JP7474718B2 - Wastewater treatment method and wastewater treatment device - Google Patents

Description

The present invention relates to a wastewater treatment method and a wastewater treatment device, and in particular to a wastewater treatment method and a wastewater treatment device that utilizes the membrane separation activated sludge process.

Conventionally, the mainstream method for treating sewage and septic tank sludge is to directly biologically treat the sludge from which impurities have been removed using methods such as the activated sludge process to remove organic matter and nitrogen, and the membrane separation activated sludge process has been adopted as one of the treatment methods. The membrane separation activated sludge process has the advantage of increasing the MLSS (mixed liquor suspended solids) concentration in the biological treatment tank and reducing the site area, because solid-liquid separation is performed using membrane separation.

In recent years, a method that reduces the burden of biological treatment by dehydrating sewage and septic tank sludge in advance, reusing the dehydrated cake as solid fuel, etc., and limiting the biological treatment to the separated liquid from the dehydration, is becoming mainstream. Furthermore, even in facilities that anaerobically digest biomass such as sludge and food waste, which have been increasing in number in recent years, and recover methane gas, it is necessary to biologically treat the anaerobic digestion sludge after dehydration. Even in such cases, the membrane separation activated sludge method is often used for biological treatment, reducing the site area.

For example, Japanese Patent No. 5868217 (Patent Document 1) describes a membrane separation activated sludge treatment method in which the water to be treated is biologically treated with activated sludge in a biological reaction tank, the mixed liquid in the biological reaction tank is solid-liquid separated in a membrane separation device, and the membrane-filtered water that has permeated the separation membrane is taken out of the tank. The method determines the organic matter concentration based on any one of COD (chemical oxygen demand), BOD (biological oxygen demand), TOC (total oxygen demand), total sugar concentration, protein concentration, uronic acid concentration, and E260 (ultraviolet light absorbance at a wavelength of 260 nm), and uses the concentration difference between the organic matter concentration in the liquid phase of the mixed liquid in the tank and the organic matter concentration in the membrane-filtered water, or the concentration ratio between the organic matter concentration in the liquid phase of the mixed liquid in the tank and the organic matter concentration in the membrane-filtered water as an adjustment index, and increases the amount of activated sludge in the biological reaction tank when the adjustment index increases, and decreases the amount of activated sludge in the biological reaction tank when the adjustment index decreases, thereby suppressing fouling of the separation membrane.

Patent Publication No. 4046661 (Patent Document 2) describes a wastewater treatment method in which organic wastewater is treated with activated sludge in a biological treatment tank, the activated sludge mixed liquid is solid-liquid separated by a submerged membrane separation device that constitutes a first separation means submerged in the biological treatment tank, and COD containing biological polymers accumulated in the biological treatment tank by the activated sludge treatment is solid-liquid separated from the activated sludge mixed liquid at appropriate times by a second separation means, and while maintaining the amount of activated sludge in the biological treatment tank at a high concentration, the amount of biological polymers in the activated sludge mixed liquid is maintained at a low concentration. In this case, the COD in the membrane filtrate that permeates the submerged membrane separation device is measured, and the COD in the filtration means filtrate obtained by filtering the activated sludge mixed liquid in the biological treatment tank with a filtration means having a predetermined diameter of pores larger than the pores of the filtration membrane of the submerged membrane separation device is measured, and when the COD difference value obtained by subtracting the COD in the membrane filtrate from the COD in the filtration means filtrate is equal to or greater than a predetermined value, the COD containing biological polymers is separated from the activated sludge mixed liquid by the second separation means.

Patent Publication No. 5822264 (Patent Document 3) describes a method for operating a membrane separation activated sludge treatment device equipped with an aeration tank in which an aeration means is submerged to diffuse aeration into the liquid being treated in the activated sludge, and a membrane separation tank in which a membrane separation device is submerged to obtain a permeate from the liquid being treated in the activated sludge, and the method describes an operation method for a membrane separation activated sludge treatment device that adjusts the amount of aeration per unit time of the aeration means based on the organic matter concentration in the supernatant liquid of the liquid being treated in the activated sludge and the BOD/SS load value, so that when the organic matter concentration is equal to or greater than a predetermined value, the amount of aeration per unit time of the aeration means is increased when the BOD/SS load value is equal to or greater than the predetermined value, and the amount of aeration per unit time of the aeration means is decreased when the BOD/SS load value is less than the predetermined value.

JP 2014-193452 A (Patent Document 4) describes a method for treating organic wastewater in which organic wastewater is introduced into a tank containing activated sludge, biologically treated, and treated water is obtained by performing solid-liquid separation using a membrane separation device installed in the tank or downstream thereof, and when the amount of extracellular ATP in the activated sludge or the rate of increase in the amount of extracellular ATP in the activated sludge reaches a predetermined reference value, the method controls at least one condition selected from the amount of diffused air supplied to the membrane separation device, the cleaning conditions of the membrane separation device, the coagulant injection conditions for injecting a coagulant into the tank containing the water to be treated, the filtration flux of the membrane separation device, and the amount of activated sludge withdrawn from the tank containing the water to be treated.

Patent No. 5868217 Japanese Patent No. 4046661 Japanese Patent No. 5822264 JP 2014-193452 A

When dehydrating organic sludge such as sewage, septic tank sludge, and anaerobic digestion sludge, inorganic coagulants and polymer coagulants (polymers) are added to the sludge to coagulate it before dehydration. The rate of polymer coagulant added is determined by the state of the coagulated flocs, but if there is an excess or deficiency due to changes in the properties of the sludge, this will lead to an increase in the concentration of suspended solids (SS) in the dehydrated separation liquid and an increase in residual polymers in the filtrate, and the concentration of polymer coagulant in the raw biological treatment water will increase.

Polymer flocculants are often resistant to degradation and are difficult to decompose by microorganisms, with molecular weights of 1 million or more. Such polymer flocculants are usually captured by the separation membrane used in the membrane separation activated sludge method, but as the concentration of the polymer flocculant in the tank increases, a layer called a gel layer may form on the separation membrane, causing fouling to progress. Therefore, measures to appropriately suppress fouling are necessary.

The invention described in Reference 1 proposes to apply the difference in concentration between the organic matter concentration in the mixed liquid in the tank and the organic matter concentration in the membrane-filtered water, and to carry out a treatment to increase the amount of activated sludge in the biological reactor when the concentration difference increases. However, with such a treatment method, there is a risk that the activated sludge or its extracellular material that has increased in the biological reactor will further bind to the polymer flocculant in the biological reactor, causing further deposition of a gel layer on the separation membrane, which may result in failure to adequately suppress fouling.

In the fouling suppression method using the second separation means as described in Patent Document 2, it is necessary to consider not only the first separation means but also the problem of membrane clogging of the second separation means, which makes maintenance processing cumbersome. The method of adjusting the aeration air volume based on the organic matter concentration and BOD/SS load value in the supernatant liquid of the treated liquid in activated sludge as described in Patent Document 3, or the fouling control method based on the amount of extracellular ATP as described in Patent Document 4, like other conventional technologies, cannot be said to be sufficient treatment from the perspective of quickly detecting signs of clogging of the separation membrane, suppressing fouling, and performing stable wastewater treatment for a long period of time.

