JP7650463B2 - Wastewater treatment device and wastewater treatment method - Google Patents

Description

The present invention relates to a wastewater treatment device and a wastewater treatment method, and in particular to a wastewater treatment device and a wastewater treatment method for treating organic wastewater such as sewage or industrial wastewater.

In the technical field of water treatment, water treatment devices are known that use forward osmosis membranes (FO membranes) to desalinize seawater, purify sewage or industrial wastewater, etc. The FO membrane method is a type of osmosis process that utilizes the phenomenon of water flowing from a concentrated solution to a diluted solution across a semipermeable membrane to desalinize saltwater and purify water containing pathogens and harmful substances.

JP 2012-223723 A (Patent Document 1) discloses that a coagulant injection device and a solids separation device are provided as pretreatment coagulation and precipitation, and an FO membrane is used as a semipermeable membrane treatment device, and that the pretreatment is controlled to reduce fouling of the FO membrane according to the amount of fouling components contained in the seawater, thereby desalination is performed.

JP 2014-61486 A (Patent Document 2) discloses an example in which sewage, human waste, and industrial wastewater are treated as the water to be treated, and concentrated water is obtained directly from the water using a forward osmosis membrane means, and a configuration is disclosed in which the concentrated water is treated with aeration in a reaction tank.

Patent Publication No. 6727056 (Patent Document 3) describes an example of a wastewater treatment device and method that can treat organic wastewater such as sewage or wastewater using a forward osmosis membrane and recover energy.

JP 2012-223723 A JP 2014-61486 A Patent No. 6727056

However, although the technology described in Patent Document 1 can suppress membrane fouling, it is a technology aimed at seawater desalination, and does not mention or suggest the recovery of energy from the treated water.

The technology described in Patent Document 2 concentrates the water being treated by directly introducing it into the FO membrane, so biofouling occurs early on on the surface of the FO membrane due to the organic solids and soluble organic matter contained in the water being treated.

The technology described in Patent Document 3 can suppress membrane fouling by supplying a bactericide to organic wastewater, and can increase the efficiency of recovering methane gas from the concentrated water by supplying wastewater or separated sludge to the concentrated water of forward osmosis membrane treatment. However, in the invention described in Patent Document 3, some of the organic matter in the wastewater or separated sludge is used to decompose the bactericide remaining in the concentrated water, which may reduce the amount of organic matter recovered as methane gas. In addition, a certain reaction time is required to fully reduce the bactericide components in the concentrated water using wastewater or separated sludge, but if sufficient reaction time cannot be ensured, the bactericide in the concentrated water will flow into the anaerobic digester, reducing the stability of the anaerobic treatment.

In view of the above problems, the present invention provides a wastewater treatment device and a wastewater treatment method that can stably perform anaerobic treatment of wastewater and fuel production while suppressing membrane fouling.

The inventors conducted extensive research to solve the above problems and discovered that it is effective to mix a bactericide with wastewater, perform forward osmosis membrane treatment, and then perform a specific treatment before anaerobic treatment of the concentrated water obtained by the forward osmosis membrane treatment.

In one aspect, an embodiment of the present invention, which has been completed based on the above findings, is a wastewater treatment device that includes forward osmosis membrane means that has a semipermeable membrane and that produces concentrated water and treated water by contacting wastewater with a driving liquid having a higher osmotic pressure than the wastewater through the semipermeable membrane, a disinfectant supply means that supplies a disinfectant to the inflow water that flows into the forward osmosis membrane means, a concentrated water storage means that stores the concentrated water, an anaerobic treatment means that anaerobically treats the concentrated water to produce fuel gas, and a circulation means that circulates the anaerobically treated liquid from the anaerobic treatment means to the concentrated water storage means.

In one embodiment, the wastewater treatment device according to the present invention further includes a wastewater supply means capable of supplying at least a portion of the wastewater to at least one of the concentrated water storage means or the anaerobic treatment means.

In another embodiment of the wastewater treatment device according to the present invention, the wastewater treatment device further comprises a first solid-liquid separation means for performing solid-liquid separation of precipitated organic matter in the wastewater, and a first sludge supply means capable of supplying the separated sludge obtained by the first solid-liquid separation means to at least one of the concentrated water storage means or the anaerobic treatment means.

In yet another embodiment, the wastewater treatment device according to the present invention further comprises a second solid-liquid separation means for performing solid-liquid separation of the wastewater or the separated liquid obtained by the first solid-liquid separation means for performing solid-liquid separation of the wastewater or the precipitated organic matter in the wastewater, and a second sludge supply means capable of supplying the separated sludge obtained by the second solid-liquid separation means to at least one of the concentrated water storage means or the anaerobic treatment means.

In yet another embodiment of the wastewater treatment device according to the present invention, the anaerobically treated liquid contains a catalyst for promoting the reduction reaction of the disinfectant.

In yet another embodiment of the wastewater treatment device according to the present invention, the anaerobically treated liquid has an oxidation-reduction potential of -500 mV to -200 mV (based on a silver-silver chloride electrode).

In yet another embodiment of the wastewater treatment device according to the present invention, the device further includes a measuring means for measuring the redox potential of the concentrated water, and a control means for controlling the circulation treatment of the anaerobically treated liquid based on the measurement results of the measuring means.

In yet another embodiment of the wastewater treatment device according to the present invention, the device includes a cleaning liquid supplying means for supplying the membrane cleaning liquid of the forward osmosis membrane means to the concentrated water storage means, and a control means for controlling the circulation treatment of the anaerobically treated liquid based on the supply of the membrane cleaning liquid to the concentrated water storage means.

In yet another embodiment of the wastewater treatment device according to the present invention, the anaerobic treatment means comprises a first anaerobic treatment tank for anaerobically treating the separated sludge to obtain digested sludge, and a second anaerobic treatment tank for anaerobically treating the concentrated water to obtain digested water, and the circulation means comprises a first circulation means for circulating the digested sludge to the concentrated water storage means, and a second circulation means for circulating the digested water to the concentrated water storage means.

In the present invention, the treated product obtained by the anaerobic treatment according to this embodiment is called anaerobically treated liquid. Anaerobically treated liquid includes, for example, so-called digested sludge obtained by anaerobically treating concentrated sludge after solid-liquid separation, and so-called digested treated water obtained by anaerobically treating wastewater (such as the concentrated water in the present invention) that is mainly composed of soluble components.

