JP6667030B2 - Water treatment method - Google Patents

Description

  The present invention relates to a water treatment method and a water treatment apparatus for biologically oxidizing and reducing ammonia nitrogen contained in water to be treated to nitrogen gas.

  Nitrogen components in wastewater are one of the causes of eutrophication in lakes and marshes and enclosed sea areas, etc., especially if the wastewater contains high concentrations of nitrogen components, remove it in the wastewater treatment process There is a need. Generally, biological treatment using microbial activated sludge is often applied.For example, in treated water containing ammonium nitrogen or organic nitrogen, ammonia nitrogen is converted into nitrite under aerobic conditions. Alternatively, nitrification and denitrification are performed in two steps: a nitrification step of oxidizing to nitrate nitrogen, and a denitrification step of reducing nitrite and nitrate nitrogen to nitrogen gas in the presence of a hydrogen donor under oxygen-free conditions. . If the water to be treated contains a large amount of organic matter, the water to be treated is supplied to the denitrification tank, and the mixture containing nitrous acid and nitrate nitrogen generated in the nitrification tank is circulated to the denitrification tank, and In some cases, the treatment is performed by a circulating nitrification denitrification method in which denitrification is performed using the organic substance as a hydrogen donor.

  In both treatment methods, nitrifying bacteria and denitrifying bacteria in microbial activated sludge are used.Denitrifying bacteria are heterotrophic bacteria capable of assimilating organic substances, whereas nitrifying bacteria use inorganic carbon. Since it is an autotrophic bacterium used as a carbon source, its growth rate is very slow as compared with denitrifying bacteria. In the case of the treatment method using microbial activated sludge, nitrification bacteria and denitrifying bacteria are mixed in the activated sludge, so the proportion of nitrifying bacteria, which is an autotrophic bacterium with a low growth rate, is very small in the sludge. It is said that. When the nitrification denitrification method is used for the treatment of the nitrogen-containing wastewater, the activity of nitrifying bacteria in the nitrification step is rate-limiting for the removal efficiency of nitrogen in the wastewater. Nitrifying bacteria are greatly affected by the water temperature, and a decrease in the water temperature may cause a significant decrease in activity. Therefore, in order to prevent the quality of the treated water from deteriorating, the nitrification rate (specific activity of ammonia oxidation) per sludge in the nitrification step needs to be lower than that in the denitrification step. For example, Non-Patent Document 1 reports that the ammonia oxidation specific activity was 0.113 mgN / (mgVSs / day) and the nitrite oxidation inactivity was 0.056 mgN / (mgVSs / day). Even in actual treatment, the volume load of the nitrification tank is often set so as to operate at a treatment rate per sludge of about 0.05 to 0.1 kgN / (kgVSS / day) at a water temperature of 20 ° C.

  On the other hand, when the treated water containing a high concentration of nitrogen is treated with microbial activated sludge containing nitrifying bacteria and denitrifying bacteria such that the nitrogen concentration in the treated water is, for example, 100 mg N / L or more, the nitrification step In some cases, the nitrification activity may decrease. If the nitrification activity decreases, the quality of the final treated water also deteriorates. Therefore, the nitrification rate per sludge in the nitrification step must be set lower than the above 0.05 to 0.1 kgN / (kgMLVSS / day), As a result, it may be difficult to keep the processing speed high.

JP 2006-272287 A

Araki et al. (1999), Counting the number of nitrifying bacteria in a biofilm and analyzing the spatial distribution by applying the FISH method; Journal of Japan Society on Water Environment, Vol. 152-159 Microbiology nitrate respiration-Genes, enzymes, and environmental distribution, Journal of Biotechnology, 155 (2011), pp.104-117 Molybdenum as a micronutrient for Nitrobacter, Journal of Bacteriology, 89 (1965), pp.123-128 Molecular analysis of ammonia oxidation and denitrification in natural environments, FEMS Microbiology Reviews, 24 (2000), pp.673-690

An object of the present invention is to perform a stable treatment at a high treatment rate even in a case where the nitrogen concentration in the treatment water is high, in the biological treatment of the treatment water containing ammonia nitrogen and organic nitrogen. It is to provide a water treatment how.

The present invention, water to be treated containing ammonia Tai窒element, a water treatment method for biologically treated, and a autotrophic ammonia-oxidizing bacteria and nitrite oxidizing bacteria contained in microbial activity sludge A nitrification step of oxidizing the ammonia nitrogen to nitrite or nitrate nitrogen with a nitrifying bacterium, wherein in the nitrification step, the molybdenum concentration with respect to the water to be treated is 0.025 mg Mo / gN or more. in the presence of a molybdenum compound, nitrification speed of the sludge, Ru der 0.11 [kgN / (kgVSS · day)] or more, a water treatment process.

  In the water treatment method, the concentration of molybdenum in the nitrification step is preferably set to 2 mgMo / L or less with respect to the water to be treated.

  In the water treatment method, the concentration of nitrogen in the water to be treated is preferably 100 mgN / L or more.

  The water treatment method preferably further includes a denitrification step of reducing nitrous acid or nitrate nitrogen generated in the nitrification step to nitrogen gas by denitrifying bacteria contained in the microbial activated sludge.

In the denitrification step of the water treatment method, the amount of the hydrogen donor added is such that the difference between the maximum concentration and the minimum concentration of the hydrogen donor during the hydraulic residence time of the treated water is 50 mgTOC / L or more. in the Rukoto allow time variation, it is preferred that the microbial activity sludge is granulated comprising said nitrifying bacteria and denitrifying bacteria.

  In the water treatment method, the denitrification step includes at least a first denitrification step and a second denitrification step, and in the denitrification step, in the hydraulic residence time of the treated water in the second denitrification step At least hydrogen is added in the first denitrification step so that the difference between the maximum concentration of the hydrogen donor in the first denitrification step and the minimum concentration of the hydrogen donor in the second denitrification step is 50 mgTOC / L or more. Preferably, a donor is provided.

  ADVANTAGE OF THE INVENTION According to this invention, in biological treatment of the to-be-processed water containing an ammonia nitrogen and an organic nitrogen, even if the nitrogen concentration in a to-be-processed water is high, it can stably process at a high processing rate.

