JP7553289B2 - Water Treatment Methods - Google Patents

Description

The present invention relates to a water treatment method.

Water to be treated, such as groundwater, often contains ammoniacal nitrogen. When the water to be treated containing ammoniacal nitrogen is to be made potable, the amount of ammoniacal nitrogen is reduced.
As a method for treating water containing ammoniacal nitrogen, for example, a water treatment method has been proposed in which the water containing ammoniacal nitrogen is subjected to nitrification treatment by a contact oxidation method using a nitrifying bacteria-supported carrier in which nitrifying bacteria are supported on a carrier.

When treating water to be treated using a nitrifying bacteria-supporting carrier, the nitrifying bacteria-supporting carrier is extracted from the nitrification tank of an existing water treatment system and introduced into the nitrification tank of a new water treatment system, i.e., the nitrifying bacteria are propagated. For this reason, the nitrifying bacteria-supporting carrier is often extracted from the nitrification tank.
However, if the nitrifying bacteria-supporting carrier is removed from the nitrification tank when the amount of nitrifying bacteria supported is insufficient, or if a large amount of the nitrifying bacteria-supporting carrier is removed from the nitrification tank at once, the absolute amount of nitrifying bacteria remaining in the nitrification tank will be significantly insufficient.Since nitrifying bacteria have a low growth rate, the nitrification efficiency in the nitrification tank will be significantly reduced until the nitrifying bacteria grow sufficiently on the surface and inside of the carrier.

Therefore, as a water treatment method that can extract the nitrifying bacteria-supporting carrier from the nitrification tank while sufficiently suppressing the decrease in nitrification efficiency, a method has been proposed in which the water to be treated is subjected to biological nitrification treatment and then subjected to ion exchange treatment with ion exchange resin (Patent Document 1).
In the water treatment method described in Patent Document 1, the decrease in nitrification efficiency in the nitrification tank after the nitrifying bacteria-supporting carrier is removed from the nitrification tank can be compensated for by the ion exchange treatment.

JP 2017-202473 A

However, in the water treatment method described in Patent Document 1, since an ion exchange treatment is performed after a biological nitrification treatment, it is likely to require a lot of work and operating costs.
An object of the present invention is to provide a water treatment method capable of extracting a nitrifying bacteria-supporting carrier from a nitrification tank easily and at low cost while sufficiently suppressing a decrease in nitrification efficiency.

The present invention has the following aspects.
[1] A water treatment method for subjecting water to biological nitrification treatment in a nitrification tank filled with a carrier carrying nitrifying bacteria, the water treatment method comprising the following steps (i) to (iii):
Step (i): A step of acclimating the carrier packed in the nitrification tank so that the bulk volume is more than 30 volume % and less than 65 volume % of the effective volume of the nitrification tank, to obtain a carrier supporting nitrifying bacteria.
Step (ii): A step of removing the nitrifying bacteria-supporting carriers from the nitrification tank while maintaining the bulk volume of the nitrifying bacteria-supporting carriers at 30 volume % or more relative to the effective volume of the nitrification tank.
Step (iii): A step of using the nitrifying bacteria-supporting carrier extracted in the step (ii) in another biological nitrification treatment.
[2] The water treatment method according to [1], wherein in the step (ii), the nitrifying bacteria-supporting carrier and the treated water treated in the nitrification tank are extracted from the nitrification tank using an air lift pump.
[3] The water treatment method according to [1] or [2] above, wherein the carrier is a sponge carrier.
[4] The water treatment method according to any one of the above [1] to [3], wherein the carrier has a rectangular shape with a short side of 3 to 10 mm.
[5] The water treatment method according to any one of [1] to [4], wherein in the step (i), the carrier is acclimatized in the nitrification tank for 4 days or more.

The present invention provides a water treatment method that can easily and inexpensively remove nitrifying bacteria-supporting carriers from a nitrification tank while sufficiently suppressing the decrease in nitrification efficiency.

FIG. 1 is a schematic diagram showing an example of a water treatment system used in the water treatment method of the present invention. 4 is a graph showing the change over time in ammonia nitrogen concentration in the water to be treated, the first treated water, and the second treated water after the nitrifying bacteria-supporting carrier is removed from the first nitrification tank in Example 1. 1 is a graph showing the change over time in ammonia nitrogen concentration in the water to be treated, the first treated water, and the second treated water after the nitrifying bacteria-supporting carrier was removed from the first nitrification tank in Comparative Example 1.

