JP2024052993A - Water treatment method and water treatment device - Google Patents

Abstract

[Problem] To provide a water treatment method and water treatment device capable of efficiently obtaining treated water of a quality that satisfies the sewage discharge standards from treated water containing septic tank sludge or sewage sludge with a smaller device. [Solution] This water treatment method comprises the steps of: supplying a separated liquid obtained by solid-liquid separation of sludge containing at least one of septic tank sludge and sewage sludge into an aeration treatment tank 2 containing a biological carrier; setting at least two aeration conditions in the treatment tank 2, namely, an aeration condition for nitrification promotion treatment and an aeration condition for denitrification promotion treatment with a smaller aeration amount than the aeration condition for the nitrification promotion treatment, so that a nitrification promotion treatment that promotes nitrification and a denitrification promotion treatment that promotes denitrification are performed in the treatment tank 2; switching the aeration conditions to alternate between the nitrification promotion treatment and the denitrification promotion treatment in the treatment tank; and diluting the biologically treated water obtained by the biological treatment so as to satisfy the sewage discharge standards. [Selected Figure] Figure 1

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 application to water treatment in which septic tank sludge and sewage sludge are treated and the treated water is discharged into a sewer system.

In order to discharge treated water obtained by treating raw water containing septic tank sludge and human waste sludge into a sewer system, it is necessary to satisfy sewage discharge standards, which are generally 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 is, for example, to remove waste (sewage residue) from sewage and other waste, dilute it to the exclusion standard, and then discharge it. In this case, the dilution rate is generally about 10 to 20 times, which results 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, dilute the dehydrated separated liquid, and then discharge it into the sewer system. In this case, the dehydrated separated liquid contains significantly less BOD, SS, nitrogen, and other components than the sludge-removed sewage, so the dilution rate can generally be about 4 to 8 times. However, there is a problem in that the water quality of the dehydrated separated liquid varies, and the amount of dilution water also varies greatly depending on the water.

The method of separating sewage and other waste into solid and liquid form using a dehydrator ultimately results in discharging 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 dehydrated separated liquid may deteriorate, making it necessary to increase the amount of dilution water. There are also cases where regulations on the amount of discharged water make it impossible to meet the discharge standards.

As another method for more reliably reducing the amount of dilution water and discharge water, a method that combines solid-liquid separation and biological treatment can be considered. For example, Japanese Patent Laid-Open Publication No. 61-50691 (Patent Document 1) describes a method in which the solid-liquid fraction obtained by solid-liquid separation of septic tank sludge is mixed with night soil wastewater, the mixed liquid is subjected to coagulation treatment, and the separated liquid is then subjected to biological treatment.

Japanese Patent Application Laid-Open No. 61-50691

The method described in Patent Document 1 does not state where the biologically treated water is discharged, but since it is described that the BOD of the treated liquid in Example 1 can be treated to 10 mg/L or less, it can be said that this treatment method is based on the premise that the water will be discharged into public water areas.

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

However, when treating about 60% of the nitrogen, about 40% of the nitrate nitrogen remains, and denitrification occurs again in the downstream settling tank where the anaerobic conditions are created, and the nitrogen gas generated causes the sludge to float, which may prevent sufficient solid-liquid separation in the settling tank. If solid-liquid separation cannot be achieved in the settling tank, the MLSS (activated sludge concentration) of the nitrification/denitrification tank cannot be maintained, and the treatment itself will deteriorate.

If all of the ammonia nitrogen contained in the dehydration separation liquid is nitrified, the tank capacity will be too large, the aeration air volume required for nitrification will be too large, and hydrogen donors such as methanol and 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 to roughly treat the treated water until the water quality falls below the sewage discharge standard, and then dilute the water before 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 is not an efficient method.

In consideration of the above problems, the present invention provides a water treatment method and water treatment device that can efficiently obtain treated water with a quality that meets sewage discharge standards from treated water containing septic tank sludge or sewage sludge using a smaller device.

As a result of intensive research by the present inventors to solve the above problems, it was discovered that it is effective to supply the separated liquid obtained by solid-liquid separation of sludge, including septic tank sludge or sewage sludge, to a treatment tank equipped with a biofilm and to treat the sludge by setting two or more aeration conditions in the treatment tank.

