Wastewater treatment method and wastewater treatment device

Abstract

To provide a waste water treatment method and apparatus which solve problems such as cost, an ozone generator, odor and chromaticity of separated water in conventional sludge solubilization techniques, and include anaerobic treatment capable of improving dewaterability of sludge and enhancing methane gas generation speed and a methane conversion rate.SOLUTION: A waste water treatment method includes: an anaerobic digestion treatment step 15 of subjecting excess sludge generated after biological treatment to anaerobic digestion treatment; and a dehydration step 16 of subjecting the sludge after the anaerobic digestion treatment to dehydration treatment. The waste water treatment method is characterized in including, so as to be performed before the anaerobic digestion treatment step 15, a pre-treatment step 14 of adding an acid to the excess sludge in the presence of 30 to 600 mg / L of iron and 1 to 150 mg / L of nitrite nitrogen (NO2-N) to adjust pH to 5 or less, and finely dividing an extracellular polymer substance of microorganisms in the excess sludge at 10°C to 25°C to obtain easily decomposable sludge.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a wastewater treatment method and apparatus, and more particularly to a wastewater treatment method and apparatus including anaerobic treatment that can improve the dewaterability of sludge and increase the methane gas production rate and methane conversion rate. [Background technology]

[0002] To reduce the volume of sludge, methods are used to solubilize the sludge introduced into methane fermentation tanks. Known sludge solubilization techniques include microwave, ozone, heat, ultrasound, and alkali treatment.

[0003] For example, Japanese Patent Laid-Open Publication No. 2009-255088 (Patent Document 1) discloses that in a solubilization treatment using ozone, an excess sludge solubilization device is provided with an aging tank in which ozone-treated excess sludge is agitated with compressed air and returned to a biological treatment tank, and a deozonation tank in which ozone is adsorbed from the ozone-containing gas generated in the aging tank, thereby sufficiently solubilizing the excess sludge with a small consumption of ozone before returning it to the biological treatment tank, thereby biodegrading substantially all of the excess sludge.

[0004] Japanese Patent Application Laid-Open Publication No. 2016-221491 (Patent Document 2) discloses that sludge is solubilized by heating it to 40°C or higher and 100°C or lower in an alkaline atmosphere of pH 11 or higher, and then subjected to anaerobic biological treatment, which generates methane gas through the decomposition of the solubilized sludge and improves the amount of methane gas generated in anaerobic treatment.

[0005] Japanese Patent Application Laid-Open Publication No. 2012-183510 (Patent Document 3) discloses that anaerobically digested sludge is solubilized by the action of high-temperature solubilizing bacteria or ultra-high-temperature solubilizing bacteria at a pH of 5 to 7 and 50 to 90°C, and the solubilized organic waste and gas containing H2 generated during solubilization are subjected to anaerobic digestion treatment, thereby increasing the yield of hydrogen and methane gas and improving the volume reduction rate of organic waste residue.

[0006] Japanese Patent Publication No. 2011-5359 (Patent Document 4) discloses a treatment method in which excess sludge after biological treatment is solubilized and then returned to biological treatment, in which ultrasonic treatment is carried out in the injection nozzle that injects the excess sludge after biological treatment, and the ultrasonically treated sludge is pressurized and injected in droplet form from the injection nozzle, thereby preventing clogging in the injection nozzle hole that occurred in conventional ultrasonic treatment and promoting solubilization through a higher cavitation effect.

[0007] However, conventional methods have issues such as cost, problems with ozone generators, odor, and color of the separated water, making them unsuitable for practical use.Alkaline treatment also has issues such as poor sludge dewaterability and color of the separated water.

[0008] The OSA process has been proposed as a method for improving energy consumption and operating costs, which are issues during high-temperature treatment and ozone treatment, and for improving treated water color and sludge dewaterability, which are issues during alkaline treatment. For example, Japanese Patent Laid-Open Publication No. 2020-142168 (Patent Document 5) discloses a method in which a portion of sludge after biological treatment is introduced into an anaerobic tank, where it is subjected to micro-aeration in the presence of iron to decompose the sludge and reconstitute it as a substrate, and then returned to the activated sludge tank for biological treatment again. The OSA process described in Patent Document 5 can shorten the retention time and reduce the size of the anaerobic tank compared to conventional OSA processes, but still has problems with a long retention time and a large anaerobic tank compared to other treatment methods. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-255088 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-221491 [Patent Document 3] Japanese Patent Application Laid-Open No. 2012-183510 [Patent Document 4] Japanese Patent Application Laid-Open No. 2011-5359 [Patent Document 5] Japanese Patent Application Publication No. 2020-142168 Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention aims to provide a wastewater treatment method and apparatus including anaerobic treatment that can solve problems associated with conventional sludge solubilization technology, such as the cost, ozone generator, odor, and color of separated water, improve sludge dewaterability, and increase the methane gas production rate and methane conversion rate. [Means for solving the problem]

[0011] In order to solve the above problems, the present inventors have conducted extensive research and have found that by treating excess sludge to be subjected to anaerobic digestion in an acidic atmosphere with a pH of 5 or less, preferably 3 or less, in the presence of nitrite nitrogen and iron, it is possible to break down extracellular polymeric substances into particles of about 1 μm without destroying the cell walls (cell membranes) of the microorganisms in the excess sludge, thereby preventing deterioration of color due to organic matter derived from the microorganisms and improving soluble COD. Cr (S-COD Cr ) components and easily decomposed COD Cr The present inventors discovered that it is possible to obtain sludge with increased components (easily decomposable sludge) and that by subjecting the easily decomposable sludge to anaerobic digestion, the amount of methane gas produced can be significantly increased, leading to the completion of the present invention.

