Water treatment method and water treatment system

Abstract

To provide a water treatment method capable of suppressing deterioration of properties of granular sludge and a decrease in biological activity when the inflow of water to be treated into a reaction tank is stopped for a certain period of time.SOLUTION: There is provided a water treatment method for repeatedly performing an operation cycle having: an inflow step of making water to be treated flow into a reaction tank 14; a biological treatment step of biologically treating the water to be treated in the reaction tank 14 using biological sludge; a settling step of settling the biological sludge in the reaction tank 14; and a discharge step of discharging the biologically treated water in the reaction tank 14, wherein when the operation cycle is stopped and the inflow of the water to be treated by the inflow step is stopped for a certain period of time, the reaction tank 14 is repeatedly switched between an aeration state in which aeration is performed with an aeration air volume that causes the biological sludge in the reaction tank 14 to flow, and a sedimentation state in which aeration is performed with an aeration air volume that causes the biological sludge in the reaction tank 14 to settle or aeration is stopped.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a technique for a water treatment method and a water treatment system. [Background technology]

[0002] Conventionally, biological wastewater treatment has used the activated sludge method, which utilizes a collection of microorganisms called flocs (aerobic biological sludge). However, in the activated sludge method, when separating the flocs (aerobic biological sludge) from the treated water in the settling tank, the surface area of ​​the settling tank may need to be very large because the settling speed of the flocs is slow. In addition, the treatment speed of the activated sludge method depends on the sludge concentration in the biological treatment tank, and although the treatment speed can be increased by increasing the sludge concentration, there are cases where the treatment cannot be maintained due to solid-liquid separation problems caused by bulking in the settling tank, etc.

[0003] On the other hand, in anaerobic biological treatment, it is common to utilize a dense collection of microorganisms called granules. Granules have a very fast settling rate and the microorganisms are densely packed together, so the sludge concentration in the biological treatment tank can be increased, making it possible to realize high-speed wastewater treatment. However, anaerobic biological treatment may have problems such as the fact that the types of wastewater to be treated are limited compared to aerobic treatment (activated sludge method) and that the treated water temperature needs to be maintained at about 30 to 35°C. In addition, when anaerobic biological treatment alone is used, the quality of the treated water is poor, and when the treated water is to be discharged into a river or the like, it may be necessary to separately carry out aerobic treatment such as the activated sludge method.

[0004] In recent years, it has become clear that by using a semi-batch treatment apparatus in which wastewater is intermittently introduced into a reaction tank, it is possible to form a granulated biological sludge (hereinafter referred to as granular sludge) with good settling properties not only from anaerobic biological sludge but also from aerobic biological sludge (see, for example, Patent Documents 1 to 4). Granular sludge has an average particle size of, for example, 0.2 mm or more. In semi-batch biological treatment, it is common to repeatedly carry out the following steps in one reaction tank: (1) introduction of wastewater, (2) biological treatment of the wastewater using biological sludge, (3) settling of the biological sludge, and (4) discharge of biologically treated water.

[0005] Patent Document 5 discloses a biological treatment method that repeats the steps of (1) inflow of wastewater and discharge of biologically treated water, (2) biological treatment of the wastewater using biological sludge, and (3) settling of the biological sludge. This allows the production of biological sludge with high settling properties like granular sludge. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2004 / 024638 [Patent Document 2] JP 2008-212878 A [Patent Document 3] Patent No. 4975541 [Patent Document 4] Patent No. 4804888 [Patent Document 5] JP 2016-77931 A Summary of the Invention [Problem to be solved by the invention]

[0007] In the case of semi-batch biological treatment, it may become impossible to flow the treated water into the reaction tank for a certain period of time due to equipment maintenance or the like. In such cases, the properties of the granular sludge in the reaction tank may deteriorate and the biological activity may decrease.

