Wastewater treatment method and device

Abstract

To provide a wastewater treatment method and device including anaerobic treatment, capable of increasing an amount of methane gas produced and improving a methane conversion rate.SOLUTION: The wastewater treatment method including an anaerobic digestion treatment step 15 of subjecting excess sludge generated after biological treatment to anaerobic digestion treatment, and a dehydration step 16 of dehydrating sludge subjected to the anaerobic digestion treatment, is characterized by including an excess sludge flocculation step 30 of injecting a flocculant to the excess sludge prior to the anaerobic digestion treatment step 15, and flocculating the excess sludge to obtain flocculated sludge, an excess sludge concentration step 19 of concentrating the flocculated sludge to obtain concentrated sludge, and a pretreatment step 14 of adding an inorganic acid to the concentrated sludge to adjust pH to 5 or below, and finely dividing extracellular polymer substances of microorganisms present in the concentrated sludge 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] Japanese Patent Application Laid-Open Publication No. 2016-221491 (Patent Document 1) 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.

[0003] Japanese Patent Application Laid-Open Publication No. 2012-183510 (Patent Document 2) 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.

[0004] Japanese Patent Application Laid-Open Publication No. 2016-117066 (Patent Document 3) discloses an anaerobic treatment method in which concentrated sludge with a sludge concentration of 4 to 12% is solubilized and subjected to acid fermentation treatment under conditions of 30 to 60°C and an HRT of 1 to 3 days, followed by methane fermentation treatment.

[0005] Japanese Patent Application Laid-Open Publication No. 2004-275813 (Patent Document 4) discloses a method for treating sludge in which an acid is added to the sludge to adjust the pH to 5 or less, and the sludge is then heated to 60°C or higher using an acid heating method, followed by anaerobic digestion. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-221491 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-183510 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-117066 [Patent Document 4] Japanese Patent Application Laid-Open No. 2004-275813 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a wastewater treatment method and apparatus including anaerobic treatment that can improve the dewaterability of sludge and increase the amount of methane gas produced and the methane conversion rate. [Means for solving the problem]

[0008] In order to solve the above problems, the present inventors have conducted extensive research and have found that by pretreating excess sludge to be subjected to anaerobic digestion, 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, and to improve the soluble COD while preventing deterioration of color due to organic matter derived from microorganisms. 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 generated can be significantly increased, leading to the completion of the present invention.

[0009] In the present invention, "easily degradable" means to break down the extracellular polymeric substances of microorganisms in sludge to make the sludge easily degradable, while leaving the cell walls (cell membranes) intact, to make the sludge easily degradable, containing the broken down extracellular polymeric substances and microorganisms. "Easily decomposable sludge" refers to easily decomposable sludge containing finely divided extracellular polymeric substances and microorganisms obtained by the easily decomposable treatment. Cr )-(Before treatment S-COD Cr )] / COD before treatment Cr"Solubilization" refers to sludge with a solubilization degree of 15% or less as determined by the above method. "Solubilization," a common technique in sludge treatment, is a technology for reducing the volume of sludge by destroying the cell walls (cell membranes) of microorganisms in sludge to increase soluble organic matter and reduce solids, and the solubilization degree exceeds 40%. "Easily degradable" and "easily degradable sludge" in this invention are distinguished from conventional "solubilization" and "solubilized sludge."

[0010] According to the present invention, there is provided a wastewater treatment method having the following aspects. [1] A wastewater treatment method including an anaerobic digestion treatment step in which excess sludge generated after biological treatment is subjected to anaerobic digestion, Before the anaerobic digestion treatment step, an excess sludge flocculation step of injecting a flocculant into the excess sludge to flocculate the excess sludge and obtain flocculated sludge; an excess sludge concentration step of concentrating the flocculated sludge to obtain concentrated sludge; A wastewater treatment method characterized by including a pretreatment step in which an inorganic acid is added to the concentrated sludge to adjust the pH to 5 or less, and the extracellular polymeric substances of the microorganisms in the concentrated sludge are broken down to obtain easily decomposable sludge. [2] The wastewater treatment method according to [1], wherein the easily decomposable sludge obtained in the pretreatment step has a solubilization degree calculated by the following formula of 15% or less.

number

[0011] According to the present invention, there is also provided a wastewater treatment device having the following aspects. [9] A wastewater treatment device, an excess sludge coagulation tank in which a coagulant is injected into excess sludge from the biological treatment tank to form coagulated sludge; an excess sludge thickening tank for thickening the flocculated sludge to form thickened sludge; a pretreatment tank in which an inorganic acid is added to the concentrated sludge from the excess sludge concentration tank to adjust the pH to 5 or less and break down the extracellular polymeric substances of the microorganisms to obtain easily degradable sludge; an anaerobic digestion tank for anaerobic digestion of the easily decomposable sludge from the pretreatment tank; A wastewater treatment device comprising:

[10] The wastewater treatment device according to [9], further comprising a dilution tank for diluting the concentrated sludge between the excess sludge concentration tank and the pretreatment tank.

[11] A solid-liquid separation means for separating the wastewater into separated water and separated sludge is provided upstream of the biological treatment tank; a separated sludge thickening tank for thickening the separated sludge from the solid-liquid separation means; The wastewater treatment device according to [9] or

[10] , characterized in that a sludge concentration and mixing tank for mixing the concentrated sludge from the separated sludge concentration tank and the easily decomposable sludge from the pretreatment tank is provided between the pretreatment tank and the anaerobic digestion treatment tank.

