JP2024099484A - Methods for Treating Organic Wastewater - Google Patents

Abstract

To provide a method for treating organic waste water that does not require special equipment or special treatment conditions for sludge volume reduction and exhibits high sludge volume reduction effect.SOLUTION: The present invention provides a method for treating organic waste water by the activated sludge process. The method includes adding, to at least one of tanks containing activated sludge, microbes that lyse activated sludge, and maintaining a redox potential of a mixture containing the organic waste water, the activated sludge, and the microbes in a range of 0 mV or more and 310 mV or less, as measured with a silver-silver chloride electrode. The tank containing activated sludge is a tank for aerobic treatment, sludge concentration or sludge storage.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for treating organic wastewater. More specifically, the present invention relates to a method for treating organic wastewater by aerobic treatment using activated sludge.

The activated sludge process, which is used in the biological treatment of organic wastewater, has the advantages of producing good quality treated water and easy maintenance. For these reasons, the activated sludge process is widely used to purify organic wastewater discharged from domestic wastewater, sewage systems, food factories, pulp factories, chemical factories, etc.

However, the microorganisms in the activated sludge grow in proportion to the amount of organic matter in the organic wastewater being treated, and about 30 to 60 percent of the decomposed BOD (also known as biochemical oxygen demand) is converted into sludge. This results in a large amount of excess sludge.

Excess sludge is discharged outside the purification treatment facility, and some of it is used as fuel for biomass power generation or as fertilizer. The majority of the rest becomes excess sludge waste. Excess sludge waste requires dehydration, transportation, and incineration, and is currently disposed of using large amounts of energy.

As a result, various technologies to reduce the volume of excess sludge are being considered from the perspectives of saving energy, reducing greenhouse gas emissions during incineration, and reducing the effort and costs involved in disposal.

For example, Patent Document 1 proposes a method in which an excess sludge reduction tank is provided in addition to the biological treatment tank used in the biological treatment process of wastewater, and excess sludge is inoculated with myxobacteria having a bacteriolytic effect and allowed to act under aerobic conditions.

Patent Document 2 proposes a method for further reducing the volume of sludge or eliminating its generation by providing a sludge decomposition process in addition to the wastewater treatment process, in which the sludge is lysed at high alkalinity and high temperature and simultaneously decomposed by microorganisms that can grow even in environments unsuitable for microbial growth.

Patent Document 3 proposes a technology for reducing the volume of excess sludge by hydrolyzing the excess sludge concentrated by membrane separation using ozone treatment to reduce its molecular weight, making it biodegradable, and then returning it to an aerobic treatment tank.

Japanese Patent Application Publication No. 6-106198 JP 2000-139449 A Japanese Unexamined Patent Publication No. 8-19789

The methods proposed in Patent Documents 1 to 3 require the addition of a separate excess sludge volume reduction facility to the facilities used in a typical activated sludge process, resulting in an increase in the size of the facilities. In addition, the sludge decomposition process in Patent Document 2 requires the continuous maintenance of a highly alkaline and high-temperature environment, which is unique to biological treatment. The ozone treatment in Patent Document 3 requires an ozone treatment device.

In view of the above, the present invention aims to provide a method for treating organic wastewater that does not require special equipment or special treatment conditions for sludge volume reduction and has a high sludge volume reduction effect.

The present invention includes the following aspects.
[1] A method for treating organic wastewater using an activated sludge method, comprising: adding a microorganism capable of lysing activated sludge to at least one tank containing activated sludge; and maintaining an oxidation-reduction potential of a mixture containing the organic wastewater, the activated sludge, and the microorganism in the tank containing the activated sludge, relative to a silver-silver chloride electrode, of 0 mV or more and 310 mV or less.
[2] The method for treating organic wastewater according to [1], wherein the tank containing the activated sludge is a tank for performing aerobic treatment.
[3] The method for treating organic wastewater according to [1], wherein the tank containing the activated sludge is a sludge concentration tank or a sludge storage tank.
[4] The method for treating organic wastewater according to any one of [1] to [3], wherein the oxidation-reduction potential is maintained at 0 mV or more and 310 mV or less by controlling an amount of diffused air.
[5] The method for treating organic wastewater according to any one of [1] to [4], wherein the microorganisms include at least one bacterium selected from the group consisting of EM bacteria, Bacillus bacteria, and Kataoka bacteria.
[6] The method for treating organic wastewater according to any one of [1] to [4], wherein the microorganism comprises at least one type of bacterium selected from the group consisting of Bacillus bacteria, Lactobacillus bacteria, Tumebacillus bacteria, Pseudomonas bacteria, Eurotium bacteria, Streptomyces bacteria, Mucor bacteria, Saccharomyces bacteria, Cellulosimicrobium bacteria, Neisseria bacteria, Exiguobacterium bacteria, Brevibacillus bacteria, Rhizopus bacteria, and Aspergillus bacteria.
[7] The method for treating organic wastewater according to any one of [1] to [6], further comprising adding photosynthetic bacteria to a tank containing the activated sludge.
[8] The method for treating organic wastewater according to any one of [1] to [7], wherein a sludge storage tank is not used.