In another aspect, an embodiment of the present invention is a wastewater treatment method that includes a step of supplying a bactericide to wastewater, a step of performing forward osmosis membrane processing in which influent containing the bactericide is brought into contact with a driving liquid having a higher osmotic pressure than the influent to obtain concentrated water and treated water, a step of anaerobically treating the concentrated water to obtain fuel gas, and a step of circulating the anaerobically treated liquid obtained by the anaerobic treatment in a storage tank that stores the concentrated water.

The present invention provides a wastewater treatment device and a wastewater treatment method that can stably perform anaerobic treatment and fuel production while suppressing membrane fouling.

1 is a schematic diagram illustrating an example of a wastewater treatment device according to a first embodiment of the present invention. FIG. 5 is a schematic diagram illustrating an example of a wastewater treatment device according to a second embodiment of the present invention. FIG. 11 is a schematic diagram illustrating an example of a wastewater treatment device according to a third embodiment of the present invention. FIG. 13 is a schematic diagram illustrating an example of a wastewater treatment device according to a fourth embodiment of the present invention. FIG. 13 is a schematic diagram illustrating an example of a wastewater treatment device according to a fifth embodiment of the present invention.

Below, we will explain wastewater treatment equipment and wastewater treatment methods according to first to fifth embodiments of the present invention with reference to the drawings. In the following description of the drawings, the same or similar parts are given the same or similar reference numerals. Note that the embodiments shown below are examples of equipment and methods for embodying the technical idea of this invention, and the technical idea of this invention does not specify the structure, arrangement, etc. of each component as described below.

(First embodiment)
As shown in FIG. 1 , the wastewater treatment apparatus according to the first embodiment of the present invention comprises forward osmosis membrane means (hereinafter also referred to as "FO membrane means") 3, disinfectant supply means 4 for supplying disinfectant to the inflow water flowing into the forward osmosis membrane means 3, concentrated water storage means 5 for storing concentrated water, anaerobic treatment means 6 for anaerobically treating the concentrated water to obtain fuel gas, and circulation means AL1 for circulating the anaerobically treated liquid obtained by the anaerobic treatment means 6 to the concentrated water storage means 5.

The type of wastewater is not particularly limited, but organic wastewater containing at least contaminants such as soluble organic matter or turbidity is preferably used. Specifically, sewage, primary sewage treatment water, secondary sewage treatment water, human waste, livestock wastewater, various manufacturing wastewater, etc. can be preferably used as wastewater in this embodiment.

The quality of the wastewater is not limited to the following, but for example, the biochemical oxygen demand (BOD) is 10-1000 mg/L, the chemical oxygen demand (CODcr) is 20-3000 mg/L, and the suspended solids (SS) is approximately 20-3000 mg/L.

The forward osmosis membrane means 3 is not particularly limited as long as it is an apparatus that has a semipermeable membrane (FO membrane) and obtains concentrated water and treated water by contacting the inflow water flowing into the forward osmosis membrane means 3 through the semipermeable membrane with a driving liquid having a higher osmotic pressure than the inflow water. In FIG. 1, the inflow water, which is wastewater (raw water), is supplied to the primary side of the FO membrane. The secondary side of the FO membrane is supplied with a driving liquid (draw solution) having a higher osmotic pressure than the inflow water. By using the forward osmosis membrane means 3, there is no need to use a large pump for pressurizing the device compared to a reverse osmosis membrane (RO) device, etc., so power can be reduced and the device can be made smaller.

When the inflow water flowing into the forward osmosis membrane means 3 comes into contact with the driving liquid via the FO membrane, concentrated water and treated water are obtained. The treated water can be discharged to the outside of the forward osmosis membrane means 3. The concentrated water is sent to the concentrated water storage means 5 via the concentrated water supply line CL. As the driving liquid to be supplied into the forward osmosis membrane means 3, seawater, concentrated water (brine) from a seawater desalination treatment facility, high salt concentration wastewater discharged from a leachate treatment facility, etc. are preferable.

The presence of trace amounts of heavy metals such as iron, cobalt, nickel, zinc, tungsten, manganese, molybdenum, selenium, and boron is important for maintaining the activity of the microorganisms that control the anaerobic treatment performed in the anaerobic treatment means 6 in FIG. 1. Normally, wastewater contains almost no of these metals. According to this embodiment, seawater containing trace amounts of heavy metals, concentrated water (brine) from a seawater desalination treatment facility, high-salt wastewater discharged from a leachate treatment facility, etc. are used as the driving liquid, and the heavy metals on the driving liquid side are caused to flow into the primary side by the diffusion phenomenon of substances due to the concentration difference, and then into the concentrated water side. This allows the heavy metals in the concentrated water to be used as a nutrient source for anaerobic treatment, making the anaerobic digestion reaction more stable. It is also possible to add a portion of the driving liquid to the primary side.

In order to more efficiently convert the concentrated water into energy (methane gasification) in the anaerobic treatment means 6, it is preferable to set the type of FO membrane and operating conditions so that the solute leakage rate of the FO membrane, defined by the following equations (1) and (2), is 0.0001 to 0.1.
Solute leakage rate = (concentrated water salinity ÷ concentration rate - raw wastewater salinity) ÷ driving liquid salinity (1)
Concentration rate = Raw wastewater flow rate ÷ Concentrated water flow rate ... (2)

The type of FO membrane is not particularly limited, and various membranes can be used. Among them, it is preferable to use a type of membrane in which the salt of the driving liquid flows partially from the secondary side to the primary side. For example, various materials such as cellulose acetate, polyamide, polysulfone, polyvinylidene fluoride, polyethylene, and vinyl chloride are used. The shape of the FO membrane is also not particularly limited, and any shape of membrane can be used, such as a flat membrane, a spiral membrane, or a hollow paper membrane.

The bactericide supplied from the bactericide supply means 4 is preferably a bactericide capable of reacting with the anaerobic treatment liquid described below, and for example, a slime control agent is preferably used. As the slime control agent, a chlorine slime control agent such as sodium hypochlorite, an oxidizing slime control agent such as hydrogen peroxide, or an organic slime control agent such as 5-chloro-methyl-isothiazolin-3-one (MIT) or a halocyanoacetamide compound can be used.

In particular, chlorine-based slime control agents such as sodium hypochlorite can deteriorate FO membrane materials due to the strong oxidizing power of beneficial residual chlorine, but in wastewater containing ammonia, they react with the ammonia to produce chloramines. These chloramines have a milder oxidizing power than beneficial residual chlorine, and can effectively suppress fouling while inhibiting oxidative deterioration of membrane materials.