In view of the above problems, the present invention provides a wastewater treatment method and wastewater treatment device that can quickly detect signs of clogging of a separation membrane and perform stable and efficient wastewater treatment while appropriately suppressing fouling.

As a result of intensive research by the inventors to solve the above problems, they discovered that it is effective to use the difference between the organic matter concentration in the liquid phase of the raw water from the membrane separation activated sludge device and the organic matter concentration in the membrane filtered water (membrane permeate water) obtained by membrane filtration as a monitoring index, and to carry out specific treatment based on this monitoring index.

In one aspect, an embodiment of the present invention, which has been completed based on the above findings, is a wastewater treatment method using a membrane bioreactor, which includes measuring the organic matter concentration in the liquid phase of the membrane-filtered raw water in the membrane bioreactor tank and the organic matter concentration in the membrane-filtered water, using the difference as a monitoring index, and adjusting the operating conditions to perform one or more of the following when the monitoring index increases: increasing the amount of cleaning air supplied to the membrane bioreactor tank, shortening the filtration duration, or increasing the amount of sludge extracted from the membrane bioreactor tank.

In one embodiment of the wastewater treatment method according to the present invention, when the monitoring index exceeds a first reference value, a process is performed to increase the amount of sludge extracted from the membrane bioreactor activated sludge tank, and when the monitoring index exceeds a second reference value that is higher than the first reference value, one or more processes are performed, either increasing the amount of cleaning air supplied to the membrane bioreactor activated sludge tank or shortening the filtration duration.

In another aspect, an embodiment of the present invention is a wastewater treatment method using a membrane bioreactor, which includes measuring the organic matter concentration in the liquid phase of membrane-filtered raw water in a membrane bioreactor tank and the organic matter concentration in the membrane-filtered water, using the difference as a monitoring index, adjusting the operating conditions to increase the amount of sludge withdrawn from the membrane bioreactor tank when the sludge retention time in the membrane bioreactor tank is 6 days or more and the monitoring index increases, and adjusting the operating conditions to reduce the amount of sludge withdrawn from the membrane bioreactor tank when the sludge retention time in the membrane bioreactor tank is less than 6 days and the monitoring index increases.

In one embodiment of the wastewater treatment method according to the embodiment of the present invention, when the sludge retention time in the membrane bioreactor tank is 6 days or more and the monitoring index increases, the operation conditions are adjusted so as to combine a process of increasing the amount of sludge extracted from the membrane bioreactor tank with one or more processes of increasing the amount of washing air supplied to the membrane bioreactor tank and shortening the filtration duration,
When the sludge retention time in the membrane bioreactor tank is less than 6 days and the monitoring index increases, the operating conditions are adjusted so as to combine a process for reducing the amount of sludge extracted from the membrane bioreactor tank with one or more of a process for increasing the amount of washing air supplied to the membrane bioreactor tank and a process for shortening the filtration duration.

In one embodiment of the wastewater treatment method according to the present invention, the organic matter concentration includes any of COD, BOD, TOC, or the organic matter concentration fractionated to a molecular weight of 20,000 or more as a result of measurement by organic carbon detection type size exclusion chromatography.

In one embodiment of the wastewater treatment method according to the present invention, when the monitoring index increases, one or more of the following processes are performed on at least a portion of the raw water from the membrane filtration: ozone decomposition process, accelerated oxidation process, or activated carbon adsorption process.

In one embodiment of the wastewater treatment method according to the present invention, the raw water from membrane filtration is a dehydrated separated liquid that is generated after dehydration treatment is performed by adding a polymer flocculant to sludge containing organic matter.

In yet another embodiment of the wastewater treatment method according to the present invention, when the monitoring index increases, the addition rate of polymer flocculant added to the membrane filtered raw water in the dehydration treatment before being supplied to the membrane separation activated sludge tank is reduced.

In one aspect, a wastewater treatment device according to an embodiment of the present invention is a wastewater treatment device that includes a membrane separation activated sludge tank in which membrane-filtered raw water is subjected to membrane separation treatment in the presence of activated sludge to obtain membrane-filtered water, an aeration device that aerates the membrane separation activated sludge tank, a suction pump that draws membrane-filtered water from the membrane separation activated sludge tank, a wastewater pump that extracts sludge from the membrane separation activated sludge tank, and a control device that controls the operating conditions of the aeration device, the suction pump, and the wastewater pump so that the difference between the organic matter concentration of the liquid phase of the membrane-filtered raw water in the membrane separation activated sludge tank and the organic matter concentration of the membrane-filtered water is used as a monitoring index, and when the monitoring index increases, one or more of the following treatments are performed: an increase in the amount of cleaning air supplied to the membrane separation activated sludge tank, a shortened filtration duration, or an increase in the amount of sludge extracted from the membrane separation activated sludge tank.

The present invention provides a wastewater treatment method and wastewater treatment device that can quickly detect signs of clogging of the separation membrane and perform stable and efficient wastewater treatment while appropriately suppressing fouling.

1 is a schematic diagram illustrating an example of a wastewater treatment device according to an embodiment of the present invention. 1 is a graph showing an example of the relationship between sludge retention time (SRT) and a monitoring index (ΔS-COD _Mn ). 1 is a graph showing an example of the relationship between SRT and transmembrane pressure. 1 is a graph showing an example of the relationship between SRT and 5C filter paper filtration amount. 1 is a graph showing an example of the relationship between BOD-SS load and ΔS-COD _Mn . 1 is a graph showing an example of the relationship between BOD-SS load and transmembrane pressure. 1 is a graph showing an example of the relationship between BOD-SS load and 5C filter paper filtration volume. 1 is a graph showing an example of the relationship between ΔS-COD _Mn and transmembrane pressure. 1 is a graph showing an example of the relationship between ΔS-COD _Mn and 5C filter paper filtration amount.

Below, we will explain the wastewater treatment equipment and wastewater treatment method according to the 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 the components as described below.

As shown in FIG. 1, the wastewater treatment device according to the embodiment of the present invention includes a membrane separation activated sludge tank 1 for treating wastewater using a membrane separation activated sludge method to obtain treated sludge and membrane filtered water, an aeration device 2 for aerating the membrane separation activated sludge tank 1, a suction pump 3 for sucking the membrane filtered water from the membrane separation activated sludge tank 1, a wastewater pump 4 for extracting sludge from the membrane separation activated sludge tank 1, and a control device 8 for controlling the operating conditions of the membrane separation activated sludge tank 1.

