisms can be accelerated | stimulated by heating excess sludge to 30-300 degreeC.

  Further, as the physicochemical treatment, solubilization of sludge microorganisms can be promoted by adding hypochlorous acid or hypochlorite to the excess sludge.

  In this case, the solubilization of sludge microorganisms is further promoted by generating hypochlorous acid or hypochlorite ions by adding chloride salts such as sodium chloride to excess sludge and performing electrolysis. Can do.

  In addition, as the physicochemical treatment, solubilization of sludge microorganisms is performed by applying at least one treatment of adding an oxidizing agent such as ozone or hydrogen peroxide, crushing by ultrasonic waves, crushing by cavitation or crushing by microwave irradiation Can be promoted.

  Hereinafter, an embodiment of a method for treating organic wastewater according to the present invention will be described with reference to the drawings.

In the organic wastewater treatment method of the present invention, the surplus sludge generated in the water treatment system is mixed with the treated sludge obtained by applying the acid solubilization method or the supernatant water of the sludge with the influent water, and the alkaline agent is added. Addition produces phosphorus sludge mainly composed of inorganic components, and removes phosphorus from wastewater by solid-liquid separation in the latter stage.
In the acid solubilization method by the present inventors, acid is added to the excess sludge generated in the water treatment system so that the pH is 5 or less (preferably 2 or less), and physicochemical treatment is performed. As a result, the cell membrane or cell wall of microorganisms in the excess sludge is damaged or destroyed, and the activity is lost. When the cell membrane or cell wall is damaged or destroyed, the cytoplasm held in the microorganism is released to the solution side, but these substances are organic substances that can be metabolized by the microorganism. Therefore, the sludge subjected to the acid solubilization treatment can be returned to the water treatment system and biodegraded by the retained activated sludge.

  Although the acid solubilization method can inactivate microorganisms, it simultaneously has the effect of releasing phosphorus contained in the microorganisms to the solution side. That is, the cytoplasm is released as described above, or the solid phosphorus component contained in the cell wall or cell membrane is eluted into the solution. The sludge treated by the acid solubilization method or the supernatant water obtained by solid-liquid separation of the sludge has a pH of 5 or less (preferably 2 or less). When added in such a manner, the phosphorus component present on the solution side precipitates as a solid and can be separated into solid and liquid at a later stage. Although the precipitate is mainly composed of inorganic substances, it contains some organic substances because other floating substances are taken in during the precipitation.

At this time, even if soluble phosphorus components other than those contained in the supernatant water obtained by the acid solubilization treatment are added, it is possible to simultaneously precipitate and separate by adding an alkaline agent.
Therefore, particularly when wastewater with a high phosphorus concentration is treated, the acid solubilized sludge is mixed with the wastewater, and an alkaline agent is added to the supernatant water obtained by solid-liquid separation, thereby producing an acid. Not only phosphorus released or eluted by the solubilization treatment but also phosphorus originally contained in the wastewater can be removed. That is, it is possible to directly remove phosphorus in wastewater without once taking it into activated sludge microorganisms.
As mentioned above, in the processing method of the organic wastewater by this invention, the phosphorus contained in inflow wastewater is removed with the following method using the characteristic of the said acid solubilization method.

That is, in a wastewater treatment facility that biologically treats organic wastewater containing phosphorus using activated sludge microorganisms, an acid is added so that the generated sludge has a pH of 4 or less (preferably 2 or less). At the same time, phosphorus is released from the activated sludge microorganisms into the solution by physicochemical treatment, followed by solid-liquid separation. The solids are returned to the biological reaction tank, and the supernatant water is mixed with the influent wastewater. By adding an alkaline agent so that the pH becomes 5 to 10 (preferably 7), phosphorus in the wastewater can be removed by precipitating and separating the phosphorus component in the mixed solution.
Alternatively, in a wastewater treatment facility that biologically treats organic wastewater containing phosphorus using activated sludge microorganisms, an acid is added so that the generated sludge has a pH of 4 or less (preferably 2 or less). At the same time, phosphorus is released from the activated sludge microorganisms into the solution by physicochemical treatment, and then the treated sludge and the influent wastewater are mixed, followed by solid-liquid separation. The activated sludge microorganisms are returned to the water and decomposed and treated, and an alkaline agent is added to the supernatant water so that the pH is 5 to 10 (preferably 7), thereby precipitating the phosphorus component in the mixture. The phosphorus in the wastewater can be removed by taking it out of the system as a solid.