An embodiment of the water treatment method according to the present invention will be described in detail below with reference to FIG.
In addition, in each drawing used in the following description, characteristic parts may be enlarged for convenience in order to make the features easier to understand, and the dimensional ratios of each component may differ from the actual ones. Furthermore, the materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto, and can be appropriately changed and implemented within the scope of the present invention.
In addition, in this specification, "ammoniacal nitrogen" refers to nitrogen contained in water as an ammonium salt, and is also called ammoniacal nitrogen.
In this specification, a carrier carrying nitrifying bacteria is also referred to as a "nitrifying bacteria-supporting carrier."

<Water treatment system>
FIG. 1 is a schematic diagram showing an example of a water treatment system used in the water treatment method according to the present invention.
The water treatment system 1 in this example comprises a first nitrification tank 10 which performs biological nitrification treatment on the water to be treated supplied from a raw water tank (not shown) through a water to be treated supply flow path 100 to produce first treated water; a second nitrification tank 20 which performs biological nitrification treatment on the first treated water transferred from the first nitrification tank 10 through a first treated water transfer flow path 101 to produce second treated water; a transfer pump (not shown) provided midway along the water to be treated supply flow path 100; and an air lift pump 30 which is immersed in the first nitrification tank 10 and extracts water from the first nitrification tank 10.
The blower and air volume adjusting means of the air diffuser, which will be described later, the transfer pump, the air lift pump 30, etc. are electrically connected to a control unit (not shown) which controls the operation of these components.

(First nitrification tank)
The first nitrification tank 10 is a so-called fluidized bed aerobic biological reaction tank, which comprises: a container-shaped tank body 11 filled with carriers 200; a solid-liquid separation device 12 provided at a point where the tank body 11 is connected to a first treated water transfer flow path 101, for separating the carriers 200 or the nitrifying bacteria-supporting carriers from the first treated water; and an aeration device 13 inserted at the bottom of the tank body 11.

The solid-liquid separator 12 may be, for example, a screen.
The air diffuser 13 comprises an air diffuser section 14 located at the bottom of the tank body 11; an air supply pipe 15 which supplies air to the air diffuser section 14; a blower 16 provided midway along the air supply pipe 15; and air volume adjustment means 17 provided midway along the air supply pipe 15 between the air diffuser section 14 and the blower 16.
The air diffusion section 14 may be, for example, an air diffusion tube or an air diffusion ball having air diffusion holes (not shown) formed therein.
The air amount adjusting means 17 may be, for example, a gate valve, a butterfly valve, or the like.

(Second nitrification tank)
The second nitrification tank 20 is a so-called fluidized bed type aerobic biological reaction tank, which comprises: a container-shaped tank body 21 filled with carriers 200; a solid-liquid separation device 22 provided at a point where the second treated water transfer flow path 102 is connected to the tank body 21, for separating the carriers 200 or the nitrifying bacteria-supporting carriers from the second treated water; and an aeration device 23 inserted at the bottom of the tank body 21.

The solid-liquid separator 22 may be, for example, a screen.
The air diffuser 23 comprises an air diffuser section 24 located at the bottom of the tank body 21; an air supply pipe 25 which supplies air to the air diffuser section 24; a blower 26 provided midway along the air supply pipe 25; and air volume adjustment means 27 provided midway along the air supply pipe 25 between the air diffuser section 24 and the blower 26.
The air diffusion section 24 may be, for example, an air diffusion tube or an air diffusion ball having air diffusion holes (not shown).
The air amount adjusting means 27 may be, for example, a gate valve, a butterfly valve, or the like.

(Air lift pump)
The air lift pump 30 comprises an air supply means (not shown); a water lifting pipe 31 whose lower end serves as a suction port that opens downward; an air supply pipe 32 whose upper end is connected to the air supply means and whose lower end is connected near the lower end of the water lifting pipe 31; and a discharge pipe 33 extending laterally from near the upper end of the water lifting pipe 31.

(Carrier)
The carrier 200 is for supporting microorganisms for biological nitrification of ammonia nitrogen contained in the water to be treated, i.e., nitrifying bacteria. By filling the first nitrification tank 10 and the second nitrification tank 20 with the carrier 200, the nitrifying bacteria in the first nitrification tank 10 and the second nitrification tank 20 are supported by the carrier 200 and grow therein, thereby increasing the concentration of nitrifying bacteria.

