Wastewater treatment method and wastewater treatment apparatus

Abstract

To provide a wastewater treatment method and apparatus, in which an anaerobic treatment is included that can solve such problems associated with a conventional sludge solubilization technology as costs, ozone generators, odor, and color of separated water, to improve sludge dewaterability and increase the methane gas production rate and methane conversion rate.SOLUTION: A wastewater treatment method includes anaerobic digestion treatment step 15 in which excess sludge generated after biological treatment is subjected to anaerobic digestion, and dehydration step 16 in which the sludge after anaerobic digestion is dehydrated. The wastewater treatment method comprises pretreatment step 14 in which an inorganic acid is added to the excess sludge to adjust pH to be 5 or lower, and extracellular polymeric material of the microorganisms in the excess sludge is fragmented at a temperatures of 50°C or below to obtain easily decomposable sludge, before the anaerobic digestion treatment step 15. The amount of inorganic acid added in the pretreatment step 14 is controlled based on one or more of the following values: alkalinity of the sludge, the organic acid concentration, the amount of generated methane gas, and the zeta potential in the anaerobic digestion treatment step 15.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 that includes anaerobic treatment capable of improving the dewatering properties of sludge and improving the methane gas generation 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 technologies include those utilizing microwaves, ozone, heat, ultrasound, and alkaline treatment.

[0003] For example, Japanese Patent Publication No. 2009-255088 (Patent Document 1) discloses a solubilization treatment using ozone in which an excess sludge solubilization device is provided that includes a maturation tank that agitates the ozone-treated excess sludge with compressed air and sends it back to a biological treatment tank, and a de-ozone tank that adsorbs ozone from the ozone-containing gas generated in the maturation tank, thereby enabling the biodegradation of substantially all excess sludge by sufficiently solubilizing it with a small amount of ozone consumption before returning it to the biological treatment tank.

[0004] Japanese Patent Publication No. 2016-221491 (Patent Document 2) discloses a method of solubilizing sludge by heating it in an alkaline atmosphere with a pH of 11 or higher at a temperature of 40°C to 100°C, and then subjecting it to anaerobic biological treatment to generate methane gas through the decomposition of the solubilized sludge, thereby improving the amount of methane gas generated in the anaerobic treatment.

[0005] Japanese Patent Publication No. 2012-183510 (Patent Document 3) discloses a method for increasing the yield of hydrogen and methane gas and improving the volume reduction rate of organic waste residue by solubilizing anaerobic digested sludge at pH 5 to 7 and 50°C to 90°C using high-temperature solubilizing bacteria or ultra-high-temperature solubilizing bacteria, and then performing anaerobic digestion treatment on the solubilized organic waste and the H2-containing gas generated during solubilization.

[0006] Japanese Patent Publication No. 2016-117066 (Patent Document 4) discloses an anaerobic treatment method in which a sludge concentrate with a sludge concentration of 4-12% is solubilized and subjected to acid fermentation treatment at 30-60°C for 1-3 days of HRT, followed by methane fermentation treatment.

[0007] Japanese Patent Publication No. 2004-275813 (Patent Document 5) discloses a sludge treatment method in which acid is added to the sludge to adjust the pH to 5 or less, an acid heating method is applied in which the sludge is heated to 60°C or higher, and then anaerobic digestion treatment is performed.

[0008] Japanese Patent Publication No. 56-16700 (Patent Document 6) discloses an anaerobic digestion method for sewage sludge, in which excess sludge is pretreated by heating it at 50-100°C for 3-24 hours to suppress the activity of obligate anaerobic bacteria, and then mixed with sludge drawn from the primary sedimentation tank and treated by a two-phase anaerobic digestion method consisting of an acid generation step and a gas generation step. It is stated that the pretreatment may be acid treatment at a pH of 4 or less for 3-24 hours. This publication is based on a two-phase system (acid treatment → acid generation → anaerobic digestion) that includes an acid generation step, and therefore the degree of sludge solubilization to be achieved in the pretreatment is small, and the pretreatment time can be shortened. However, if the acid treatment is performed directly followed by anaerobic digestion without acid generation after the acid treatment, the degree of sludge solubilization to be achieved will be large, and the pretreatment time will be prolonged.

