JP2012020262A - Nitritification treatment method of ammoniacal nitrogen - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for controlling oxidation from a nitrous acid to a nitric acid and maintaining generation of a nitrous acid which is necessary for anammox treatment for a long time duration by nitritification treatment using a method of fixed-bed carrier.SOLUTION: The nitrification treatment method of ammoniacal nitrogen includes the steps of: obtaining fixed-bed filtering medium by sticking a carrier on ammonia oxidization bacteria; nitritifying ammoniacal nitrogen by contacting the fixed-bed filtering medium to wastewater containing the ammoniacal nitrogen; and removing sludge accumulating on the fixed-bed filtering medium. The carrier has a network structure or a pore structure.

Description

  The present invention relates to a method for nitritation of ammoniacal nitrogen.

  Ammonia nitrogen contained in the wastewater of organic waste including sewage treatment is one of the causative substances of eutrophication in rivers, oceans, etc., and therefore needs to be removed. Conventionally, there is a biological nitrification denitrification method as a treatment method of wastewater containing ammonia nitrogen. In this treatment method, ammoniacal nitrogen in wastewater is oxidized to nitrite by ammonia oxidizing bacteria (Nitrosomonas, Nitrosospira, etc.) under aerobic conditions, and nitrite is nitrated by nitrite oxidizing bacteria (Nitrobactor, Nitrospina, etc.). It is oxidized to (nitrification reaction). Finally, under anaerobic conditions, nitric acid is reduced to nitrogen gas by a denitrifying bacterium (denitrification reaction), and ammonia nitrogen in the wastewater is removed.

  However, this method requires aeration of a large amount of oxygen (air) in the nitrification reaction, requires a large amount of organic matter such as methanol as an electron donor in the denitrification reaction, and generates a large amount of sludge. In addition, since the reaction rate is slow, a large-scale processing facility is required, and the processing cost increases.

As an alternative method, a biological nitrogen removal method using an anaerobic ammonia oxidizing bacterium which is an autotrophic bacterium has been proposed. This method uses ammonia as an electron donor and nitrous acid as an electron acceptor. 1NH ₄ ⁺ + 1.32NO ₂ ⁻ + 0.066HCO ₃ ⁻ + 0.13H ⁺ → 1.02N ₂ + 0.26NO ₃ ⁻ + 0.066CH ₂ In this method, denitrification is performed by a reaction formula of O _0.5 N _0.15 + 2.03H ₂ O. However, since there is almost no nitrous acid contained in the wastewater, a nitritation treatment that converts part of the ammonia nitrogen in the wastewater into nitrous acid is performed as a pretreatment, and then the ammonia in the wastewater. Nitrogen is removed by anaerobic ammonia oxidation treatment that converts nitrous acid into nitrogen gas. Since this method does not require supply of a large amount of oxygen (air) and addition of organic substances, it is a very economical nitrogen removal method. In the anaerobic ammonia oxidation treatment, the reaction is efficiently performed by controlling the molar ratio of ammonia to nitrous acid in the wastewater to about 1: 1 to 1: 1.5 (preferably about 1: 1.32). proceed. This method is generally referred to as an anaerobic ammonia oxidation method or an anammox method.

  In order to perform the anammox treatment, it is necessary to control the molar ratio of ammonia and nitrous acid in waste water containing ammonia nitrogen to about 1: 1 to 1: 1.5 as described above. As a method for controlling the molar ratio of ammonia and nitrous acid to about 1: 1 to 1: 1.5, it is included by introducing the entire amount of waste water into the nitritation tank and controlling the reaction amount in the tank. A method that nitrites about half of the ammonia and a part of the wastewater is introduced into the nitritation tank, and all of the ammonia contained in the introduced wastewater is nitritized, and nitritized according to the reaction amount A method of controlling the amount of waste water that bypasses the tank is mentioned. Furthermore, since the anammox treatment is performed by a reaction between ammonia and nitrous acid, it is necessary to suppress oxidation from nitrous acid to nitric acid.

  Nitrite treatment is performed by the activated sludge method (floating biological method), fluidized bed type carrier method (microorganism immobilization carrier method), fixed bed type carrier method (biofilm method), etc. .

  The activated sludge method is the most common method for removing nitrogen. However, in this method, it is necessary to retain the wastewater for a long time, and the processing capacity is also low.