As the carrier, a porous carrier is preferred because it can support nitrifying bacteria on the surface and inside of the carrier, i.e., it can support a large number of nitrifying bacteria and improve nitrification efficiency. Among them, a sponge carrier is more preferred because it can maintain the support of nitrifying bacteria well, can increase nitrification efficiency, and can minimize damage to pumps and piping.
Examples of materials for the sponge carrier include polyvinyl alcohol, polystyrene rubber, and polyurethane.

Examples of the shape of the carrier include a square shape (e.g., a rectangular parallelepiped, a cube, etc.), a sphere, a cylinder, a filament, etc. Among these, a square shape is preferred from the viewpoint of ease of molding, and a cube is particularly preferred.
The short side of the rectangular carrier is preferably 3 to 10 mm, more preferably 3 to 7 mm, even more preferably 3 to 6 mm, and particularly preferably 3 to 5 mm. If the short side is equal to or greater than the above-mentioned lower limit, the nitrifying bacteria can be sufficiently supported. If the short side is equal to or less than the above-mentioned upper limit, the carrier is easily flowed in the first nitrification tank 10 and the second nitrification tank 20. In addition, the carrier is easily separated in the solid-liquid separator 12 and the solid-liquid separator 22.
The long side of the rectangular carrier is not particularly limited, but is preferably 3 to 10 mm, more preferably 3 to 7 mm, further preferably 3 to 6 mm, and particularly preferably 3 to 5 mm.

The nitrifying bacteria include known nitrifying bacteria used in biological nitrification of ammonia nitrogen. Nitrosobacter is an autotroph, and basically uses carbon dioxide as the only carbon source. It does not require an organic substrate and can grow in the presence of ammonia, but its growth rate is extremely slow. Therefore, in order to maintain a high biological nitrification reaction, it is necessary to perform an operation to hold a large amount of nitrifying bacteria in the nitrification tank. Therefore, it is preferable to hold the nitrifying bacteria in a state supported on a carrier rather than as suspended cells.
An example of a method for supporting nitrifying bacteria on a carrier is to introduce the carrier into a nitrification tank of an existing water treatment system and grow the nitrifying bacteria on the surface of the carrier.

<Water treatment method>
The water treatment method of the present invention is a method in which water to be treated is subjected to biological nitrification in a nitrification tank filled with a carrier.
The water to be treated often contains ammoniacal nitrogen, and the water treatment method according to this embodiment is particularly suitable for subjecting the water to be treated containing ammoniacal nitrogen to biological nitrification treatment.
The water to be treated is not particularly limited, and examples thereof include groundwater, river water, lake water, and the like, with groundwater being preferred. Examples of groundwater include well water, hot spring water, spring water, mineral water, and mineral spring water.

When the carrier carrying nitrifying bacteria (nitrifying bacteria-supporting carrier) is extracted from the nitrification tank, the following steps (i) to (iii) are carried out. That is, the water treatment method of the present invention includes the following steps (i) to (iii).
Step (i): A step of acclimating the carrier packed in the nitrification tank so that the bulk volume is more than 30 volume % and less than 65 volume % of the effective volume of the nitrification tank, to obtain a carrier supporting nitrifying bacteria.
Step (ii): A step of removing the nitrifying bacteria-supporting carriers from the nitrification tank while maintaining the bulk volume of the nitrifying bacteria-supporting carriers at 30 volume % or more relative to the effective volume of the nitrification tank.
Step (iii): A step of using the nitrifying bacteria-supporting carrier extracted in the step (ii) in another biological nitrification treatment.

Hereinafter, there will be described an example of a water treatment method using the water treatment system 1. In the water treatment method of the present embodiment, the nitrifying bacteria-supporting carrier is extracted from the first nitrification tank 10.
A transfer pump provided in the middle of the treated water supply flow path 100 is driven to supply the treated water from a raw water tank (not shown) through the treated water supply flow path 100 to the first nitrification tank 10, and the treated water is stored in the first nitrification tank 10. Furthermore, the first treated water discharged from the first nitrification tank 10 is transferred through the first treated water transfer flow path 101 to the second nitrification tank 20, and the first treated water is stored in the second nitrification tank 20.