[0012] In the present invention, "easily degradable" refers to the process of breaking down the extracellular polymeric substances of microorganisms in sludge to make it easily degradable sludge, while leaving the cell walls (cell membranes) intact, to make it easily degradable sludge containing the broken down extracellular polymeric substances and microorganisms. "Easily degradable sludge" refers to easily degradable sludge containing the broken down extracellular polymeric substances and microorganisms obtained by the easily degradable treatment, and [(S-COD after treatment] Cr )-(Before treatment S-COD Cr )] / COD before treatment Cr"Solubilization" generally used in sludge treatment is a technology for reducing the volume of sludge by destroying the cell walls (cell membranes) of microorganisms in the sludge to increase soluble organic matter and reduce solid matter, and "easily degradable" and "easily degradable sludge" in this invention are distinguished from "solubilization" and "solubilized sludge".

[0013] According to the present invention, there are provided a wastewater treatment method and a wastewater treatment device having the following aspects. [1] A wastewater treatment method including an anaerobic digestion treatment step of anaerobic digestion of excess sludge generated after biological treatment, and a dehydration step of dehydrating the sludge after anaerobic digestion treatment, This wastewater treatment method is characterized by including, prior to the anaerobic digestion treatment step, a pretreatment step in which, in the presence of 30 mg / L to 600 mg / L of iron and 1 mg / L to 150 mg / L of nitrite nitrogen (NO2-N), an acid is added to adjust the pH to 5 or less, and the extracellular polymeric substances of the microorganisms in the excess sludge are broken down at 10°C to 25°C to obtain easily degradable sludge. [2] The wastewater treatment method according to [1] above, wherein nitrite nitrogen (NO2-N) and / or iron is added to the excess sludge in the pretreatment step. [3] The wastewater treatment method according to [1] or [2] above, further comprising an excess sludge concentration step of concentrating the excess sludge prior to the pretreatment step. [4] A wastewater treatment method according to any one of [1] to [3] above, comprising a solid-liquid separation step of separating wastewater into sludge and separated water, a biological treatment step of biologically treating the separated water from the solid-liquid separation step, a separated sludge concentration step of concentrating the separated sludge from the solid-liquid separation step, and a sludge mixing step of mixing the concentrated sludge from the separated sludge concentration step with the easily decomposable sludge from the pretreatment step before the anaerobic digestion treatment step. [5] The method further comprises a nitrite nitrogen (NO2-N) production step of oxidizing ammonia nitrogen contained in the separated liquid from the dehydration step to produce nitrite nitrogen (NO2-N), The wastewater treatment method according to any one of the above [1] to [4], wherein the nitrite from the nitrite nitrogen (NO2-N) production step is added to the pretreatment step. [6] A wastewater treatment device, a pretreatment tank in which the excess sludge after biological treatment is treated in the presence of 30 mg / L to 600 mg / L of iron and 1 mg / L to 150 mg / L of nitrite nitrogen (NO2-N), and the pH is adjusted to 5 or less by adding acid, and the extracellular polymeric substances of the microorganisms are broken down at 10°C to 25°C to obtain easily degradable sludge; an anaerobic digestion tank for anaerobic digestion of the easily decomposable sludge from the pretreatment tank; a dehydrator that dehydrates sludge from the anaerobic digestion tank; An anaerobic digestion treatment device comprising: [7] The wastewater treatment device according to [6] above, further comprising an excess sludge concentration tank for concentrating the excess sludge, located upstream of the pretreatment tank. [8] A first solid-liquid separation means for separating wastewater into separated sludge and separated water; a biological treatment tank for biologically treating the separated water from the first solid-liquid separation means; a separated sludge thickening tank for thickening the separated sludge from the first solid-liquid separation means; The wastewater treatment device according to [6] or [7] above, further comprising a sludge concentration and mixing tank upstream of the anaerobic treatment device, which mixes the concentrated sludge from the separated sludge concentration tank and the easily decomposable sludge from the pretreatment tank. [9] The system further includes a nitrite nitrogen generation tank that oxidizes ammonia nitrogen contained in the separated liquid from the dehydrator to generate nitrite nitrogen (NO2-N), The wastewater treatment device according to any one of the above [6] to [8], wherein nitrite nitrogen (NO2-N) from the nitrite nitrogen generation tank is added to the pretreatment tank. [Effects of the Invention]

[0014] The wastewater treatment method of the present invention, unlike conventional sludge solubilization techniques, fragments extracellular polymeric substances but does not destroy cell walls, thereby reducing energy consumption and operating costs compared to conventional solubilization methods, providing excellent operability by shortening the time required for anaerobic digestion (retention time), improving the color of treated water and dewatering of sludge, and increasing the methane gas production rate and methane conversion rate. When the extracellular polymeric substances of microorganisms are fragmented during pretreatment, the bound water held in the extracellular polymeric substances becomes free water, improving dewatering in the sludge treatment process.