The subject of treatment is not particularly limited as long as it is organic wastewater containing organic matter. In particular, the dehydrated separated liquid of organic sludge containing one or more of sewage/septic tank sludge, sewage initial settling sludge, excess sewage sludge, food waste, and anaerobic digestion sludge of biomass can be used as the raw water for membrane filtration in the membrane separation activated sludge tank 1. Since the dehydrated separated liquid contains a polymer flocculant, as the dehydrated separated liquid is treated for a long period of time in the membrane separation activated sludge tank 1, the polymer flocculant contained in the dehydrated separated liquid aggregates to form a gel layer, which adheres to the separation membrane 10 in the membrane separation activated sludge tank 1, making it more likely to cause fouling. By performing biological treatment using the wastewater treatment device according to the embodiment of the present invention, it is possible to perform stable treatment for a long period of time while suppressing fouling.

The dehydrated separate liquid may contain suspended solids (SS), soluble COD (COD _Cr , COD _Mn ), BOD, TOC, nitrogen, phosphorus, calcium (Ca), and the like. Although not limited thereto, the dehydrated separate liquid typically contains 100 to 3000 mg/L of SS, more typically 500 to 2000 mg/L. The dehydrated separate liquid contains 500 to 10000 mg/L of COD _Cr , more typically 1000 to 5000 mg/L, and 100 to 5000 mg/L of COD _Mn , more typically 500 to 3000 mg/L. The dehydrated separate liquid contains 500 to 5000 mg/L of BOD, more typically 1000 to 3000 mg/L, and 250 to 4000 mg/L of TOC, more typically 500 to 2000 mg/L. The dehydrated separate liquid contains total nitrogen (TN) of 10 to 500 mg/L, more typically 50 to 300 mg/L, and total phosphorus (TP) of 5 to 200 mg/L, more typically 10 to 100 mg/L. The dehydrated separate liquid contains organic nitrogen (concentration calculated by subtracting inorganic nitrogen (NO _x -N, NH ₄ -N) from TN) of 5 to 300 mg/L, more typically 50 to 200 mg-N/L.

In particular, if the dehydrated separated liquid contains 500 mg/L or more of SS, the polymer flocculant adsorbed to the SS will have a greater effect of flowing into the membrane separation activated sludge tank 1, making fouling more likely to progress. In addition, if the dehydrated separated liquid contains 50 mg/L or more of organic nitrogen, polymeric organic matter will be produced during the protein decomposition process, making it more likely that a gel layer will form.

In addition, when the BOD/COD _Cr in the dewatered separated liquid is 0.7 or less, the proportion of persistent and slow decomposable organic matter increases, and it is easy for the organic matter to form a gel layer by combining with the polymer flocculant in the membrane separation activated sludge tank. Furthermore, when the type of sludge to be subjected to the dewatering process includes sludge that has not been subjected to biological treatment, such as sludge from the primary sedimentation tank of sewage, sewage sludge, and food waste, the sludge properties are likely to fluctuate. Pre-dewatering such sludge results in an excess or shortage of polymer addition rate, and the polymer flocculant is likely to flow into the filtrate side.

When the dehydrated separated liquid is used as the raw water for membrane filtration, the following pretreatment operation is performed on the organic sludge before it is supplied to the membrane separation activated sludge tank 1. As a pretreatment operation, for example, a polymer flocculant and, if necessary, an inorganic flocculant are added to the organic sludge, and the mixture is stirred to perform flocculation treatment. Since the surface of organic sludge is generally negatively charged, it is preferable to use a cationic polymer as the polymer flocculant. The molecular weight of the polymer flocculant is preferably 1 to 15 million, preferably 1 to 10 million, and more preferably 1 to 7.5 million. The addition rate of the polymer flocculant is typically 0.2 to 4 w/w% of TS, and more preferably 0.5 to 3 w/w%. It is preferable to determine the selection and addition rate of the polymer flocculant by a flocculation test or a laboratory-scale dehydration test.

As inorganic coagulants, polyferric sulfate, ferric chloride, aluminum sulfate, polyaluminum chloride (PAC), etc. can be used. Among them, the use of polyferric sulfate is preferable from the viewpoint of chemical costs and corrosion prevention. The addition rate of the inorganic coagulant should also be determined by coagulation tests, laboratory-scale dehydration tests, etc.

Methods for adding inorganic coagulants include a pre-addition method in which inorganic coagulants are added before the sludge is concentrated, a post-addition method in which inorganic coagulants are added after the sludge is concentrated, and a dual-addition method in which inorganic coagulants are added both before and after the sludge is concentrated. Of these, the post-addition method or dual-addition method is preferable from the viewpoint of improving dewaterability and supplying phosphorus required for the downstream biological treatment to the separated liquid side.

The flocculated sludge obtained by the above flocculation process can be thickened before dehydration to improve the processing speed during dehydration and reduce the moisture content of the dehydrated sludge. Mechanical thickening can be used for the thickening process, and any of the following types of thickeners can be used: rotary, gravity, or pressure.

The concentrated sludge obtained in the concentration process is further dehydrated to obtain dehydrated sludge and dehydrated separated liquid. The dehydrator that can be used includes a centrifugal dehydrator, a belt press type dehydrator, a filter press type dehydrator, a screw press type dehydrator, a rotary press type dehydrator, an electro-osmosis type dehydrator, etc.

In particular, screw press dehydrators are preferred because they can achieve a low moisture content with low power. Inside a cylindrical outer tube, a screw press dehydrator has a screw shaft and screw blades that are concentric with the cylindrical outer tube, and is formed with a thickening section on the mixed sludge supply side and a squeezing section on the dehydrated cake discharge side where the space between the cylindrical outer tube and the screw shaft gradually narrows in the direction in which the mixed sludge moves, and the cylindrical outer tube has multiple openings for discharging separated liquid.

Among them, the shaft-sliding screw press dehydrator has a mechanism in which the screw shaft moves parallel to the direction of the dehydrated sludge outlet and forcibly discharges the dehydrated sludge. By using a screw press dehydrator, the moisture content of the dehydrated cake can be significantly reduced. In addition to dehydration devices that combine an independent screen and dehydrator, dehydration devices that include a thickening section that performs the screen function in the front stage and a squeezing section in the rear stage and have an integrated screen and dehydrator are preferable because they do not require a separate screen and the device configuration is simplified.

In the membrane separation activated sludge tank 1, the raw water that flows into the membrane-filtered activated sludge tank 1 is subjected to organic matter removal using microbial reactions using activated sludge, and nitrogen removal by nitrification-denitrification, and biological treatment is carried out using the membrane separation activated sludge method, which separates the activated sludge from the treated water by membrane separation.

The biological treatment in the membrane bioreactor tank 1 may be any method capable of removing organic matter and nitrogen, such as a circulating nitrification-denitrification method or a step-flow nitrification-denitrification method. The sludge concentration (MLSS) of the activated sludge is typically 4,000 to 18,000 mg/L, and more preferably 6,000 to 15,000 mg/L, and is desirably adjusted according to the treatment conditions. For example, a sludge concentration meter 7 for measuring the MLSS in the membrane bioreactor tank 1 may be disposed in the membrane bioreactor tank 1.

When performing nitrogen removal treatment on membrane filtered water, if there is insufficient alkalinity during nitrification, an alkaline agent such as sodium hydroxide solution (NaOH) can be added using a chemical addition means (not shown), and if there is a shortage of organic matter during denitrification, an organic matter such as methanol ( _CH3OH ) can be added.