In one aspect, the water treatment method according to the embodiment of the present invention, which was completed based on the above findings, is a water treatment method comprising: supplying the separated liquid obtained by solid-liquid separation of sludge containing at least one of septic tank sludge and sewage sludge into an aeration tank containing a biological carrier; setting at least two aeration conditions in the treatment tank, namely, aeration conditions for nitrification promotion treatment and aeration conditions for denitrification promotion treatment with a lower aeration volume than the aeration conditions for nitrification promotion treatment, so that a nitrification promotion treatment that promotes nitrification and a denitrification promotion treatment that promotes denitrification are performed in the treatment tank; switching the aeration conditions to perform biological treatment so that the nitrification promotion treatment and the denitrification promotion treatment are alternately switched in the treatment tank; and diluting the biologically treated water obtained by the biological treatment so as to meet the sewage discharge standards.

In one embodiment of the water treatment method according to the present invention, the separated liquid is intermittently supplied to the treatment tank.

In another embodiment of the water treatment method according to the present invention, in the denitrification promotion treatment, the separated liquid contained in the treatment tank is caused to flow by at least one of the following: flowing the separated liquid from the bottom of the treatment tank, stirring with a stirrer, or aeration.

In one aspect, the water treatment device according to the embodiment of the present invention is a water treatment device that includes a solid-liquid separation device 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 contains a biofilm that biologically treats the separated liquid and performs a nitrification promotion treatment that promotes nitrification and a denitrification promotion treatment that promotes denitrification on the separated liquid, a control means that sets and controls the aeration conditions of the treatment tank to at least two conditions, the aeration conditions for the nitrification promotion treatment and the aeration conditions for the denitrification promotion treatment, so that the nitrification promotion treatment and the denitrification promotion treatment are performed alternately in the treatment tank, and a dilution tank that dilutes the biologically treated water treated in the treatment tank so that it meets the sewage discharge standards.

In another embodiment of the water treatment device according to the present invention, the water treatment device is provided with a separation liquid supply pipe that is connected to the bottom of the treatment tank and can supply separation liquid into the treatment tank so as to generate a liquid flow within the treatment tank.

The present invention provides a water treatment method and a water treatment device that can efficiently produce treated water with a quality that meets sewage discharge standards from treated water containing septic tank sludge or sewage sludge using a smaller device.

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.

The following describes an embodiment of the present invention with reference to the drawings. The embodiment described below is an example of an apparatus and method for embodying the technical concept of the present invention, and the technical concept of the present invention does not limit the structure, arrangement, etc. of the components to those described below.

<Water treatment method>
A water treatment method according to an embodiment of the present invention includes subjecting a separated liquid obtained by solid-liquid separation of sludge containing at least one of septic tank sludge and sewage sludge to biological treatment using a biofilm (biological carrier), and diluting the biologically treated water obtained by the biological treatment so as to satisfy sewage discharge standards.

As the raw water, a separated liquid obtained by solid-liquid separation of sludge containing at least one of septic tank sludge and night soil sludge can be used. Typically, dehydrated separated liquid of septic tank sludge, night soil sludge, or a mixture of these can be suitably used. When mixing septic tank sludge and night soil sludge, the mixed liquid of the septic tank sludge and night soil sludge may be subjected to solid-liquid separation, but it is preferable to perform solid-liquid separation separately for the septic tank sludge and night soil sludge and mix the separated liquids obtained by the solid-liquid separation.

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

By performing a concentration process in which a polymer flocculant is added before solid-liquid separation, the sludge concentration (TS) of the concentrated sludge can be concentrated to a maximum of approximately 10-12% by mass, which allows for a more compact treatment device. If the highly concentrated concentrated sludge is further dehydrated using a dehydrator, dehydrated sludge (separated sludge) with a moisture content of 70% or less can be obtained, resulting in a more significant sludge volume reduction effect. As this dehydrated sludge with a low moisture content has a high calorie content, it can be spontaneously combusted without auxiliary fuel during incineration, resulting in energy savings and low costs.

Typical raw water qualities include, but are not limited to, a BOD of 600-2000 mg/L, a T-N of 240-1000 mg/L, and more typically a T-N of 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 T-N of 240-600 mg/L.

The separated liquid is supplied to a treatment tank equipped with a biofilm in which microorganisms are attached to the surface of a carrier, and biological treatment using the biofilm method is carried out in the treatment tank. The biofilm method 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 methods, etc. The latter include trickling filter methods, fluidized carrier methods, rotating disk methods, and fixed bed methods (contact oxidation methods). Of these, it is preferable to use a type of biofilm method in which the amount of biofilm is autonomously controlled during treatment as the biological treatment according to the embodiment of the present invention, and this is defined in this specification as a "non-blocking biofilm method."