It is a schematic structure figure showing an example of a water treatment device concerning an embodiment of the present invention. It is a schematic structure figure showing other examples of a water treatment device concerning an embodiment of the present invention. It is a schematic structure figure showing other examples of a water treatment device concerning an embodiment of the present invention. It is a schematic structure figure showing other examples of a water treatment device concerning an embodiment of the present invention. It is a schematic structure figure showing other examples of a water treatment device concerning an embodiment of the present invention. In Example 1 and Comparative Example 1, nitrification tank volume loading for age ^{[day] [kgN / (m} 3 · d)], it is a graph showing the ammonium nitrogen concentration in the nitrification tank [mgN / L]. In Example 1 and Comparative Example 1, denitrification tank volume loading for age ^{[day] [kgN / (m} 3 · d)], it is a graph showing the total nitrogen concentration in the treated water [mgN / L]. 10 is a graph showing the nitrification tank volume load [kgN / (m ³ · d)] and the ammonia nitrogen concentration [mgN / L] of the nitrification tank with respect to the elapsed days [day] in Comparative Example 2. In Comparative Example 2, the denitrification tank volume loading for age ^{[day] [kgN / (m} 3 · d)], is a graph showing the total nitrogen concentration in the treated water [mgN / L]. In Example 2, nitrification tank volume loading for age ^{[day] [kgN / (m} 3 · d)], it is a graph showing the ammonium nitrogen concentration in the nitrification tank [mgN / L]. In Example 2, the denitrification tank volume loading for age ^{[day] [kgN / (m} 3 · d)], is a graph showing the total nitrogen concentration in the treated water [mgN / L]. In Example 3, nitrification tank volume loading for age ^{[day] [kgN / (m} 3 · d)], it is a graph showing the ammonium nitrogen concentration in the nitrification tank [mgN / L]. In Example 3, denitrification tank volume loading for age ^{[day] [kgN / (m} 3 · d)], it is a graph showing the total nitrogen concentration in the treated water [mgN / L]. It is a graph which shows the nitrification rate [kgN / (kgVS.d)] with respect to the molybdenum concentration (Mo / N [mg / g]) in an Example.

  An embodiment of the present invention will be described below. The present embodiment is an example for implementing the present invention, and the present invention is not limited to the present embodiment.

  FIG. 1 schematically shows an example of a water treatment apparatus according to an embodiment of the present invention, and the configuration thereof will be described.

  The water treatment apparatus 1 is a water treatment apparatus for biologically treating water to be treated containing ammonia nitrogen and organic nitrogen, and comprises autotrophic ammonium oxidizing bacteria and nitrite contained in microbial activated sludge. As a nitrification means for oxidizing ammonia nitrogen to nitrite or nitrate nitrogen by nitrifying bacteria containing oxidizing bacteria, a nitrification device 10 is provided. This is a device in which a molybdenum compound is present so as to be 025 mgMo / gN or more. The water treatment device 1 may further include a denitrification device 12 as denitrification means for reducing nitrite or nitrate nitrogen generated in the nitrification device 10 to nitrogen gas by denitrification bacteria contained in the microbial activated sludge. Good.

  The water treatment apparatus 1 includes a solid-liquid separator 14 and a solid-liquid separator 14 that separates treated water from microbial activated sludge to obtain treated water. A sludge return pipe 24 may be further provided as return means for returning to the preceding stage.

  In the water treatment apparatus 1 of FIG. 1, a pipe 16 is connected to an inlet of the nitrification apparatus 10, an outlet of the nitrification apparatus 10 is connected to an inlet of the denitrification apparatus 12 by a pipe 18, and an outlet of the denitrification apparatus 12 is provided. The solid-liquid separator 14 is connected to an inlet of the solid-liquid separator 14 by a pipe 20, and the treated water outlet of the solid-liquid separator 14 is connected to a pipe 22. The sludge outlet of the solid-liquid separator 14 and the pipe 16 are It is connected by a return pipe 24. A molybdenum compound supply pipe 26 is connected to the pipe 16, and a hydrogen donor supply pipe 28 is connected to the denitrification apparatus 12.

  The operation of the water treatment method and the water treatment device 1 according to the present embodiment will be described.

  The water to be treated containing ammonia nitrogen and organic nitrogen is sent to the nitrification device 10 through the pipe 16. In the nitrification apparatus 10, ammonia nitrogen contained in the water to be treated is oxidized to nitrite or nitrate nitrogen by nitrifying bacteria containing autotrophic ammonium oxidizing bacteria and nitrite oxidizing bacteria contained in the microbial activated sludge. (Nitrification step). Here, in the pipe 16, the molybdenum compound is supplied to the water to be treated through the molybdenum compound supply pipe 26, and the molybdenum compound is present so that the molybdenum concentration becomes 0.025 mg Mo / gN or more (molybdenum compound supply step). . The nitrification liquid is sent to the denitrification device 12 through the pipe 18.

  In the denitrification device 12, a hydrogen donor is supplied through a hydrogen donor supply pipe 28, and nitrite or nitrate generated in the nitrification device 10 (nitrification step) by heterotrophic denitrifying bacteria contained in the microbial activated sludge. Nitrogen is reduced to nitrogen gas (denitrification step). The denitrification liquid is sent to the solid-liquid separator 14 through the pipe 20.

  In the solid-liquid separator 14, the treated water is separated from the microbial activated sludge of the denitrification liquid to obtain treated water (solid-liquid separation step). The treated water obtained by the solid-liquid separation is discharged through a pipe 22. On the other hand, at least a part of the sludge obtained by the solid-liquid separation is returned to the pipe 16 through the sludge return pipe 24 and mixed with the water to be treated. The sludge may be returned to the previous stage of the solid-liquid separation device 14 (solid-liquid separation step). For example, the sludge may be returned to the nitrification device 10, the denitrification device 12, or may be returned to the pipes 18 and 20. . At least a part of the sludge obtained by the solid-liquid separation may be discharged from the solid-liquid separation device 14 to the outside of the system.

  The present inventors have proposed to treat nitrogen-containing treated water containing ammonia nitrogen and organic nitrogen, particularly high-concentration nitrogen-containing treated water having a nitrogen concentration of, for example, 100 mgN / L or more in autotrophic water. In the method of treating with a microbial activated sludge containing nitrifying bacteria containing ammonia oxidizing bacteria and nitrite oxidizing bacteria, the metabolic activity of microorganisms is reduced, and when the treatment speed is reduced, The presence of the molybdenum compound so that the molybdenum concentration becomes 0.025 mg Mo / gN or more greatly restores and further improves the metabolic activity of nitrifying bacteria, and makes it possible to stably obtain a high processing rate. I found that. In addition, the present inventors independently treat nitrogen-containing treated water containing ammonia nitrogen and organic nitrogen, particularly high-concentration nitrogen-containing treated water having a nitrogen concentration in the treated water of, for example, 100 mgN / L or more. In the method using microbial activated sludge containing nitrifying bacteria containing vegetative ammonium oxidizing bacteria and nitrite oxidizing bacteria and heterotrophic denitrifying bacteria, the metabolic activity of microorganisms is reduced and the processing speed is reduced. When the molybdenum compound is present in the water to be treated so that the molybdenum concentration becomes 0.025 mg Mo / gN or more, the metabolic activities of nitrifying bacteria and denitrifying bacteria are largely recovered, further improved, and stabilized. High processing speed can be obtained.

