Water treatment method, water treatment apparatus, and start-up method for water treatment apparatus

Abstract

To provide a water treatment method capable of efficiently and stably biologically treating ammoniacal nitrogen in organic wastewater containing night soil or septic tank sludge using nitrite type nitrification and denitrification treatment, a water treatment apparatus, and a method for starting up the water treatment apparatus.SOLUTION: A water treatment method includes a nitrifying step of oxidizing ammonium nitrogen in organic wastewater containing human waste or septic tank sludge into nitrite nitrogen using organism-attaching carriers in which ammonium-oxidizing bacteria are attached to carriers, an extracting step of extracting some of the organism-attaching carriers in the nitrifying step, a nitrite treatment step of suppressing activity and growth of the nitrite-oxidizing bacteria attached to the organism-attaching carriers by bringing the extracted organism-attaching carriers into contact with a nitrite aqueous solution, and a returning step of returning the organism-attaching carriers obtained in the nitrite treatment step to the nitrifying step.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a water treatment method, a water treatment device, and a method for starting up 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 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, such as five days or more, to kill nitrite-oxidizing bacteria. This method kills not only nitrite-oxidizing bacteria but also some ammonia-oxidizing bacteria. Therefore, when treating ammonia nitrogen in organic wastewater containing human waste or septic tank sludge, it may be difficult to efficiently and stably treat the ammonia nitrogen using a nitrite-based nitrification / denitrification process.

[0009] In view of the above problems, the present invention provides a water treatment method, a water treatment device, and a method for starting up a water treatment device, which are capable of efficiently and stably biologically treating ammonia nitrogen in organic wastewater containing sewage or septic tank sludge using a nitrite-type nitrification-denitrification process. [Means for solving the problem]

[0010] As a result of intensive research into solving the above problems, the present inventors have discovered that it is useful to treat organic wastewater containing sewage or septic tank sludge with nitritation using a bioattached carrier to which microorganisms have been attached, to subject the bioattached carrier to nitrite treatment in such a way that the activity and proliferation of ammonia-oxidizing bacteria attached to the bioattached carrier is dominant while suppressing the activity and proliferation of nitrite-oxidizing bacteria attached to the bioattached carrier, and to return the bioattached carrier after nitrite treatment to the nitritation treatment.

[0011] In order to solve the above problems, in one aspect, the present invention provides a water treatment method comprising: a nitritation step in which ammonia nitrogen in organic wastewater containing sewage or septic tank sludge is oxidized to nitrite nitrogen using bioattached carriers having ammonia-oxidizing bacteria attached to the carriers; an extraction step in which a portion of the bioattached carriers from the nitritation step is extracted; a nitrite treatment step in which the extracted bioattached carriers are brought into contact with an aqueous nitrite solution to suppress the activity and proliferation of the nitrite-oxidizing bacteria attached to the bioattached carriers; and a return step in which the bioattached carriers obtained in the nitrite treatment step are returned to the nitritation step.

[0012] In one embodiment, the water treatment method according to the present invention further comprises a denitrification step of reducing nitrite nitrogen contained in the treated water from the nitritation step to nitrogen using a bioattached carrier having denitrifying bacteria attached thereto.

[0013] In another embodiment, the water treatment method according to the present invention further comprises a step of recycling a portion of the organic wastewater containing the bioattached carriers to the denitrification step after returning the bioattached carriers to the nitritation step.

[0014] In yet another embodiment, the water treatment method according to the present invention further comprises a step of heating the bioattached carriers removed in the nitrite treatment step.

[0015] In yet another embodiment of the water treatment method according to the present invention, the nitrite treatment step is carried out during the nitritation step, and the withdrawal step and return step involve withdrawing and returning the bioattached carriers without stopping the nitritation step.

[0016] In yet another embodiment, the water treatment method according to the present invention includes a step of suspending the withdrawal and return of the bioattached carrier for a certain period of time based on the free nitrite concentration or nitrite concentration in the nitrification tank where the nitritation step is carried out.

[0017] In yet another embodiment of the water treatment method according to the present invention, the organic wastewater is a separated liquid obtained by dehydrating organic wastewater containing human waste or septic tank sludge.

[0018] In yet another embodiment, the water treatment method according to the present invention comprises subjecting the extracted bioattached carriers to solid-liquid separation, introducing the bioattached carriers separated by solid-liquid separation into a nitrite treatment step, and returning the separated liquid by solid-liquid separation to the nitritation step.

[0019] In yet another embodiment, the water treatment method according to the present invention comprises subjecting the bioattached carriers obtained in the nitrite treatment step to solid-liquid separation, returning the bioattached carriers separated by solid-liquid separation to the nitritation step, and returning the separated liquid separated by solid-liquid separation to the nitrite treatment step.

[0020] In yet another embodiment of the water treatment method according to the present invention, the pH and nitrite concentration of the water to be treated in the nitrite treatment tank in which the nitrite treatment step is carried out are measured, and the pH of the water to be treated is controlled to be less than 6.0, the nitrite nitrogen concentration to be less than 250 mg-N / L, and the residence time of the bioattached carrier to be within 48 hours.

[0021] In another aspect, the present invention provides a water treatment device comprising: a nitrification tank that holds bioattached carriers with ammonia-oxidizing bacteria attached to the carriers, and that oxidizes ammonia nitrogen in organic wastewater containing sewage or septic tank sludge to nitrite nitrogen using the bioattached carriers; extraction means that extracts a portion of the bioattached carriers from the nitrification tank; a nitrite treatment tank that suppresses the activity and proliferation of the nitrite-oxidizing bacteria attached to the bioattached carriers by contacting the bioattached carriers extracted by the extraction means with an aqueous nitrite solution; and return means that returns the bioattached carriers in the nitrite treatment tank to the nitrification tank.

