JP7767178B2 - Water treatment method and water treatment device - Google Patents

Description

The present invention relates to a water treatment method and a water treatment device, and in particular to a water treatment method and a water treatment device suitable for use in treating septic tank sludge and human waste sludge and discharging the treated water into a sewer system.

In order to discharge treated water obtained by treating raw water containing septic tank sludge and human waste sludge into the sewer system, it is necessary for the water to meet sewage discharge standards, which are generally known to be more lenient than standards for discharge into public water bodies. For example, standards for discharge into public water bodies are BOD (biochemical oxygen demand) 10 mg/L, T-N (total nitrogen) 10 mg/L, and SS (suspended solids) 10 mg/L, while sewage discharge standards are BOD 600 mg/L, T-N 240 mg/L, and SS 600 mg/L.

A conventional treatment method involves removing the waste (sewage residue) contained in sewage, diluting it to the exclusion standard, and then discharging it. In this case, the dilution rate is generally around 10 to 20 times, resulting in excessive amounts of dilution water and discharge into the sewer system.

Another treatment method is to separate the sewage and other waste into solids and liquids using a dehydrator, then dilute the separated liquid before discharging it into the sewer. In this case, the separated liquid contains significantly less BOD, SS, nitrogen, and other components than sludge-removed sewage, so the dilution ratio can generally be around 4 to 8 times. However, because the quality of the separated liquid varies, the amount of water used for dilution also varies greatly depending on the water.

In addition, the method of separating sewage and other waste into solids and liquids using a dehydrator ultimately results in a discharge of 6 to 9 times the amount brought in, so its effectiveness in reducing the amount discharged into the sewer system is limited. Furthermore, because soluble components are difficult to remove using solid-liquid separation, if the sewage and other waste contains a large amount of soluble components, the quality of the separated liquid from the dehydrated waste may deteriorate, making it necessary to increase the amount of dilution water. In some cases, regulations on the amount of discharged water may make it impossible to meet discharge standards.

Another method for more reliably reducing the amount of dilution water and discharged water is to combine solid-liquid separation with biological treatment. For example, Japanese Patent Application Laid-Open No. 61-50691 (Patent Document 1) describes a method in which the solids obtained by solid-liquid separation of septic tank sludge are mixed with human wastewater, the mixed liquid is subjected to coagulation treatment, and the separated liquid is then subjected to biological treatment.

Japanese Patent Application Publication No. 61-50691

The method described in Patent Document 1 does not state where the biologically treated water is discharged, but since it states that the BOD of the treated water in Example 1 can be reduced to 10 mg/L or less, it can be said that this treatment method is intended for discharge into public water bodies.

However, treated water discharged into sewer systems does not necessarily have to have the same quality as that required for discharge into public waters, as mentioned in Patent Document 1. Meanwhile, in water treatment that combines solid-liquid separation and biological treatment, as described in Patent Document 1, there is a problem in that it is difficult to achieve partial treatment that meets sewage rejection standards. For example, when performing nitrification-denitrification as biological treatment, possible methods include treating only about 60% of the nitrogen rather than the entire amount, or nitrifying all of the ammoniacal nitrogen contained in the dehydrated separated liquid and then denitrifying only 60% of that.

However, when treating about 60% of the nitrogen, about 40% of the nitrate nitrogen remains, which can cause denitrification to occur again in the downstream settling tank when the system becomes anaerobic. The nitrogen gas generated can cause the sludge to float, preventing sufficient solid-liquid separation in the settling tank. If solid-liquid separation is not possible in the settling tank, the MLSS (activated sludge concentration) in the nitrification/denitrification tank cannot be maintained, and the treatment itself will deteriorate.

If all of the ammonia nitrogen contained in the dehydrated separated liquid is nitrified, the tank capacity will be excessive, the aeration air volume required for nitrification will be excessive, and hydrogen donors such as methanol or ethanol, which are necessary for denitrification, will need to be added, resulting in excessive equipment and operating costs compared to the required treated water quality.

Another method for efficiently treating septic tank sludge and sewage sludge is to use biological treatment using the activated sludge method, which involves roughly treating the treated water until the water quality falls below the sewage discharge standards, then diluting the water and discharging it into the sewer. However, biological treatment using the activated sludge method requires a large tank capacity, and the aeration air volume required to deal with the high BOD load is also excessive, so this method cannot be said to be efficient in terms of treatment efficiency.

In consideration of the above-mentioned problems, the present invention provides a water treatment method and water treatment device that can efficiently and stably produce treated water of a quality that meets sewage discharge standards from treated water containing septic tank sludge or human waste sludge using simpler equipment.

As a result of extensive research conducted by the inventors to solve the above problems, they discovered that it would be useful to carry out biological treatment in a single treatment tank that alternates between nitrification promotion treatment, which promotes nitrification, and denitrification promotion treatment, which promotes denitrification, and to discharge and dilute the biologically treated water from the treatment tank at predetermined times.

In one aspect, a water treatment method according to an embodiment of the present invention, which was developed based on the above findings, includes: supplying a separated liquid obtained by solid-liquid separation of sludge containing at least one of septic tank sludge and sewage sludge into a treatment tank containing a biological carrier; setting at least two aeration conditions in the treatment tank, namely, aeration conditions for nitrification-promoting treatment and aeration conditions for denitrification-promoting treatment with a lower aeration rate than the aeration conditions for nitrification-promoting treatment, so that a nitrification-promoting treatment that promotes nitrification and a denitrification-promoting treatment that promotes denitrification are carried out in the treatment tank; switching the aeration conditions in the treatment tank so that the nitrification-promoting treatment and the denitrification-promoting treatment alternate within the treatment tank; and discharging the biologically treated water from the treatment tank after the nitrification-promoting treatment is completed and before the denitrification-promoting treatment begins, and diluting the biologically treated water to meet sewage discharge standards.

In one embodiment of the water treatment method according to the present invention, the biologically treated water is discharged from the middle layer region in the vertical direction within the treatment tank to the outside of the treatment tank.

In another embodiment of the water treatment method according to the present invention, after the nitrification promotion treatment is completed, the aeration treatment is stopped, the treatment tank is allowed to stand, the biologically treated water is discharged from the treatment tank, and then the separated liquid is supplied into the treatment tank, after which the denitrification promotion treatment is started.

In yet another embodiment of the water treatment method according to the present invention, the biological carriers are contained in the treatment tank at a volumetric concentration of 40% or less.