  In biological treatment of nitrogen-containing treated water, especially high-concentration nitrogen-containing treated water, molybdenum compounds improve the activity of nitrifying bacteria, including ammonia-oxidizing bacteria and nitrite-oxidizing bacteria, in microbial activated sludge. Even if the concentration of nitrogen in the water to be treated is high, the treatment can be performed stably at a high treatment speed. In the biological treatment of nitrogen-containing treated water, particularly high-concentration nitrogen-containing treated water, molybdenum compounds not only reduce the activity of denitrifying bacteria in microbial activated sludge, but also reduce ammonia oxidizing bacteria and nitrite oxidizing bacteria. By improving the activity of the nitrifying bacteria contained, even if the nitrogen concentration in the water to be treated is high, it can be stably treated at a high treatment rate.

  Generally, when performing biological treatment of wastewater, it is necessary to maintain a nutrient balance in the treated water in order to maintain the growth and metabolic reaction of microorganisms. Carbon (C), oxygen (O), nitrogen (N), hydrogen (H), and phosphorus (P), which are called "biological parent elements", are essential components as nutrients as constituents of cells. In addition, sulfur (S), potassium (K), sodium (Na), calcium (Ca), magnesium (Mg), chlorine (Cl), and iron (Fe) are relatively less required than the biological parent element. Since it is a necessary component as a cell component, when the content of various elements in the water to be treated is low, it is preferable to add and replenish them. On the other hand, although the required amount is small, it is preferable that trace elements involved in the enzyme metabolism of microorganisms are also present. For example, fluorine (F), silica (Si), vanadium (V), chromium (Cr), manganese (Mn) ), Cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), arsenic (As), selenium (Se), molybdenum (Mo), iodine (I) and the like. However, since the required amount of these trace elements including heavy metals is extremely small, and it is assumed that a necessary and sufficient amount is present in the water to be treated, the water to be treated is usually supplied from outside. There is no need to add it in the treatment system. On the other hand, when biologically treating wastewater discharged from a factory that uses ultrapure water in the semiconductor industry, etc., if it is assumed that the above trace elements are insufficient, In some cases, replenishment is performed by mixing tap water, industrial water, well water, and the like with water. However, when the nitrogen concentration in the water to be treated is relatively high (for example, 100 mgN / L or more), the nitrification rate may decrease. Even in such a state, in the water treatment method and the water treatment apparatus according to the present embodiment, by supplying a predetermined amount of the molybdenum compound into the treatment system, the treatment can be stabilized and the high-speed treatment can be performed.

The reaction relating to the treatment of the nitrogen component in the water to be treated is mainly divided into the following.
1. NH ₄ ⁺ → NO ₂ ⁻ (ammonia oxidizing bacteria)
2. NO ₂ ⁻ → NO ₃ ⁻ (Nitrite oxidizing bacteria)
3. NO ₃ ⁻ → N ₂ (denitrifying bacteria)

Biochemical studies on the catabolic reduction reaction of nitric acid (nitrate respiration) under anoxic conditions by denitrifying bacteria are under way. For example, according to Non-Patent Document 2, the reaction from nitric acid to nitrogen gas is subdivided into [NO ₃ ⁻ → NO ₂ ⁻ → NO → N ₂ O → N ₂ ] and [NO ₃ ⁻ → NO ₂ ^−]. It has been clarified that molybdenum element is involved in the enzyme that catalyzes the reduction reaction [1]. That is, although the required amount of the denitrification reaction in the wastewater treatment system is not clear, it is assumed that the activity is improved by adding a molybdenum compound.

Research has also shown that nitrite oxidizing bacteria require molybdenum when oxidizing nitrite. For example, Non-Patent Document 3 discusses the culture conditions of Nitrobacter, which is a nitrite-oxidizing bacterium. At least 10 ⁻⁹ M of molybdenum increases nitrite utilization and cell growth by Nitrobacter by 11 times. It is shown. That is, regarding the nitrite oxidation reaction in the wastewater treatment system, it is assumed that the activity is improved by adding the molybdenum compound.

  On the other hand, enzymes involved in the ammonia oxidative metabolism of ammonia oxidizing bacteria include ammonium monooxygenase (AMO), which oxidizes ammonia to produce hydroxylamine, and hydroxylamine oxidoreductase (HAO), which produces nitrite from the produced hydroxylamine. Although it is said that two kinds of enzymes are involved, there is no report that these are enzyme reactions involving molybdenum (see Non-Patent Document 4).

  Patent Literature 1 describes a method in which cobalt is coexisted in water to be treated and molybdenum, calcium, and a magnesium component are coexistent in order to efficiently promote a nitrification reaction. In the example of Patent Document 1, a nitrification denitrification treatment test of water to be treated having a relatively low nitrogen concentration of 70 mgN / L was verified in the presence of 1 mgMo / L (14.3 mgMo / gN) as a molybdenum concentration. Under the condition that the sludge load of nitrification is 2.5 gN / (kgMLSS · hour) [= 0.06 kgN / (kgMLSS · day)], the result that the nitrogen removal rate is 90% is obtained. It is unlikely that a remarkable effect was obtained.

