Water treatment method and water treatment apparatus

Abstract

To provide a water treatment method and a water treatment apparatus capable of efficiently adjusting sludge used for nitrification and denitrification treatment to a state suitable for nitrite type nitrification and denitrification treatment by a simple method.SOLUTION: A water treatment method for biologically treating raw water containing a nitrite component includes a nitrite formation step of oxidizing ammoniacal nitrogen in the raw water to nitrite nitrogen by sludge containing ammonia-oxidizing bacteria, a nitrite treatment step of extracting a part of the sludge in the nitrite formation step and bringing the extracted sludge into contact with a nitrite aqueous solution containing nitrite to suppress activity and growth of the nitrite-oxidizing bacteria contained in the sludge, and a return step of returning the sludge after the nitrite treatment step to the nitrite formation step. In the water treatment method, the water quality in the nitrous acid treatment tank 10 is controlled based on the water quality measurement result by the water quality measuring means 21.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a water treatment method and a water treatment device. [Background technology]

[0002] Biological nitrification denitrification is used as a method for treating raw water containing nitrogen components. In this method, ammonia-oxidizing bacteria oxidize ammonia nitrogen (NH4-N) in the raw water to nitrite nitrogen (NO2-N) in an aerobic nitrification tank, and nitrite-oxidizing bacteria further oxidize NO2-N to nitrate nitrogen (NO3-N). Next, the treated water from the nitrification tank is supplied to an anaerobic denitrification tank, where heterotrophic denitrifying bacteria reduce the NO3-N in the treated water to nitrogen gas. In the denitrification tank, organic matter is used as an electron donor.

[0003] However, this conventional nitrification denitrification method (hereafter referred to as complete nitrification denitrification) requires the oxidation of NH4-N to NO2-N, which is then further oxidized to NO3-N. This requires a large amount of oxygen, and a large amount of power is required for aeration to supply the oxygen. Furthermore, during denitrification, NO3-N is reduced to NO2-N, which is then reduced to nitrogen gas, requiring a large amount of organic matter (electron donor). In particular, when the amount of organic matter in the raw water is low, it is necessary to add organic matter such as methanol, which increases treatment costs.

[0004] Nitrite-type nitrification denitrification methods (hereafter referred to as nitrite-type nitrification denitrification) are also known. In nitrite-type nitrification denitrification, NH4-N in the raw water is oxidized to NO2-N by ammonia-oxidizing bacteria in an aerobic nitritation tank, but NO2-N is not oxidized to NO3-N. The treated water from the nitritation tank is supplied to an anaerobic denitrification tank, where the NO2-N in the treated water from the nitritation tank is reduced to nitrogen gas by denitrifying bacteria, which are heterotrophic bacteria.

[0005] Nitrite-type nitrification denitrification simply oxidizes NH4-N to NO2-N, so it requires less oxygen than complete nitrification denitrification. This has the advantage of reducing the aeration power required to supply oxygen. Furthermore, nitrite-type nitrification denitrification simply reduces NO2-N to N2 gas, so it requires less organic matter (electron donor) during denitrification than complete nitrification denitrification. If there is little organic matter in the raw water, it also has the advantage of reducing the cost of organic matter such as methanol.

[0006] As a technology for performing nitrite-type nitrification denitrification, for example, Patent Document 1 describes a biological treatment method for biologically treating nitrogen components contained in organic wastewater, in which ammonia nitrogen contained in the organic wastewater is subjected to nitrite-type nitrification denitrification treatment in a first reaction tank using ammonia-oxidizing bacteria contained in activated sludge, the activated sludge is supplied from the first reaction tank to a sterilization tank, the nitrite-oxidizing bacteria contained in the activated sludge supplied from the first reaction tank are sterilized with nitrite nitrogen while maintaining the pH in the sterilization tank at or below neutral, and the sterilized activated sludge is supplied to the first reaction tank. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2023 / 095399 Summary of the Invention [Problem to be solved by the invention]

[0008] However, in the invention described in Patent Document 1, activated sludge supplied from a reaction tank is exposed to a high concentration of nitrite of about 1000 mgN / L for a long period of time, such as five days or more, to kill nitrite-oxidizing bacteria. This method kills not only nitrite-oxidizing bacteria but also ammonia-oxidizing bacteria, which may result in the inability to stably maintain the activity and proliferation of ammonia-oxidizing bacteria. As a result, it may be difficult to maintain appropriate and stable production of nitrite nitrogen in the nitritation process.

[0009] Furthermore, the invention described in Patent Document 1 uses nitrous acid as a chemical to kill nitrite-oxidizing bacteria, but because nitrite is unstable and easily decomposes, it may not be possible to achieve a significant effect in killing nitrite-oxidizing bacteria. If a high concentration of nitrous acid is used, as in the invention described in Patent Document 1, it is conceivable that the impact of nitrite decomposition would be small. However, the use of high concentrations of nitrous acid increases the cost of the chemical. Because nitrite is designated as a deleterious substance, it must be handled with care, and waste liquid treatment is required after use. Therefore, waste liquid treatment is also problematic in that it is time-consuming.

[0010] In view of the above problems, the present invention provides a water treatment method and a water treatment device that can efficiently adjust sludge used in nitrification-denitrification treatment to a state suitable for nitrite-type nitrification-denitrification treatment using a simple method. [Means for solving the problem]

[0011] The present inventors conducted extensive research to solve the above problems and found that treating sludge with nitrite is a suitable method for creating an environment suitable for nitrite-based nitrification and denitrification by suppressing the activity of nitrite-oxidizing bacteria while suppressing the decrease in the activity of ammonia-oxidizing bacteria due to nitrite treatment. However, because nitrite aqueous solution is rapidly decomposed in the nitrite treatment tank, it was found that using a low-concentration nitrite aqueous solution reduces the free nitrite concentration in the nitrite treatment tank by about 70% in 24 hours, for example. Therefore, the present inventors found that strict control of specific conditions in the nitrite treatment tank is important for efficiently performing nitrite treatment of sludge used in nitrification and denitrification treatment using a simple method, and this finding led to the present invention.

[0012] In order to solve the above-mentioned problems, in one aspect, the present invention is a water treatment method for biologically treating raw water containing nitrogen components, comprising: a nitritation step in which ammoniacal nitrogen in the raw water is oxidized to nitrite nitrogen using sludge containing ammonia-oxidizing bacteria; a nitrite treatment step in which a portion of the sludge from the nitritation step is extracted and brought into contact with an aqueous nitrite solution containing nitrite, thereby suppressing the activity and proliferation of nitrite-oxidizing bacteria contained in the sludge; and a return step in which the sludge after the nitrite treatment step is returned to the nitritation step, wherein water quality measurement means including a nitrite concentration meter for measuring nitrite concentration and a water thermometer for measuring water temperature is placed in a nitrite treatment tank in which the nitrite treatment step is carried out, and the water quality in the nitrite treatment tank is controlled based on the water quality measurement results obtained by the water quality measurement means.

[0013] In one embodiment of the water treatment method according to the present invention, the nitrite concentration measuring meter includes any one of a nitrite sensor, an ORP meter, and a conductivity meter.

[0014] In another embodiment of the water treatment method according to the present invention, the water quality measuring means further includes a pH meter for measuring pH.

[0015] In yet another embodiment of the water treatment method of the present invention, the water quality measurement means further includes a dissolved oxygen concentration meter for measuring the dissolved oxygen concentration, and includes controlling the dissolved oxygen concentration of the mixed liquid in the nitrite treatment tank to be less than 0.5 mg / L based on the measurement results of the dissolved oxygen concentration meter.

[0016] In yet another embodiment of the water treatment method according to the present invention, the sludge containing ammonia oxidizing bacteria comprises a bioattached carrier in which the sludge is attached to a carrier.

[0017] In yet another embodiment, the water treatment method according to the present invention further comprises subjecting the sludge after the nitrite treatment step to solid-liquid separation before returning the sludge to the nitritation step, and returning the separated liquid obtained by the solid-liquid separation to the treatment tank of the nitrite treatment step.

[0018] In yet another embodiment of the water treatment method according to the present invention, the water quality measuring means measures the quality of treated water from the nitritation step continuously or every 24 hours or less.

[0019] In yet another embodiment of the water treatment method of the present invention, the water quality measurement means further includes an alkalinity measurement device that measures the alkalinity of the water to be treated in the nitrite treatment tank, and when the alkalinity in the nitrite treatment tank becomes equal to or higher than a standard value, the nitrite aqueous solution in the nitrite treatment tank is replaced with a new nitrite aqueous solution.

[0020] In yet another embodiment, the water treatment method of the present invention includes monitoring the water quality control results in the nitrite treatment tank, and if the water quality control results deviate from a predetermined target value, changing the treatment conditions of the nitritation step or issuing an alarm about an abnormality in the nitritation step.

[0021] In yet another embodiment of the water treatment method according to the present invention, at least one of the supply of a pH adjuster to the nitrite treatment tank, the supply of an aqueous nitrite solution, and water temperature adjustment is controlled based on the results of water quality measurement so that the pH of the treatment tank in the nitrite treatment step is less than 6.0, the nitrite nitrogen concentration is less than 250 mg-N / L, and the water temperature is 20 to 28°C, and the sludge in the treatment tank is returned to the nitritation step within 48 hours after the extracted sludge is introduced into the treatment tank.