According to the above aspect, it is possible to provide a method for treating organic wastewater that does not require special equipment for sludge volume reduction or special treatment conditions for sludge volume reduction and has a high sludge volume reduction effect.

FIG. 1 is a schematic diagram illustrating a method for treating organic wastewater according to an embodiment.

In this specification, the design values of the pollution load L _{P and D} are values specific to the organic wastewater treatment facility. More specifically, the design values of the pollution load L _{P and D} of the organic wastewater treatment facility are values determined from the design stage of the organic wastewater treatment facility, taking into consideration the treatment load expected, for example, from the production capacity of the factory and the number of households to be treated, the aerobic treatment method adopted, and the concentration standards and total water quality regulations to be achieved in the area where the treatment facility is located. Generally, they are described in the completion documents of the facility, or can be easily calculated from the values in the completion documents.

Aerobic treatment is defined herein as contacting microorganisms with pollutants contained in organic wastewater under conditions where oxygen is supplied to the system by means of aeration, stirring, or the like (e.g., under aeration). Aerobic treatment includes contacting microorganisms with pollutants contained in organic wastewater, for example, in an aeration tank. Anaerobic treatment is defined herein as contacting microorganisms with organic wastewater pollutants in a state where the supply of oxygen is cut off.

The organic wastewater treatment method of this embodiment is a method for treating organic wastewater by an activated sludge method, which includes adding microorganisms that lyse activated sludge to at least one tank containing activated sludge, and maintaining the oxidation-reduction potential of the mixture containing the organic wastewater, the activated sludge, and the microorganisms in the tank containing the activated sludge against a silver-silver chloride electrode at 0 mV or more and 310 mV or less.

The organic wastewater treatment method of this embodiment includes treatment by the activated sludge method. FIG. 1 is a schematic diagram illustrating the organic wastewater treatment method according to one embodiment of the present invention. The organic wastewater treatment method according to one embodiment of the present invention can be performed in an organic wastewater treatment facility 1. The organic wastewater treatment facility 1 includes a tank 12 for performing aerobic treatment, a settling tank 13, a sludge concentration tank 14, a sludge storage tank 15, and a pump 16. The tank 12 for performing aerobic treatment is generally called an aeration tank or a reaction tank. The organic wastewater 11 is decomposed in the tank 12 for performing aerobic treatment containing activated sludge, and the pollutants contained in the organic wastewater 11 are decomposed to produce treated water. The treated water discharged from the tank 12 for performing aerobic treatment is left to stand in the settling tank 13. Of the treated water left to stand in the settling tank 13, the supernatant 17 is discharged directly or after appropriate post-treatment such as neutralization, disinfection, and filtration into the environment such as a river, sea, or sewage. The sediment in the settling tank 13, i.e., sludge, is pumped up by a pump 16, and most of it is returned as returned sludge to the tank 12 where aerobic treatment is performed and reused. A part of the sludge is sent to the sludge concentration tank 14 as excess sludge. After being pumped up by the pump 16, the excess sludge is concentrated in the sludge concentration tank 14, where the water is removed and the sludge is concentrated to become concentrated sludge. The eluate 18, which is the water removed during concentration, is either discharged directly or after appropriate post-treatment such as neutralization, disinfection, and filtration, like the treated water, into the environment such as a river, the sea, or a sewage system, or is returned to the tank 12 where aerobic treatment is performed. The concentrated sludge 19 is stored in the sludge storage tank 15 and is finally discharged outside the organic wastewater treatment facility 1 and disposed of.

The organic wastewater treatment equipment 1 in FIG. 1 has two tanks 12 for performing aerobic treatment, but this embodiment is not limited to this, and the tank 12 for performing aerobic treatment may be one or three or more. Also, the organic wastewater treatment equipment 1 in FIG. 1 has a sludge concentration tank 14 and a sludge storage tank 15, but this embodiment is not limited to this, and the sludge concentration tank 14 and the sludge storage tank 15 may not be provided. Also, in the organic wastewater treatment equipment 1 in FIG. 1, the organic wastewater 11 is introduced into the tank 12 for performing aerobic treatment, but this embodiment is not limited to this. If necessary, garbage and sand may be separated from the organic wastewater 11 in a settling tank (sand basin) before being introduced into the tank 12 for performing aerobic treatment. The organic wastewater 11 may first be introduced into a separately provided tank for performing anaerobic treatment, where anaerobic treatment is performed, and then treated in the tank 12 for performing aerobic treatment.

The present embodiment will be described below with reference to FIG. 1. The tank 12 for performing aerobic treatment used in this embodiment is introduced with organic wastewater 11 discharged from domestic wastewater, sewage, food factories, chemical factories, etc. The organic wastewater 11 is mixed with the activated sludge contained in the tank 12 for performing aerobic treatment, and air is diffused through an aeration tube or an aerator, and the tank is appropriately stirred with a pump or a screw, thereby decomposing the organic matter in the organic wastewater 11. The tank 12 for performing aerobic treatment can be a tank for performing aerobic treatment using a conventional standard activated sludge method or various other modified methods. In addition, in order to perform high-load treatment, the tank 12 for performing aerobic treatment can be a fixed bed in which sludge is supported on a fixed carrier, or a fluidized bed in which sludge is supported and flows in the tank with a water current. Furthermore, the tank 12 for performing aerobic treatment can be a tank that is capable of performing solid-liquid separation treatment, as described below, specifically a membrane separation type or batch type tank.

