Treatment method for wastewater containing organic matter

Abstract

Provided is a treatment method for wastewater containing organic matter which comprises: a biotreatment step in which wastewater containing organic matter is aerobically biotreated using a fluidized-bed support; a flocculation step in which a flocculant is added to the biotreated water from the biotreatment step to conduct flocculation; and a solid-liquid separation step in which the flocculation-treated water from the flocculation step is subjected to solid-liquid separation by membrane separation. The treatment method for wastewater containing organic matter is characterized in that the number of microanimals, including metazoan animals, deposited on the fluidized-bed support is kept at 20,000 or less per mL of the tank volume.

Description

Method for treating wastewater containing organic matter

[0001] The present invention relates to a method for treating wastewater containing organic matter, and more particularly to a method for treating wastewater containing organic matter by biological treatment, followed by coagulation treatment and solid-liquid separation.

[0002] In recent years, methods for treating wastewater containing organic matter, in which wastewater containing organic matter is subjected to aerobic biological treatment in an aeration tank followed by coagulation treatment or membrane separation treatment, have been used in a wide range of fields. By subjecting the biologically treated water to solid-liquid separation using an ultrafiltration (UF) membrane or advanced treatment mainly using a reverse osmosis (RO) membrane, organic and inorganic substances in the biologically treated water can be removed and reused. However, if substances that clog the membrane enter the water, it becomes difficult for the water to pass through, making cleaning necessary.

[0003] Fluctuations in the quality of the biologically treated water will also cause fluctuations in the quality of the water flowing into the membrane separation treatment process at the subsequent stage, leading to membrane blockage. To prevent this, the raw water quality of the biological treatment, the treated water quality, the inflow load, and operating conditions (water temperature, pH, DO, etc.) are carefully controlled. However, changes in the biota that cannot be fully understood by these parameters can cause the membrane treatment situation to deteriorate, making it necessary to urgently clean the membrane or reduce the amount of water passing through.

[0004] Biological treatment includes activated sludge (biological sedimentation type) that uses suspended microorganisms (sludge), membrane activated sludge, etc., and biofilm method that uses sludge attached to a carrier. Among the biofilm methods, the fluidized bed method that uses a fluidized carrier is capable of high-load operation and requires a small installation area, so it has been widely adopted in biological treatment for wastewater recovery in recent years.

[0005] Because the fluidized bed method is easier to operate than the suspended sludge method, there has been little attention paid to the management and control of biota, and the impact of inadequate management of biota on subsequent advanced treatment has been overlooked.

[0006] JP 2013-121558 A

[0007] When organic matter-containing wastewater is biologically treated and then subjected to membrane separation to recover treated water, membrane separation devices with fine pores, such as reverse osmosis (RO) membranes, are widely used because they can remove even high-molecular-weight organic substances and produce high-quality treated water. However, because the membrane pores of these membrane separation devices are small, if the concentration of organic matter entering the device is high, the organic matter tends to accumulate on the membrane surface, causing a significant increase in filtration resistance and making it difficult for water to pass through.

[0008] The present invention aims to stabilize the treatment in the membrane separation step in a method for treating organic matter-containing wastewater, which involves aerobic biological treatment using a fluidized bed carrier, followed by coagulation treatment and then membrane separation.

[0009] The present invention has the following gist.

[0010] [1] A method for treating wastewater containing organic matter, comprising: a biological treatment step of aerobic biological treatment of wastewater containing organic matter using a fluidized bed carrier; a flocculation treatment step of flocculating the biologically treated water from the biological treatment step by adding a flocculant; and a solid-liquid separation step of separating the flocculated water from the flocculation treatment step into solid and liquid form by membrane separation, wherein the number of microscopic animals, including metazoans, attached to the fluidized bed carrier is maintained at 20,000 or less per mL of tank volume.

[0011] [2] The method for treating organic matter-containing wastewater according to [1], further comprising an advanced treatment step of removing dissolved substances contained in the water by membrane separation downstream of the solid-liquid separation step.

[0012] [3] The method for treating wastewater containing organic matter according to [1], wherein the fluidized bed carrier is a sponge carrier.