[0008] An object of the present disclosure is to provide a water treatment method and a water treatment system that can suppress deterioration of the properties of granular sludge and a decrease in biological activity when the inflow of treated water into the reaction tank is stopped for a certain period of time. [Means for solving the problem]

[0009] One aspect of the present disclosure is a water treatment method that repeatedly performs an operating cycle having an inflow process of inflowing water to be treated into a reaction tank, a biological treatment process of biologically treating the water to be treated in the reaction tank with biological sludge, a sedimentation process of settling the biological sludge in the reaction tank, and a discharge process of discharging the biologically treated water from the reaction tank, characterized in that when the operating cycle is stopped and the inflow of water to be treated through the inflow process is stopped for a certain period of time, the water treatment method repeatedly performs an aeration state in which the reaction tank is aerated with an aeration air volume that causes the biological sludge in the reaction tank to flow, and aeration with an aeration air volume that causes the biological sludge in the reaction tank to settle or a settling state in which aeration is stopped.

[0010] In the water treatment method, it is preferable that the aeration is stopped during the settling state.

[0011] In the water treatment method, the time for each of the aeration state and the settling state is preferably within the range of 1 hour or more and less than 24 hours.

[0012] In the water treatment method, it is preferable that when the pH in the reaction tank becomes equal to or lower than a predetermined value during the aeration state, the aeration state is switched to the sedimentation state.

[0013] In the water treatment method, when the pH in the reaction tank becomes 5 or less during the aeration state, it is preferable to adjust the pH in the reaction tank to 6 or more and 8 or less.

[0014] In the water treatment method, in order to adjust the pH in the reaction tank to 6 or more and 8 or less, it is preferable to supply biologically treated water obtained by biologically treating the water to be treated to the reaction tank.

[0015] Moreover, one aspect of the present disclosure is a water treatment device having a semi-batch treatment device which has a reaction tank and repeats an operating cycle including an inflow process of inflowing water to be treated into the reaction tank, a biological treatment process of biologically treating the water to be treated in the reaction tank with biological sludge, a sedimentation process of settling the biological sludge in the reaction tank, and a discharge process of discharging the biologically treated water in the reaction tank, wherein when the operating cycle is stopped and the inflow of water to be treated through the inflow process is stopped for a certain period of time, the semi-batch treatment device is a water treatment system characterized in that it repeats an aeration state in which the reaction tank is aerated with an aeration air volume at which the biological sludge in the reaction tank flows, and aeration with an aeration air volume at which the biological sludge in the reaction tank settles or a settling state in which aeration is stopped. Effect of the Invention

[0016] According to the present disclosure, it is possible to provide a water treatment method and a water treatment system that can suppress deterioration of the properties of granular sludge and a decrease in biological activity when the inflow of treated water into the reaction tank is stopped for a certain period of time. [Brief description of the drawings]

[0017] [Figure 1] 1 is a schematic configuration diagram illustrating an example of a water treatment system according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a schematic configuration diagram showing another example of the water treatment system according to the present embodiment. [Diagram 3] 1 is a graph showing the results of nitrification activity in a nitrification activity test and the average particle size of biological sludge in an example and comparative examples 1 and 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] An embodiment of the present disclosure will be described below. The embodiment is an example of implementing the present disclosure, and the present disclosure is not limited to the embodiment.

[0019] FIG. 1 is a schematic diagram showing an example of a water treatment system according to the present embodiment. The water treatment system 1 includes a semi-batch treatment device 10 and a control device 12. The semi-batch treatment device 10 includes a reaction tank 14. A supply pipe 18 is connected to the inlet of the reaction tank 14. A pump 20 and a valve 22 are installed in the supply pipe 18. A treated water pipe 26 is connected to the outlet of the reaction tank 14. A valve 28 is installed in the treated water pipe 26. A sludge outlet of the reaction tank 14 is connected to a sludge discharge pipe 30. A sludge discharge pump 32 and a valve 33 are installed in the sludge discharge pipe 30.

[0020] An aeration device 34 is provided near the bottom of the reaction tank 14. A gas supply pipe 36 is connected to the aeration device 34, and a blower 38 is installed in the gas supply pipe 36.

[0021] The control device 12 is composed of, for example, a microcomputer and electronic circuitry, which is composed of a CPU that executes a predetermined program, and a ROM and RAM that store programs and calculation results, etc., and reads out a predetermined program stored in the ROM, etc., and executes the program to control the operation of the semi-batch processing device 10. The control device 12 is connected to each valve, each pump, and blower, and controls the operation and stopping of the pumps and blowers, the opening and closing of the valves, etc.

[0022] An example of the operation of the water treatment system 1 will be described below.