In the organic wastewater treatment method of this embodiment, microorganisms that lyse activated sludge are added to at least one tank containing activated sludge. The tank to which the microorganisms are added may be any tank containing activated sludge, including the tank 12 for aerobic treatment, the settling tank 13, the sludge concentration tank 14, and the sludge storage tank 15. Adding microorganisms that lyse activated sludge to the tank 12 for aerobic treatment is preferable because it reduces the amount of sludge sent to the subsequent treatment stage. The microorganisms that lyse activated sludge lyse the activated sludge (also called excess sludge) that has grown by treating the organic wastewater 11 under aerobic conditions, thereby reducing the volume of the excess sludge. The activated sludge that has been lysed and dissolved in water is again treated by the microorganisms in the activated sludge in the same way as the organic matter in the organic wastewater 11.

The microorganisms that lyse activated sludge are not particularly limited as long as they can lyse activated sludge, but if microorganisms with a high ability to lyse sludge are used, the sludge can be stably reduced in volume even if the pollution load is high, making high-load treatment possible. Specifically, aerobic bacteria and facultative anaerobic bacteria can be mentioned, and from the viewpoint of a high ability to lyse sludge, at least one or more bacteria selected from the group consisting of EM bacteria (e.g., manufactured by EM Research Institute Co., Ltd. or Ueta Lab Co., Ltd.), Bacillus bacteria, and Kataoka bacteria (e.g., manufactured by Kataoka Bio Research Institute Co., Ltd.) are preferable. These bacteria or groups of bacteria tend to produce hydrolytic enzymes (e.g., lysozyme, etc.) of the cell walls (mainly peptidoglycan) of bacteria that make up activated sludge. Therefore, when these bacteria or groups of bacteria are added to activated sludge, they selectively decompose sludge that has become dead and can no longer produce the viscous substance that protects the cell walls, and efficiently lyse excess sludge. In addition, the microorganisms that lyse activated sludge include bacteria of the genus Bacillus, bacteria of the genus Lactobacillus, bacteria of the genus Tumebacillus, bacteria of the genus Pseudomonas, bacteria of the genus Eurotium, bacteria of the genus Streptomyces, bacteria of the genus Mucor, bacteria of the genus Saccharomyces, and the like. The bacteria may be at least one type of bacteria selected from the group consisting of bacteria of the genus Cellulosimicrobium, bacteria of the genus Neisseria, bacteria of the genus Exiguobacterium, bacteria of the genus Brevibacillus, bacteria of the genus Rhizopus, and bacteria of the genus Aspergillus.

The amount of microorganisms that lyse activated sludge added per day, M _D (units/day), is preferably between the design pollution load of the organic wastewater treatment facility, L _P,D × 1.0 × 10 ⁶ and the design pollution load of the organic wastewater treatment facility, L _P,D × 2.0 × 10 ⁹ , and more preferably between the design pollution load of the organic wastewater treatment facility, L _P,D × 2.0 × 10 ⁶ and the design pollution load of the organic wastewater treatment facility, L _P,D × 7.0 × 10 ⁸ .

The microorganisms that lyse activated sludge are added to the tank containing the activated sludge by an appropriate method. For example, the form of the microorganisms that lyse activated sludge to be added may be a microbial preparation that is a powder or liquid containing a large amount of the microorganisms that lyse activated sludge, or a microbial preparation that further contains at least one of additives and preservative stabilizers to improve activity. The microbial preparation may be added to the tank containing activated sludge periodically in small amounts, or a predetermined amount may be added all at once.

It is also preferable to add photosynthetic bacteria to a tank containing activated sludge to which microorganisms that lyse activated sludge are added. Photosynthetic bacteria are bacteria that prey on sulfate-reducing bacteria that may be contained in activated sludge to produce amino acids. Photosynthetic bacteria have the effect of suppressing the generation of hydrogen sulfide, which is the cause of odors generated by the metabolism of sulfate-reducing bacteria, and the effect of the amino acids they produce improving the activity of microorganisms that lyse activated sludge and promoting sludge volume reduction. Specific examples of photosynthetic bacteria include purple sulfur bacteria, green sulfur bacteria, purple non-sulfur bacteria, and green non-sulfur bacteria. Among them, purple sulfur bacteria are preferred because they have a high ability to use sulfide ions as an electron acceptor, can be grown under microaerobic conditions, and are inexpensively available.

The amount of photosynthetic bacteria added is preferably 5.00×10 ³ to 5.00×10 ⁹ per kg of activated sludge (MLSS; Mixed Liquor Suspended Solids) held in a tank containing activated sludge to which microorganisms that lyse activated sludge are added, more preferably 5.00×10 ³ to 5.00×10 ⁶ , and particularly preferably 1.00×10 ⁴ to 1.00×10 ^6. When the amount of photosynthetic bacteria added is 5.00×10 ³ per kg of MLSS, the above-mentioned effect is easily obtained. When the amount of photosynthetic bacteria added is 5.00×10 ⁹ or less per kg of MLSS, the odor emitted by photosynthetic bacteria can be suppressed. In this specification, the method for measuring the ratio of photosynthetic bacteria is in accordance with the method described in JIS K0350-10-10 (2002) General bacteria testing method for water and wastewater. The inoculation method is the smear method.