[0022] In yet another aspect, the present invention provides a start-up method for a water treatment device equipped with a nitrification tank for nitritating ammonia nitrogen in organic wastewater containing sewage or septic tank sludge using bioattached carriers having ammonia-oxidizing bacteria attached thereto. The start-up method for a water treatment device includes a first start-up process in which organic wastewater is passed through the nitrification tank containing the carriers and the nitrogen load is increased stepwise until the nitrogen removal rate reaches a predetermined value, thereby causing a biofilm containing ammonia-oxidizing bacteria and nitrite-oxidizing bacteria to attach to the surface of the carriers and retaining the biofilm-attached bioattached carriers in the nitrification tank; and a second start-up process in which some of the bioattached carriers are removed from the nitrification tank and brought into contact with an aqueous nitrite solution to perform nitrite treatment to suppress the activity and growth of the nitrite-oxidizing bacteria attached to the bioattached carriers, and the bioattached carriers after nitrite treatment are returned to the nitrification tank.

[0023] In one embodiment of the start-up method for a water treatment device according to the present invention, the second start-up process adjusts the aeration air volume in the nitrite treatment tank so that the dissolved oxygen concentration of the water to be treated in the nitrification tank where the nitrite treatment is performed is 5.0 mg / L or less.

[0024] In another embodiment of the start-up method for a water treatment device according to the present invention, the second start-up treatment is carried out so that the amount of organism attachment carriers extracted from the nitrification tank is 20V% or more.

[0025] In another embodiment of the start-up method for a water treatment device according to the present invention, the second start-up treatment is controlled so that the pH in the nitrite treatment tank where the nitritation treatment is performed is less than 6.0, the nitrite nitrogen concentration is less than 250 mg-N / L, and the residence time of the bioattached carrier is within 48 hours. [Effects of the Invention]

[0026] According to the present invention, there are provided a water treatment method, a water treatment device, and a method for starting up a water treatment device, which are capable of efficiently and stably biologically treating ammonia nitrogen in organic wastewater containing sewage or septic tank sludge using a nitrite-type nitrification-denitrification process. [Brief explanation of the drawings]

[0027] [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] 1 is a schematic diagram illustrating an example of a water treatment system 100 according to an embodiment of the present invention. [Figure 3] FIG. 1 is a schematic diagram showing a test device of Example 1. [Figure 4] FIG. 2 is a schematic diagram showing the change in the nitrite accumulation rate in the nitrification tank and the nitrogen removal rate during start-up and steady-state operation in Example 1. [Figure 5] 1 is a graph showing the change over time in the aeration air volume and the nitrite accumulation rate in the nitrification tank in Example 1. [Figure 6] 1 is a graph evaluating the nitrogen removal rate in the denitrification tank when the amount of electron donor added (amount of organic matter added) supplied to the denitrification tank is reduced stepwise during steady operation in Example 1. [Figure 7] Figure 7(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 7(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 process. DETAILED DESCRIPTION OF THE INVENTION

[0028] 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.

[0029] (Water treatment equipment) 1 is a schematic diagram showing 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 organic wastewater containing human waste or septic tank sludge is oxidized to nitrite nitrogen using bioattached carriers having ammonia-oxidizing bacteria attached to the carriers, extraction means L1 for extracting a portion of the bioattached carriers from the nitrification tank, a nitrite treatment tank 10 in which the bioattached carriers extracted by the extraction means L1 are contacted with an aqueous nitrite solution to suppress the activity and proliferation of the nitrite-oxidizing bacteria attached to the bioattached carriers, and return means L2 for returning the bioattached carriers after the nitrite treatment step to the nitritation step.

[0030] As the raw water, organic wastewater containing human waste or septic tank sludge is used. In this embodiment, it is particularly preferable to use wastewater containing human waste and septic tank sludge as the raw water, and it is more preferable to use organic wastewater containing a higher proportion of septic tank sludge than human waste. The wastewater containing at least human waste or septic tank sludge may include sludge generated in agricultural village wastewater treatment.

[0031] For example, the ratio (volume ratio) of human waste to septic tank sludge is: 3 In contrast, septic tank sludge is 0 to 1 m 3 , preferably 0.2 to 0.8 m 3 , and more preferably 0.4 to 0.6 m 3This method can treat wastewater containing ammonium nitrate, ammonium nitrate, and ammonium nitrate, particularly stably and efficiently. Examples of such raw water include wastewater with 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 to 3, 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 to 10, preferably 3.5 to 8. Raw water with a BOD / NH4-N ratio of less than 3 is particularly 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, requiring the addition of external organic matter such as methanol.

[0032] The ratio (volume ratio) of human waste to septic tank sludge is: 3 For example, 1m of septic tank sludge 3 Over 99m 3 Less than 2 to 50 m, preferably 3 This 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 to 6, preferably 3.5 to 5. As mentioned above, raw water with a BOD / NH4-N ratio below 3 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.

[0033] The nitrification tank 6 contains bioattached carriers, in which sludge containing ammonia-oxidizing bacteria is attached to the carriers. Examples of bioattached carriers that can be used include entrapping immobilization carriers in which ammonia bacteria and nitrite-oxidizing bacteria are entrappingly immobilized using a resin or the like, and bonded immobilization carriers in which a biofilm containing ammonia-oxidizing bacteria and nitrite-oxidizing bacteria is attached to the surface of the carrier. In this embodiment, the use of bonded immobilization carriers is preferred because it is easy to adjust the carriers so that specific microorganisms become dominant and to stably retain them.

[0034] By retaining the bioattached carriers in the nitrification tank 6, the efficiency of transporting the substrate to the microorganisms in the nitrification tank 6 increases. Therefore, the reaction rate of the nitrification / denitrification treatment is faster in the nitrification tank 6 retaining the bioattached carriers than in a tank retaining activated sludge. As a result, the nitrification / denitrification treatment during steady operation is stabilized, and the start-up treatment at the time of starting up the equipment, which will be described later, is completed quickly.

[0035] 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.