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

- Contact oxidation method (fixed bed method) -
In the contact oxidation method, a fixed bed carrier is immersed in the liquid contained in a treatment tank, and the separated liquid is passed through while aeration is performed, thereby forming a biofilm on the surface of the carrier and decomposing the organic matter in the separated liquid to obtain biologically treated water. Since treatment is performed using a biofilm attached to the carrier, there is no need to control the amount of sludge by returning it, as compared with the activated sludge method, and maintenance and management are easy. The BOD volume load is preferably 0.1 to 1.0 kg-BOD/m ³ /d, and operation at a high load may 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 for the contact oxidation method, and any device can be used. The carrier can be made of polyethylene, plastic, etc., and can be in any shape, such as a tube, string, net, plate, or ball.

The specific surface area of the carrier in the contact oxidation method is preferably 50 to 200 ^m2 / ^m3 , more preferably 70 to 150 ^m2 / ^m3 , 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 specific surface area.

In general, when treatment is performed using the contact oxidation method, continuous aeration is the basis, and switching to low aeration air volume is not performed. This is because when the aeration air volume is low, the agitation in the tank weakens, and the raw water may short-pass and cause untreated wastewater to flow out. In contrast, in the water treatment method according to the embodiment of the present invention, the contact oxidation method is used as a method of roughly treating the separated liquid, which is obtained by separating sludge into solids and liquids to reduce the load on biological treatment, for discharge into a sewerage system. Therefore, even if the mixing of the water in the tank becomes insufficient during times of low aeration air volume or no aeration, the intended treatment can be sufficiently achieved. Furthermore, in order to further improve the mixing in the tank, at least one of the following can be implemented: flowing raw water in from the bottom of the treatment tank, stirring with a stirrer, or aeration, which can promote the flow of the separated liquid contained in the treatment tank in the tank, making it possible to achieve even more stable treatment.

- Fluid carrier method -
The floating 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 water to be treated, thereby obtaining biologically treated water. A treatment tank using the floating carrier method may be newly constructed, or carriers and an aeration device may be installed in an existing storage tank. There are no particular limitations on the carriers used as floating carriers, but the following are some typical examples:

The carrier used may be any carrier to which microorganisms can adhere and which can flow due to aeration. Any carrier material may be used as long as it can flow due to aeration, and examples of the carrier material include plastics (polyurethane (PU), polyethylene (PE), polyethylene glycol (PEG), polyvinyl alcohol (PVA)), wood chips, sand, etc. The carrier may be in the form of a sponge, gel, solid, etc.

The carriers can be of any shape, including spherical, cubic, cylindrical, honeycomb, etc. In particular, by using a bonded immobilization carrier that attaches microorganisms to the outer surface of the carrier, microorganisms suited to the environment in the treatment tank can be attached to the carrier, making it less susceptible to fluctuations in the properties of the inflow water and enabling more stable biological treatment.

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

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 capable of maintaining a specific surface area of 50 ^m2 /m3 or ^more even after sludge is filled in the voids of the carrier. The porosity of the carrier (i.e., the volume calculated from the volume of the voids 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 carriers.

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 flow uniformly while being mixed, and more preferably 20 to 40% by volume (V%). A filling rate of 20% by volume or more allows a large amount of microorganisms to be retained in the tank, and a filling rate of 40% by volume or less allows for adequate voids to be created, allowing good fluidity to be maintained.

It is also possible to fill two types of carriers in the treatment tank, one for nitrification promotion treatment and the other for denitrification promotion treatment. In that case, it is preferable to fill 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 fill a carrier 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 a carrier 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 a carrier, the carrier can be made to flow in the treatment tank even by micro-aeration, intermittent aeration, or supplying a separated liquid into the treatment tank, and the denitrification treatment can be suitably carried out.

The BOD volume 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 T-N 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 carrier method is less likely to cause blockage under higher loads than the contact oxidation method. Activated sludge may be added to the treatment tank in addition to the fluidized carrier. By adding activated sludge to the treatment tank, it is possible to suppress changes in the properties of the biologically treated water caused by changes in the properties of the raw water, and more stable treatment can be achieved.