The photosynthetic bacteria are added to the tank containing the activated sludge in an appropriate manner, similar to the microorganisms that lyse the activated sludge. For example, the photosynthetic bacteria to be added may be in the form of a photosynthetic bacteria preparation that is a powder or liquid containing a large amount of photosynthetic bacteria, or may be a photosynthetic bacteria preparation that further contains at least one of additives and storage stabilizers to improve activity. The photosynthetic bacteria preparation may be added periodically in small amounts to the tank containing the activated sludge, or a predetermined amount may be added all at once.

In order to improve the activity of the microorganisms that lyse activated sludge, it is preferable to add an additive to the tank containing activated sludge together with the microorganisms that lyse activated sludge. Examples of the additive include carbohydrates, amino acids, minerals, steroids, and their glycosides and humic substances. Here, in low-temperature environments such as winter or high-latitude regions where the water temperature drops to about 10°C to 15°C, and in high-temperature environments such as summer or low-latitude regions where the water temperature rises to about 40°C to 45°C, the activity of the microorganisms that lyse activated sludge decreases to about 50% to 80% compared to environments relatively comfortable for microorganisms such as spring or autumn. From the viewpoint of maintaining a high lysis effect even in such low-temperature or high-temperature environments, the additive is preferably at least one of saponin and fulvic acid, and it is more preferable to add both saponin and fulvic acid as additives to the tank containing activated sludge.
Saponin is a general term for glycosides consisting of saponin, which is made up of a steroid or triterpene skeleton, and sugar, and is generally extracted from plant roots, leaves, stems, etc. Examples of saponins include soybean saponin, ginseng saponin, and tea seed saponin.
Fulvic acid is a general term for the acid-soluble components of humic substances, the final products of the decomposition of plants by microorganisms.

The amount of saponin added is preferably 5 mg to 500 mg, more preferably 10 mg to 250 mg, per 1.0 x 10 ⁶ bacteria of the microorganism that lyses activated sludge. When the amount of saponin added is 5 mg or more per 1.0 x 10 ⁶ bacteria of the microorganism that lyses activated sludge, a high lysis effect can be maintained even in a low or high temperature environment. When the amount of saponin added is 500 mg or less per 1.0 x 10 ⁶ bacteria of the microorganism that lyses activated sludge, the surface activity effect of saponin suppresses foaming of the treated water, making it easier to separate solid and liquid. In this specification, the method for measuring the proportion of bacteria that lyse activated sludge is in accordance with the method described in JIS K0350-10-10 (2002) General bacteria test method for water and wastewater. The inoculation method is the smear method.

The amount of fulvic acid added is preferably 0.5 mg to 50 mg, more preferably 1 mg to 25 mg, per 1.0 x ¹⁰⁶ of the microorganisms that lyse activated sludge. If the amount of fulvic acid added is 0.5 mg or more per 1.0 x ¹⁰⁶ of the microorganisms that lyse activated sludge, a high lysis effect can be maintained even in low or high temperature environments. If the amount of fulvic acid added is 50 mg or less per 1.0 x ¹⁰⁶ of the microorganisms that lyse activated sludge, the fulvic acid dissolves sufficiently in water and the pH of the water is unlikely to change.

When both saponin and fulvic acid are added, it is preferable that the amount of saponin added is 5 mg to 500 mg per 1.0 x ¹⁰⁶ added bacteria number of the microorganisms that lyse the activated sludge, and the amount of fulvic acid added is 0.5 mg to 50 mg per 1.0 x 106 added bacteria number of the microorganisms that lyse the activated sludge, and it is more preferable that the amount of saponin added is 10 mg to 250 mg per 1.0 x ¹⁰⁶ added bacteria number of the microorganisms that lyse the activated sludge, and the amount of fulvic acid added is 1 mg to 25 mg per 1.0 x 106 added bacteria number of the microorganisms that lyse the activated sludge.

In the organic wastewater treatment method of this embodiment, the oxidation-reduction potential of the mixture containing the organic wastewater, the activated sludge, and the microorganisms in the tank containing activated sludge is maintained at 0 mV or more and 310 mV or less against a silver-silver chloride electrode. If the oxidation-reduction potential is 0 mV or more, the putrefaction of the sludge is suppressed, thereby suppressing the generation of odor, and the effect of reducing the volume of the sludge by the microorganisms that lyse the activated sludge can be maintained at a high level. Furthermore, if the oxidation-reduction potential is 310 mV or less, the disintegration of flocs in the activated sludge is suppressed, and the separation of the treated water and the sludge in the solid-liquid separation process described later can be facilitated. A preferred oxidation-reduction potential may be 50 mV or more, 200 mV or less, or 150 mV or less. In particular, when the oxidation-reduction potential is adjusted in the sludge concentration tank described later, the sludge is prone to putrefaction, so that the oxidation-reduction potential is preferably 50 mV or more, and more preferably 80 mV or more. The upper and lower limit values of the oxidation-reduction potential can be combined arbitrarily.