[0023] <(1) Inflow process> By operating the pump 20 and opening the valve 22, a predetermined amount of water to be treated is supplied from the supply pipe 18 into the reaction tank 14. The water to be treated may be, for example, wastewater containing organic matter, and specific examples thereof include organic wastewater containing biodegradable organic matter, such as wastewater from food processing plants, chemical plants, semiconductor plants, machine factories, sewage, and human waste.

[0024] <(2) Biological treatment process> The pump 20 is stopped and the valve 22 is closed. Then, the blower 38 is operated and an oxygen-containing gas such as air is supplied from the gas supply pipe 36 through the aeration device 34 into the reaction tank 14. In the reaction tank 14, the water to be treated is biologically treated by the biological sludge under aerobic conditions. Note that the biological treatment is not limited to aerobic conditions, but may be anaerobic conditions or a combination of aerobic and anaerobic conditions. When biological treatment is performed under anaerobic conditions, it is desirable to not supply air or the like into the reaction tank 14 and to agitate the water to be treated and the biological sludge, for example, by an agitator (not shown).

[0025] <(3) Sedimentation process> The blower 38 is stopped (if stirring is performed by an agitator, the agitator is stopped), and the inside of the reaction tank 14 is left to stand for a predetermined period of time to allow the biological sludge in the reaction tank 14 to settle. As a result, biologically treated supernatant water is obtained.

[0026] <(4) Discharge process> Depending on a predetermined end time of the settling process, the valve 28 is opened and the supernatant water obtained in the settling process is discharged from the reaction tank 14 to the treated water pipe 26 as treated water.

[0027] In the water treatment system 1 shown in FIG. 1, an operation cycle including steps (1) to (4) is repeatedly performed, whereby the water to be treated is treated in the reaction tank 14 and granular sludge that has undergone self-granulation is formed.

[0028] 1 may also periodically carry out a sludge discharge process for discharging biological sludge (including granular sludge) in the reaction tank 14. The sludge discharge process is carried out, for example, once a day to once a week. When carrying out the sludge discharge process, the sludge discharge pump 32 is operated and the valve 33 is opened. As a result, a predetermined amount of biological sludge in the reaction tank 14 is drawn out from the sludge discharge piping 30.

[0029] However, there are cases where the operation cycle including the above steps (1) to (4) is stopped due to maintenance of the water treatment system, etc. If the water to be treated cannot be supplied to the reaction tank 14 for a certain period of time due to the stop of the operation cycle, the properties of the granular sludge in the reaction tank 14 may deteriorate and the biological activity may decrease. Therefore, in this embodiment, the following control is performed while the operation cycle is stopped.

[0030] For example, when an operator stops the operation cycle for maintenance of the water treatment system 1 and stops the inflow of the water to be treated in the inflow step for a certain period (for example, one day or more), the operator transmits a signal to the control device 12 to notify the stop of the operation cycle, for example, using an input device (not shown) connected to the control device 12. The control device 12, which has received the signal to stop the operation cycle, controls the blower 38 so as to repeat an aeration state in which the reaction tank 14 is aerated with an aeration air volume at which at least a part of the biological sludge (including granular sludge) in the reaction tank 14 flows, and an aeration state in which the biological sludge (including granular sludge) in the reaction tank 14 is aerated with an aeration air volume at which the biological sludge (including granular sludge) in the reaction tank 14 settles, or a settling state in which aeration is stopped. Here, the aeration air volume at which at least a part of the biological sludge (including granular sludge) flows is an aeration air volume equal to or greater than the minimum aeration air volume required to ensure a state in which at least a part of the biological sludge in the reaction tank 14 flows and to prevent all of the biological sludge from accumulating at the bottom of the reaction tank 14. The aeration air volume at which the biological sludge in the reaction tank 14 settles is less than the minimum aeration air volume described above. The minimum aeration air volume is determined based on, for example, the results of a preliminary experiment or the results of actual operation using an actual device.

[0031] In this way, when the operating cycle is stopped and the inflow of treated water through the inflow process is stopped for a certain period of time, the above-mentioned aeration state and sedimentation state are repeatedly performed for the reaction tank 14, thereby making it possible to suppress deterioration of the properties of the granular sludge in the reaction tank 14 and a decrease in biological activity.