[0015] The treatment device of the present invention requires a short residence time to convert excess sludge into easily decomposable sludge, so that a pretreatment tank smaller than a conventional solubilization tank can be used, and a pretreatment tank with a tubular structure such as a line mixer can also be used. Furthermore, a treatment device with a simple configuration that achieves a high methane production rate and methane conversion rate can be provided without requiring special equipment such as a heater or ozone generator. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a schematic explanatory diagram of a wastewater treatment method of the present invention. [Figure 2] 1 is a schematic explanatory diagram of one embodiment of a wastewater treatment method of the present invention. [Figure 3] FIG. 2 is a schematic explanatory diagram of another embodiment of the wastewater treatment method of the present invention. [Figure 4] FIG. 2 is a schematic explanatory diagram of still another embodiment of the wastewater treatment method of the present invention. [Figure 5] 1 is a schematic explanatory diagram of a wastewater treatment device of the present invention. [Figure 6] 1 is a schematic explanatory diagram of an embodiment of a wastewater treatment device of the present invention. [Figure 7] FIG. 4 is a schematic explanatory diagram of another embodiment of the wastewater treatment device of the present invention. [Figure 8] FIG. 10 is a schematic explanatory view of still another embodiment of the wastewater treatment device of the present invention. [Figure 9] 1 is a graph showing changes in iron concentration and nitrite nitrogen (NO2-N) concentration. [Figure 10] 1 is a graph showing changes in iron concentration and nitrate nitrogen (NO3-N) concentration. [Figure 11] 1 is a graph showing the results of measuring S-CODCr concentrations in an example. Preferred Embodiments

[0017] The present invention will be described in detail below with reference to the accompanying drawings. The embodiments shown in the accompanying drawings are representative examples of the present invention, and the present invention is not limited thereto.

[0018] A basic treatment flow diagram of the wastewater treatment method of the present invention is shown in Figure 1. The wastewater treatment method of the present invention includes an anaerobic digestion step 15 in which excess sludge generated after biological treatment is anaerobically digested, and a dehydration step 16 in which the sludge after anaerobic digestion is dehydrated. The method is characterized by including a pretreatment step 14, prior to the anaerobic digestion step 15, in which an acid is added to adjust the pH to 5 or less in the presence of 30 mg / L to 600 mg / L of iron and 1 mg / L to 150 mg / L of nitrite nitrogen (NO-N), and the extracellular polymeric substances of microorganisms in the excess sludge are broken down at 10°C to 25°C to obtain easily degradable sludge. The illustrated embodiment includes a first solid-liquid separation step 11 for solid-liquid separation of wastewater into separated sludge and separated water, a biological treatment step 12 for biologically treating the separated water from the first solid-liquid separation step 11, and a second solid-liquid separation step 13 for separating the biologically treated water from the biological treatment step 12 into sludge and treated water, with a portion of the sludge from the second solid-liquid separation step 13 being returned to the biological treatment step 12 as returned sludge and the remainder of the sludge being sent to the pretreatment step 14 as excess sludge. The first solid-liquid separation step is not essential and can be omitted.

[0019] Fig. 2 shows an outline of a treatment flow of one embodiment of the wastewater treatment method of the present invention. The wastewater treatment method shown in Fig. 2 includes a first solid-liquid separation step 11 for performing solid-liquid separation of wastewater into sludge and separated water, a biological treatment step 12 for biologically treating the separated water from the first solid-liquid separation step 11, an anaerobic digestion treatment step 15 for anaerobic digestion of excess sludge generated after the biological treatment, and a dewatering step 16 for dewatering the sludge after the anaerobic digestion treatment. Prior to the anaerobic digestion treatment step 15, the wastewater is treated with sludge in the presence of 30 mg / L to 600 mg / L of iron and 1 mg / L to 150 mg / L of nitrite nitrogen (NO2-N). The method further comprises a pretreatment step 14 in which an acid is added to adjust the pH to 5 or less and the extracellular polymeric substances of the microorganisms in the excess sludge are broken down at 10°C to 25°C to obtain easily degradable sludge, a separated sludge concentration step 17 in which the separated sludge from the first solid-liquid separation step 11 is concentrated, and a sludge mixing step 18 in which the concentrated sludge from the separated sludge concentration step 17 is mixed with the easily degradable sludge from the pretreatment step 14, and the mixed sludge of the concentrated sludge and the easily degradable sludge is supplied to an anaerobic digestion treatment step 15 for treatment. The biologically treated water from the biological treatment step 12 is separated into sludge and treated water in a second solid-liquid separation step 13, and a portion of the sludge is returned to the biological treatment step 12 as returned sludge, while the remainder is sent to the pretreatment step 14 as excess sludge. The method may further comprise an excess sludge concentration step 19 in which the excess sludge is concentrated before being supplied to the pretreatment step 14. When the excess sludge concentration step 19 is included in the stage preceding the pretreatment step 14, the efficiency of the pretreatment step can be improved (reduced amount of acid used, smaller pretreatment tank).

[0020] FIG. 3 shows an outline of the treatment flow of another embodiment of the wastewater treatment method of the present invention. The wastewater treatment method shown in Figure 3 includes an anaerobic digestion process 15 in which excess sludge generated after biological treatment is anaerobically digested, and a dehydration process 16 in which the sludge after anaerobic digestion is dehydrated. Prior to the anaerobic digestion process 15, a pretreatment process 14 is included in which an acid is added to adjust the pH to 5 or less in the presence of 30 mg / L to 600 mg / L of iron and 1 mg / L to 150 mg / L of nitrite nitrogen (NO2-N), and the excess sludge is broken down at 10°C to 25°C to obtain easily degradable sludge. The method further includes a nitrite nitrogen (NO2-N) production process 20 in which ammonia nitrogen contained in the separated liquid from the dehydration process 16 is oxidized to produce nitrite nitrogen (NO2-N). The nitrite nitrogen (NO2-N) from the nitrite nitrogen (NO2-N) production process 20 is added to the pretreatment process 14. The illustrated embodiment includes a first solid-liquid separation step 11 for separating wastewater into sludge and separated water, a biological treatment step 12 for biologically treating the separated water from the first solid-liquid separation step 11, and a second solid-liquid separation step 13 for separating the biologically treated water from the biological treatment step 12 into sludge and treated water. A portion of the sludge from the second solid-liquid separation step 13 is returned to the biological treatment step 12 as returned sludge, and the remainder is sent to the pretreatment step 14 as excess sludge. An excess sludge concentration step 19 for concentrating the excess sludge before supplying it to the pretreatment step 14 may also be included. Including the excess sludge concentration step 19 prior to the pretreatment step 14 can improve the efficiency of the pretreatment step (reducing the amount of acid used and downsizing the pretreatment tank). The first solid-liquid separation step 11 is not essential and can be omitted.