A separation membrane 10 is contained in the membrane separation activated sludge tank 1. The type of membrane used for the separation membrane 10 may be either a microfiltration (MF) membrane or an ultrafiltration (UF) membrane. In particular, by using a membrane with a pore size of 0.2 μm or more or a molecular weight cutoff of 1 million or more, stable operation over the long term can be achieved while suppressing fouling.

As for the membrane material, organic membranes can be PSF (polysulfone), PE (polyethylene), CA (cellulose acetate), PAN (polyacrylonitrile), PP (polypropylene), PVDF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene), etc., and inorganic membranes can be ceramic membranes. The filtration method can be either full-flow filtration or cross-flow filtration, but from the viewpoint of suppressing fouling, it is preferable to adopt cross-flow filtration. There are no particular restrictions on the type of membrane module, and hollow fiber type, flat membrane type, spiral type, tubular type, monolith type, etc. can be used.

The aeration device 2 may include an aeration device 21 housed in the membrane bioreactor activated sludge tank 1 and a blower 22 for supplying a gas such as air to the aeration device 21. The aeration device 2 is disposed so that air bubbles hit the membrane surface of the separation membrane 10. Typically, the aeration device 2 is disposed below or at the bottom of the separation membrane 10, and is configured to send gas into the membrane bioreactor activated sludge tank 1 to peel off deposits (gel layer) accumulated on the membrane surface and perform membrane cleaning. The amount of aeration air during membrane cleaning is not limited to the following, but is typically 6 to 30 L/m ² /min, more preferably 9 to 25 L/m ² /min.

The aeration device 2 may be of a continuous aeration type, or an intermittent aeration type in which a predetermined rest period is provided between one aeration period and the next. When intermittent aeration is performed, the operating conditions are set so that membrane cleaning by aeration using the aeration device 2 is performed during the rest period when the suction pump 3, which sucks the membrane filtered water from the membrane separation activated sludge tank 1, is turned off (suction stop), thereby making it possible to more effectively obtain the membrane cleaning effect of the aeration device 2.

From the viewpoint of suppressing fouling of the separation membrane 10, it is desirable to set up an intermittent timer to perform intermittent suction of the membrane filtrate water by the suction pump 3. During membrane filtration, organic matter is adsorbed onto the membrane surface of the separation membrane 10, which makes fouling more likely to progress. Therefore, particularly when the progression of fouling is expected, it is desirable to increase the flux and reduce the ratio of suction time as much as possible. Specifically, if one cycle is an operation in which one ON time and one OFF time are performed, it is desirable to set the ratio of ON and OFF times of the intermittent timer per cycle to 59:1 to 4:1, and more preferably 29:1 to 9:1.

The sludge discharge pump 4 is connected to an extraction pipe 41 that is connected to the bottom of the membrane separation activated sludge tank 1, and extracts treated sludge generated in the membrane separation activated sludge tank 1 as excess sludge. The amount of sludge extracted by the sludge discharge pump 4 can be adjusted appropriately according to the operating conditions input from the control device 8.

As shown in FIG. 1, an organic matter concentration measuring device 5 for measuring the organic matter concentration of the membrane-filtered raw water contained in the membrane separation activated sludge tank 1, and an organic matter concentration measuring device 6 for measuring the organic matter concentration of the membrane-filtered water sucked from inside the membrane separation activated sludge tank 1 to the outside of the membrane separation activated sludge tank 1 via the suction pump 3 may be provided.

As the organic matter concentration measuring devices 5 and 6, organic matter measuring devices such as a COD meter, a BOD meter, a TOC meter, and a size exclusion chromatography with organic carbon detection (LC-OCD) can be used. Of course, it is also possible to manually or automatically sample the membrane-filtered raw water and the membrane-filtered water without using the organic matter concentration measuring devices 5 and 6, and measure the organic matter concentration of the membrane-filtered raw water and the membrane-filtered water in a separate facility other than the wastewater treatment device in Figure 1.

The measurement results of the organic matter concentration of the membrane-filtered raw water and the membrane-filtered water are input to the control device 8. The membrane separation activated sludge tank 1 may be further provided with measuring devices including a thermometer for measuring the temperature of the membrane-filtered raw water and a pressure gauge for measuring the transmembrane pressure difference of the separation membrane 10, and the measurement results from each measuring device may be configured to be input to the control device 8.

The mechanism of membrane clogging of the separation membrane 10 is presumed to be that membrane clogging-causing substances, such as polymeric flocculants contained in the raw water from the membrane filtration, flow into the biological treatment and bind to the activated sludge or its extracellular substances to form a polymeric and difficult-to-decompose complex, which then forms a gel layer on the membrane surface. Furthermore, once this gel layer is formed, the presence of the gel layer makes it easier for the membrane clogging-causing substances to be captured on the membrane surface, causing fouling to progress rapidly. Therefore, in order to prevent the occurrence of fouling, it is important to monitor the behavior of the organic matter that causes membrane clogging.

In the wastewater treatment device and wastewater treatment method according to the embodiment of the present invention, the difference between the organic matter concentration in the liquid phase of the membrane filtrate raw water and the organic matter concentration in the membrane filtrate water is used as an indicator of the membrane clogging causative substance. This is because this difference greatly affects the organic matter concentration captured by the separation membrane 10, i.e., the organic matter concentration that forms the gel layer that accumulates on the membrane surface of the separation membrane 10. As an indicator of fouling, the filtration amount of a filter paper with a predetermined pore size (e.g., 5C) over a predetermined time (e.g., 5 minutes) can also be measured, but this method may be affected by measurement conditions such as water temperature and how the filter paper is folded, and operating conditions such as MLSS during measurement. As described above, measuring the organic matter concentration difference can reduce errors and is superior in terms of quantitativeness.

The raw water from the membrane-filtered activated sludge tank 1 is usually high-concentration activated sludge. Therefore, when measuring the organic matter concentration in the liquid phase of the raw water from the membrane-filtered activated sludge tank 1 as a monitoring index, the raw water from the membrane-filtered activated sludge tank 1 is filtered to remove the activated sludge from the raw water from the membrane-filtered activated sludge tank before analysis. When collecting the raw water from the membrane-filtered activated sludge tank 1, easily decomposable organic matter in the raw water that has not been decomposed and methanol added during denitrification remain in the upstream part of the membrane-filtered activated sludge tank 1, which may affect the measurement of the difference in organic matter concentration. Therefore, it is preferable to collect the sludge immediately before it is subjected to membrane separation downstream of the membrane-filtered activated sludge tank 1. When filtering, it is preferable to use a filter with a pore size equal to or larger than the pore size of the membrane used in the membrane-filtered activated sludge tank 1, typically with a pore size of about 0.45 to 1 μm.

As mentioned above, COD, BOD, TOC, etc. can be used as organic matter concentrations to be measured as monitoring indicators, but it is more preferable to use COD _Mn , COD _Cr and TOC as indicators for more appropriately measuring organic matter that causes the formation of a gel layer and for appropriately measuring difficult-to-decompose polymers.