[0022] In another aspect, the present invention provides a water treatment device comprising: a nitrification tank in which ammonia nitrogen in raw water containing nitrogen components is oxidized to nitrite nitrogen using sludge containing ammonia-oxidizing bacteria; a nitrite treatment tank in which a portion of the sludge from the nitrification tank is extracted and brought into contact with an aqueous nitrite solution containing nitrite to suppress the activity and proliferation of nitrite-oxidizing bacteria contained in the sludge; return means for returning the sludge treated in the nitrite treatment tank to the nitrification tank; water quality measurement means including a nitrite concentration meter for measuring the nitrite concentration in the nitrite treatment tank and a water thermometer for measuring water temperature; and water quality control means for controlling the water quality in the nitrite treatment tank by controlling at least one of the supply of a pH adjuster to the nitrite treatment tank, the supply of an aqueous nitrite solution, and water temperature adjustment to the nitrite treatment tank based on the water quality measurement results obtained by the water quality measurement means. [Effects of the Invention]

[0023] According to the present invention, a water treatment method and a water treatment device can be provided that can efficiently adjust sludge used in the nitrification-denitrification treatment of raw water to a state suitable for nitrite-type nitrification-denitrification treatment using a simple method. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a schematic diagram illustrating an example of a water treatment device according to an embodiment of the present invention. [Figure 2] FIG. 10 is a schematic diagram illustrating a modified example of a water treatment device according to an embodiment of the present invention. [Figure 3] 1 is a schematic diagram illustrating a water treatment system according to an embodiment of the present invention. [Figure 4] FIG. 10 is a schematic diagram illustrating a modified example of a water treatment system according to an embodiment of the present invention. [Figure 5] FIG. 10 is a schematic diagram illustrating another modified example of the water treatment system according to the embodiment of the present invention. [Figure 6] 1 is a schematic diagram showing a test device using a water treatment method according to an embodiment of the present invention. [Figure 7] 7 is a graph showing the change over time in the concentration of free nitrite in the nitrite treatment tank provided in the testing device of FIG. 6. [Figure 8] 7 is a graph showing the change over time in the concentration of free nitrite in the nitrite treatment tank during continuous operation using the test apparatus of FIG. 6. [Figure 9] 7 is a graph showing the change over time in the nitrite conversion rate in the nitrification tank during continuous operation using the test apparatus of FIG. 6. [Figure 10] Figure 10(a) is a graph showing the change in the nitrification rate of ammonia oxidizing bacteria and nitrite oxidizing bacteria in the nitrification tank when the nitrite concentration of the water to be treated in the nitrite treatment tank is controlled at 100 to 250 mg / L, and Figure 10(b) is a graph showing the nitrite concentration in the nitrite treatment tank and the activity of ammonia oxidizing bacteria and nitrite oxidizing bacteria in the nitrite treatment tank during the nitrite treatment process. DETAILED DESCRIPTION OF THE INVENTION

[0025] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following description of the drawings, identical or similar parts are designated by identical or similar reference numerals. Note that the embodiments shown below are merely examples of devices and methods for embodying the technical concept of the present invention, and the technical concept of the present invention does not limit the structure, arrangement, etc. of component parts to those described below.

[0026] (Water treatment equipment) 1 is a schematic diagram illustrating an example of a water treatment apparatus W capable of carrying out a water treatment method according to an embodiment of the present invention. The water treatment apparatus W according to the embodiment of the present invention includes a nitrification tank 6 in which ammonia nitrogen in raw water containing nitrogen components is oxidized to nitrite nitrogen using sludge containing ammonia-oxidizing bacteria, a nitrite treatment tank 10 in which a portion of the sludge in the nitrification tank 6 is extracted and contacted with an aqueous nitrite solution containing nitrite to suppress the activity and proliferation of the nitrite-oxidizing bacteria contained in the sludge, a return means L2 for returning the sludge treated in the nitrite treatment tank 10 to the nitrification tank 6, a water quality measurement means 21 including a nitrite concentration meter for measuring the nitrite concentration in the nitrite treatment tank 10 and a water thermometer for measuring the water temperature, and a water quality control means 22 for controlling the water quality in the nitrite treatment tank 10 during the nitritation treatment step by controlling at least one of the supply of a pH adjuster, the supply of an aqueous nitrite solution, and water temperature adjustment to the nitrite treatment tank 10 based on the water quality measurement results obtained by the water quality measurement means 21.

[0027] The water quality control means 22 is connected to chemical supply means 23 for supplying chemicals such as a pH adjuster and an aqueous nitrite solution to the nitrite treatment tank 10 to adjust the water quality in the nitrite treatment tank 10. Sludge is extracted from the nitrification tank 6 to the nitrite treatment tank 10 by extraction means L1 connected between the nitrification tank 6 and the nitrite treatment tank 10.

[0028] The raw water is not particularly limited as long as it contains nitrogen components. For example, sewage, leachate, dehydrated separated liquid of anaerobic digestion sludge of sewage sludge, wastewater containing at least human waste or septic tank sludge, concentrated separated liquid of human waste or septic tank sludge, wastewater generated in food factories and various factories, wastewater containing organic waste, dehydrated separated liquid of anaerobic digestion sludge of organic waste, etc. can be used as raw water. Furthermore, wastewater containing at least human waste or septic tank sludge may also contain sludge generated in agricultural wastewater treatment. In this embodiment, it is particularly preferable to use wastewater containing at least human waste or septic tank sludge as raw water. For example, raw water can have a BOD of 360 to 3,000 mg / L, preferably 500 to 2,000 mg / L, an ammonia nitrogen concentration (NH4-N) of 100 to 1,200 mg / L, preferably 300 to 600 mg / L, an M alkalinity of 300 to 3,000 mg / L, preferably 400 to 2,000 mg / L, a BOD / NH4-N ratio of 0.5 to 5.0, preferably 1.0 to 3.0, a phosphate concentration of 0.5 to 100 mg / L, preferably 1 to 50 mg / L, and an M alkalinity / NH4-N ratio of 3.0 to 10.0, preferably 3.5 to 8.0. In particular, raw water with a BOD / NH4-N ratio below 3.0 is suitable for this method because, when a conventional complete nitrification-type nitrification denitrification method is used, denitrification cannot be completed by the BOD alone in the raw water, and external organic matter such as methanol must be added.

[0029] The ratio (volume ratio) of human waste to septic tank sludge is: 3 In contrast, septic tank sludge is 1 to 99 m 3 , preferably 2 to 50 m 3This method can treat wastewater containing septic tank sludge in a stable and efficient manner. Generally, a higher ratio of septic tank sludge to human waste tends to lower the BOD / NH4-N ratio of the raw water for biological treatment, making it suitable for this method. Examples of such raw water include wastewater with a BOD of 360 to 2,000 mg / L, preferably 500 to 1,300 mg / L, an ammonia nitrogen concentration (NH4-N) of 200 to 800 mg / L, preferably 300 to 500 mg / L, an M alkalinity of 600 to 2,000 mg / L, preferably 900 to 1,500 mg / L, a BOD / NH4-N ratio of 1.5 to 4.0, preferably 1.8 to 3, a phosphate concentration of 10 to 30 mg / L, preferably 15 to 20 mg / L, and an M alkalinity / NH4-N ratio of 3.0 to 6.0, preferably 3.5 to 5.0. In particular, raw water with a BOD / NH4-N ratio below 3.0 is suitable for this method because, when a conventional complete nitrification-type nitrification denitrification method is used, denitrification cannot be completed with the BOD in the raw water alone, and external organic matter such as methanol must be added.

[0030] Sludge containing ammonia-oxidizing bacteria is maintained in the nitrification tank 6. Examples of sludge containing ammonia-oxidizing bacteria include activated sludge, self-granulated granular sludge, and bioattached carriers in which sludge containing ammonia-oxidizing bacteria is attached to a carrier. Examples of bioattached carriers include entrapping immobilization carriers in which ammonia-oxidizing bacteria and nitrite-oxidizing bacteria are entrappingly immobilized in resin or the like, and attached immobilization carriers in which a biofilm containing ammonia-oxidizing bacteria and nitrite-oxidizing bacteria is attached to the surface of a carrier of various shapes and materials. However, the use of attached immobilization carriers is preferred for the dominant and stable retention of specific microorganisms. Plastics (polyethylene, polypropylene, polyurethane, polyethylene glycol, polyvinyl alcohol, etc.) can be used as materials for the attached immobilization carriers. The attached immobilization carriers can be shaped like sponges, gels, cylinders, honeycombs, nets, spheres, rectangles, etc. Activated sludge, granular sludge, and bioattached carriers can be used in combination or individually. For example, by adding a small amount of activated sludge to the nitrification tank 6 in addition to the organism attachment carriers, more stable biological treatment can be achieved.

[0031] The nitrification tank 6 is provided with agitation means (not shown). The agitation means may be a mechanical agitator equipped with agitation blades or the like. Instead of a mechanical agitator, a gas supply means for supplying gas into the nitrification tank 6 may be provided at the bottom of the nitrification tank 6, and the water to be treated in the nitrification tank 6 may be agitated by the gas supplied from the gas supply means. The agitation means does not need to be driven constantly, and intermittent agitation may be performed.

[0032] Nitrite-type nitrification treatment is preferably carried out in the nitrification tank 6, in which ammonia nitrogen in the raw water is oxidized to nitrite nitrogen by sludge containing ammonia-oxidizing bacteria. By carrying out nitrite-type nitrification treatment in the nitrification tank 6, the concentration of nitrite nitrogen in the nitrification tank 6 increases. By subjecting the treated water (nitrified liquid) in the nitrification tank 6, which contains a large amount of nitrite nitrogen, to denitrification treatment in the denitrification tank 5, the amount of electron donor such as methanol required for denitrification treatment can be reduced, thereby improving treatment efficiency.