In one aspect, a water treatment device according to an embodiment of the present invention includes a solid-liquid separator that separates sludge, including at least one of septic tank sludge and sewage sludge, into separated sludge and separated liquid by solid-liquid separation; a treatment tank that houses a biofilm that biologically treats the separated liquid and subjects the separated liquid to a nitrification-promoting treatment that promotes nitrification and a denitrification-promoting treatment that promotes denitrification, thereby obtaining biologically treated water; control means that sets and controls the aeration conditions of the treatment tank by setting at least two aeration conditions, namely aeration conditions for the nitrification-promoting treatment and aeration conditions for the denitrification-promoting treatment, so that the nitrification-promoting treatment and the denitrification-promoting treatment are alternately performed in the treatment tank; overflow means that allows the biologically treated water to overflow from the treatment tank; discharge means that discharges the biologically treated water from the treatment tank through a middle layer region in the vertical direction of the treatment tank; and a dilution tank that dilutes the biologically treated water to meet sewage discharge standards.

The present invention provides a water treatment method and water treatment device that can efficiently and stably produce treated water of a quality that meets sewage discharge standards from treated water containing septic tank sludge or human waste sludge using simpler equipment.

1 is a schematic diagram illustrating a water treatment device according to an embodiment of the present invention. FIG. 1 is a schematic diagram showing a process flow in a laboratory test.

Embodiments of the present invention will be described below with reference to the drawings. The embodiments shown below are examples of devices and methods that embody the technical concept of this invention, and the technical concept of this invention does not limit the structure, arrangement, etc. of component parts to those described below.

<Water treatment method>
A water treatment method according to an embodiment of the present invention includes performing solid-liquid separation on sludge containing at least one of septic tank sludge and sewage sludge, subjecting the separated liquid obtained by the solid-liquid separation to biological treatment, and diluting the biologically treated water obtained by the biological treatment so that it meets sewage removal standards.

The raw water can be a separated liquid obtained by solid-liquid separation of sludge containing at least one of septic tank sludge and human waste sludge. Typically, the separated liquid obtained by dehydration of septic tank sludge, human waste sludge, or a mixture of these can be suitably used. When mixing septic tank sludge and human waste sludge, the mixed liquid obtained by mixing the septic tank sludge and human waste sludge can be subjected to solid-liquid separation, but it is preferable to perform solid-liquid separation on the septic tank sludge and human waste sludge separately and then mix the separated liquids obtained by solid-liquid separation.

A variety of solid-liquid separation devices can be used for solid-liquid separation treatment. Among these, using a dehydrator to separate raw water into separated sludge and separated liquid is preferred in terms of equipment and operating costs. It is also preferable to subject the raw water to a concentration treatment before solid-liquid separation. Both gravity concentration and mechanical concentration are effective concentration methods.

By performing a concentration process with the addition of a polymer flocculant before solid-liquid separation, the sludge concentration (TS) of the concentrated sludge can be increased to a maximum of approximately 10-12% by mass, allowing for a more compact treatment facility. Further dehydration of the highly concentrated sludge using a dehydrator produces dehydrated sludge (separated sludge) with a moisture content of 70% or less, resulting in a more significant sludge volume reduction effect. Because this dehydrated sludge with a low moisture content has a high calorie content, it can be self-burned during incineration without supplemental fuel, resulting in energy savings and lower costs.

Typical raw water qualities include, but are not limited to, a BOD of 600-2000 mg/L and a TN of 240-1000 mg/L, more typically 240-700 mg/L. The quality of the biologically treated water before dilution obtained by the biological treatment described below is a BOD of 200-1500 mg/L and a TN of 240-600 mg/L.

The separated liquid is supplied to a treatment tank containing a biofilm, which is made up of microorganisms attached to the surface of a carrier, where biological treatment using the biofilm method takes place. Biofilm methods according to this embodiment can be broadly divided into those that require a periodic cleaning process for the carrier, and those in which the amount of biofilm is autonomously controlled during treatment. The former includes biofilm filtration, while the latter includes trickling filters, fluidized bed methods, rotating disk methods, and fixed bed methods (contact oxidation). Of these, it is preferable to use a biofilm method in which the amount of biofilm is autonomously controlled during treatment, which is defined herein as a "non-blocking biofilm method."

In particular, in this embodiment, an aerated biofilm process is preferably used, which performs biological treatment in an aeration treatment tank (hereinafter also referred to as the "treatment tank") containing biological carriers, including a contact oxidation process using fixed-bed carriers (fixed-bed process) or a fluidized-bed process using fluidized-bed carriers. The "aerated biofilm process" refers to biological treatment in which the biological carriers contained in the treatment tank are fluidized while aeration is used to diffuse the air within the treatment tank. An example of an aerated biofilm process suitable for this embodiment is shown below.

-Catalytic oxidation method (fixed bed method)-
In the contact oxidation method, fixed-bed carriers are immersed in a liquid contained in a treatment tank, and aeration is performed while the separated liquid is passed through, thereby forming a biofilm on the surface of the carrier and decomposing the organic matter in the separated liquid to obtain biologically treated water. Because treatment is carried out using a biofilm attached to the carrier, there is no need to control the amount of sludge by returning it, as opposed to activated sludge methods, and maintenance is easier. The BOD load is preferably 0.1 to 1.0 kg-BOD/m ³ /d, and operation at a high load can cause the biofilm to swell and clog the contact material. The T-N load is preferably 0.06 to 0.3 kg-N/m ³ /d.

There are no particular restrictions on the material or specific shape of the carrier used in the contact oxidation method, and any device can be used. Materials that can be used for the carrier include polyethylene and plastic, and the carrier can be in any shape, such as a tube, string, mesh, plate, or ball.

The specific surface area of the carrier for the contact oxidation method is preferably 50 to 200 ^m / ^m , more preferably 70 to 150 ^m / ^m , from the viewpoint of adhesion of microorganisms. The porosity of the carrier is preferably 97 to 99.5%, more preferably 97.5 to 99%, from the viewpoint of achieving both prevention of clogging and the above-mentioned specific surface area.

Generally, when using contact oxidation for treatment, continuous aeration is the norm, and switching to low aeration airflow is not performed. This is because low aeration airflow weakens agitation within the tank, potentially causing the raw water to short-pass and resulting in the outflow of untreated wastewater. In contrast, the water treatment method according to an embodiment of the present invention utilizes contact oxidation as a method for roughly treating the separated liquid, which is obtained by solid-liquid separation of sludge to reduce the load on biological treatment and prepare it for discharge into a sewer system. Therefore, the desired treatment can be achieved sufficiently even if the water in the tank is insufficiently mixed during periods of low aeration airflow or no aeration. Furthermore, to further improve mixing within the tank, raw water can be introduced from the bottom of the treatment tank, agitated with an agitator, or aerated, which can promote the flow of the separated liquid contained in the treatment tank within the tank, thereby enabling even more stable treatment.