[0021] Fig. 4 shows an outline of a treatment flow of another embodiment of the wastewater treatment method of the present invention. The wastewater treatment method shown in Fig. 4 includes a first solid-liquid separation step 11 for separating wastewater into sludge and separated water, a biological treatment step 12 for biologically treating the separated water from the first solid-liquid separation step 11, an anaerobic digestion treatment step 15 for anaerobic digestion of excess sludge generated after the biological treatment, and a dehydration step 16 for dehydrating the sludge after the anaerobic digestion treatment. Prior to the anaerobic digestion treatment step 15, an acid is added to adjust the pH to 5 or less in the presence of 30 mg / L to 600 mg / L of iron and 1 mg / L to 150 mg / L of nitrite nitrogen (NO2-N), and the extracellular polymeric substances of microorganisms in the excess sludge are broken down at 10°C to 25°C to obtain easily degradable sludge. The method further comprises a pretreatment step 14 in which the separated sludge from the first solid-liquid separation step 11 is concentrated, a separated sludge concentration step 17 in which the separated sludge from the first solid-liquid separation step 11 is concentrated, and a sludge mixing step 18 in which the concentrated sludge from the separated sludge concentration step 17 is mixed with the easily decomposable sludge from the pretreatment step 14, and the mixed sludge of the concentrated sludge and the easily decomposable sludge is supplied to an anaerobic digestion treatment step 15 for treatment, and a nitrite nitrogen (NO2-N) production step 20 in which ammonia nitrogen contained in the separated liquid from the dehydration step 16 is oxidized to produce nitrite nitrogen (NO2-N), and the nitrite nitrogen (NO2-N) from the nitrite nitrogen (NO2-N) production step 20 is added to the pretreatment step 14. The biologically treated water from the biological treatment process 12 is separated into sludge and treated water in a second solid-liquid separation process 13, and part of the sludge is returned to the biological treatment process 12 as returned sludge, while the remainder is sent as excess sludge to a pretreatment process 14. An excess sludge concentration process 19 may be further included in which the excess sludge is concentrated before being supplied to the pretreatment process 14.

[0022] First, we will explain the pretreatment step 14 in the wastewater treatment method of the present invention, as shown in Figures 1 to 4. In the pretreatment step, excess sludge is treated at room temperature in the presence of iron and nitrite nitrogen (NO2-N) in an acidic atmosphere with a pH of 5 or less, preferably 4 or less, and more preferably 3 or less and 2 or more. This allows the action of eluted metal ions, in addition to iron ions, nitrite ions, and hydroxyl radicals, to fragment the extracellular polymeric substances (EPMS) of microorganisms in the excess sludge, producing easily degradable sludge (easily degradable sludge). Furthermore, the bound water held by the extracellular polymeric substances is converted into free water and released, improving the dewaterability of the anaerobically digested sludge in the dewatering step and contributing to sludge volume reduction.

[0023] The iron content of the excess sludge to be treated in the pretreatment step should be 30 mg / L to 600 mg / L, preferably 50 mg / L or more. The higher the iron concentration, the more fragmented the extracellular polymeric substances become. However, if the iron concentration is too high, the amount of sludge generated after anaerobic digestion increases, which increases the amount of iron to be added and increases costs. Therefore, it is preferable to set the upper limit at 600 mg / L.

[0024] Iron may be contained in the excess sludge to be treated, originating from the sludge or the coagulant, but in the present invention, the concentration is adjusted to 30 mg / L or more and 600 mg / L or less. By lowering the pH to 5 or less, preferably 3 or less, the iron contained in the excess sludge dissolves and becomes trivalent iron ions (Fe 3+ However, iron ions absorbed in the organic matter in the excess sludge are difficult to dissolve, so if there is a shortage, iron must be added. Iron compounds such as FeCl3 and FeCl2 can be preferably used as the added iron.

[0025] In the pretreatment process, the nitrite nitrogen (NO2-N) in the excess sludge to be treated should be between 1 mg / L and 150 mg / L, preferably 50 mg / L or less. If there is an excess of nitrite nitrogen (NO2-N), organic matter is consumed by denitrification in the anaerobic digester, reducing the amount of methane gas produced. By lowering the pH to 5 or less, preferably 3 or less, free nitrite (FNA) is produced. 1 mg / L of nitrite nitrogen (NO2-N) produces 0.02 mg / L of free nitrite (FNA) and 0.98 mg / L of nitrite ions (NO2 - Therefore, this can also be said to mean that the free nitrite (FNA) present in the excess sludge is 0.02 mg / L or more and 3 mg / L or less, preferably 1 mg / L or less.