Although not limited to the following example, for example, when the difference value of COD _Mn (ΔCOD _Mn ) is used as the monitoring index, the standard for ΔCOD _Mn is set to 70 mg/L or less, further 60 mg/L or less, and further 40 mg/L or less, and when the measured value exceeds the preset standard, the operating conditions of the membrane bioreactor activated sludge tank 1 are changed, thereby making it possible to suppress membrane clogging in the membrane bioreactor activated sludge tank 1 for a long period of time.

When circulating nitrification and denitrification is performed in the membrane separation activated sludge tank 1 to remove nitrogen, easily decomposable organic matter may remain in the membrane filtered water depending on the nitrification and denitrification conditions. If easily decomposable organic matter remains, it becomes difficult to quantitatively evaluate the substances causing membrane blockage in the separation membrane 10. In such cases, it is preferable to measure the non-decomposable organic matter as an index of the organic matter concentration to be measured. Here, non-decomposable organic matter is defined as organic matter remaining after aerating the wastewater to be measured for a certain period of time (e.g., 1 to 8 hours). Measuring the concentration of such organic matter makes it possible to more quickly detect signs of blockage in the separation membrane 10.

Another method for measuring organic matter that causes membrane blockage is to use size exclusion chromatography with organic carbon detection: Liquid Chromatography-Organic Carbon Detector (LC-OCD), an analytical device that combines molecular weight fractionation and organic matter concentration distribution. It is known that organic matter that causes membrane blockage is a substance that is fractionated by LC-OCD to a molecular weight of 20,000 or more, and by measuring the concentration in the liquid phase of the raw membrane filtration water and the membrane filtrate water, it is possible to narrow down and quantify the target.

LC-OCD is a liquid chromatograph organic carbon meter equipped with a size exclusion column, and the dissolved organic matter in a sample is fractionated into the molar mass fractions (1) to (5) below using a liquid chromatograph equipped with a size exclusion column, and each fraction is then measured with an organic carbon meter to quantify the organic carbon.
(1) Biopolymers: Biopolymers, fractions with a molar mass of 20,000 g/mol or more (2) Humic Substances: Humic substances, fractions with a molar mass of 500 to 20,000 g/mol (3) Building Blocks: Building blocks (basic elements), fractions with a molar mass of 300 to 500 g/mol (4) LMW Acids: Low molecular weight organic acids, fractions of organic acids with a molar mass of 350 g/mol or less (5) LMW Neutrals: Low molecular weight neutral substances, fractions other than organic acids with a molar mass of 350 g/mol or less

Analysis using LC-OCD can be performed, for example, by the following procedure. First, the collected sample is filtered through a 0.45 μm pore size PTFE membrane filter, and diluted with ultrapure water (Milli-Q water) so that the TOC is 1-2 mg/L to prepare an analytical sample, which is then passed through a size exclusion column of an LC-OCD device. The sample is fractionated into four fractions with a molar mass of 20,000 g/mol or more, a molar mass of 500-20,000 g/mol, a molar mass of 300-500 g/mol, and a molar mass of 350 g/mol or less, and the organic carbon concentration of each fraction is measured. Phosphate buffer (pH 6.58) is used as the mobile phase. For example, an LC-OCD Model 8 from DOC-Labor can be used as the LC-OCD analyzer.

After fractionation by the size exclusion column, the analytical sample of each fraction is subjected to removal of inorganic carbon in a thin-film reactor (Glanzel thin-film reactor, DOC-Labor) using an acidifying solution (phosphoric acid, pH 1.5) and nitrogen gas purging, and then oxidized by an ultraviolet lamp (DOCOX lamp, DOC-Labor) and detected as _CO2 by an NDIR (non-dispersive infrared) detector. The analytical results obtained are analyzed using dedicated software (ChromCALC, DOC-Labor). Specifically, the peaks are divided based on the shape of the chromatogram, and the concentration is calculated from the area of each peak, allowing the quantification of organic matter fractionated into molecular weights of 20,000 or more by LC-OCD.

-How to improve driving-
In this embodiment, when the monitoring index increases, the following three measures are implemented as an operation improvement method for reducing fouling: (1) reducing the inflow of substances causing membrane blockage, (2) promoting the peeling off of the gel layer adhering to the membrane, and (3) decomposing or discharging the membrane blockage substances in the membrane bioreactor activated sludge tank 1. Of course, when the following improvement measures are appropriately implemented so that the monitoring index falls within an appropriate range, the treatment conditions may be returned to normal conditions.

(1) Reduction of the inflow of membrane blocking causative substances When the membrane blocking causative substances in the membrane filtration raw water, for example, the polymer flocculant in the dehydration separation liquid is excessive, the concentration of the polymer flocculant contained in the dehydration separation liquid increases, and it binds to the activated sludge and its extracellular substances to form a complex, which makes it easier for membrane blocking to occur. Therefore, when the monitoring index increases, the optimal addition rate of additives such as polymer flocculants added to the membrane filtration raw water before flowing into the membrane separation activated sludge tank 1, for example in the pretreatment operation, is confirmed, and if possible in order to maintain stable operation, the addition rate of the additives is reduced, and the operating conditions are adjusted to reduce the inflow of membrane blocking causative substances.

For example, when using dehydrated separated liquid as the raw water for membrane filtration, if the measured value of the monitoring index exceeds a preset standard, the operating conditions are adjusted to reduce the addition rate of polymer flocculant for the dehydration treatment. The optimal addition rate of polymer flocculant can be confirmed by flocculation tests or analysis of the colloidal charge of the dehydrated separated liquid. Other methods that can be taken in response to an increase in the monitoring index include reselecting the type of polymer flocculant, adjusting the addition rate of inorganic flocculant, and reducing the amount of food waste, etc., that is accepted.

(2) Promotion of peeling of gel layer adhering to membrane One method for promoting the destruction of the gel layer adhering to the separation membrane 10 contained in the membrane bioreactor activated sludge tank 1 is to adjust the filtration duration of the separation membrane 10 by adjusting the timing of the suction of the membrane filtrate water by the suction pump 3 using an intermittent timer. When the monitoring index increases, the ratio of the ON and OFF times of the intermittent timer per cycle can be adjusted within an appropriate range of 59:1 to 4:1, preferably 29:1 to 9:1, so that the filtration duration (ON time) is shortened and the filtration pause time (OFF time) is lengthened, and the time for which air bubbles are applied to the membrane surface during the OFF time is lengthened, thereby promoting the peeling of the gel layer.

Another method for promoting the destruction of the gel layer adhering to the separation membrane 10 contained in the membrane bioreactor 1 is to adjust the aeration time in the membrane bioreactor 1 by the aeration device 2. For example, when the monitoring index increases, the amount of cleaning air supplied to the membrane bioreactor 1 can be increased by about 5 to 50% compared to normal operation at an aeration air volume in the range of 6 to 30 L/ ^m2 /min, more preferably 9 to 25 L/ ^m2 /min, to promote the peeling off of the gel layer adhering to the surface of the separation membrane 10.