The carrier 200 is packed in the first nitrification tank 10 and the second nitrification tank 20 of the water treatment system 1 .
The filling rate of the carrier 200 in the first nitrification tank 10 is an amount such that the bulk volume of the carrier 200 is more than 30 volume % and less than 65 volume % of the effective volume of the first nitrification tank 10, preferably 35 to 60 volume %, and more preferably 40 to 60 volume %.
In the first nitrification tank 10, if the bulk volume of the carrier 200 is equal to or greater than the lower limit, the nitrification efficiency can be maintained at a good level even after the nitrifying bacteria-supporting carrier is removed from the first nitrification tank 10. The carrier 200 also has excellent fluidity in the first nitrification tank 10. In the first nitrification tank 10, if the bulk volume of the carrier 200 is equal to or less than the upper limit, the carrier 200 can easily rotate in the first nitrification tank 10, the nitrifying bacteria can be sufficiently contacted with the water to be treated, and a decrease in the nitrification reaction rate can be suppressed. In addition, an increase in the energy consumption when the carrier 200 is fluidized in the first nitrification tank 10 can be suppressed.

The filling rate of the carrier 200 in the second nitrification tank 20 is not particularly limited, but is preferably an amount such that the bulk volume of the carrier 200 is 20 volume % or more and less than 65 volume % relative to the effective volume of the second nitrification tank 20, more preferably an amount such that the bulk volume of the carrier 200 is more than 30 volume % and less than 65 volume %, even more preferably 35 to 60 volume %, and particularly preferably 40 to 60 volume %.
In the second nitrification tank 20, if the bulk volume of the carrier 200 is equal to or larger than the lower limit, the first treated water can be sufficiently biologically nitrified. In the second nitrification tank 20, if the bulk volume of the carrier 200 is equal to or smaller than the upper limit, the carrier 200 can easily rotate in the second nitrification tank 20, the nitrifying bacteria can be sufficiently contacted with the first treated water, and a decrease in the nitrification reaction rate can be suppressed. In addition, an increase in energy consumption when the carrier 200 is caused to flow in the second nitrification tank 20 can be suppressed.
In the present invention, the "effective volume of the nitrification tank" refers to the volume of the liquid (the water to be treated or the first treated water) contained in the nitrification tank.
In the present invention, the "bulk volume" refers to the apparent volume including voids.

While a preset amount of water to be treated is supplied to the first nitrification tank 10, the air diffuser 13 is driven to diffuse a preset amount of air from the air diffuser 14 into the first nitrification tank 10.
When air is diffused from the bottom of first nitrification tank 10, a swirling flow consisting of an upward current and a downward current is generated in first nitrification tank 10, and carriers 200 flow freely in the water to be treated in first nitrification tank 10.
The flow velocity of the upward flow in the swirling flow is preferably 0.1 to 0.5 m/s.
The amount of air diffused from the air diffusion section 14 of the air diffusion device 13 can be adjusted to a desired amount by the air amount adjustment means 17 .

The first treated water discharged from the first nitrification tank 10 through the solid-liquid separation device 12 is transferred to the second nitrification tank 20 through the first treated water transfer flow path 101, while the aeration device 23 is driven to diffuse a preset amount of air from the aeration section 24 into the second nitrification tank 20.
When air is diffused from the bottom of the second nitrification tank 20, a swirling current consisting of an upward current and a downward current is generated in the second nitrification tank 20, and the carriers 200 flow freely in the first treated water in the second nitrification tank 20.
The flow velocity of the upward flow in the swirling flow is preferably 0.1 to 0.5 m/s.
The amount of air diffused from the air diffusion section 24 of the air diffusion device 23 can be adjusted to a desired amount by the air amount adjustment means 27 .

