Water treatment method and water treatment apparatus

Abstract

To provide a water treatment method and a water treatment apparatus capable of more appropriately and stably maintaining the generation of nitrite nitrogen in a nitritation process and more efficiently performing nitrification and denitrification treatment.SOLUTION: A water treatment method for biologically treating raw water containing a nitrite component includes a nitrite-forming step of oxidizing ammoniacal nitrogen in the raw water to nitrite-nitrogen with biosludge containing ammonia-oxidizing bacteria, a nitrite treatment step of extracting a part of the biosludge in the nitrite-forming step and bringing the extracted biosludge into contact with a nitrite aqueous solution at a pH6 of less than 0 and a nitrite-nitrogen concentration of less than 250mg - N / L for 48 hours or less to suppress activity and proliferation of the nitrite-oxidizing bacteria contained in the biosludge, and a returning step of returning the biosludge after the nitrite treatment step to the nitrite-forming step.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 raw water contains little organic matter, 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 heterotrophic denitrifying bacteria reduce the NO2-N in the treated water from the nitritation tank to nitrogen gas.

[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 of 5 days or more to kill nitrite-oxidizing bacteria. This method kills not only nitrite-oxidizing bacteria but also ammonia-oxidizing bacteria, so the activity and proliferation of ammonia-oxidizing bacteria may not be stably maintained, and it may be difficult to maintain appropriate and stable production of nitrite nitrogen in the nitritation process.

[0009] In view of the above problems, the present invention provides a water treatment method and a water treatment device that can maintain the production of nitrite nitrogen in the nitritation step more appropriately and stably and can perform nitrification-denitrification treatment more efficiently. [Means for solving the problem]

[0010] As a result of intensive research to solve the above problems, the inventors discovered that it is useful to treat sludge from the nitritation step with nitrite under certain specific conditions and return the nitrite-treated sludge to the nitritation treatment, and thus arrived at the present invention.

[0011] 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 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 the extracted 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, 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.

[0012] In one embodiment of the water treatment method according to the present invention, the dissolved oxygen concentration in the nitrite treatment step is controlled to 0.5 mg / L or less.

[0013] In yet another embodiment of the water treatment method according to the present invention, 10 to 50 wt % of the sludge from the nitritation step is extracted.

[0014] 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.

[0015] In yet another embodiment, the water treatment method according to the present invention further comprises a solid-liquid separation step of performing solid-liquid separation on the treated liquid treated in the nitritation step, and a step of withdrawing a portion of the sludge separated in the solid-liquid separation step and supplying the withdrawn sludge to a nitrite treatment step.

[0016] In still another embodiment of the water treatment method according to the present invention, sludge from the nitritation step is extracted and subjected to solid-liquid separation, and the sludge after solid-liquid separation is supplied to the nitrite treatment step.

[0017] In still another embodiment of the water treatment method according to the present invention, the sludge after the nitrite treatment step is subjected to solid-liquid separation, and the separated liquid after solid-liquid separation is returned to the nitrite treatment step.

[0018] In yet another embodiment, the water treatment method according to the present invention further comprises a denitrification step in which nitrite nitrogen contained in the treated water from the nitritation step is reduced to nitrogen by denitrifying bacteria.

[0019] In one aspect, the present invention provides a water treatment device comprising: a nitrification tank in which ammonia nitrogen contained in raw water containing nitrogen components is oxidized to nitrite nitrogen by sludge containing ammonia oxidizing bacteria; extraction means for extracting the sludge containing ammonia oxidizing bacteria from the nitrification tank; a nitrite treatment tank into which the sludge extracted by the extraction means is introduced and which suppresses the activity and proliferation of nitrite oxidizing bacteria contained in the sludge by contacting the sludge 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; and return means for returning the sludge treated in the nitrite treatment tank to the nitrification tank.

[0020] In one embodiment, the water treatment device according to the present invention further comprises a DO meter that measures the dissolved oxygen concentration in the nitrite treatment tank, and a control means that controls the dissolved oxygen concentration in the nitrite treatment tank to be 0.5 mg / L or less.

[0021] In another embodiment of the water treatment device according to the present invention, the sludge containing ammonia oxidizing bacteria comprises a bioattached carrier in which the sludge is attached to a carrier.

[0022] In one embodiment, the water treatment device according to the present invention further comprises a separated liquid returning means for performing solid-liquid separation on the sludge treated in the nitrite treatment tank and returning the separated liquid after solid-liquid separation to the nitrite treatment tank.