(3) Decomposition or discharge of membrane-clogging substances in the membrane bioreactor tank 1 Even if the gel layer attached to the membrane of the separation membrane 10 is peeled off, the substances that cause the gel layer are difficult to decompose and decompose slowly, have a large molecular weight, and tend to be captured by the membrane surface. Therefore, as long as the substances that are easily captured remain in the tank, they may adhere to the membrane surface again, and the state in which fouling tends to progress may continue. Therefore, the following methods are preferably used as measures to promote the decomposition or discharge of membrane-clogging substances in the membrane bioreactor tank 1.

- SRT control -
The substances that cause the gel layer are likely to be captured by the membrane and remain in the tank together with the sludge. Therefore, the longer the SRT, the more the substances that cause the gel layer are concentrated, and the more likely fouling progresses. For this reason, it is desirable to lower the SRT to the extent that the nitrification, denitrification, and other processes do not deteriorate, and to discharge the substances that cause the gel layer out of the tank.

As described in Patent Document 1, the presence of extracellular substances in activated sludge is known to be one of the causes of fouling, and measures to prevent fouling are sometimes taken to lengthen the SRT and reduce the sludge load. However, when the raw water contains persistent gel layer-causing substances, such as dehydration separation liquid, and the SRT is already within an appropriate range, lengthening the SRT may actually promote the concentration of fouling-causing substances. For this reason, in this embodiment, it is preferable to adopt a method of preventing the concentration of gel layer-causing substances by deliberately lowering the SRT while maintaining an appropriate SRT.

By adjusting the amount of extracted sludge so that the SRT is typically 35 days or less, more typically 30 days or less, it becomes possible to carry out stable and efficient wastewater treatment for a long period of time while appropriately suppressing fouling. On the other hand, if the SRT is made too short, the transmembrane pressure difference of the separation membrane 10 becomes high, and stable treatment may not be possible. Typically, it is preferable to control the SRT to 6 days or more, and preferably 10 days or more. This allows stable treatment while appropriately suppressing fouling of the separation membrane 10 in the membrane separation activated sludge tank 1.

-Physical and chemical treatment-
When the monitoring index increases, at least a portion of the raw membrane-filtered water is subjected to one or more of ozone decomposition treatment, advanced oxidation treatment, and activated carbon adsorption treatment to decompose and remove the causative substances in the gel layer.

-Adjusting the volume of activated sludge in membrane bioreactor tank 1 based on the measured values of monitoring indices-
Even if the SRT is appropriately adjusted, the state in the membrane bioreactor activated sludge tank 1 may temporarily become one that promotes the formation of a gel layer due to fluctuations in the membrane-filtered raw water, etc. In this embodiment, when the monitoring index increases, it is preferable to control the operating conditions for adjusting the sludge pump 4 so as to increase the amount of sludge extracted from the membrane bioreactor activated sludge tank 1 and reduce the amount of activated sludge in the membrane bioreactor activated sludge tank 1, thereby discharging the substances that cause the gel layer adhering to the separation membrane 10 out of the tank.

Although not limited to the following, if the monitoring index increases during the first sludge retention time (typically SRT is 6 to 35 days) when the transmembrane pressure difference of the separation membrane 10 is maintained within a preset allowable range and biological treatment by the membrane separation activated sludge method can be stably performed, the amount of sludge extracted from the membrane separation activated sludge tank 1 is increased. If the monitoring index increases during the second sludge retention time (typically less than 6 days) when the transmembrane pressure difference of the separation membrane 10 exceeds the allowable range, it is preferable to reduce the amount of sludge extracted from the membrane separation activated sludge tank 1. If the monitoring index increases when the SRT exceeds 35 days, reducing the amount of sludge extracted may lengthen the SRT and affect the treatment. If the monitoring index increases when the SRT exceeds 35 days, the amount of sludge extracted from the membrane separation activated sludge tank 1 is increased.

The control device 8 is electrically connected to the blower 22 of the aeration device 2, the sludge pump 4, the suction pump 3, the organic matter concentration measuring devices 5 and 6, and the sludge concentration meter 7, and controls the operating conditions of the aeration device 2, the suction pump 3, and the sludge pump 4 so that when the monitoring index increases, one or more of the following processes are performed: increasing the amount of cleaning air supplied to the membrane separation activated sludge tank 1, shortening the filtration duration, or increasing the amount of sludge extracted from the membrane separation activated sludge tank 1.

The wastewater treatment method according to the embodiment of the present invention can be carried out using the wastewater treatment device shown in Figure 1. That is, the wastewater treatment method according to the embodiment includes measuring the organic matter concentration in the liquid phase of the membrane-filtered raw water in the membrane separation activated sludge tank 1 and the organic matter concentration in the membrane-filtered water, using the difference as a monitoring index, and adjusting the operating conditions so that, when the monitoring index increases, one or more of the following processes are performed: increasing the amount of cleaning air supplied to the membrane separation activated sludge tank 1, shortening the filtration duration, or increasing the amount of sludge extracted from the membrane separation activated sludge tank 1.

Thus, according to the wastewater treatment device and wastewater treatment method of the embodiment of the present invention, the organic matter concentration of the liquid phase of the membrane-filtered raw water in the membrane separation activated sludge tank 1 and the organic matter concentration of the membrane-filtered water are measured, and the difference between them is used as a monitoring index. If the monitoring index increases, at least one of the following processes is carried out: reducing the inflow of substances that cause membrane blockage, promoting the peeling off of the gel layer adhering to the membrane, and decomposing or discharging the membrane-blocking substances in the membrane separation activated sludge tank 1. This makes it possible to quickly detect signs of separation membrane blockage and perform stable and efficient wastewater treatment while appropriately suppressing fouling.

(Modification)
As a method for suppressing clogging of the separation membrane 10 in the membrane bioreactor activated sludge tank 1, it is preferable to optimize at least one of the timing of increasing the amount of cleaning air supplied to the membrane bioreactor activated sludge tank 1, shortening the filtration duration, or increasing the amount of sludge extracted from the membrane bioreactor activated sludge tank, or the treatment time, for each treatment, when the monitoring index increases.

For example, the increase in the amount of cleaning air supplied to the membrane bioreactor activated sludge tank 1 and the shortening of the filtration duration are processes for physically removing the gel layer attached to the separation membrane 10, and as measures for suppressing fouling, they tend to be effective in a relatively short time. On the other hand, the control of the sludge pump 4 for increasing the amount of sludge extracted from the membrane bioreactor activated sludge tank 1 is a process for adjusting the concentration in the membrane bioreactor activated sludge tank 1 so as not to increase the concentration of substances that tend to form a gel layer on the membrane surface of the separation membrane 10, and as a measure for suppressing fouling, it is necessary to continue it for a relatively long time to obtain an effect. Therefore, it is preferable to combine short-term measures and long-term measures for suppressing fouling depending on the degree of contamination of the separation membrane 10 and the concentrations of the sludge and inflow polymer flocculant in the membrane bioreactor activated sludge tank 1.