[0013] [4] The method for treating organic matter-containing wastewater according to [1], wherein the number of microscopic animals is the sum of the number of metazoans and the number of protozoans multiplied by a predetermined coefficient (a value selected from 0.01 to 0.5).

[0014] [5] The method for treating organic matter-containing wastewater according to [1], wherein when the number of microscopic animals per mL of the tank volume exceeds 20,000, the amount of flocculant added in the flocculation step is increased to 1.2 to 3.0 times the average amount added up to that point.

[0015] [6] The method for treating organic matter-containing wastewater according to [1], wherein when the number of microscopic animals per mL of the tank volume exceeds 20,000, aeration volume in the biological treatment step is increased to 1.2 to 2.5 times the average aeration volume up to that point.

[0016] [7] The method for treating wastewater containing organic matter according to [1], wherein when the number of microscopic animals per mL of the tank volume exceeds 20,000, the aeration volume in the biological treatment step is increased to 0.7 to 0.9 times the average aeration volume up to that point, and measures are taken to maintain the dissolved oxygen concentration at 2 mg / L or more.

[0017] [8] The method for treating organic matter-containing wastewater according to [1], wherein, when the number of microscopic animals per mL of the tank volume exceeds 20,000, intermittent aeration is performed as a countermeasure, with the ratio of aeration time / aeration stop time in the biological treatment step being 2 to 10.

[0018] [9] The method for treating wastewater containing organic matter according to any one of [5] to [8], wherein the countermeasure is continued until the number of microscopic animals per mL of the total volume is reduced to 10,000 or less.

[0019]

[10] The BOD volume load of the biological treatment tank in which the biological treatment step is performed is 0.2 to 2.0 kg / m ３ / d [1] The method for treating organic matter-containing wastewater.

[0020]

[11] A method for treating wastewater containing organic matter according to [1], in which the correlation between the number of microscopic animals attached to the fluidized bed carrier and the number of microscopic animals in the biologically treated water from the biological treatment process is obtained in advance, the number of microscopic animals attached to the fluidized bed carrier is estimated from the measured value of the number of microscopic animals in the biologically treated water and the correlation, and this estimated value is used as the number of microscopic animals.

[0021] In the present invention, when organic matter-containing wastewater is subjected to aerobic biological treatment using a fluidized bed carrier, followed by coagulation treatment and then membrane separation, the number of microscopic animals adhering to the fluidized bed carrier can be appropriately controlled, thereby enabling stable membrane separation. For example, it becomes possible to operate the membrane separation means at a normal membrane cleaning frequency.

[0022] 1 is a flow diagram of a method for treating wastewater containing organic matter according to an embodiment.

[0023] The method for treating organic matter-containing wastewater of the present invention includes a biological treatment step in which organic matter-containing wastewater is aerobic biologically treated using a fluidized bed carrier, a coagulation treatment step in which a coagulant is added to the biologically treated wastewater from the biological treatment step to perform coagulation treatment, and a solid-liquid separation step in which the coagulated treated water from the coagulation treatment step is separated into solid and liquid forms by membrane separation. Examples of organic matter-containing wastewater to be treated in the present invention include, but are not limited to, wastewater from electronics, liquid crystal, and semiconductor factories, and wastewater from chemical factories. The organic matter concentration in the organic matter-containing wastewater is preferably, but is not limited to, about 20 to 1000 mg / L, particularly about 50 to 500 mg / L, in terms of TOC.

[0024] The present invention will now be described in more detail with reference to Figure 1. Figure 1 shows an example of the present invention.

[0025] Wastewater containing organic matter undergoes aerobic biological treatment in a biological treatment tank (aeration tank) 1. Reference numeral 1a denotes an aeration pipe, and 1b denotes a carrier. The biologically treated water undergoes coagulation treatment in a first coagulation tank 2 where a coagulant is added, and then the water is slowly stirred in a second coagulation tank 3 to allow a coagulation reaction to proceed, after which it undergoes sedimentation treatment in a settling tank 4, where the precipitate containing solids resulting from the biological treatment is separated into solids and liquids.

[0026] Examples of inorganic flocculants used in the flocculation treatment include iron-based flocculants such as ferric chloride and polyiron sulfate, and aluminum-based flocculants such as aluminum sulfate, aluminum chloride and polyaluminum chloride, with iron-based flocculants being preferred in terms of flocculation effect. These inorganic flocculants may be used alone or in combination of two or more.