If the amount of bactericide added is too large, it may adversely affect the anaerobic treatment of the concentrated water in the downstream anaerobic treatment means 6, and if the amount is too small, the membrane fouling suppression effect may not be obtained advantageously. The amount of bactericide added to the separated liquid supplied to the forward osmosis membrane means 3 is preferably 0.1 to 100 mg/L, for example, and more preferably about 0.5 to 50 mg/L.

It is preferable to control the amount of bactericide added according to the component fluctuation of the inflow water supplied to the forward osmosis membrane means 3. For example, a differential pressure gauge (not shown) that measures the transmembrane pressure difference of the forward osmosis membrane is placed in the forward osmosis membrane means 3, and when the transmembrane pressure difference value or the membrane permeation flow rate value calculated from the detection result of the differential pressure gauge becomes equal to or less than a predetermined value (predefined value), the amount of bactericide added can be controlled continuously or intermittently by using the control means 8 that can send a control signal to the bactericide supply means 4 to increase the amount of bactericide added. This makes it possible to suppress membrane fouling for a longer period of time even if the water quality of the wastewater changes. In addition, the control means 8 can measure the inlet pressure and concentrated water pressure of the forward osmosis membrane module provided in the forward osmosis membrane means 3, and send a control signal to the bactericide supply means 4 to increase the amount of bactericide added according to the differential pressure (pressure loss).

In terms of efficient energy recovery from the concentrated water, it is most preferable that the bactericide supplied from the bactericide supply means 4 maintains the bactericidal effect of the membrane until it reaches the outlet where the concentrated water is discharged from the forward osmosis membrane means 3, and that the bactericidal effect is not maintained after it is discharged from the forward osmosis membrane means 3. On the other hand, from the viewpoint of more appropriately suppressing fouling of the forward osmosis membrane, it is preferable to add a sufficient amount of bactericide to the inflow water supplied to the forward osmosis membrane means 3. Therefore, components of the bactericide may remain in the concentrated water obtained from the forward osmosis membrane means 3.

In this embodiment, the concentrated water obtained by the forward osmosis membrane means 3 can be stored in the concentrated water storage means 5 for a certain period of time to the extent that the effect of the bactericide is inactivated. For example, the concentrated water stored in the concentrated water storage means 5 can be left to stand in the air, or an aeration means (not shown) can be provided in the concentrated water storage means 5 and the concentrated water can be aerated and stirred to decompose the bactericide, causing it to lose its effectiveness.

The concentrated water storage means 5 includes, for example, a storage tank for storing concentrated water for a certain period of time. The concentrated water storage means 5 may be connected to a wastewater supply means 10 capable of supplying organic matter contained in the wastewater (raw water) to at least one of the concentrated water storage means 5 or the anaerobic treatment means 6. The wastewater supply means 10 includes a supply line OL1 for supplying at least a portion of the wastewater to the concentrated water storage means 5, and a supply line SL1 for supplying at least a portion of the wastewater to the anaerobic treatment means 6.

By supplying at least a portion of the wastewater to the concentrated water storage means 5 via the supply line OL1, it is possible to obtain the effect of decomposing the disinfectant in the concentrated water by utilizing the organic matter contained in the wastewater. By supplying at least a portion of the wastewater to the anaerobic treatment means 6 via the supply line SL1, it is possible to increase the amount of organic matter flowing into the anaerobic treatment means 6, and therefore the amount of fuel gas generated by anaerobic treatment can be increased.

The supply of wastewater to the concentrated water storage means 5 via the supply line OL1 may be omitted. By supplying the wastewater to the anaerobic treatment means 6 via the supply line SL1 instead of to the concentrated water storage means 5, the amount of organic matter flowing into the anaerobic treatment means 6 can be increased, improving the energy recovery efficiency.

When at least a portion of the wastewater is supplied to the concentrated water storage means 5 via the supply line OL1, it is preferable to adjust the supply amount so that the amount of organic matter in the wastewater flowing into the concentrated water storage means 5 is about 0.1 to 10 times (weight ratio) the effective bactericidal amount contained in the concentrated water. Here, the effective bactericidal amount is defined as the amount of organic matter (weight) in the inflow liquid that acts on the amount of bactericide (weight). For example, if the residual chlorine is 0.5 mg/L, an organic matter amount of 0.05 to 5 mg/L is required as an effective bactericidal amount. In this way, by mixing the organic matter contained in the wastewater with the concentrated water, the storage time can be shortened, and the volume of the storage tank of the concentrated water storage means 5 can be reduced.

On the other hand, if organic matter contained in the wastewater is supplied to the concentrated water storage means 5 via the supply line OL1, the amount of organic matter that can be recovered as fuel gas in the anaerobic treatment means 6 may decrease. In addition, a certain residence time is required to sufficiently reduce the fungicide in the concentrated water using the organic matter in the wastewater in the concentrated water storage means 5, and the capacity of the storage tank for promoting the reduction reaction by mixing the wastewater and concentrated water may also become excessive. Furthermore, when the inflow of concentrated water and wastewater into the concentrated water storage means 5 increases due to fluctuations in the amount of raw water, it is no longer possible to ensure sufficient reaction time, and as a result, the fungicide flows into the anaerobic treatment means 6 before it loses its effect, which causes a problem of impairing the stability of anaerobic treatment.

The wastewater treatment device according to the first embodiment includes a circulation means AL1 that circulates the anaerobically treated liquid from the anaerobic treatment means 6 to the concentrated water storage means 5. The refractory organic matter contained in the anaerobically treated liquid acts as a reducing agent for the bactericide in the concentrated water. In addition, iron sulfide, magnesium ammonium phosphate (MAP), aluminum phosphate ( _AlPO4 ), iron (II) phosphate ( _Fe3 ( _PO4 ) ₂ ), and calcium hydrogen phosphate hydrate ( _CaHPO4.2H2O ) contained in the anaerobically treated liquid act as catalysts _that promote the reduction reaction of the bactericide remaining in the concentrated water. Therefore, by circulating the anaerobically treated liquid to the concentrated water storage means 5 via the circulation means AL1, the bactericide in the concentrated water can be decomposed more quickly and the device can be made smaller than when the residence time is adjusted or wastewater is used.