Increasing the amount of cleaning air can suppress fouling in the short term, but it has problems such as increased running costs due to increased power consumption of the blower 22 and dismantling of activated sludge. Increasing the duration of filtration can suppress fouling in the short term, but it has a limit to the amount of filtration per hour. In addition, it takes several days to several tens of days for the sludge to be replaced, and it takes time for the SRT control to be effective. Therefore, in this embodiment, a process of increasing the amount of cleaning air and shortening the duration of filtration, which is effective in the short term, is combined with a process of adjusting the amount of extracted sludge, which is effective in the long term. For example, in the early stage of fouling, short-term and long-term measures are taken simultaneously, and after the replacement of sludge progresses and the monitoring index decreases, the short-term measures are returned to the state before the treatment, thereby immediately suppressing fouling, reducing running costs over the long term, and enabling stable operation.

Specific examples include, but are not limited to, a first reference value and a second reference value greater than the first reference value are preset for the monitoring index, and when the monitoring index increases and exceeds the first reference value, a process is started to increase the amount of sludge extracted from the membrane bioreactor activated sludge tank 1, and when the monitoring index further exceeds the second reference value, the operating conditions can be adjusted to start a first process to increase the amount of cleaning air supplied to the membrane bioreactor activated sludge tank 1 and/or shorten the filtration duration.

Specifically, when the transmembrane pressure difference of the separation membrane 10 in the membrane bioreactor tank 1 falls within the standard range and the monitoring index increases during the first sludge retention time (typically 6 to 35 days) during which stable biological treatment is performed, the operating conditions can be adjusted to perform at least one of a first process, which increases the amount of cleaning air supplied to the membrane bioreactor tank 1 and/or a second process, which increases the amount of sludge extracted from the membrane bioreactor tank 1.

In addition, when the monitoring index increases during the second sludge retention time (typically less than 6 days) when the transmembrane pressure difference of the separation membrane 10 exceeds the above-mentioned reference range, the operating conditions can be adjusted to perform at least one of the following: a first process for increasing the amount of cleaning air supplied to the membrane bioreactor activated sludge tank 1 and/or shortening the filtration duration, and a third process for reducing the amount of sludge extracted from the membrane bioreactor activated sludge tank 1. In addition, when the monitoring index increases when the SRT exceeds 35 days, the operating conditions can be adjusted to perform at least one of the following: a first process for increasing the amount of cleaning air supplied to the membrane bioreactor activated sludge tank 1 and/or shortening the filtration duration, and a second process for increasing the amount of sludge extracted from the membrane bioreactor activated sludge tank 1.

This allows biological treatment in the membrane separation activated sludge tank 1 to proceed more stably, and while it is possible to stably and continuously obtain treated water that meets the standard values, even if there is a sudden change in the properties of the membrane filtration raw water, it is possible to quickly detect signs of blockage of the separation membrane 10 and perform wastewater treatment stably and efficiently over a long period of time while appropriately suppressing fouling.

If the above measures are taken and a decrease in the monitoring index is confirmed, the first process, which involves one or more of increasing the amount of cleaning air supplied to the membrane bioreactor activated sludge tank 1 and shortening the filtration duration, can be gradually returned to the state before the measures were taken, thereby reducing running costs and stabilizing the process.

The following examples of the present invention are presented together with comparative examples, but these examples are provided to provide a better understanding of the present invention and its advantages, and are not intended to limit the invention.

(Test 1) SRT control Dehydrated separated liquid from sewage and septic tank sludge was used as the membrane-filtered raw water to be supplied to a membrane separation activated sludge tank equipped with a denitrification tank and a nitrification tank (membrane separation tank), and the MLSS of the dehydrated separated liquid was changed from 3,000 to 20,000 mg/L to examine the membrane filtration performance due to changes in SRT, T-N-SS load in the nitrification tank (hereinafter referred to as "T-N-SS load"), and BOD-SS load in all tanks (hereinafter referred to as "BOD-SS load"), and the treatment test conditions and measurement results are shown in Table 1. Note that the BOD-SS load represents the total BOD-SS load in the denitrification tank and the nitrification tank.

In Test 1, an organic flat membrane with a pore size of 0.4 μm was used. The filtration flux was fixed at 0.4 m/d, and the degree of contamination of the membrane was evaluated based on the change in transmembrane pressure difference, the difference in organic matter concentration (ΔS-COD _Mn ), and the amount of filtration using 5C filter paper. The amount of filtration using 5C filter paper is the result of measuring the amount of filtrate that can be filtered in 5 minutes using 5C filter paper with a diameter of 150 mm. ΔS-COD _Mn indicates the difference between the soluble COD _Mn (S-COD _Mn ) in the mixed liquid in the membrane separation tank and the COD _Mn of the membrane separated water. The higher the ΔS-COD _Mn , the higher the degree of contamination of the membrane surface, indicating that filtration is more difficult. ΔTransmembrane pressure difference, ΔS-COD _Mn , and the amount of filtration using 5C filter paper are all measured values after 3 months of continuous treatment.

2 to 4 show the relationship between the SRT obtained in Test 1 and ΔS-COD _Mn , which is an index showing the degree of membrane fouling, transmembrane pressure difference, and 5C filter paper filtration volume. When the SRT was 10 to 30 d, ΔS-COD _Mn was relatively stable at 22 to 40 mg/L, and it was determined that there was little membrane fouling. On the other hand, when the SRT was as short as 6 d, ΔS-COD _Mn was high at 82 mg/L, and it was determined that there was a lot of membrane fouling. Also, when the SRT was as long as 40 d, ΔS-COD _Mn was high at 90 mg/L, indicating that there was a lot of membrane fouling.

From the relationship between SRT and transmembrane pressure shown in Figure 3, it is estimated that an SRT of 6 to 35 d, or even 10 to 30 d, is appropriate to keep the transmembrane pressure at 25 KPa or less, and preferably 20 KPa or less, which is the benchmark for stable treatment. From the relationship between SRT and 5C filter paper filtration volume shown in Figure 4, it is determined that an SRT of 6 to 35 d, or even 10 to 30 d, is preferable to maintain a 5C filter paper filtration volume of 8 ml or more, and preferably 10 ml or more, which is the benchmark for good filtration performance.

Figures 5 to 7 show the relationship between the BOD-SS load of the entire reactor, ΔS-COD _Mn , transmembrane pressure, and 5C paper filter filtration volume obtained in the test. In terms of the BOD-SS load, it is considered appropriate to set the BOD-SS load at 0.05 to 0.2 kg/kg/d in order to suppress membrane fouling. Figure 8 shows the relationship between ΔS-COD _Mn and transmembrane pressure, and Figure 9 shows the relationship between ΔS-COD _Mn and 5C paper filter filtration volume. A good correlation was observed between ΔS-COD _Mn and transmembrane pressure. In order to keep the transmembrane pressure at 20 KPa or less, which is the guideline for stable treatment, it is preferable to operate and manage ΔS-COD _Mn at 40 mg/L or less. A similar good correlation was observed between ΔS-COD _Mn and 5C paper filter filtration volume. In order to keep the 5C paper filter filtration volume at 10 ml or more, which is the guideline for good filtration performance, it is preferable to operate and manage ΔS-COD _Mn at approximately 70 mg/L or less.