[12] A biological desulfurization device that receives the biogas from the anaerobic digestion tank and produces inorganic acids from odorous components in the biogas, and / or a biological deodorization device that produces inorganic acids from odorous gases generated in a wastewater treatment facility; an inorganic acid supply line that supplies inorganic acid from the biological desulfurization device and / or the biological deodorization device to the pretreatment tank; The wastewater treatment device according to any one of [9] to

[11] , comprising:

[13] A pH meter for measuring the pH of the concentrated sludge in the pretreatment tank; a control device that controls the addition of inorganic acid to the pretreatment tank based on the pH measured by the pH meter; The wastewater treatment device according to any one of [9] to

[12] , further comprising: [Effects of the Invention]

[0012] The wastewater treatment method of the present invention, unlike conventional sludge solubilization techniques, fragments extracellular polymeric substances (EPS) without destroying cell walls. This reduces energy consumption and operating costs compared to conventional solubilization methods, provides excellent operability by shortening the time required for anaerobic digestion (retention time), improves the color of treated water, and improves the dewaterability of sludge, as well as the methane gas production rate and methane conversion rate. When microbial EPS is fragmented during pretreatment, the bound water held in the EPS becomes free water, improving dewaterability in the sludge treatment process. This is particularly effective in anaerobic fermentation, which involves the methane fermentation of high-concentration sludge.

[0013] By carrying out the predetermined pretreatment in the wastewater treatment method of the present invention, the excess sludge to be subjected to anaerobic digestion treatment can be made to have optimal properties for improving the methane production rate and methane conversion rate.

[0014] The treatment device of the present invention requires a short residence time to convert excess sludge into easily decomposable sludge, and therefore can be used as a tubular pretreatment tank such as a line mixer, eliminating the need for large-volume tanks such as conventional solubilization tanks. Furthermore, it is possible to provide a treatment device with a simple configuration that achieves a high methane production rate and methane conversion rate without requiring special equipment such as a heater or ozone generator. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a schematic explanatory diagram of one embodiment of a wastewater treatment method of the present invention. [Figure 2] FIG. 2 is a schematic explanatory diagram of another embodiment of the 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 another embodiment of the wastewater treatment method of the present invention. [Figure 5] FIG. 2 is a schematic explanatory diagram of another embodiment of the wastewater treatment method of the present invention. [Figure 6] FIG. 2 is a schematic explanatory diagram of another embodiment of the wastewater treatment method of the present invention. [Figure 7] FIG. 2 is a schematic explanatory diagram of another embodiment of the wastewater treatment method of the present invention. [Figure 8] FIG. 2 is a schematic explanatory diagram of another embodiment of the wastewater treatment method of the present invention. [Figure 9] FIG. 2 is an explanatory diagram showing the relationship between the pH and zeta potential of sludge. [Figure 10] 1 is a schematic explanatory diagram of an embodiment of a wastewater treatment device of the present invention. [Figure 11] FIG. 4 is a schematic explanatory diagram of another embodiment of the wastewater treatment device of the present invention. [Figure 12] FIG. 4 is a schematic explanatory diagram of another embodiment of the wastewater treatment device of the present invention. [Figure 13] FIG. 4 is a schematic explanatory diagram of another embodiment of the wastewater treatment device of the present invention. [Figure 14] FIG. 4 is a schematic explanatory diagram of another embodiment of the wastewater treatment device of the present invention. [Figure 15]FIG. 4 is a schematic explanatory diagram of another embodiment of the wastewater treatment device of the present invention. [Figure 16] FIG. 4 is a schematic explanatory diagram of another embodiment of the wastewater treatment device of the present invention. [Figure 17] FIG. 4 is a schematic explanatory diagram of another embodiment of the wastewater treatment device of the present invention. [Figure 18] FIG. 2 is a schematic explanatory diagram showing a mechanism for controlling the amount of inorganic acid added to a pretreatment tank. [Figure 19] 1 is a graph showing methane conversion when a flocculant is added according to the wastewater treatment method of the present invention in Example 1. [Figure 20] 1 is a graph showing the methane conversion rate when no flocculant was added in Example 1 (Comparative Example). Preferred Embodiments

[0016] 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.

[0017] Figure 1 shows an outline of the basic treatment flow of the wastewater treatment method of the present invention. This wastewater treatment method includes an anaerobic digestion step 15 in which excess sludge generated after biological treatment is anaerobically digested. The method is characterized by the following steps: an excess sludge flocculation step 30 in which a flocculant is added to the excess sludge to produce flocculated sludge; an excess sludge concentration step 19 in which the flocculated sludge is concentrated to obtain concentrated sludge; and a pretreatment step 14 in which an inorganic acid is added to the concentrated sludge to adjust the pH to 5 or less, preferably 4 or less, and more preferably 3 or less, to break down the extracellular polymeric substances (EPS) of the microorganisms in the concentrated sludge to obtain easily degradable sludge. Figure 1 also shows an embodiment of the wastewater treatment method further including a dehydration step 16 in which the sludge after anaerobic digestion is dehydrated.

[0018] In the excess sludge flocculation step 30, a flocculant is added to the excess sludge to flocculate the excess sludge. Suitable flocculants include organic polymer flocculants, particularly cationic or amphoteric organic polymer flocculants, such as amidine-based flocculants, acrylamide-based flocculants, acrylic acid-based flocculants, acrylic acid ester-based flocculants, and methacrylic acid ester-based flocculants. In particular, flocculants with a molecular weight of approximately 2,000,000 to 10,000,000 and a viscosity of 100 to 800 mPa·s when dissolved at 0.2% are preferred.