In the present invention, phosphorus is precipitated and separated by adding an alkali agent and adjusting the pH to about 5 to 10, preferably about 7. However, precipitation and separation of phosphorus can be achieved by adding a flocculant at the same time. Can be promoted.
Moreover, as a physicochemical process, an excess sludge can be heated to 30-300 degreeC, Preferably it is 60-100 degreeC.

Or the disinfection power by hypochlorous acid or hypochlorite ion can be utilized by adding hypochlorous acid or hypochlorite to excess sludge. Hypochlorous acid or hypochlorite ions can be generated by adding a chloride salt such as sodium chloride to the excess sludge and performing electrolysis.
In addition, as other physicochemical treatment, chemical treatment using an oxidizing agent such as ozone or hydrogen peroxide, or physical treatment such as ultrasonic crushing, cavitation, or microwave irradiation can be performed.

As an example of the physicochemical treatment in the acid solubilization method, the results of experiments on the release of phosphorus from microorganisms and the precipitation and recovery of phosphorus sludge when heat treatment is performed are shown.
For example, FIG. 1 shows the phosphorus release rate when the excess sludge is treated at 80 ° C. for 1 hour by changing the pH of the treatment by changing the amount of sulfuric acid to be added. As shown in the figure, the lower the pH, the higher the release rate of phosphorus from the microorganism. In particular, the release rate was 50% or more at pH 2 or lower.

Next, FIG. 2 shows a result of an experiment in which the released phosphorus is increased in pH by adding an alkaline agent to the solution and precipitated. As shown in the figure, when the pH is 4 or more, the recovery rate increases rapidly, and when the pH is 7, the maximum is about 80%. However, if the pH is further raised further, the recovery rate gradually decreases, and if the pH exceeds 10, it becomes 70% or less.
The recovery rate is the same even when the phosphoric acid solution is added so that the phosphorus concentration is doubled and the alkaline agent is added so that the pH is about 7 with respect to the supernatant water of the acid-solubilized sludge. About 80%.

In this example, sulfuric acid was exemplified as the acid to be added, but similar results were obtained with other hydrochloric acid, nitric acid, and the like.
In addition, when a method that generates hypochlorous acid or hypochlorite ions is adopted as the physicochemical treatment, the release rate of phosphorus from microorganisms increases when the pH is 5 or less. confirmed.

FIG. 3 shows a treatment flow in the case of a wastewater treatment facility that does not have an initial settling basin, that is, a small amount of solid matter in the wastewater to be treated.
The pollutant contained in the waste water A containing a large amount of phosphorus flowing into the treatment facility stays in the biological reaction tank 1 for a certain period of time, but during this time, it is decomposed and removed by the activated sludge microorganisms held in the biological reaction tank 1, It flows out into the final sedimentation basin 2 of the latter stage as the mixed liquid B. In the final sedimentation basin 2, solid-liquid separation is achieved, the supernatant is discharged out of the system as treated water C, and the solid is sent as surplus sludge D to the subsequent solubilization tank 3.

Excess sludge D undergoes an acid solubilization treatment in the solubilization tank 3, but in this embodiment, a case where a heat treatment is performed as a physicochemical treatment combined with acid addition is illustrated. In acid solubilization combined with heat treatment, microorganisms in surplus sludge D are sterilized and phosphorus contained in the microorganisms is eluted to the solution side. There is also an effect that is significantly improved.
Accordingly, the treated sludge E flowing out of the solubilization treatment tank 3 is subjected to solid-liquid separation in the solid-liquid separation tank 4, and the solid content is returned to the biological reaction tank 1 as the solubilized sludge F and retained activated sludge. Degraded by microorganisms.
On the other hand, the supernatant water G obtained in the solid-liquid separation tank 4 contains high-concentration soluble phosphorus, and is similarly sent to the phosphorus precipitation tank 5 together with the influent water A containing high-concentration phosphorus. In the phosphorus precipitation tank 5, the pH is adjusted to about 7 by adding the alkaline agent H. During this time, soluble phosphorus in the supernatant water G is precipitated as a solid and can be separated by precipitation.