  The present inventors, using a microbial activated sludge coexisting with ammonia oxidizing bacteria and nitrite oxidizing bacteria, particularly in a method of treating water to be treated containing a high concentration of nitrogen, by supplying a predetermined amount of molybdenum, It has been found that not only the nitrite oxidation reaction but also the activity of ammonia oxidizing bacteria can be significantly improved. In addition, the present inventors have proposed a method of treating water to be treated containing a particularly high concentration of nitrogen by using microbial activated sludge in which various bacteria including ammonia oxidizing bacteria, nitrite oxidizing bacteria, and denitrifying bacteria coexist. It has been found that, by supplying a predetermined amount of molybdenum, not only the nitrite oxidation and denitrification (nitrate reduction) reaction but also the activity of ammonia-oxidizing bacteria can be significantly improved. In microbial activated sludge in which autotrophic bacteria such as ammonia oxidizing bacteria and nitrite oxidizing bacteria coexist with heterotrophic bacteria such as denitrifying bacteria capable of assimilating organic substances, the growth rate is higher than that of nitrifying bacteria. Heterotrophic bacteria, including nitrifying bacteria, dominate in the sludge. Normally, it is assumed that the metabolic reaction of each bacterium is independent, but in a microbial activated sludge in which a plurality of functional microorganisms are present, each bacterium has a coexistence relationship. The mechanism by which the addition of molybdenum compounds improves the ammonia oxidation activity is not clear, but the metabolic activity of heterotrophic bacteria such as denitrifying bacteria can be increased by the supply of molybdenum. It is presumed that it is also connected to.

  In the present embodiment, the water to be treated is nitrogen-containing water containing ammonia nitrogen, particularly nitrogen-containing water containing ammonia nitrogen at a high concentration, and further contains organic nitrogen. Examples of the water to be treated include industrial effluents such as electronic industrial effluents, metal smelting plant effluents, and power plant effluents, and effluents including digestion desorbed water discharged in a sludge treatment process. Here, the electronic industrial wastewater contains various chemicals, and the components in the wastewater vary greatly depending on the products to be manufactured. Examples of the nitrogen-containing water include wafer cleaning wastewater. This wastewater often contains, in addition to ammonia, tetramethylammonium hydroxide (TMAH), hydrogen peroxide, fluorine ions, isopropyl alcohol (IPA), and the like.

  The molybdenum concentration in the water to be treated is, for example, 0.0001 mgMo / L or less. Further, when the nitrogen concentration in the water to be treated is preferably 100 mgN / L or more, and more preferably 400 mgN / L or more, the water treatment method and the water treatment apparatus according to the present embodiment are suitably applied. .

  In biologically treating such nitrogen-containing water, inhibitory substances such as hydrogen peroxide and fluorine ions have an inhibitory effect on living organisms, and thus it is desirable to remove them in advance. As a method for treating these inhibitory substances, existing techniques can be used.For example, in the treatment of hydrogen peroxide, a method of adding an enzyme, a method of injecting a reducing agent, a method of contacting with activated carbon, and the like are exemplified. No. In the treatment of fluorine ions, a method of adding calcium to remove as calcium fluoride, a method of treating with an ion-exchange resin, and the like can be given.

  Nitrogen-containing water from which inhibitory substances such as hydrogen peroxide and fluorine ions have been removed is temporarily stored in a water tank before the biological treatment process is performed, and the flow rate and water quality of the biological treatment process are stabilized. Preferably, the pH is adjusted to an appropriate pH (for example, pH 6.5 to 8.0) with a pH adjuster such as an alkali or an acid. Then, nitrogen-containing water (water to be treated) whose flow rate, water quality, pH and the like are adjusted is sent to the biological treatment step.

  In the nitrification step in the nitrification apparatus 10, the water to be treated is supplied to a nitrification section (for example, a nitrification tank), and ammonia nitrogen such as ammonium ions in the water to be treated is aerobically (for example, in the presence of oxygen) nitrite. Alternatively, it is a step of oxidizing to nitrate nitrogen. For example, an air introduction pipe is connected to the nitrification unit, and has a structure capable of supplying an oxygen-containing gas such as air to the water to be treated in the nitrification unit. Then, in the nitrification section, nitric acid is used to nitrify ammonium nitrogen such as ammonium ion in the water to be treated into nitrite or nitrate nitrogen. Here, nitrifying bacteria are a general term for autotrophic ammonium oxidizing bacteria that oxidize ammonium nitrogen such as ammonium ions to nitrite ions, and autotrophic nitrite oxidizing bacteria that oxidize nitrite ions to nitrate ions. Means

  When molybdenum contained in the water to be treated is insufficient, a molybdenum compound may be externally added. The molybdenum compound is supplied to the water to be treated through the molybdenum compound supply pipe 26, for example, as a molybdenum compound solution, and is supplied into the system by mixing the molybdenum compound with the water to be treated. The molybdenum compound may be supplied, for example, in proportion to the amount of nitrogen to be treated. By supplying a predetermined amount of the molybdenum compound, the activities of nitrifying bacteria (ammonium oxidizing bacteria and nitrite oxidizing bacteria) and denitrifying bacteria are maintained at a high level, and stable operation or high-speed treatment can be performed.

  Examples of the molybdenum compound include molybdate compounds such as sodium molybdate, potassium molybdate, and ammonium molybdate. The form of the molybdenum compound is not particularly limited, but, for example, if it is in the form of a solution, the bacteria in the microbial activated sludge can be easily used.For example, an aqueous solution of sodium molybdate or potassium molybdate is prepared and added in advance. Is preferred.

  The molybdenum compound may be added to the pipe 16 before the nitrification treatment is performed, or may be supplied to the nitrification apparatus 10 in which the water to be treated and the microbial activated sludge are mixed. Also, considering that the added molybdenum compound is returned as return sludge before the solid-liquid separation step and circulates in the system, the molybdenum compound supply pipe is connected to the pipe 18 and the denitrification apparatus 12 and supplied. It can be done.

  In the nitrification device 10 (nitrification step), the molybdenum compound is present in the water to be treated so that the molybdenum concentration becomes 0.025 mg Mo / gN or more, but the molybdenum compound becomes so that the molybdenum concentration becomes 0.1 mg Mo / gN or more. Preferably, the compound is present. The upper limit of the molybdenum concentration is not particularly limited, but is, for example, 0.25 mgMo / gN or less. In the nitrification apparatus 10 (nitrification step), when the molybdenum concentration is less than 0.025 mgMo / gN with respect to the water to be treated, the activity maintaining effect of nitrifying bacteria (ammonium oxidizing bacteria and nitrite oxidizing bacteria) and denitrifying bacteria does not appear. There are cases.

  In the nitrification apparatus 10 (nitrification step), the molybdenum concentration is preferably set to 2 mgMo / L or less with respect to the water to be treated. If the molybdenum concentration exceeds 2 mgMo / L with respect to the water to be treated, the nitrification reaction may be hindered.

  A carrier for supporting microorganisms may be provided in the nitrification section. The carrier on which the microorganisms are carried is not particularly limited, but it is preferable to use, for example, a carrier made of resin such as plastic or polyurethane.