-Floating carrier method-
The fluidized carrier method is a method in which carriers are placed in a treatment tank and allowed to flow in the liquid in the treatment tank, thereby bringing microorganisms into contact with organic matter and oxygen in the liquid to be treated, thereby obtaining biologically treated water. A treatment tank using the fluidized carrier method may be newly constructed, or carriers and an aeration device may be installed in an existing storage tank. There are no particular restrictions on the carriers used in fluidized carriers, but typical examples include the following:

The carrier used can be any carrier that can accommodate microorganisms and can be fluidized by aeration. Any carrier material can be used as long as it can be fluidized by aeration, such as plastic (polyurethane (PU), polyethylene (PE), polyethylene glycol (PEG), polyvinyl alcohol (PVA)), wood chips, sand, etc. The carrier can be in a sponge-like, gel-like, solid, or other form.

Carriers can be any shape, including spherical, cubic, cylindrical, and honeycomb. By using bonded immobilized carriers, which attach microorganisms to the outer surface of the carrier, microorganisms suited to the environment in the treatment tank can be attached to the carrier, resulting in more stable biological treatment that is less susceptible to fluctuations in the properties of the influent water.

The carrier structure preferably takes into consideration the contact efficiency between the carrier and raw water in both nitrification promotion treatment, which promotes nitrification, and denitrification promotion treatment, which promotes denitrification, in the treatment tank. For example, to efficiently promote nitrification promotion treatment, it is preferable to use a carrier with a large specific surface area to increase the contact efficiency between nitrifying bacteria and oxygen. On the other hand, since denitrification treatment does not require contact with oxygen, it is preferable to have a carrier structure in which microorganisms are packed into the voids inside the carrier in order to promote denitrification promotion treatment.

Therefore, the specific surface area of the carrier suitable for nitrification promotion treatment and denitrification promotion treatment is preferably 50 to 5000 ^m2 / ^m3 , and more preferably 100 to 3000 ^m2 / ^m3 , and it is preferable that the carrier has a structure that can maintain a specific surface area of 50 ^m2 / ^m3 or more even after sludge is filled in the void spaces of the carrier. The porosity of the carrier (i.e., the volume calculated from the volume of the void spaces of the carrier divided by the external dimensions of the carrier) is preferably 50 to 99%, and more preferably 70 to 99%. The effective diameter of the carrier is preferably 3 to 10 mm, which allows stable separation by a screen for separating the carrier.

The specific gravity of the carrier is preferably 1.00 to 1.10 g/ ^cm3, which allows the carrier to flow quickly and uniformly in the treatment tank even when intermittent aeration or micro-aeration is performed in the treatment tank, and more preferably 1.01 to 1.05 g/ ^cm3 .

The carrier filling rate is preferably 10 to 50% by volume (V%), which allows the carrier to be uniformly mixed and flowable, and even more preferably 20 to 40% by volume (V%). A filling rate of 20% by volume (V%) or more can retain a large number of microorganisms within the tank, while a filling rate of 40% by volume (V%) or less, which creates an appropriate amount of void space, can maintain good fluidity.

It is also possible to pack two types of carriers into the treatment tank, one for nitrification promotion treatment and one for denitrification promotion treatment. In this case, it is preferable to pack a carrier with a relatively large specific surface area for the nitrification promotion treatment, and a carrier with a relatively high porosity and packing rate for the denitrification promotion treatment. Although not limited to the following, it is preferable to pack, for example, carriers with a specific surface area of 100 m ² /m ³ or more, preferably 300 m ² /m ³ or more, at 20 to 35% by volume of the tank volume for the nitrification promotion treatment, and carriers with a porosity of 70% or more, preferably 85% or more, at 5 to 20% by volume of the tank volume for the denitrification promotion treatment. By using such carriers, the carriers can be fluidized within the treatment tank by micro-aeration, intermittent aeration, or by supplying a separated liquid into the treatment tank, thereby allowing the denitrification treatment to proceed smoothly.

The BOD load is preferably 0.1 to 2.0 kg-BOD/m ³ /d, and more preferably 0.2 to 1.0 kg-BOD/m ³ /d. The TN load is preferably 0.03 to 0.7 kg-N/m ³ /d, and more preferably 0.06 to 0.3 kg-N/m ³ /d. The fluidized bed carrier method has a higher load and is less likely to clog than the contact oxidation method. Activated sludge may be added to the treatment tank in addition to the fluidized bed carrier. Adding activated sludge to the treatment tank can suppress fluctuations in the properties of the biologically treated water caused by fluctuations in the properties of the raw water, allowing for more stable treatment.

As with the contact oxidation process described above, continuous aeration is generally the norm when using the fluidized bed carrier method, and switching to low aeration airflow is generally not performed. This is because low aeration airflow weakens agitation within the tank, potentially causing the raw water to short-pass and resulting in the outflow of untreated wastewater. In contrast, the water treatment method according to an embodiment of the present invention utilizes the fluidized bed carrier method as a method for roughly treating the separated liquid, which is obtained by solid-liquid separation of sludge to reduce the load on biological treatment and prepare it for discharge into a sewer system. Therefore, even if insufficient mixing of the water within the tank occurs during periods of low aeration airflow or no aeration, resulting in settling of the carriers, the intended treatment can still be fully achieved. Furthermore, to further improve mixing within the tank, raw water can be introduced from the bottom of the treatment tank, agitation can be performed with an agitator, or aeration can be performed to promote the flow of the separated liquid contained in the treatment tank within the tank, thereby achieving even more stable treatment.

Furthermore, in general, in treatments using flow-through carriers, separating the nitrification tank and denitrification tank and allowing nitrifying bacteria to predominate in the nitrification tank carriers and denitrifying bacteria to predominate in the denitrification tank carriers allows for a higher treatment speed than when both nitrifying and denitrifying bacteria are attached to a single carrier, so single-tank systems are rarely used. In contrast, the water treatment method according to the embodiment of the present invention aims to roughly treat treated water containing septic tank sludge or sewage sludge so that the water quality meets sewage discharge standards. Therefore, by adopting a single-tank system that switches between the nitrification and denitrification processes, a water treatment method and water treatment device can be provided that can efficiently produce treated water using a smaller device.

In accordance with the water treatment method according to an embodiment of the present invention, by using an aerated biofilm process containing fixed-bed or fluidized bed carriers in the treatment tank, the settling tank installed downstream of the biological treatment can be omitted or downsized, thereby enabling the overall treatment equipment to be downsized. When the settling tank is omitted, there is no need to consider the problem of poor sludge settling due to some of the BOD components or T-N components remaining in the settling tank, thereby improving treatment efficiency. Even when a settling tank is installed downstream of the treatment tank, a compact settling tank is sufficient because only an auxiliary tank is required. Even if some activated sludge flows out due to poor settling, stable treatment is possible at all times because sufficient carriers are retained within the treatment tank. Furthermore, while typical batch activated sludge processes require a certain amount of settling time to settle MLSS, maintain microorganisms, and obtain clear treated water, this method uses biological carriers, which eliminates or shortens the settling time, thereby enabling a higher treatable load than treatment using activated sludge.