  The fluidized bed type carrier method is a method in which ammonia oxidizing bacteria are immobilized on a floating carrier and ammonia is nitrified. This method has advantages such as no clogging by the biofilm and high contact efficiency between the waste water and the floating carrier. However, since the floating carrier is not fixed to the nitritation tank unlike the fixed bed, there are problems such as requiring a screen for preventing the outflow and circulating power for flowing the floating carrier. Moreover, in order to maintain the activity of the ammonia-oxidizing bacteria, a part of the floating carrier may be extracted and heat-treated, which makes the operation complicated.

  The fixed bed type carrier method does not require a screen for preventing carrier outflow, and is a treatment method that is easy to maintain and manage, such as obtaining suspended solid (SS) -free treated water. However, simply attaching ammonia oxidizing bacteria to the surface of the fixed bed has a problem that the ammonia oxidizing bacteria are easily detached from the fixed bed. Furthermore, it is susceptible to the inflowing SS, and if SS accumulates in the apparatus, the environmental conditions inside the sludge such as pH may deviate from the control set value and cannot be controlled.

  Furthermore, as a means to suppress oxidation of nitrite to nitric acid by ammonia oxidizing bacteria, reaction inhibition by free ammonia and free nitrous acid, control of dissolved oxygen (DO) amount, heat treatment, use of nitrite oxidizing bacteria inhibitor, etc. (Patent Documents 1 to 6). However, in these methods, the control is unstable and the running cost increases.

JP 2000-61494 A JP 2003-2111177 A Japanese Patent Laid-Open No. 2004-298441 JP 2003-10883 A JP 2006-88057 A JP 2001-170684 A

  It is an object of the present invention to provide a method for suppressing the oxidation of nitrous acid to nitric acid in a nitritation treatment using a fixed bed type carrier method and sustaining the production of nitrous acid necessary for anammox treatment for a long period of time. And

  The present invention provides a method for nitritation treatment of ammonia nitrogen, which comprises fixing a ammonia oxidizing bacterium to a carrier to obtain a fixed bed filter medium; waste water containing the fixed bed filter medium and ammonia nitrogen; And nitrating the ammoniacal nitrogen; and removing the sludge deposited on the fixed bed filter medium, the carrier having a network structure or a pore structure.

  In one embodiment, the carrier has a cylindrical shape, a square shape, or a flat plate shape.

  In one embodiment, the average mesh diameter or pore average diameter of the carrier is 1 mm to 20 mm.

  In one embodiment, the carrier is made of at least one synthetic resin selected from the group consisting of polyester resins, polyvinyl resins, polyvinylidene resins, and acrylic resins.

  In certain embodiments, the carrier is coated with an ionic material.

  In a certain embodiment, the said fixed bed filter medium is installed uniformly at equal intervals.

  In another embodiment, the step of removing the sludge is performed by mechanical means, bubble generating means or water flow generating means.

  In still another embodiment, in the step of removing the sludge, the removing means is provided at a lower part and a central part of the fixed bed filter medium.

  In another embodiment, the method further includes a step of removing suspended substances in the wastewater containing the ammoniacal nitrogen before the step of nitrating the ammoniacal nitrogen.

  Furthermore, the present invention provides a fixed bed filter medium used for nitritation treatment of wastewater containing ammonia nitrogen, and the fixed bed filter medium has ammonia-oxidizing bacteria fixed to a carrier having a network structure or a pore structure. Yes.

  According to the present invention, there is provided a method for suppressing the oxidation of nitrous acid to nitric acid and maintaining the generation of nitrous acid required for anammox treatment for a long period in nitritation using a fixed bed type carrier method. .

  Furthermore, when a carrier made of synthetic resin fibers having a high affinity for ammonia-oxidizing bacteria is used, the ammonia-oxidizing bacteria are more firmly fixed to the carrier and are less likely to be detached.

  Moreover, since the removed microbial mass is easily precipitated as a large sludge floc, it can be easily removed.