  In the denitrification step in the denitrification apparatus 12, for example, a hydrogen donor is supplied to a completely mixed type denitrification unit (for example, a denitrification tank), and nitrous acid or nitrate nitrogen generated in the nitrification unit is subjected to anoxic condition. This is the step of reducing to nitrogen gas. In the denitrification unit (for example, a denitrification tank), nitrite or nitrate nitrogen is reduced to nitrogen gas by the action of heterotrophic bacteria, denitrification bacteria. In the denitrification section, it is preferable to provide a stirrer for mixing the nitrifying solution and the microorganism-activated sludge under oxygen-free conditions in order to perform the treatment efficiently.

  A carrier for supporting microorganisms may be provided in the denitrification unit. The carrier on which the microorganisms are supported is not particularly limited, but it is preferable to use a carrier made of resin such as plastic or polyurethane.

  Examples of the hydrogen donor for denitrification used in the present embodiment include alcohols such as methanol, ethanol, and isopropyl alcohol, organic acids such as acetic acid, hydrogen gas, acetone, glucose, ethyl methyl ketone, and tetramethyl hydroxide. One or more of ammonium (TMAH) and the like may be mentioned, but not limited thereto, and any conventionally known hydrogen donors may be used. As the hydrogen donor, an organic substance or the like contained in the water to be treated may be used.

  The solid-liquid separation step in the solid-liquid separation device 14 separates the denitrification liquid whose nitrogen component has been nitrified and denitrified by the nitrifying bacteria and denitrifying bacteria in the microbial activated sludge into treated water and microbial activated sludge. This is a step of obtaining treated water.

  The solid-liquid separation device 14 is not particularly limited, and examples thereof include separation devices such as sedimentation separation, pressure flotation, filtration, and membrane separation. In the solid-liquid separation step, the treated water is obtained, and the microorganism-activated sludge after separation is also obtained. Part of the microorganism-activated sludge is drawn out of the system as excess sludge, and part is, for example, a nitrification device 10 (nitrification step). By returning to the system, the amount of microbial activated sludge in the system can be maintained.

  The hydrogen donor is added in the denitrification device 12, but if there is a concern that the hydrogen donor remains after the denitrification treatment and the quality of the treated water deteriorates, the denitrification device 12 (denitrification step) is used. An oxidizing device may be provided as an oxidizing means for aerobically treating the hydrogen donor between the solid-liquid separating device 14 (solid-liquid separating step).

  FIG. 2 shows an example of such a water treatment apparatus. In the water treatment device 3 of FIG. 2, an oxidizing device 30 is provided between the denitrification device 12 (denitrification process) and the solid-liquid separation device 14 (solid-liquid separation process). The outlet of the denitrification device 12 and the inlet of the oxidizer 30 are connected by a pipe 32, and the outlet of the oxidizer 30 and the inlet of the solid-liquid separator 14 are connected by a pipe 34.

  The denitrification liquid obtained in the denitrification device 12 (denitrification step) is sent to the oxidizing device 30 through the pipe 32. In the oxidation step in the oxidation device 30, a hydrogen donor is aerobically treated in an oxidation section (for example, an oxidation tank). An air introduction pipe is connected to the oxidizing section (for example, the oxidizing tank), similarly to the nitrifying section, and has a structure capable of supplying an oxygen-containing gas such as air to the water to be treated in the oxidizing section. .

  The oxidizing solution oxidized in the oxidizing device 30 is sent to the solid-liquid separating device 14 through the pipe 34, and thereafter, the treatment is performed in the same manner as in the water treatment device 1 of FIG.

  When the water to be treated contains organic matter and nitrogen, the denitrification reaction may be caused by using the organic matter in the water to be treated as a hydrogen donor without adding a hydrogen donor for the denitrification reaction from the outside.

  FIG. 3 shows an example of such a water treatment apparatus. In the water treatment apparatus 5 shown in FIG. 3, a pipe 36 is connected to an inlet of the denitrification apparatus 12, and an outlet of the denitrification apparatus 12 and an inlet of the nitrification apparatus 10 are connected by a pipe 38, and an outlet of the nitrification apparatus 10 is provided. And the inlet of the solid-liquid separator 14 are connected by a pipe 40, the treated water outlet of the solid-liquid separator 14 is connected to a pipe 42, and the sludge outlet of the solid-liquid separator 14 and the pipe 36 are connected to the sludge It is connected by a return pipe 44. The pipe 36 is connected to the molybdenum compound supply pipe 26. The pipe 40 and the denitrification device 12 are connected by a nitrification liquid return pipe 46.

  In the water treatment device 5, the water to be treated containing ammonia nitrogen and organic nitrogen is sent to the denitrification device 12 through the pipe 36. On the other hand, at least a part of the nitrification liquid is sent to the denitrification apparatus 12 through the nitrification liquid return pipe 46 from the subsequent nitrification apparatus 10. Here, in the pipe 36, the molybdenum compound is supplied to the water to be treated through the molybdenum compound supply pipe 26, and the molybdenum compound is present so that the molybdenum concentration becomes 0.025 mgMo / gN or more (molybdenum compound supply step). .

  In the nitrification apparatus 10, ammonia nitrogen contained in the water to be treated is oxidized to nitrite or nitrate nitrogen by nitrifying bacteria containing autotrophic ammonium oxidizing bacteria and nitrite oxidizing bacteria contained in the microbial activated sludge. (Nitrification step). In the denitrification apparatus 12, nitrite or nitrate nitrogen generated in the nitrification apparatus 10 (nitrification step) is reduced to nitrogen gas by heterotrophic denitrifying bacteria contained in the microbial activated sludge (denitrification step). . The denitrification liquid is sent to the nitrification device 10 through the pipe 38, and at least a part of the nitrification liquid is sent to the solid-liquid separation device 14 through the pipe 40. Thereafter, the processing is performed in the same manner as in the water treatment apparatus 1 of FIG.

  To further reduce the nitrogen concentration of the treated water, a post-denitrification device as a post-denitrification device and an oxidization device as an oxidization device are provided between the nitrification device 10 and the solid-liquid separation device 14 in the water treatment device 5 in FIG. And a device.