[0019] According to the treatment method of the present invention, in which a flocculant is added to excess sludge to produce flocculated sludge, the flocculated sludge is concentrated, and then pretreated and subjected to anaerobic digestion, as shown in the examples and comparative examples described below, the methane conversion rate and methane gas generation rate can be significantly improved compared to when excess sludge concentrated without the addition of a flocculant is pretreated and then subjected to anaerobic digestion, or when excess sludge concentrated with the addition of a flocculant is subjected to anaerobic digestion. While the details of the mechanism by which the addition of a flocculant increases methane gas generation are not clear, it is thought that adding a flocculant in the excess sludge flocculation step changes the flocculation state of the organic matter in the excess sludge, for example, resulting in a form that makes extracellular organic matter more susceptible to elution during the acid treatment in the subsequent pretreatment step.

[0020] The excess sludge thickening step 19 thickens the flocculated sludge obtained by flocculating the excess sludge in the excess sludge flocculation step 30. As a thickening method, a method of dehydrating and thickening the excess sludge using a known thickening device such as a belt thickener, a centrifugal thickener, or an elliptical thickener can be suitably used.

[0021] The thickened sludge to be subjected to the pretreatment step 14 desirably has a sludge concentration of 1.5 wt% or more and less than 4.5 wt%, preferably 1.8 wt% or more, more preferably 2 wt% or more, preferably 4.3 wt% or less, and more preferably 4 wt% or less. In the pretreatment step 14, if the sludge concentration of the thickened sludge is too high, the thickened sludge cannot be mixed sufficiently with the acid.

[0022] In the pretreatment step 14, an inorganic acid is added to the concentrated sludge to adjust the pH to 5 or less, preferably 4 or less, and more preferably 3 or less, thereby breaking down the extracellular polymeric substances (EPMS) of microorganisms in the concentrated sludge and producing easily degradable sludge. In the pretreatment step 14, a pH of 2 or greater is preferred because a too low pH necessitates the addition of a large amount of alkaline agents to adjust the pH to the optimal pH (neutral) for anaerobic digestion, resulting in increased costs. Typically, when high-concentration sludge is subjected to anaerobic digestion, the alkalinity increases due to the production of ammoniacal nitrogen (NH4-N) during anaerobic digestion, which deteriorates dewaterability. Therefore, lowering the pH before anaerobic digestion is also effective for treating high-concentration sludge. Furthermore, this method prevents the release of organic matter, which constitutes the cells and 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 dewatering process, effectively preventing color deterioration. Furthermore, compared to solubilized sludge obtained by conventional alkaline treatment, the easily decomposable sludge produced by pretreatment using inorganic acid addition according to the present invention has less odor, lower viscosity, and higher fluidity, making it easier to handle and reducing the power required for transportation and stirring.

[0023] In the pretreatment process 14, adding inorganic acid to the concentrated sludge breaks down the extracellular polymeric substances (EPMS) of the microorganisms that make up the activated sludge, turning it into easily degradable sludge (easily degradable sludge). Scale components such as phosphate-type phosphorus (PO4-P), ammoniacal nitrogen (NH4-N), Ca, Mg, Fe, and Mn, which were held by the EPS, are eluted as ions, breaking down the activated sludge particles. Furthermore, the bound water held by the EPS is converted into free water and released, improving the dewaterability of the anaerobically digested sludge in the dewatering process and contributing to sludge volume reduction.

[0024] Examples of inorganic acids that can be added in the pretreatment process include hydrochloric acid, sulfuric acid, nitric acid, nitrous acid, and phosphoric acid. Hydrochloric acid is particularly 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 methane gas generation less than hydrochloric acid, but have a similar sludge volume reduction effect. Sulfuric acid obtained by biological desulfurization of odorous components such as hydrogen sulfide contained in biogas generated in the anaerobic digestion process can also be used, as can sulfuric acid, nitric acid, and nitrous acid obtained by biological deodorization of odorous components generated in wastewater treatment facilities. Acids are also less expensive than alkaline agents, which has the advantage of reducing chemical costs.

[0025] In the pretreatment step 14, the pH of the concentrated sludge is measured and the amount of inorganic acid added is controlled. For example, if the pH is higher than the above range, inorganic acid is added, and if the pH is within the range, the addition of inorganic acid is stopped. The inorganic acid may be added manually or automatically by a control device.