[0033] If the DO in the nitrification tank 6 becomes too high, the proliferation and activity of nitritizing bacteria in the sludge will increase, making it difficult to maintain stable nitrite-type nitrification and denitrification. When activated sludge is used, the DO in the nitrification tank 6 is preferably adjusted to 2.0 mg / L or less, and more preferably 1.0 mg / L or less. On the other hand, when attached immobilization carriers are used, the biofilm is thicker than activated sludge, and the DO inside the biofilm is lower due to the rate-limiting effect of substrate diffusion. Therefore, the dissolved oxygen concentration (DO) in the nitrification tank 6 is preferably adjusted to 5.0 mg / L or less, more preferably 4.0 mg / L or less, and even more preferably 3.0 mg / L or less. As mentioned above, the DO in the nitrification tank 6 does not need to be strictly controlled, but the lower the DO, the more nitrite-type nitrification and denitrification can proceed predominantly in the nitrification tank 6. Therefore, even when attached immobilization carriers are used, the DO in the nitrification tank 6 may be adjusted to 1.0 mg / L or less, or even 0.5 mg / L or less.

[0034] Although not limited to the following, for example, in the nitrification tank 6, the nitritation treatment is preferably carried out under the following conditions: nitrogen load 0.01 to 0.1 kg / kg-MLSS / day, pH 6.0 to 9.0, preferably 6.3 to 8.0, and more preferably 6.5 to 7.5, MLSS 1,000 to 10,000 mg / L, preferably 2,000 to 8,000 mg / L, and more preferably 3,000 to 6,000 mg / L, retention time (HRT) 0.5 to 3 days, preferably 1 to 2 days, and dissolved oxygen concentration (DO) 0.1 to 5.0 mg / L. Furthermore, particularly when a carrier is used, the nitrogen volume load is, for example, 0.3 to 0.4 kg / m 3 days, the pH is set to 6.0 to 8.0, preferably 7.0 to 8.0, and more preferably 7.0 to 7.5, and the ammonia oxidation rate is set to 0.4 to 0.5 kg / m 3 -day, nitrite oxidation rate 0.1-0.2 kg / m 3 It is preferable to carry out the nitritation treatment so that the dissolved oxygen concentration (DO) is 5.0 mg / L or less, typically 2.0 to 3.0 mg / L. The ammonia oxidation rate and nitrite oxidation rate can be evaluated by a batch oxidation rate measurement test in which the carrier in the nitrification tank 6 is placed in a specified container and the activity of the bacteria attached to the carrier is tested. The batch oxidation rate can be measured in accordance with the "Nitrification Rate Test Method" described in Chapter 2, Section 2 of the Sewage Testing Methods.

[0035] The extraction means L1 is connected to the nitrification tank 6. The sludge extracted by the extraction means L1 is subjected to nitrite treatment in the nitrite treatment tank 10, and the sludge after the nitrite treatment is returned to the nitrification tank 6 via the return means L2. As shown in FIG. 2 , the extraction means L1 is equipped with lines L11 and L12, and a solid-liquid separator 9 is preferably disposed between the nitrification tank 6 and the nitrite treatment tank 10. The lines L11 and L12 are each composed of a pump, piping, etc. The extraction means L1 is preferably configured to supply the sludge treated in the nitrification tank 6 to the solid-liquid separator 9 via line L11, and to supply the sludge separated into solid and liquid form in the solid-liquid separator 9 to the nitrite treatment tank 10 via line L12. The solid-liquid separation of the sludge in the solid-liquid separator 9 reduces the amount of digested liquid adhering to the sludge that enters the nitrite treatment tank 10, thereby facilitating the adjustment of the concentration of the water to be treated in the nitrite treatment tank 10. The separated liquid (nitrified liquid) obtained by solid-liquid separation in the solid-liquid separator 9 is returned to the nitrification tank 6 via a line L31 that serves as a separated liquid returning means.

[0036] The extraction means L1 preferably extracts 10 to 50 wt% of the sludge from the nitritation step and brings the extracted sludge into contact with the aqueous nitrite solution in the nitrification tank 6. The amount of sludge extracted is more preferably 10 to 45 wt%, even more preferably 15 to 35 wt%, and even more preferably 15 to 30 wt%. The amount of sludge extracted by the extraction means L1 can be adjusted appropriately depending on the type of sludge in the nitrification tank 6 and the treatment status in the nitrification tank 6.

[0037] The specific configuration of the solid-liquid separator 9 is not particularly limited. For example, when performing solid-liquid separation of activated sludge or the like, a mechanical thickener that mechanically thickens the sludge can be suitably used. For example, such a mechanical thickener can be one that rotates multiple elliptical rotating plates to transport the sludge while discharging separated liquid through gaps between the rotating plates, thereby making it possible to reduce the size of the entire apparatus and improve the efficiency of processing. Furthermore, when performing solid-liquid separation of bioattached carriers, a screw conveyor equipped with a screen for capturing the bioattached carriers can be suitably used.

[0038] The nitrite treatment tank 10 is a treatment tank (FNA treatment tank) into which the sludge extracted by the extraction means L1 is introduced and into which the sludge is brought into contact with an aqueous nitrite solution. The aqueous nitrite solution supplied to the nitrite treatment tank 10 can be an aqueous solution in which a nitrite salt, such as sodium nitrite, is dissolved in water. By contacting the sludge with the aqueous nitrite solution, some of the nitrite-oxidizing bacteria in the sludge are killed, creating an environment in which ammonia-oxidizing bacteria are predominantly active in the sludge. By returning this sludge containing an environment in which ammonia-oxidizing bacteria are predominantly active to the nitrification tank 6 and subjecting it to nitritation treatment, nitrite-type nitrification and denitrification treatment can be promoted in the nitrification tank 6.

[0039] Nitrite used in nitrite treatment is unstable and easily decomposed. Therefore, in order to properly and stably maintain nitrite-type nitrification treatment in nitrification tank 6, it is necessary to properly and strictly control the nitrite treatment process in nitrite treatment tank 10. This embodiment is equipped with water quality measurement means 21 that measures the water quality in nitrite treatment tank 10, and water quality control means 22 that controls the water quality in the nitrite treatment tank based on the measurement results of water quality measurement means 21.

[0040] The water quality measuring means 21 includes at least a nitrite concentration measuring meter for measuring the nitrite concentration in the nitrite treatment tank 10 and a water thermometer for measuring the water temperature. The water thermometer is not particularly limited as long as it is an instrument that can measure the water temperature of the water to be treated in the nitrite treatment tank 10.

[0041] The nitrite concentration measuring meter preferably includes at least one of a commonly available nitrite sensor, an ORP meter, and a conductivity meter that can measure the nitrate concentration or nitrite concentration in the nitrite treatment tank 10. Because ORP meters and conductivity meters are less expensive than nitrite sensors, using these ORP meters and conductivity meters instead of nitrite sensors makes it possible to obtain an economical water treatment device W.

[0042] In the nitrite treatment tank 10, in the presence of dissolved oxygen, free nitrite is oxidized to nitric acid according to the following reaction formula (1). HNO2 - +H2O→NO3-+3H+ +2e - ···(1) The ORP and (Eh) in the nitrite treatment tank 10 are expressed by the following Nernst equation (2).

[0043]

number

[0044] When measuring the free nitrite concentration using a conductivity meter, for example, a relational expression between the measured value of the free nitrite concentration in the nitrite treatment tank 10 and the change in conductivity due to that measurement is measured in advance. Then, the change in the conductivity of the water to be treated in the nitrite treatment tank 10 is measured with the conductivity meter, and the measurement result is compared with the previously measured relational expression, thereby measuring the nitrite concentration in the nitrite treatment tank 10.

[0045] If the pH of the water to be treated in the nitrite treatment tank 10 becomes 6.0 or higher, the effect of suppressing the activity and proliferation of nitrite-producing bacteria attached to the sludge may not be sufficiently obtained. Therefore, it is preferable that the water quality measurement means 21 further includes a pH meter for measuring the pH of the water to be treated in the nitrite treatment tank 10.

[0046] The water quality control means 22 controls the pH of the water to be treated in the nitrite treatment tank 10 to less than 6.0, preferably 5.8 or less, more preferably 5.6 or less, and even more preferably 5.5 or less. There is no particular lower limit for the pH of the water to be treated in the nitrite treatment tank 10; however, if the pH falls below 4.0, the activity and proliferation of ammonia-oxidizing bacteria contained in the sludge may be inhibited if the sludge is in contact with the nitrite aqueous solution for an extended period of time. The water quality control means 22 controls the pH of the water to be treated in the nitrite treatment tank 10 to preferably 4.0 or higher, more preferably 4.2 or higher, and even more preferably 4.5 or higher. The pH of the water to be treated in the nitrite treatment tank 10 can be adjusted by supplying inorganic acids such as sulfuric acid and hydrochloric acid, or organic acids such as acetic acid and citric acid, into the nitrite treatment tank 10 as a pH adjuster according to the pH meter reading. Note that inorganic acids are preferable because organic acids may promote denitrification in the nitrite treatment tank and reduce the nitrite concentration in the tank. The waste liquid containing sulfuric acid discharged from a deodorizing device can also be used as a pH adjuster.

[0047] In the nitrite treatment tank 10, as the dissolved oxygen concentration in the water being treated increases, the activity of nitrite-producing bacteria in the sludge increases, reducing the effect of suppressing the activity and proliferation of nitrite bacteria due to contact between the sludge and the nitrite aqueous solution. As a result, it may become difficult to stably maintain the nitrite-based nitrification treatment in the nitrification tank 6. The water quality measurement means 21 preferably further includes a dissolved oxygen concentration meter for measuring the dissolved oxygen concentration. Furthermore, the water quality control means 22 preferably controls the dissolved oxygen concentration (DO) of the mixed liquor in the nitrite treatment tank 10 based on the measurement results of the dissolved oxygen concentration meter, for example, by adjusting the aeration air volume so that the DO of the mixed liquor in the nitrite treatment tank 10 is less than 0.5 mg / L. Adjusting the aeration air volume, for example, can be achieved by performing intermittent stirring, in which stirring in the nitrite treatment tank 10 is stopped when the DO in the nitrite treatment tank 10 reaches 0.5 mg / L or higher. This allows for both suppression of nitrification in the nitrite treatment tank 10 and reduction of stirring power.