[0036] 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, carriers with many micropores or numerous irregularities on the surface allow ammonia-oxidizing bacteria and nitrite-oxidizing bacteria to adhere and fix quickly, thereby achieving high nitrification and denitrification performance in a short period of time and allowing the bacteria to be maintained at a high concentration in the tank for a long period of time.

[0037] 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 Preferably, the carrier has a specific gravity of 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 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 for uniform mixing and flow.

[0038] 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.

[0039] In the nitrification tank 6, treatment conditions are adjusted so that a nitrite-type nitrification-denitrification process is carried out, in which ammonia nitrogen in the raw water is oxidized to nitrite nitrogen by the bioattached carrier. As a result of the nitrite-type nitrification-denitrification process being carried out in the nitrification tank 6, the concentration of nitrite nitrogen in the nitrification tank 6 increases. By subjecting the treated water (nitrification liquid) in the nitrification tank 6, which contains a large amount of nitrite nitrogen, to denitrification in the denitrification tank 5, the amount of electron donor such as methanol required for denitrification can be reduced, thereby improving treatment efficiency.

[0040] If the dissolved oxygen concentration (DO) in the nitrification tank 6 becomes too high, the proliferation and activity of nitrite-oxidizing bacteria attached to the activated sludge will increase, making it difficult to stably maintain nitritation-type nitrification treatment. In this embodiment, because the bioattached carrier is maintained in the nitrification tank 6, the DO in the nitrification tank 6 does not need to be controlled as strictly as when activated sludge is maintained in the nitrification tank 6. Although not limited thereto, 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 described above, the DO in the nitrification tank 6 does not need to be strictly controlled, but the lower the DO, the more the nitrite-type nitrification denitrification treatment can proceed predominantly in the nitrification tank 6. The DO in the nitrification tank 6 may be set to 1.0 mg / L or less, or 0.5 mg / L or less.

[0041] In the nitrification tank 6, the nitrogen volume load is, for example, but not limited to, 0.3 to 0.4 kg / m 3 pH is 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 0.4 to 0.5 kg / m 3 · day, nitrite oxidation rate is 0.1-0.2 kg / m 3 Nitritation treatment is carried out so that the dissolved oxygen concentration (DO) is 5.0 mg / L or less, typically 2.0 to 3.0 mg / L per day. The ammonia oxidation rate and nitrite oxidation rate can be evaluated by a batch oxidation rate measurement test, in which the bioattached carrier in the nitrification tank is placed in a designated container and the activity of each bacterium is tested. The batch oxidation rate was measured according to the "Nitrification Rate Test Method" described in Chapter 2, Section 2 of the Sewage Testing Methods.

[0042] The withdrawal means L1 is connected between the nitrification tank 6 and the nitrite treatment tank 10. The withdrawal means L1 is equipped with lines L11 and 12. 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 configured to supply sludge treated in the nitrification tank 6 to the solid-liquid separator 9 via line L11, and to supply the sludge that has been solid-liquid separated 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 line L31, which serves as separated liquid returning means.

[0043] 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 treatment status in the nitrification tank 6. As the solid-liquid separator 9, a screw conveyor equipped with a screen for capturing solid-adhered carriers or the like is suitably used.

[0044] The nitrite treatment tank 10 is a treatment tank (FNA treatment tank) into which the bioattached carriers extracted by the extraction means L1 are introduced and into which the sludge is brought into contact with the nitrite aqueous solution. In order to maintain the nitrite-type nitrification and denitrification treatment in the nitrification tank 6 appropriately and stably, it is necessary to appropriately and strictly control the contact conditions between the bioattached carriers and the nitrite aqueous solution in the nitrite treatment tank 10.

[0045] 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 in the bioattached carrier 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 lower, more preferably 5.6 or lower, and even more preferably 5.5 or lower. There is no particular lower limit for 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.

[0046] 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 and denitrification treatment will be suppressed, which may make it difficult to perform appropriate and stable nitrite-type nitrification and denitrification 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-N / 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.

[0047] 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).

[0048]

number

[0049] 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 active growth of nitrite-oxidizing bacteria can be more reliably suppressed, and the activity and growth 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.

[0050] 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 and denitrification treatment will be suppressed, which may make it difficult to perform appropriate and stable nitrite-type nitrification and denitrification 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 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.

[0051] 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 heated to be maintained at 20 to 28°C, more preferably heated to be maintained at 23 to 27°C, and even more preferably heated to be maintained 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, and then returning the heated separated liquid to the nitrite treatment tank 10.

[0052] 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.

[0053] In the nitrite treatment tank 10, as the dissolved oxygen concentration in 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 / denitrification treatment in the nitrification tank 6. The dissolved oxygen concentration of the water to be treated in the nitrite treatment step 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 so that the DO in the nitrite treatment tank 10 is low, the properties of the sludge can be adjusted so that the nitrite-type nitrification / denitrification step in the nitrification tank 6 is performed efficiently.

[0054] The nitrite treatment tank 10 preferably comprises a water quality measuring means 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.

[0055] Return means L2 is connected to nitrite treatment tank 10. Return means L2 is equipped with 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 (nitrified liquid) obtained by solid-liquid separation in solid-liquid separator 9 is returned to nitrite treatment tank 10 via line L32, which serves as separated liquid return means.

[0056] 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.

[0057] (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 an embodiment of the present invention comprises a nitritation step in which ammonia nitrogen in organic wastewater containing sewage or septic tank sludge is oxidized to nitrite nitrogen using bioattached carriers having ammonia-oxidizing bacteria attached to the carriers, an extraction step in which a portion of the bioattached carriers from the nitritation step is extracted, a nitrite treatment step in which the extracted bioattached carriers are contacted with an aqueous nitrite solution to suppress the activity and growth of the nitrite-oxidizing bacteria attached to the bioattached carriers, and a return step in which the bioattached carriers obtained in the nitrite treatment step are returned to the nitritation step.