[0009] Japanese Patent Publication No. 2007-330881 (Patent Document 7) describes crushed food waste and sludge concentration equipment. The description states that a portion of the concentrated separated liquid from the wastewater and excess sludge are introduced into a solubilization tank, where the pH decrease is buffered and the wastewater is treated at 30-60°C for a residence time of 1-48 hours to suppress the generation of biogas in the solubilization tank and obtain solubilized food waste, which is then subjected to methane fermentation in a subsequent methane fermentation tank to generate biogas.

[0010] Japanese Patent Application Laid-Open No. 52-5960 (Patent Document 8) describes an anaerobic digestion method in which acid heat pretreatment is performed at pH 3.5 or lower, 60°C or higher and 160°C or lower, for 1 minute or longer and 10 minutes or shorter, and then the mixture is held at a constant temperature for several days in a liquefaction tank under anaerobic conditions.

[0011] However, the conventional methods have problems such as cost, problems with ozone generators, odors, and the chromaticity of separated water, and thus have not been widely used in practice. Also, in the case of alkali treatment, there are problems such as deterioration of sludge dewaterability and chromaticity of separated water.

[0012] The OSA process has been proposed as a method to improve the energy consumption and operating cost, which are problems during high-temperature treatment and ozone treatment, and to improve the chromaticity of treated water and sludge dewaterability, which are problems during alkali treatment. For example, Japanese Patent Application Laid-Open No. 2020-142168 (Patent Document 9) discloses a method in which a part of the sludge after biological treatment is introduced into an anaerobic tank, micro-aerated treatment is performed in the presence of iron to decompose and re-substrate the sludge, and then it is returned to an activated sludge tank for biological treatment again. The OSA process described in Patent Document 5 can shorten the residence time and reduce the size of the anaerobic tank compared to the conventional OSA process, but there remains a problem that the residence time is still long and the anaerobic tank is still large compared to other treatment methods.

Prior Art Documents

Patent Documents

[0013]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

[0014] The present invention solves problems such as costs, ozone generators, odors, and colority of separated water in conventional sludge solubilization technologies, improves the dewaterability of sludge, and aims to provide a wastewater treatment method and apparatus including anaerobic treatment that can improve the methane gas generation rate and methane conversion rate. [Means for Solving the Problems]

[0015] In order to solve the above problems, as a result of intensive research by the present inventors, surplus sludge to be subjected to anaerobic digestion treatment is treated in an acidic atmosphere at 50°C or lower with an inorganic acid added to a pH of 5 or lower, preferably 4 or lower, more preferably 3 or lower, so that extracellular polymeric substances can be subdivided to about 1 μm without destroying the cell walls (cell membranes) of microorganisms in the surplus sludge, preventing deterioration of colority due to organic substances derived from microorganisms, while increasing the dissolved COD Cr (S-COD Cr ) component and the COD Cr component that can be easily decomposed (easily decomposed sludge) can be obtained. By subjecting the easily decomposed sludge to anaerobic digestion treatment, it was found that the amount of methane gas generated can be significantly increased, and the present invention was completed.

[0016] In the present invention, "easily decomposed" means making the extracellular polymeric substances of microorganisms in sludge into sludge that is subdivided and easily decomposed, but leaving the cell walls (cell membranes) intact, and making the sludge that contains the subdivided extracellular polymeric substances and microorganisms and is easily decomposed. "Easily decomposed sludge" is easily decomposed sludge containing subdivided extracellular polymeric substances and microorganisms obtained by the easily decomposed treatment, [(S-COD after treatment Cr)-(Pre-treatment S-COD Cr ) / Pre-treatment COD Cr refers to sludge with a solubilization degree of less than 10% obtained thereby. In general "solubilization" in sludge treatment is a technique 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 solids. The "easily decomposable" and "easily decomposable sludge" in the present invention are distinguished from "solubilized" and "solubilized sludge".