In the first nitrification tank 10, air containing oxygen is supplied from the aeration section 14 of the aeration device 13. When oxygen is supplied into the first nitrification tank 10, the ammoniacal nitrogen in the water to be treated is oxidized to ammonia (nitrified) by nitrifying bacteria in the first nitrification tank 10 to become nitric acid. In this manner, the water to be treated containing ammoniacal nitrogen is treated in the first nitrification tank 10 to produce first treated water.
In the second nitrification tank 20, air containing oxygen is supplied from the aeration section 24 of the aeration device 23. When oxygen is supplied into the second nitrification tank 20, the ammoniacal nitrogen remaining in the first treated water is oxidized to ammonia (nitrified) by nitrifying bacteria in the second nitrification tank 20 to become nitric acid. In this manner, the first treated water containing ammoniacal nitrogen is treated in the second nitrification tank 20 to produce the second treated water.
Ammonia oxidation is represented by the following formula:
NH ₄ ⁺ + 2O ₂ + HCO ₃ ^- → HNO ₃ + H ₂ CO ₃ + H ₂ O

In addition, by continuously supplying the water to be treated to the first nitrification tank 10, the nitrifying bacteria are supported on the carriers and grow therein. That is, the carriers 200 are acclimated in the first nitrification tank 10 and become carriers supporting nitrifying bacteria (step (i)). The carriers 200 are also acclimated in the second nitrification tank 20 and become carriers supporting nitrifying bacteria.
In the step (i), the carrier is preferably acclimatized for 4 days or more in the first nitrification tank 10. If the acclimatization period is 4 days or more, the nitrifying bacteria can grow sufficiently.
The nitrifying bacteria are supported at least on the surface of the carrier 200. When the carrier 200 is porous, the nitrifying bacteria may be supported inside the carrier 200 as well.

After the carrier 200 is acclimatized in the first nitrification tank 10, when it becomes necessary to remove the acclimatized carrier 200, i.e., the nitrifying bacteria-supporting carrier, the nitrifying bacteria-supporting carrier is removed from the first nitrification tank 10. Specifically, the air lift pump 30 is driven manually by an operator or automatically by a control device (not shown), and the nitrifying bacteria-supporting carrier in the first nitrification tank 10 is removed from the first nitrification tank 10 while maintaining the bulk volume of the nitrifying bacteria-supporting carrier relative to the effective volume of the first nitrification tank 10 at 30 volume% or more (step (ii)). When removing the nitrifying bacteria-supporting carrier from the first nitrification tank 10, the supply of the water to be treated to the first nitrification tank 10 may be continued or stopped.
When the nitrifying bacteria-supporting carriers are removed from the first nitrification tank 10, the bulk volume of the nitrifying bacteria-supporting carriers is kept at 30 volume % or more relative to the effective volume of the first nitrification tank 10, thereby preventing a decrease in nitrification efficiency after the nitrifying bacteria-supporting carriers are removed.
In addition, since the carrier 200 is much larger than the size of the nitrifying bacteria, the bulk volume is almost the same even if the nitrifying bacteria are supported on the carrier 200. In other words, the bulk volume of the carrier 200 and the bulk volume of the nitrifying bacteria-supporting carrier are almost the same.

In the air lift pump 30, air supplied from the air supply means is supplied to the lift pipe 31 from the vicinity of the lower end of the lift pipe 31 through the air supply pipe 32, and rises in the lift pipe 31. At this time, the first treated water containing the nitrifying bacteria carrier is sucked up from the suction port at the lower end of the lift pipe 31, and rises while being mixed with the air rising in the lift pipe 31. The mixture of the first treated water containing the nitrifying bacteria carrier and the air that has risen in the lift pipe 31 is discharged from the discharge pipe 33 extending laterally from the vicinity of the upper end of the lift pipe 31. In this way, the nitrifying bacteria carrier is extracted from the first nitrification tank 10. After a preset amount of the nitrifying bacteria carrier is extracted, the air lift pump 30 is stopped, that is, the supply of air from the air supply means is stopped, thereby stopping the extraction of the nitrifying bacteria carrier from the first nitrification tank 10.
In the case where the supply of the water to be treated to first nitrification tank 10 was stopped when the nitrifying bacteria-supporting carriers were removed from first nitrification tank 10, the supply is resumed.

The extracted nitrifying bacteria-supporting carriers are used in another biological nitrification treatment (step (iii)).
The second treated water discharged from the second nitrification tank 20 through the solid-liquid separator 22 is discharged to the outside of the water treatment system 1 through the second treated water transfer flow path 102 .