[0048] It is more preferable that the water quality control means 22 controls the dissolved oxygen concentration of the water to be treated in the nitrite treatment tank 10 to be less than 0.5 mg / L, even more preferably 0.3 mg / L or less, and even more preferably 0.1 mg / L or less. By more strictly controlling the DO in the nitrite treatment tank 10 so that the DO in the nitrite treatment tank 10 is low, the properties of the sludge can be adjusted so that an efficient nitrite-type nitrification and denitrification process can be carried out in the nitrification tank 6.

[0049] It is preferable that the water quality control means 22 controls the pH, nitrite concentration, free nitrite concentration, water temperature, and sludge retention time in the nitrite treatment tank 10 based on the measurement values ​​of the water quality measurement means 21 so that conditions in the nitrite treatment tank 10 are suitable for nitrite treatment of the sludge.

[0050] If the nitrite nitrogen concentration of the water to be treated in the nitrite treatment tank 10 is too high, the activity and proliferation of not only nitrite-oxidizing bacteria but also ammonia-oxidizing bacteria necessary for nitrite-type nitrification treatment will be suppressed, which may make it difficult to perform appropriate and stable nitrite-type nitrification treatment in the nitrification tank 6. The water quality control means 22 controls the nitrite nitrogen concentration of the water to be treated in the nitrite treatment tank 10 to 300 mg-N / L or less, preferably 250 mg-N / L or less, more preferably less than 250 mg-N / L, even more preferably 240 mg-N / L or less, still more preferably 210 mg-N / L or less, and even more preferably 150 mg-L or less. On the other hand, if the nitrite nitrogen concentration of the water to be treated in the nitrite treatment tank 10 is too low, the effect of suppressing the activity and proliferation of nitrite-oxidizing bacteria may not be significantly obtained. The water quality control means 22 preferably controls the nitrite nitrogen concentration of the water to be treated in the nitrite treatment tank 10 to 30 mg-N / L or more, more preferably 50 mg-N / L or more, and even more preferably 100 mg-N / L or more.

[0051] It is more preferable to adjust the free nitrite concentration of the water to be treated to fall within an appropriate range inside the nitrite treatment tank 10. The free nitrite concentration of the water to be treated in the nitrite treatment tank 10 can be calculated using the following equation (3).

[0052]

number

[0053] The water quality control means 22 preferably controls the free nitrite concentration of the water to be treated in the nitrite treatment tank 10 to 5.0 mg-N / L or less, preferably 4.0 mg-N / L or less, more preferably 3.0 mg-N / L or less, even more preferably 1.20 mg-N / L or less, further preferably 1.15 mg-N / L or less, and even more preferably 1.10 mg-N / L or less. The water quality control means 22 preferably controls the free nitrite concentration to a lower limit of 0.70 mg-N / L or more, more preferably 0.77 mg-N / L or more, even more preferably 0.80 mg-N / L or more, and even more preferably 1.0 mg-N / L or more. By adjusting the free nitrite concentration of the water to be treated in the nitrite treatment tank 10 within an appropriate range, sludge in which ammonia-oxidizing bacteria are more prevalent than nitrite-oxidizing bacteria can be more efficiently produced in the nitrite treatment tank 10.

[0054] If the retention time of sludge in the nitrite treatment tank 10 is too long, the activity and proliferation of ammonia-oxidizing bacteria necessary for nitrite-type nitrification treatment will be suppressed, making it difficult to perform appropriate and stable nitrite-type nitrification treatment in the nitrification tank 6. Furthermore, if the retention time of sludge in the nitrite treatment tank 10 is too long, the volume required for treatment in the nitrite treatment tank 10 will increase, resulting in an increase in the size of the apparatus. The water quality control means 22 controls the retention time of sludge in the nitrite treatment tank 10 to 48 hours or less, preferably 24 hours or less, more preferably 12 hours or less, and even more preferably 8 hours or less. The water quality control means 22 preferably controls the retention time of sludge in the nitrite treatment tank 10 to 3 hours or more, more preferably 5 hours or more, and even more preferably 6 hours or more. Typically, in this embodiment, the retention time of sludge in the nitrite treatment tank 10 can be approximately 12 to 24 hours.

[0055] Since the activity of nitrite-oxidizing bacteria decreases when heated, heating the aqueous nitrite solution has the effect of selectively suppressing the activity and proliferation of nitrite-oxidizing bacteria while maintaining the activity and proliferation of ammonia-oxidizing bacteria in a dominant position. The temperature of the water to be treated in the nitrite treatment tank 10 is preferably maintained at 20 to 28°C, more preferably at 23 to 27°C, and even more preferably at 24 to 26°C. The temperature of the water to be treated in the nitrite treatment tank 10 can be adjusted, for example, by wrapping a heater around line L32, which functions as a separated liquid return means for returning the separated liquid returned from the solid-liquid separator 9 back into the nitrite treatment tank 10, to heat the separated liquid.

[0056] In the nitrite treatment tank 10, moderate agitation is desirable to maintain contact between the sludge surface and the nitrite aqueous solution. When using a bio-adhered carrier with microorganisms attached to the carrier as the sludge, contaminants in the water are not removed as in the conventional activated sludge process. Therefore, weaker agitation than in nitritation treatment using activated sludge is sufficient; a slight flow of the carrier, or even a flow of the water without the carrier flow, is acceptable and is also desirable from the perspective of power saving. Any agitation method, such as mechanical aeration, air aeration, or gas agitation, is acceptable. In particular, in the nitrite treatment tank 10, it is preferable to circulate the gas in the nitrite treatment tank 10 by gas agitation in order to recover NO gas generated in the nitrite treatment step and to suppress an increase in DO and the progression of nitrification.

[0057] As shown in Figure 1, return means L2 is connected between nitrite treatment tank 10 and nitrification tank 6. As shown in Figure 2, return means L2 preferably includes lines L21 and L22. Lines L21 and L22 are each composed of a pump, piping, etc. Return means L2 is configured to supply sludge treated in nitrite treatment tank 10 to solid-liquid separator 9 via line L21, and to supply sludge that has been solid-liquid separated in solid-liquid separator 9 to nitrification tank 6 via line L22. The separated liquid (nitrification liquid) obtained by solid-liquid separation in solid-liquid separator 9 is returned to nitrite treatment tank 10 via line L32.

[0058] The water treatment device W further includes a denitrification tank 5 connected to a nitrification tank 6, in which nitrite nitrogen contained in the treated water from the nitrification tank 6 is reduced to nitrogen by denitrifying bacteria. A portion of the treated water in the nitrification tank 6 is circulated to the denitrification tank 5 as a circulating liquid via a circulation line L4. The denitrification tank 5 receives raw water and treated water from the nitrification tank 6, and oxidizes the nitrite nitrogen in the raw water or treated water to nitrogen in the presence of the denitrifying bacteria. The nitrification tank 6 is further connected to a settling tank 8. The treated water that has been nitritized in the nitrification tank 6 is supplied to the settling tank 8, where solid-liquid separation is carried out to separate the treated water from excess sludge.

[0059] In water treatment equipment W according to an embodiment of the present invention, water quality measurement means 21 measures the quality of the water to be treated in nitrite treatment tank 10, and water quality control means 22 controls the environment in nitrite treatment tank 10 based on the measurement results of water quality measurement means 21 so that the environment in nitrite treatment tank 10 is suitable for nitrite treatment of sludge, and therefore sludge in which the activity and proliferation of ammonia oxidizing bacteria are dominant over nitrite bacteria is produced in nitrite treatment tank 10. As a result, water treatment equipment W according to an embodiment of the present invention makes it possible to efficiently adjust sludge to a state suitable for nitrite-type nitrification and denitrification treatment using a simple method.

[0060] (Control by water quality control means 22) An example of a method for controlling the environment in the nitrite treatment tank 10 using the water quality control means 22 according to the embodiment of the present invention will be described below. The control methods available include (1) pH control, (2) nitrite concentration control, (3) nitrite concentration / water temperature control, and (4) free nitrite control (FNA control).

[0061] (1) A specific example of pH control is a method in which an acid is added as a pH adjuster to the nitrite treatment tank 10 when the pH reaches a predetermined value (e.g., 6.0 or higher). Using biological treatment wastewater, such as wastewater from biological deodorization treatment containing sulfuric acid, nitric acid, etc., as the pH adjuster can reduce the amount of chemicals used for pH adjustment, leading to more effective use of resources. However, with pH control alone, nitrification and denitrification can occur in the nitrite treatment tank 10, and when the nitrite concentration in the nitrite treatment tank 10 drops, sufficient nitrite treatment of the sludge cannot be performed, making it difficult to precisely control the conditions in the nitrite treatment tank 10.

[0062] (2) A specific example of nitrite concentration control is a method in which, when the nitrite concentration in the water to be treated falls below a predetermined value based on the above-mentioned nitrite concentration meter, an aqueous solution of nitrite is added to nitrite treatment tank 10 via chemical supply means 23. For example, when the NO2-N concentration falls below a predetermined value (e.g., 300 mg-N / L or less, preferably 250 mg / L or less, more preferably 200 mg / L or less), an aqueous solution of nitrite is added to nitrite treatment tank 10. However, it may be difficult to maintain the nitrite concentration at a stable constant level solely by controlling based on the nitrite concentration in nitrite treatment tank 10.

[0063] (3) Nitrite concentration and water temperature control is a method of controlling the water quality in the nitrite treatment tank based on the results of water quality measurement by water quality measurement means 21, which includes a nitrite concentration meter for measuring nitrite concentration and a water thermometer for measuring water temperature. As a specific example, if the water temperature or nitrite concentration is not within the appropriate range based on the results of water temperature and nitrite concentration measurement by water quality measurement means 21, water quality control means 22 controls chemical supply means 23 to add an aqueous solution of nitrite to nitrite treatment tank 10. Water temperature is adjusted by heating the aqueous solution of nitrite added to nitrite treatment tank 10 with a heater or the like, or by heating the separated liquid returned from solid-liquid separation device 9 shown in FIG. 2 to nitrite treatment tank 10 with a heater or the like. This (3) nitrite concentration and water temperature control enables the activation of ammonia-oxidizing bacteria in sludge to be suppressed, while predominantly activating them.