As with the contact oxidation method described above, in general, when treatment is performed using the fluidized carrier method, continuous aeration is the basis, and switching to low aeration air volume is not performed as a rule. This is because when the aeration air volume is low, the agitation in the tank weakens, and the raw water may short-pass and cause untreated wastewater to flow out. On the other hand, the water treatment method according to the embodiment of the present invention uses the fluidized carrier method as a method for roughly treating the separated liquid, which is obtained by separating sludge into solid and liquid and reducing the load on biological treatment, for discharge into a sewerage. Therefore, even if the mixing of the water in the tank becomes insufficient during times of low aeration air volume or no aeration, and the carrier settles, the treatment aimed for by this method can be sufficiently achieved. Furthermore, in order to further improve mixing in the tank, at least one of the following can be performed: raw water is introduced from the bottom of the treatment tank, agitation is performed with an agitator, or aeration is performed, which can promote the flow of the separated liquid contained in the treatment tank in the tank, making it possible to achieve even more stable treatment.

Furthermore, in general, in treatments using flow carriers, the single-tank system is rarely used because the treatment speed can be increased by dividing the nitrification tank and denitrification tank into separate tanks and placing nitrifying bacteria predominantly in the nitrification tank carrier and denitrifying bacteria predominantly in the denitrification tank carrier, compared to when both nitrifying and denitrifying bacteria are attached to a single carrier. In contrast, the water treatment 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 satisfies the sewage discharge standards, so by adopting a system of switching between the nitrification process and the denitrification process in a single tank system, a water treatment method and water treatment device that can efficiently obtain treated water can be provided using a smaller device.

According to the water treatment method according to the embodiment of the present invention, by using an aeration biofilm method in which a fixed bed carrier or a fluidized carrier is contained in the treatment tank, the settling tank set in the downstream of the biological treatment can be omitted or made smaller, so that the overall treatment device can be made smaller. When the settling tank is omitted, there is no need to consider the problem of poor settling of sludge caused by a portion of the BOD components or T-N components of the biological treatment water remaining in the settling tank, so the treatment can be made more efficient. Even when a settling tank is provided in the downstream of the treatment tank, a compact settling tank is sufficient because only an auxiliary settling tank is required. Even if a portion of the activated sludge flows out due to poor settling, the carrier is sufficiently held in the treatment tank, so stable treatment can be performed at all times. Furthermore, in the general batch activated sludge method, a certain amount of settling time is required to settle the MLSS, maintain the microorganisms, and obtain clear treated water, but in this method, since a biological carrier is used, a settling time is not required or can be shortened, so the load that can be treated can be increased compared to the case of treatment using activated sludge.

In the biofilm method, in which biological treatment is carried out by placing fixed bed carriers or fluidized carriers in a treatment tank, oxygen is supplied by aeration, which also has a tank stirring function, in order to ensure the fluidity of the carriers or the contact efficiency between the raw water and the carriers. However, when nitrification and denitrification are carried out in the same tank, it is generally difficult to keep the aeration volume in the denitrification treatment to a low level suitable for denitrification. In this embodiment, since it is sufficient to obtain treated water in which some of the BOD components and T-N components remain in the biological treatment, BOD removal and nitrogen removal from the raw water can be carried out simply and efficiently in a single tank by simply manipulating the aeration volume in the treatment tank. This makes it possible to efficiently obtain treated water of the desired quality (for sewage discharge) using small 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 one treatment tank, the water treatment method of this embodiment can be applied even when only one treatment tank is available due to modifications to existing facilities, etc., so the economic burden for modifications can be reduced and treated water suitable for discharge into a sewerage system can be obtained more efficiently and stably.

The aeration conditions in the treatment tank should be at least two, 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 for the denitrification promotion treatment with a lower aeration volume than the aeration condition for the nitrification promotion treatment. Of course, three or more conditions or four or more conditions may be set as necessary.

- Nitrification promotion treatment -
In the nitrification promotion treatment, it is preferable to set the aeration conditions so that the dissolved oxygen (DO) in the tank is 1.0 to 5.0 mg/L, and more preferably 2.5 to 4.0 mg/L. This can promote 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 ^m3 / ^m2 /min, more preferably 0.3 to 0.5 ^m3 / ^m2 /min, in order to cause the carriers to flow throughout the entire tank. This allows the carriers to flow throughout the entire tank, increasing the reaction efficiency. Preferably, the aeration air volume is fine-tuned so that the set DO is achieved within the above range while confirming that the carriers flow throughout the entire tank at the above-mentioned aeration air volume, thereby making it possible to more efficiently carry out the nitrification promotion treatment according to the present invention.

When the fluidized carrier method is used as the biofilm method, in the nitrification promotion treatment, the carriers are fluidized throughout the tank by aeration, which increases the contact efficiency with the raw water and increases the 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, and it is sufficient that nitrification is completed to the target concentration.