<Action and effect>
The water treatment method of the present invention explained above includes the above-mentioned steps (i) to (iii), and therefore after the nitrifying bacteria-supporting carriers are removed from the nitrification tank, the nitrifying bacteria-supporting carriers have a bulk volume of 30% or more of the effective volume of the nitrification tank, thereby making it possible to suppress a decrease in the nitrification efficiency in the nitrification tank after the nitrifying bacteria-supporting carriers are removed from the nitrification tank.
Moreover, in the water treatment method of the present invention, since the nitrifying bacteria-supporting carriers having a bulk volume of 30% or more remain in the nitrification tank after the nitrifying bacteria-supporting carriers are removed, there is no need to perform an ion exchange treatment after the biological nitrification treatment. Furthermore, in order to continue the biological nitrification treatment after the nitrifying bacteria-supporting carriers are removed, there is no need to fill the nitrification tank after the nitrifying bacteria-supporting carriers have been removed with carriers that do not support nitrifying bacteria. Therefore, according to the water treatment method of the present invention, the nitrifying bacteria-supporting carriers can be removed from the nitrification tank easily and at low cost while sufficiently suppressing a decrease in nitrification efficiency.
However, when the nitrifying bacteria-supporting carriers are to be extracted again from the nitrification tank after the nitrifying bacteria-supporting carriers have been extracted, it is preferable to fill the nitrification tank after the nitrifying bacteria-supporting carriers have been extracted with carriers not carrying nitrifying bacteria in an amount substantially equal to the amount of the nitrifying bacteria-supporting carriers extracted from the nitrification tank, and after acclimation, remove the nitrifying bacteria-supporting carriers. When the nitrifying bacteria-supporting carriers are filled in the nitrification tank, the amount is set so that the total bulk volume of the carriers and the nitrifying bacteria-supporting carriers is more than 30 volume % and less than 65 volume % of the effective volume of the nitrification tank.

<Other embodiments>
The water treatment method of the present invention is not limited to the above-described embodiment.
Although the illustrated water treatment system 1 has two nitrification tanks, the number of nitrification tanks is not limited to two and may be one, or three or more.
In the illustrated water treatment system 1, the nitrifying bacteria-supporting carrier is extracted only from the first nitrification tank 10, but the nitrifying bacteria-supporting carrier may be extracted only from the second nitrification tank 20, or may be extracted from both the first nitrification tank 10 and the second nitrification tank 20. When the nitrifying bacteria-supporting carrier is extracted from the second nitrification tank 20, the packing rate of the carrier 200 in the second nitrification tank 20 is set to an amount such that the bulk volume of the carrier 200 is more than 30 volume % and less than 65 volume % with respect to the effective volume of the second nitrification tank 20. The nitrifying bacteria-supporting carrier is extracted from the second nitrification tank 20 while remaining therein so that the bulk volume of the nitrifying bacteria-supporting carrier with respect to the effective volume of the second nitrification tank 20 is maintained at 30 volume % or more. Furthermore, it is preferable to extract the nitrifying bacteria-supporting carriers from the second nitrification tank 20 after the carriers 200 have been acclimatized for at least four days in the second nitrification tank 20, and it is preferable to use an air lift pump for this purpose.
When the water treatment system has three or more nitrification tanks, the nitrifying bacteria-supporting carriers may be extracted from any of the nitrification tanks, but the above-mentioned steps (i) to (iii) are carried out in the nitrification tank from which the nitrifying bacteria-supporting carriers are extracted.

The air diffuser is not limited to the one shown in the figure, and any known air diffuser may be used.
The pump used to extract the nitrifying bacteria-supporting carriers from the nitrification tank is not limited to an air lift pump. However, an air lift pump is preferred because it is highly energy efficient, can simplify the equipment, and is less likely to crush the nitrifying bacteria-supporting carriers.

In the illustrated water treatment system 1, a water flow rate measuring device (not shown) may be provided in the treated water supply flow path 100 to measure the amount of treated water supplied to the first nitrification tank 10. Examples of the water flow rate measuring device include a rotameter and an electromagnetic flow meter.
Furthermore, a concentration measuring device (not shown) for measuring the ammonia concentration or ammoniacal nitrogen concentration of the treated water may be provided in the first treated water transfer flow path 101 and the second treated water transfer flow path 102. An example of the concentration measuring device 50 is an ion selective sensor. The concentration measuring device may be directly immersed in the first nitrification tank 10 and the second nitrification tank 20.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these.