When the redox potential becomes low, the redox potential can be increased by administering an additive to the microorganism, adding symbiotic bacteria, or increasing the amount of diffused air. Examples of additives include those mentioned above. Examples of symbiotic bacteria include the bacteria mentioned above. When the redox potential becomes high, the redox potential can be decreased by closing a tank containing activated sludge with a lid or by reducing the amount of diffused air. From the viewpoint of immediate effect and easy control, it is preferable to control the redox potential by controlling the amount of diffused air. The redox potential may be maintained between 0 mV and 310 mV using a system that inputs the measured value of the redox potential and automatically increases or decreases the amount of diffused air based on the input measured value.

In the organic wastewater treatment method of this embodiment, the BOD-SS load of the tank performing aerobic treatment may be kept below 0.1 kg/kg-day. By keeping the BOD-SS load of the tank performing aerobic treatment below 0.1 kg/kg-day, the amount of excess sludge generated can be significantly reduced, and under some conditions, the generation of excess sludge can be eliminated. A more preferable range of the BOD-SS load of the tank performing aerobic treatment is below 0.08 kg/kg-day. There is no particular limit to the lower limit of the BOD-SS load of the tank performing aerobic treatment, but a sufficient amount of wastewater can be treated as long as the BOD-SS load is 0.015 kg/kg-day or more.

In this specification, the BOD-SS load is defined as the mass of BOD (biochemical oxygen demand) per day introduced into the aerobic treatment tank divided by the mass of activated sludge. In typical organic wastewater treatment using the activated sludge method, the BOD-SS load is often operated at 0.2 to 0.4 kg/kg per day.

In order to keep the BOD-SS load of the aerobic treatment tank low, it is possible to reduce the amount of sludge withdrawn from the aerobic treatment tank or to operate the tank without withdrawing any sludge at all, but simply reducing the amount of sludge withdrawn will lead to the disintegration of flocs, reducing solid-liquid separation and resulting in sludge flowing into the treated water. By adding microorganisms that lyse activated sludge to the aerobic treatment tank, the disintegrated sludge is lysed, and by reducing the amount of sludge withdrawn from the aerobic treatment tank or operating the tank without withdrawing any sludge at all, it is possible to prevent a decline in the quality of the effluent and make it easier to keep the BOD-SS load low.

When aeration is performed in a tank containing activated sludge, air bubbles should be diffused at a bubble number concentration of 15,000 bubbles/ ^m3 or more. The bubble number concentration (number of bubbles per unit volume) is an index of the average diameter of bubbles, and the higher the bubble number concentration, the smaller the average diameter of the bubbles. The bubble number concentration is determined by using a transparent tank containing a specified amount of water, diffusing for 60 seconds, stopping the aeration, and then counting the number of bubbles by image analysis of a photograph taken immediately after.

Here, bubbles with a large particle size have a strong ability to rise in water and reach the water surface in a short time after generation. On the other hand, bubbles with a small particle size have a weak ability to rise in water and have the property of remaining in water for a long time after generation. In addition, when the same volume of air is diffused, the sum of the surface areas of bubbles with a smaller average particle size is larger. Therefore, by diffusing air with a small average particle size, the oxygen dissolving ability in the tank can be improved, the amount of air to be diffused can be reduced, and the input energy can be reduced, and even if the tank in which the aerobic treatment is performed is shallow, oxygen can be easily dissolved sufficiently. The upper limit of the bubble number concentration of the diffused air bubbles is not particularly limited, but it is preferable that the bubble number concentration is 400,000 pieces/m3 or ^less from the viewpoint of stable diffusion from the air diffuser without being defeated by the pressure of water or sludge.

The treated water treated in the aerobic treatment tank 12 is subjected to solid-liquid separation together with the sludge dispersed in the treated water. The solid-liquid separation may be performed by a known method. For example, the solid-liquid separation may be performed by precipitating and separating the treated water and sludge in a settling tank 13 separate from the aerobic treatment tank 12 as shown in FIG. 1, or may be performed by separating the treated water and sludge using a membrane separation module. The membrane separation module may be provided outside the aerobic treatment tank 12 or within the aerobic treatment tank 12.

The sludge separated in the settling tank 13 in FIG. 1 by the solid-liquid separation process is mostly returned to the tank 12 where aerobic treatment is performed again. If necessary, an appropriate amount of sludge may be extracted and disposed of as excess sludge. The disposal of the excess sludge may be performed by a known method, for example, by concentrating it in the sludge concentration tank 14 as shown in FIG. 1 and then drying it, or by using a separate sludge volume reduction tank. By using the organic wastewater treatment method of this embodiment, the amount of excess sludge generated can be significantly reduced or even eliminated, so that special equipment for disposing of the excess sludge and the treatment of the excess sludge can be omitted.