  FIG. 4 shows an example of such a water treatment apparatus. The water treatment apparatus 7 in FIG. 4 further includes a post-denitrification device 48 as post-denitrification means, and an oxidization device 30 as oxidization means. An outlet of the nitrification device 10 and an inlet of the post-denitrification device 48 are connected by a pipe 50, and an outlet of the post-denitrification device 48 and an inlet of the oxidizer 30 are connected by a pipe 52, and connected to an outlet of the oxidizer 30. The inlet of the solid-liquid separator 14 is connected by a pipe 54. The hydrogen donor supply pipe 28 is connected to the post-denitrification device 48. The pipe 50 and the denitrification device 12 are connected by a nitrification liquid return pipe 46.

  In the water treatment device 7, the water to be treated containing ammonia nitrogen and organic nitrogen is sent to the denitrification device 12 through the pipe 36. On the other hand, at least a part of the nitrification liquid is sent to the denitrification apparatus 12 through the nitrification liquid return pipe 46 from the subsequent nitrification apparatus 10. Here, in the pipe 36, the molybdenum compound is supplied to the water to be treated through the molybdenum compound supply pipe 26, and the molybdenum compound is present so that the molybdenum concentration becomes 0.025 mgMo / gN or more (molybdenum compound supply step). .

  In the nitrification apparatus 10, ammonia nitrogen contained in the water to be treated is oxidized to nitrite or nitrate nitrogen by nitrifying bacteria containing autotrophic ammonium oxidizing bacteria and nitrite oxidizing bacteria contained in the microbial activated sludge. (Nitrification step). In the denitrification apparatus 12, nitrite or nitrate nitrogen generated in the nitrification apparatus 10 (nitrification step) is reduced to nitrogen gas by heterotrophic denitrifying bacteria contained in the microbial activated sludge (denitrification step). . The denitrification liquid is sent to the nitrification apparatus 10 through the pipe 38, and at least a part of the nitrification liquid is sent to the post-denitrification apparatus 48 through the pipe 50. Nitrite or nitrate nitrogen generated in the nitrification device 10 (nitrification step) is reduced to nitrogen gas (denitrification step). The denitrification liquid is sent to the oxidizer 30 through the pipe 52. Thereafter, the processing is performed in the same manner as in the water treatment apparatus 3 of FIG. At least a part of the nitrification liquid is sent to the denitrification device 12 through the nitrification liquid return pipe 46.

  In the denitrification step, the addition amount of the hydrogen donor is varied with time so that the difference between the maximum concentration and the minimum concentration of the hydrogen donor during the hydraulic residence time of the treated water is 50 mgTOC / L or more. It is preferable to granulate microbial activated sludge containing bacteria and denitrifying bacteria. By varying the concentration of the hydrogen donor to be added in the denitrification reaction, granules self-granulated by the denitrifying bacteria can be easily formed.

  Furthermore, by circulating the granules in a nitrogen-containing water treatment system for nitrification and denitrification, all bacterial groups such as nitrifying bacteria are granulated, and the entire treatment apparatus for the nitrogen-containing water to be treated is substantially reduced. Can be processed with the same granules.

  Further, in the case where the difference between the maximum concentration and the minimum concentration of the hydrogen donor in the denitrification step is increased to efficiently promote the granulation of the microbial activated sludge, the denitrification step is performed at least in the first denitrification step. And two or more steps including a second denitrification step. The denitrification step includes at least a first denitrification step and a second denitrification step, and in the denitrification step, the hydrogen donor in the first denitrification step at the hydraulic residence time of the treated water in the second denitrification step The hydrogen donor may be supplied at least in the first denitrification step so that the difference between the maximum concentration of the hydrogen donor and the minimum concentration of the hydrogen donor in the second denitrification step is 50 mgTOC / L or more.

  An example of such a water treatment apparatus is shown in FIG. 5 includes a first denitrification device 58 and a second denitrification device 60 as denitrification means. The outlet of the nitrification device 10 and the inlet of the first denitrification device 58 are connected by a pipe 62, the outlet of the first denitrification device 58 and the inlet of the second denitrification device 60 are connected by a pipe 64, 2 The outlet of the denitrification device 60 and the inlet of the oxidation device 30 are connected by a pipe 66.

  The nitrification liquid obtained by the nitrification device 10 is sent to the first denitrification device 58 through the pipe 62. In the first denitrification apparatus 58, after the hydrogen donor is supplied through the hydrogen donor supply pipe 28 and is brought into contact with heterotrophic denitrifying bacteria contained in the microbial activated sludge, the mixed solution is passed through the pipe 64 to the second The liquid is sent to the denitrification device 60, and in the second denitrification device 60, the nitrite or nitrate nitrogen generated in the nitrification device 10 (nitrification process) is reduced to nitrogen gas by the denitrifying bacteria (denitrification process). . The denitrification liquid is sent to the oxidizer 30 through the pipe 66. Thereafter, the processing is performed in the same manner as in the water treatment apparatus 3 of FIG.

  Hereinafter, the present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to the following examples.

  Examples and comparative examples using a continuous water flow tester are shown below. In addition, all were implemented on the conditions which controlled room temperature at 20 degreeC.

<Example 1>
In Example 1, a bench scale tester having the configuration of the water treatment device 9 shown in FIG. 5 was used. The nitrifying bacteria and the denitrifying bacteria were granulated, and the simulated wastewater was subjected to a nitrification denitrification treatment test. The simulated wastewater used was a solution prepared by dissolving ammonia nitrogen in pure water at a concentration of 400 mgN / L, and a phosphoric acid and a trace element chemical solution added as other nutrient sources. The trace element chemical solution used in this test did not contain molybdenum. Methanol was used as a hydrogen donor for denitrification and was added intermittently to the first denitrification tank, and the difference between the maximum methanol concentration in the first denitrification tank and the minimum methanol concentration in the second denitrification tank was 50 mgTOC. / L or more. A pH controller was installed in the nitrification tank, the first denitrification tank, and the second denitrification tank, and the pH in the tank was adjusted to 7 to 7.5 using hydrochloric acid or sodium hydroxide. The concentrated sludge obtained from the solid-liquid separation tank was returned to the nitrification tank. From day 0 to day 45, molybdenum was not added (Comparative Example 1), and from day 46, a molybdenum compound (sodium molybdate) was added to the water to be treated so as to be 0.1 mgMo / L. Started (Example 1). The concentration of molybdenum is 0.25 mgMo / gN with respect to the nitrogen concentration of the water to be treated. The results are shown in FIGS. 6, the nitrification tank volume loading ^{[kgN / (m 3 · d} )], shows changes in ammonium nitrogen concentration of the nitrification tank [mgN / L], FIG. 7, the denitrification tank volume loading for age [day] ^{[kgN / (m 3 · d} )], shows changes in total nitrogen concentration in the treated water [mgN / L].