In the biofilm process, which performs biological treatment by placing fixed-bed or fluidized bed carriers in a treatment tank, oxygen is supplied by aeration, which also functions as agitation within the tank, to ensure the fluidity of the carriers and the contact efficiency between the raw water and the carriers. However, when nitrification and denitrification are performed in the same tank, it is generally difficult to keep the aeration volume during denitrification to a low level suitable for denitrification. In this embodiment, since it is sufficient for biological treatment to produce treated water in which some BOD components and T-N components remain, BOD removal and nitrogen removal from raw water can be easily and efficiently performed in a single tank by simply adjusting the aeration volume within the treatment tank. This allows treated water of the desired quality (for sewage discharge) to be obtained efficiently using compact equipment.

In the biological treatment of this embodiment, two or more aeration conditions are set in the treatment tank so that nitrification promotion treatment, which promotes nitrification, and denitrification promotion treatment, which promotes denitrification, are alternately performed in the treatment tank. By setting two or more aeration conditions in the treatment tank and performing nitrification promotion treatment and denitrification promotion treatment in a single treatment tank, the water treatment method of this embodiment can be applied even if only one treatment tank is available due to modifications to existing equipment, thereby reducing the financial burden of modifications and making it possible to more efficiently and stably obtain treated water suitable for discharge into a sewerage system.

The aeration conditions in the treatment tank should be set at least two conditions, one for each of the nitrification promotion treatment and the denitrification promotion treatment, i.e., one aeration condition for the nitrification promotion treatment and one aeration condition for the denitrification promotion treatment with a lower aeration volume than the aeration condition for the nitrification promotion treatment. However, it is of course possible to set three or more conditions or four or more conditions as needed.

- Nitrification promotion treatment -
In the nitrification promotion treatment, it is preferable to set the aeration conditions so that the dissolved oxygen (DO) in the treatment tank is 1.0 to 5.0 mg/L, and preferably 2.5 to 4.0 mg/L, thereby promoting the nitrification reaction that oxidizes ammonia nitrogen (NH ₄ -N) contained in the raw water to nitrate nitrogen (NO _x -N).

The aeration air volume is preferably 0.2 to 0.6 ^{m 3} /m ² /min, and more preferably 0.3 to 0.5 ^{m 3} /m ² /min, in order to ensure that the carriers flow throughout the entire tank. This ensures that the carriers flow throughout the entire tank, increasing reaction efficiency. Preferably, the aeration air volume is finely adjusted so that the set DO falls within the above range, while confirming that the carriers flow throughout the entire tank at the above aeration air volume, thereby enabling the nitrification promotion treatment according to the present invention to be carried out more efficiently.

When the fluidized carrier method is used as the biofilm method, aeration causes the carriers to flow throughout the tank during nitrification promotion treatment, increasing contact efficiency with the raw water and improving nitrification capacity. Note that since the purpose of the nitrification promotion treatment in this embodiment is to roughly remove BOD and T-N, it is not necessary to nitrify the entire amount; it is sufficient to complete nitrification to the target concentration.

According to the inventors' research, it is preferable to adjust the pH of the ammonia-nitrogen-containing wastewater in the tank so that the free ammonia concentration is maintained at 1.0 to 10 mg/L, more preferably 2.0 to 10 mg/L. This allows ammonia-oxidizing bacteria to preferentially attach to the carrier while suppressing the growth of nitritizing bacteria, resulting in stable nitritation treatment.

In nitritation treatment, NH ₄ -N is normally nitrified to NO ₃ -N, but the reaction can be stopped at NO ₂ -N, which increases the efficiency of oxygen use and reduces the amount of oxygen supplied. Furthermore, under the latter denitrification conditions, in addition to dependent denitrification using BOD and other substances in the raw water as electron donors, denitrification by anaerobic ammonium oxidation reactions involving NO ₂ -N and NH ₄ -N also progresses in part, thereby increasing the efficiency of nitrogen removal. The free ammonia concentration can be calculated according to equation (1).

As can be seen from equation (1), the free ammonia concentration is affected by changes in pH, NH ₄ —N concentration, and water temperature. Stable nitritation treatment can be carried out by measuring changes in the pH, NH ₄ —N concentration, and water temperature of the separated liquid in the treatment tank and adjusting the pH based on the measurement results so that the free ammonia concentration is 1.0 to 10 mg/L.

- Denitrification promotion treatment -
In the denitrification promotion treatment, aeration conditions are set so that the DO in the treatment tank is less than 1.0 mg/L, preferably 0.5 mg/L or less, and further, it is preferable to promote the denitrification reaction at an ORP (oxidation-reduction potential) of 0 to -300 mV, preferably -100 to -200 mV. Under denitrification conditions, aeration may be completely stopped and fluidization by mechanical agitation may be performed, or micro-aeration may be performed within a range where the DO is less than 1.0 mg/L, or a combination of mechanical agitation and micro-aeration may be used. By using such aeration conditions, the nitrification rate is slower than under nitrification conditions, but nitrification and denitrification are expected to proceed simultaneously.

When using fluidized carriers in the biological treatment of the present invention, the agitation conditions for denitrification can be achieved by creating a water flow in the tank rather than fluidizing the carriers throughout the entire tank, thereby increasing the contact efficiency between the carriers and the water being treated. Some of the fluidized carriers may remain submerged. In conventional fluidized carrier methods, it is ideal to uniformly fluidize the fluidized carriers throughout the tank to ensure proper treatment. On the other hand, the water treatment method of the present invention aims to roughly treat septic tank sludge or sewage sludge. Therefore, the progress of the target reaction is more important than the degree of fluidization of the fluidized carriers within the tank typically required in fluidized carrier methods. The progress of the reaction can be confirmed using sensors such as an ORP meter or a _NOx -N meter, as described below. To facilitate operational management and reduce running costs, it is desirable not to add electron donors such as methanol from the outside. However, depending on the target treated water quality, methanol or other suitable substances may be added.

The aeration air volume when stirring by aeration may be, for example, 0.2 m ³ /m ² /min or less, since there is no need to fluidize the biological carriers for the reasons mentioned above.

When controlling aeration air volume due to micro-aeration conditions, inverter control has a lower control limit due to the discharge pressure, and precise control may not be possible. Therefore, it is preferable to perform intermittent aeration using the aeration blower's timer, which allows for more precise adjustment of the air volume.

When using a timer to adjust the operating time ratio, setting both the on (aeration operation) time and off (aeration stop) time to less than one minute can place a strain on the blower, making stable operation over long periods of time difficult. To reduce the strain on the blower and ensure stable treatment that is favorable for denitrification, it is preferable to set the off time to five minutes or more. Aeration should be carried out for one to five minutes, preferably one to two minutes, after the raw water is supplied. This is because the above time is sufficient to agitate the tank, and prolonged aeration promotes aerobic decomposition of organic matter in the raw water, leading to a decrease in the amount of denitrification.