It is a systematic diagram which shows one embodiment of the anaerobic ammonia oxidation processing system (Anamox processing system) which uses the nitritation processing method of this invention. It is a systematic diagram which shows the other embodiment of the anaerobic ammonia oxidation processing system (Anamox processing system) which uses the nitritation processing method of this invention. (A) is a schematic diagram showing an embodiment of a fixed bed filter used in the nitritation treatment method of the present invention, and (B) is a microorganism other than ammonia-oxidizing bacteria attached to the fixed bed filter shown in (A). It is a schematic diagram which shows the state which carried out. It is a schematic diagram which shows the method of removing microorganisms other than the ammonia oxidation bacteria adhering to the fixed bed filter medium used for the nitritation processing method of this invention, (A) is a schematic diagram which shows a mechanical system, (B) Is a schematic diagram showing a bubble system, and (C) is a schematic diagram showing a water flow system. In Example 2, it is a graph which shows the relationship between the removal amount of ammonia in a nitritation tank, and the production amount of nitrous acid and nitric acid. In Example 2, it is a graph which shows the conversion rate to nitrous acid after separating excess sludge, and the conversion rate to nitric acid.

  The nitritation treatment method of wastewater containing ammonia nitrogen according to the present invention (hereinafter sometimes simply referred to as “nitritation treatment method”) will be described with reference to the accompanying drawings. The nitritation treatment method of the present invention is performed, for example, in an anaerobic ammonia oxidation treatment system shown in FIGS.

  In this specification, an organic substance means an organic substance that can be oxidized by a living organism such as a microorganism, and is used synonymously with biological oxygen demand (BOD). Therefore, the organic substance may be described as BOD.

  As described above, the method for nitrating ammonia nitrogen of the present invention is a step of fixing ammonia oxidizing bacteria to a carrier to obtain a fixed bed filter medium; contacting the fixed bed filter medium with waste water containing ammonia nitrogen. The carrier has a network structure or a pore structure. The process comprises: nitrifying ammoniacal nitrogen; and removing sludge deposited on a fixed bed filter medium.

(Step of obtaining a fixed floor filter)
FIG. 3 (A) shows one embodiment of the fixed bed filter used in the present invention. In the fixed bed filter used in the present invention, ammonia-oxidizing bacteria 12 are fixed to the carrier 11.

  The carrier 11 has a network structure or a pore structure. This network structure or pore structure may be provided at least on the surface of the carrier 11. However, in order to increase the fixing area of the ammonia-oxidizing bacteria 12, it is preferable that the entire support 11 has a network structure or a pore structure. Furthermore, it is preferable to have a communicating hole.

  The size of the mesh or pore is not particularly limited. However, from the viewpoint of preventing clogging by a biofilm, a large mesh or pore is preferable in order to prevent the ammonia-oxidizing bacteria 12 and other microorganisms from attaching excessively densely. For example, the average diameter of the carrier network or the average diameter of the pores may be preferably 1 mm to 20 mm.

  Although the shape of the support | carrier 11 is not specifically limited, Shapes, such as a cylindrical shape, a square shape, and a flat plate shape, are preferable. For example, a cylindrical, rectangular, flat plate or other carrier having a mesh structure formed of synthetic resin fibers, which will be described later, and a cylindrical, rectangular, flat plate or other carrier having pores may be mentioned. What is necessary is just to set the magnitude | size of the support | carrier 11 suitably according to the magnitude | size of the nitritation tank in which the fixed bed filter medium obtained is installed.

  The carrier 11 is not particularly limited as long as it is a material having high affinity with the ammonia-oxidizing bacteria 12 and excellent permeability. Examples of such a carrier 11 include a carrier made of a synthetic resin or the like molded to have pores or meshes. Examples of the synthetic resin include polyester resins, polyvinyl resins, polyvinylidene resins, and acrylic resins.

  The carrier 11 may be used as it is, but it is preferably coated with an ionic substance, particularly preferably a cationic substance, from the viewpoint of firmly fixing the ammonia-oxidizing bacteria 12. By coating the carrier 11 with an ionic substance, the affinity between the ammonia-oxidizing bacteria 12 and the carrier 11 is improved. Therefore, the ammonia-oxidizing bacteria 12 can adhere to the carrier 11 more firmly. Examples of ionic substances include inorganic substances such as zeolite and organic ion exchange resins. Such an ionic substance is used by applying to a synthetic resin, for example. Alternatively, it may be kneaded into a synthetic resin and formed into a fiber shape.

  Next, a method for fixing the ammonia-oxidizing bacteria 12 to the carrier 11 will be described. The method for fixing the ammonia-oxidizing bacteria 12 to the carrier 11 is not particularly limited, and the method described below is merely an example. Ammonia-oxidizing bacteria are bacteria that oxidize ammonia to nitrite, and examples include Nitrosomonas and Nitrosospira described above.