[0017] According to the present invention, there are provided wastewater treatment methods and wastewater treatment apparatuses in the following aspects. [1] A wastewater treatment method including an anaerobic digestion treatment step of subjecting excess sludge generated after biological treatment to anaerobic digestion treatment, and a dehydration step of dehydrating the sludge after anaerobic digestion treatment, wherein before the anaerobic digestion treatment step, an inorganic acid is added to the excess sludge to adjust the pH to 5 or less, and at 50°C or less, a pretreatment step is included of subdividing extracellular polymeric substances of microorganisms in the excess sludge to obtain easily decomposable sludge. [2] In the pretreatment step, the easily decomposable sludge is such that [(Post-treatment S-COD Cr )-(Pre-treatment S-COD Cr )] / Pre-treatment COD Cr has a solubilization degree of less than 10%, which is the wastewater treatment method according to [1] above. [3] The residence time in the pretreatment step is 0.01 hour or more and 24 hours or less, which is the wastewater treatment method according to [1] or [2] above. [4] Further including an excess sludge concentration step of concentrating the excess sludge before the pretreatment step, wherein in the pretreatment step, the concentrated excess sludge is treated, which is the wastewater treatment method according to any one of [1] to [3] above. [5] A wastewater treatment method according to any one of [1] to [4] above, further 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. [6] The wastewater treatment method according to any one of [1] to [5] above, characterized in that an inorganic acid obtained by biological desulfurization of odor components in biogas from the anaerobic digestion treatment step, and / or an inorganic acid obtained by biological deodorization of odor gas generated in the wastewater treatment facility is supplied to the pretreatment step. [7] A wastewater treatment device, A pretreatment tank is used to obtain easily decomposable sludge by adding inorganic acid to excess sludge from a biological treatment tank to adjust the pH to 5 or less, and by breaking down extracellular polymers of microorganisms at a temperature of 50°C or less. An anaerobic digester for anaerobic digestion of the easily decomposable sludge from the pretreatment tank, A dewatering machine for dewatering the sludge from the anaerobic digester, An anaerobic digestion apparatus characterized by comprising the following: [8] The anaerobic digestion apparatus according to [7], characterized in that the pretreatment tank is a line mixer. [9] The anaerobic digestion apparatus according to [7] or [8], further comprising an excess sludge thickening tank for concentrating excess sludge from a biological treatment tank, prior to the pretreatment tank.

[10] Furthermore, a solid-liquid separation means for separating wastewater into separated water and separated sludge is provided upstream of the biological treatment tank, A sludge thickening tank for concentrating the sludge separated from the solid-liquid separation means, A sludge concentration and mixing tank for mixing the concentrated sludge from the separated sludge concentration tank with the easily decomposable sludge from the pretreatment tank is provided between the solid-liquid separation means and the anaerobic digestion treatment tank. A wastewater treatment apparatus according to any one of the above [7] to [9], characterized in that

[11] A biological desulfurization apparatus that introduces biogas from the anaerobic digester and generates inorganic acids from odor components in the biogas, and / or a biological deodorization apparatus that generates inorganic acids from odor gases generated in a wastewater treatment facility, An inorganic acid supply line for supplying inorganic acid from the biological desulfurization apparatus and / or biological deodorization apparatus to the pretreatment tank, An anaerobic digestion apparatus according to any one of the above [7] to

[10] , characterized by comprising: [Effects of the Invention]

[0018] Unlike conventional sludge solubilization technologies, the wastewater treatment method of the present invention subdivides extracellular polymers without destroying the cell walls, thereby reducing energy consumption and operating costs compared to conventional solubilization methods, shortening the time required for anaerobic digestion (residence time), improving operability, improving the color of treated water and the dewatering properties of sludge, and enhancing the methane gas production rate and methane conversion rate. When extracellular polymers of microorganisms are subdivided during pretreatment, the bound water held by the extracellular polymers becomes free water, improving the dewatering properties in the sludge treatment process. This is particularly effective in anaerobic fermentation treatment of high-concentration sludge for methane fermentation.