[0026] The amount of inorganic acid added in the pretreatment step 14 is preferably set so that the divalent or higher soluble metals (e.g., S-Ca, S-Mg, S-Fe, S-Mn) contained in the excess thickened sludge account for 30 wt% or more, preferably 40 wt% or more, of the total amount of metals contained in the excess sludge. As the anaerobic digestion reaction progresses, ammoniacal nitrogen (NH4-N) is generated, causing the pH to rise to 6-8.5. However, adjusting the pH to 5 or less in the pretreatment step can suppress this increase in pH during the anaerobic digestion treatment step. The amount of inorganic acid added in the pretreatment step can also be controlled depending on the pH of the sludge in the anaerobic digestion treatment step. For example, if the pH of the sludge in the anaerobic digestion step is 7.5 or higher, the amount of inorganic acid added is adjusted to a pH of 2-3 in the pretreatment step. If the pH of the sludge in the anaerobic digestion step is 7.0-7.5, the amount of inorganic acid added is adjusted to a pH of 3-4 in the pretreatment step. If the pH of the sludge in the anaerobic digestion process is 7.0 or less, an inorganic acid is added in the pretreatment process in an amount sufficient to bring the sludge pH to 4 to 5. Lowering the pH of the sludge by adding an inorganic acid in the pretreatment process suppresses the increase in the pH of the sludge in the anaerobic digestion process, thereby suppressing the elution of scale components derived from soluble metals and preventing scale formation in the anaerobic digestion process. Alternatively, the amount of inorganic acid added in the pretreatment process may be controlled using indicators such as the alkalinity of the sludge in the anaerobic digestion process, the organic acid concentration, and the amount of methane gas generated. Furthermore, the amount of inorganic acid added may be controlled by measuring the zeta potential of the sludge in the anaerobic digestion process. The surface charge of sludge is negative at alkaline or near-neutral pH, but approaches zero (the isoelectric point) as the pH is lowered. It is preferable to control the amount of inorganic acid added so as to approach the isoelectric point. Since the zeta potential varies depending on the properties of the sludge, for example, in the case of sludge having the relationship between zeta potential and pH shown in Figure 9(a), inorganic acid can be added so that the zeta potential is in the range of -4.5mV to 1mV, and in the case of sludge having the relationship between zeta potential and pH shown in Figure 9(b), inorganic acid can be added so that the zeta potential is in the range of -7mV to 1mV.The amount of inorganic acid added in the pretreatment step is preferably determined by collecting and analyzing the excess sludge to be treated in advance to determine the amount of acid required to be added, and then determining an appropriate amount to be added.

[0027] Because the decomposition reaction by adding inorganic acids is highly rapid, the retention time in the pretreatment process can be very short, from 0.01 to 24 hours, preferably 12 hours or less, and particularly preferably 3 hours or less. Furthermore, when high-concentration sludge is subjected to anaerobic digestion, the alkalinity increases due to the ammoniacal nitrogen (NH4-N) produced during anaerobic digestion, which deteriorates dewaterability. Therefore, lowering the pH before anaerobic digestion is also effective in treating high-concentration sludge. Furthermore, this method prevents the release of organic matter constituting the cells that cause 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 dewatering process, effectively preventing color deterioration. Furthermore, compared to solubilized sludge obtained by conventional alkaline treatment, the decomposition-enhancing sludge produced by pretreatment with inorganic acids according to 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] Furthermore, the pretreatment step 14 is preferably carried out at a temperature of 50°C or lower, more preferably at a temperature of 10°C or higher and 25°C or lower.

[0029] The easily decomposable sludge obtained in the pretreatment process 14 is different from the solubilized sludge (solubility degree of approximately 40% or more) obtained in conventional sludge solubilization processes, and it is desirable that the solubilization degree calculated by the following formula is 15% or less, preferably less than 15%, and more preferably 10% or less.

[0030]

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[0031] The wastewater treatment method of the present invention can use excess sludge generated by general biological treatment as the post-biological treatment excess sludge. For example, the embodiment shown in Figure 1 includes a first solid-liquid separation step 11 for 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, 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 may be returned to the biological treatment step 12 as returned sludge, and the remainder of the sludge may be supplied to a pretreatment step 14 as excess sludge. The first solid-liquid separation step 11 is not essential and can be omitted.

[0032] As shown in FIG. 2, the wastewater treatment method of the present invention further includes an inorganic acid production step 20 in which odorous components such as hydrogen sulfide contained in the biogas from the anaerobic digestion treatment step 15 are treated with microorganisms to generate acids such as sulfuric acid, and the inorganic acid from the inorganic acid production step 20 can also be added to the pretreatment step 14.

[0033] As shown in FIG. 3, the wastewater treatment method of the present invention may further include, before the anaerobic digestion treatment step 15, a separated sludge concentration step 17 for concentrating the separated sludge separated in the first solid-liquid separation step 11, and a sludge mixing step 18 for mixing the concentrated sludge concentrated in the separated sludge concentration step 17 with the easily decomposable sludge from the pretreatment step 14.

[0034] As shown in FIG. 4 , the wastewater treatment method of the present invention further includes, before the anaerobic digestion treatment step 15, a separated sludge concentration step 17 for concentrating the separated sludge separated in the first solid-liquid separation step 11, and a sludge mixing step 18 for mixing the concentrated sludge concentrated in the separated sludge concentration step 17 with the easily decomposable sludge from the pretreatment step 14. The method also includes an inorganic acid production step 20 for treating odorous components such as hydrogen sulfide contained in the biogas from the anaerobic digestion treatment step 15 with microorganisms to generate acids such as sulfuric acid, and the inorganic acid from the inorganic acid production step 20 can also be added to the pretreatment step 14.

[0035] Fig. 5 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 Fig. 5 includes an anaerobic digestion treatment step 15 in which excess sludge generated after biological treatment is subjected to anaerobic digestion, and a dehydration step 16 in which the sludge after anaerobic digestion is dehydrated. Prior to the anaerobic digestion treatment step 15, the method includes an excess sludge flocculation step 30 in which a flocculant is added to the excess sludge to produce flocculated sludge, an excess sludge concentration step 19 in which the flocculated sludge is concentrated to obtain concentrated sludge, a dilution step 40 in which the concentrated sludge is diluted to a predetermined concentration, and a pretreatment step 14 in which an inorganic acid is added to the concentrated sludge to adjust the pH to 5 or less, preferably 4 or less, and more preferably 3 or less, to break down the extracellular polymeric substances of the microorganisms in the concentrated sludge to obtain easily degradable sludge.