[0032] Whether or not the aeration air volume supplied into the reaction tank 14 reaches a preset aeration air volume in the aeration state or the settling state may be determined based on a value directly measured by an air volume meter (not shown) installed in the gas supply pipe 36 or the like, or based on an air volume estimated from an index such as DO (amount of dissolved oxygen) measured by a DO meter (not shown) installed in the reaction tank 14. When estimating, for example, correlation information that predefines the relationship between an index (e.g., DO, etc.) and the air volume is prepared, and the index measured by a meter (e.g., DO meter) is applied to the correlation information to estimate the air volume. In either case, the control device 12 increases the output of the blower 38 if the directly measured or estimated air volume during the aeration state or the settling state does not reach the preset aeration air volume, or decreases the output of the blower 38 if it exceeds the preset aeration air volume.

[0033] In the settling state, aeration may be performed with an aeration volume at which the biological sludge in the reaction tank 14 settles, but it is preferable to stop aeration and allow the biological sludge to settle. Stopping aeration can better prevent the biological sludge from breaking down, and therefore can better prevent the deterioration of the properties of the granular sludge.

[0034] The duration of each aeration state and each settling state is preferably in the range of 1 hour or more and less than 24 hours, and more preferably in the range of 2 hours or more and 12 hours or less, in order to further prevent deterioration of the properties of the biological sludge or to further prevent a decrease in biological activity.

[0035] In this embodiment, the following control may be performed based on the pH in the reaction tank 14. The pH in the reaction tank 14 is measured by, for example, a pH meter 40 installed in the reaction tank 14.

[0036] For example, when the pH in the reaction tank 14 becomes equal to or lower than a predetermined value during the aeration state, the control device 12 preferably controls the blower 38 to transition from the aeration state to the sedimentation state. This further suppresses the decrease in biological activity associated with the dismantling of biological sludge. The predetermined value of pH is, for example, 5. When the aeration state is transitioned to the sedimentation state and the sedimentation state is performed for a preset time, if the pH in the reaction tank 14 is still equal to or lower than a predetermined value, it is preferable to adjust the pH in the reaction tank 14 so that it exceeds the predetermined value before transitioning to the aeration state. There is no particular limitation on the pH adjuster used to adjust the pH in the reaction tank 14. However, since the pH of the reaction tank 14 in the aeration state is preferably in the neutral range (for example, in the range of pH 6 to 8), when the pH is adjusted from an acidic state to the neutral range, biologically treated water obtained by biologically treating the water to be treated may be used. The biologically treated water may be biologically treated water obtained in the water treatment system 1, or biologically treated water obtained in a separate treatment system.

[0037] Furthermore, for example, when the pH in the reaction tank 14 becomes 5 or less during the aeration state, it is preferable to adjust the pH in the reaction tank 14 to 6 or more and 8 or less. This further suppresses the decrease in biological activity associated with the dismantling of biological sludge. The pH adjuster used at this time may be biologically treated water obtained by biologically treating the water to be treated. The biologically treated water may be biologically treated water obtained in the water treatment system 1, or biologically treated water obtained in a separate treatment system.

[0038] The pH adjustment in the reaction tank 14 may be performed by an operator or by a pH adjuster supplying device. The pH adjuster supplying device is composed of, for example, a tank that stores a pH adjuster, a pipe that connects the tank to the reaction tank 14, and a pump installed in the pipe. In the pH adjuster supplying device, the tank is operated based on the pH value measured by a pH meter 40 installed in the reaction tank 14, and a required amount of pH adjuster is supplied from the tank through the pipe to the reaction tank 14, thereby adjusting the pH in the reaction tank 14.

[0039] Fig. 2 is a schematic diagram showing another example of a water treatment system according to an embodiment. In the water treatment system 2 shown in Fig. 2, the same components as those in the water treatment system 1 shown in Fig. 1 are denoted by the same reference numerals. In the water treatment system 2 shown in Fig. 2, an outlet to which a treated water pipe 26 is connected is provided at a position at the water level height of the reaction tank 14. Note that the water treatment system 2 shown in Fig. 2 does not have a sludge discharge pipe 30 connected to a sludge outlet of the reaction tank 14.

[0040] An example of the operation of the water treatment system 2 shown in FIG. 2 will be described.