[0019] Because the treatment apparatus of the present invention requires a short time (residence time) to convert excess sludge into easily decomposable sludge, it 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 can provide a treatment apparatus with a high methane production rate and methane conversion rate with a simple configuration, without requiring special equipment such as heaters or ozone generators. [Brief explanation of the drawing]

[0020] [Figure 1] This is a schematic diagram illustrating the wastewater treatment method of the present invention. [Figure 2] This is a schematic diagram illustrating one embodiment of the wastewater treatment method of the present invention. [Figure 3] This is a schematic diagram illustrating another embodiment of the wastewater treatment method of the present invention. [Figure 4] This is a schematic diagram illustrating yet another embodiment of the wastewater treatment method of the present invention. [Figure 5] This graph shows an example of the relationship between sludge pH and zeta potential. [Figure 6] This is a schematic diagram illustrating the wastewater treatment apparatus of the present invention. [Figure 7] This is a schematic diagram illustrating the wastewater treatment apparatus of the present invention, which is equipped with an immersion-type membrane separation device in a biological treatment tank. [Figure 8] This is a schematic diagram illustrating one embodiment of the wastewater treatment apparatus of the present invention. [Figure 9] This is a schematic diagram illustrating another embodiment of the wastewater treatment apparatus of the present invention. [Figure 10] This is a schematic diagram illustrating yet another embodiment of the wastewater treatment apparatus of the present invention. [Figure 11] This graph shows the relationship between pH and dissolved iron (S-Fe) concentration. [Figure 12] This graph shows the relationship between pH and soluble manganese (S-Mn) concentration. [Figure 13] This graph shows the relationship between pH, dissolved magnesium (S-Mg) concentration, and dissolved calcium (S-Ca) concentration. [Figure 14] This graph shows the relationship between pH and CODCr. [Figure 15] This graph shows the relationship between pH and CST. [Figure 16] This graph shows the relationship between pH and chromaticity. [Figure 17] This graph shows the relationship between pH and solubility. [Figure 18] This graph shows the relationship between pH and cumulative methane gas emissions. [Figure 19] This graph shows the relationship between pH and methane gas conversion rate. [Figure 20] This graph shows the relationship between temperature and methane gas production. Preferred Embodiment

[0021] The present invention will be described in detail below with reference to the attached drawings. The embodiments shown in the attached drawings are representative examples of the present invention, and the present invention is not limited thereto.

[0022] Figure 1 shows a schematic of the basic treatment flow of the wastewater treatment method of the present invention. The wastewater treatment method of the present invention includes an anaerobic digestion treatment step 15 for anaerobic digestion treatment of excess sludge generated after biological treatment, and a dewatering step 16 for dewatering treatment of the sludge after anaerobic digestion treatment, and is characterized by including a pretreatment step 14 before the anaerobic digestion treatment step 15 in which an inorganic acid is added to adjust the pH to 5 or less, preferably 4 or less, more preferably 3 or less, and at 50°C or less, preferably 10°C to 25°C, to subdivide the extracellular polymer substances of microorganisms in the excess sludge and obtain easily decomposable sludge. In the embodiment shown in Figure 1, the process 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 biological treatment of 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 return sludge, and the remaining sludge is supplied to the pretreatment step 15 as excess sludge. The first solid-liquid separation step 11 is not mandatory and can be omitted.

[0023] Figure 2 shows a schematic diagram of the treatment flow of one embodiment of the wastewater treatment method of the present invention. The wastewater treatment method shown in Figure 2 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 biological treatment of the separated water from the first solid-liquid separation step 11, an anaerobic digestion treatment step 15 for anaerobic digestion treatment of excess sludge generated after biological treatment, and a dewatering step 16 for dewatering treatment of the sludge after anaerobic digestion treatment. Prior to the anaerobic digestion treatment step 15, an inorganic acid is added to adjust the pH to 5 or less, preferably 4 or less, more preferably 3 or less, and the temperature is 50°C or below. The process further includes a pretreatment step 14, preferably at a temperature of 10°C to 25°C, to subdivide the extracellular polymer substances of microorganisms in the excess sludge to obtain easily decomposable sludge; a separated sludge concentration step 17 for concentrating the sludge separated from the first solid-liquid separation step 11; and a sludge mixing step 18 for mixing the concentrated sludge from the separated sludge concentration step 17 with the easily decomposable sludge from the pretreatment step 14. The mixed sludge of concentrated sludge and easily decomposable sludge is then supplied to an anaerobic digestion step 15 for treatment. The biologically treated water from the biological treatment step 12 is separated into sludge and treated water in the second solid-liquid separation step 13. A portion of the sludge is returned to the biological treatment step 12 as return sludge, and the remainder is sent to the pretreatment step 14 as excess sludge. The process may further include an excess sludge concentration step 19 for concentrating the excess sludge before supplying it to the pretreatment step 14. If the pretreatment process 14 is preceded by the excess sludge concentration process 19, the efficiency of the pretreatment process can be improved (reduction in acid usage, miniaturization of the pretreatment tank).