In early Comparative Example 1, although the volume load in the nitrification tank starts passing water in ^{0.2kgN / (m 3 · d)} , the ammonia nitrogen in the nitrification tank is left 5~60mgN / L, increasing the load can not, nitrification rate was stagnant at ^{0.15~0.25kgN / (m 3 · d)} . With the stagnation of the nitrification rate, stagnated in denitrification rate also ^{0.3~0.5kgN / (m 3 · d)} . During the period of Comparative Example 1, the processing speed per sludge at which stable operation was possible was 0.05 kgN / (kgVSs · d) by nitrification.

Next, when the addition of molybdenum to the water to be treated was started, an increase in the treatment rate was observed, and a nitrification rate of 1.1 kgN / (m ³ · d) at maximum was confirmed. In addition, during the period of Example 1, the ammonia nitrogen in the nitrification tank constantly changed to 1 mgN / L or less. As the nitrification rate increased, the denitrification rate also increased, reaching 2.2 kgN / (m ³ · d) at the maximum. During the period of Comparative Example 1, it was confirmed that the processing speed per sludge exhibiting sludge activity was stable at 0.24 kgN / (kgVSs · d) for nitrification and 0.54 kgN / (kgVSS · d) for denitrification. .

<Comparative Example 2>
In Comparative Example 2, a continuous water flow test was performed using a bench scale tester having the configuration of the water treatment apparatus 9 shown in FIG. As the simulated wastewater, pure water adjusted to 800 mgN / L as ammonia nitrogen and added with phosphoric acid and a trace element chemical solution (not containing molybdenum) was used. For the purpose of replenishing trace elements, molybdenum-free well water (molybdenum concentration: 0.0001 mg Mo / L or less (below detection limit)) was used from day 120 to day 215, and molybdenum was used from day 216 to day 280. The replenishment was carried out by adding 10% flow rate of the water to be treated to the processing water containing. The molybdenum concentration in the working water was 0.0006 mgMo / L. The molybdenum concentration in well water and industrial water was measured using ICP mass spectrometry (ICP-MS).

Nitrification tank volume loading ^{[kgN / (m 3 · d} )] and FIG changes in the residual ammonia nitrogen concentration in the nitrification tank [mgN / L] 8, denitrification tank volume loading ^{[kgN / (m 3 · d} )] and treated FIG. 9 shows changes in the total nitrogen concentration [mgN / L] of water. As can be seen from FIG. 8, the nitrification tank volume load was increased to 0.8 kgN / (m ³ · d), but the ammonia nitrogen concentration remained 41 mgN / L on the 169th day, and 130 mgN / L on the 171th day. / L. Thereafter, the nitrification performance was not stable, and 10 to 40 mgN / L sometimes remained in the nitrification tank, and stable operation was not possible. The nitrification activity during the test period was 0.02 to 0.075 kgN / (kgVSs · d). Due to the instability of nitrification, the treatment for denitrification was not stable, and the treated water TN increased to a maximum of about 150 mgN / L.

<Example 2>
Using a simulated drainage and test apparatus under the same conditions as in Comparative Example 2, the effect of adding the molybdenum compound solution was verified by a continuous water flow test. The molybdenum addition concentration was 0.02 mgMo / L with respect to the ammonia nitrogen concentration of the water to be treated of 800 mgN / L, and the addition was started on the 175th day. The molybdenum addition concentration is 0.025 mgMo / gN with respect to the nitrogen concentration of the water to be treated.

Nitrification tank volume loading ^{[kgN / (m 3 · d} )] and the transition of the remaining ammonia nitrogen concentration in the nitrification tank [mgN / L] 10, a denitrification tank volume loading ^{[kgN / (m 3 · d} )] and treated FIG. 11 shows the transition of the total nitrogen concentration [mgN / L] of water. As a result, during the period in which molybdenum was not added, the volume load of the nitrification tank was not increased, and 55 mgN / L of ammonia nitrogen remained on the 175th day. Was started addition of Mo compound 175 days, reduction of the ammonia nitrogen concentration is confirmed, can be stably operated even by increasing the volumetric loading up to 0.8~0.9kgN / ^(m 3 · d) Met. The nitrification activity before the addition of molybdenum was 0.05 kgN / (kg VSS · d) at the maximum, but the nitrification activity after the start of the addition of molybdenum increased to 0.11 kgN / (kg VSS · d). Regard denitrification, rose as denitrification volume loading up ^{1.4kgN / (m 3 · d)} .

<Example 3>
Using a simulated drainage and test apparatus under the same conditions as in Example 2, the effect of adding the molybdenum compound was verified by a continuous water flow test. The addition concentration was 0.1 mgMo / L with respect to the ammonium nitrogen concentration of the water to be treated being 800 mgN / L, and the addition was started on the 343rd day. The concentration of molybdenum is 0.125 mgMo / gN with respect to the nitrogen concentration of the water to be treated.

Nitrification tank volume loading ^{[kgN / (m 3 · d} )] and 12 changes in the residual ammonia nitrogen concentration in the nitrification tank [mgN / L], denitrification tank volume loading ^{[kgN / (m 3 · d} )] and treated FIG. 13 shows changes in the total nitrogen concentration [mgN / L] of water. As a result, during the period in which molybdenum was not added, the load could not be increased because about 70 to 80 mg / L of nitrate nitrogen was detected in the denitrification tank, and the load on the nitrification tank was 0.25 kgN / (m ³ ·· d) It was stagnant. On day 343, the addition of the molybdenum solution was started to try to increase the load. On the 346th day, 37 mg N / L of ammonia nitrogen was detected in the nitrification tank, and thereafter, a decrease in the concentration of ammonia nitrogen was observed. Although increasing the nitrification tank volume loading up ^{0.86kgN / (m 3 · d)} , never ammonia nitrogen is detected in the nitrification tank, treated water TN in denitrification tank remained below 5mgN / L. The nitrification activity before the addition of molybdenum was stagnant at 0.05 to 0.06 kgN / (kgVSs · d), but after the start of the addition of molybdenum, the activity was improved, and the activity increased to 0.2 kgN / (kgVSs · d). The rise was confirmed.

  FIG. 14 shows a summary of the above results. FIG. 14 shows that in the nitrification step, the molybdenum compound is preferably present in the water to be treated so that the molybdenum concentration becomes 0.025 mgMo / gN or more.