  First, the support | carrier 11 is installed in a nitritation tank. It is preferable that the support | carrier 11 is installed uniformly at equal intervals so that 5-30% of the volume of a nitritation tank may be occupied. Next, waste water containing ammonia-oxidizing bacteria 12 is introduced into the nitritation tank. When waste water containing ammonia nitrogen is used as the waste water, the nitritation treatment described later can be continued.

The wastewater containing the ammonia-oxidizing bacteria 12 preferably has a BOD of 300 mg / L or less, more preferably 100 mg / L or less, and an SS of 150 mg / L or less, more preferably 100 mg / L or less. The waste water is preferably controlled to have a pH of 7 to 8, a nitrogen volume load of 0.5 to 2 kg / m ³ / day, and a water temperature of 30 to 40 ° C., and is introduced into the nitritation tank.

  When the waste water is supplied into the nitritation tank, the ammonia oxidizing bacteria 12 adhere to the environment and adhere to the surface of the carrier 11. In FIG. 3A, the ammonia-oxidizing bacteria 12 are adhered only to the surface layer of the carrier 11, but the ammonia-oxidizing bacteria 12 can also adhere to the inside of the pores, the fiber surface of the mesh, and the like. That is, in this specification, the “surface of the carrier” means not only the surface layer portion of the carrier, but also all surfaces that can be contacted with waste water, such as the inside of the network structure and the inside of the pores.

  Ammonia-oxidizing bacteria 12 adhered to the carrier 11 grow as dominant species and inhibit the growth of other microorganisms (for example, nitrated bacteria). About one to two weeks after supplying the wastewater, ammonia-oxidizing bacteria become the dominant species and become fixed.

  In this way, a fixed bed filter medium used in the present invention is obtained by forming a flora of ammonia-oxidizing bacteria 12 on the carrier 11.

(Nitrite process)
In the nitritation step, ammonia nitrogen is oxidized to nitrous acid by bringing the fixed bed filter medium into contact with waste water containing ammonia nitrogen.

  The anaerobic ammonia oxidation treatment system shown in FIG. 1 includes a raw water tank 14, a distribution tank 15, a nitritation tank 16, a regulation tank 17, and an anaerobic ammonia oxidation tank 18.

  The raw water tank 14 stores waste water to be treated by the nitritation method of the present invention. Examples of the ammonia-containing wastewater include, but are not limited to, sewage sludge digestion dehydrated filtrate, livestock manure, methane fermentation wastewater such as food waste, semiconductor factory wastewater, and waste leachate.

  Waste water stored in the raw water tank 14 is supplied to the distribution tank 15. The distribution tank 15 distributes the waste water to the nitritation tank 16 and the adjustment tank 17.

  Waste water supplied to the nitritation tank 16 is brought into contact with a fixed bed filter medium. Ammonia-oxidizing bacteria adhering to the fixed bed filter medium oxidize ammoniacal nitrogen contained in the wastewater to nitrous acid. In the nitritation tank 16, at least about half of the supplied ammoniacal nitrogen, preferably almost all, is oxidized to nitrous acid.

(Process to remove sludge)
When performing a nitritation treatment using a fixed bed filter medium, when the SS concentration of waste water to be nitritation treatment is high or when used for a long period of time, as shown in FIG. In some cases, microorganisms 13 (such as BOD-oxidizing bacteria) other than ammonia-oxidizing bacteria may be coated on the surface. When the ammonia-oxidizing bacteria 12 are coated with microorganisms 13 other than the ammonia-oxidizing bacteria, the activity is reduced due to lack of DO, or the environmental conditions (such as pH) inside the sludge deviate from the control set value, so that they are nitrated. Bacteria may become the dominant species.

  The nitritation treatment method of the present invention includes a step of removing sludge (such as microorganisms other than ammonia-oxidizing bacteria) deposited on a fixed bed filter medium. By removing sludge containing microorganisms other than the ammonia-oxidizing bacteria, the ammonia-oxidizing bacteria fixed to the fixed bed filter medium are activated.

  Sludge removal means may preferably be provided in the nitritation tank. In particular, when the nitritation tank is deep (high water depth), sludge removal means are provided at the lower part and the central part of the fixed bed filter medium. Examples of such microorganism removing means include mechanical means such as a scraper, bubble generating means, water flow generating means, etc., as shown in FIG.