[0058] The use of bioattached carriers, especially bonded immobilized carriers, in the nitritation process creates a thick biofilm, which creates a concentration gradient of dissolved oxygen within the biofilm. Because the activity of nitrite-oxidizing bacteria is suppressed at low DO, the use of carriers makes it possible to maintain nitritation treatment in the nitritation process more stably than activated sludge, which has a thin biofilm and is less likely to create a concentration gradient.

[0059] The water treatment method according to the embodiment of the present invention preferably further includes a denitrification step in which nitrite nitrogen contained in the treated water from the nitritation treatment is reduced to nitrogen using a bioattached carrier having denitrifying bacteria attached thereto. As in the nitritation step, the use of a bioattached carrier in the denitrification step creates a thickness in the biofilm, creating a concentration gradient of dissolved oxygen within the biofilm. This allows the denitrification treatment to proceed stably. The denitrification treatment liquid after the denitrification treatment is further subjected to nitritation treatment in the nitritation step. In this case, to prevent activated sludge from being mixed into the nitrification tank, the denitrification treatment liquid after the denitrification treatment is preferably subjected to solid-liquid separation using a solid-liquid separator or the like before being introduced into the nitrification tank 6.

[0060] It is preferable to further include a circulation step in which, after returning the bioattached carriers to the nitritation step, a portion of the water to be treated containing the bioattached carriers is circulated to the denitrification step. In this case, in order to prevent the bioattached carriers from being mixed into the denitrification tank 5 from the nitrification tank 6, it is preferable to circulate the portion of the water to be treated containing the bioattached carriers to the denitrification tank 5 after solid-liquid separation.

[0061] The nitrite treatment step preferably further includes a step of heating the removed bioattached carrier. At low temperatures, the activity and proliferation of nitrite-oxidizing bacteria are promoted, which may prevent the nitritation step from being carried out stably. According to this embodiment, by heating the aqueous nitrite solution, it is possible to selectively suppress the activity and proliferation of nitrite-oxidizing bacteria while maintaining the activity and proliferation of ammonia-oxidizing bacteria dominant, regardless of the season, even at low temperatures. The water to be treated in the nitrite treatment tank 10 is heated so that the temperature is maintained at, for example, 20 to 28°C.

[0062] In this embodiment, a nitrite treatment step is preferably performed during the nitritation step, and the bioattached carriers are preferably withdrawn and returned in the withdrawal and return steps without stopping the nitritation step. That is, in the water treatment method according to this embodiment, the nitritation step and denitrification step are continuously performed in the nitrification tank 6 and denitrification tank 5 (main stream), while the nitrite treatment of the bioattached carriers using an aqueous nitrite solution is performed in the nitrite treatment tank 10 (substream). The bioattached carriers in the nitrite treatment tank 10 are then returned to the nitrification tank 6 (main stream) at appropriate timing, and the bioattached carriers in the nitritation tank 6 are withdrawn and supplied to the nitrite treatment tank 10 (substream), allowing efficient treatment without stopping the nitrification / denitrification treatment of the organic wastewater. If the timing of withdrawing and returning the bioattached carriers is too long, the activity and proliferation of the microorganisms attached to the bioattached carriers may be impaired. The timing of removing and returning the bioattached carrier is, for example, preferably every 12 to 24 hours, more preferably every 16 to 24 hours, and even more preferably every 16 to 18 hours. Typically, the timing of removing and returning the bioattached carrier can be every 24 hours, every 18 hours, or every 12 hours.

[0063] In the case of a nitrification tank 6 containing activated sludge, if the nitrite-treated activated sludge is not returned for a certain period of time, it may become difficult to stably perform nitrite-type nitrification / denitrification treatment in the nitrification tank 6. In this embodiment, because a bioattached carrier is used, it is possible to suspend the withdrawal and return of the bioattached carrier for a certain period of time based on the free nitrite concentration or nitrite concentration in the nitrite treatment tank 19 during the nitritation step. For example, according to this embodiment, nitrite-type nitrification / denitrification treatment can be stably performed in the nitrification tank 6 even if the system is stopped for 30 days or more. Therefore, even in situations where the system must be temporarily stopped, such as during the New Year holidays, the nitrification tank 6 can be maintained in a state where nitrite-type nitrification / denitrification treatment is possible.

[0064] In one aspect of this embodiment, it is preferable to separate the bioadherent carriers extracted in the extraction step into solid-liquid separation, introduce the separated bioadherent carriers into the nitrite treatment step, and return the separated liquid to the nitritation step. The commonly used activated sludge takes time to thicken by gravity, and even after thickening, the solids (SS) concentration is often about 1 wt%, with about 99 wt% remaining water. When using activated sludge, solid-liquid separation can be achieved by adding inorganic flocculants or polymers or by using mechanical thickening, but this increases running costs.

[0065] On the other hand, the bioattached carriers used in this embodiment can be separated into solids and liquids in a shorter time than activated sludge, and can achieve high concentration. Therefore, when the bioattached carriers separated by solid-liquid separation are introduced into the nitrite treatment step, the amount of water in the nitrification liquid accompanying the bioattached carriers can be minimized, and the overall amount of water in the nitrification liquid mixed into the nitrite treatment tank can be reduced. As a result, fluctuations in the treatment conditions in the nitrite treatment tank due to the mixing of nitrification liquid can be reduced.

[0066] In one aspect of this embodiment, it is preferable to separate the bioattached carriers after the nitrite treatment step into a solid-liquid state, return the bioattached carriers separated by solid-liquid separation to the nitritation 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 bioattached carriers after the nitrite treatment step can be reused as an aqueous nitrite solution in the nitrite treatment step, thereby reducing the amount of nitrite such as sodium nitrite required to adjust the treatment conditions in the nitrite treatment step and the amount of pH adjuster such as sulfuric acid required to adjust the pH.