  As described above, according to the method of the embodiment, in the biological treatment of the treated water containing ammonia nitrogen and organic nitrogen, even if the concentration of nitrogen in the treated water is high, the treatment is stably performed at a high treatment rate. We were able to.

  Hereinafter, the inhibitory effect of the addition concentration of the molybdenum compound on the ammonia oxidation reaction and the denitrification reaction was evaluated by a batch test.

[Test for the effect of adding molybdenum on denitrification (batch test)]
Using nitrifying denitrification sludge, the effect of molybdenum addition on the denitrification reaction was verified by a batch test using simulated wastewater. The specific method of the experiment is described below.

1. As the simulated wastewater used for the batch test, well water was used in which nitrate ion was added to 60 mgN / L and phosphate phosphorus to 1 mgP / L. In addition, the molybdenum content concentration in the simulated wastewater was 0.0001 mg / L or less.
2. A sludge that had been previously acclimated to methanol as a hydrogen donor and washed with pure water was suspended in simulated wastewater, and dispensed into five beakers.
3. A sodium molybdate solution was added to the mixed liquid of the sludge and the simulated drainage so as to be 0 mgMo / L, 1 mgMo / L, 5 mgMo / L, 10 mgMo / L, and 20 mgMo / L in each beaker, followed by stirring.
4. While stirring, methanol as a hydrogen donor was added to each beaker in the same amount, and the reduction rate of nitric acid was evaluated.

  As a result of the denitrification activity test, the denitrification activity calculated from the nitric acid reduction rate and the amount of sludge in the beaker was 0.43 gN / gSS / day in the series where no Mo was added, and 0 in the series where the Mo concentration was 1 mgMo / L. 0.46 gN / gSS / day in the series of 0.46 g, 5 mgMo / L, 0.42 gN / gSS / day in the series of 10 mgMo / L, and 0.41 gN / gSS / day in the series of 20 mgMo / L. Although the activity was slightly higher than that of the series in which no was added, the activity was improved by 8% in the series in which the Mo concentration was 1 mgMo / L and 5% in the series in which the Mo concentration was 5 mgMo / L. On the other hand, in the series in which the Mo concentration was added as high as 20 mg Mo / L, only about 10% decrease in the activity was confirmed as compared with the series in which the Mo concentration was 1 mg Mo / L, which had the highest denitrification activity. No significant inhibition of Mo was observed.

[Test for the effect of adding Mo on ammonia oxidation (batch test)]
Using nitrification denitrification sludge, the effect of molybdenum addition on the ammonia oxidation reaction was verified by a batch test using simulated wastewater. The specific method of the experiment is described below.

1. As the simulated wastewater used for the batch test, well water was used in which ammonium ion was added to 60 mgN / L and phosphoric acid phosphorus to 1 mgP / L. In addition, the molybdenum content concentration in the simulated wastewater was 0.0001 mg / L or less.
2. The test sludge washed with pure water was suspended in simulated drainage water and dispensed into five beakers.
3. Add sodium molybdate solution to the mixture of sludge and simulated waste water to each beaker to be 0mgMo / L, 0.1mgMo / L, 0.5mgMo / L, 2mgMo / L, 10mgMo / L, and start aeration respectively. did.
4. The reduction rate of each ammonia nitrogen concentration was evaluated.

  As a result of the ammonia oxidation activity test, the ammonia oxidation activity calculated from the ammonia reduction rate and the amount of sludge in the beaker was 0.18 gN / gSS / day in the series without adding Mo, and the series in which the Mo concentration was 0.1 mgMo / L. In the series of 0.13 gN / gSS / day and 0.5 mgMo / L, 0.13 gN / gSS / day in the series of 2 mgMo / L, and 0.13 gN / gSS / day in the series of 10 mgMo / L. The result was 10 gN / gSS / day, and the result was that the ammonia oxidation activity of the series without addition of Mo was the highest, and the ammonia oxidation activity of the series with 10 mgMo / L was the lowest. From the results of the batch test, it was considered that the concentration is preferably 2 mgMo / L or less in order to suppress the inhibition of the molybdenum concentration on the ammonia oxidation reaction.

  1,3,5,7,9 water treatment device, 10 nitrification device, 12 denitrification device, 14 solid-liquid separation device, 16,18,20,22,32,34,36,38,40,42,50, 52, 54, 62, 64, 66 piping, 24, 44 sludge return piping, 26 molybdenum compound supply piping, 28 hydrogen donor supply piping, 30 oxidizer, 46 nitrification liquid return piping, 48 post-denitrification device, 58 first Denitrification device, 60 Second denitrification device.

Claims (6)

A water treatment method for biologically treating water to be treated containing ammonia nitrogen,
By nitrifying bacteria containing autotrophic ammonium oxidizing bacteria and nitrite oxidizing bacteria contained in the microbial activated sludge, including a nitrification step of oxidizing the ammonia nitrogen to nitrite or nitrate nitrogen,
In the nitrification step, a molybdenum compound is present in the water to be treated so that the molybdenum concentration becomes 0.025 mgMo / gN or more,
A water treatment method, wherein a nitrification rate per sludge is 0.11 [kgN / (kgVSs / day)] or more. The water treatment method according to claim 1,
A water treatment method, wherein the molybdenum concentration in the nitrification step is 2 mgMo / L or less with respect to the water to be treated. The water treatment method according to claim 1 or 2,
The water treatment method, wherein the nitrogen concentration in the water to be treated is 100 mgN / L or more. The water treatment method according to any one of claims 1 to 3,
A water treatment method, further comprising a denitrification step of reducing nitrite or nitrate nitrogen generated in the nitrification step to nitrogen gas by denitrifying bacteria contained in the microorganism-activated sludge. The water treatment method according to claim 4,
In the denitrification step, the addition amount of the hydrogen donor is varied with time so that the difference between the maximum concentration and the minimum concentration of the hydrogen donor during the hydraulic residence time of the treated water is 50 mgTOC / L or more. A water treatment method, wherein the microbial activated sludge containing the nitrifying bacteria and the denitrifying bacteria is granulated. The water treatment method according to claim 5, wherein
The denitrification step includes at least a first denitrification step and a second denitrification step,
In the denitrification step, the maximum concentration of the hydrogen donor in the first denitrification step and the minimum concentration of the hydrogen donor in the second denitrification step in the hydraulic residence time of the treated water in the second denitrification step A water treatment method comprising supplying a hydrogen donor at least in the first denitrification step so that the difference is 50 mgTOC / L or more .