[0023] In one embodiment, the water treatment device according to the present invention further comprises a denitrification tank connected to the nitrification tank, in which nitrite nitrogen contained in the treated water of the nitrification tank is reduced to nitrogen by denitrifying bacteria. [Effects of the Invention]

[0024] According to the present invention, it is possible to provide a water treatment method and a water treatment device that can maintain the production of nitrite nitrogen in the nitritation step more appropriately and stably and can carry out nitrification-denitrification treatment more efficiently. [Brief explanation of the drawings]

[0025] [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. 1 is a schematic diagram illustrating an example of a water treatment system 100 using activated sludge. [Figure 3] FIG. 1 is a schematic diagram showing a modified example of a water treatment system 100 using activated sludge. [Figure 4] FIG. 1 is a schematic diagram illustrating an example of a water treatment system 100 using a bioattaching carrier. [Figure 5] FIG. 1 is a schematic diagram showing a test device of Example 1. [Figure 6] 1 is a graph showing changes in the nitrite accumulation rate in the nitrification tank and the nitrogen removal rate during start-up and steady operation in Example 1. [Figure 7] 1 is a graph showing the change in the aeration air volume in the nitrification tank and the nitrite accumulation rate in the nitrification tank during nitrite treatment using activated sludge of Example 1. [Figure 8] FIG. 1 is a schematic diagram showing a test device of Example 2. [Figure 9] FIG. 1 is a schematic diagram showing the change in the nitrite accumulation rate in the nitrification tank and the nitrogen removal rate in Example 2. [Figure 10]1 is a graph showing the change over time in the aeration air volume in the nitrification tank and the nitrite accumulation rate in the nitrification tank in Example 2. [Figure 11] 10 is a graph showing an example of evaluation of the nitrogen removal rate in the denitrification tank when the amount of electron donor added to the denitrification tank is gradually reduced during steady operation in Example 2. [Figure 12] Figure 12(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 12(b) is a graph showing the relationship between 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. DETAILED DESCRIPTION OF THE INVENTION

[0026] 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.

[0027] (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 ammoniacal nitrogen contained in raw water containing nitrogen components (or ammoniacal nitrogen generated by decomposition of nitrogen components in the raw water) is oxidized to nitrite nitrogen by sludge containing ammonia-oxidizing bacteria, extraction means L1 for extracting the sludge containing ammonia-oxidizing bacteria from the nitrification tank 6, a nitrite treatment tank 10 into which the sludge extracted by the extraction means L1 is introduced and contacted 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, thereby suppressing the activity and proliferation of the nitrite-oxidizing bacteria contained in the sludge, and return means L2 for returning the sludge treated in the nitrite treatment tank 10 to the nitritation step.

[0028] The raw water is not particularly limited as long as it contains nitrogen. Examples of usable raw water include sewage, leachate, dehydrated separated liquid from anaerobic digestion of sewage sludge, wastewater containing at least human waste or septic tank sludge, wastewater generated in food factories and various other factories, dehydrated separated liquid from human waste or anaerobic digestion sludge, and wastewater containing at least human waste or septic tank sludge. Wastewater containing at least human waste or septic tank sludge may also contain sludge generated in agricultural wastewater treatment. Also usable raw water includes separated liquids obtained by solid-liquid separation (sedimentation, coagulation sedimentation, flotation, screen separation, dehydration, etc.) from sewage, leachate, and various types of wastewater, and separated liquids from dehydrated anaerobic digestion sludge.

[0029] For example, wastewater with a BOD concentration of 50 to 2,000 mg / L, preferably 100 to 1,500 mg / L, a total nitrogen concentration (TN) of 10 to 2,000 mg / L, preferably 200 to 1,000 mg / L, an ammoniacal nitrogen concentration (NH4-N) of 10 to 1,500 mg / L, preferably 150 to 800 mg / L, an M alkalinity of 20 to 6,000 mg / L, preferably 400 to 5,000 mg / L, a BOD / TN ratio of 0.5 to 5, preferably 1 to 3, a total phosphorus concentration (TP) of 1 to 200 mg / L, preferably 2 to 150 mg / L, a phosphate concentration of 0.5 to 150 mg / L, preferably 1 to 100 mg / L, and an M alkalinity / TN ratio of 1 to 10, preferably 2 to 8, can be used as raw water.

[0030] In wastewater containing at least human waste or septic tank sludge, the volume ratio of septic tank sludge to the total wastewater is preferably 10 to 100%, more preferably 20 to 100%, and even more preferably 40 to 100%. Usable wastewater containing at least sewage or septic tank sludge has 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 with the BOD in the raw water alone, and external organic matter such as methanol must be added.

[0031] Wastewater consisting of a mixture of human waste and septic tank sludge: 1 m of human waste 3 In contrast, septic tank sludge is 1 to 99 m 3 , preferably 2 to 50 m 3 Wastewater containing septic tank sludge may also be used. Generally, a higher ratio of septic tank sludge than 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 3.0, 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.

[0032] 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 a gel-like 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. Materials for the attached immobilization carriers include plastics (polyethylene, polypropylene, polyurethane, polyethylene glycol, polyvinyl alcohol, etc.). Shapes of the attached immobilization carriers include sponge, gel, cylindrical, honeycomb, net, spherical, rectangular, etc. The use of attached immobilization carriers is preferable for maintaining specific microorganisms predominantly and stably. Activated sludge, granular sludge, and bioattached carriers may 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.

[0033] 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.

[0034] Nitritization 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.