[0064] (4) Free nitrite control is a combination of the above (1) pH control and (2) nitrite concentration control. Specifically, the free nitrite concentration is calculated using the above formula (3) based on the water temperature and nitrite concentration measured by the water quality measurement means 21. When the free nitrite concentration falls below a predetermined value, an aqueous solution of nitrite is added to the nitrite treatment tank 10. According to this (4) free nitrite control, the free nitrite concentration can be calculated using formula (3), which takes into account pH, nitrite concentration, and water temperature, which are the main factors related to the decomposition of nitrite. Therefore, by controlling the water quality in the nitrite treatment tank 10 based on this free nitrite concentration, more accurate water quality control can be achieved. This allows for efficient and appropriate activation of ammonia-oxidizing bacteria while suppressing the activation of nitrite-producing bacteria in the sludge.

[0065] (Water treatment method) A water treatment method according to an embodiment of the present invention can be carried out using a water treatment device W shown in Figures 1 and 2. The water treatment method according to the embodiment of the present invention is a water treatment method for biologically treating raw water containing nitrogen components, and includes: a nitritation step in which ammonia nitrogen in the raw water is oxidized to nitrite nitrogen using sludge containing ammonia-oxidizing bacteria; a nitrite treatment step in which a portion of the sludge from the nitritation step is extracted and contacted with an aqueous nitrite solution containing nitrite to suppress the activity and proliferation of the nitrite-oxidizing bacteria contained in the sludge; and a return step in which the sludge after the nitrite treatment step is returned to the nitritation step. The water treatment method also includes a step of disposing water quality measurement means 21 including a nitrite concentration meter for measuring nitrite concentration and a water thermometer for measuring water temperature in nitrite treatment tank 10 in which the nitrite treatment step is performed, and controlling the water quality in the nitrite treatment tank based on the water quality measurement results obtained by water quality measurement means 21.

[0066] In one aspect of this embodiment, the nitrite concentration meter preferably includes any one of a nitrite sensor, an ORP meter, and a conductivity meter. Furthermore, it is preferable that the water quality measurement means 21 further includes a pH meter for measuring pH. Furthermore, it is preferable that the water quality measurement means 21 further includes a dissolved oxygen concentration meter for measuring the dissolved oxygen concentration, and that the dissolved oxygen concentration of the mixed solution in the nitrite treatment tank is controlled to be less than 0.5 mg / L based on the measurement results of the dissolved oxygen concentration meter.

[0067] In one aspect of this embodiment, the sludge containing ammonia-oxidizing bacteria used in the nitritation step preferably includes a bioattached carrier in which the sludge is attached to a carrier. The use of a bioattached carrier in the nitritation step creates a thick biofilm, creating a dissolved oxygen concentration gradient within the biofilm. This allows the nitritation treatment to proceed stably. Furthermore, by performing nitrite treatment on the bioattached carrier in the nitrite treatment tank 10, it becomes possible to more strictly control the environment within the tank to be more suitable for nitrite treatment than when conventional nitrite treatment is performed on activated sludge.

[0068] In one aspect of this embodiment, it is preferable to subject the sludge extracted in the extraction step to solid-liquid separation, introduce the sludge separated by solid-liquid separation into the nitrite treatment step, and return the separated liquid to the nitrite treatment step. By supplying the sludge after solid-liquid separation into the nitrite treatment tank 10, the overall amount of nitrification liquid mixed into the nitrite treatment tank can be reduced. As a result, fluctuations in treatment conditions in the nitrite treatment tank 10 due to the mixing of nitrification liquid can be reduced.

[0069] In one aspect of this embodiment, it is preferable to separate the sludge after the nitrite treatment step into a solid-liquid state, return the sludge separated by solid-liquid separation to the nitritation treatment step, and return the separated liquid to the nitrite treatment step. The separated liquid from the bioattached carriers separated by solid-liquid separation of the sludge after the nitrite treatment step can be reused as a nitrous acid solution in the nitrite treatment step, thereby reducing the amount of chemicals used to adjust the treatment conditions in the nitrite treatment step.

[0070] Furthermore, in one aspect of this embodiment, it is preferable to measure the pH and nitrite concentration of the water to be treated in the treatment tank (nitrite treatment tank 10) for the nitrite treatment step, and control the withdrawal and return of the bioadherent carriers so that the pH of the water to be treated is less than 6.0, the nitrite nitrogen concentration is less than 250 mg-N / L, and the residence time of the bioadherent carriers is within 48 hours. Because the nitrite concentration in the nitrite treatment tank 10 fluctuates greatly, if the nitrite treatment using the bioadherent carriers is not performed at an appropriate pH and nitrite concentration in the nitrite treatment tank 10, stable nitrite-type nitrification and denitrification may not be achieved in the nitrification tank 6 to which the water is returned. According to this embodiment, the withdrawal and return of the bioadherent carriers are controlled so that the pH of the water to be treated is less than 6.0, the nitrite nitrogen concentration is less than 250 mg-N / L, and the residence time of the bioadherent carriers is within 48 hours, thereby maintaining the microorganisms attached to the bioadherent carriers in a form more suitable for nitrite-type nitrification and denitrification treatment.

[0071] According to the water treatment device W and water treatment method of the embodiment of the present invention, it is possible to efficiently adjust the sludge used in the nitrification-denitrification treatment of raw water to a state suitable for nitrite-type nitrification-denitrification treatment using a simple method, and it is possible to maintain the nitrite-type nitrification treatment in the nitrification tank 6 appropriately and stably.

[0072] (Water treatment system using activated sludge) 3 is a schematic diagram illustrating an example of a water treatment system 100 according to an embodiment of the present invention. The water treatment system 100 according to the embodiment of the present invention includes a night soil receiving tank 1a for receiving night soil, a septic tank sludge receiving tank 1b for receiving septic tank sludge, a mixed sludge tank 2 for mixing the night soil and septic tank sludge, a dehydrator 3 for dehydrating the mixed sludge of night soil and septic tank sludge by adding a flocculant as needed to obtain a dehydrated cake and a separated liquid, and a separated liquid storage tank 4 for storing the separated liquid treated in the dehydrator 3.

[0073] A water treatment system 100 according to an embodiment of the present invention further comprises a first denitrification tank 5 for biologically treating a concentrated separated liquid of sewage and septic tank sludge, a nitrification tank 6 connected to the first denitrification tank 5, a second denitrification tank 7a connected to the nitrification tank 6, an aeration tank 7b connected to the second denitrification tank 7a, a settling tank 8 connected to the aeration tank 7b, a solid-liquid separator 9 for performing solid-liquid separation of excess sludge in the settling tank 8, and a nitrite treatment tank 10 for bringing the sludge separated in the solid-liquid separator 9 into contact with an aqueous nitrite solution.

[0074] The separated liquid in the separated liquid storage tank 4 is introduced into a first denitrification tank 5 which holds activated sludge containing denitrifying bacteria. In the first denitrification tank 5, the separated liquid is brought into contact with activated sludge containing denitrifying bacteria, whereby denitrification treatment of the separated liquid is carried out. The treated liquid after denitrification treatment (denitrified liquid) is introduced into a nitrification tank 6 which holds activated sludge containing ammonia-oxidizing bacteria.

[0075] In the nitrification tank 6, a nitritation treatment is carried out, in which ammonia nitrogen contained in the denitrification treatment liquid is oxidized to nitrite nitrogen. A portion of the treatment liquid (nitrification liquid) after the nitrite treatment is circulated as a circulating liquid to the first denitrification tank 5 via the circulation line L4. The remaining treatment liquid after the nitrite treatment is subjected to a denitrification treatment in the second denitrification tank 7a in the presence of methanol added as an electron donor and activated sludge containing denitrifying bacteria. The denitrification treatment liquid in the second denitrification tank 7a is subjected to a predetermined aeration treatment in the aeration tank 7b. The treated water after the aeration treatment is subjected to solid-liquid separation in the settling tank 8, where it is separated into effluent and separated sludge. A portion of the separated sludge is returned to the first denitrification tank 5, or is introduced into the nitrite treatment tank 10 after undergoing solid-liquid separation in the solid-liquid separator 9. The remaining separated sludge is discharged to the outside as excess sludge.

[0076] Sludge supplied from solid-liquid separator 9 is introduced into nitrite treatment tank 10, and the sludge is brought into contact with an aqueous nitrite solution at a pH of less than 6.0 and a nitrite nitrogen concentration of less than 250 mg-N / L for 48 hours or less, whereby water quality is controlled by water quality control means 22 based on the results of water quality measurement by water quality measurement means 21 so as to perform nitrite treatment that suppresses the activity and proliferation of nitritizing bacteria contained in the sludge. The sludge that has been treated with nitrite in nitrite treatment tank 10 is returned to nitrification tank 6. The treatment conditions in nitrite treatment tank 10 are the same as those in water treatment equipment W shown in FIG. 1.

[0077] In the water treatment system 100 according to the embodiment of the present invention, separated sludge obtained by solid-liquid separation in the settling tank 8 is subjected to nitrite treatment in the nitrite treatment tank 10, thereby producing sludge in which the activity of ammonia-oxidizing bacteria is selectively increased over that of nitrite-oxidizing bacteria, and this sludge can be returned to the nitrification tank 6. This creates an environment in the nitrification tank 6 in which ammonia-oxidizing bacteria are predominantly present, thereby enabling more appropriate and stable nitrite-type nitrification treatment in the nitrification tank 6. Furthermore, by introducing the separated sludge obtained from the settling tank 8 into the nitrite treatment step, the sludge can be effectively utilized for biological treatment, thereby reducing the amount of excess sludge discharged from the settling tank 8. In the water treatment system 100 of FIG. 2, activated sludge is used as the sludge, allowing biological treatment to be carried out more cheaply and economically than when using a fluidized bed method.