The wastewater supplied through the supply line OL1 typically has an oxidation-reduction potential (ORP: silver-silver chloride electrode standard) of about 50 mV to -250 mV. However, wastewater is easily affected by water quality fluctuations due to climate change, etc., and ORP fluctuations can be large. On the other hand, the anaerobically treated liquid obtained in the anaerobic treatment means 6 has an oxidation-reduction potential of about -500 mV to -200 mV, and has a lower ORP and higher reducing power than the wastewater supplied through the supply line OL1. Furthermore, the anaerobically treated liquid obtained in the anaerobic treatment means 6 has a long HRT, generally 20 to 30 days, depending on the retention time in the anaerobic treatment tank, and is therefore less susceptible to ORP fluctuations than wastewater, which is prone to water quality fluctuations.

According to the first embodiment, by circulating the anaerobically treated liquid to the concentrated water storage means 5 via the circulation means AL1, the disinfectant in the concentrated water can be reduced more quickly and reliably with a smaller supply volume than when wastewater is supplied. As a result, the amount of circulated anaerobically treated liquid can be reduced, the capacity of the pump required for circulation and the storage tank required for storage can be reduced, and the reaction time for decomposing the disinfectant can be shortened. Since the ORP fluctuation of the anaerobically treated liquid is smaller than that of wastewater, more stable treatment can be performed than when wastewater is supplied to the concentrated water storage means 5.

In the anaerobic treatment circulation process, the more the circulation amount of the treatment liquid is increased, the shorter the decomposition time of the bactericide, and the less the circulation amount is, the more the power of the circulation pump can be reduced. The circulation amount can be determined based on the capacity of the circulation pump and the capacity of the tank equipped in the wastewater treatment device. Specifically, in the circulation process of the supply anaerobic treatment liquid, it is preferable to supply the anaerobic treatment liquid in a volume ratio of about 0.01 to 0.25 times the concentrated water, and about 0.05 to 0.1 times is optimal. By mixing the difficult-to-decompose organic matter and catalyst components contained in the anaerobic treatment liquid with the concentrated water in which the bactericide remains, the storage time in the concentrated water storage means 5 can be within several tens of minutes, more typically within 20 minutes, even within 10 minutes, and even about 5 minutes. This also makes it possible to reduce the volume of the concentrated water storage means 5. In order to obtain or eliminate the bactericidal effect more quickly, it is preferable to adjust the circulation amount so that, for example, the COD in the anaerobically treated liquid is reliably supplied to the concentrated water storage means 5 at about 20 to 100 mg/L, more preferably about 30 to 60 mg/L per 1 mg/L of bactericide. Furthermore, it is preferable to adjust the circulation amount so that the ratio of the supply amount of concentrated water supplied to the concentrated water storage means 5 via the concentrated water supply line CL to the supply amount of the anaerobically treated liquid circulated to the concentrated water storage means 5 via the circulation means AL1 is 250:1 to 150:1.

The anaerobically treated liquid does not necessarily have to contain the above-mentioned difficult-to-decompose organic matter and catalyst. This is because anaerobically treated liquid generally has a lower redox potential than wastewater and a higher reduction effect than wastewater, and can therefore decompose the disinfectant in the concentrated water more efficiently than using wastewater with a high redox potential. The anaerobically treated liquid does not necessarily have to be the anaerobic treated liquid generated in the wastewater treatment device of FIG. 1, and the anaerobic treated liquid brought in from outside the wastewater treatment device of FIG. 1 may be supplied to the concentrated water storage means 5 via the circulation means AL1.

Many disinfectants that react with anaerobically treated liquid have oxidizing power and function as an oxidizing agent. In this embodiment, the anaerobically treated liquid has reducing power, so by mixing the anaerobically treated liquid with a disinfectant that functions as a reducing agent, a greater effect can be obtained.

The anaerobic treatment means 6 may be a device capable of decomposing at least a portion of the concentrated water supplied from the concentrated water storage means 5 and the wastewater supplied via the supply line SL1 through anaerobic treatment using microorganisms, and converting them into fuel gas such as methane gas or carbon dioxide gas.

In anaerobic treatment, temperature and pH control are extremely important for maintaining a stable level of anaerobic bacteria. For example, the anaerobic treatment in the anaerobic treatment means 6 can be a medium temperature treatment in which the optimum pH in the anaerobic treatment tank is 6.5 to 7.5 and the optimum temperature is 30 to 35°C, or a high temperature treatment in which the optimum pH in the anaerobic treatment tank is 6.5 to 7.5 and the optimum temperature is 50 to 55°C.

Following the anaerobic treatment means 6 is a solid-liquid separation means 7 that separates the anaerobically treated liquid from the anaerobic treatment means 6 into solid and liquid. The treated water obtained by the solid-liquid separation means 7 is discharged. The separated sludge separated by the solid-liquid separation means 7 can be dried or carbonized and converted into energy such as fuel sludge.

(Wastewater treatment method)
Wastewater treatment according to the first embodiment can be performed using the wastewater treatment device shown in Fig. 1. That is, the wastewater treatment method according to the first embodiment includes at least a step of supplying a bactericide to wastewater, a step of performing forward osmosis membrane treatment in which influent (wastewater) containing the bactericide is brought into contact with a driving liquid having a higher osmotic pressure than the influent to obtain concentrated water and treated water, a step of anaerobically treating the concentrated water to obtain a fuel gas, and a step of circulating the anaerobically treated liquid obtained by the anaerobically treating process in a storage tank that stores the concentrated water.

According to the wastewater treatment device and wastewater treatment method of the first embodiment, in order to sterilize the semipermeable membrane of the inflow water flowing into the forward osmosis membrane means 3, a bactericide is supplied from the bactericide supply means 4, and the anaerobically treated liquid of the anaerobic treatment means 6 is circulated to the concentrated water storage means 5 via the circulation means AL1. This allows the bactericide remaining in the concentrated water to be reduced in a short residence time and in a small storage tank. In this way, the bactericide can be more reliably removed from the concentrated water flowing into the concentrated water storage means 5, so that the anaerobic bacteria used in the anaerobic treatment means 6 can be stably maintained in the anaerobic treatment tank, and fuel gas can be stably obtained.

Second Embodiment
2, the wastewater treatment device according to the second embodiment includes a cleaning liquid supplying means BL that is connected between the forward osmosis membrane means 3 and the concentrated water storage means 5 and supplies a membrane cleaning liquid from the forward osmosis membrane means 3 to the concentrated water storage means 5, and a control means 8 that controls the circulation treatment of the anaerobically treated liquid based on the supply of the membrane cleaning liquid to the concentrated water storage means 5. As the rest is substantially the same as the wastewater treatment device according to the first embodiment, a description thereof will be omitted.