[0035] If the DO in the nitrification tank 6 becomes too high, the proliferation and activity of nitrite-oxidizing bacteria in the sludge will increase, making it difficult to stably maintain nitrite-type nitrification treatment. When activated sludge is used in the nitrification tank 6, 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 a bioadhesive carrier is used in the nitrification tank 6, the biofilm is thicker than activated sludge, and the DO inside the biofilm is lowered due to the rate-limiting effect of substrate diffusion. Therefore, the 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 treatment can proceed predominantly in the nitrification tank 6. Therefore, 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, even when an attached immobilization carrier is used.

[0036] Although not limited to the following, for example, in the nitrification tank 6, 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, for example, the nitrogen volume load is 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 3It 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.

[0037] The withdrawal means L1 is connected to the nitrification tank 6. The withdrawal means L1 is equipped with a line L11 and a line L12. The lines L11 and L12 are each composed of a pump, piping, etc. A solid-liquid separator 9 is preferably disposed between the nitrification tank 6 and the nitrite treatment tank 10. The withdrawal means L1 is preferably configured to supply 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 in the solid-liquid separator 9 to the nitrite treatment tank 10 via line L12. The separated liquid (nitrification liquid) obtained by solid-liquid separation in the solid-liquid separator 9 is returned to the nitrification tank 6 via a separated liquid returning means (line L31).

[0038] When a biologically attached carrier is used in the nitrification tank 6, 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.

[0039] When activated sludge is used in the nitrification tank 6, as shown in Fig. 2, activated sludge obtained in a settling tank 8 used for a solid-liquid separation step in which solid-liquid separation is performed on the treated liquid from the nitritation step can be extracted, in addition to the nitrification tank 6, and the extracted sludge can be supplied to a nitrite treatment tank 10 for the nitrite treatment step, for treatment. When activated sludge is used in the nitrification tank 6, the amount of sludge extracted relative to the total amount of sludge in the nitrification tank 6 and / or settling tank 8 from which activated sludge is extracted is 5 to 50 wt%, more preferably 10 to 45 wt%, even more preferably 15 to 35 wt%, and even more preferably 15 to 30 wt%.

[0040] 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.

[0041] 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. In order to maintain appropriate and stable nitrite-type nitrification treatment in the nitrification tank 6, it is necessary to appropriately and strictly control the contact conditions between the sludge and the aqueous nitrite solution in the nitrite treatment tank 10.

[0042] If the pH of the water to be treated in the nitrite treatment tank 10 is 6.0 or higher, the effect of suppressing the activity and proliferation of nitrite-oxidizing bacteria attached to the sludge may not be sufficiently achieved. The pH of the water to be treated in the nitrite treatment tank 10 is 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 limit on the lower limit of the pH of the water to be treated in the nitrite treatment tank 10, but if the pH is below 4.0, the activity and proliferation of ammonia-oxidizing bacteria contained in the sludge may be suppressed if the sludge is in contact with the nitrite aqueous solution for a long period of time. The pH of the water to be treated in the nitrite treatment tank 10 is 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 measured using a commonly available pH meter or the like.

[0043] 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 nitrite nitrogen concentration of the water to be treated in the nitrite treatment tank 10 is less than 250 mg-N / L, more preferably 240 mg-N / L or less, even 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 achieved. The nitrite nitrogen concentration of the water to be treated in the nitrite treatment tank 10 is preferably 30 mg-N / L or more, more preferably 50 mg-N / L or more, and even more preferably 100 mg-N / L or more. The nitrite nitrogen concentration in the nitrite treatment tank 10 can be measured using a commonly available nitrite sensor or the like.

[0044] 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 (1).

[0045]

number

[0046] The free nitrite concentration of the water to be treated in the nitrite treatment tank 10 is preferably 1.20 mg-N / L or less, more preferably 1.15 mg-N / L or less, and even more preferably 1.10 mg-N / L or less. The lower limit of the free nitrite concentration is preferably 0.70 mg-N / L or more, more preferably 0.77 mg-N / L or more, and even more preferably 0.80 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 the activity of ammonia-oxidizing bacteria is dominant over that of nitrite-oxidizing bacteria can be more efficiently produced in the nitrite treatment tank 10. Furthermore, by simultaneously adjusting the nitrite nitrogen concentration and the free nitrite concentration of the water to be treated in the nitrite treatment tank 10, the activity and proliferation of nitrite-oxidizing bacteria can be more reliably suppressed, while the activity and proliferation of ammonia-oxidizing bacteria can be maintained. Furthermore, even if the pH or temperature of the water to be treated fluctuates, the activity and proliferation of ammonia-oxidizing bacteria can be maintained to a certain extent by adjusting either the nitrite concentration or the free nitrite concentration. Furthermore, chemical costs can be reduced by appropriately adjusting the injection amount of nitrite or pH adjuster to adjust the nitrite concentration or free nitrite concentration in accordance with the unit price of the nitrite and pH adjuster.