According to the inventors' investigations, it is preferable to adjust the pH so that the free ammonia concentration of the ammonia nitrogen-containing wastewater in the tank is maintained at 1.0 to 10 mg/L, more preferably 2.0 to 10 mg/L. This allows the ammonia oxidizing bacteria to preferentially attach to the carrier while suppressing the growth of nitritizing bacteria, thereby achieving a 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 the BOD in the raw water as an electron donor, denitrification by anaerobic ammonia oxidation reaction of 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 formula (1).

As can be seen from formula (1), the free ammonia concentration is affected by changes in pH, _NH4 -N concentration, and water temperature. A stable nitritation treatment can be carried out by measuring changes in the pH, _NH4 -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, it is preferable to set the aeration conditions so that the DO in the treatment tank is less than 1.0 mg/L, preferably 0.5 mg/L or less, to promote the denitrification reaction. Under the denitrification conditions, aeration may be completely stopped and fluidization may be performed by mechanical agitation, or micro-aeration may be performed in 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 setting such aeration conditions, the nitrification rate is lower than that under the nitrification conditions, but it is expected that nitrification and denitrification will proceed simultaneously.

When a fluidized carrier is used in the biological treatment according to the embodiment of the present invention, the agitation of the denitrification conditions is not necessarily required to flow the carrier throughout the entire tank, but rather to create a flow of water in the tank to increase the contact efficiency between the carrier and the water to be treated. Some of the fluidized carrier may remain submerged. In the case of a conventional general fluidized carrier method, it is ideal to flow the fluidized carrier uniformly throughout the tank in order to proceed with the treatment appropriately. On the other hand, in the water treatment method according to the present embodiment, since the purpose is the rough treatment of septic tank sludge or sewage sludge, it is sufficient that the target reaction proceeds rather than the degree of flow of the fluidized carrier in the tank that is generally required in the fluidized carrier method. The progress of the reaction can be confirmed by a sensor such as an ORP meter or a NO _x -N meter described later. In order to facilitate operation management and reduce running costs, it is desirable not to add an electron donor such as methanol from the outside, but depending on the target quality of the treated water, methanol or the like 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 the aeration air volume due to micro-aeration conditions, control using an inverter may not be able to achieve precise control because there is a limit to the lower control value due to the discharge pressure. Therefore, it is preferable to perform intermittent aeration using the aeration blower timer, which allows for more precise adjustment of the air volume.

When using a timer to adjust the operating time ratio, setting the on (aeration operation) time and off (aeration stop) time to less than one minute can place a strain on the blower, making it difficult to operate stably for long periods of time. In order to reduce the strain on the blower and perform stable processing that creates conditions suitable for denitrification, it is preferable to set the off time to five minutes or more.

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 the size of the tank, it is more preferable to set the off-time to 5 minutes or more, and even more preferably 10 minutes or more, so that the DO in the tank is 0.5 mg/L or less, and a period suitable for denitrification can be provided.

On the other hand, if the off time is set too long, scale is likely to adhere to the aeration device, carrier, etc. in the tank during the aeration stop period if mechanical stirring is not performed. The aeration stop time in the nitritation treatment is 60 minutes or less, more preferably 45 minutes or less, and even more preferably 30 minutes or less.
There is no particular restriction 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 a flow carrier is used, it is even more preferable to set the DO to 2.5 mg/L or more. In addition, when the dehydrator is not operated on holidays and there is no inflow of raw water, it is possible to promote endogenous denitrification by extending the ratio of aeration stop time under micro-aeration conditions.

In the denitrification promotion process, it is preferable to at least one of the following: introduce raw water from the bottom of the treatment tank, stir with an agitator, or aerate, to generate a liquid flow in the raw water contained in the treatment tank and move the carriers in the treatment tank. This increases the contact efficiency between the flowing carriers and the raw water, promoting the nitrogen removal process.

It is preferable to supply raw water to the treatment tank intermittently with a predetermined supply suspension period. "Intermittently" may 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 septic tank sludge and sewage sludge are to be treated, raw water flows in from 9:00 to 17:00 on weekdays, and there are periods during the early morning hours of weekday nights, holidays, and public holidays when raw water does not flow in, depending on the operating conditions of the treatment facility. By supplying raw water to the treatment tank intermittently in accordance with the operating conditions of the treatment facility, such as the raw water inflow time, a storage tank for storing raw water can be omitted, which contributes to the miniaturization of the entire device. In addition, by controlling the aeration conditions in the treatment tank to switch between aeration conditions for nitrification promotion treatment and aeration conditions for denitrification promotion treatment in accordance with the timing of the raw water inflow, nitrification and denitrification of raw water can be performed more efficiently in a single treatment tank. For example, raw water is fed under denitrification-promoting conditions, and after it has been confirmed that denitrification has been completed using a set time, ORP sensor, etc., as described below, the feed of raw water is stopped and the conditions are switched to nitrification-promoting conditions, allowing the BOD components in the raw water to be effectively denitrified and reducing the amount of oxygen used for BOD oxidation.