The concentrated water storage means 5 is provided with a measuring means 51 for measuring the oxidation-reduction potential of the concentrated water in the storage tank of the concentrated water storage means 5. There are no particular limitations on the measuring means 51, and a commercially available ORP meter or the like can be used. The control means 8 receives the output of the ORP value measurement result from the measuring means 51 and controls the circulation treatment of the anaerobically treated liquid.

The backwash method, in which cleaning liquid is introduced from the driving liquid side by osmosis counterflow, is known as a method for cleaning FO membranes. Although not limited to the following, for example, backwashing with cleaning liquid for about 1 to 30 minutes can remove foulants (dirt components) such as organic matter that have accumulated inside the FO membrane and restore the permeation flow rate.

Backwash wastewater (also called membrane cleaning liquid) contains high concentrations of organic matter and bactericides, so it is necessary to perform pretreatment before discharging the backwash wastewater. The wastewater treatment device according to the second embodiment is equipped with a cleaning liquid supply means BL that supplies the backwash wastewater to the concentrated water storage means 5. This eliminates the need to perform pretreatment before discharging the backwash wastewater, improving the overall treatment efficiency of the wastewater treatment.

In addition, because the backwash wastewater contains a large amount of organic matter, the amount of organic matter for energy recovery can be increased by adding the backwash wastewater to the concentrated water and subjecting this concentrated water to anaerobical treatment using the anaerobic treatment means 6.

When the backwash wastewater is fed into the concentrated water storage means 5, highly concentrated wastewater flows into the concentrated water storage means 5 in a short period of time. In the wastewater treatment device and wastewater treatment method according to the second embodiment, the circulation treatment of the anaerobically treated liquid is controlled in accordance with the timing of the backwash wastewater flowing into the concentrated water storage means 5. This makes it possible to quickly remove the disinfectant contained in the backwash wastewater and also to recover methane from the organic matter contained in the backwash wastewater.

The ORP of the backwash wastewater is, for example, about -100 mV to 0 mV, but is not limited to the following. The measurement means 51 measures the ORP value of the solution in the concentrated water storage means 5 and detects changes in the ORP value, thereby enabling the timing of the supply of the backwash wastewater to be confirmed. The control means 8 controls the amount of anaerobically treated liquid circulated by the circulation means AL1 in accordance with the timing of the supply of this backwash wastewater. For example, when the supply of backwash wastewater causes the ORP of the concentrated water in the concentrated water storage means 5 to exceed a predetermined reference value, the control means 8 can control the pumps, valves, etc. provided in the circulation means AL1 to start circulating the anaerobically treated liquid or increase the amount circulated.

The wastewater treatment method according to the second embodiment can be carried out using the wastewater treatment device shown in FIG. 2. That is, the wastewater treatment method according to the second embodiment includes a step of supplying a bactericide to the wastewater, a step of performing forward osmosis membrane treatment in which the influent (wastewater) containing the bactericide is brought into contact with a driving liquid having a higher osmotic pressure than the influent to obtain concentrated water and treated water, a step of anaerobically treating the concentrated water to obtain a fuel gas, a step of circulating the anaerobically treated liquid obtained by the anaerobic treatment in a storage tank that stores the concentrated water, a step of supplying the membrane cleaning liquid of the forward osmosis membrane means 3 to the concentrated water storage means 5 via the cleaning liquid supply means BL, and a step of controlling the circulation treatment of the anaerobically treated liquid based on the supply of the membrane cleaning liquid to the concentrated water storage means 5.

According to the wastewater treatment device and wastewater treatment method using the same according to the second embodiment, the backwash wastewater from the forward osmosis membrane means 3 can be treated in the concentrated water storage means 5. In addition, because the backwash wastewater is added to the concentrated water, the amount of organic matter in the inflow water flowing into the anaerobic treatment means 6 increases, improving the energy recovery efficiency.

Third Embodiment
The wastewater treatment device and the wastewater treatment method according to the third embodiment of the present invention further comprises a first solid-liquid separation means 1, which is disposed in a stage preceding the disinfectant supply means 4 and separates precipitated organic matter in the wastewater into solid-liquid separation to obtain separated sludge and separated liquid, and a first sludge supply means 11. The rest of the wastewater treatment device and the wastewater treatment method are the same as those according to the second embodiment.

As the first solid-liquid separation means 1, for example, a primary sedimentation tank is preferably used. The specific means of solid-liquid separation is not particularly limited, and any means such as gravity settling separation, centrifugation, flotation separation, coagulation separation, membrane separation, etc. can be used. The separated liquid obtained in the first solid-liquid separation means 1 is sent to the forward osmosis membrane means 3 as inflow water to the forward osmosis membrane means 3. The first solid-liquid separation means 1 can have a hydraulic retention time (HRT) of 0.5 to 2.0 hours, more preferably 1.0 to 2.0 hours. This allows the organic matter contained in the wastewater to be efficiently transferred to the separated sludge side used for energy recovery.

The first solid-liquid separation means 1 is connected to a first sludge supply means 11 capable of supplying the separated sludge obtained by the first solid-liquid separation means 1 to the concentrated water storage means 5 or the anaerobic treatment means 6. The first sludge supply means 11 includes a supply line OL2 for supplying the separated sludge to the concentrated water storage means 5, and a supply line SL2 for supplying the separated sludge to the anaerobic treatment means 6.

By supplying the separated sludge to the concentrated water storage means 5 via the supply line OL2, the decomposition effect of the disinfectant in the concentrated water using the organic matter contained in the separated sludge can be accelerated. By supplying the separated sludge to the anaerobic treatment means 6 via the supply line SL2, the amount of organic matter flowing into the anaerobic treatment means 6 can be increased, and therefore the amount of fuel gas generated by anaerobic treatment can be increased. To increase the efficiency of recovery of the fuel gas, the supply line OL2 may be omitted.

When the separated sludge is supplied to the concentrated water storage means 5 via the supply line OL2, it is preferable to adjust the supply amount so that the amount of organic matter in the separated sludge flowing into the concentrated water storage means 5 is approximately 0.1 to 10 times (by weight) the effective sterilization amount contained in the concentrated water.