[0078] The amount of sludge supplied from the settling tank 8 to the solid-liquid separator 9 is preferably 5 to 60V% of the total separated sludge subjected to solid-liquid separation in the settling tank 8, more preferably 10 to 50V%, and even more preferably 15 to 40V%.

[0079] As shown in a water treatment system 100 according to a modified example in FIG. 4, activated sludge extracted from the nitrification tank 6 may be introduced into the nitrite treatment tank 10 via a solid-liquid separator 9 connected to the nitrification tank 6. The sludge extracted from the nitrification tank 6 is supplied to the solid-liquid separator 9 via line L11, where it undergoes solid-liquid separation. The sludge that has undergone solid-liquid separation in the solid-liquid separator 9 is supplied to the nitrite treatment tank 10 via line L12. The separated liquid obtained by solid-liquid separation in the solid-liquid separator 9 is returned to the nitrite treatment tank 6 via line L31. The sludge that has been treated with nitrite in the nitrite treatment tank 10 is supplied to the solid-liquid separator 9 via line L21, where it undergoes solid-liquid separation. The sludge separated in the solid-liquid separator 9 is returned to the nitrite treatment tank 6 via line L22. The separated liquid obtained by solid-liquid separation in the solid-liquid separator 9 is returned to the nitrite treatment tank 10 via line L32. Although the example in Figure 3 shows an example having one solid-liquid separator 9, two or more separate solid-liquid separators 9 may be provided, one for performing solid-liquid separation of the sludge extracted from the nitrification tank 6 and the other for performing solid-liquid separation of the sludge that has been treated with nitrite in the nitrite treatment tank 10.

[0080] According to the water treatment system 100 shown in Figure 4, activated sludge containing ammonia-oxidizing bacteria extracted from the nitrification tank 6 is treated with nitrite in the nitrite treatment tank 10, and the activated sludge after the nitrite treatment is returned to the nitrification tank 6. As a result, ammonia-oxidizing bacteria are present more predominantly than nitrite-producing bacteria in the activated sludge in the nitrification tank 6, and the nitrite-type nitrification treatment in the nitrification tank 6 is carried out more appropriately and stably. As a result, it is possible to maintain the production of nitrite nitrogen in the nitritation step more appropriately and stably, and more efficient nitrification and denitrification treatment can be carried out.

[0081] (Water treatment system using the fluidized carrier method) The water treatment system 100 shown in Figure 5 differs from the water treatment system 100 shown in Figure 4 in that flowing carriers are held in the first denitrification tank 5 and the nitrification tank 6, and bio-attached carriers with microorganisms attached to them are used as the flowing carriers. In addition, screens 15 are placed in the circulation line L4 of the first denitrification tank 5 and the nitrification tank 6 and in the piping leading to the second denitrification tank 7a to prevent the outflow of the bio-attached carriers in the tanks. As the rest is the same as the water treatment system shown in Figure 4, repeated description will be omitted.

[0082] The carriers held in the first denitrification tank 5 and the nitrification tank 6 are bioattached carriers on which microorganisms gradually attach and become immobilized as biological treatment progresses. More specifically, bioattached carriers are preferably those on which microorganisms are immobilized by an attachment immobilization method, in which microorganisms attach or grow on the outer surface of the carrier. The use of bioattached carriers increases the efficiency of transport of the substrate to the microorganisms, thereby enabling a faster reaction rate than when activated sludge is used. This stabilizes the nitrification / denitrification process, allowing the start-up process to be completed more quickly, even during the start-up of the equipment.

[0083] The carrier material is preferably a freely flowing hydrophilic polymer carrier material, such as synthetic polymers such as polyvinyl alcohol (PVA), polyethylene glycol (PEG), polyacrylamide, and photocurable resins, gel carriers using polymers such as carrageenan and sodium alginate, and carriers such as polyethylene, polyurethane, and polypropylene.

[0084] The carrier may be spherical, rectangular, or cylindrical in shape, and its effective diameter φ is 1 to 20 mm, more preferably 3 to 15 mm, and even more preferably 3 to 10 mm, which allows stable separation by a screen installed at the outlet of the reaction tank. Regarding the surface properties of the carrier, those with many micropores or numerous irregularities on the surface allow for rapid attachment and fixation of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria, thereby achieving high nitrification and denitrification performance in a short period of time and enabling the bacteria to be maintained at a high concentration in the tank for a long period of time.

[0085] The specific surface area of ​​the carrier is 200 to 30,000 m 2 / m 3 , more preferably 200 to 20,000 m 2 / m 3 , more preferably 200 to 10,000 m 2 / m 3 The carrier specific gravity is preferably 1.01 to 1.15, more preferably 1.01 to 1.10, and even more preferably 1.01 to 1.05, which allows uniform flow within the tank. The carrier filling amount in the first denitrification tank 5 and the nitrification tank 6 is preferably 5 to 40 V%, more preferably 10 to 35 V%, which allows uniform mixing and flow.

[0086] In the nitrification tank 6, a nitritation treatment is carried out in which ammoniacal nitrogen contained in the denitrification treatment liquid is oxidized to nitrite nitrogen. A portion of the treated liquid (nitrification liquid) after the nitrite treatment is circulated as a circulating liquid via a circulation line L4 to the first denitrification tank 5. The circulating flow rate of the nitrification liquid relative to the raw water flow rate depends on the control values ​​of the ammoniacal nitrogen concentrations of the raw water and the treated water, but is preferably 2.0 to 20.0 times (circulation ratio 2.0 to 20.0), more preferably 5.0 to 15.0 times (circulation ratio 5.0 to 15.0), and even more preferably 7.0 to 13.0 times (circulation ratio 7.0 to 13.0).

[0087] The remaining treated liquid after the nitrite treatment is subjected to denitrification treatment in the second denitrification tank 7a in the presence of methanol added as an electron donor and activated sludge containing denitrifying bacteria. The denitrified liquid in the second denitrification tank 7a is subjected to a predetermined aeration treatment in the aeration tank 7b. The treated water after the aeration treatment undergoes solid-liquid separation in the settling tank 8, where it is separated into effluent and separated sludge. A portion of the separated sludge is returned to the first denitrification tank 5, or is introduced into the nitrite treatment tank 10 after undergoing solid-liquid separation in the solid-liquid separator 9. The remaining separated sludge is discharged to the outside as excess sludge.

[0088] The bioattached carriers supplied from the solid-liquid separator 9 are introduced into the nitrite treatment tank 10, and preferably the bioattached carriers are brought into contact with an aqueous nitrite solution at a pH of less than 6.0 and a nitrite nitrogen concentration of 250 mg-N / L or less for 48 hours or less to perform nitrite treatment, which suppresses the activity and proliferation of nitrite-oxidizing bacteria contained in the bioattached carriers. The bioattached carriers that have been treated with nitrite in the nitrite treatment tank 10 are returned to the nitrification tank 6.

[0089] The separated sludge that has been separated into solid and liquid in the settling tank 8 is supplied to the second denitrification tank 7a via line L81. The line L81 that supplies the separated sludge to the second denitrification tank 7a is further connected to the first denitrification tank 5. During the start-up process of the water treatment system 100, the separated sludge is supplied to the first denitrification tank 5 via line L82. After the start-up process of the water treatment system 100, the separated sludge is supplied to the second denitrification tank 7a via line L81.

[0090] According to the water treatment system 100 shown in FIG. 5 , fluidized carriers are retained in the denitrification step in the first denitrification tank 5 and in the nitrification tank 6. Compared to biological treatment using an activated sludge method, biological treatment using a fluidized carrier method facilitates solid-liquid separation and provides higher solid-liquid separation efficiency. Therefore, the amount of nitrite solution adhering to the sludge after nitrite treatment in the nitrite treatment tank 10 that is mixed into the nitrification tank 6 is significantly less than when activated sludge is used as sludge. As a result, deterioration of the biological treatment performance in the first denitrification tank 5 and nitrification tank 6 due to the inflow of high-concentration nitrite from the nitrite treatment tank 10 into the first denitrification tank 5 and nitrification tank 6 is suppressed, thereby stably maintaining the treatment performance of the first denitrification tank 5 and nitrification tank 6. Furthermore, because biological treatment using a fluidized carrier method facilitates solid-liquid separation as described above, the sludge after nitrite oxidation treatment is subjected to solid-liquid separation before being returned to the nitritation step, and the separated liquid obtained by solid-liquid separation can be returned to the nitrite treatment tank 10 for reuse. This reduces the amount of chemicals used.

[0091] Generally, biological treatment using the activated sludge process, such as the water treatment system 100 shown in Figures 2 and 3, requires thickening of the activated sludge before and after nitrite treatment. In contrast, biological treatment using the fluidized bed carrier process, such as the water treatment system 100 shown in Figure 5, can achieve stable solid-liquid separation of sludge simply by using a solid-liquid separator such as a screen, thereby improving treatment efficiency and simplifying the equipment compared to the activated sludge process. Furthermore, the fluidized bed carrier process can form a thicker biofilm than the activated sludge process. Therefore, ammonia-oxidizing bacteria can be stably maintained and controlled in the tank by controlling the DO concentration, which also stabilizes the nitrite treatment in the nitrification tank 6.