[0041] (1) Inflow process and discharge process: The pump 20 is operated and the valve 22 is opened, so that the water to be treated is supplied from the supply pipe 18 into the reaction tank 14. When the water to be treated flows in, the valve 28 is opened and the treated water (supernatant water) in the reaction tank 14 obtained in the settling process is pushed out by the inflowing water to be treated and discharged to the treated water pipe 26.

[0042] (2) Biological treatment step: The pump 20 is stopped, and the valves 22 and 28 are closed. In the reaction tank 14, the water to be treated is biologically treated by the biological sludge. As described above, the biological treatment is carried out, for example, under aerobic conditions, anaerobic conditions, or a combination thereof.

[0043] (3) Sedimentation step: The blower 38 is stopped (if stirring is performed by an agitator, the agitator is stopped), and the inside of the reaction tank 14 is left to stand for a predetermined period of time to allow the biological sludge in the reaction tank 14 to settle. As a result, biologically treated supernatant water is obtained.

[0044] In the water treatment system 2 shown in Fig. 2, an operation cycle including steps (1) to (3) is repeatedly performed, whereby the water to be treated is treated and granular sludge that has undergone self-granulation is formed in the reaction tank 14. Note that the operation cycle including steps (1) to (3) simply includes the inflow step and the discharge step together, but still includes four steps, namely, the inflow step, biological treatment step, sedimentation step, and discharge step.

[0045] 2 may also periodically perform a sludge discharge step of discharging biological sludge (including granular sludge) in the reaction tank 14. When performing the sludge discharge step, a gas such as an oxygen-containing gas is supplied into the reaction tank 14, an agitator or the like is operated, and the biological sludge in the reaction tank 14 is stirred. Then, the pump 20 is operated and the valves 22 and 28 are opened. As a result, the biological sludge in the reaction tank 14 is pushed out by the incoming treated water and discharged from the treated water piping 26.

[0046] In the water treatment system 2 shown in FIG. 2, when the operating cycle having the above steps (1) to (3) is stopped for maintenance or the like, and the inflow of the water to be treated through the inflow step is stopped for a certain period of time (e.g., one day or more), the aeration state and the sedimentation state are repeated in the reaction tank 14 in the same manner as described above.

[0047] The granular sludge formed by this embodiment is, for example, sludge with an average particle size of 0.1 mm or more, or with a settling index SVI5 of 80 mL / g or less. SVI is a settling index of biological sludge, and is calculated by the following method. First, 1 L of sludge is put into a 1 L measuring cylinder, gently stirred so that the sludge concentration becomes as uniform as possible, and then the sludge interface is measured after leaving it to stand for 5 minutes. Then, the volume ratio (%) of the sludge in the measuring cylinder is calculated. Next, the MLSS (mg / L) of the sludge is measured. These are applied to the following formula to calculate SVI5. To calculate SVI30, the above 5 minutes of standing can be changed to 30 minutes of standing. SVI5 (mL / g) = Volume ratio of sludge x 10,000 / MLSS

[0048] The volumetric load of the reactor 14 is 0.15 kg BOD / m 3 / day ~ 1.00kgBOD / m 3 / day, and preferably in the range of 0.30 kg BOD / m 3 / day ~ 0.60kgBOD / m 3By setting the volume load of the reaction vessel 14 in the above range, it is possible to form better granules.

[0049] The dissolved oxygen (DO) in the reaction tank 14 is preferably set to 0.5 mg / L or more, particularly preferably 1 mg / L or more, under aerobic conditions.

[0050] In order to promote the granulation of biological sludge, Fe is added to the water to be treated in the reaction tank 14 or to the water to be treated before being introduced into the reaction tank 14. 2+ , Fe 3+ , Ca 2+ , Mg 2+ It is also possible to add ions that can form hydroxides, including the above ions. By adding the above ions, it is possible to promote the nucleation of granules. EXAMPLES

[0051] The present disclosure will be described more specifically and in detail below with reference to examples, but the present disclosure is not limited to the following examples.

[0052] (Example) The sludge mixture was poured into a 250 mL beaker so that the MLSS concentration was 3000 mg / L. The sludge mixture was wastewater containing organic matter, including granular sludge with an average particle size of 473 μm.