[0027] During the coagulation treatment, a pH adjuster is added as necessary to adjust the pH to a suitable level for the inorganic coagulant used. Regarding pH conditions, for example, it is effective to react iron-based coagulants at a pH of 5 to 7, preferably 5.5 to 6.5, while it is effective to react aluminum-based coagulants at a pH of 4.5 to 5.5 and then adjust the pH to 6 to 7. Examples of pH adjusters include hydrochloric acid (HCl), sulfuric acid (H ２ SO ４ Examples include acids such as ammonium hydroxide (NaOH) and alkalis such as sodium hydroxide (NaOH). Polymer flocculants can be either anionic or nonionic, but when increasing the amount of inorganic flocculant, it is necessary to conduct a flocculation test and adjust the amount added.

[0028] The separated water (supernatant water) obtained in the settling tank 4 is filtered in a filtration device 5, and the filtrate is subjected to membrane separation treatment in an RO membrane separation device 6 as an advanced treatment means, and the permeate is used as treated water.

[0029] Instead of the settling tank 4, a pressure flotation device, an ultrafiltration (UF) membrane device or the like may be used as a solid-liquid separation means.

[0030] When a UF (ultrafiltration) membrane device is used instead of an RO (reverse osmosis) membrane device, the filtration device 5 can be omitted, but it is preferable to perform a sterilization treatment before the UF membrane treatment. Depending on the use of the treated water, the UF membrane permeate may be used as the treated water, in which case the UF membrane treatment is the final treatment. In this case, the UF membrane device, like the RO membrane, is also a device that is affected by organic matter remaining in the biologically treated water.

[0031] When an RO membrane separation device is used as the advanced treatment means, it is preferable to remove SS from the water by providing a filtration device 5 in the upstream stage of the RO membrane separation device 6 as shown in Figure 1. The filtration device 5 may be a packed bed type filtration device filled with a filter material such as sand, anthracite, or activated carbon.

[0032] In this embodiment, the biological treatment tank 1 is a fluidized bed type, and treatment is performed by a pass-through method (without returning sludge). The shape of the fluidized bed carrier may be any shape, such as a sphere, pellet, hollow cylinder, filament, or plate, and the size preferably has an average diameter of about 0.1 to 10 mm. The material may be any material, such as a natural material, inorganic material, or polymer material, and a gel-like substance may also be used.

[0033] Particularly preferred is a polyurethane sponge carrier consisting of cubes of about 3 to 10 mm square, which, from the viewpoint of sludge retention, has a cell count of 30 cells / 25 mm or more and a specific surface area of ​​2000 m ２ / m ３ In order to maintain the fluidity of the carrier even when sludge settles at a high concentration, the density of the polyurethane sponge carrier should be 20 to 60 kg / m ３ In order to prevent the sludge from peeling off due to excessive deformation of the carrier, the volume expansion rate of the polyurethane sponge carrier is preferably about 100 to 120%.

[0034] By filling the carrier, the BOD volume load of the biological treatment tank 1 is reduced to 0.75 kg / m ３ / d or more (0.3kg / m at TOC load) ３ The carrier loading rate is preferably 50% or less, and more preferably 20 to 40%.

[0035] Unlike the flotation method, this fluidized bed treatment method does not require management of sludge retention volume, is capable of high-load operation, and has been considered an easy-to-operate treatment method.Furthermore, when treating and recovering wastewater, it is possible to operate at high loads compared to the flotation method, and is therefore widely adopted.

[0036] However, in the case of fluidized bed treatment, if the management of attached sludge is insufficient, depending on the operating conditions, higher microorganisms called microorganisms (metazoans, protozoans) will grow rapidly, and the metabolites they produce will increase the organic matter concentration after biological treatment, affecting the subsequent advanced treatment (RO membrane treatment in the case of Figure 1). With a flotation treatment method, the number of microorganisms can be controlled by managing the SRT, but with a fluidized bed treatment method, as mentioned above, if the management of attached sludge is insufficient, the number of microorganisms in the fluidized bed carrier will periodically increase sharply, requiring management specific to fluidized beds.