[0067] 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 nitrite treatment tank 10, where the nitrite treatment step is performed, 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 of the bioadherent carriers is not performed at an appropriate pH and nitrite concentration in the nitrite treatment tank 10, stable nitrite-type nitrification 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 denitrification treatment.

[0068] As described above, 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 bioattached carriers removed from the nitrification tank 6 are 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 in which ammonia-oxidizing bacteria thrive, and therefore nitrite-type nitrification-denitrification treatment in the nitrification tank 6 is carried out appropriately and stably. As a result, a water treatment method and water treatment device W are obtained that can efficiently and stably treat ammonia nitrogen in organic wastewater containing sewage or septic tank sludge by nitrite-type nitrification-denitrification treatment.

[0069] (Water treatment system) 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.

[0070] Furthermore, the water treatment system 100 according to the embodiment of the present invention includes a first denitrification tank 5 for biologically treating the concentrated / dehydrated separated liquid obtained by concentrating / dehydrating 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 contacting carriers separated in the solid-liquid separator 9 with an aqueous nitrite solution. Screens 15 are provided in the circulation line L4 between the first denitrification tank 5 and the nitrification tank 6 and in the piping connecting to the second denitrification tank 7a to prevent carriers from flowing out of the tanks.

[0071] This water treatment system 100 is an example of a system using a direct dehydration (pre-dehydration) type denitrification treatment method, which directly dehydrates incoming night soil and septic tank sludge and biologically treats the resulting separated liquid. This system directly dehydrates night soil and septic tank sludge and reduces the moisture content of the dehydrated cake, making it a very useful technology in that the resulting dehydrated cake can be effectively used as a combustion improver. Meanwhile, this system directly dehydrates night soil and septic tank sludge, removing organic matter derived from solids (SS). This reduces the BOD / N ratio of the treated water before nitrification / denitrification treatment. Therefore, organic matter such as methanol may be added to promote denitrification. According to this embodiment, the amount of organic matter required to promote denitrification is sufficient to reduce nitrite nitrogen to nitrogen, which is less than the amount required to reduce nitrite nitrogen to nitrogen. Therefore, applying this direct dehydration (pre-dehydration) type denitrification treatment method has the advantage of reducing the amount of organic matter, such as methanol, added during denitrification treatment and further reducing the aeration air volume.

[0072] The separated liquid in the separated liquid storage tank 4 is introduced into a first denitrification tank 5 that holds a bio-attached carrier with denitrifying bacteria attached. In the first denitrification tank 5, the separated liquid is brought into contact with the bio-attached carrier, 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 that holds a bio-attached carrier with ammonia-oxidizing bacteria attached.

[0073] 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 treatment 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 flow rate of the raw water 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).

[0074] 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.

[0075] 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. The treatment conditions in the nitrite treatment tank 10 are the same as those in the water treatment device W shown in FIG. 1.

[0076] 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.

[0077] According to the water treatment system 100 shown in FIG. 2 , bioattached carriers are retained in the denitrification process in the first denitrification tank 5 and in the nitrification tank 6. Compared to biological treatment using activated sludge, biological treatment using a fluidized bed carrier method with bioattached carriers 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 enters the nitrification tank 6 is significantly less than when activated sludge is used. 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 maintaining stable treatment performance in the first denitrification tank 5 and nitrification tank 6. Furthermore, the separated liquid obtained by solid-liquid separation of the bioattached carriers after nitrite treatment can be returned to the nitrite treatment tank 10 for reuse. This reduces the amount of chemicals used.

[0078] In conventional biological treatment using the activated sludge method, when the nitritation treatment of activated sludge is stopped, the nitrite accumulation rate in the nitrification tank 6 drops sharply, which can lead to unstable treatment. In contrast, in biological treatment using the fluidized carrier method, such as the water treatment system 100 shown in Figure 2, the nitrite accumulation rate is less likely to drop sharply 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.

[0079] Furthermore, in the start-up process of biological treatment using the activated sludge method, it takes about 14 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 the start-up process of biological treatment using the fluidized carrier method, such as the water treatment system 100 shown in Figure 2, it takes about 5 to 10 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.

[0080] Furthermore, in the water treatment system 100 according to this embodiment, even if the environment suitable for nitrite-type nitrification and denitrification in the nitrification tank 6 is disrupted due to fluctuations in the properties of the raw water or the extraction of the organism-attached carrier from the nitrification tank 6 and its return to the nitrification tank 6, recovery is faster than in biological treatment using activated sludge, and therefore the production of nitrite nitrogen in the nitritation step can be maintained more appropriately and stably. Furthermore, even when it is desired to change the treatment in the nitrification tank 6 from nitrite-type nitrification and denitrification treatment to conventional nitrification and denitrification treatment, the environment in the nitrification tank 6 can be adjusted quickly by aerating the nitrification tank 6 to promote the activity and proliferation of nitrite-oxidizing bacteria.

[0081] Furthermore, calculations have shown that in the water treatment system 100 shown in Figure 2, the amount of sulfuric acid used to adjust the pH in the nitrite treatment tank can be reduced by about 99% compared to biological treatment using activated sludge. Calculations have also shown that the amount of nitrite concentration added to adjust the pH in the nitrite treatment tank, 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.

[0082] When using a conventional activated sludge process, 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 and denitrification treatment in the nitrification tank 6. In contrast, with the biological treatment using the fluidized carrier process shown in the water treatment system 100 shown in Figure 2, the DO in the nitrification tank 6 does not need to be strictly controlled to 0.5 mg / L or less, and the nitrite-type nitrification and denitrification treatment in the nitrification tank 6 can be stably maintained, making it easy to control the treatment conditions.