[0036] In the excess sludge flocculation step 30, a flocculant is added to the excess sludge to flocculate the excess sludge. As the flocculant, an organic polymer flocculant, particularly a cationic or amphoteric organic polymer flocculant, can be suitably used.

[0037] According to the treatment method of the present invention, in which a flocculant is added to excess sludge to produce flocculated sludge, the flocculated sludge is concentrated, and then pretreated and subjected to anaerobic digestion, as shown in the examples and comparative examples described below, the methane conversion rate and methane gas generation rate can be significantly improved compared to when excess sludge concentrated without the addition of a flocculant is pretreated and then subjected to anaerobic digestion, or when excess sludge concentrated with the addition of a flocculant is subjected to anaerobic digestion. While the details of the mechanism by which the addition of a flocculant increases methane gas generation are not clear, it is thought that adding a flocculant in the excess sludge flocculation step changes the flocculation state of the organic matter in the excess sludge, for example, resulting in a form that makes extracellular organic matter more susceptible to elution during the acid treatment in the subsequent pretreatment step.

[0038] The excess sludge thickening step 19 thickens the flocculated sludge obtained by flocculating the excess sludge in the excess sludge flocculation step 30. As a thickening method, a method of dehydrating and thickening the excess sludge using a known thickening device such as a belt thickener, a centrifugal thickener, or an elliptical thickener can be suitably used.

[0039] The thickened sludge to be subjected to the pretreatment step 14 has a sludge concentration of 1.5 wt% or more but less than 4.5 wt%, preferably 1.8 wt% or more, more preferably 2 wt% or more, preferably 4.3 wt% or less, and more preferably 4 wt% or less. If the sludge concentration is too high, mixing of the thickened sludge with acid does not proceed sufficiently in the subsequent pretreatment step 14. If the sludge concentration of the thickened sludge obtained in the concentration step 19 is 4.5 wt% or more, dilution water is added to the thickened sludge in the subsequent dilution step 40 to adjust the sludge concentration to a predetermined range.

[0040] In the dilution step 40, the concentrated sludge obtained in the excess sludge concentration step 19 is diluted with dilution water to adjust the sludge concentration to the predetermined value. Suitable dilution water includes groundwater, tap water, treated water, and the like.

[0041] The pretreatment step 14 and the excess sludge after biological treatment have been described in detail with reference to FIG. 1, and therefore a detailed description thereof will be omitted here.

[0042] As shown in FIG. 6, the wastewater treatment method of the present invention further includes an inorganic acid production step 20 in which odorous components such as hydrogen sulfide contained in the biogas from the anaerobic digestion treatment step 15 are treated with microorganisms to generate acids such as sulfuric acid, and the inorganic acid from the inorganic acid production step 20 can also be added to the pretreatment step 14.

[0043] As shown in FIG. 7, the wastewater treatment method of the present invention may further include, before the anaerobic digestion treatment step 15, a separated sludge concentration step 17 for concentrating the separated sludge separated in the first solid-liquid separation step 11, and a sludge mixing step 18 for mixing the concentrated sludge concentrated in the separated sludge concentration step 17 with the easily decomposable sludge from the pretreatment step 14.

[0044] As shown in FIG. 8 , the wastewater treatment method of the present invention further includes, prior to the anaerobic digestion treatment step 15, a separated sludge concentration step 17 for concentrating the separated sludge separated in the first solid-liquid separation step 11, and a sludge mixing step 18 for mixing the concentrated sludge concentrated in the separated sludge concentration step 17 with the easily decomposable sludge from the pretreatment step 14. The method also includes an inorganic acid production step 20 for treating odorous components such as hydrogen sulfide contained in the biogas from the anaerobic digestion treatment step 15 with microorganisms to generate acids such as sulfuric acid, and the inorganic acid from the inorganic acid production step 20 can also be added to the pretreatment step 14.

[0045] The wastewater treatment device of the present invention is an apparatus capable of carrying out the wastewater treatment method described above. The wastewater treatment device shown in Figure 10 is characterized by comprising an excess sludge coagulation tank 130 in which a coagulant is injected into excess sludge from a biological treatment tank 112 to form coagulated sludge; an excess sludge concentration tank (mechanical thickener) 119 in which the coagulated sludge is thickened to form thickened sludge; a pretreatment tank 114 in which an inorganic acid is added to the thickened sludge from the excess sludge concentration tank 119 to adjust the pH to 5 or less and break down the extracellular polymeric substances of the microorganisms to obtain easily biodegradable sludge; and an anaerobic digestion tank 115 in which the easily biodegradable sludge from the pretreatment tank 114 is subjected to anaerobic digestion. Figure 10 shows an embodiment of the wastewater treatment device further comprising a dehydrator 116 for dehydrating the sludge from the anaerobic digestion tank 115.

[0046] The biological treatment system includes a first solid-liquid separation means (primary sedimentation tank) 111 for separating wastewater into sludge and separated water, a biological treatment tank 112 for biologically treating the separated water from the first solid-liquid separation means (primary sedimentation tank) 111, and a second solid-liquid separation means (final sedimentation tank) 113 for separating the biologically treated water from the biological treatment tank 112 into sludge and treated water. A portion of the sludge from the second solid-liquid separation means (final sedimentation tank) 113 is returned to the biological treatment tank 112 as returned sludge, and the remaining sludge can be reused. In the illustrated embodiment, the system is configured to suppress the amount of excess sludge generated and maintain a sufficiently high sludge concentration during biological treatment. The first solid-liquid separation means 111 can be omitted. The second solid-liquid separation means 113 may be provided separately from the biological treatment tank 112 as shown, or it may be provided within the biological treatment tank 112. A primary settling tank is used as the first solid-liquid separation means 111, and a final settling tank is used as the second solid-liquid separation means 113, but the present invention is not limited to these, and any solid-liquid separation means commonly used in wastewater treatment can be used, such as a gravity filter, a compression filter, a vacuum filter, an atmospheric flotation thickener, a centrifugal thickener, a belt-type filter thickener, a submerged membrane separator, an external membrane separator, etc. When installed inside the biological treatment tank 112, the second solid-liquid separation means 113 is preferably a submerged membrane separator.