It is preferable to adjust the off-time so that the DO in the tank is 0.5 mg/L or less, thereby ensuring that the progress of nitrification can be stopped. For example, although this depends on the blower capacity and tank size, by setting the off-time to 5 minutes or more, and more preferably 10 minutes or more, it is possible to keep the DO in the tank at 0.5 mg/L or less and provide a period suitable for denitrification.

On the other hand, if the off time is set too long, and mechanical agitation is not performed, scale is likely to form on the aeration diffuser, carriers, etc. in the tank during the aeration stop period. The aeration stop time in nitritation treatment is 60 minutes or less, more preferably 45 minutes or less, and even more preferably 30 minutes or less. There are no particular restrictions on the on time, as long as it is 1 minute or more. During the on time, it is preferable to set the DO to 1 mg/L or more, and when using a flow carrier, it is even more preferable to set the DO to 2.5 mg/L or more. Furthermore, if the dehydrator is not operated on holidays and there is no inflow of raw water, endogenous denitrification can be promoted by extending the aeration stop time ratio under micro-aeration conditions.

In denitrification promotion treatment, it is preferable to at least one of the following methods to generate a liquid flow in the raw water contained in the treatment tank and cause the carriers to flow within the treatment tank: by introducing raw water into the treatment tank from the bottom, stirring with an agitator, or aerating the water. This increases the contact efficiency between the flowing carriers and the raw water, accelerating the nitrogen removal process.

It is preferable to supply raw water to the treatment tank intermittently, with a specified supply suspension period in place. "Intermittently" can mean, for example, a supply suspension period of several hours (e.g., 1 to 20 hours) or a supply suspension period of several days (e.g., 1 to 7 days). For example, when treating septic tank sludge and sewage sludge, depending on the operating conditions of the treatment facility, raw water inflow may occur between 9:00 and 17:00 on weekdays, with no raw water inflow occurring during weekday nights and early mornings, weekends, and holidays. By supplying raw water intermittently to the treatment tank in accordance with the operating conditions of the treatment facility, such as the raw water inflow time, it is possible to omit a storage tank for storing raw water, contributing to the miniaturization of the entire system.

Furthermore, by controlling the aeration conditions in the treatment tank to switch between aeration conditions for nitrification-promoting treatment and aeration conditions for denitrification-promoting treatment in accordance with the timing of the raw water inflow, nitrification and denitrification of raw water can be carried out more efficiently in a single treatment tank. For example, by introducing raw water under denitrification-promoting conditions, and then stopping the introduction of raw water after confirming that denitrification has been completed using a sensor such as an ORP meter for a set period of time (described below), and switching to nitrification-promoting conditions, the BOD components in the raw water can be effectively denitrified, and the amount of oxygen used for BOD oxidation can also be reduced.

In biological treatment, it is preferable to switch aeration conditions so that nitrification promotion treatment and denitrification promotion treatment are alternately performed within a single treatment tank. By switching aeration conditions and repeatedly performing nitrification promotion treatment and denitrification promotion treatment alternately, BOD and T-N in raw water can be roughly removed efficiently in a single treatment tank.

Aeration conditions can be switched manually by an operator, or automatically using a timer, sensor, etc. The following methods, for example, can be used to switch the conditions:

-Switching by time-
Since the ratio of the nitrification rate to the denitrification rate is about 1:1 to 1:5, it is possible to maintain appropriate nitrification and denitrification times by setting the time ratio of nitrification promotion treatment to denitrification promotion treatment to 1:1 to 5:1, preferably 2:1 to 4:1. When using a flow-through carrier in biological treatment, it is possible to manually or automatically set the optimal ratio of nitrification and denitrification times depending on the adhesion state of microorganisms by sampling the acclimatized carrier and determining the nitrification and denitrification rates, for example, through the following batch test.

(Nitrification batch test)
For example, 20 vol% of the carrier collected from the treatment tank is placed in a 1-L measuring cylinder, and the raw water is diluted to a total volume of 1 L. Sodium carbonate is then added so that the alkalinity (mg-CaCO ₃ /L) is 7 times the ammonia concentration (mg-N/L) of the raw water. Aeration is then started, and samples are taken at regular intervals (e.g., 0, 30, 60, 90, and 120 minutes) for water quality analysis. The nitrification rate per carrier volume (mg-N/L-carrier/h) is calculated from the change in NO _x -N concentration.

(Denitrification batch test)
For example, 20 vol% of the carrier collected from the treatment tank is placed in a 1 L sealable container, and the raw water is diluted to a total volume of 1 L. _NaNO3 is then added so that the _NO3 -N concentration is 50 to 100 mg/L. After the gas in the container is replaced with nitrogen gas, stirring is started, and samples are taken at regular intervals (for example, 0, 30, 60, 90, and 120 minutes) and water quality is analyzed. The denitrification rate per carrier volume (mg-N/L-carrier/h) is calculated from the change in _NOx -N concentration.

-Sensor-based control 1 (feedback control)-
Control methods using sensors include pH control, alkalinity control, ORP control, DO control, NH ₄ —N control, NO _x —N control, etc., and combinations thereof. Of these, ORP control and NO _x —N control are particularly suitable for the reasons described below.

(pH and alkalinity control)
In nitrification promotion treatment, alkalinity decreases and pH drops as the nitrification reaction progresses. If nitrification is performed using only the alkalinity in the raw water without adding alkali, the nitrification process will stop once the alkalinity falls below a certain value, and the decrease in pH will also stop. Therefore, by checking the change in pH or alkalinity over time, the point at which the change per hour falls below a certain value can be considered the point at which nitrification is complete, and the treatment can be switched to denitrification promotion treatment.

In denitrification promotion treatment, alkalinity increases as the denitrification reaction progresses, causing the pH to rise. Therefore, by checking the change in pH or alkalinity over time, the point at which the change per hour reaches a certain range can be considered the point at which denitrification is complete, and the treatment can be switched to nitrification promotion treatment.

(ORP control)
In nitrification promotion treatment, ORP increases with the oxidation of organic matter and the nitrification of ammonia. The slope of the change in ORP over time differs during the period when the oxidation and nitrification of organic matter are proceeding simultaneously, the period when the oxidation of organic matter is complete and only nitrification is proceeding, and after nitrification is complete. Therefore, the amount of change can be confirmed, and the point at which the amount of change per hour falls below a certain value can be considered to be the point at which nitrification is completed, and the treatment can be switched to denitrification promotion treatment.

In denitrification treatment, ORP decreases with the inflow of raw water and the denitrification of _NOx -N, and the rate of change decreases once denitrification is complete. Therefore, the point at which the rate of change per hour falls below a certain value is considered the point at which denitrification is complete, and the system can switch to nitrification treatment. Compared to pH and DO, ORP control has a wider range of possible values and higher resolution (generally -400mV to +400mV), allowing for more accurate control. Furthermore, unlike NH4 _- N or _NOx -N, ORP is an index that comprehensively evaluates the state of organic matter, nitrogen compounds, and other substances in the system. Therefore, in cases where the goal is to roughly remove BOD and T-N, as in this case, and BOD and nitrogen remain in the tank, ORP is an ideal index for evaluating the condition of the tank.