[0026] Nitrite nitrogen may be contained in excess sludge, but in the present invention, the nitrite nitrogen (NO-N) is adjusted to 1 mg / L to 150 mg / L, preferably 50 mg / L or less. Nitrite nitrogen (NO-N) may be added to excess sludge as nitrite, or ammonia nitrogen contained in excess sludge or the separated liquid of anaerobically digested sludge may be oxidized to produce nitrite nitrogen (NO-N). In the present invention, it is preferable to use nitrite nitrogen (NO-N) generated in existing water or sludge treatment processes as at least a portion of the nitrite nitrogen (NO-N). Furthermore, in the pretreatment step, a portion of the ammonia nitrogen (NH-N) in excess sludge can be oxidized to nitrite nitrogen (NO-N).

[0027] In the pretreatment process, acid is added to adjust the pH of excess sludge to 5 or less, preferably 4 or less, and more preferably 3 or less. If the pH is too low, a large amount of alkaline agent must be added to adjust the pH to the optimal pH (neutral) for anaerobic digestion, resulting in increased costs. Therefore, a pH of 2 or greater is preferred. Typically, when high-concentration sludge is subjected to anaerobic digestion, the alkalinity increases due to the ammoniacal nitrogen (NH4-N) produced during anaerobic digestion, reducing the dehydration effect. Therefore, lowering the pH before anaerobic digestion is also effective for treating high-concentration sludge. Furthermore, this method prevents the release of organic matter constituting cells, which causes color deterioration, a problem with conventional alkaline and high-temperature treatments. Furthermore, humic substances tend to aggregate in acidic regions and can be removed during the dehydration process, effectively preventing color deterioration. Furthermore, compared to solubilized sludge produced by conventional alkaline treatment, the easily degradable sludge produced by the pretreatment process of the present invention has less odor, lower viscosity, and higher fluidity, making it easier to handle and reducing the power required for transportation and agitation.

[0028] Acids that can be added in the pretreatment step include hydrochloric acid, sulfuric acid, nitric acid, nitrous acid, and phosphoric acid. Among these, hydrochloric acid is preferred because it consumes less organic matter through reduction reactions during anaerobic digestion and can increase the amount of methane gas generated. Other acids increase the amount of methane gas generated less than hydrochloric acid, but have the same sludge volume reduction effect. Acids are also cheaper than alkaline agents, which has the advantage of reducing chemical costs.

[0029] In the pretreatment process, the pH is 5 or less, and the trivalent iron ion Fe 3+ is nitrite ion (NO2 - ) and nitrate ions (NO3 - ) and divalent iron ion Fe 2+ and hydrogen ions H + It is thought that this produces a reaction that dissolves the extracellular polymeric substances of the microorganisms in the sludge. This is a gentle reaction that breaks down the extracellular polymeric substances of the microorganisms but does not completely dissolve the cell walls, and the pretreatment residence time is preferably 0.01 to 24 hours, and more preferably 0.5 to 12 hours.

[0030] The pretreatment step can be carried out at an ORP (standard electrode potential) of -50 mV to 150 mV, and active anaerobic treatment or aeration is not required. Furthermore, room temperature (approximately 10°C to 25°C) is sufficient, and heating is not required. Because the easily decomposable sludge from the pretreatment step is acidic, the alkalinity of the anaerobic digested sludge after anaerobic digestion does not become too high, and the amount of coagulant added during dehydration can be reduced. The wastewater treatment method including the pretreatment step of the present invention can reduce the amounts of pH adjuster and coagulant added, as well as the energy required for heating and ORP control, compared to conventional anaerobic wastewater treatment including a sludge solubilization step, thereby reducing operating costs.

[0031] Next, other processing steps in FIGS. 1 to 4 will be described. The first solid-liquid separation step 11 is a step of separating the wastewater to be treated into sludge and separated water, and since the amount of wastewater to be treated is large, gravity settling separation is preferred.

[0032] The biological treatment step 12 is preferably an aerobic biological treatment using activated sludge treatment. The second solid-liquid separation step 13 is a step in which the sludge after biological treatment, including activated sludge, is subjected to solid-liquid separation into excess sludge and treated water, and gravity settling separation or membrane separation is preferred.

[0033] The excess sludge thickening step 19 is a step of thickening the excess sludge from the second solid-liquid separation step 13 before subjecting it to pretreatment, and mechanical thickening is preferred. By subjecting the thickened excess sludge to pretreatment, the amount of extracellular polymer-derived organic matter (soluble COD) per unit volume can be reduced. Cr This increases the amount of methane produced by anaerobic digestion, improving the efficiency of methane gas production.

[0034] The anaerobic digestion step 15 is a step in which excess sludge is subjected to anaerobically digested treatment to generate methane gas. In the present invention, the amount of methane gas generated can be increased by subjecting easily degradable sludge to anaerobically digested treatment. Typically, the anaerobic digestion step is carried out at a pH of 6 to 8.5. In the present invention, easily degradable sludge obtained by adjusting the pH to 5 or less at room temperature in the pretreatment step is introduced into the anaerobic digestion step, so the free ammonia concentration is reduced and the alkalinity of the easily degradable sludge is not too high, making it possible to reduce the amount of pH adjuster that is usually required to lower the pH. For example, when the alkalinity of sludge in a typical anaerobic digestion process is 3000 mg / L, the ammoniacal nitrogen concentration is approximately 500 to 1000 mg / L, and when highly concentrated sludge is subjected to anaerobically digested treatment, the alkalinity of the sludge in the anaerobic digestion process often rises to 6000 mg / L and the ammoniacal nitrogen concentration to approximately 5000 mg / L.However, when the easily decomposable sludge of the present invention is supplied to the anaerobic digestion process, the alkalinity of the anaerobically digested sludge is not too high, so the amount of coagulant added during dehydration treatment can be reduced.