[0047] The biological treatment tank 112 can preferably be an activated sludge tank, a trickling filter, an oxidation ditch tank, or the like. A submerged membrane separation device can also be immersed inside the biological treatment tank 112. The anaerobic digestion tank 115 can preferably be a UASB-type anaerobic tank, an anaerobic membrane separation tank, a carrier-loaded digestion tank, or the like. The dehydrator 116 can preferably be 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.

[0048] The pretreatment tank 114 may be any tank with a volume large enough to treat the excess sludge from the biological treatment tank 112 at a pH of 5 or less, preferably between 2 and 3, preferably at 50°C or less, and more preferably at room temperature (10°C to 25°C), and to ensure the residence time required to break down the extracellular polymeric substances of microorganisms in the excess sludge. The tank shape is not limited, and a tubular shape such as a line mixer may be used. Because the pretreatment step breaks down the extracellular polymeric substances of microorganisms but does not completely dissolve the cell walls, a short treatment time is preferable. The residence time is preferably 0.01 to 24 hours, preferably 12 hours or less, more preferably 3 hours or less, and even more preferably 0.01 to 2 hours. The volume of the pretreatment tank may be small. The pretreatment tank 114 may be equipped with a pH adjuster addition means in addition to an acid addition means. It is also preferable to equip the pretreatment tank with an agitation means (not shown) to ensure uniform contact between the sludge and the acid. A line mixer, which can be installed in the piping, is particularly preferable because it does not require heating and has a short residence time. 18, the pretreatment tank 114 is preferably equipped with sludge analysis means such as a pH meter 151 and a zeta potential meter 152, acid addition amount adjustment means 153 for adjusting the amount of inorganic acid added, and pH adjuster addition amount adjustment means (not shown) for appropriately controlling the amount of inorganic acid added. Because the easily decomposed reaction using inorganic acid is highly effective, it is efficient to add the inorganic acid near the inlet of the pretreatment tank 114, where the sludge concentration is high. Inorganic acid may also be added upstream of the pretreatment tank 114, upstream of the excess sludge concentration tank 119.

[0049] Another embodiment shown in FIG. 11 further includes, in addition to the apparatus configuration of FIG. 10, a biological desulfurization unit 120 that receives biogas from the anaerobic digestion tank 115 and produces inorganic acid from odorous components in the biogas, and / or a biological deodorization unit (not shown) that produces inorganic acid from odorous gases generated within the wastewater treatment facility, and an inorganic acid supply line that supplies inorganic acid from the biological desulfurization unit 120 and / or the biological deodorization unit to the pretreatment tank 114.

[0050] In another embodiment shown in Figure 12, in addition to the device configuration of Figure 10, a solid-liquid separation means (primary sedimentation tank) 111 that separates wastewater into separated water and separated sludge is provided upstream of the biological treatment tank 112, and a separated sludge concentration tank (gravity thickener) 117 that thickens the separated sludge from the solid-liquid separation means (primary sedimentation tank) 111, and a sludge concentration mixing tank (sludge storage tank) 118 that mixes the concentrated sludge from the separated sludge concentration tank (gravity thickener) 117 with the easily decomposable sludge from the pretreatment tank 114 are provided between the pretreatment tank 114 and the anaerobic digestion treatment tank 115.

[0051] In another embodiment shown in FIG. 13, in addition to the device configuration of FIG. 10, a solid-liquid separation means (primary sedimentation tank) 111 for separating wastewater into separated water and separated sludge is provided in the upstream stage of the biological treatment tank 112, a separated sludge thickening tank (gravity thickener) 117 for thickening the separated sludge from the solid-liquid separation means (primary sedimentation tank) 111, and a sludge thickening and mixing tank (sludge storage tank) for mixing the thickened sludge from the separated sludge thickening tank (gravity thickener) 117 with the easily decomposable sludge from the pretreatment tank 114. The wastewater treatment facility further comprises a biological desulfurization unit 120 that receives biogas from the anaerobic digestion tank 115 and produces inorganic acid from odorous components in the biogas, and / or a biological deodorization unit (not shown) that produces inorganic acid from odorous gases generated within the wastewater treatment facility, and an inorganic acid supply line that supplies inorganic acid from the biological desulfurization unit 120 and / or the biological deodorization unit to the pretreatment tank 114.

[0052] In the embodiment shown in FIG. 14, in addition to the device configuration shown in FIG. 10, a dilution tank 140 for diluting concentrated sludge is further provided between the excess sludge concentration tank 119 and the pretreatment tank 114.

[0053] In the embodiment shown in FIG. 15, in addition to the device configuration shown in FIG. 11, a dilution tank 140 for diluting concentrated sludge is further provided between the excess sludge concentration tank 119 and the pretreatment tank 114.

[0054] In the embodiment shown in FIG. 16, in addition to the device configuration shown in FIG. 12, a dilution tank 140 for diluting concentrated sludge is further provided between the excess sludge concentration tank 119 and the pretreatment tank 114.