[0083] (Method for starting up a water treatment device) A start-up method for a water treatment system according to an embodiment of the present invention is a start-up method for a water treatment system having a nitrification tank 6, which uses bio-attached carriers with ammonia-oxidizing bacteria attached to them to nitrite the ammonia nitrogen in organic wastewater containing sewage or septic tank sludge. This start-up method includes a first start-up process in which organic wastewater is passed through the nitrification tank 6, which contains seed activated sludge containing ammonia-oxidizing bacteria and nitrite-oxidizing bacteria, and carriers. The nitrogen load is gradually increased until the nitrogen removal rate reaches a predetermined value, thereby forming bio-attached carriers in the nitrification tank 6, with a biofilm containing ammonia-oxidizing bacteria and nitrite-oxidizing bacteria attached to the carrier surface. A second start-up process is performed in which a portion of the bio-attached carriers in the nitrification tank 6 is removed and contacted with an aqueous nitrite solution to perform a nitrite treatment that suppresses the activity and growth of the nitrite-oxidizing bacteria attached to the bio-attached carriers, and the bio-attached carriers after the nitrite treatment are returned to the nitrification tank 6.

[0084] In the first start-up process, nitrification tank sludge from a sewage treatment plant or other facility operating normally and unacclimated seed carriers are introduced into the nitrification tank 6 as seed sludge. For example, nitrification tank sludge with an ammonia oxidation rate of 3.6 mg-N / g-MLSS / h, a nitrite oxidation rate of 5.1 mg-N / g-MLSS / h, and a denitrification rate of 16.3 mg-N / g-MLSS / h or greater can be introduced as seed sludge. The ammonia oxidation rate of the seed sludge is preferably 0.05 to 20 mg-N / g-MLSS / h, and more preferably 0.1 to 10 mg-N / g-MLSS / h. These values ​​can be expressed more accurately using MLVSS as the standard. For example, if a large amount of inorganic coagulant is added to activated sludge, increasing the proportion of inorganic sludge and resulting in a somewhat low MLVSS ratio of 60%, a concentration of 0.08 to 33 mg-N / g-MLVSS / h is preferred, with 0.16 to 17 mg-N / g-MLVSS / h being more preferred. Furthermore, the nitrite oxidation rate of seed sludge is preferably no more than three times the ammonia oxidation rate, more preferably no more than two times, and even more preferably no more than one time.

[0085] Seed sludge is added so that the MLSS of the nitrification tank sludge is 2000 to 3000 mg / L. Raw water is continuously passed through the nitrification tank 6, which holds the nitrification tank sludge and unacclimated carriers, while the nitrogen load and sludge load in the nitrification tank 6 are gradually increased. The larger the amount of carriers, the higher the initial total nitrogen (TN) concentration in the nitrification tank 6 can be set, and the shorter the start-up period. The carriers are preferably added to the nitrification tank 6 so that the concentration is 5 to 50 V%, more preferably 15 to 30 V%. In the first start-up process, for example, the total nitrogen load is increased to, for example, 0.3 to 0.5 kg-N / m while checking the nitrogen removal rate. 3 / d, 0.4 kg-N / m in the embodiment 3 The load is increased stepwise to reach 10V% / day and 20V% / day. The carrier extraction rate is increased stepwise to 10V% / day and 20V% / day while checking the nitrite accumulation rate in the nitrification tank 6. When the nitrogen removal rate of the raw water in the nitrification tank 6 reaches a stable 90%, the first start-up process (Run 0) is completed.

[0086] In the second start-up treatment, a portion of the bioattached carriers is extracted from the nitrification tank 6 and subjected to solid-liquid separation. The separated bioattached carriers are then supplied to the nitrite treatment tank 10, where nitrite treatment is carried out. The water to be treated in the nitrite treatment tank 10 is controlled so that the pH of the water is less than 6.0 and the nitrite nitrogen concentration is less than 250 mg-N / L, and the residence time of the bioattached carriers is adjusted so that the residence time is within 48 hours. The dissolved oxygen concentration of the water to be treated in the nitrite treatment tank 10 is also adjusted to 0.5 mg / L or less. After the nitrite treatment, the bioattached carriers are subjected to solid-liquid separation in the solid-liquid separator 9, and the separated bioattached carriers are returned to the nitrification tank 6. The separated liquid is returned to the nitrite treatment tank 10 via line L32. By returning the separated liquid obtained by solid-liquid separation to the nitrite treatment tank 10, the amount of nitrite newly added to the nitrite treatment tank 10 can be reduced, thereby reducing the amount of chemicals used and improving the efficiency of the treatment.

[0087] On the other hand, the concentration of nitrite nitrogen in the separated liquid returned to the nitrite treatment tank 10 gradually decreases due to biological reactions (oxidation of nitrite, denitrification), and the concentration of free nitrite also decreases. Therefore, when nitrite treatment is continued for a long period of time while returning the separated liquid to the nitrite treatment tank 10, it may become difficult to effectively suppress nitrite-oxidizing bacteria over time. Therefore, in this embodiment, the concentration of nitrite nitrogen in the separated liquid returned to the nitrite treatment tank 10 is periodically measured, and the amount of nitrite nitrogen lost in the separated liquid is replenished. This allows efficient and stable treatment in the nitrite treatment tank 10.

[0088] When the nitrite accumulation rate in the nitrification tank 6 reaches a stable level of 70 to 80%, the nitrite treatment process is temporarily stopped to confirm stability. When the nitrite accumulation rate in the nitrification tank 6 reaches a stable level of 70 to 80% even after the nitrite treatment process has been stopped, the second start-up process (Run 1) is completed.

[0089] According to the start-up method for a water treatment system according to an embodiment of the present invention, by using a biologically attached carrier, it is possible to perform early start-up treatment so that ammoniacal nitrogen in organic wastewater containing human waste or septic tank sludge can be efficiently and stably biologically treated using a nitrite-type nitrification / denitrification process in the nitrification tank 6.

[0090] Although the present invention has been described with reference to the above-described 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-described embodiments, and components can be combined and modified to be embodied within the scope of the gist of the present disclosure.