In biological treatment, it is preferable to switch aeration conditions so that nitrification promotion treatment and denitrification promotion treatment are alternately performed in one treatment tank. By repeatedly switching aeration conditions and alternately performing nitrification promotion treatment and denitrification promotion treatment, it is possible to efficiently roughly remove BOD and T-N from raw water in one treatment tank.

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

-Time-based switching-
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 the nitrification promotion treatment to the denitrification promotion treatment to be 1:1 to 5:1, preferably 2:1 to 4:1. When using a flowing carrier in biological treatment, it is possible to manually or automatically set the optimal ratio of the nitrification time and the denitrification time according to the adhesion state of the microorganisms by sampling the acclimatized carrier and determining the nitrification rate and the denitrification rate by, for example, a batch test such as the one described below.

(Nitrification batch test)
For example, 20 vol% of the carrier collected from the treatment tank is placed in a 1 L graduated 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 (for example, 0, 30, 60, 90, and 120 minutes) and water quality is analyzed. 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. NaNO ₃ is then added so that the NO ₃ -N concentration is 50-100 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 NO _x -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, alkalinity control)
In the nitrification promotion process, alkalinity decreases and pH drops as the nitrification reaction proceeds. If nitrification is performed using only the alkalinity in the raw water without adding alkali, the nitrification process stops and the pH drop also stops when the alkalinity falls below a certain value. For this reason, the change in pH or alkalinity over time can be confirmed, and the point at which the change per hour falls below a certain value can be regarded as the point at which nitrification is completed, and the denitrification promotion process can be switched to.

In denitrification promotion treatment, alkalinity increases with the denitrification reaction, and so does the pH. For this reason, by checking the change in pH or alkalinity over time and determining when the rate of change per hour falls below a certain value, denitrification is considered complete and the treatment can be switched to nitrification promotion treatment.

(ORP Control)
In the nitrification promotion treatment, the 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 of organic matter and nitrification are proceeding simultaneously, during the period when the oxidation of organic matter is completed and only nitrification is proceeding, and after nitrification is completed. Therefore, the amount of change is confirmed, and the point at which the amount of change per hour falls below a certain value is regarded as the point at which nitrification is completed, and the treatment can be switched to the denitrification promotion treatment.

In the denitrification promotion process, ORP decreases with the inflow of raw water and the denitrification of NO _x -N, and the change in ORP decreases when denitrification is completed. Therefore, the time when the change in the amount per hour is below a certain value is considered to be the time when nitrification is completed, and the nitrification promotion process can be switched to. Compared with pH and DO, ORP control has a wider range of possible values and a higher resolution (generally -400mV to +400mV), allowing for more accurate control. In addition, unlike NH ₄ -N and NO _x -N, ORP is not the concentration of a single substance, but is an index that can comprehensively evaluate the state of organic matter, nitrogen compounds, etc. in the system. Therefore, when the purpose is to roughly remove BOD and nitrogen as in this case and BOD and nitrogen remain in the tank, ORP is a suitable index for evaluating the state in 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 is 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.

( _NH4 -N control)
In nitrification promotion treatment, the change in _NH4 -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 when the amount of change in _NH4 -N becomes small, it is possible to consider that nitrification is complete and to move on to denitrification promotion treatment.

( _NOx -N)
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 when the amount of change in NO _x -N becomes small, nitrification can be considered to be complete and denitrification promotion treatment can be started.

In the denitrification promotion process, the amount of change is large while denitrification is progressing, and 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 deemed to be complete, and the process can be switched to nitrification promotion. Compared to DO control and NH ₄ -N control, control based on the NO _x -N concentration is preferable for this embodiment in that the completion time of the denitrification promotion process can be confirmed with a single sensor.

-Sensor-based control 2 (combined with feedforward)-
Since most of the nitrogen components in the raw water are NH ₄ -N, the nitrogen concentration of the 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-denitrification from the relationship between the raw water NH ₄ -N concentration and the target T-N concentration of the treated water. Since the nitrification rate and denitrification rate per hour can be calculated from the NH ₄ -N concentration or NO _x -N concentration in the treatment tank, the time required for the nitrification and denitrification conditions to reach the target concentration can be calculated by a calculation device and the blower can be controlled. This makes it possible to keep the nitrification time to a minimum and reduce the aeration air volume.