[0055] In the embodiment shown in FIG. 17, in addition to the device configuration shown in FIG. 13, a dilution tank 140 for diluting concentrated sludge is further provided between the excess sludge concentration tank 119 and the pretreatment tank 114.

[0056] 18 shows a mechanism for controlling the addition of inorganic acid to the pretreatment tank 114. The pretreatment tank 114 is provided with a pH meter 151 that measures the pH of the thickened sludge in the pretreatment tank 114. The pH value measured by the pH meter 151 is sent to a control device 153, which then sends a signal to the inorganic acid adding device 150 indicating whether or not inorganic acid needs to be added and the amount to be added. The inorganic acid adding device 150 adds a predetermined amount of inorganic acid to the pretreatment tank 114 or stops adding inorganic acid according to instructions from the control device 153.

[0057] A zeta potential meter 152 may be provided to measure the zeta potential of the thickened sludge in the pretreatment tank 114 and the treated sludge from the pretreatment tank 114. FIG. 18 shows an embodiment in which the sludge after zeta potential measurement is added to a sludge thickening and mixing tank (sludge storage tank) 118 and treated together. There is a correlation between pH and zeta potential, but the value varies depending on the properties of the sludge. The relationship between the pH and zeta potential of the target sludge can be investigated in advance, and the addition of inorganic acid can be controlled by measuring the zeta potential instead of measuring the pH. [Example]

[0058] The present invention will be specifically described below using examples, but the present invention is not limited to these examples.

[0059] [Example 1] An organic polymer flocculant (Evergrose C-104G, manufactured by Swing Co., Ltd.) was added to the excess sludge shown in Table 1 to form flocculated sludge. The flocculated sludge was then mechanically thickened to form a 2.5-fold diluted sludge (referred to as "flocculant-added thickened sludge"), and the sludge was returned to the laboratory without flocculant and thickened three-fold (referred to as "flocculant-free unthickened sludge"). Hydrochloric acid was added to each sludge at 25°C to adjust the pH to 2 or 4, and after undergoing an easily biodegradable treatment, the sludge was added to a container containing the digested sludge shown in Table 1 and subjected to anaerobic digestion at 35°C under a load of approximately 0.45 g-VS / g-VSS. The properties of each sludge are shown in Table 1.

[0060] [Table 1]

[0061] [Solubilization degree] Using the excess sludge shown in Table 1, the pH was adjusted to 2 or 4 by adding hydrochloric acid, and the excess sludge was shaken in a thermostatic chamber at 25°C for 24 hours. Cr ("Before treatment COD Cr "), soluble COD of excess sludge Cr ("Pre-processing S-COD Cr "), and the soluble COD of the treated sludge Cr ("S-COD after treatment" Cr The soluble COD was measured and the degree of solubilization was calculated. Cr (S-COD Cr The solubilization rate was measured using the filtrate obtained by filtering through a filter paper with a pore size of 1 μm. The results are shown in Table 2. The solubilization rate was calculated using the following formula.

[0062]

number

[0063] [Table 2]

[0064] For thickened sludge treated with coagulant, the solubilization degree of untreated sludge at pH 6 was 6.3%, at pH 2 it was 7.0%, and at pH 4 it was 6.2%. Alkaline treatment at pH 12 not only destroyed extracellular polymeric substances but also cell walls (cell membranes), releasing intracellular organic matter, resulting in a high solubilization degree of 44.2%. For thickened sludge without coagulant, the solubilization degree of untreated sludge at pH 6 was 7.1%, at pH 2 it was 12.4%, and at pH 4 it was 9.7%.

[0065] [Methane conversion rate] 100 mL each of easily decomposable sludge treated at pH 2 or 4 and untreated sludge at pH 6 was added to 400 mL of anaerobically digested sludge, and the mixture was stirred in a constant temperature bath at 35°C for about one month. Cr and the amount of gas generated were measured, and the COD Cr The methane conversion rate was calculated using the formula: where the methane gas conversion coefficient is a constant (0.35).

[0066]

number

[0067] FIG. 19 shows the methane conversion rate when a flocculant was added by the wastewater treatment method of the present invention, and FIG. 20 shows the methane conversion rate when no flocculant was added.

[0068] For the concentrated sludge treated with coagulant, the sludge treated for easy decomposition at pH 2 achieved a methane conversion rate of over 40% in approximately 100 hours (4 days), and after 330 hours (14 days) it was 57%. For the sludge treated for easy decomposition at pH 4, a methane conversion rate of over 40% was achieved in approximately 100 hours (4 days), similar to pH 2, and after 288 hours (12 days) it was 50%. In contrast, for the untreated sludge at pH 6, a methane conversion rate of 33% was achieved in approximately 100 hours (4 days), similar to the other conditions, but after 192 hours (8 days) it was 37%. It can be seen that the addition of coagulant and inorganic acid significantly improves methane conversion.

[0069] For unconcentrated sludge with added coagulant, the untreated sludge at pH 6 had a methane conversion rate of 32% after approximately 100 hours (4 days) and 39% after approximately 310 hours (13 days), similar to that of concentrated sludge with added coagulant. For sludge treated for easy biodegradation at pH 2, the methane conversion rate was 36% after approximately 100 hours (4 days) and 44% after approximately 380 hours (16 days). For sludge treated for easy biodegradation at pH 4, the methane conversion rate was 38% after approximately 100 hours (4 days) and 44% after 192 hours (8 days). While adjusting the pH improves methane conversion, it was confirmed that this method is less effective at improving methane conversion than adding a coagulant.