[0091] For example, in the second start-up treatment, it is preferable to monitor the water quality in the nitrification tank 6. If, for example, an increase in the ammonia concentration is observed as a result of monitoring the water quality in the nitrification tank 6, this indicates that the nitritation process in the mainstream nitrification tank 6 is not working properly (as a result of the activity of ammonia-oxidizing bacteria being suppressed in the nitrification tank 6). In this embodiment, in order to prevent deterioration of the treatment performance of the mainstream, the water quality in the nitrification tank 6 is monitored, and if the monitoring result shows that the ammonia concentration in the nitrification tank 6 exceeds a predetermined reference value, the nitrite treatment process is stopped and the water quality in the nitrification tank 6 is monitored. Thereafter, if a decrease in the ammonia concentration in the nitrification tank 6 is confirmed, it is preferable to resume the nitrite treatment process and resume the withdrawal and return of the carrier.

[0092] Alternatively, the amount of acid used and the amount of nitrite added during the nitrite treatment process in the nitrite treatment tank 10 are constantly monitored. If the amount of acid or nitrite added per treatment is higher than normal, 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 indicates that acid-resistant nitrite-oxidizing bacteria may 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.

[0093] Therefore, in this embodiment, during the second start-up treatment, the water quality control results in the nitrite treatment tank 10 (amounts of acid and nitrite added per treatment) are monitored, and if the water quality control results deviate from predetermined target values, the treatment conditions for the nitritation step in the nitrification tank 6 are changed or an alarm is issued for an abnormality in the nitritation step in the nitrification tank 6. In this way, if a change from the normal chemical usage is observed, it leads 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 becomes possible to prevent a decline in performance. Note that the above-mentioned monitoring of the water quality in the nitrification tank 6 and the nitrite treatment tank 10 may also be performed during steady-state operation when carrier removal from the nitrification tank 6 and nitrite treatment in the nitrite treatment tank 10 are performed. [Example]

[0094] 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.

[0095] Example 1 A continuous test of nitritation treatment of raw water using the fluidized carrier method was conducted using the test equipment shown in Figure 3, which is equipped with a raw water tank, denitrification tank, nitrification tank, nitrite treatment tank, and treated water tank. The effective solution volumes for 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: approximately 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 × 10) were 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).

[0096] As shown in Table 1, the nitrification tank was subjected to a first start-up process (Run 0) and a second start-up process (Run 1) to adjust the environment in the nitrification tank so that nitrite-type nitrification and denitrification treatment could be performed in the nitrification tank. Steady-state operation (Runs 2 and 3) was then initiated. Polyethylene glycol hydrogel (specific gravity 1.025) with an effective diameter of 3.4 to 5.2 mm (standard diameter 4.2 mm) was used as the carrier. This carrier was introduced into the nitrification tank and denitrification tank, respectively. In the nitrification tank, the first start-up process (Run 0) for nitrification and denitrification treatment was first performed for 14 days, allowing a biofilm (sludge) containing nitrite-oxidizing bacteria and ammonia-oxidizing bacteria to adhere to the carrier.

[0097] [Table 1]

[0098] In the first start-up process (Run 0), the nitrogen load and sludge load in the nitrification tank were gradually increased. The first start-up process of the nitrification / denitrification process in the nitrification tank was completed in 14 days, when the nitrogen removal rate in the nitrification tank reached approximately 90%. 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 every 10 days, from 10 V% / day to 20 V% / day.

[0099] Nitrite treatment was performed by measuring the water quality in the nitrite treatment tank using water quality measuring devices such as a pH meter, water thermometer, and nitrite sensor. 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.

[0100] 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 carrier separated in the solid-liquid separator was returned to the nitrification tank. The separated liquid from the solid-liquid separator 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%.

[0101] Figure 4 shows the time course of the nitrite accumulation rate in the nitrification tank (nitrite accumulation rate in the nitrification tank) and the nitrogen removal rate for each treatment (Run 0 to Run 2). The "nitrite accumulation rate" and "nitrogen removal rate" were calculated using the following formulas (2) and (3).

[0102]

number

[0103]

number

[0104] In Example 1, the nitrogen removal rate reached 90% or more in about 5 days, indicating that the start-up process for conventional nitrification / denitrification treatment could be completed quickly. The example in Figure 4 shows the results when the sludge withdrawal rate from the nitrification tank was increased to 10 V% / day from the 30th day and to 20 V% / day from the 40th day, changing the environment in the nitrification tank from conventional nitrification / denitrification treatment to one for nitrite-type nitrification / denitrification treatment. It can be seen that the nitrite accumulation rate in the nitrification tank gradually increased after the sludge withdrawal rate was increased to 20 V% / day, exceeding 70% after 60 days. By performing the water treatment method according to this embodiment, nitrite-type nitrification / denitrification treatment in the nitrification tank is possible.

[0105] In order to confirm the stability of the nitritation step, in a continuous test shown in Fig. 4, the removal and return of the carrier was stopped after 70 days to stop the nitrite treatment to the carrier, and even on the 100th day, the nitrite accumulation rate in the nitrification tank was maintained at approximately 67 to 79%. In other words, according to this embodiment, even when the removal and return of the carrier is stopped, nitrite-type nitrification denitrification treatment can be carried out in the nitrification tank for at least approximately 30 days.

[0106] 5 is a graph showing the time-dependent changes in aeration air volume and nitrite accumulation rate when steady-state operation is carried out after start-up treatment of a nitrite-based nitrification denitrification treatment by the water treatment method according to this embodiment. After 50 days, when the nitrite accumulation rate in the nitrification tank exceeds 50%, the nitrite accumulation rate in the nitrification tank can be maintained high even when the aeration air volume is reduced. Calculations have shown 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.

[0107] (Example 2: Effect on the reduction of the amount of electron donor added in denitrification treatment) Figure 6 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). 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, enabling stable denitrification treatment. 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, enabling stable denitrification treatment. It can be seen that this embodiment allows for a reduction in the amount of electron donor required for nitrification / denitrification treatment compared to conventional complete nitrification / denitrification.