(DO control)
In the nitrification promotion treatment, if the same amount of air is blown in, after the oxidation and nitrification of the organic matter in the tank are completed, the DO will be higher than when the reaction is in progress. By checking the change in DO over time, it is possible to consider the nitrification to be complete when the DO increases, and to move on to the denitrification promotion treatment.

(NH ₄ -N control)
In the nitrification promotion treatment, the change in NH ₄ -N concentration over time increases in the following order: the period when nitrification is complete, the period when oxidation of organic matter and nitrification are proceeding simultaneously, and the period when oxidation of organic matter is completed and nitrification is proceeding. By checking this change over time and determining when the amount of change in NH ₄ -N becomes small, nitrification can be considered complete and the treatment can be switched to denitrification promotion treatment.

(NO _x -N control)
In nitrification promotion treatment, the change in NO _x -N concentration over time increases in the following order: the period when nitrification is complete, the period when oxidation of organic matter and nitrification are proceeding simultaneously, and the period when oxidation of organic matter is completed and nitrification is proceeding. By checking this change over time and determining when the change in NO _x -N becomes small, nitrification can be considered complete and the process can be switched to denitrification promotion treatment.

In the denitrification promotion treatment, the amount of change is large while denitrification is progressing, and then decreases when denitrification is complete. This change over time is confirmed, and when the amount of change in NO _x -N becomes small, denitrification is considered to be complete, and the nitrification promotion treatment can be started. Compared to DO control or NH ₄ -N control, control based on the NO _x -N concentration is preferable to this embodiment in that the completion point of the denitrification promotion treatment can be confirmed with a single sensor.

- Sensor-based control 2 (combined with feedforward) -
Since most of the nitrogen components in raw water are NH ₄ -N, the nitrogen concentration in raw water can be measured with an NH ₄ -N meter. Therefore, if a target value for the nitrogen concentration of the treated water is set, it is possible to calculate the required amount of nitrification and denitrification based on the relationship between the raw water NH ₄ -N concentration and the target treated water TN concentration. Because the hourly nitrification rate and denitrification rate can be calculated from the NH ₄ -N concentration or NO _x -N concentration in the treatment tank, the time required for nitrification and denitrification conditions to reach the target concentration can be calculated using a computing device, and the blower can be controlled accordingly. This allows the nitrification time to be kept to a minimum, making it possible to reduce the aeration air volume.

- Addition of nutrients (phosphorus) -
In the water treatment method according to the embodiment of the present invention, biological treatment is used to treat the separated liquid (raw water), and therefore it may be necessary to supply phosphorus in an amount sufficient for the growth of organisms. Generally, in biological treatment, approximately 1 mg/L of phosphorus is required for a BOD of 100 mg/L, and it is desirable to supply phosphorus so as to satisfy this ratio relative to the raw water BOD. If treatment is satisfactory, the phosphorus concentration may be reduced to 1 mg/L or less, preferably 0.7 mg/L or less, and more preferably 0.5 mg/L or less.

In particular, when dewatering septic tank sludge and sewage sludge as a solid-liquid separation process, using iron- or aluminum-based coagulants can result in phosphorus being absorbed into the sludge, reducing the phosphorus concentration in the separated liquid and making it necessary to add phosphorus. In rough treatment for BOD and T-N removal, the supply of nutrients such as phosphorus tends to be overlooked, but a phosphorus deficiency can make it almost impossible to remove BOD, so it is preferable to supply phosphorus even in rough treatment. There are no particular restrictions on the form of phosphorus to be added, but methods include adding chemicals such as phosphoric acid or potassium dihydrogen phosphate, or adding a portion of sewage sludge.

-Discharge of biologically treated water-
The biologically treated water obtained by the above biological treatment is discharged from the treatment tank. The timing of discharging the biologically treated water can be determined as appropriate. However, when raw water flows into the treatment tank and the biologically treated water in the treatment tank is pushed out, some of the untreated raw water may be pushed out along with the biologically treated water, resulting in unstable treated water quality.

In an embodiment of the present invention, after the completion of nitrification promotion treatment and before the start of denitrification promotion treatment, the biologically treated water that has been biologically treated in the treatment tank is discharged outside the treatment tank. By pre-discharging some or all of the biologically treated water outside the treatment tank after the completion of nitrification promotion treatment and before the start of denitrification promotion treatment, space can be secured within the treatment tank to accommodate inflowing raw water. This prevents untreated raw water from flowing out of the treatment tank, even if untreated raw water is allowed to flow into the treatment tank at a predetermined time. This prevents major fluctuations in the quality of the treated water, and biologically treated water of stable quality can be obtained.

The biologically treated water is preferably discharged after the nitrification promotion treatment, after stopping aeration and allowing the treatment tank to stand. Because the carrier method is used in the treatment tank, the standing time can be short. For example, it is preferable to allow the tank to stand for no more than 10 minutes, typically around 5 minutes, after stopping aeration, to allow the carriers in the treatment tank to sink. This allows the biologically treated water to be discharged while preventing the carriers from spilling out of the treatment tank.

Even when aeration in the treatment tank is stopped and the tank is left to stand, some of the carriers may rise to the surface. For this reason, it is preferable to discharge the biologically treated water from the middle layer region in the vertical direction of the treatment tank. This prevents the carriers from flowing into the screen installed at the outlet when the biologically treated water is discharged from the tank to the outside, making it possible to prevent clogging of the screen. Here, the "middle layer region in the vertical direction of the treatment tank" refers to the area excluding the bottom of the treatment tank and the area within 10%, and even 15%, of the water surface in the vertical direction of the treatment tank.

A portion of the biologically treated water may be discharged by overflowing through a screen from the upper layer region, which is higher in the vertical direction of the treatment tank than the middle layer region. If the amount of water in the treatment tank increases due to the inflow of raw water, by discharging a portion of the biologically treated water from the upper layer region of the treatment tank through a screen, the water level in the treatment tank can be maintained at a predetermined height while suppressing the outflow of carriers outside the treatment tank. When the discharge treatment of the biologically treated water is carried out in the middle and upper layers of the treatment tank, it is preferable to keep the biological carriers in the treatment tank at 40% by volume or less, and even 30% by volume or less.

(Dilution ratio)
The biologically treated water obtained by the above biological treatment is sent to a dilution tank, where it is mixed with dilution water and diluted to meet the sewage rejection standards. The biologically treated water can be diluted by a dilution ratio of typically 1 to 4 times, more typically 1 to 3 times, even 1 to 2 times, or even 1 to 1.5 times to meet the sewage rejection standards. Dilution may be performed continuously, or only when necessary to meet the sewage rejection standards. This allows for more efficient and appropriate treatment in accordance with the water quality standards for sewerage discharge, using a smaller amount of dilution water than conventional methods.