The wastewater treatment method according to the third embodiment can be carried out using the wastewater treatment device shown in FIG. 3. First, in the first solid-liquid separation means 1, the precipitated organic matter in the wastewater, which is raw water, is separated into solid and liquid to obtain separated sludge and separated liquid. Next, a bactericide is supplied from a bactericide supply means 4 to the separated liquid obtained in the first solid-liquid separation means 1, and the separated liquid is supplied to a forward osmosis membrane means 3 equipped with a semipermeable membrane. In the forward osmosis membrane means 3, concentrated water and treated water are obtained by contacting the separated liquid with a driving liquid having a higher osmotic pressure than the separated liquid through a semipermeable membrane. The separated sludge obtained in the first solid-liquid separation means 1 can be supplied to the anaerobic treatment means 6 via a supply line SL2. The subsequent treatment steps can be substantially similar to those in the wastewater treatment method according to the second embodiment, and therefore will not be described.

According to the wastewater treatment device and wastewater treatment method of the third embodiment, by providing the first solid-liquid separation means 1, organic matter contained in the wastewater can be efficiently transferred to the separated sludge side, and the organic matter contained in the separated sludge can be efficiently converted into fuel by the anaerobic treatment means 6. In addition, by arranging the first solid-liquid separation means 1 in front of the forward osmosis membrane means 3, the amount of organic matter in the inflow water flowing into the forward osmosis membrane means 3 can be reduced, so that the amount of organic matter adhering to the semipermeable membrane can be further reduced, and stable wastewater treatment can be performed for a long period of time while suppressing membrane fouling.

(Fourth embodiment)
4, the wastewater treatment device according to the fourth embodiment of the present invention further comprises a second solid-liquid separation means 2 for performing solid-liquid separation of the wastewater or the separated liquid of the first solid-liquid separation means 1, and a second sludge supply means 13 capable of supplying separated sludge obtained in the second solid-liquid separation means 2 to the concentrated water storage means 5 or the anaerobic treatment means 6. The rest is the same as the wastewater treatment device according to the third embodiment.

The second solid-liquid separation means 2 is not particularly limited as long as it is a device that separates organic matter in the inflow water into solid and liquid. For example, the second solid-liquid separation means 2 can be a coagulation sedimentation device, a coagulation sand filter device, a coagulation membrane filter device, a sand filter or a membrane filter device. Among them, by using a high-speed coagulation sedimentation device that can add a coagulant and perform coagulation sedimentation treatment in a short time as the second solid-liquid separation means 2, the retention time of the inflow water can generally be within one hour, making it possible to treat in a very short time.

The separated liquid from the first solid-liquid separation means 1 contains fine organic solids and soluble organic matter that cannot be completely removed by the first solid-liquid separation means 1. Therefore, by further performing solid-liquid separation in the second solid-liquid separation means 2, the organic matter concentration in the inflow water flowing into the forward osmosis membrane means 3 can be reduced, thereby suppressing the growth of microorganisms that use organic matter as a substrate in the forward osmosis membrane means 3 and suppressing membrane fouling for a longer period of time. In addition, since the amount of disinfectant supplied can be reduced compared to the third embodiment, the disinfectant supply means 4 can be made more compact.

In the example shown in FIG. 4, the second solid-liquid separation means 2 includes a first reaction tank 21 that adds a flocculant to the separated liquid from the first solid-liquid separation means 1, a second reaction tank 22 that adds a flocculation aid to the separated liquid flowing out from the first reaction tank 21, and a flocculation and settling tank 23 that separates the separated liquid flowing out from the second reaction tank 22 into solid and liquid. In the example shown in FIG. 4, an example is shown that includes two reaction tanks 21 and 22, but the number of reaction tanks 21 and 22 is not particularly limited, and may be, for example, a single reaction tank.

As the flocculant, a commonly used organic coagulant can be used. Compared to conventional inorganic flocculants, organic coagulants are mainly composed of organic matter and can be decomposed by anaerobic digestion. Examples of organic coagulants include condensation polyamines, dicyandiamide-formaldehyde condensates, polyethyleneimine, polyvinyl imidaline, polyvinylpyridine, diallylamine salt-sulfur dioxide copolymers, polydimethyldiallylammonium salts, polydimethyldiallylammonium salt-sulfur dioxide copolymers, polydimethyldiallylammonium salt-acrylamide copolymers, polydimethyldiallylammonium salt-diallylamine hydrochloride derivative copolymers, and allylamine salt polymers.

Specific examples of condensation polyamines include condensates of alkylene dichloride and alkylene polyamine, condensates of aniline and formalin, condensates of alkylene diamine and epichlorohydrin, and condensates of ammonia and epichlorohydrin. Examples of alkylene diamines that condense with epichlorohydrin include dimethylamine, diethylamine, methylpropylamine, methylbutylamine, and dibutylamine. Organic coagulants can turn relatively small molecular weight polymers (representative molecular weights are more than 4,000 and 1.5 million or less), colloidal particles in the treated water, and SS into small flocs. The amount of these organic coagulants injected ranges from 1 to 1,000 mg/L, depending on the quality of the raw water.

Instead of the organic coagulant, an inorganic coagulant may be used alone. In order to further strengthen the flocs and improve solid-liquid separation, an organic coagulant and an inorganic coagulant may be used in combination. In general, the inorganic coagulant may be an iron- or aluminum-based inorganic coagulant that is already in use. Specific examples include aluminum sulfate, polyaluminum chloride (PAC), aluminum chloride, polyferric sulfate (polyferric iron), ferric chloride, and mixtures of these. The amount of these inorganic coagulants to be injected ranges from 1 to 1000 mg/L, depending on the quality of the raw water.

As the flocculation aid, a nonionic, anionic, cationic or amphoteric polymer flocculant such as poly(meth)acrylamide, its hydrolysate, poly(meth)acrylic acid, (meth)acrylamide and alkylamino(meth)acrylamide copolymer, or the like can be used. The amount of polymer flocculant added is usually about 0.5 to 5 mg/L relative to the amount of wastewater. As the solid-liquid separation method in the flocculation settling tank 23, any solid-liquid separation method such as precipitation, pressure flotation, or membrane can be used.

A second sludge supply means 13 is connected to the coagulation and sedimentation tank 23, which can supply the separated sludge obtained by solid-liquid separation in the coagulation and sedimentation tank 23 to the concentrated water storage means 5 or the anaerobic treatment means 6. The second sludge supply means 13 includes a supply line OL3 for supplying the separated sludge obtained in the coagulation and sedimentation tank 23 to the concentrated water storage means 5, and a supply line SL3 for supplying the separated sludge to the anaerobic treatment means 6.