Therefore, the phosphorus concentration contained in the dephosphorization waste water I flowing out from the phosphorus precipitation tank 5 can be lowered, and the load on the biological reaction tank 1, that is, the amount of phosphorus to be removed in the tank is reduced, that is, water treatment. The excess sludge that is inevitably generated to remove phosphorus in the system can be reduced.
Moreover, although the solid substance obtained in the phosphorus precipitation tank 5 is discharged out of the system as phosphorus sludge J, its generation amount is smaller than the surplus sludge amount subjected to the acid solubilization treatment, and the sludge generation amount as a whole is small. Is possible.

In addition, when the solid substance density | concentration in inflow wastewater is high, as shown in FIG. 4, wastewater A flows into the sedimentation tank 6 first, and is separated into solid and liquid.
The obtained supernatant is first introduced into the phosphorus precipitation tank 5 as sedimentation basin effluent K, and after the phosphorus component is removed, it is sent to the biological reaction tank 1 as dephosphorization waste water I.
The first sedimentation basin sludge L, which is the solid content obtained in the first sedimentation basin 6, is usually discharged out of the system, but when there are many components that can be decomposed by the acid solubilization treatment, it is sent to the solubilization tank 3 and the acid. Solubilization treatment can be performed.

Next, FIG. 5 shows an embodiment in which techniques for generating hypochlorous acid are combined as physicochemical treatment.
As in Example 1, the pollutant in the wastewater A is treated in the biological reaction tank 1 and flows out as a mixed solution B, and then solid-liquid separated in the final sedimentation basin 2, and separated into treated water C and excess sludge D. Is done.
The treated water C is discharged outside the system after undergoing further treatment such as sterilization treatment, and the excess sludge D is subjected to acid solubilization treatment in the solubilization tank 3 at the subsequent stage.

In the solubilization tank 3, sodium chloride M is added and electrolytic treatment is performed, and thereby hypochlorous acid is generated. Since the solubilization tank 3 is maintained at a pH of 5 or less, most of the hypochlorous acid that is not ionized is generated by electrolysis.
Hypochlorous acid sterilizes microorganisms in excess sludge D by its strong oxidizing power, and releases or elutes phosphorus in microorganisms to the dissolution side, together with the effect of maintaining the pH at 5 or less. .

On the other hand, the acid solubilization method shown in this example is less effective in improving the solid-liquid separation property by the treatment than the acid solubilization method in which the acid addition and the heat treatment shown in Example 2 are combined, and is sufficient as it is. It is difficult to achieve solid-liquid separation.
Therefore, the sedimentation performance is improved by diluting and washing the treated sludge E with the wastewater A flowing into the treatment facility. That is, the treated sludge E that has flowed out of the solubilization tank 3 flows into the mixing / separation tank 7 in the subsequent stage, but the tank 7 is divided into two tanks by the separation wall 8, and the waste water A and the treated sludge are disposed in the former stage portion. E is introduced, mixed and diluted.
As a result, the phosphorus component present on the solution side of the treated sludge E is mixed with the wastewater A, and the solids concentration is reduced by dilution, so that solid-liquid separation in the subsequent stage of the mixing / separation tank 7 is easy. Become. Further, by mixing the waste water A and the treated sludge E, the extracellular polymer that inhibits the precipitation separation is washed out from the inactivated microorganisms contained in the treated sludge E, and the solid-liquid separation property can be improved.

The solid content separated in the mixing / separation tank 7 is returned to the biological reaction tank 1 as the solubilized sludge F and decomposed by the retained activated sludge microorganisms.
On the other hand, the solution containing a large amount of phosphorus is led to the subsequent phosphorus precipitation tank 5 as the supernatant water G, and by adding the alkaline agent H, the phosphorus in the solution is precipitated, and the solid-liquid separation is performed. It can be discharged out of the system as sludge J. In addition, about the solution from which phosphorus was removed, it is guide | induced to the biological reaction tank 1 as dephosphorization wastewater I, and the phosphorus and other organic substance which remain | survive are decomposed | disassembled and removed by activated sludge microorganisms.