[0035] The dewatering step 16 is a step in which the sludge after anaerobic digestion is dewatered and separated into dewatered sludge and separated liquid. In the present invention, by subjecting the easily decomposable sludge to anaerobic digestion, the amount of sludge from the anaerobic digestion step 15 is reduced, which increases the dewatering efficiency in the dewatering step and reduces the amount of dewatered sludge ultimately discharged. Because the ammonia concentration of the anaerobically treated sludge resulting from the anaerobic treatment of easily decomposable sludge is not too high, the amount of flocculant added can be reduced, thereby suppressing operating costs.

[0036] The separated sludge concentration step 17 is a step of concentrating the sludge from the first solid-liquid separation step 11, and gravity concentration can be preferably used. The sludge mixing step 18 is a step of mixing the concentrated sludge and easily decomposable sludge before the anaerobic digestion treatment, and can adjust the sludge concentration during the anaerobic digestion treatment to within a suitable range.

[0037] The nitrite nitrogen (NO2-N) production step 20 is a step in which ammonia nitrogen (NH4-N) contained in the separated liquid from the dehydration step 16 is oxidized to nitrite nitrogen (NO2-N). It is desirable to prevent the nitrite nitrogen (NO2-N) from being oxidized before being introduced into the pretreatment step after the nitrite nitrogen (NO2-N) production step. Preferred methods for inhibiting the oxidation of nitrite nitrogen (NO2-N) include lowering the pH to approximately 5 to 6.5, maintaining nitrite nitrogen (NO2-N) at 100 mg / L or more, and generating free nitrite; raising the pH to approximately 7 to 8.5, maintaining ammonia nitrogen (NH4-N) at 100 mg / L or more, and generating free ammonia; inhibiting the oxidation of nitrite nitrogen (NO2-N) by maintaining dissolved oxygen (DO) at 1 mg / L or less; and shortening the retention time by, for example, withdrawing sludge every 1 to 2 days as an SRT, and thereby maintaining a high concentration of nitrite nitrogen (NO2-N).

[0038] Next, the wastewater treatment device of the present invention will be described with reference to FIGS. Figures 5 to 8 are schematic explanatory diagrams showing the configuration of a wastewater treatment device suitable for carrying out the wastewater treatment methods shown in Figures 1 to 4. Figures 5 to 8 specifically illustrate the device configuration for carrying out each step shown in Figures 1 to 4, and the same reference numerals are used for the components for carrying out each step.

[0039] As shown in FIG. 5, the wastewater treatment device of the present invention is characterized by comprising a pretreatment tank 14 in which excess sludge after biological treatment is treated in the presence of 30 mg / L to 600 mg / L of iron and 1 mg / L to 150 mg / L of nitrite nitrogen (NO-N) by adding acid to adjust the pH to 5 or less, and then treating the excess sludge at 10°C to 25°C to break down the extracellular polymeric substances of the microorganisms in the excess sludge to obtain easily degradable sludge; an anaerobic digestion tank 15 in which the easily degradable sludge from the pretreatment tank 14 is anaerobically digested; and a dehydrator 16 in which the sludge from the anaerobic digestion tank 15 is dehydrated. The illustrated embodiment includes a first solid-liquid separation means 11 for separating wastewater into solids and liquids, a biological treatment tank 12 for biologically treating the separated water from the first solid-liquid separation means 11, a second solid-liquid separation means 13 for separating the biologically treated water from the biological treatment tank 12 into sludge and treated water, and a sludge return line 12a for returning sludge from the second solid-liquid separation means 13 to the biological treatment tank 12 in order to suppress the amount of excess sludge generated and maintain a sufficiently high sludge concentration during biological treatment. The first solid-liquid separation means 11 can be omitted. The second solid-liquid separation means 13 may be provided separately from the biological treatment tank 12 as shown, or a submerged separation membrane or the like may be provided within the biological treatment tank 12.

[0040] As shown in FIG. 6, the wastewater treatment device of the present invention may further include, between the first solid-liquid separation means 11 and the anaerobic digestion treatment tank 15, a separated sludge concentration tank 17 for concentrating the separated sludge from the first solid-liquid separation means 11, and a sludge concentration and mixing tank 18 for mixing the concentrated sludge from the separated sludge concentration tank 17 with the easily decomposable sludge from the pretreatment tank 14.

[0041] As shown in FIG. 7, the wastewater treatment device of the present invention may further include a nitrite nitrogen generation tank 20 that oxidizes ammonia nitrogen contained in the separated liquid from the dehydrator 16 to generate nitrite nitrogen, and the nitrite nitrogen (NO-N) from the nitrite nitrogen generation tank 20 may be added to the pretreatment tank 14.

[0042] 5 to 8, a primary settling tank is used as the first solid-liquid separation means 11, and a final settling tank is used as the second solid-liquid separation means 13, but the present invention is not limited to these, and solid-liquid separation means commonly used in wastewater treatment, such as gravity filtration, compression filtration, vacuum filtration, atmospheric flotation thickener, centrifugal thickener, belt-type filtration thickener, submerged membrane separator, and external membrane separator, can also be used. Furthermore, the solid-liquid separation means 11 and 13 can be provided with a flocculant addition means for adding a flocculant to facilitate settling of sludge, or a coagulation tank can be provided upstream of the solid-liquid separation means 11 and 13, in which a flocculant is added to the sludge to cause coagulation.