[Example 1]
A water treatment system similar to the water treatment system 1 shown in FIG. 1 was prepared.
The tank body of the nitrification tank was a completely mixed type transparent acrylic tank. The tank was a one-sided swirling flow square tank with a width of 120 mm, a depth of 60 mm, a height of 270 mm, and an effective volume of 1.75 L. An air diffuser was provided at the bottom of the tank.

Ammonium chloride and sodium bicarbonate were added to the groundwater to prepare treated water with an ammoniacal nitrogen concentration of 4.4 mg/L and an alkalinity of 70 mg/L.
Water to be treated at a temperature of 18° C. was supplied to the first nitrification tank and stored in the first nitrification tank.

As the carrier, a foamed polyurethane sponge carrier (Technoform Japan, Waterflex, 5 mm square, product number: AQ-14) was packed into the first nitrification tank and the second nitrification tank. The loading rate of the carrier in the first nitrification tank was an amount such that the bulk volume of the carrier was 60% by volume of the effective volume of the first nitrification tank. The loading rate of the carrier in the second nitrification tank was an amount such that the bulk volume of the carrier was 30% by volume of the effective volume of the second nitrification tank.

While the water to be treated was being supplied to the first nitrification tank, air was diffused into the first nitrification tank from the air diffuser ball. Also, while the first treated water discharged from the first nitrification tank was being transferred to the second nitrification tank, air was diffused into the second nitrification tank from the air diffuser ball. The water to be treated was supplied so that the water residence time (HRT) was 0.43 hours. The ammoniacal nitrogen load was 0.25 kg-N/m ³ /d. Excessive air was diffused so that the dissolved oxygen in each nitrification tank was saturated.
The water to be treated was continuously supplied to the first nitrification tank to be subjected to biological nitrification treatment, while the carrier was acclimated in the first nitrification tank and the second nitrification tank to support nitrifying bacteria on the carrier, thereby forming a nitrifying bacteria-supporting carrier.

After four days had passed since the start of the supply of the water to be treated to the first nitrification tank (i.e., the acclimation period was four days), the supply of the water to be treated to the first nitrification tank was stopped, and the air lift pump was then operated to remove the nitrifying bacteria-supporting carriers from the first nitrification tank 10 while leaving the bulk volume of the nitrifying bacteria-supporting carriers in the first nitrification tank at 30% by volume relative to the effective volume of the first nitrification tank 10. The amount of nitrifying bacteria-supporting carriers removed was equivalent to approximately half of the amount of carriers filled in the first nitrification tank.

Next, the air lift pump was stopped, and the supply of the water to be treated to the first nitrification tank was resumed.
An ion-selective sensor (ISEmax CAS40D, manufactured by Endress+Hauser Japan) was placed in the raw water tank (not shown), the first nitrification tank, and the second nitrification tank, respectively, and the ammonia nitrogen concentrations of the water to be treated, the first treated water, and the second treated water were measured from immediately after restart until the seventh day. The changes in the ammonia nitrogen concentrations of the water to be treated, the first treated water, and the second treated water are shown in FIG. 2.
As a result, the ammonia nitrogen concentration of the first treated water was about 0.8 mg/L immediately after the restart, and the ammonia nitrogen concentration of the second treated water was 0 mg/L immediately after the restart, and the decrease in nitrification efficiency was sufficiently suppressed.

[Comparative Example 1]
The same water treatment system as in Example 1 was used.
Water to be treated, prepared in the same manner as in Example 1 and having a temperature of 18° C., was supplied to the first nitrification tank and stored in the first nitrification tank.
The nitrifying bacteria-supporting carriers extracted from the nitrification tank of the existing water treatment system were packed into the first nitrification tank and the second nitrification tank. The packing rate of the nitrifying bacteria-supporting carriers was set so that the bulk volume of the nitrifying bacteria-supporting carriers was 30% by volume relative to the effective volume of each nitrification tank.

While the water to be treated was being supplied to the first nitrification tank, air was diffused into the first nitrification tank from the air diffuser ball. Also, while the first treated water discharged from the first nitrification tank was being transferred to the second nitrification tank, air was diffused into the second nitrification tank from the air diffuser ball. The water to be treated was supplied so that the water residence time (HRT) was 0.43 hours. The ammoniacal nitrogen load was 0.25 kg-N/m ³ /d. Excessive air was diffused so that the dissolved oxygen in each nitrification tank was saturated.