[0092] In biological treatment using the activated sludge method, such as the water treatment system 100 shown in Figures 2 and 3, when the nitrite oxidation treatment of activated sludge is stopped, the nitrite accumulation rate in the nitrification tank 6 drops sharply, and the treatment may become unstable. In contrast, in biological treatment using the fluidized carrier method, such as the water treatment system 100 shown in Figure 5, a sharp drop in the nitrite accumulation rate is less likely to occur even after the nitrite oxidation treatment of the fluidized carrier is stopped, compared to biological treatment using the activated sludge method. This makes it possible to maintain more appropriate and stable production of nitrite nitrogen in the nitrification step.

[0093] In biological treatment using the activated sludge method, such as the water treatment system 100 shown in Figures 2 and 3, it takes about 30 days (activated sludge withdrawal amount: 47%, DO: 0.5 mg / L) for the activated sludge in the nitrite treatment tank to be treated with nitrite and for the conditions in the sludge in the nitrite treatment tank to become dominantly ammonia-oxidizing bacteria. In contrast, in biological treatment using the fluidized carrier method, such as the water treatment system 100 shown in Figure 4, it takes only about 5 days for the conditions in the sludge in the nitrite treatment tank to become dominantly ammonia-oxidizing bacteria. This allows for a shorter treatment time.

[0094] 5, trial calculations have shown that the amount of sulfuric acid added to adjust the pH in the nitrite treatment tank 10 can be reduced by about 99% compared to biological treatment using activated sludge. Trial calculations have also shown that the amount of nitrite concentration added to adjust the pH in the nitrite treatment tank 10, or the amount of sodium nitrite added to control the free nitrite concentration, can be reduced by about 65% compared to biological treatment using activated sludge.

[0095] 2 and 3, it is necessary to keep the DO levels low in both the nitrite treatment tank 10 and the nitrification tank 6 in order to stably maintain the nitrite-type nitrification and denitrification treatment in the nitrification tank 6. In contrast, in the biological treatment using the fluidized carrier method shown in the water treatment system 100 shown in FIG. 5, the DO level in the nitrification tank 6 does not need to be strictly controlled to 0.5 mg / L or less, and the treatment conditions can be easily controlled because the nitrite-type nitrification and denitrification treatment in the nitrification tank 6 can be stably maintained.

[0096] (Control by water quality control means 22) By adjusting the pH and nitrite concentration in the nitrite treatment step in the nitrite treatment tank 10 using the water quality control means 22, there is also the secondary benefit of being able to estimate the state of the microorganisms attached to the carriers.

[0097] For example, if the amount of acid used and the amount of nitrite added in the nitrite treatment process are constantly monitored, and the amount of acid or nitrite added per treatment is higher than usual, it can be assumed that the state of the microorganisms attached to the carrier is not desirable. That is, a high amount of nitrite added may indicate that acid-resistant nitrite-oxidizing bacteria have attached to the carrier, making it difficult to maintain nitrite-type nitrification, or that the nitrite conversion rate in the nitrification tank has decreased, resulting in a decrease in the nitrite concentration in the liquid contained within the carrier. Furthermore, a high amount of acid added may indicate that metal salts such as calcium scale have attached to the carrier, reducing the amount of microorganisms attached.

[0098] The water quality control means 22 can be configured to monitor the water quality control results in the nitrite treatment tank 10 (amount of acid added per treatment, amount of nitrite added) and, if the water quality control results deviate from predetermined target values, change the treatment conditions for the nitritation step in the nitrification tank 6 or issue an alarm about an abnormality in the nitritation step in the nitrification tank 6. In this way, if a change from the normal amount of chemical usage is observed, it will lead to early detection of signs of a decline in the performance of nitrite-type nitrification-denitrification, and by changing the carrier amount, residence time, and treatment conditions in the nitrite treatment step, it will be possible to prevent a decline in performance.

[0099] Furthermore, in this embodiment, the sludge after the nitrite treatment step is subjected to solid-liquid separation, and the separated liquid is returned to the nitrite treatment tank 10 for reuse, but repeated return of the separated liquid increases the buffer capacity in the liquid, making it difficult to obtain the effect of pH adjustment by adding acid and causing the growth of bacteria in the nitrite treatment tank 10. To prevent such problems, it is preferable to replace the aqueous nitrite solution in the nitrite treatment tank 10 with a new aqueous nitrite solution at regular intervals.

[0100] Therefore, it is preferable that the water quality measurement means 21 further includes an alkalinity measurement device that measures the alkalinity of the water to be treated in the nitrite treatment tank. When the alkalinity measurement by the alkalinity measurement device indicates that the alkalinity in the nitrite treatment tank 10 is equal to or higher than a reference value, the water quality control means 22 preferably issues a predetermined control signal to replace the nitrite aqueous solution in the nitrite treatment tank 10 with a new nitrite aqueous solution, or issues a warning to urge the operator. For example, when the alkalinity in the nitrite treatment tank 10 reaches a reference value (e.g., 1000 mg / L or higher), the water quality control means 22 can issue a predetermined warning to urge the operator to replace the nitrite aqueous solution in the nitrite treatment tank with a new nitrite aqueous solution.

[0101] Furthermore, the water quality measurement means 21 includes an MLSS measurement device that measures the MLSS of the water to be treated in the nitrite treatment tank, and the water quality control means 22 can issue a predetermined warning or the like to urge the operator to replace the nitrite aqueous solution in the nitrite treatment tank with a new nitrite aqueous solution when the MLSS in the nitrite treatment tank 10 reaches a standard value (for example, 500 mg / L or more).

[0102] Furthermore, in one embodiment, the water quality control means 22 controls at least one of the supply of a pH adjuster to the nitrite treatment tank 10, the supply of an aqueous nitrite solution, and water temperature adjustment based on the water quality measurement results by the water quality measurement means 21 so that the pH of the treatment tank in the nitrite treatment step is less than 6.0, the nitrite nitrogen concentration is less than 250 mg-N / L, and the water temperature is 20 to 28°C, and it is preferable that the sludge in the treatment tank is returned to the nitritation step within 48 hours after the extracted sludge is introduced into the treatment tank. Furthermore, in order to prevent decomposition of nitrite in the nitrite treatment tank 10, it is preferable that the water quality measurement means 21 measures the water quality of the treated water in the nitritation step continuously or every 24 hours, more preferably every 12 hours, and even more preferably every 6 hours.

[0103] Although the present invention has been described using the above embodiments, the descriptions and drawings that form part of this disclosure should not be understood to limit the present invention. The present disclosure is not limited to the above embodiments, and components can be combined and modified to achieve specific embodiments without departing from the spirit of the present disclosure. For example, the "water treatment method for biologically treating raw water containing nitrogen components" according to the embodiments of the present invention may include not only treatment during steady-state operation for biologically treating raw water, but also treatment when starting up a nitrite-type nitrification denitrification treatment, or treatment temporarily performed when the nitrite-type nitrification denitrification treatment is temporarily unstable, as shown in the following examples. [Example]

[0104] Examples of the present invention will be described below together with comparative examples. These examples are provided for a better understanding of the present invention and its advantages, and are not intended to limit the invention.

[0105] (Nitritation treatment) A continuous test of nitritation treatment of raw water using a flow carrier was conducted using the test equipment shown in Figure 6, which includes a raw water tank, denitrification tank, nitrification tank, nitrite treatment tank, and treated water tank. In Example 1, the raw water used was simulated wastewater with NH4-N: 500 mg / L, M alkalinity: 1500-1800 mg / L, BOD: 750-1500 mg / L, PO4-P: 3.8 mg / L, M alkalinity / NH4-N: approximately 3.0, and BOD / NH4-N: 1.5-3.0, to which trace elements such as ethylenediaminetetraacetic acid (EDTA): 6.0 mg / L, Fe: 1.0 mg / L, Zn: 0.01 mg / L, Co: 0.06 mg / L, Mn: 0.28 mg / L, Cu: 0.06 mg / L, Mo: 0.07 mg / L, and Ni: 0.05 mg / L were added. In the nitrification tank, ammonia-oxidizing bacteria (amoA: 1.02 × 10 5 copies / g-MLVSS, 16SrRNA:3,15×10 8 copies / g-MLVSS) and nitrite-oxidizing bacteria (Nitrobacter sp.: 1.58 × 10 4 copies / g-MLVSS, Nitrospira sp.:2.17×10 7 copies / g-MLVSS) was introduced.

[0106] As the flow carrier, polyethylene glycol hydrogel (specific gravity 1.025) with an effective diameter of φ3.4 to 5.2 mm (standard diameter φ4.2 mm) was used. This carrier was introduced into the nitrification tank and denitrification tank, and a start-up process (Run 0) for nitrification and denitrification treatment was performed, causing a biofilm (sludge) containing nitrite-oxidizing bacteria and ammonia-oxidizing bacteria to attach to the carrier.

[0107] In Example 1, continuous biological treatment tests using the fluidized bed carrier method were conducted under the conditions (Runs 0 to 3) shown in Table 1 according to the objectives. In the first start-up process (Run 0), the nitrogen load and sludge load in the nitrification tank were gradually increased, and the first start-up process of nitrification and denitrification treatment in the nitrification tank was completed in 14 days when the nitrogen removal rate in the nitrification tank reached 90%. In the second start-up process (Run 1), to perform nitrite-type nitrification and denitrification treatment in the nitrification tank, the amount of carrier withdrawn and returned from the nitrification tank to the nitrite treatment tank was withdrawn at 10 V% / day for 10 days, followed by 20 V% / day for 10 days. The carrier withdrawn from the nitrification tank was subjected to nitrite treatment, and the nitrite-treated carrier was returned to the nitrification tank.

[0108] In the second start-up process (Run 1), a portion of the carrier with attached biofilm in the first start-up process (Run 0) was extracted and subjected to solid-liquid separation. The separated liquid was returned to the nitrification tank, and only the carrier obtained by solid-liquid separation was transferred to the nitrite treatment tank for nitrite treatment. In the second start-up process (Run 1), the amount of carrier extracted from the nitrification tank and the amount of carrier returned to the nitrite treatment tank were increased stepwise to 10V% / day, 20V% / day, and 30V% / day.