[0043] An activated sludge tank, a trickling filter, an oxidation ditch tank, or the like can be preferably used as the biological treatment tank 12. A UASB-type anaerobic tank, an anaerobic membrane separation tank, a carrier-feed digester, or the like can be preferably used as the anaerobic digester 15. A dehydrator used in ordinary sludge treatment, such as a gravity filtration dehydrator, a compression filtration dehydrator, a vacuum filtration dehydrator, a screw press dehydrator, a centrifugal dehydrator, a filter press dehydrator, or a belt press dehydrator, can be preferably used as the dehydrator 16.

[0044] The pretreatment tank 14 is not limited to a specific tank shape, and may be a tubular tank such as a line mixer, as long as it has a volume large enough to treat the excess sludge from the biological treatment tank 12 at room temperature at a pH of 5 or less by adding acid in the presence of iron and nitrite nitrogen, thereby achieving fragmentation of the extracellular polymeric substances (EPMS) of microorganisms in the excess sludge. Because the pretreatment step fragments the microbial cellular polymeric substances but does not completely dissolve the cell walls, a short treatment time is preferred, with a retention time of 0.01 to 24 hours, preferably 0.5 to 12 hours. In addition to the acid addition means 14a, the pretreatment tank 14 preferably includes an iron addition means 14b and a nitrite nitrogen addition means 14c, and may further include a pH adjuster addition means (not shown). A stirring means (not shown) is also preferably provided to uniformly contact the sludge with the acid, iron, and nitrite nitrogen. Furthermore, although not shown, the pretreatment tank 14 is preferably equipped with an iron detection means and a nitrite nitrogen detection means for detecting the amounts of iron and nitrite present in the sludge being treated, as well as a pH meter, and the acid addition means, pH adjuster addition means, iron addition means, and nitrite nitrogen addition means are preferably equipped with acid addition amount adjustment means, pH adjuster addition amount adjustment means, iron addition amount adjustment means, and nitrite nitrogen addition amount adjustment means, which adjust the amounts added according to the amounts present in the sludge. Furthermore, the pretreatment tank 14 may be equipped with a micro-aeration means for oxidizing a portion of the ammoniacal nitrogen in the excess sludge to nitrite nitrogen.

[0045] The treatment device of the present invention preferably further includes a nitrite nitrogen generation tank 20 for generating nitrite nitrogen to be added to the pretreatment tank 14. The nitrite nitrogen generation tank 20 preferably includes a separated liquid introduction means 20a for introducing a portion of the separated liquid from the dehydrator 16, and a nitrite nitrogen addition means 14c for supplying the generated nitrite nitrogen to the pretreatment tank 14. The nitrite nitrogen generation tank 20 is not particularly limited as long as it can oxidize ammoniacal nitrogen (NH4-N) in the separated liquid to nitrite nitrogen (NO2-N) and supply the nitrite nitrogen to the pretreatment tank 14. For example, the tank may include an aeration means for oxidizing ammoniacal nitrogen, a pH adjustment means for maintaining the pH of the separated liquid within a predetermined range, and an aeration adjustment means for maintaining the dissolved oxygen (DO) concentration at 1 mg / L or less. [Example]

[0046] Pure water and sodium nitrite were added to a container in a thermostatic chamber at 25°C to adjust the nitrite nitrogen (NO2-N) concentration to 150 mg / L. The amount of ferric chloride added was varied to adjust the iron concentration to 0 mg / L, 65 mg / L, 280 mg / L, or 580 mg / L. The sludge was stirred for four days. The changes in the concentrations of nitrite nitrogen (NO2-N) and nitrate nitrogen (NO3-N) in the sludge before treatment, after one day, and after four days of treatment were measured. The results are shown in Figures 9 and 10. In the absence of iron, no changes were observed in the concentrations of nitrite nitrogen (NO2-N) and nitrate nitrogen (NO3-N). In the presence of iron, the nitrite nitrogen (NO2-N) decreased and the nitrate nitrogen (NO3-N) increased with increasing time after ferric chloride addition. The decrease in nitrite nitrogen (NO2-N) and the increase in nitrate nitrogen (NO3-N) increased with increasing iron concentration, indicating that the following reactions were occurring:

[0047] [ka]

[0048] Next, 300 mg / L of excess sludge having the properties shown in Table 1, hydrochloric acid, sodium nitrite, and ferric chloride were added to a 500 mL plastic container and shaken in a thermostatic chamber at 25°C for 24 hours. Hydrochloric acid was added so that the pH of the excess sludge after addition was 3 or 5. Sodium nitrite was added so that the nitrite nitrogen concentration in the excess sludge after addition was 0 or 2 mg / L. Ferric chloride was added to reduce the Fe in the excess sludge after addition. 3+ The concentration was 30 mg / L or 100 mg / L. S-COD was measured 10 minutes after addition before shaking and 24 hours after addition after shaking. Cr The results of measuring the concentration are shown in Table 2 and FIG.

[0049] [Table 1]

[0050] [Table 2]

[0051] A comparison of No. 3 and No. 5, where the pH is 5 and the nitrite nitrogen concentration is 0 mg / L, and a comparison of No. 4 and No. 6, where the pH is 5 and the nitrite nitrogen concentration is 2 mg / L, shows that the nitrite nitrogen concentration is a more important factor in the S-COD than the iron concentration. Cr It can be seen that the effect of increasing concentration is large.