-Addition of nutrients (phosphorus)-
In the water treatment method according to the embodiment of the present invention, since biological treatment is used to treat the separated liquid, which is raw water, it may be necessary to supply phosphorus in an amount necessary for the growth of organisms. Generally, in biological treatment, about 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 with respect to the raw water BOD. If the treatment is good, 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 before supplying it.

In particular, when performing dehydration treatment as a solid-liquid separation process for septic tank sludge and sewage sludge, if iron- or aluminum-based coagulants are used, phosphorus is absorbed into the sludge, reducing the phosphorus concentration in the separated liquid, and phosphorus may need to be added. In rough treatment for removing BOD and T-N, 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 and potassium dihydrogen phosphate, or adding a portion of sewage sludge.

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

According to this embodiment, by carrying out the above-mentioned biological treatment, the biologically treated water may be treated to such an extent that dilution of the biologically treated water is not necessary. In such a case, it goes without saying that the biologically treated water after biological treatment using the biofilm method may be discharged directly into the sewerage without dilution.

According to the water treatment method of the embodiment of the present invention, the separated liquid obtained by solid-liquid separation of sludge containing at least one of septic tank sludge and sewage sludge is subjected to a nitrification promotion process and a denitrification promotion process to promote denitrification. By setting two or more aeration conditions in the treatment tank and roughly removing BOD and nitrogen, and then diluting the separated liquid, it is possible to more efficiently and stably obtain treated water suitable for discharge into the sewerage system using a small amount of dilution water and a small and simple device.

<Water Treatment Device>
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 performs solid-liquid separation of sludge, which includes at least one of septic tank sludge and sewage sludge, into separated sludge and separated liquid, a treatment tank (aeration treatment tank) 2 that contains a biofilm that biologically treats the separated liquid and performs a nitrification promotion treatment that promotes nitrification and a denitrification promotion treatment that promotes denitrification on the separated liquid, a control means 4 that sets and controls the aeration conditions of the treatment tank 2 to at least two conditions so that the nitrification promotion treatment and the denitrification promotion treatment are performed alternately in the treatment tank 2, and a dilution tank 3 that dilutes the biologically treated water treated in the treatment tank 2 so as to meet the sewage discharge standards.

Various devices can be used for the solid-liquid separation device 1, and among them, 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 use a concentration machine (not shown) upstream of the solid-liquid separation device 1 to perform concentration treatment on the raw water before solid-liquid separation.

The treatment tank 2 contains a biofilm with microorganisms attached to the surface of the carrier. Typically, a contact oxidation tank containing a fixed bed carrier inside or a fluidized carrier tank containing a fluidized carrier inside can be suitably used as the treatment tank 2 according to 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 lower part of the treatment tank 2. The aeration means 22 can be composed of a pump for sending 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, which causes the biofilm (fluidized carrier or fixed bed carrier) in the treatment tank to flow.

A measuring means 23 may be disposed in the treatment tank 2 for measuring the water quality of the separated liquid in 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 of these may be used as the measuring means 23. A measuring means 24 may be disposed in 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 of these may be used as the measuring means 24.

For example, by measuring the NH ₄ —N concentration in the separated liquid with measuring means 24 and measuring the NH ₄ —N concentration or NO _x —N concentration with measuring means 23 and determining a target value for the nitrogen concentration in the biological treatment water, the required amount of nitrification and denitrification can be calculated from the relationship between the NH ₄ —N concentrations in the separated liquid and the biological treatment water. By determining the nitrification promotion treatment time and denitrification promotion treatment time based on the calculation results and determining 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 provided with a separation liquid supply pipe 11 that is connected to the lower part of the treatment tank 2 and can supply the separation liquid into the treatment tank 2 so as to generate a liquid flow in the treatment tank 2. By connecting the separation liquid supply pipe 11 to the lower part of the treatment tank 2 and configuring it so that the separation liquid is supplied from the lower part to the upper part of the treatment tank 2, it is possible to suppress the 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 increase the contact efficiency between the carrier and the separation liquid. By connecting the separation liquid supply pipe 11 at a height that is relatively 0 (tank bottom) to 0.5, more preferably 0 to 0.3, when the height of the treatment tank 2 is 1, it is possible to improve the efficiency of generating a liquid flow in the tank by supplying the 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 the nitrification promotion treatment and the denitrification promotion treatment are alternately performed in the treatment tank 2. The control means 4 may be connected to the agitator 21, and may also control the agitation conditions of the agitator 21. Furthermore, the control means 4 may 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. The control means 4 controls the aeration in the treatment tank 2 by the aeration means 22 based on the measurement results of the water quality by the measurement means 23, 24, thereby performing a more stable biological treatment that can follow the occurrence of 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 is diluted with dilution water in the dilution tank 3 to obtain treated water.