[0037] In the present invention, the number of microscopic animals, including metazoans, attached to the carrier is measured, and the number of microscopic animals is maintained at 20,000 or less per mL of tank volume.

[0038] Among microorganisms, protozoa, which prey on fewer bacteria per individual, have a smaller impact than metazoans, so management may be based solely on the number of metazoan individuals. However, it is preferable to also measure the number of protozoans, which have a relatively small impact, and multiply the counted number of protozoan individuals by a predetermined coefficient and add the result to the number of metazoans to determine the number of microorganisms.

[0039] That is, in the present invention, the "number of microscopic animals" does not simply refer to the total number of microscopic animals (metazoans and protozoans), but rather, taking into consideration the magnitude of the impact on downstream processing, it is preferable to define the number of microscopic animals as the sum of the number of metazoans and the number obtained by multiplying the number of protozoans by a predetermined coefficient (a value selected from 0.01 to 0.5). The coefficient by which the counted value of protozoans is multiplied is set within the range of 0.01 to 0.5, preferably 0.05 to 0.2. For example, when the coefficient is set to 0.1, the number of microscopic animals is expressed as "number of metazoans + number of protozoans × 0.1." Note that, as mentioned above, when managing only the number of metazoan individuals, the predetermined coefficient is set to 0. Therefore, including this case, the number of microscopic animals is defined as the sum of the number of metazoans and the number obtained by multiplying the number of protozoans by a predetermined coefficient (a value selected from 0 to 0.5).

[0040] The method for measuring the number of microscopic animals is as follows. First, 1 to 10 carriers, for example, 5 carriers, are collected from the fluidized-bed biological treatment tank. Then, each carrier is immersed in 1 mL of pure water, and any adhering solid matter is removed. The number of microscopic animals per carrier is calculated from the number of microscopic animals in the liquid. If the carrier is a sponge carrier, the carrier is crushed to push out the sludge inside and count it. The number of microscopic animals is preferably counted using a hemocytometer and counted under a microscope. Then, the amount of carriers per liter of biological treatment tank is calculated and converted into the number of microscopic animals per mL of biological treatment tank. The frequency of this measurement should be at least once every four weeks, particularly at least once every two weeks, and especially 1 to 4 times every two weeks.

[0041] Furthermore, such microscopic animal count measurements take time, and depending on the site, frequent measurements may be difficult. In such cases, the number of microscopic animals leaking into the treated water may be measured at least once a month, preferably at least once every two weeks, and particularly preferably 1 to 4 times every two weeks, and only when a count of 1,000 or more microscopic animals per mL of treated water is confirmed may the microscopic animals attached to the carrier be measured. Note that if a correlation is observed between the number of microscopic animals in the treated water and the number of microscopic animals in the carrier, the number of microscopic animals in the treated water may be used as a management indicator.

[0042] The microscopic animals that grow in fluidized-bed biological treatment are not aggregate-feeding animals that break down sludge, but rather filter-feeding animals that create water currents to suck up and prey on microscopic bacteria. The microscopic animals that affect the quality of treated water are primarily aggregate-feeding animals, and the impact of filter-feeding animals on the growth of conventional wastewater treatment has not been taken into consideration. However, in wastewater reclamation systems equipped with advanced treatment at a later stage, the sludge is rapidly preyed upon by the microscopic animals, and even an increase of just 1 mg-TOC / L in the organic matter concentration produced during this process can have an impact on the advanced treatment at the later stage. For this reason, in the present invention, the number of microscopic animals is measured without distinguishing between filter-feeding animals and aggregate-feeding animals.

[0043] When the number of microscopic animals per 1 mL of tank volume exceeds 20,000, one of the following measures (1) to (4) is taken to reduce the amount of sludge adhering to the carrier to 80 to 90% of the normal amount and reduce the number of microscopic animals.

[0044] Countermeasure (1): The amount of flocculant added in the flocculation process is increased to at least 1.2 times, preferably 1.2 to 3.0 times, more preferably 1.3 to 2.9 times the average amount added up to that point (for example, the average weekly amount added), to reduce the organic matter in the flocculation-treated water.