[0024] Figure 3 shows a schematic diagram 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 treatment step 15 for anaerobic digestion treatment of excess sludge generated after biological treatment, and a dewatering step 16 for dewatering treatment of the sludge after anaerobic digestion treatment. It further includes a pretreatment step 14 in which an inorganic acid is added before the anaerobic digestion treatment step 15 to adjust the pH to 5 or less, preferably 4 or less, more preferably 3 or less, and at 50°C or less, preferably 10°C to 25°C, to subdivide the extracellular polymer substances of microorganisms in the excess sludge to obtain easily decomposable sludge. It further includes an inorganic acid generation step 20 in which odor components such as hydrogen sulfide contained in the biogas from the anaerobic digestion treatment step 15 are treated by microorganisms to generate acids such as sulfuric acid, and the inorganic acid from the inorganic acid generation step 20 is added to the pretreatment step 14. In the illustrated embodiment, the process 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 biological treatment of 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 return sludge, and the remainder is sent to the pretreatment step 14 as excess sludge. The process may further include an excess sludge concentration step 19 for concentrating the excess sludge before supplying it to the pretreatment step 14. Including the excess sludge concentration step 19 prior to the pretreatment step 14 improves the efficiency of the pretreatment process (reduction in acid usage, miniaturization of the pretreatment tank). This can be achieved. The first solid-liquid separation step 11 is not essential and can be omitted.

[0025] Figure 4 shows a schematic diagram of the treatment flow of another embodiment of the wastewater treatment method of the present invention. The wastewater treatment method shown in Figure 4 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 biological treatment of the separated water from the first solid-liquid separation step 11, an anaerobic digestion treatment step 15 for anaerobic digestion treatment of excess sludge generated after biological treatment, and a dewatering step 16 for dewatering treatment of the sludge after anaerobic digestion treatment. Prior to the anaerobic digestion treatment step 15, an inorganic acid is added to adjust the pH to 5 or less, preferably 3 or less, and the extracellular polymer substances of microorganisms in the excess sludge are subdivided at 50°C or less, preferably 10°C to 25°C to obtain easily decomposable sludge. The process includes a processing step 14, further comprising a separated sludge concentration step 17 for concentrating the separated sludge from the first solid-liquid separation step 11, a sludge mixing step 18 for mixing the concentrated sludge from the separated sludge concentration step 17 with the easily decomposable sludge from the pretreatment step 14, and supplying the mixed sludge of concentrated sludge and easily decomposable sludge to an anaerobic digestion step 15 for treatment, and further comprising an acid generation step 20 for treating odor components such as hydrogen sulfide contained in the biogas from the anaerobic digestion step 15 with microorganisms to generate acids such as sulfuric acid, and adding the acid from the acid generation step 20 to the pretreatment step 14. The biologically treated water from the biological treatment step 12 is separated into sludge and treated water in the second solid-liquid separation step 13, a portion of the sludge is returned to the biological treatment step 12 as return sludge, and the remaining excess sludge is sent to the pretreatment step 14. The process may further include an excess sludge concentration step 19, which concentrates the excess sludge before supplying it to the pretreatment step 14. Including the excess sludge concentration step 19 prior to the pretreatment step 14 can improve the efficiency of the pretreatment process (reduction in acid usage, miniaturization of the pretreatment tank).