From the above results, it has been found that, according to one embodiment of the present invention, in order to suppress membrane fouling and maintain stable filtration performance, it is preferable to control the operating conditions so that ΔS-COD _Mn is at least 70 mg/L or less, preferably 40 mg/L or less, and to appropriately adjust SRT and BOD-SS.

(Test 2) Study of aeration air volume under appropriate SRT conditions and appropriate suction time conditions Using the same equipment as in Test 1, dehydrated separated liquid of human waste and septic tank sludge was used as the raw water for membrane filtration, and the MLSS of the dehydrated separated liquid was changed from 3,000 to 20,000 mg/L to study the membrane filtration performance according to the change in the amount of washing air and the duration of filtration (suction timer). The results are shown in Table 2.

Under conditions of SRT18d, repeated suction for 9 minutes and 1 minute without suction, and a cleaning air volume of 7-18 L/ ^m2 /min, the transmembrane pressure remained low at 17-23 Kpa and the 5C paper filtration volume was also high at 15-21 mL, showing good conditions (No. 11-13).When the cleaning air volume was reduced to 4 L/ ^m2 /min, the transmembrane pressure rose to 26 Kpa and the 5C paper filtration volume was also low at 9.2 mL, showing signs of membrane blockage (No. 14).

Under conditions of SRT18d and cleaning air volume 18 L/ ^m2 /min, by stopping suction for 1 minute every 20 to 40 minutes, the transmembrane pressure was maintained at a low value of 24 to 32 Kpa and the 5C filter paper filtration volume was also high at 16 to 20 mL, which was in a good condition (No. 15, 16). However, when suction was stopped for 1 minute every 60 minutes, the transmembrane pressure rose to 31 Kpa and the 5C filter paper filtration volume was also low at 8.4 mL, indicating signs of membrane blockage (No. 17).

(Test 3) Control Based on Monitoring Indicators Continuous operation was carried out under the following three conditions, and the number of days until the transmembrane pressure reached 30 kPa or more was examined.
(1) Comparative Example 1: Normal operation, no control based on monitoring index (cleaning air volume 7 L/ ^m2 /min, suction timer On:Off=12:1, SRT 18 days)
(2) Example 1: Control based on monitoring indicators (short-term treatment only)
(When ΔS-COD _Mn reached 40 mg/L or more, the cleaning air volume was changed from normal operation to 9 L/m ² /min and the suction timer On:Off=9:1, and when ΔS-COD _Mn reached 30 mg/L or less, the operation was returned to normal.)
(3) Example 2: Control based on monitoring indicators (combination of short-term and long-term treatments)
If ΔS-COD _Mn reached 40 mg/L or above, the system changed from normal operation to a scrubbing air volume of 9 L/ ^m2 /min, suction timer On:Off = 9:1, and SRT to 14 days by operating the sludge pump; if ΔS-COD _Mn fell below 30 mg/L, control was performed to return to normal operation; if S-COD _Mn fell below 30 mg/L, control was performed to return to normal operation conditions, with the scrubbing air volume changed to 7 L/ ^m2 /min, suction timer On:Off = 12:1, and SRT to 14 days; and if S-COD _Mn was below 20 mg/L, control was performed to return to normal operating conditions.

When only the short-term treatment of Example 1 was performed, the operation time was 234 days, and when the short-term and long-term treatments of Example 2 were combined, the operation time was 381 days, confirming that the operation could be stably continued for a longer period of time compared to Comparative Example 1 where no control was performed.

1...membrane separation activated sludge tank 2...aeration device 3...suction pump 4...sludge pump 5, 6...organic matter concentration measuring device 7...sludge concentration meter 8...control device 10...separation membrane 21...aeration device 22...blower 41...drawing pipe

Claims (6)

A wastewater treatment method using a membrane bioreactor, comprising measuring an organic matter concentration in the liquid phase of membrane-filtered raw water in a membrane bioreactor tank and an organic matter concentration in membrane-filtered water, using the difference between the organic matter concentrations as a monitoring index, and adjusting operating conditions so as to perform one or more of the following treatments when the monitoring index increases: increasing the amount of cleaning air supplied to the membrane bioreactor tank, shortening the filtration duration, or increasing the amount of sludge extracted from the membrane bioreactor tank;
When the monitoring index exceeds a first reference value, a process for increasing the amount of sludge extracted from the membrane bioreactor activated sludge tank is performed;
When the monitoring index exceeds a second standard value that is higher than the first standard value, the wastewater treatment method comprises carrying out one or more of the following: increasing an amount of cleaning air to be supplied to the membrane bioreactor tank or shortening the filtration duration. The wastewater treatment method according to claim 1, further comprising measuring the organic matter concentration as any one of COD, BOD, TOC, or the concentration of organic matter fractionated to a molecular weight of 20,000 or more as measured by organic carbon detection type size exclusion chromatography. The wastewater treatment method according to claim 1 or 2 , further comprising subjecting at least a portion of the membrane-filtered raw water to one or more of ozone decomposition treatment, advanced oxidation treatment, and activated carbon adsorption treatment when the monitoring index increases. The wastewater treatment method according to any one of claims 1 to 3 , wherein the membrane filtrated raw water is a dehydrated separated liquid generated after a polymer flocculant is added to sludge containing organic matter and the dehydration treatment is performed. The wastewater treatment method according to any one of claims 1 to 4, further comprising performing a process of reducing an addition rate of a polymer flocculant added to the membrane-filtered raw water in a dehydration process before the raw water is supplied to the membrane separation activated sludge tank when the monitoring index increases. a membrane separation activated sludge tank for subjecting the membrane-filtered raw water to membrane separation treatment in the presence of activated sludge to obtain membrane-filtered water;
an aeration device for aerating the membrane separation activated sludge tank;
a suction pump that sucks the membrane-filtered water from the membrane separation activated sludge tank;
a sludge pump for extracting sludge from the membrane bioreactor tank;
a control device which controls the operating conditions of the aeration device, the suction pump and the discharge sludge pump so that a difference value between an organic matter concentration in the liquid phase of the membrane-filtered raw water in the membrane bioreactor tank and an organic matter concentration in the membrane-filtered water is used as a monitoring index, and when the monitoring index increases, one or more of the following processes are performed: an increase in the amount of cleaning air supplied to the membrane bioreactor tank, a shortened filtration duration, or an increase in the amount of sludge extracted from the membrane bioreactor tank ;
the control device, when the monitoring index exceeds a first reference value, performs a process of increasing the amount of sludge extracted from the membrane bioreactor tank, and, when the monitoring index exceeds a second reference value higher than the first reference value, performs one or more of the following processes: increasing the amount of cleaning air supplied to the membrane bioreactor tank or shortening the filtration duration .