[0045] Countermeasure (2): The aeration volume in the biological treatment tank is temporarily increased to 1.2 to 2.5 times, preferably 1.5 to 2.0 times, the normal treatment volume (e.g., the average weekly aeration volume), and this is continued for at least one week, preferably two to three weeks, to push out the microorganisms attached to the carriers. However, care must be taken not to excessively reduce the amount of attached sludge. When the amount of sludge attached to the carriers falls to 70% or less of the normal amount, the aeration volume should be returned to normal treatment. Therefore, it is preferable to periodically measure not only the number of microorganisms but also the amount of attached sludge on the carriers. Methods for measuring the amount of attached sludge include drying the SS detached from the carriers and measuring it, or eluting the sludge from the carriers with sodium hydroxide, measuring the amount of protein, and converting it to the sludge volume (VSS).

[0046] Countermeasure (3): The aeration volume should be 0.7 to 0.9 times that during normal treatment (for example, the average weekly aeration volume) and DO should be 2 mg / L or more.

[0047] Countermeasure (4): Intermittent aeration with aeration time / aeration stop time = 2 to 10.

[0048] It is desirable to continue these measures (1) to (4) until the number of microscopic animals per mL of tank volume is reduced to 10,000 or less, particularly 5,000 or less.

[0049] In addition, when multiple once-through type fluidized bed aerobic treatment tanks are installed, it is preferable to apply the present invention to the second and subsequent biological treatment tanks. ３ / d or less, particularly 1.0 kg / m ３ This is likely to occur when the load is less than 1 / d, so the present invention is suitable for operation under such a load.

[0050] <Initial Operation> The organic matter-containing wastewater treatment system shown in FIG. 1 was operated for two months under the following conditions.

[0051] Raw water: IPA, TMAH, MEA mixed raw water. CODcr = 500 mg / L, BOD = 250 mg / L, TOC = 200 mg / L. Nutrients: Urea, phosphate, and sulfate were added to achieve a BOD:N:P:S ratio of 100:5:1:0.1. Minerals: Ca, Mg, and K were added as elements to achieve 1 mg / L or more per raw water. Trace metals: Fe was added as an element to achieve 0.1 mg / L or more per raw water, and Cu, Zn, Mo, Mn, Ni, and Co were added as elements to achieve 0.005 mg / L or more per raw water. Water volume: 1000 L / d. Aeration tank (fluidized bed type): 500 L. BOD volume load: 0.5 kg / m ３ / d Carrier: 3mm square polyurethane sponge carrier. Filling rate: 40% Aeration tank DO: 4mg / L Aeration rate: 50L / min First coagulation tank: 120L (stirring speed: 150rpm) Coagulant: 38% ferric chloride aqueous solution 300mg / L added Coagulation pH: 6.0 Second coagulation tank: 20L (stirring speed: 50rpm) Polymer coagulant: anionic coagulant 1mg / L added Settling tank: Lv = 20m / d Filtration device: filled with activated carbon as filter material. Lv = 1m / hr RO membrane separation device: RO membrane device manufactured by Nitto Denko Corporation

[0052] After two months of operation under the above conditions, Bdellovibrio (metazoa) multiplied in the carriers, reaching 25,000 per mL of tank volume, and the S.TOC (soluble TOC) of the treated water reached 10 mg / L.

[0053] [Comparative Example 1] The above initial operation was continued. As a result, the RO flux decline rate was 17% after 5 days, 24% after 15 days, and exceeded 30% after 30 days, making cleaning necessary. The flux decline continued even after cleaning. Cleaning was continued once a month for three months, and the number of microscopic animals naturally decreased, and the flux decline thereafter became gradual.

[0054] [Example 1] After the second month of the initial operation, the amount of 38% aqueous ferric chloride solution added to the first coagulation tank was increased by 1.5 times (450 mg / L). As a result, the flux reduction rate after 30 days was only 11%, and operation was possible without cleaning, unlike Comparative Example 1. However, it was necessary to continue adding 1.5 times the amount of coagulant until the number of microfauna naturally decreased.

[0055] [Example 2] After two months of operation, the amount of 38% ferric chloride solution added to the first coagulation tank was increased by 1.5 times (450 mg / L), and the aeration rate in the biological treatment tank was increased by 1.3 times for two weeks. As a result, the number of microorganisms per mL of tank volume decreased to less than 10,000, and the S.TOC of the treated water decreased to 5 mg / L. After that, the aeration rate and coagulant addition rate were returned to their original levels. After 30 days, the RO flux declined by only about 12%. The system was then able to operate for another three months without cleaning, but the amount of sludge generated increased by 1.1 times.