  The fixed-bed filter medium in which the ammonia-oxidizing bacteria 12 is coated with the microorganisms 13 other than the ammonia-oxidizing bacteria does not have to be sterilized by the heat treatment or the like, for example, (A) to FIG. By the method as shown in (C), the microorganisms 13 other than the ammonia-oxidizing bacteria can be easily removed. This is because ammonia-oxidizing bacteria firmly adhere to the fixed bed filter medium of the present invention.

  FIG. 4A is a schematic diagram showing a method for removing microorganisms 13 other than ammonia-oxidizing bacteria by a mechanical method (mechanical means). The mechanical method is a method of stripping off microorganisms 13 other than the ammonia oxidizing bacteria attached to the ammonia oxidizing bacteria 12 using a scraper or the like.

  FIG. 4B is a schematic diagram showing a method of removing microorganisms 13 other than the ammonia-oxidizing bacteria by a bubble method (bubble generation means). The bubble method is a method in which microorganisms 13 other than the ammonia oxidizing bacteria attached to the ammonia oxidizing bacteria 12 are removed by floating with bubbles.

  FIG. 4C is a schematic diagram showing a method of removing microorganisms 13 other than the ammonia-oxidizing bacteria by a water flow method (water flow generating means). The water flow method is a method in which the microorganisms 13 other than the ammonia oxidizing bacteria attached to the ammonia oxidizing bacteria 12 are washed away by a water flow.

  The removal method of the microorganisms 13 other than the ammonia-oxidizing bacteria is not limited to the mechanical method, the bubble method, and the water flow method, and any method that can remove the microorganisms 13 other than the ammonia-oxidizing bacteria is adopted.

  As described above, since the ammonia-oxidizing bacteria 12 are firmly fixed due to the affinity with the carrier 11, even if these methods are used, the ammonia-oxidizing bacteria 12 hardly peels off from the carrier 11, or even if they are peeled off. It is trace amount and does not affect nitritation treatment. And since the lump of the other microorganisms removed (or sludge containing microorganisms) tends to settle as a big sludge floc, it can remove easily.

  The wastewater treated with nitritation in the nitritation tank 16 is supplied to the adjustment tank 17 and mixed with untreated wastewater, and the molar ratio of ammonia and nitrous acid, which is the stoichiometric ratio of the anaerobic ammonia oxidation reaction. The ratio is adjusted to be about 1: 1 to 1: 1.5, preferably about 1: 1.32.

  The wastewater mixed and adjusted in the adjustment tank 17 is supplied to the anaerobic ammonia oxidation tank 18, and ammonia and nitrous acid in the wastewater are converted into nitrogen gas. From the anaerobic ammonia oxidation tank 18, treated water containing no ammonia nitrogen is discharged into the natural world such as a river.

(Process to remove suspended solids)
The nitritation treatment method of the present invention preferably further includes a step of removing suspended substances in the wastewater containing ammonia nitrogen before the nitritation step.

  Examples of the method for removing suspended solids include a method of precipitating suspended solids (such as coagulation sedimentation) in wastewater before subjecting the wastewater to a nitritation step. For example, as shown in FIG. 2, a precipitation tank 19 is provided in the upstream (upstream side) of the nitritation tank 16 to precipitate floating substances in the wastewater. If it is the upstream of the nitritation tank 16, the position of the precipitation tank 19 is not specifically limited. In FIG. 2, the settling tank 19 is provided between the raw water tank 14 and the distribution tank 15.

  EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to the following examples.

  Nitrite of ammonia nitrogen in wastewater was performed by the nitritation treatment method of the present invention.

(Example 1: Preparation of fixed bed filter medium)
First, the support was placed in the nitritation tank so as to occupy about 20% of the volume of the nitritation tank. The carrier had a structure in which an acrylic resin was processed into a mesh shape. Surplus sludge from a sewage treatment plant containing ammonia-oxidizing bacteria was supplied to a nitritation tank in which the carrier was installed, and was fixed to the carrier. As waste water, digested dehydrated filtrate of sewage sludge was used. This wastewater had an ammoniacal nitrogen concentration of 700 to 990 mg / L, an SS concentration of 50 to 250 mg / L, and a BOD of 20 to 160 mg / L.

  When surplus sludge was supplied into the nitritation tank, it started to adhere to the carrier in about 12 hours, and a fixed bed filter medium was obtained. However, at this time, other microorganisms such as BOD oxidizing bacteria are also mixed together with the ammonia oxidizing bacteria.