[0052] Comparison of No. 3 and No. 4, which have a pH of 5 and an iron concentration of 30 mg / L, and No. 5 and No. 6, which have a pH of 5 and an iron concentration of 100 mg / L, reveals the S-COD after 10 minutes and 24 hours. Cr The concentration of nitrite nitrogen in No. 4 and No. 6, which had 2 mg / L, was higher than that in No. 3 and No. 5, which had 0 mg / L, indicating that the presence of nitrite nitrogen improved the ease of decomposition. In particular, the S-COD of No. 4 after 24 hours Cr The concentration is No.3 S-COD Cr The concentration increased by about four times, and the S-COD of No. 6 after 24 hours Cr The concentration is No.5 S-COD CrThe concentration increased to about six times the normal concentration, indicating that sufficient contact between the sludge and nitrite nitrogen promotes the easy decomposition reaction.

[0053] S-COD after 10 minutes and 24 hours from No. 2 and No. 4, where the nitrate nitrogen concentration is 2 mg / L and the iron concentration is 30 mg / L. Cr The concentration was higher in No. 2, which had a pH of 3, than in No. 4, which had a pH of 5, and it can be seen that the lower the pH, the greater the effect. In particular, the S-COD of No. 2 after 10 minutes Cr The concentration is the No. 1 S-COD of untreated sludge. Cr The concentration increased by more than 10 times, indicating that the lower the pH, the more immediate the effect.

[0054] S-COD Cr The concentration of S-COD is known to be correlated with the amount of methane gas generated and to be an indicator of the amount of methane gas generated. When the pH is below 5, the coexistence of iron and nitrite nitrogen increases the S-COD. Cr Since a significant increase in concentration has been observed, it is thought that the amount of methane gas produced will also increase significantly.

[0055] As shown in Table 3, the wastewater treatment method using the pretreatment of the present invention consumes less energy than wastewater treatment using conventional solubilization treatment, reduces operating costs, is easy to operate, produces clearer colored treated water, and has good dewaterability of sludge.

[0056] [Table 3] [Explanation of symbols]

[0057] 11: First solid-liquid separation process (primary sedimentation tank) 12: Biological treatment process (biological treatment tank) 13: Second solid-liquid separation process (final sedimentation tank) 14: Pretreatment process (pretreatment tank) 15: Anaerobic digestion process (anaerobic digestion tank) 16: Dehydration process (dehydrator) 17: Separated sludge thickening process (gravity thickener) 18: Sludge mixing process (sludge mixing tank) 19: Excess sludge thickening process (mechanical thickening device) 20: Nitrite nitrogen generation process (nitrite nitrogen generation tank)

Claims

1. A wastewater treatment method including an anaerobic digestion treatment step of anaerobic digestion of excess sludge generated after biological treatment, and a dehydration step of dehydrating the sludge after the anaerobic digestion treatment, This wastewater treatment method is characterized by including a pretreatment step in which, prior to the anaerobic digestion treatment step, the iron content in the excess sludge is adjusted to between 30 mg / L and 600 mg / L and the nitrite nitrogen (NO 2 -N) is adjusted to between 1 mg / L and 150 mg / L, and an acid is added to adjust the pH to 5 or less, and the extracellular polymeric substances of the microorganisms in the excess sludge are broken down at between 10°C and 25°C to obtain easily decomposable sludge.

2. In the pretreatment step, nitrite nitrogen (NO ２ 2. The wastewater treatment method according to claim 1, wherein the wastewater treatment method further comprises adding nitrite (-N) and / or iron.

3. 3. The wastewater treatment method according to claim 1, further comprising an excess sludge concentration step of concentrating the excess sludge before the pretreatment step.

4. 3. The wastewater treatment method according to claim 1, further comprising a sludge mixing step of concentrating the separated sludge from the solid-liquid separation step and mixing it with the easily decomposable sludge from the pretreatment step, prior to the anaerobic digestion treatment step.

5. The ammonia nitrogen contained in the separated liquid from the dehydration step is oxidized to produce nitrite nitrogen (NO ２ Nitrite nitrogen (NO ) ２ -N) further comprising a generating step, In the pretreatment step, the nitrite nitrogen (NO ２ 3. The wastewater treatment method according to claim 1, wherein nitrite from the (-N) production step is added.

6. A wastewater treatment device, a pretreatment tank in which the excess sludge after biological treatment is treated to adjust the iron content to 30 mg / L to 600 mg / L and the nitrite nitrogen (NO 2 -N) to 1 mg / L to 150 mg / L, and the pH to 5 or less by adding an acid, and the extracellular polymeric substances of the microorganisms are broken down at 10°C to 25°C to obtain easily degradable sludge; an anaerobic digestion tank for anaerobic digestion of the easily decomposable sludge from the pretreatment tank; a dehydrator that dehydrates sludge from the anaerobic digestion tank; An anaerobic digestion treatment device comprising:

7. 7. The wastewater treatment apparatus according to claim 6, further comprising an excess sludge concentration tank for concentrating the excess sludge, located upstream of the pretreatment tank.

8. a first solid-liquid separation means for separating wastewater into separated sludge and separated water; a biological treatment tank for biologically treating the separated water from the first solid-liquid separation means; a separated sludge thickening tank for thickening the separated sludge from the first solid-liquid separation means; 8. The wastewater treatment device according to claim 6, further comprising a sludge concentration and mixing tank upstream of the anaerobic treatment device, for mixing the concentrated sludge from the separated sludge concentration tank and the easily decomposable sludge from the pretreatment tank.

9. The ammonia nitrogen contained in the separated liquid from the dehydrator is oxidized to produce nitrite nitrogen (NO ２ -N), Nitrite nitrogen (NO ) from the nitrite nitrogen generation tank is added to the pretreatment tank. ２ 8. The wastewater treatment device according to claim 6, wherein a nitrate-N is added.

Images