[0047] 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, and appropriate and stable nitrite-type nitrification treatment in the nitrification tank 6 may become difficult. 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 retention time of sludge in the nitrite treatment tank 10 is 48 hours or less, preferably 24 hours or less, more preferably 12 hours or less, and even more preferably 8 hours or less. The lower limit of the retention time of sludge in the nitrite treatment tank 10 is preferably 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.

[0048] 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 separated from the solid-liquid separator 9 to the nitrite treatment tank 10, to heat the separated liquid.

[0049] The nitrite treatment tank 10 is provided with agitation means 11. The agitation means 11 may be a mechanical agitator equipped with a stirring blade or the like. The agitation means 11 may be a gas supply means provided at the bottom of the nitrification tank 6 for supplying gas into the nitrification tank 6. The agitation means 11 does not need to be driven constantly and may be driven intermittently.

[0050] In the nitrite treatment tank 10, when the DO of the water to be treated increases, the activity of nitrite-oxidizing bacteria in the sludge increases, and the effect of inhibiting the activity and proliferation of nitrite-oxidizing bacteria due to contact between the sludge and the nitrite aqueous solution decreases. As a result, it may become difficult to stably maintain the nitrite-type nitrification treatment in the nitrification tank 6. In the nitrite treatment step, the dissolved oxygen concentration of the water to be treated in the nitrification tank 6 is preferably 0.5 mg / L or less, more preferably less than 0.5 mg / L, even more preferably 0.3 mg / L or less, still more preferably 0.2 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 to maintain a low DO, the properties of the sludge can be adjusted so that efficient nitrite-type nitrification and denitrification treatment in the nitrification tank 6 is carried out.

[0051] The nitrite treatment tank 10 preferably comprises a water quality measuring means (not shown) including a DO meter that measures the dissolved oxygen concentration in the nitrite treatment tank 10, and a control means C that controls the dissolved oxygen concentration in the nitrite treatment tank 10 to be 0.5 mg / L or less. The control means C controls the aeration air volume in the nitrite treatment tank 10 so that the dissolved oxygen concentration in the nitrite treatment tank 10 is always 0.5 mg / L or less, and thereby sludge is produced in the nitrite treatment tank 10 in which the activity and proliferation of ammonia oxidizing bacteria are dominant over nitrite oxidizing bacteria.

[0052] Return means L2 is connected to nitrite treatment tank 10. Return means L2 is equipped with line L21 and line L22. Line L21 and line 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.

[0053] The water treatment device W according to the embodiment of the present invention further includes a denitrification tank 5 connected to the 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 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.

[0054] (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 Fig. 1. 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 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, 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.

[0055] Regarding the sludge in the nitritation step, when a bio-attached carrier is used in the nitrification tank 6, it is preferable to remove 10 to 50 wt% of the sludge in the nitritation tank 6. When activated sludge is used in the nitritation tank 6, it is preferable to remove 5 to 50 wt% of the sludge in the nitritation tank 6 and / or the settling tank 8 from which the activated sludge is removed, based on the total sludge volume. Furthermore, the sludge containing ammonia-oxidizing bacteria used in the nitritation step preferably contains a bio-attached carrier in which sludge is attached to a carrier. Furthermore, it is preferable to separate the sludge in the nitritation step into solid and liquid form, and supply the separated sludge to the nitrite treatment step. Furthermore, it is preferable to separate the sludge after the nitrite treatment step into solid and liquid form, and return the separated liquid to the nitrite treatment step. In the nitrite treatment step, it is preferable to control the dissolved oxygen concentration in the water to be treated to 0.5 mg / L or less.

[0056] The water treatment device W and water treatment method according to the embodiment of the present invention include a nitrite treatment tank 10 in which sludge extracted from a nitrification tank 6 is brought into contact with an aqueous nitrite solution under predetermined conditions. In the nitrite treatment tank 10, a treatment is carried out to suppress the activity and proliferation of nitrite-oxidizing bacteria. The nitrification tank 6, to which the sludge treated in the nitrite treatment tank 10 is returned, becomes an environment inhabited by many ammonia-oxidizing bacteria, so that nitrite-type nitrification treatment in the nitrification tank 6 is carried out appropriately and stably. As a result, a water treatment device W and water treatment method are obtained that can maintain the production of nitrite nitrogen in the nitritation step more appropriately and stably.

[0057] (Water treatment system using activated sludge method) 2 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] The sludge supplied from the solid-liquid separator 9 is introduced into the nitrite treatment tank 10, where 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, thereby carrying out nitrite treatment to suppress the activity and proliferation of nitrite-oxidizing bacteria contained in the sludge. The sludge treated with nitrite in the nitrite treatment tank 10 is returned to the nitrification tank 6. The treatment conditions in the nitrite treatment tank 10 are the same as those in the water treatment device W shown in FIG. 1.

[0062] In the water treatment system 100 according to the embodiment of the present invention, the separated sludge obtained by solid-liquid separation in the settling tank 8 is subjected to a specific 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 performed more cheaply and economically than when using a fluidized bed method.

[0063] 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%.