[0109] Steady-state operation (Runs 2 and 3) was performed using a nitrite-based nitrification and denitrification process based on the process established in Run 1. To evaluate the reduction in the required BOD, the raw water BOD was gradually reduced (30% reduction in Run 2, 50% reduction in Run 3). In these steady-state operations (Runs 2 and 3), the carrier removed from the nitrification tank was separated into solids and liquids using a solid-liquid separator, and the separated carrier was fed to the nitrification tank. The separated liquid was returned to the nitrification tank. The carrier treated in the nitrite treatment tank was fed to the solid-liquid separator, and the separated carrier was returned to the nitrification tank. The separated liquid was returned to the nitrite treatment tank for reuse. A portion of the treated water (nitrification liquid) in the nitrification tank was circulated as a circulating liquid to the denitrification tank, where raw water from the raw water tank was denitrified and the denitrification liquid was fed to the nitrification tank. The amount of activated sludge withdrawn from the nitrification tank to the nitrite treatment tank during steady-state operation was set to 30V%.

[0110] [Table 1]

[0111] The nitrite treatment tank was not aerated, and the carrier was left in the tank while it was shaken. The nitrite-treated carrier was then subjected to solid-liquid separation, and only the carrier obtained by solid-liquid separation was returned to the nitrification tank. The separated liquid was returned to the nitrite treatment tank. The water temperature, pH, DO, ORP, SS, and M alkalinity in the nitrite treatment tank were continuously measured using a water quality measuring device, and water quality was managed by adding sulfuric acid and sodium nitrite based on the following control items.

[0112] In Example 1, the water quality of the water to be treated in the nitrite treatment tank was controlled based on the measurement results of the water quality measuring device in the nitrite treatment tank so that the free nitrite concentration was 1.0 to 2.0 mg-N / L, pH was 5.0 to 5.5, water temperature was 25°C, and nitrite concentration was 150 to 200 mg-N / L. In Example 2, the quality of the water to be treated in the nitrite treatment tank was controlled so that the nitrite concentration was 150 to 200 mg-N / L and the water temperature was 25°C. In Comparative Example 1, the quality of the water to be treated in the nitrite treatment tank was controlled so that the pH in the nitrite treatment tank was 5.0 to 5.5. In Comparative Example 2, the quality of the water to be treated in the nitrite treatment tank was not controlled. The aqueous nitrite solution in the nitrite treatment tank was replaced with a new aqueous nitrite solution once a week.

[0113] Figure 7 shows the daily change in free nitrite concentration in the nitrite treatment tank. In Example 1, in which the free nitrite concentration was controlled based on pH, water temperature, and nitrite concentration, the FNA concentration stabilized in the range of 1.0 to 1.4 mg-N / L. In Example 2, in which the nitrite concentration and water temperature were controlled, the FNA concentration decreased more than in Example 1, but stabilized at 1.0 mg-N / L or higher. On the other hand, in Comparative Examples 1 and 2, the free nitrite concentration decreased significantly within 24 hours.

[0114] Figure 8 shows the change in free nitrite concentration in the nitrite treatment tank when nitrite treatment of the fluid carrier was carried out while exchanging the aqueous nitrite solution every week. In Example 1, the FNA concentration was adjusted to 1.0 to 1.4 mg-N / L, a concentration range suitable for nitrite treatment. In Example 2, in which the nitrite concentration and water temperature were controlled, the FNA concentration decreased slightly more than in Example 1 over time, but stabilized at 0.8 mg-N / L or higher. On the other hand, in Comparative Examples 1 and 2, the free nitrite concentration decreased significantly within one week.

[0115] 9 is a graph showing the change in the nitritation rate in the nitrification tank when the carriers used in the nitrite treatment in Examples 1 and 2 and Comparative Examples 1 and 2 are returned to the nitrification tank to perform nitrite-type nitrification and denitrification treatment. X The results of evaluation using -N are shown below. In Example 1, the nitritation rate was able to be maintained at 90% or higher. In Example 2, although the nitritation rate decreased slightly over time, a stable nitrite-type nitrification and denitrification treatment was achieved. In Comparative Examples 1 and 2, the nitritation rate decreased significantly after 14 days.

[0116] (Changes in nitrification rate in the nitrification tank due to nitrite treatment) Figure 10(a) shows the change in nitrification rate of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria in the nitrification tank when the nitrite concentration of the treated water in the nitrite treatment tank is maintained at 100 to 250 mg / L. Figure 10(b) shows the nitrite concentration in the nitrite treatment tank and the activity rate (%) of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria in the nitrite treatment process. In Figure 10(a), the "nitrification rate" was evaluated using a batch ammonia oxidation rate measurement test (for ammonia-oxidizing bacteria activity) and a batch nitrite oxidation rate measurement test (for nitrite-oxidizing bacteria activity). In Figure 10(b), the "activity rate" was evaluated by comparing the activity after nitrite treatment with the activity before nitrite treatment. The batch oxidation rate was measured according to the "Nitrification Rate Test Method" described in Chapter 2, Section 2 of the Sewage Testing Methods. As shown in Figure 10(a), the activity of nitrite-oxidizing bacteria in the nitrification tank decreases as the nitrite concentration in the nitrite treatment tank increases, but when the nitrite concentration is 250 mg / L, the activity of ammonia-oxidizing bacteria in the nitrification tank is also reduced by half compared to before nitrite treatment.Furthermore, as shown in Figure 10(b), when the nitrite concentration in the nitrite treatment tank is 250 mg / L, the activity of nitrite-oxidizing bacteria is reduced to about one-quarter compared to when the nitrite concentration is 100 mg / L, but the activity of ammonia-oxidizing bacteria is also reduced by about 40%. [Explanation of symbols]

[0117] 1a: Night soil receiving tank 1b: Septic tank sludge receiving tank 2: Mixed sludge tank 3: Dehydrator 4:Separated liquid storage tank 5:Denitrification tank (first denitrification tank) 6: Nitrification tank 7a:Second denitrification tank 7b: Aeration tank 8: Sedimentation tank 9: Solid-liquid separator 10: Nitrite treatment tank 15: Screen 21:Water quality measurement means 22: Water quality control measures 100: Water treatment system L1: Extraction means L2: Return method L4: Circulation line W: Water treatment equipment

Claims

1. A water treatment method for biologically treating raw water containing nitrogen components, comprising: a nitritation step in which ammonia nitrogen in the raw water is oxidized to nitrite nitrogen by sludge containing ammonia-oxidizing bacteria; a nitrite treatment step in which a portion of the sludge from the nitritation step is extracted and the extracted sludge is brought into contact with an aqueous nitrite solution containing nitrite, thereby suppressing the activity and proliferation of nitrite-oxidizing bacteria contained in the sludge; a returning step of returning the sludge after the nitrite treatment step to the nitritation step; and a water quality measuring means including a nitrite concentration measuring meter for measuring the nitrite concentration and a water thermometer for measuring the water temperature is disposed in the nitrite treatment tank in which the nitrite treatment step is carried out; A water treatment method comprising controlling the water quality in the nitrite treatment tank based on the results of water quality measurement by the water quality measurement means.

2. 2. The water treatment method according to claim 1, wherein the nitrite concentration measuring meter includes one of a nitrite sensor, an ORP meter, and a conductivity meter.

3. 2. The water treatment method according to claim 1, wherein the water quality measuring means further includes a pH meter for measuring pH.

4. the water quality measuring means further includes a dissolved oxygen concentration meter for measuring a dissolved oxygen concentration; 2. The water treatment method according to claim 1, further comprising controlling the dissolved oxygen concentration of the mixed solution in the nitrite treatment tank to be less than 0.5 mg / L based on the measurement result of the dissolved oxygen concentration meter.

5. 2. The water treatment method according to claim 1, wherein the sludge containing ammonia-oxidizing bacteria comprises a carrier on which the sludge is attached.

6. 2. The water treatment method according to claim 1, further comprising subjecting the sludge after the nitrite treatment step to solid-liquid separation before returning the sludge to the nitritation step, and returning the separated liquid obtained by the solid-liquid separation to the nitrite treatment tank.

7. 2. The water treatment method according to claim 1, wherein the water quality measuring means measures the water quality in the nitrite treatment tank continuously or every 24 hours.

8. The water treatment method of claim 1, wherein the water quality measuring means further includes an alkalinity measuring device that measures the alkalinity of the water to be treated in the nitrite treatment tank, and when the alkalinity in the nitrite treatment tank becomes equal to or higher than a standard value, the nitrite solution in the nitrite treatment tank is replaced with a new nitrite solution.

9. 2. The water treatment method according to claim 1, further comprising monitoring the water quality control results in the nitrite treatment tank, and if the water quality control results deviate from a predetermined target value, changing the treatment conditions of the nitritation step or issuing an alarm about an abnormality in the nitritation step.

10. a nitrification tank in which ammonia nitrogen in raw water containing nitrogen components is oxidized to nitrite nitrogen using sludge containing ammonia-oxidizing bacteria; a nitrite treatment tank in which a portion of the sludge in the nitrification tank is withdrawn and the withdrawn sludge is brought into contact with an aqueous nitrite solution containing nitrite to suppress the activity and proliferation of nitrite-oxidizing bacteria contained in the sludge; a return means for returning the sludge treated in the nitrite treatment tank to the nitrification tank; a water quality measuring means including a nitrite concentration measuring meter for measuring the nitrite concentration in the nitrite treatment tank and a water thermometer for measuring the water temperature; a water quality control means for controlling the water quality in the nitrite treatment tank by controlling at least one of the supply of a pH adjuster to the nitrite treatment tank, the supply of an aqueous nitrite solution, and water temperature adjustment based on the water quality measurement results obtained by the water quality measurement means; A water treatment device comprising:

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