[0056] [Example 3] After two months of operation, the amount of 38% ferric chloride solution added to the first coagulation tank was increased by 1.5 times (450 mg / L), and the aeration rate in the biological treatment tank was reduced to 2 mg / L. After 10 days, the number of microfauna decreased to less than 10,000 / ml of tank volume, and the treated water S.TOC was 6 mg / L. The aeration rate and coagulant addition rate were then returned to their original levels. After 30 days, the RO flux reduction rate was only about 10.5%, and the system could be operated for three months without cleaning.

[0057] Although the present invention has been described in detail using specific embodiments, it will be apparent to those skilled in the art that various modifications are possible within the scope of the invention. This application is based on Japanese Patent Application No. 2024-028879 filed on February 28, 2024, the entire contents of which are incorporated by reference.

[0058] REFERENCE SIGNS LIST 1 Biological treatment tank 2 First coagulation tank 3 Second coagulation tank 4 Sedimentation tank 5 Filtration device 6 RO membrane separation device

Claims

1. A method for treating wastewater containing organic matter, comprising: a biological treatment step in which wastewater containing organic matter is subjected to aerobic biological treatment using fluidized bed carriers; a coagulation treatment step in which a coagulant is added to the biologically treated water from the biological treatment step to subject the water to coagulation treatment; and a solid-liquid separation step in which the coagulated water from the coagulation treatment step is separated into solid and liquid by membrane separation, wherein the number of microscopic animals, including metazoans, attached to the fluidized bed carriers is maintained at 20,000 or less per mL of tank volume.

2. The method for treating wastewater containing organic matter according to claim 1, further comprising an advanced treatment step downstream of the solid-liquid separation step for removing dissolved substances contained in the water by membrane separation.

3. The method for treating wastewater containing organic matter according to claim 1, wherein the fluidized bed carrier is a sponge carrier.

4. A method for treating wastewater containing organic matter according to claim 1, wherein the number of microscopic animals is the sum of the number of metazoans and the number of protozoans multiplied by a predetermined coefficient (a value selected from the range of 0.01 to 0.5).

5. A method for treating wastewater containing organic matter according to claim 1, wherein when the number of microscopic animals per mL of tank volume exceeds 20,000, the amount of flocculant added in the flocculation step is increased to 1.2 to 3.0 times the average amount added up to that point.

6. A method for treating wastewater containing organic matter according to claim 1, wherein when the number of microscopic animals per mL of tank volume exceeds 20,000, a measure is taken to increase the aeration volume in the biological treatment process to 1.2 to 2.5 times the average aeration volume up to that point.

7. A method for treating wastewater containing organic matter according to claim 1, wherein when the number of microscopic animals per mL of tank volume exceeds 20,000, the aeration volume in the biological treatment process is increased to 0.7 to 0.9 times the average aeration volume up to that point, and measures are taken to maintain the dissolved oxygen concentration at 2 mg / L or higher.

8. A method for treating wastewater containing organic matter according to claim 1, wherein, when the number of microscopic animals per mL of tank volume exceeds 20,000, intermittent aeration is performed in the biological treatment process with a ratio of aeration time to aeration stop time of 2 to 10 as a countermeasure.

9. A method for treating wastewater containing organic matter according to any one of claims 5 to 8, wherein the measures are continued until the number of microscopic animals per mL of total volume is reduced to 10,000 or less.

10. The BOD volume load of the biological treatment tank in which the biological treatment step is carried out is 0.2 to 2.0 kg / m ３ 2. The method for treating wastewater containing organic matter according to claim 1, wherein the organic matter content is 0.1 to 0.5 wt %.

11. A method for treating wastewater containing organic matter as described in claim 1, in which the correlation between the number of microscopic animals attached to the fluidized bed carrier and the number of microscopic animals in the biologically treated water from the biological treatment process is obtained in advance, the number of microscopic animals attached to the fluidized bed carrier is estimated from the measured value of the number of microscopic animals in the biologically treated water and this correlation, and this estimated value is used as the number of microscopic animals.

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