According to this embodiment, by performing the above-described biological treatment, the biologically treated water may be treated to an extent that dilution of the biologically treated water is no longer necessary. In such cases, it goes without saying that the biologically treated water after biological treatment using the biofilm method may be discharged directly into the sewer without dilution.

In accordance with the water treatment method of an embodiment of the present invention, two or more aeration conditions are set in the treatment tank so that the separated liquid obtained by solid-liquid separation of sludge containing at least one of septic tank sludge and sewage sludge undergoes a nitrification promotion process to promote nitrification and a denitrification promotion process to promote denitrification. By roughly removing BOD and T-N and then diluting the separated liquid, it is possible to more efficiently and stably obtain treated water of a quality suitable for discharge into a sewer system using a small amount of dilution water and a small, simple device.

<Water treatment equipment>
As shown in FIG. 1, a water treatment device according to an embodiment of the present invention includes a solid-liquid separation device 1 that separates sludge, including at least one of septic tank sludge and sewage sludge, into separated sludge and separated liquid by solid-liquid separation; a treatment tank 2 that contains a biofilm that biologically treats the separated liquid and subjects the separated liquid to a nitrification-promoting treatment that promotes nitrification and a denitrification-promoting treatment that promotes denitrification, thereby obtaining biologically treated water; control means 4 that controls the aeration conditions in the treatment tank 2 by setting at least two aeration conditions, i.e., aeration conditions for the nitrification-promoting treatment and aeration conditions for the denitrification-promoting treatment, so that the nitrification-promoting treatment and the denitrification-promoting treatment are alternately performed in the treatment tank 2; overflow means 25 that allows the biologically treated water to overflow from the treatment tank 2; discharge means 26 that discharges the biologically treated water from the treatment tank 2 through a middle layer region in the vertical direction of the treatment tank 2; and a dilution tank 3 that dilutes the biologically treated water discharged from the treatment tank 2 so that it meets sewage discharge standards.

A variety of devices can be used for the solid-liquid separation device 1, but it is preferable to use a dehydrator as the solid-liquid separation device 1 in terms of equipment and operating costs. It is also preferable to install a concentration machine (not shown) upstream of the solid-liquid separation device 1 to perform a concentration process on the raw water before solid-liquid separation.

The treatment tank 2 contains a biofilm with microorganisms attached to the surface of carriers. Typically, a contact oxidation tank containing fixed-bed carriers or a fluidized-bed carrier tank containing fluidized-bed carriers can be used as the treatment tank 2 in this embodiment. The treatment tank 2 may be provided with an agitator 21 for agitating the separated liquid supplied to the treatment tank 2. The agitator 21 may be omitted. An aeration means 22 for aerating the treatment tank 2 is connected to the bottom of the treatment tank 2. The aeration means 22 may be composed of a pump for supplying gas into the treatment tank 2 and an aeration pipe (not shown), etc. Aeration by the aeration means 22 generates a liquid flow in the separated liquid in the treatment tank 2, thereby fluidizing the biofilm (fluidized or fixed-bed carriers) in the treatment tank 2.

A measuring means 23 may be disposed within the treatment tank 2 for measuring the water quality of the separated liquid within the treatment tank 2. For example, a pH meter, an ORP meter, a DO meter, an NH ₄ —N concentration meter, a NO _x —N concentration meter, or a combination thereof may be used as the measuring means 23. A measuring means 24 may be disposed within the treatment tank 2 for measuring the water quality of the separated liquid flowing into the treatment tank 2. For example, a pH meter, an ORP meter, a DO meter, an NH ₄ —N concentration meter, a NO _x —N concentration meter, or a combination thereof may be used as the measuring means 24.

For example, if the NH ₄ -N concentration of the separated liquid is measured by measuring means 24, and the NH ₄ -N concentration or NO _x -N concentration meter is measured by measuring means 23, and a target value for the nitrogen concentration of the biologically treated water is determined, the required amount of nitrification and denitrification can be calculated from the relationship between the NH ₄ -N concentrations of the separated liquid and the biologically treated water.By determining the nitrification promotion treatment time and denitrification promotion treatment time based on the calculation results and setting the aeration conditions for each treatment, it is possible to efficiently roughly remove BOD and T-N from the separated liquid.

The treatment tank 2 is preferably equipped with a separation liquid supply pipe 11 connected to the bottom of the treatment tank 2 and capable of supplying separation liquid into the treatment tank 2 to generate a liquid flow within the treatment tank 2. By connecting the separation liquid supply pipe 11 to the bottom of the treatment tank 2 and configuring it so that the separation liquid is supplied from below to above the treatment tank 2, it is possible to suppress settling of the fluidized carrier or fixed-bed carrier contained in the treatment tank 2 to the bottom of the treatment tank 2, increase the fluidity of the carrier, and improve the contact efficiency between the carrier and the separation liquid. By connecting the separation liquid supply pipe 11 at a height that is relatively between 0 (tank bottom) and 0.5, more preferably between 0 and 0.3, assuming the height of the treatment tank 2 to be 1, it is possible to improve the efficiency of generating a liquid flow within the tank due to the supply of separation liquid.

The control means 4 is composed of a general-purpose computer or the like and is connected to the aeration means 22. The control means 4 controls the aeration conditions so that nitrification promotion treatment and denitrification promotion treatment are alternately performed in the treatment tank 2. The control means 4 may be connected to the agitator 21 and may control the agitation conditions of the agitator 21. The control means 4 may also be connected to the measurement means 23, 24 and may control the aeration by the aeration means 22 based on the measurement results of the water quality (pH, ORP, DO, NH ₄ -N concentration, NO _x -N concentration, etc.) by the measurement means 23, 24. By controlling the aeration in the treatment tank 2 by the aeration means 22 based on the water quality measurement results by the measurement means 23, 24, more stable biological treatment can be performed that can follow fluctuations in the water quality of the raw water. The biologically treated water treated in the treatment tank 2 is stored in the dilution tank 3 and diluted with dilution water in the dilution tank 3 to obtain treated water.

The specific configuration of the overflow means 25 is not particularly limited. For example, the overflow means 25 may be configured to include an overflow weir (not shown) of an appropriate height near the water surface in the upper layer region of the treatment tank 2, e.g., near the water surface, and a screen 27. When the water level of the biologically treated water in the treatment tank 2 rises due to an inflow of raw water, etc., the biologically treated water can be discharged from the treatment tank 2 by overflow. In this embodiment, the upper layer region of the treatment tank 2 refers to a height within approximately 10% of the water surface in the treatment tank 2. The biologically treated water flowing out of the overflow means 25 is sent to the dilution tank 3 via the overflow line L1. The discharge means 26 is not particularly limited as long as it is configured to withdraw the biologically treated water from the middle layer region of the treatment tank 2 via the screen 28, a pump, an on-off valve, etc. The biologically treated water flowing out of the discharge means 26 is sent to the dilution tank 3 via the withdrawal line L2.