[0064] As shown in the water treatment system 100 of FIG. 3, activated sludge withdrawn 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 withdrawn 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.

[0065] According to the water treatment system 100 shown in Figure 3, 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-oxidizing 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.

[0066] (Water treatment system using the fluidized carrier method) The water treatment system 100 shown in Figure 4 differs from the water treatment system 100 shown in Figure 3 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 3, repeated description will be omitted.

[0067] The carriers used 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 a bonding 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.

[0068] 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, allowing for stable separation by a screen installed at the outlet of the reaction tank. Regarding the surface properties of the carrier, carriers with many micropores or numerous irregularities on the surface allow for rapid attachment and fixation of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria. This allows for high nitrification and denitrification performance to be achieved in a short period of time, and allows bacteria to be maintained at a high concentration in the tank for a long period of time.

[0069] 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 3The 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 for uniform flow within the tank. The carrier filling rate 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 for uniform mixing and flow.

[0070] 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.

[0071] According to the water treatment system 100 shown in FIG. 4 , 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 solid-liquid separation is easy in biological treatment using a fluidized carrier method as described above, the separated liquid obtained by solid-liquid separation of the sludge after nitrite treatment can be returned to the nitrite treatment tank 10 and reused. This reduces the amount of chemicals used.

[0072] Generally, biological treatment using the activated sludge method, 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 method, such as the water treatment system 100 shown in Figure 4, 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 method. Furthermore, the fluidized bed carrier method can form a thicker biofilm than the activated sludge method. 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.

[0073] In biological treatment using the activated sludge method, such as the water treatment system 100 shown in Figures 2 and 3, when the nitritation 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 4, a sharp drop in the nitrite accumulation rate is less likely to occur even after the nitritation 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 nitritation step.

[0074] In biological treatment using the activated sludge method, such as the water treatment system 100 shown in Figures 2 and 3, it takes about 14 days (activated sludge withdrawal amount: 47%, DO: 0.5 mg / L) for the activated sludge in the nitrite treatment tank 10 to be treated with nitrite and for the conditions in the sludge in the nitrite treatment tank 10 to become predominantly 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 10 to become predominantly ammonia-oxidizing bacteria. This allows for a shorter treatment time.

[0075] In addition, in the water treatment system 100 shown in Figure 4, trial calculations showed that the amount of sulfuric acid added to adjust the pH in the nitritation treatment tank could be reduced by about 99% compared to biological treatment using activated sludge. Trial calculations also showed that the amount of nitrite concentration added to adjust the pH in the nitritation treatment tank, or the amount of sodium nitrite added to control the free nitrite concentration, could be reduced by about 65% compared to biological treatment using activated sludge.

[0076] Furthermore, in the biological treatment utilizing the activated sludge method in the water treatment system 100 shown in Figures 2 and 3, it is necessary to keep the DO in both the nitrite treatment tank 10 and the nitrification tank 6 low in order to stably maintain the nitrite-type nitrification-denitrification treatment in the nitrification tank 6. In contrast, in the biological treatment utilizing the fluidized carrier method shown in the water treatment system 100 shown in Figure 4, the DO 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 in that the nitrite-type nitrification-denitrification treatment in the nitrification tank 6 can be stably maintained.

[0077] 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" in this specification and claims 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, as shown in the following examples, or treatment temporarily performed when the nitrite-type nitrification denitrification treatment is temporarily unstable. [Example]

[0078] 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.

[0079] (Example 1: Nitritation treatment using activated sludge) A continuous test of nitrite-type nitrification and denitrification treatment of raw water (Runs 1 and 2 (see Table 2)) was conducted using the test equipment shown in Figure 5, which includes a raw water tank, denitrification tank, nitrification tank, settling tank, treated water tank, and nitritation tank. The effective volumes of each equipment were 5.0 L for the denitrification tank, 11.5 L for the nitrification tank, and 2.0 L for the nitrite treatment 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: 3.0, and BOD / NH4-N: 1.5-3.0, to which an appropriate amount of trace element-containing activated sludge treatment chemical (ENP®-92 or ENP®-91, manufactured by Swing Co., Ltd.) had been 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 Activated sludge containing 100 copies / g-MLVSS was added.

[0080] Using the test equipment shown in Figure 5, a start-up process (Run 0) was conducted to gradually increase the nitrogen load and sludge load in the nitrification tank to perform normal nitrification and denitrification treatment in the nitrification tank. The activated sludge withdrawal rate during the start-up process was increased in stages to 12V% / day, 23V% / day, and 47V% / day. Note that when the activated sludge withdrawal rate was set to 12V% / day and 23V% / day, the start-up process of the nitrification tank did not progress. By treating with an activated sludge withdrawal rate of 47V% / day in the nitrification tank, a nitrogen removal rate of 90% was achieved. The start-up process (Run 0) was conducted for 30 days.