By supplying the separated sludge to the concentrated water storage means 5 via the supply line OL3, the decomposition effect of the disinfectant in the concentrated water using the organic matter contained in the separated sludge can be accelerated. By supplying the separated sludge to the anaerobic treatment means 6 via the supply line SL3, the amount of organic matter flowing into the anaerobic treatment means 6 can be increased, and therefore the amount of fuel gas generated by anaerobic treatment can be increased.

Compared to the solids that naturally settle in the first solid-liquid separation means 1 such as the initial settling tank, the microorganisms contained in the SS suspended in the separated liquid obtained by the first solid-liquid separation means 1 have high biological activity. This high activity effectively increases the oxidation-reduction potential of the solution, creating a reducing atmosphere in the solution.

According to the wastewater treatment device of the fourth embodiment, by disposing the second solid-liquid separation means 2, the organic matter in the separated liquid separated by the first solid-liquid separation means 1 can be further coagulated and precipitated, thereby reducing the amount of microorganisms supplied to the forward osmosis membrane means 3. When a membrane filtration device is used for the second solid-liquid separation means 2, a bactericide may be added upstream of the second solid-liquid separation means 2 or at any point in the second solid-liquid separation means 2.

In addition, according to the wastewater treatment device of the fourth embodiment, at least a portion of the separated sludge obtained in the coagulation and settling tank 23 is supplied to the concentrated water storage means 5 via the supply line OL3, thereby accelerating the decomposition of the bactericide remaining in the concentrated water. This allows stable energy recovery without killing the anaerobic bacteria in the anaerobic treatment means 6 with the bactericide.

The wastewater treatment method according to the fourth embodiment can be carried out using the wastewater treatment device shown in FIG. 4. First, in the first solid-liquid separation means 1, precipitated organic matter in the wastewater, which is raw water, is separated into solid and liquid to obtain separated sludge and separated liquid. Next, in the second solid-liquid separation means 2, a flocculant is added to the separated liquid obtained in the first solid-liquid separation means 1 to perform solid-liquid separation. Furthermore, a bactericide is supplied from a bactericide supply means 4 to the separated liquid after solid-liquid separation by adding the flocculant, and the bactericide is supplied to a forward osmosis membrane means 3 equipped with a semipermeable membrane. In the forward osmosis membrane means 3, concentrated water and treated water are obtained by contacting the separated liquid with a driving liquid having a higher osmotic pressure than the separated liquid through a semipermeable membrane. The separated sludge obtained in the second solid-liquid separation means 2 can be supplied to the anaerobic treatment means 6 via a supply line SL3. The subsequent treatment steps can be substantially similar to those of the wastewater treatment method according to the first embodiment, so a description thereof will be omitted.

Fifth embodiment
As shown in Fig. 5, the wastewater treatment device according to the fifth embodiment differs from the wastewater treatment device shown in Fig. 4 in that the anaerobic treatment means 6 includes a first anaerobic treatment tank 61 and a second anaerobic treatment tank 62. Other aspects are substantially similar to those of the wastewater treatment device shown in Fig. 4, and therefore will not be described.

The separated sludge obtained by the first solid-liquid separation means 1 or the second solid-liquid separation means 2 and the concentrated water obtained by the forward osmosis membrane means 3 have different water concentrations and organic matter concentrations, and therefore the optimal residence time in the anaerobic treatment tank is different. Therefore, it is preferable from the viewpoint of efficiency to treat the separated sludge generated by the first solid-liquid separation means 1 and the second solid-liquid separation means 2 individually, such as treating the separated sludge in the first anaerobic treatment tank 61 and treating the concentrated water in the second anaerobic treatment tank 62. Then, the digested sludge obtained in the first anaerobic treatment tank 61 is circulated to the concentrated water storage means 5 via the first circulation means AL11, and the digested treated water obtained in the second anaerobic treatment tank 62 is circulated to the concentrated water storage means 5 via the second circulation means AL12, so that the digested sludge or the digested treated water can be effectively used.

Anaerobic digestion can be carried out in the first anaerobic treatment tank 61. The tank is heated to maintain a temperature of approximately 55°C or approximately 25°C. In the anaerobic digestion tank, acid fermentation bacteria and methane fermentation bacteria work to decompose sludge into methane gas, carbon dioxide, hydrogen sulfide and other gases, water-soluble nitrogen, phosphorus, and other substances. The methane gas generated can be recovered and used as energy. Separated sludge is easily decomposed and easily fermented into methane, so the amount of methane gas generated can be increased. The retention time of the sludge is approximately 10 to 40 days, and can be any time depending on the decomposition performance of the sludge.

A biological treatment device can be used for the second anaerobic treatment tank 62. As the biological treatment device, there are devices that adopt high-load anaerobic treatment methods such as the anaerobic fixed bed method, the anaerobic fluidized bed method, and the upflow sludge bed method (UASB method, EGSB method), but any of these devices may be used. The second anaerobic treatment tank 62 may be a single-tank type in which acid fermentation and methane fermentation are performed in one tank, or a two-tank type in which acid fermentation and methane fermentation are performed in separate reaction tanks. In order to maintain anaerobic bacteria, temperature control and pH control are extremely important. For example, it is preferable to detect the temperature and pH of the second anaerobic treatment tank 62, the raw water for methane fermentation (concentrated water and concentrated sludge), the treated water, etc., and operate the tank while controlling each of them by feeding back or feeding forward the values. The residence time of the inflow water flowing into the second anaerobic treatment tank 62 varies depending on the organic matter concentration, but if the organic matter concentration is high, it is preferable to circulate the treated water, etc. to reduce the concentration before treatment, and the treatment time is generally about 0.1 to 10 hours.

The treated material obtained in the first anaerobic treatment tank 61 is separated into solid and liquid by solid-liquid separation means 71, and the treated water is discharged to the outside. The treated material obtained in the second anaerobic treatment tank 62 is separated into solid and liquid by solid-liquid separation means 7, and the treated water is discharged to the outside.

According to the wastewater treatment device of the fifth embodiment, anaerobic digestion can be performed using separate devices for the anaerobically treated liquid and the concentrated water, so that methane fermentation processing can be carried out more efficiently with an optimal retention time.

The present invention has been described using the above embodiments, but the descriptions and drawings that form part of this disclosure should not be understood as limiting this invention. In other words, this disclosure is not limited to the above-mentioned embodiments, and it goes without saying that the components can be combined and modified to be embodied within the scope of the gist of the disclosure.