The water treatment device according to an embodiment of the present invention is equipped with a control means 4 that sets and controls at least two aeration conditions in the treatment tank 2 so that nitrification promotion treatment and denitrification promotion treatment are alternately performed in the treatment tank 2. This allows for rough removal of BOD and T-N from the solid-liquid separated liquid of sludge containing septic tank sludge or human waste sludge in a single treatment tank 2, making it possible to efficiently obtain treated water of a quality that meets sewage discharge standards from sludge containing septic tank sludge or human waste sludge using a compact device.

Furthermore, with the water treatment device according to the embodiment of the present invention, the discharge means 26 is used to discharge the biologically treated water that has been biologically treated in the treatment tank 2 out of the treatment tank 2 after the nitrification promotion treatment is completed and before the denitrification promotion treatment is started, and then raw water is allowed to flow in, thereby preventing large amounts of poor-quality water mixed with raw water from overflowing from the treatment tank 2. Furthermore, by locating the discharge means 26 in the middle layer region of the treatment tank 2, it is possible to smoothly withdraw only the treated water without relying on a screen or the like, preventing the biological carriers in the treatment tank 2 from flowing out of the treatment tank 2. Furthermore, by providing the overflow means 25, even if the amount of water in the treatment tank 2 temporarily increases, the biologically treated water can be smoothly discharged from the treatment tank 2 without causing large amounts of overflow.

The following examples of the present invention are provided to provide a better understanding of the present invention and its advantages, and are not intended to limit the invention.

According to the flow chart shown in Figure 2, water treatment was carried out for the example (experimental system) and an example (control system) for evaluating the effect of the water treatment of the example. The raw water used was dehydrated separated liquid from an actual sewage treatment facility. Nutrients ( _{monopotassium} phosphate ( _KH2PO4 )) were added to the raw water as appropriate. Representative examples of the raw water conditions during the experiment are shown in Table 1.

Raw water was passed through each test system to achieve a BOD volumetric load of 0.5 kg-BOD/m ³ /d and a TN load of 0.29 kg-N/m ³ /d. Figure 2 shows the treatment flow of the laboratory test. A control system was also compared with a system in which the biologically treated water was simply allowed to overflow after the completion of nitrification promotion treatment and before the start of denitrification promotion treatment (hereinafter referred to as the "treated water withdrawal process"), without undergoing the process of discharging the biologically treated water from the middle layer region of the treatment tank. The carriers were polyethylene honeycomb carriers (φ25 mm, thickness 4 mm, specific surface area 800 m ² /m ³ ) and filled at 30% by volume of each tank capacity.

The raw water inflow conditions, treated water withdrawal conditions, and aeration conditions were set as shown in Table 2. In the experimental system, the same amount of biologically treated water as the amount of raw water input that day was discharged. Note that for the "nitrification conditions" in Table 2, the aeration volume was set so that the set DO in the treatment tank was 4.0 mg/L, and mechanical agitation was not performed. For the "denitrification (micro-aeration) conditions," the aeration volume was set so that the set DO in the treatment tank was 0.2 mg/L, and mechanical agitation was performed. In the control system, the water that overflowed when raw water inflowed was used as biologically treated water.

The experimental results are shown in Table 3. For the control system, a sample of biologically treated water mixed with one day's worth of biologically treated water was analyzed. The required dilution ratio was calculated based on discharge standards of BOD: 600 mg/L and T-N: 240 mg/L. In the experimental system with a treated water withdrawal process, the average T-N value of the treated water was 235 mg/L, and the average required dilution ratio was below 1. On the other hand, in the test system without a treated water withdrawal process, BOD and T-N were also removed, but the average dilution ratio was 1.3, slightly higher than that of the experimental system.

These results demonstrate that more stable nitrogen removal and a reduced dilution rate can be achieved by setting and switching between two or more aeration conditions to alternate between nitrification and denitrification, and by discharging the biologically treated water from the treatment tank after the nitrification promotion process is completed and before the denitrification promotion process begins.

1... solid-liquid separation device 2... treatment tank (aeration treatment tank)
3...Dilution tank 4...Control means 11...Separated liquid supply pipe 21...Agitator 22...Aeration means 23...Measuring means 24...Measuring means

Claims (4)

A separated liquid obtained by solid-liquid separation of sludge containing at least one of septic tank sludge and sewage sludge is supplied into a treatment tank containing a biological carrier;
In the treatment tank, aeration conditions in the treatment tank are set to at least two conditions, including aeration conditions for the nitrification promotion treatment and aeration conditions for the denitrification promotion treatment having an aeration amount smaller than that of the aeration conditions for the nitrification promotion treatment, so that a nitrification promotion treatment for promoting nitrification and a denitrification promotion treatment for promoting denitrification are performed on the separated liquid,
The biological treatment is performed by switching the aeration conditions in the treatment tank so that the nitrification promotion treatment and the denitrification promotion treatment are alternately switched in the treatment tank,
After the nitrification promotion treatment is completed and before the denitrification promotion treatment is started, the biologically treated water in the treatment tank is discharged to the outside of the treatment tank through a discharge means disposed in a middle layer region in the height direction of the treatment tank and an overflow .
A water treatment method comprising diluting the biologically treated water in a dilution tank so that the water meets sewage rejection standards. 2. The water treatment method according to claim 1, further comprising the steps of: stopping the aeration treatment after the completion of the nitrification promotion treatment, allowing the treatment tank to stand, discharging the biologically treated water from the treatment tank, and then supplying the separated liquid into the treatment tank, and then starting the denitrification promotion treatment. 3. The water treatment method according to claim 1, further comprising storing the biological carriers in the treatment tank at 40% by volume or less. a solid-liquid separator that separates sludge containing at least one of septic tank sludge and sewage sludge into separated sludge and separated liquid;
a treatment tank that accommodates a biofilm that biologically treats the separated liquid, and subjects the separated liquid to a nitrification promotion treatment that promotes nitrification and a denitrification promotion treatment that promotes denitrification, thereby obtaining biologically treated water;
a control means for controlling the aeration conditions of the treatment tank by setting at least two conditions, i.e., aeration conditions for the nitrification promotion treatment and aeration conditions for the denitrification promotion treatment, so that the nitrification promotion treatment and the denitrification promotion treatment are alternately performed in the treatment tank;
an overflow means for allowing the biologically treated water to overflow from the treatment tank;
a discharge means for discharging the biologically treated water from the treatment tank through a middle layer region in the height direction of the treatment tank;
A water treatment device comprising a dilution tank for diluting the biologically treated water so that it meets sewage removal standards.