In a typical activated sludge process, the sludge separated by solid-liquid separation has a solid content of about 0.4 to 1% by mass, and is mostly water. Therefore, the sludge is subjected to a sludge concentration process, and is concentrated until the solid content reaches about 1 to 4% by mass. The sludge concentration process is carried out in a sludge concentration tank 14 provided separately from the tank in which the aerobic treatment is carried out. The sludge concentration process may be carried out by a known method, such as gravity concentration, centrifugal concentration, atmospheric flotation concentration, and belt filtration concentration. However, with the organic wastewater treatment method of this embodiment, the amount of excess sludge generated can be significantly reduced or eliminated, so the sludge concentration process can be omitted.

In the sludge concentration treatment in the sludge concentration tank 14, microorganisms that lyse activated sludge may be added in the same manner as described in the tank containing activated sludge, and the volume of the sludge may be reduced by maintaining the oxidation-reduction potential of the mixture containing the organic wastewater, the activated sludge, and the microorganisms in the tank containing the activated sludge against a silver-silver chloride electrode at 0 mV or more and 310 mV or less. Also, microorganisms that lyse activated sludge may be added in both the tank 12 performing the aerobic treatment tank and the sludge concentration tank 14, and the oxidation-reduction potential of the mixture containing the organic wastewater, the activated sludge, and the microorganisms in the tank containing the activated sludge (i.e., the tank 12 performing the aerobic treatment tank and the sludge concentration tank 14) against a silver-silver chloride electrode at 0 mV or more and 310 mV or less may be reduced. Here, the lower limit of the oxidation-reduction potential of the mixture in the sludge concentration tank 14 is preferably 50 mV or more, more preferably 80 mV or more. In addition, the upper limit of the oxidation-reduction potential of the mixture in the sludge concentration tank 14 is preferably 200 mV or less, and more preferably 150 mV or less.

In addition, because sludge is prone to putrefaction, adding photosynthetic bacteria to the sludge concentration tank 14 provides a higher effect than adding photosynthetic bacteria to the tank 12 where the aerobic treatment described above is performed. The type and amount of photosynthetic bacteria added can be the same as those described above.

In most cases, the sludge concentration tank 14 is equipped with an air pipe that is used during cleaning. Therefore, the air pipe can be used to diffuse air into the sludge concentration tank 14, thereby adjusting the redox potential.

In a typical activated sludge process, the concentrated excess sludge is discharged outside the purification treatment facility system, but is sent to and stored in the sludge storage tank 15 where the excess sludge is temporarily stored until it is treated or transported. Here, particularly when sludge concentration is performed by gravity concentration, the concentration property changes due to the influence of air and water temperature, and the water quality of the elution liquid 18 may not be stable. When applying the embodiment of the present invention to an organic wastewater treatment facility having a sludge storage tank 15, it may be difficult to control the water quality when trying to reduce the volume of sludge in the sludge concentration tank 14 by the above-mentioned method. In such a case, or in the case of an organic wastewater treatment facility that does not simply have a sludge concentration tank 14, microorganisms that lyse activated sludge may be added to the sludge storage tank 15 in the same manner as described for the sludge concentration tank 14, and the oxidation-reduction potential of the mixture containing the organic wastewater, the activated sludge, and the microorganisms in the tank containing the activated sludge may be maintained at 0 mV or more and 310 mV or less to reduce the volume of sludge.

As described above, by using the organic wastewater treatment method of this embodiment, it is possible to lyse excess sludge with high efficiency in at least one tank containing activated sludge. Therefore, no special equipment for sludge volume reduction or special treatment conditions for sludge volume reduction are required, and a high sludge volume reduction effect can be obtained. In particular, by using the organic wastewater treatment method of this embodiment, it is possible to eliminate the generation of excess sludge in some cases. In such cases, it is possible to treat organic wastewater by the activated sludge method without using the sludge storage tank 15 in which the excess sludge is stored until it is treated. If the sludge storage tank 15 can be omitted, it is preferable because it provides even more advantages such as a smaller facility, reduced energy input, and suppression of odor generation caused by the decay of the stored sludge.

The present invention will be further explained below with reference to examples, but the present invention is not limited to these examples in any way.

(Examples 1 to 3)
A mixture of organic wastewater flowing into an agricultural village drainage facility in Komatsu City and sewage sludge (activated sludge) was prepared in a 1000 mL beaker so that the activated sludge concentration was 5000 to 10000 mg/L. A microbial preparation was added to this mixture as a microorganism that lyses the activated sludge to prepare a sample. The amount of the microbial preparation added was 0.6 mL per 1 g of the solid mass (MLSS) of the activated sludge. Thereafter, the redox potential of the sample in the 1000 mL beaker against a silver-silver chloride electrode was controlled to be 0 mV or more and 200 mV or less for 7 days. The water temperature at this time was 25°C. The redox potential against a silver-silver chloride electrode was measured with an ORP meter (manufactured by Horiba Ltd., D-200-1, electrode: 9300-10D). The microbial preparations used were microbial preparations containing EM bacteria (Uetadron, manufactured by Ueta Lab, Ltd., Example 1), Bacillus bacteria (Elbic BZ, manufactured by Hinode Sangyo Co., Ltd., Example 2), or Kataoka bacteria (manufactured by Kataoka Bio Research Institute Co., Ltd., Example 3) at a general bacterial count of 1.0 x ¹⁰⁶ cells/mL.