[0081] In the nitrite treatment (Run 1) following the start-up treatment (Run 0), nitritation-type nitrification / denitrification treatment was performed in the nitrification tank. As shown in Table 1, the sludge withdrawal and return rates from the nitrification tank to the nitrite treatment tank were set at 10 V% / cycle, and the nitrite treatment was performed for 10 days (first FNA treatment: FNA(1) in Table 1). Then, the nitrite treatment was performed for 10 days at 20 V% / cycle (second FNA treatment: FNA(2) in Table 1). Then, the nitrite treatment was performed for 35 days at 40 V% / cycle (third FNA treatment: FNA(3) in Table 1). After the third FNA treatment, the FNA treatment was stopped for 15 days. After the FNA treatment was stopped, the sludge withdrawal and return rates from the nitrification tank to the nitrite treatment tank were set at 40 V% / cycle, and the nitrite treatment was performed again for 30 days (fourth FNA treatment: FNA(4) in Table 1). The retention time of activated sludge in the nitrite treatment tank was set to 24 hours.

[0082] During the nitrite treatment (Run 1), the DO in the nitrification tank was controlled, and the pH, nitrite nitrogen concentration, and free nitrite concentration in the nitrite treatment tank were controlled to be below the control values ​​in Table 1. The water temperature was set to 25°C. DO was measured by placing a DO meter in the nitrification tank. pH, nitrite nitrogen concentration, and water temperature were measured by placing a nitrite sensor, pH, and water thermometer in the nitrite treatment tank. The free nitrite concentration was calculated by substituting the measurements of the nitrite sensor, pH, and water thermometer installed in the nitrite treatment tank into the following equation (1).

[0083]

number

[0084] [Table 1]

[0085] Figure 6 shows the changes in the nitrite accumulation rate and nitrogen removal rate in the nitrification tank in Example 1. In Figure 6, the "nitrite accumulation rate" and "nitrogen removal rate" were calculated using the following equations (2) and (3).

[0086]

number

[0087]

number

[0088] As shown in Figure 6, in the FNA(1) and FNA(2) processes, in which the amount of sludge extracted is small, the nitrite accumulation rate does not increase. However, in the FNA(3) process, in which the sludge extraction rate is 40V% per cycle, the pH of the solution in the nitrite treatment tank is 5.5, the nitrite concentration is 170mg-N / L, the FNA concentration is 1.2mg-N / L, and the retention time is 24 hours, and the nitrite-treated sludge is returned to the nitrification tank, the nitrite accumulation rate increases over the course of days, and the nitrite accumulation rate in the nitrification tank can be increased to a maximum of over 80%.

[0089] In the treatment using activated sludge, as shown in the FNA stop period and FNA(4) treatment in Figure 6, when the FNA treatment is stopped, the nitrite accumulation rate drops significantly, and even if the activated sludge is subsequently subjected to nitrite treatment (FNA(4)) again, the nitrite accumulation rate does not improve. Furthermore, in the FNA(4) treatment, even if the FNA concentration is increased to 3.5 mg / L and the DO in the nitrification tank is increased to 3.0 mg / L, the nitrite accumulation rate increases slightly, but only by a maximum of about 20%, and the nitrogen removal rate also decreases slightly.

[0090] FIG. 7 is a graph showing the change in aeration air volume in the nitrification tank and the nitrite accumulation rate (nitrite accumulation rate in the nitrification tank) in the treatment using the activated sludge of Example 1. As shown in the treatment of FNA(3), during the period from 51 to 85 days, during which nitrite treatment was performed with DO in the nitrification tank controlled at 0.5 mg / L and the nitrite concentration in the nitrite treatment tank controlled at 170 mg / L or less, the nitrite accumulation rate in the nitrification tank gradually increased, reaching a maximum of approximately 87%, indicating favorable results. On the other hand, during the period from 101 to 130 days, during which nitrite treatment was performed with DO in the nitrification tank controlled at 3.0 mg / L and the nitrite concentration in the nitrite treatment tank controlled at 500 mg / L or less, a slight increase in the nitrite accumulation rate was observed, but the nitrite accumulation rate was lower than that during the period from 51 to 85 days. Thus, it can be seen that the lower the DO in the nitrification tank during nitrite treatment of activated sludge, the higher the nitrite accumulation rate. Optimization of the aeration air volume into the nitrification tank was investigated based on the results of Figure 7, and it was found that the DO in the nitrification tank is preferably 1.0 mg / L or less, and more preferably about 0.5 mg / L. Although not limited to the following, trial calculations were made for the case where the nitrite treatment according to this embodiment was applied and the case where a conventional nitrite treatment was applied, and it was found that the aeration air volume in the nitrification tank can be reduced by about 37.2% according to this embodiment.