[0053] The beaker was aerated for 3 hours with an aeration airflow rate (1L / min) that caused the biological sludge in the beaker to move, and then the aeration was stopped for 3 hours to allow the biological sludge in the beaker to settle. This aeration and aeration stop was repeated while the biological sludge was stored in the beaker for 10 days. After that, the biological sludge was collected from the beaker and a nitrification activity test and average particle size measurement were performed. The nitrification activity test was performed by measuring the rate of decrease in ammonia concentration in a batch test, and the average particle size was measured using a laser diffraction particle size distribution analyzer.

[0054] Comparative Example 1 The conditions were the same as in Example, except that the biological sludge was stored in the beaker for 10 days while constantly aerating the beaker with an aeration airflow rate (1 L / min) that caused the biological sludge in the beaker to flow. The biological sludge was then sampled from the beaker and subjected to a nitrification activity test and measurement of the average particle size in the same manner as in Example.

[0055] Comparative Example 2 The conditions were the same as in Example 1, except that the biological sludge was stored in the beaker for 10 days without aeration. The biological sludge was then sampled from the beaker, and the nitrification activity test and the average particle size measurement were carried out in the same manner as in Example 1.

[0056] FIG. 3 shows the results of the nitrification activity and the average particle size of the biological sludge in the nitrification activity test in the Example and Comparative Examples 1 and 2. FIG. 3 also shows the results of the biological sludge in the beaker before storage for 10 days as the initial value. In Comparative Example 1, the average particle size of the biological sludge was reduced by 26% from the initial value, and the nitrification activity was reduced by 49% from the initial value. In Comparative Example 2, the average particle size of the biological sludge was suppressed to a decrease of about 8% from the initial value, but the nitrification activity was reduced by 47% from the initial value. On the other hand, in the Example, the average particle size of the biological sludge was reduced by about 8% from the initial value, and the nitrification activity was suppressed to a decrease of 18% from the initial value, showing better results than Comparative Examples 1 and 2.

[0057] In Comparative Examples 1 and 2, when the pH in the beaker after 10 days of storage was checked, it was found to have dropped to 4. In Comparative Example 1, it is believed that the pH dropped because the biological sludge was broken down by constant aeration, ammonia was eluted, and the nitrification reaction proceeded. In Comparative Example 2, it is believed that the pH dropped because the inside of the beaker became anaerobic, and organic acids were generated as a result of acid fermentation proceeding. In any case, a drop in pH leads to a drop in biological activity, so it is believed that the nitrification activity was significantly reduced in Comparative Examples 1 and 2. On the other hand, in the Example, the pH in the beaker after 10 days of storage was 6, and was maintained in the neutral region, so it is believed that the nitrification activity was maintained higher than in the Comparative Example.

[0058] As described above, in the embodiment, even if no supply of treated water is expected for a certain period of time, it is possible to suppress deterioration of the properties of the granular sludge and a decrease in biological activity. Furthermore, in cases where a decrease in pH is caused, it is believed that the decrease in biological activity can be suppressed by increasing the pH. In addition, in cases where it is difficult to adjust the pH using chemicals such as alkaline agents in large-scale sewage treatment plants, the pH may be increased to about neutral by introducing biologically treated water that does not inhibit organisms. In this way, activity can be maintained at low cost without installing extra equipment.