[0026] First, the pretreatment step 14 in the wastewater treatment method of the present invention, shown in Figures 1-4, will be explained. In the pretreatment step, an inorganic acid is added to treat the excess sludge in an acidic atmosphere with a pH of 5 or lower, preferably 4 or lower, more preferably 3 or higher, at a room temperature of 50°C or lower, preferably 10°C to 25°C or lower. This process subdivides the extracellular polymers of the microorganisms constituting the activated sludge, making it easier to decompose (easily decomposable sludge). Scale components such as phosphated phosphorus (PO4-P), ammoniacal nitrogen (NH4-N), Ca, Mg, Fe, and Mn, which were held by the extracellular polymers, dissolve as ions, and the activated sludge particles are subdivided. Furthermore, the bound water held by the extracellular polymers is converted into free water and released, improving the dewatering ability of the anaerobic digested sludge in the dewatering step and contributing to sludge volume reduction.

[0027] In the pretreatment step, an inorganic acid is added to adjust the pH of the excess sludge to 5 or less, preferably 4 or less, and more preferably 3 or less. If the pH is too low, it becomes necessary to add a large amount of alkaline agent to adjust it to the pH (neutral) optimal for anaerobic digestion treatment, which increases costs, so it is preferable to keep the pH at 2 or higher. Since the decomposition reaction by adding inorganic acid is highly rapid, the residence time in the pretreatment step can be very short, between 0.01 hours and 24 hours, preferably within 12 hours, and particularly preferably within 3 hours. In addition, when high-concentration sludge is subjected to anaerobic digestion treatment, alkalinity increases due to the ammoniacal nitrogen (NH4-N) produced during anaerobic digestion, worsening dewatering. Therefore, lowering the pH before anaerobic digestion treatment is also effective in treating high-concentration sludge. Furthermore, the release of organic matter constituting cells, which causes color deterioration, a problem in conventional alkaline treatment and high-temperature treatment, is prevented, and since humic substances tend to aggregate in the acidic range and can be removed in the dewatering step, color deterioration can be effectively prevented. Furthermore, compared to solubilized sludge produced by conventional alkaline treatment, the easily decomposable sludge produced by the inorganic acid pretreatment of the present invention has less odor, lower viscosity, and higher fluidity, making it easier to handle and reducing the power required for transfer and stirring.

[0028] In the pretreatment process, inorganic acids such as hydrochloric acid, sulfuric acid, nitric acid, nitrite, and phosphoric acid can be added. Among these, hydrochloric acid is particularly useful for reducing organic matter during anaerobic digestion. Hydrochloric acid is preferable because it is inexpensive and can increase methane gas production. Other acids increase methane gas production less than hydrochloric acid, but their sludge volume reduction effect is equivalent. In addition, sulfuric acid obtained by biological desulfurization of odor components such as hydrogen sulfide contained in biogas generated in the anaerobic digestion process can be used, or sulfuric acid, nitric acid, or nitrite obtained by biological deodorization of odor components generated in wastewater treatment facilities can be used. Furthermore, acids are cheaper than alkaline agents, which has the advantage of suppressing chemical costs.