The MLSS concentration of the sample immediately after preparation was measured using an MLSS meter (IM-100P, manufactured by Iijima Electronics Co., Ltd.) after zero calibration and was recorded as the initial MLSS concentration (mg/L). The MLSS concentration of the sample after 7 days was measured using the same MLSS meter and was recorded as the MLSS concentration after 7 days (mg/L).
The sludge volume reduction rate (%) was calculated using the following formula.
Sludge volume reduction rate (%) = {(MLSS concentration after 7 days/initial MLSS concentration) - 1} x 100

(Examples 4 to 6)
A photosynthetic bacteria preparation containing 1.0 x ¹⁰⁶ cells/mL of purple sulfur bacteria, which are photosynthetic bacteria, was added to the sample to prepare the sample. The amount of photosynthetic bacteria preparation added was 0.6 μL per 1 g of activated sludge solid mass (MLSS). Except for using this sample, the experiments of Examples 4 to 6 were carried out in the same manner as Examples 1 to 3.

(Example 7)
The experiment was carried out in the same manner as in Example 3, except that the redox potential against the silver-silver chloride electrode was controlled to be 240 mV for 7 days.

(Comparative Example 1)
The experiment was carried out in the same manner as in Example 1, except that the samples were prepared without adding a microbial preparation.

(Comparative Example 2)
The experiment was carried out in the same manner as in Comparative Example 1, except that the redox potential against the silver-silver chloride electrode was controlled to be less than 0 mV for 7 days.

(Comparative Example 3)
The experiment was carried out in the same manner as in Example 4, except that no microbial preparation was added.

(Comparative Example 4)
The experiment was carried out in the same manner as in Comparative Example 3, except that the redox potential against the silver-silver chloride electrode was controlled to be less than 0 mV for 7 days.

Each example and comparative example was performed five times. Table 1 shows the microbial preparations for Examples 1 to 7 and Comparative Examples 1 to 4, the presence or absence of addition of photosynthetic bacteria, and the minimum and maximum values of the oxidation-reduction potential and sludge volume reduction rate.

As shown in Table 1, the sludge volume reduction rate was higher in Examples 1 to 7 than in Comparative Examples 1 to 4. Furthermore, a comparison of Examples 1 to 3 with Comparative Examples 1 and 4 showed that the sludge volume reduction rate was lower when microorganisms that lyse activated sludge were not added. Furthermore, a comparison of Comparative Example 1 with Comparative Example 2 showed that the sludge volume reduction rate was even lower when the oxidation-reduction potential was set to less than 0 mV.

Comparing Examples 1 to 3 with Examples 4 to 6, it was found that the sludge volume reduction rate improved when photosynthetic bacteria were further added.

(Examples 8 to 10)
A mixture of organic wastewater flowing into an agricultural village drainage facility in Komatsu City and sewage sludge (activated sludge) was adjusted to a total volume of 1.8 L in a test vessel so that the activated sludge concentration was 5000 to 10000 mg/L. A microbial preparation was added to this mixture as a microorganism that lyses the activated sludge to prepare a sample. The amount of the microbial preparation added was 0.6 mL per 1 g of solid mass (MLSS) of activated sludge. Thereafter, the redox potential of the sample in the test vessel against a silver-silver chloride electrode was controlled to be 100 mV or more and 140 mV or less for 14 days. The water temperature at this time was 25°C. The redox potential against a silver-silver chloride electrode was measured with an ORP meter (manufactured by Horiba Ltd., D-200-1, electrode: 9300-10D). As the microbial preparations, a microbial preparation containing EM bacteria (Uetadron manufactured by Ueta Lab, Ltd., Example 8), a microbial preparation containing Bacillus bacteria (Elbic BZ manufactured by Hinode Sangyo Co., Ltd., Example 9), or a microbial preparation containing Kataoka bacteria (Kataoka Bio Research Co., Ltd., Example 10) was used after adjusting the general bacterial count to 1.0 x ¹⁰⁶ /mL. After 14 days, the samples were evaluated for sludge volume reduction rate, activated sludge settling rate (SV30), and transparency.

[Sludge volume reduction rate]
The MLSS concentration of the sample immediately after preparation was measured using an MLSS meter (IM-100P, manufactured by Iijima Electronics Co., Ltd.) after zero calibration, and was recorded as the initial MLSS concentration (mg/L). The MLSS concentration of the sample after 14 days was measured using the same MLSS meter, and was recorded as the MLSS concentration after 14 days (mg/L).
The sludge volume reduction rate (%) was calculated using the following formula.
Sludge volume reduction rate (%) = {(MLSS concentration after 14 days/initial MLSS concentration) - 1} x 100

[Activated sludge settling rate (SV30)]
The activated sludge settling rate (SV30) was measured according to the method described in JIS B9944 (1987). The smaller the SV30, the easier the solid-liquid separation of the treated water is.

[Transparency]
The transparency was measured according to the method described in JIS K0102 (2019). It can be seen that the higher the transparency, the higher the treated water quality.