[0059] [Note] (1) A water treatment method that repeatedly performs an operation cycle including an inflow process of inflowing water to be treated into a reaction tank, a biological treatment process of biologically treating the water to be treated in the reaction tank with biological sludge, a sedimentation process of sedimenting the biological sludge in the reaction tank, and a discharge process of discharging the biologically treated water in the reaction tank, This water treatment method is characterized in that, when the operating cycle is stopped and the inflow of treated water through the inflow process is stopped for a certain period of time, the reaction tank is repeatedly subjected to an aeration state in which the biological sludge in the reaction tank is aerated with an aeration air volume that causes the biological sludge in the reaction tank to flow, and a sedimentation state in which the biological sludge in the reaction tank is aerated with an aeration air volume that causes the biological sludge in the reaction tank to settle or aeration is stopped. (2) The water treatment method according to (1) above, wherein the aeration is stopped during the settling state. (3) The water treatment method according to (1) or (2) above, wherein the time for each of the aeration state and the settling state is in the range of not less than 1 hour and less than 24 hours, respectively. (4) The water treatment method according to any one of (1) to (3) above, characterized in that when the pH in the reaction tank becomes equal to or lower than a predetermined value during the aeration state, the aeration state is switched to the sedimentation state. (5) The water treatment method according to any one of (1) to (3) above, characterized in that when the pH in the reaction tank becomes 5 or less during the aeration state, the pH in the reaction tank is adjusted to 6 or more and 8 or less. (6) The water treatment method according to (5) above, characterized in that biologically treated water obtained by biologically treating water to be treated is supplied to the reaction tank in order to adjust the pH in the reaction tank to a range of 6 or more and 8 or less. (7) A water treatment system having a semi-batch treatment apparatus which repeats an operation cycle including an inflow process of inflowing water to be treated into the reaction tank, a biological treatment process of biologically treating the water to be treated in the reaction tank with biological sludge, a sedimentation process of sedimenting the biological sludge in the reaction tank, and a discharge process of discharging the biologically treated water in the reaction tank, The semi-batch treatment device is a water treatment system characterized in that, when the operating cycle is stopped and the inflow of water to be treated through the inflow process is stopped for a certain period of time, the semi-batch treatment device repeatedly switches between an aeration state in which the biological sludge in the reaction tank is aerated with an aeration air volume that causes the biological sludge in the reaction tank to flow, and a sedimentation state in which the biological sludge in the reaction tank is aerated with an aeration air volume that causes the biological sludge in the reaction tank to settle or aeration is stopped. [Explanation of symbols]

[0060] 1, 2 water treatment system, 10 semi-batch treatment device, 12 control device, 14 reaction tank, 18 supply piping, 20 pump, 22, 28, 33 valve, 26 treated water piping, 30 sludge discharge piping, 32 sludge discharge pump, 34 aeration device, 36 gas supply piping, 38 blower, 40 pH meter.

Claims

1. A water treatment method that repeatedly performs an operation cycle including an inflow process of inflowing water to be treated into a reaction tank, a biological treatment process of biologically treating the water to be treated in the reaction tank with biological sludge, a sedimentation process of sedimenting the biological sludge in the reaction tank, and a discharge process of discharging the biologically treated water in the reaction tank, A water treatment method characterized in that, when the operating cycle is stopped and the inflow of treated water through the inflow process is stopped for a certain period of time, the reaction tank is repeatedly subjected to an aeration state in which an aeration air volume is provided to cause at least a portion of the biological sludge in the reaction tank to flow, and a sedimentation state in which the biological sludge in the reaction tank is aerated at an aeration air volume to cause it to settle, or aeration is stopped.

2. 2. The water treatment method according to claim 1, wherein the aeration is stopped during the settling state.

3. The water treatment method according to claim 1 or 2, wherein the duration of each of the aeration state and the settling state is within a range of 1 hour or more and less than 24 hours.

4. 3. The water treatment method according to claim 1, wherein when the pH in the reaction tank becomes equal to or lower than a predetermined value during the aeration state, the reaction tank transitions from the aeration state to the sedimentation state.

5. 3. The water treatment method according to claim 1, wherein when the pH in the reaction tank becomes 5 or less during the aeration state, the pH in the reaction tank is adjusted to 6 or more and 8 or less.

6. 6. The water treatment method according to claim 5, wherein biologically treated water obtained by biologically treating water to be treated is supplied to the reaction tank in order to adjust the pH in the reaction tank to 6 or more and 8 or less.

7. A water treatment system having a semi-batch treatment apparatus which repeats an operation cycle including an inflow process of inflowing water to be treated into the reaction tank, a biological treatment process of biologically treating the water to be treated in the reaction tank with biological sludge, a sedimentation process of sedimenting the biological sludge in the reaction tank, and a discharge process of discharging the biologically treated water in the reaction tank, The semi-batch treatment device is a water treatment system characterized in that, when the operating cycle is stopped and the inflow of treated water through the inflow process is stopped for a certain period of time, the semi-batch treatment device repeatedly performs an aeration state in which the reaction tank is aerated with an aeration air volume that causes at least a portion of the biological sludge in the reaction tank to flow, and aeration with an aeration air volume that causes the biological sludge in the reaction tank to settle or a settling state in which aeration is stopped.

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