[0070] The above results confirm that the addition of a flocculant increased the methane conversion rate by approximately 4 to 8 points after 4 days, ultimately increasing it by 13 points at pH 2 and 6 points at pH 4. This confirms that adding a flocculant to excess sludge and concentrating it can improve the methane conversion rate in a short period of time. Therefore, the wastewater treatment method of the present invention can recover methane gas more efficiently than a methane fermentation treatment method in which concentration is performed without adding a flocculant. [Explanation of symbols]

[0071] 11: First solid-liquid separation step 12: Biological treatment process 13: Second solid-liquid separation process 14: Pre-treatment process 15: Anaerobic digestion process 16: Dehydration process 17: Separated sludge thickening process 18: Sludge mixing process 19: Excess sludge thickening process 20: Acid generation process 30: Excess sludge coagulation process 40: Dilution process 111: First solid-liquid separation means (first sedimentation tank) 112: Biological treatment tank 113: Second solid-liquid separation means (final settling tank, submerged membrane separation device) 114: Pretreatment tank 115: Anaerobic digestion treatment tank 116: Dehydrator 117: Gravity concentrator 118: Sludge mixing tank 119: Excess sludge thickener (mechanical thickener) 120: Biological desulfurization equipment 130: Excess sludge coagulation tank 140: Dilution tank

Claims

1. A wastewater treatment method including an anaerobic digestion treatment step of anaerobic digestion of excess sludge generated after biological treatment, Before the anaerobic digestion treatment step, an excess sludge flocculation step of injecting a flocculant into the excess sludge to flocculate the excess sludge and obtain flocculated sludge; an excess sludge concentration step of concentrating the flocculated sludge to obtain concentrated sludge; A wastewater treatment method characterized by including a pretreatment step in which an inorganic acid is added to the concentrated sludge to adjust the pH to 5 or less, and the extracellular polymeric substances of the microorganisms in the concentrated sludge are broken down to obtain easily decomposable sludge.

2. 2. The wastewater treatment method according to claim 1, wherein the easily decomposable sludge obtained in the pretreatment step has a solubilization degree calculated by the following formula of 15% or less. [Equation 1]

3. 2. The wastewater treatment method according to claim 1, wherein the concentrated sludge subjected to the pretreatment step has a sludge concentration of 1.5 wt % or more and less than 4.5 wt %.

4. 3. The wastewater treatment method according to claim 1, wherein the pretreatment step is carried out at a temperature of 50° C. or less.

5. 3. The wastewater treatment method according to claim 1, further comprising a dilution step between the excess sludge concentration step and the pretreatment step, in which the concentrated sludge from the excess sludge concentration step is diluted.

6. 3. The wastewater treatment method according to claim 1, further comprising: a solid-liquid separation step for performing solid-liquid separation of wastewater into sludge and separated water; a biological treatment step for biologically treating the separated water from the solid-liquid separation step; a separated sludge concentration step for concentrating the separated sludge from the solid-liquid separation step; and a sludge mixing step for mixing the concentrated sludge from the separated sludge concentration step with the easily decomposable sludge from the pretreatment step prior to the anaerobic digestion treatment step.

7. 3. The wastewater treatment method according to claim 1, wherein inorganic acids obtained by biological desulfurization of odorous components in the biogas from the anaerobic digestion treatment step and / or inorganic acids obtained by biological deodorization of odorous gases generated in the wastewater treatment facility are supplied to the pretreatment step.

8. 3. The wastewater treatment method according to claim 1, wherein the pH of the concentrated sludge in the pretreatment step is measured and the addition of the inorganic acid in the pretreatment step is controlled.

9. A wastewater treatment device, an excess sludge coagulation tank in which a coagulant is injected into excess sludge from the biological treatment tank to form coagulated sludge; an excess sludge thickening tank for thickening the flocculated sludge to form thickened sludge; a pretreatment tank in which an inorganic acid is added to the concentrated sludge from the excess sludge concentration tank to adjust the pH to 5 or less and break down the extracellular polymeric substances of the microorganisms to obtain easily degradable sludge; an anaerobic digestion tank for anaerobic digestion of the easily decomposable sludge from the pretreatment tank; A wastewater treatment device comprising:

10. The wastewater treatment device according to claim 9, further comprising a dilution tank for diluting the concentrated sludge between the excess sludge concentration tank and the pretreatment tank.

11. A solid-liquid separation means for separating the wastewater into separated water and separated sludge is provided upstream of the biological treatment tank, a separated sludge thickening tank for thickening the separated sludge from the solid-liquid separation means; 11. The wastewater treatment device according to claim 9 or 10, wherein a sludge concentration and mixing tank for mixing the concentrated sludge from the separated sludge concentration tank and the easily decomposable sludge from the pretreatment tank is provided between the pretreatment tank and the anaerobic digestion treatment tank.

12. a biological desulfurization device that receives the biogas from the anaerobic digestion tank and produces inorganic acids from odorous components in the biogas, and / or a biological deodorization device that produces inorganic acids from odorous gases generated in a wastewater treatment facility; an inorganic acid supply line that supplies inorganic acid from the biological desulfurization device and / or the biological deodorization device to the pretreatment tank; The wastewater treatment device according to claim 9 or 10, further comprising:

13. a pH meter for measuring the pH of the concentrated sludge in the pretreatment tank; a control device that controls the addition of inorganic acid to the pretreatment tank based on the pH measured by the pH meter; The wastewater treatment device according to claim 9 or 10, further comprising:

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