The following examples of the present invention are provided to provide a better understanding of the present invention and its advantages, and are not intended to limit the invention.

The ratio of the concentrated water obtained by the forward osmosis membrane treatment of wastewater shown in Figure 3 to the anaerobic treatment liquid in the anaerobic digester was changed, and the chloramine (available chlorine) concentration was measured after 5 minutes, 10 minutes, and 30 minutes. The results are shown in Table 1.

When no anaerobically treated liquid was added, there was almost no decrease in the chloramine concentration, but when 10% anaerobically treated liquid was added, the chloramine concentration fell to 1 mg/L or less in 5 minutes. As the ratio of anaerobically treated liquid was increased, the chloramine concentration fell more rapidly. This shows that by mixing anaerobically treated liquid with concentrated water, chloramine, a disinfectant, can be decomposed quickly and efficiently.

By adding a chlorine slime control agent as a disinfectant, chloramines are generated by the reaction between ammonia and free residual chlorine, and the disinfecting effect of this generates chloramines, which suppresses fouling of the forward osmosis membrane. By circulating the anaerobically treated liquid to the concentrated water storage means and mixing the concentrated water with the anaerobically treated liquid, the chloramines remaining in the concentrated water can be quickly decomposed by reduction, thereby suppressing the inflow of chloramines into the anaerobic digester. This makes it possible to provide a wastewater treatment device and a wastewater treatment method that can efficiently recover energy while suppressing membrane fouling.

1...First solid-liquid separation means 2...Second solid-liquid separation means 3...Forward osmosis membrane means 4...Bactericide supply means 5...Concentrated water storage means 6...Anaerobic treatment means 7, 71...Solid-liquid separation means 8...Control means 10...Wastewater supply means 11...First sludge supply means 13...Second sludge supply means 21...First reaction tank 22...Second reaction tank 23...Coagulation and sedimentation tank 51...Measurement means 61...First anaerobic treatment tank 62...Second anaerobic treatment tank AL1...Circulation means AL11...First circulation means AL12...Second circulation means

Claims (14)

a forward osmosis membrane means for producing a concentrated water and a treated water by contacting a wastewater with a driving liquid having a higher osmotic pressure than the wastewater through the semipermeable membrane;
a disinfectant supply means for supplying a disinfectant to the inflow water flowing into the forward osmosis membrane means;
A concentrated water storage means for storing the concentrated water;
an anaerobic treatment means for anaerobically treating the concentrated water to obtain a fuel gas;
and a circulation means for circulating the anaerobically treated liquid from the anaerobically treated liquid to the concentrated water storage means so that the amount of organic matter in the anaerobically treated liquid is 0.1 to 10 times the effective sterilization amount contained in the concentrated water. The wastewater treatment device according to claim 1, further comprising a wastewater supply means capable of supplying at least a portion of the wastewater to at least one of the concentrated water storage means or the anaerobic treatment means. The wastewater treatment device according to claim 1 or 2, wherein the circulation means circulates the anaerobically treated liquid to the concentrated water storage means so that the mixture ratio of the anaerobically treated liquid to the concentrated water is 25:75 to 90:10 by volume. The wastewater treatment device according to claim 1 or 2, wherein the circulation means circulates the anaerobically treated liquid at a volume ratio of 0.01 to 0.25 times the volume of the concentrated water. The wastewater treatment device according to any one of claims 1 to 4, wherein the circulation means includes circulating the anaerobically treated liquid to the concentrated water storage means so that the contact time between the concentrated water and the anaerobically treated water is 5 minutes or more. A first solid-liquid separation means for separating precipitated organic matter in the wastewater into solid and liquid;
The wastewater treatment device according to any one of claims 1 to 5, further comprising: a first sludge supply means capable of supplying separated sludge obtained in the first solid-liquid separation means to at least one of the concentrated water storage means or the anaerobic treatment means. a second solid-liquid separation means for performing solid-liquid separation of the wastewater or a separated liquid obtained by a first solid-liquid separation means for performing solid-liquid separation of a precipitated organic matter in the wastewater;
The wastewater treatment device according to any one of claims 1 to 6, further comprising: a second sludge supply means capable of supplying separated sludge obtained in the second solid-liquid separation means to at least one of the concentrated water storage means or the anaerobic treatment means. The wastewater treatment device according to any one of claims 1 to 7, wherein the anaerobically treated liquid contains any one of iron sulfide, magnesium ammonium phosphate, aluminum phosphate, iron (II) phosphate, and calcium hydrogen phosphate hydrate as a catalyst for promoting the reduction reaction of the bactericide. The wastewater treatment device according to any one of claims 1 to 8, wherein the anaerobically treated liquid has an oxidation-reduction potential (based on a silver-silver chloride electrode) of -500 mV to -200 mV. A measuring means for measuring the oxidation-reduction potential of the concentrated water;
The wastewater treatment device according to any one of claims 1 to 9, further comprising: a control means for controlling the circulating treatment of the anaerobically treated liquid based on the measurement result of the measurement means. a cleaning liquid supplying means for supplying a membrane cleaning liquid of the forward osmosis membrane means to the concentrated water storage means;
and a control means for controlling the circulation treatment of the anaerobically treated liquid based on the supply of the membrane cleaning liquid to the concentrated water storage means. The anaerobic treatment means comprises:
a first anaerobic treatment tank for subjecting the separated sludge to the anaerobically treating process to obtain digested sludge;
a second anaerobic treatment tank for subjecting the concentrated water to the anaerobically treating process to obtain digested water;
Equipped with
The circulation means is
a first circulation means for circulating the digested sludge to the concentrated water storage means;
A second circulation means for circulating the septic treated water to the concentrated water storage means;
The wastewater treatment device according to any one of claims 6 to 11, comprising: providing a disinfectant to the wastewater;
a step of performing a forward osmosis membrane process to obtain a concentrate and a treated water by contacting the influent containing the bactericide with a driving liquid having a higher osmotic pressure than the influent;
a step of anaerobically treating the concentrated water to obtain a fuel gas;
and circulating the anaerobically treated liquid obtained by the anaerobically treated process in a storage tank for storing the concentrated water so that the amount of organic matter in the anaerobically treated liquid is 0.1 to 10 times the effective sterilizing amount contained in the concentrated water. The wastewater treatment method according to claim 13, further comprising circulating the anaerobically treated liquid into the storage tank so that the mixing ratio of the anaerobically treated liquid to the concentrated water is 25:75 to 90:10 by volume.