[0091] (Example 2: Nitritation treatment using a flow carrier (biofouling carrier)) A continuous test of nitritation treatment of raw water using a fluidized carrier was conducted using the test equipment shown in Figure 8, which is equipped with a raw water tank, denitrification tank, nitrification tank, nitrite treatment tank, and treated water tank. In Example 2, 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: 3.0, and BOD / NH4-N: 1.5-3.0, to which an appropriate amount of activated sludge treatment chemical (manufactured by Suing Co., Ltd., ENP (registered trademark)-92 or ENP (registered trademark)-91) had been added. As in Example 1, ammonia-oxidizing bacteria (amoA: 1.02 x 10) was added as seed sludge in the nitrification tank. 5 copies / g-MLVSS, 16SrRNA:3,15×10 8copies / g-MLVSS) and nitrite-oxidizing bacteria (Nitrobacter sp.: 1.58 × 10 4 copies / g-MLVSS, Nitrospira sp.:2.17×10 7 The nitrification solution circulation rate was 13 times the raw water flow rate (circulation ratio 13).

[0092] 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 for nitrification and denitrification treatment (Run 0) was performed, allowing a biofilm (sludge) containing nitrite-oxidizing bacteria and ammonia-oxidizing bacteria to adhere to the carrier.

[0093] In Example 2, continuous biological treatment tests using the fluidized bed carrier method were conducted under the conditions (Runs 0 to 3) shown in Table 2 according to the respective objectives. The first start-up process (Run 0) was conducted to perform nitrification and denitrification treatment in the nitrification tank, similar to Example 1. 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 for nitrification and denitrification treatment in the nitrification tank was completed when the nitrogen removal rate in the nitrification tank reached 90%. In the second start-up process (Run 1), in order to perform nitritation-type nitrification and denitrification treatment in the nitrification tank, the amount of carrier withdrawn and the amount of carrier returned from the nitrification tank to the nitrite treatment tank were gradually increased from 10 V% / time to 20 V% / time, while nitrite treatment and carrier withdrawal and return processes were conducted.

[0094] In the second start-up process (Run 1), a portion of the carrier to which the biofilm had attached 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 and subjected to nitrite treatment.

[0095] Nitrite treatment was performed by measuring the quality of the water in the nitrite treatment tank using water quality measuring devices such as a pH meter, water thermometer, and nitrite sensor installed in the nitrite treatment tank. Based on the measurement results, sulfuric acid and sodium nitrite were added to maintain the pH of the solution in the nitrite treatment tank at 5.5, the nitrite concentration at 170 mg-N / L, the free nitrite concentration at 1.20 mg-N / L, and the water temperature at 25°C. The nitrite treatment tank was not aerated, and the carrier was left in the tank for 18 hours while being shaken. The nitrite-treated carrier was then subjected to solid-liquid separation, and only the carrier obtained from solid-liquid separation was returned to the nitrification tank. The separated liquid from solid-liquid separation was returned to the nitrite treatment tank.

[0096] In steady-state operation (Runs 2 and 3), the BOD of the raw water was gradually reduced (30% reduction in Run 2, 50% reduction in Run 3) to evaluate the effect of reducing the required BOD. In steady-state operation (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 supplied to the nitrification tank. The separated liquid from the solid-liquid separator was returned to the nitrification tank. The carrier treated in the nitrite treatment tank was supplied to the solid-liquid separator, and the separated carrier was returned to the nitrification tank. The separated liquid from the solid-liquid separation tank was returned to the nitrite treatment tank for reuse. A portion of the water to be treated (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 supplied to the nitrification tank. The amount of activated sludge withdrawn from the nitrification tank to the nitrite treatment tank during steady-state operation was 30V%.

[0097] [Table 2]

[0098] FIG. 9 is a schematic diagram showing the changes in the nitrite accumulation rate and nitrogen removal rate in the nitrification tank in Example 2. The evaluation of the nitrite accumulation rate and nitrogen removal rate in FIG. 9 was calculated based on the above-mentioned formulas (2) and (3). In the treatment in Example 2 using the fluidized carrier method, the nitrogen removal rate reached 90% or more in about 10 days, and the start-up process for the conventional nitrification denitrification treatment, which is a conventional complete nitrification-type nitrification denitrification, was completed within 14 days. In the example in FIG. 9, the sludge withdrawal rate from the nitrification tank was increased to 10 V% / day from the 30th day, 20 V% / day from the 40th day, and 40 V% / day from the 50th day. The nitrite accumulation rate in the nitrification tank gradually increased after the sludge withdrawal rate reached 20 V% / day, and exceeded 70% after 60 days. Therefore, by performing the water treatment method according to this embodiment, nitrite-type nitrification denitrification treatment in the nitrification tank is possible.

[0099] 9, in order to confirm the stability of the nitritation step, the nitrite treatment to the carrier was stopped by stopping the removal and return of the carrier after 70 days, and the nitrite accumulation rate in the nitrification tank was maintained at about 67 to 79% even on the 100th day. In other words, this embodiment demonstrates that nitrite-type nitrification denitrification treatment in the nitrification tank is possible for at least about 30 days even when the removal and return of the carrier is stopped.

[0100] 10 is a graph showing the time-dependent change in aeration air volume and nitrite accumulation rate when steady-state operation was carried out after start-up treatment of the nitrite-type nitrification denitrification treatment in Example 2. After 50 days, when the nitrite accumulation rate in the nitrification tank exceeded 50%, the nitrite accumulation rate in the nitrification tank could be maintained at a high level even with aeration air volume low. Calculations showed that the water treatment method according to this embodiment can reduce the aeration air volume by about 20% compared to conventional complete nitrification denitrification, reducing the power required for treatment and enabling efficient biological treatment.