The waste water was then supplied to the nitritation tank. The inside of the nitritation tank is controlled to a temperature of 30 to 35 ° C. and a pH of 7.7 to 7.8, and waste water is continuously supplied to the nitritation tank at a nitrogen volume load of 1.0 kg / m ³ / day. In about one week, ammonia-oxidizing bacteria became the dominant species, and a fixed bed filter medium was obtained.

(Example 2: Nitrite treatment)
Using the nitritation tank provided with the fixed-bed filter medium obtained in Example 1 as it was, the above-mentioned sewage sludge digested dehydrated filtrate was subjected to nitritation treatment.

  When the SS concentration of the digested and dehydrated filtrate of sewage sludge supplied to the nitrification tank is controlled to a relatively low concentration (50 to 160 mg / L), excess sludge is almost accumulated in the fixed bed filter medium. There wasn't. The environmental conditions in the nitritation tank were maintained at the control setpoint, and the activity of nitrite oxidizing bacteria (nitrating bacteria) was suppressed. About 85% of the ammonia nitrogen in the wastewater is converted to nitrite nitrogen. As shown in FIG. 5, the amount of nitrite nitrogen in the treated water is about 630 to about 890 mg / L, and the nitrate nitrogen is 0. To about 50 mg / L.

  When the SS concentration of the digested and dehydrated filtrate of sewage sludge supplied to the nitrification tank became high (160 to 250 mg / L), excessive sludge accumulated on the fixed bed filter medium. As a result, the environmental conditions in the nitrification tank such as pH deviated from the control set value inside the sludge, the nitric acid conversion rate increased, and the nitrous acid conversion rate decreased. Therefore, when the excessively accumulated sludge was peeled off by the water flow method shown in FIG. 4 (c), the conversion to nitric acid was quickly suppressed as shown in FIG. 6, and the nitrite conversion rate increased. In the graph of FIG. 6, the left bar shows the conversion rate to nitrous acid (NO2-N conversion rate), and the right bar shows the conversion rate to nitric acid (NO3-N conversion rate).

  According to the present invention, there is provided a method for suppressing the oxidation of nitrous acid to nitric acid and maintaining the generation of nitrous acid required for anammox treatment for a long period in nitritation using a fixed bed type carrier method. . Therefore, it is extremely useful in the field of wastewater treatment containing ammonia nitrogen.

DESCRIPTION OF SYMBOLS 11 Carrier 12 Ammonia oxidizing bacteria 13 Microorganisms other than ammonia oxidizing bacteria 14 Raw water tank 15 Distribution tank 16 Nitrite tank 17 Adjustment tank 18 Anaerobic ammonia oxidation tank 19 Precipitation tank

Claims (10)

A method for nitritation of ammoniacal nitrogen,
A step of fixing ammonia-oxidizing bacteria to a carrier to obtain a fixed bed filter medium;
Contacting the fixed bed filter medium with waste water containing ammonia nitrogen to nitrite the ammonia nitrogen; and removing sludge deposited on the fixed bed filter medium;
Including
The carrier has a network structure or a pore structure;
Method.   The method of claim 1, wherein the carrier has a cylindrical shape, a square shape, or a flat plate shape.   The method according to claim 1 or 2, wherein an average diameter of the mesh of the carrier or an average diameter of the pores is 1 mm to 20 mm.   The said support | carrier consists of at least 1 sort (s) of synthetic resin selected from the group which consists of a polyester-type resin, a polyvinyl-type resin, a polyvinylidene-type resin, and an acrylic resin. Method.   The method according to claim 1, wherein the carrier is coated with an ionic substance.   The method according to any one of claims 1 to 5, wherein the fixed bed filter medium is uniformly installed at equal intervals.   The method according to any one of claims 1 to 6, wherein the step of removing the sludge is performed by mechanical means, bubble generation means, or water flow generation means.   The method according to claim 7, wherein in the step of removing the sludge, the removing means is provided at a lower part and a central part of the fixed bed filter medium.   The method according to any one of claims 1 to 8, further comprising a step of removing suspended substances in the wastewater containing the ammonia nitrogen before the step of nitrating the ammonia nitrogen. A fixed-bed filter medium used for nitritation treatment of wastewater containing ammonia nitrogen,
A fixed-bed filter medium in which ammonia-oxidizing bacteria are fixed to a carrier having a network structure or a pore structure.

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