  As mentioned above, although the processing method of the organic wastewater of this invention was demonstrated based on the several Example, this invention is not limited to the structure described in the said Example, The structure described in each Example is shown. The configuration can be changed as appropriate within a range that does not depart from the spirit of the invention, for example, as appropriate.

  As described above, the organic wastewater treatment method of the present invention has a characteristic that the solid content returning to the biological reaction tank and the phosphorus content of the inflow wastewater can be reduced in advance and the dephosphorization efficiency is improved. Therefore, it can be suitably used for economical treatment of organic wastewater containing a large amount of phosphorus.

In the processing method of the organic wastewater of this invention, it is a graph which shows the relationship between pH by addition of an acid, and phosphorus release rate. In the processing method of the organic wastewater of this invention, it is a graph which shows the relationship between pH by addition of an alkali, and phosphorus recovery rate. In the processing method of the organic wastewater of this invention, when the solid substance density | concentration in inflow wastewater is low, it is a processing flowchart which shows the example which used acid solubilization processing and heat processing together. In the processing method of the organic wastewater of this invention, when the solid substance density | concentration in inflow wastewater is high, it is a processing flow figure which shows the example which used acid solubilization processing and heat processing together. In the processing method of the organic wastewater of this invention, when the solid substance density | concentration in inflow wastewater is low, it is a processing flowchart which shows the example which used the acid solubilization process and the electrolytic treatment together.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Biological reaction tank 2 Final sedimentation tank 3 Solubilization tank 4 Solid-liquid separation tank 5 Phosphorus precipitation tank 6 First sedimentation tank 7 Mixing / separation tank 8 Separation wall A Waste water B Mixed liquid C Treated water D Surplus sludge E Treated sludge F Solubilization Sludge G Supernatant water H Alkaline agent I Dephosphorization wastewater J Phosphorus sludge K First sedimentation basin effluent L First sedimentation basin sludge M Sodium chloride

Claims (8)

  In an organic wastewater treatment method in which organic wastewater containing phosphorus is biologically treated using activated sludge microorganisms, an acid is added so that the pH is 4 or less with respect to excess sludge generated in the wastewater treatment facility. After releasing phosphorus from the activated sludge microorganisms into the solution by physicochemical treatment, solid-liquid separation is performed, the solid content is returned to the biological reaction tank, and the supernatant water is mixed with the influent wastewater. By adding an alkaline agent so that the pH becomes 5 to 10, the phosphorus component in the mixed solution is precipitated and taken out as a solid matter, and then the mixed solution is returned to the biological reaction tank. A method for treating organic wastewater, characterized by:   In an organic wastewater treatment method in which organic wastewater containing phosphorus is biologically treated using activated sludge microorganisms, an acid is added so that the pH is 4 or less with respect to excess sludge generated in the wastewater treatment facility. After physicochemical treatment, phosphorus is released from the activated sludge microorganisms into the solution, and then this treated sludge and the influent wastewater are mixed, followed by solid-liquid separation. It is returned to the tank and decomposed and treated by activated sludge microorganisms, and an alkaline agent is added to the supernatant water so that the pH is 5 to 10, and the phosphorus component in the supernatant water is precipitated to form a solid matter outside the system. And then, the supernatant water is returned to the biological reaction tank.   The method for treating organic wastewater according to claim 1 or 2, wherein an acid is added so that the pH is 2 or less, and an alkali agent is added so that the pH is about 7.   The method for treating organic wastewater according to claim 1, 2 or 3, wherein a flocculant is added together with the alkali agent.   The method for treating organic wastewater according to claim 1, 2, 3, or 4, wherein as the physicochemical treatment, surplus sludge is heated to 30 to 300 ° C.   The organic wastewater treatment method according to claim 1, 2, 3 or 4, wherein hypochlorous acid or hypochlorite is added to the excess sludge as the physicochemical treatment.   The method of treating organic wastewater according to claim 6, wherein hypochlorite or hypochlorite ions are generated by adding a chloride salt such as sodium chloride to excess sludge and performing electrolysis. .   The physicochemical treatment includes at least one treatment of adding an oxidizing agent such as ozone or hydrogen peroxide, crushing by ultrasonic waves, crushing by cavitation, or crushing by microwave irradiation. 3. A method for treating organic wastewater according to 3 or 4.

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