[0101] (Example 3: Effect on the reduction of the amount of electron donor added in denitrification treatment) FIG. 11 shows the results of evaluating the nitrogen removal rate of the denitrification tank when the amount of electron donor (organic matter) added to the denitrification tank was gradually reduced during steady-state operation (Runs 2 and 3) of Example 2. Sodium acetate was added as the organic matter. Although treatment performance temporarily decreased slightly immediately after the amount of organic matter added was reduced by 20 to 25% compared to the initial amount, treatment performance gradually recovered thereafter, and stable denitrification treatment was again possible. Furthermore, when the amount of organic matter added was reduced by 30 to 35% compared to the initial amount, treatment performance also temporarily decreased slightly, but treatment performance gradually recovered thereafter, and stable denitrification treatment was again possible. It can be seen that this embodiment allows the supply of electron donor required for nitrification / denitrification treatment to be reduced compared to conventional complete nitrification / denitrification.

[0102] (Example 4: Change in nitrification rate in the nitrification tank due to nitrite treatment) Figure 12(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 water being treated in the nitrite treatment tank was maintained at 100 to 250 mg / L in Example 2. Figure 12(b) shows the nitrite concentration in the nitrite treatment tank and the activity (%) of ammonia-oxidizing bacteria and BOD in the nitrite treatment step. In Figure 12(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 12(b), the "activity rate" was evaluated by the activity after nitrite treatment relative to the activity before nitrite treatment. As shown in Figure 12(a), the activity of nitrite-oxidizing bacteria in the nitrification tank decreased as the nitrite concentration in the nitrite treatment tank increased. However, when the nitrite concentration was 250 mg / L, the activity of ammonia-oxidizing bacteria in the nitrification tank was also reduced by half compared to before nitrite treatment. Furthermore, as shown in Figure 12(b), when the nitrite concentration in the nitrite treatment tank is 250 mg / L, the activity of the nitrite oxidizing bacteria is suppressed to about one-quarter compared to when the nitrite concentration is 100 mg / L, but the activity of the ammonia oxidizing bacteria is also suppressed by about 40%. [Explanation of symbols]

[0103] 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 11: Stirring means 15: Screen 100: Water treatment system L1: Extraction means L2: Return method L31, L32: Separation liquid return means 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 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, 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; A water treatment method comprising:

2. 2. The water treatment method according to claim 1, wherein the dissolved oxygen concentration in the nitrite treatment step is controlled to 0.5 mg / L or less.

3. 2. The water treatment method according to claim 1, wherein 10 to 50 wt % of the sludge from the nitritation step is extracted.

4. 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.

5. a solid-liquid separation step of separating the treated liquid from the nitritation step into solid and liquid; A step of extracting a portion of the sludge separated in the solid-liquid separation step and supplying the extracted sludge to the nitrite treatment step.

2. The water treatment method according to claim 1, further comprising:

6. 2. The water treatment method according to claim 1, wherein the sludge from the nitritation step is extracted and subjected to solid-liquid separation, and the sludge after the solid-liquid separation is supplied to the nitrite treatment step.

7. The water treatment method according to claim 1, further comprising subjecting the sludge after the nitrite treatment step to solid-liquid separation, and returning the separated liquid after the solid-liquid separation to the nitrite treatment step.

8. 8. The water treatment method according to claim 1, further comprising a denitrification step of reducing nitrite nitrogen contained in the treated water from the nitritation step to nitrogen by denitrifying bacteria.

9. a nitrification tank in which ammonia nitrogen contained in raw water containing nitrogen components is oxidized to nitrite nitrogen using sludge containing ammonia-oxidizing bacteria; an extracting means for extracting the sludge containing the ammonia-oxidizing bacteria from the nitrification tank; a nitrite treatment tank into which the sludge extracted by the extraction means is introduced and which contacts the sludge 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, thereby suppressing 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 treatment device comprising:

10. a DO meter for measuring the dissolved oxygen concentration in the nitrite treatment tank; a control means for controlling the dissolved oxygen concentration in the nitrite treatment tank to be 0.5 mg / L or less; The water treatment device according to claim 9, further comprising:

11. 10. The water treatment device according to claim 9, wherein the sludge containing ammonia oxidizing bacteria includes a bioattached carrier in which the sludge is attached to a carrier.

12. 10. The water treatment device according to claim 9, further comprising a separated liquid returning means for performing solid-liquid separation on the sludge treated in the nitrite treatment tank and returning the separated liquid after solid-liquid separation to the nitrite treatment tank.

13. 13. The water treatment device according to claim 9, further comprising a denitrification tank connected to the nitrification tank, in which nitrite nitrogen contained in the treated water of the nitrification tank is reduced to nitrogen by denitrifying bacteria.

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