(Examples 11 to 13)
A photosynthetic bacteria preparation containing 1.0 x ¹⁰⁶ cells/mL of purple sulfur bacteria, which are photosynthetic bacteria, was added to the samples of Examples 8 to 10 to prepare samples. The amount of photosynthetic bacteria added was 0.6 μL per 1 g of activated sludge solid mass (MLSS). Except for using these samples, the experiments of Examples 11 to 13 were carried out using the same procedures as those of Examples 8 to 10.

(Examples 14 to 15)
The experiments of Examples 14 and 15 were carried out in the same manner as in Examples 8 and 11, except that the redox potential against a silver-silver chloride electrode was controlled to be 0 mV or more and 40 mV or less for 14 days.

(Examples 16 to 17)
The experiment was carried out in the same manner as in Examples 8 and 10, except that the redox potential against the silver-silver chloride electrode was controlled to be 210 mV or more and 250 mV or less for 14 days.

(Examples 18 to 19)
The experiments of Examples 18 and 19 were carried out in the same manner as in Examples 8 and 10, except that the redox potential against a silver-silver chloride electrode was controlled to be 270 mV or more and 310 mV or less for 14 days.

(Comparative Example 5)
The experiment was carried out in the same manner as in Example 8, except that the samples were prepared without adding the microbial preparation.

(Comparative Example 6)
The experiment was carried out in the same manner as in Comparative Example 5, except that the redox potential against the silver-silver chloride electrode was controlled to be −50 mV or more and −10 mV or less for 14 days.

(Comparative Examples 7 to 8)
The experiment was carried out in the same manner as in Examples 8 and 11, except that the redox potential against the silver-silver chloride electrode was controlled to be −50 mV or more and −10 mV or less for 14 days.

(Comparative Example 9)
The experiment was carried out in the same manner as in Example 8, except that the redox potential against the silver-silver chloride electrode was controlled to be 320 mV or more and 350 mV or less for 14 days.

(Comparative Example 10)
The experiment was carried out in the same manner as in Example 11, except that the samples were prepared without adding the microbial preparation.

(Comparative Example 11)
The experiment was carried out in the same manner as in Comparative Example 10, except that the redox potential against the silver-silver chloride electrode was controlled to be −50 mV or more and −10 mV or less for 14 days.

The microbial preparations used in Examples 8 to 19, the presence or absence of photosynthetic bacteria, the redox potential, the sludge volume reduction rate, the activated sludge settling rate (referred to as SV30), and the transparency are shown in Table 2, while the microbial preparations used in Comparative Examples 5 to 11, the presence or absence of photosynthetic bacteria, the redox potential, the sludge volume reduction rate, the activated sludge settling rate (referred to as SV30), and the transparency are shown in Table 3.

In Examples 8 to 19, in which the redox potential was 0 mV or more and 310 mV or less, the sludge volume reduction rate was high and the SV30 value was low. In contrast, in Comparative Examples 5, 6, 10, and 11, which did not contain a microbial preparation, and Comparative Examples 7 and 8, in which the redox potential was less than 0 V, the sludge volume reduction rate was low. In Comparative Example 9, in which the redox potential was more than 310 mV, the SV30 value was high.

1...organic wastewater treatment equipment, 11...organic wastewater, 12...aerobic treatment tank, 13...sedimentation tank, 14...sludge thickening tank, 15...sludge storage tank, 16...pump, 17...supernatant, 18...supernatant, 19...thickened sludge.

Claims (8)

A method for treating organic wastewater using an activated sludge process, comprising adding microorganisms that lyse activated sludge to at least one tank containing activated sludge, and maintaining the oxidation-reduction potential of the mixture containing the organic wastewater, the activated sludge, and the microorganisms in the tank containing the activated sludge against a silver-silver chloride electrode between 0 mV and 310 mV. The method for treating organic wastewater according to claim 1, wherein the tank containing the activated sludge is a tank for performing aerobic treatment. The method for treating organic wastewater according to claim 1, wherein the tank containing the activated sludge is a sludge concentration tank or a sludge storage tank. The method for treating organic wastewater according to any one of claims 1 to 3, wherein the oxidation-reduction potential is maintained between 0 mV and 310 mV by controlling the amount of aeration. The method for treating organic wastewater according to any one of claims 1 to 3, wherein the microorganisms include at least one bacterium selected from the group consisting of EM bacteria, Bacillus bacteria, and Kataoka bacteria. The method for treating organic wastewater according to any one of claims 1 to 3, wherein the microorganisms include at least one type of bacteria selected from the group consisting of bacteria of the genus Bacillus, bacteria of the genus Lactobacillus, bacteria of the genus Tumebacillus, bacteria of the genus Pseudomonas, bacteria of the genus Eurotium, bacteria of the genus Streptomyces, bacteria of the genus Mucor, bacteria of the genus Saccharomyces, bacteria of the genus Cellulosimicrobium, bacteria of the genus Neisseria, bacteria of the genus Exiguobacterium, bacteria of the genus Brevibacillus, bacteria of the genus Rhizopus, and bacteria of the genus Aspergillus. The method for treating organic wastewater according to any one of claims 1 to 3, further comprising adding photosynthetic bacteria to the tank containing the activated sludge. The method for treating organic wastewater according to any one of claims 1 to 3, which does not use a sludge storage tank.

Images