[0108] (Example 3: Changes in ammonia oxidation rate and nitrite oxidation rate in the nitrification tank due to nitrite treatment) Figure 7(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 was maintained at 100 to 250 mg / L. Figure 7(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 7(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 7(b), the "activity rate" was evaluated by comparing the activity after nitrite treatment with the activity before nitrite treatment. As shown in Figure 7(a), the activity of nitrite-oxidizing bacteria in the nitrification tank decreased as the nitrite concentration in the nitrite treatment tank increased. However, at a nitrite concentration of 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 7(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]

[0109] 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 L32: Nitrous acid aqueous solution return line L4: Circulation line W: Water treatment equipment

Claims

1. a nitritation step in which ammonia nitrogen in organic wastewater containing sewage or septic tank sludge is oxidized to nitrite nitrogen using a bioattached carrier having ammonia-oxidizing bacteria attached to the carrier; A withdrawal step of withdrawing a portion of the bioattached carrier in the nitritation step; a nitrite treatment step in which the extracted bioadherent carrier is brought into contact with an aqueous nitrite solution to suppress the activity and proliferation of nitrite-oxidizing bacteria attached to the bioadherent carrier; a returning step of returning the bioattached carrier obtained in the nitrite treatment step to the nitritation step; A water treatment method comprising the steps of:

2. 2. 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 using a bioattached carrier having denitrifying bacteria attached thereto.

3. 3. The water treatment method according to claim 2, further comprising a step of circulating a portion of the organic wastewater containing the bioattached carriers to the denitrification step after returning the bioattached carriers to the nitritation step.

4. 2. The water treatment method according to claim 1, further comprising a step of heating the removed organism-attaching carrier in the nitrite treatment step.

5. 2. The water treatment method according to claim 1, wherein the nitrite treatment step is carried out during the nitritation step, and the extraction step and the return step involve extracting and returning the bioattached carrier without stopping the nitritation step.

6. 2. The water treatment method according to claim 1, further comprising a step of suspending the extraction and return of the bioattached carrier for a certain period of time based on the free nitrite concentration or the nitrite concentration in the nitrification tank where the nitritation step is carried out.

7. 2. The water treatment method according to claim 1, wherein the organic wastewater is a separated liquid obtained by dehydrating organic wastewater containing human waste or septic tank sludge.

8. The extracted bioadherent carrier is subjected to solid-liquid separation, The bioattached carrier separated by the solid-liquid separation is introduced into the nitrite treatment step, 2. The water treatment method according to claim 1, further comprising returning the separated liquid obtained by the solid-liquid separation to the nitritation step.

9. The bioadherent carrier obtained in the nitrite treatment step is subjected to solid-liquid separation, The bioattached carrier separated by the solid-liquid separation is returned to the nitritation step, 2. The water treatment method according to claim 1, further comprising returning the separated liquid obtained by the solid-liquid separation to the nitrite treatment step.

10. The water treatment method according to any one of claims 1 to 9, characterized in that the pH and nitrite concentration of the water to be treated in the nitrite treatment tank in which the nitrite treatment step is carried out are measured, and the pH of the water to be treated is controlled to be less than 6.0, the nitrite nitrogen concentration is less than 250 mg-N / L, and the residence time of the bioattachment carrier is controlled to be within 48 hours.

11. a nitrification tank that holds a bioattached carrier having ammonia-oxidizing bacteria attached thereto, and oxidizes ammonia nitrogen in organic wastewater containing human waste or septic tank sludge to nitrite nitrogen using the bioattached carrier; An extracting means for extracting a part of the organism attachment carrier in the nitrification tank; a nitrite treatment tank in which the bioattached carriers extracted by the extraction means are brought into contact with an aqueous nitrite solution to suppress the activity and proliferation of nitrite-oxidizing bacteria attached to the bioattached carriers; a return means for returning the organism-attached carrier in the nitrite treatment tank to the nitrification tank; A water treatment device comprising:

12. A method for starting up a water treatment device equipped with a nitrification tank for nitritizing ammonia nitrogen in organic wastewater containing human waste or septic tank sludge using a bioattached carrier having ammonia-oxidizing bacteria attached thereto, comprising: a first start-up process in which the organic wastewater is passed through the nitrification tank containing a carrier, and the nitrogen load is gradually increased until the nitrogen removal rate reaches a predetermined value, thereby causing a biofilm containing ammonia-oxidizing bacteria and nitrite-oxidizing bacteria to adhere to the surface of the carrier, and the bio-attached carrier with the attached biofilm is retained in the nitrification tank; a second start-up treatment in which a portion of the bioattached carriers is extracted from the nitrification tank, the extracted bioattached carriers are brought into contact with an aqueous nitrite solution, and thereby a nitrite treatment is carried out to suppress the activity and proliferation of the nitrite-oxidizing bacteria attached to the bioattached carriers, and the bioattached carriers after the nitrite treatment are returned to the nitrification tank; A method for starting up a water treatment device, comprising:

13. The start-up method for a water treatment device according to claim 12, characterized in that the second start-up process adjusts the aeration air volume in the nitrite treatment tank so that the dissolved oxygen concentration of the water to be treated in the nitrification tank where the nitrite treatment is performed is 5.0 mg / L or less.

14. 13. The start-up method for a water treatment apparatus according to claim 12, wherein the second start-up process comprises extracting the amount of the organism attachment carrier from the nitrification tank at least 20V%.

15. The start-up method for a water treatment device according to any one of claims 12 to 14, characterized in that the second start-up process is controlled so that the pH in the nitrite treatment tank where the nitrite treatment is performed is less than 6.0, the nitrite nitrogen concentration is less than 250 mg-N / L, and the residence time of the bioattached carrier is 48 hours or less.

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