According to the water treatment device of the embodiment of the present invention, a control means 4 is provided for setting and controlling the aeration conditions of the treatment tank 2 to at least two conditions so that nitrification promotion treatment and denitrification promotion treatment are alternately performed in the treatment tank 2. This allows rough removal of BOD and T-N from the solid-liquid separation liquid of sludge containing septic tank sludge or sewage sludge in one treatment tank 2, so that treated water with a quality that meets the sewage discharge standards can be efficiently obtained from sludge containing septic tank sludge or sewage sludge using a small device.

The following examples of the present invention are provided to aid in the 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 according to the Example (experimental system), water treatment according to the Example, and water treatment according to a Reference Example (control system) for comparing the effects of the water treatment according to the Example were carried out. 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. A representative example of the state of the raw water during the experiment is shown in Table 1.

Raw water was passed through each test system so that the BOD volumetric loading was 0.6 kg-BOD/ ^m3 /d and the TN loading was 0.23 kg-N/ ^m3 /d. Figure 2 shows the treatment flow of the laboratory test. In addition, a control system was used for comparison with a system in which constant aeration was performed. The carriers were polyethylene honeycomb carriers (φ25 mm, thickness 4 mm, specific surface area 800 ^m2 / ^m3 ), which were filled at 30% by volume of each tank.

The raw water inflow conditions and aeration conditions were set for weekdays (Monday to Friday) and holidays (Saturday and Sunday) as shown in Table 2. Note that for "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 mixing was not performed, and for "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 mixing was performed.

The experimental results are shown in Table 3. A sample of treated water mixed with one day's worth of treated water was analyzed. The necessary dilution ratio was calculated based on discharge standards of BOD: 600 mg/L and T-N: 240 mg/L. In the experimental system in which the aeration conditions were switched, the BOD concentration was slightly higher than in the control system, but the T-N concentration was 320 mg-N/L on average, and the nitrogen removal rate was 52.2%. In the control system in which the aeration conditions were not switched, the T-N concentration of the treated water was 648 mg/L on average, and almost no nitrogen was removed. The dilution ratio of the dilution water used for discharge into the sewerage system was also reduced to 1.3 times in the experimental system.

These results demonstrate that stable nitrogen removal and reduced dilution rates can be achieved by setting and switching between at least two aeration conditions to alternate between nitrification and denitrification.

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 (5)

A separated liquid obtained by solid-liquid separation of sludge containing at least one of septic tank sludge and sewage sludge is supplied into an aeration treatment tank containing a biological carrier;
In the treatment tank, aeration conditions in the treatment tank are set to at least two conditions, namely, aeration conditions for the nitrification promotion treatment and aeration conditions for the denitrification promotion treatment having an aeration amount less 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 in the treatment tank;
The aeration conditions are switched to perform biological treatment so that the nitrification promotion treatment and the denitrification promotion treatment are alternately switched in the treatment tank;
The water treatment method further comprises diluting the biologically treated water obtained by the biological treatment so that the water meets sewage discharge standards. The water treatment method according to claim 1, characterized in that the separated liquid is intermittently supplied to the treatment tank. The water treatment method according to claim 1 or 2, characterized in that in the denitrification promotion treatment, the separated liquid contained in the treatment tank is caused to flow by at least one of the following: flowing the separated liquid from the bottom of the treatment tank, stirring the separated liquid with a stirrer, or aeration. a solid-liquid separation device that separates sludge, including 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 performs a nitrification promotion treatment that promotes nitrification and a denitrification promotion treatment that promotes denitrification on the separated liquid;
A control means for controlling the aeration conditions of the treatment tank by setting at least two conditions, that is, 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;
A water treatment device comprising: a dilution tank for diluting the biologically treated water treated in the treatment tank so that the water meets sewage removal standards. The water treatment device according to claim 4, further comprising a separated liquid supply pipe connected to the lower part of the treatment tank and capable of supplying the separated liquid into the treatment tank so as to generate a liquid flow within the treatment tank.