An ion-selective sensor (ISEmax CAS40D, manufactured by Endress+Hauser Japan) was placed in the second nitrification tank to measure the ammonia nitrogen concentration of the second treated water. After about one month, the condition stabilized, i.e., the ammonia nitrogen concentration of the second treated water in the second nitrification tank was 0.1 mg/L or less for seven consecutive days.

After the supply of the water to be treated to the first nitrification tank was stopped, the air lift pump was operated and the nitrifying bacteria-supporting carriers in the first nitrification tank were removed from the first nitrification tank 10 while leaving the bulk volume of the nitrifying bacteria-supporting carriers at 15% by volume relative to the effective volume of the first nitrification tank 10. The amount of nitrifying bacteria-supporting carriers removed was equivalent to approximately half of the amount of nitrifying bacteria-supporting carriers filled in the first nitrification tank.

Next, the air lift pump was stopped, and a new polyurethane foam sponge carrier (Technoform Japan, Waterflex, 5 mm square, product number: AQ-14) in an amount equal to the amount of the nitrifying bacteria-supporting carrier removed was added to the first nitrification tank 10, and the supply of the water to be treated was resumed.
An ion-selective sensor (ISEmax CAS40D, manufactured by Endress+Hauser Japan) was placed in the raw water tank (not shown), the first nitrification tank, and the second nitrification tank, respectively, and the ammonia nitrogen concentrations of the water to be treated, the first treated water, and the second treated water were measured from immediately after restarting until the seventh day. The changes in the ammonia nitrogen concentrations of the water to be treated, the first treated water, and the second treated water are shown in FIG.
As a result, the ammonia nitrogen concentration of the first treated water finally reached about 1 mg/L on the sixth day after the restart. In particular, the ammonia nitrogen concentration of the first treated water was high from the first day after the restart. In addition, the ammonia nitrogen concentration of the second treated water immediately after the restart was about 1.8 mg/L, and it was not until the second day that the ammonia nitrogen concentration of the second treated water finally reached about 0.1 mg/L. It took six days from the restart for the ammonia nitrogen concentration of the second treated water to reach 0 mg/L. Thus, the decrease in nitrification efficiency after the removal of the nitrifying bacteria-supporting carrier could not be sufficiently suppressed.

The water treatment method of the present invention is useful as a water treatment method that reduces ammoniacal nitrogen by treating water containing a large amount of ammoniacal nitrogen, particularly groundwater, through biological nitrification.

LIST OF SYMBOLS 1 Water treatment system 10 First nitrification tank 11 Tank body 12 Solid-liquid separation device 13 Aeration device 14 Aeration section 15 Air supply pipe 16 Blower 17 Air amount adjustment means 20 Second nitrification tank 21 Tank body 22 Solid-liquid separation device 23 Aeration device 24 Aeration section 25 Air supply pipe 26 Blower 27 Air amount adjustment means 30 Air lift pump 31 Lifting pipe 32 Air supply pipe 33 Discharge pipe 100 Treated water supply flow path 101 First treated water transfer flow path 102 Second treated water transfer flow path 200 Carrier

Claims (5)

A water treatment method for subjecting water to biological nitrification treatment in a nitrification tank filled with a carrier carrying nitrifying bacteria, comprising:
A water treatment method comprising the following steps (i) to (iii):
Step (i): A step of acclimating the carrier filled in the nitrification tank so that the bulk volume is 60 volume % or more and less than 65 volume % of the effective volume of the nitrification tank, to obtain a carrier for supporting nitrifying bacteria.
Step (ii): A step of extracting the nitrifying bacteria-supporting carrier from the nitrification tank while maintaining the bulk volume of the nitrifying bacteria-supporting carrier at 30 volume % or more relative to the effective volume of the nitrification tank. Step (iii): A step of using the nitrifying bacteria-supporting carrier extracted in the step (ii) in another biological nitrification treatment. The water treatment method according to claim 1, wherein in step (ii), the nitrifying bacteria carrier and the treated water treated in the nitrification tank are extracted from the nitrification tank using an air lift pump. The water treatment method according to claim 1 or 2, wherein the carrier is a sponge carrier. The water treatment method according to any one of claims 1 to 3, wherein the carrier has a rectangular shape with a short side of 3 to 10 mm. The water treatment method according to any one of claims 1 to 4, wherein in step (i), the carrier is acclimatized in the nitrification tank for 4 days or more.

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