[0029] The amount of inorganic acid added in the pretreatment step should preferably be such that the amount of divalent or higher soluble metals (S-Ca, S-Mg, S-Fe, S-Mn, etc.) in the excess sludge accounts for 30 wt% or more, preferably 40 wt% or more, of the total amount of metals in the excess sludge. As the anaerobic digestion reaction progresses, ammoniacal nitrogen (NH4-N) is generated and the pH rises to 6-8.5, but by adjusting the pH to 5 or less in the pretreatment step, the rise in pH in the anaerobic digestion treatment step can be suppressed. The amount of inorganic acid added in the pretreatment step can also be controlled according to the pH of the sludge in the anaerobic digestion treatment step. For example, if the pH of the sludge in the anaerobic digestion treatment step is 7.5 or higher, an amount of inorganic acid should be added in the pretreatment step so that the pH of the sludge in the pretreatment step becomes 2-3, and if the pH of the sludge in the anaerobic digestion treatment step is 7.0-7.5, an amount of inorganic acid should be added in the pretreatment step so that the pH of the sludge in the pretreatment step becomes 3-4. If the pH of the sludge in the anaerobic digestion process is 7.0 or lower, an inorganic acid is added to the pretreatment process in an amount that lowers the sludge pH to 4-5. By lowering the pH of the sludge by adding an inorganic acid in the pretreatment process, the rise in pH of the sludge in the anaerobic digestion process is suppressed, thereby suppressing the elution of scale components derived from dissolved 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, organic acid concentration, and methane gas generation amount in the anaerobic digestion process. 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 to neutral pH, but approaches zero (isoelectric point) when the pH is lowered. It is preferable to control the amount of inorganic acid added so that it approaches the isoelectric point. Since the zeta potential varies depending on the properties of the sludge, for example, in the case of sludge with the relationship between zeta potential and pH shown in Figure 5(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 with the relationship between zeta potential and pH shown in Figure 5(b), inorganic acid can be added so that the zeta potential is in the range of -7mV to 1mV. It is preferable to determine the appropriate amount of inorganic acid to be added in the pretreatment step by taking a sample of the excess sludge to be treated in advance, analyzing it, and determining the necessary amount of acid to add.

[0030] The pretreatment process can be carried out at an ORP (Standard Electrode Potential) of -50mV to 150mV, and active anaerobic treatment or aeration is not required. Furthermore, since it can be performed at 50°C or below, preferably at room temperature (around 10°C to 25°C), heating is not necessary. Because the easily decomposable sludge from the pretreatment process is acidic, the alkalinity of the anaerobic digested sludge from the anaerobic digestion treatment does not become too high, and the amount of coagulant added during the dewatering treatment can also be reduced. The wastewater treatment method including the pretreatment process in the present invention can reduce the amount of pH adjusters and coagulants added, as well as the energy required for heating and ORP control, compared to conventional anaerobic wastewater treatment including a sludge solubilization process, thus reducing operating costs.

[0031] Next, we will explain the other processing steps shown in Figures 1-4. The first solid-liquid separation step 11 is a step in which the wastewater to be treated is separated into sludge and separated water. Since the amount of wastewater to be treated is large, gravity sedimentation 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 biologically treated water containing activated sludge is separated into sludge and treated water, and gravity sedimentation separation or membrane separation can preferably be used.

[0033] The excess sludge concentration step 19 is a step in which the excess sludge is concentrated before being subjected to pretreatment, and mechanical concentration is preferred. By subjecting the concentrated excess sludge to pretreatment, the amount of organic matter (soluble COD) derived from extracellular polymers per unit volume is reduced. Cr The components increase, and the efficiency of methane gas generation through anaerobic digestion can be improved.

[0034] The anaerobic digestion step 15 is a step in which excess sludge is subjected to anaerobic digestion to generate methane gas. In the present invention, the amount of methane gas generated can be increased by subjecting easily decomposable sludge to anaerobic digestion. Normally, the anaerobic digestion step is performed at a pH of 6 to 8.5. In the present invention, easily decomposable sludge obtained by adjusting the pH to 5 or less, preferably 4 or less, and more preferably 3 or less, at a temperature of 50°C or less, preferably at room temperature, in the pretreatment step is introduced into the anaerobic digestion step. As a result, the free ammonia concentration is reduced, and ammonia inhibition in the anaerobic digester can be reduced. For example, in a typical anaerobic digestion process, if the alkalinity of the sludge is 3000 mg / L, the ammoniacal nitrogen concentration is about 500-1000 mg / L. When highly concentrated sludge is subjected to anaerobic digestion, the alkalinity of the sludge in the anaerobic digestion process often rises to about 6000 mg / L and the ammoniacal nitrogen concentration to about 5000 mg / L. However, when the easily decomposable sludge of the present invention is supplied to the anaerobic digestion process, the alkalinity of the sludge in the anaerobic digestion process does not rise too high, and the pH adjusting agent that is normally required to lower the pH can be reduced. Because the alkalinity of the anaerobic digested sludge is not too high, the amount of coagulant added during dewatering can be reduced.

[0035] The dewatering step 16 is a step in which the sludge after anaerobic digestion treatment is dewatered and separated into dewatered sludge and a separate liquid. In the present invention, by anaerobic digestion treatment of easily decomposable sludge, the amount of sludge from the anaerobic digestion step is reduced, which increases the dewatering efficiency in the dewatering step and ultimately reduces the amount of dewatered sludge discharged. Since the ammonia concentration of the anaerobic treated sludge from the anaerobic treatment of easily decomposable sludge is not too high, the amount of coagulant added can be reduced, and operating costs can be suppressed.

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

[0037] Although not shown in Figures 1-4, it is also preferable to include a decarboxylation aeration treatment step immediately after the anaerobic digestion treatment step 15, prior to the dewatering step 16, in which the anaerobic digested sludge is aerated and decarboxylated to increase the pH of the anaerobic digested sludge, thereby promoting the generation of CaCO3 and separating it by sedimentation, thereby suppressing scale formation in subsequent stages. Alternatively, a step may be included in which magnesium is added to crystallize and recover the MAP.

[0038] Next, the wastewater treatment apparatus of the present invention will be described with reference to Figures 6 to 10. Figures 6-10 are schematic diagrams illustrating the configuration of a wastewater treatment apparatus suitable for implementing the wastewater treatment method shown in Figures 1-4. Figures 6-10 specifically describe the apparatus configuration for implementing each step shown in Figures 1-4, and the same reference numerals are used for the components for each step.

[0039] As shown in Figure 6, the wastewater treatment apparatus of the present invention is characterized by comprising: a pretreatment tank 14 that adds an inorganic acid to excess sludge from a biological treatment tank 12 to adjust the pH to 5 or less, preferably 4 or less, more preferably 3 or less, and subdivides the extracellular polymer substances of microorganisms in the excess sludge at 50°C or less, preferably 10°C to 25°C to obtain easily decomposable sludge; an anaerobic digester 15 that anaerobically digests the easily decomposable sludge from the pretreatment tank 14; and a dewatering machine 16 that dewaters the sludge from the anaerobic digester 15. In the illustrated embodiment, a first solid separates the wastewater into solid and liquid components. The system includes a liquid separation means 11, a biological treatment tank 12 for biologically treating the water separated 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 separated water, and a line 12a for returning the returned sludge from the second solid-liquid separation means 13 to the biological treatment tank 12. This configuration suppresses the generation of excess sludge and maintains 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 in the figure, or it may be provided inside the biological treatment tank 12 as shown in Figure 7.

[0040] As shown in Figure 8, the wastewater treatment apparatus 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 thickening tank 17 for thickening the sludge separated from the first solid-liquid separation means 11, and a sludge thickening and mixing tank 18 for mixing the thickened sludge from the separated sludge thickening tank 17 with the easily decomposable sludge from the pretreatment tank 14. Alternatively, it may also include an excess sludge thickening device 19 for thickening excess sludge before supplying it to the pretreatment tank 14.

[0041] As shown in Figure 9, the wastewater treatment apparatus of the present invention further includes a biological desulfurization apparatus 20 that deodorizes odor components such as hydrogen sulfide contained in biogas from an anaerobic digester 15 by contacting them with microorganisms supported on a packing material to produce inorganic acids such as sulfuric acid, and / or a biological deodorization apparatus (not shown) that produces inorganic acids from odor gases generated in the wastewater treatment facility. The apparatus may also be configured to add inorganic acids from the biological desulfurization apparatus 20 and / or the biological deodorization apparatus to the pretreatment tank 14. As the biological desulfurization apparatus or biological deodorization apparatus, any general biological desulfurization apparatus or biological deodorization apparatus used for deodorizing odor components generated in the wastewater treatment facility can be used without limitation. By reusing odor components generated in the wastewater treatment facility, the amount of inorganic acid required can be reduced.

[0042] In the embodiments shown in Figures 6-10, a primary sedimentation tank is used as the first solid-liquid separation means 11 and a final sedimentation tank is used as the second solid-liquid separation means 13. However, the system is not limited to these, and any solid-liquid separation means commonly used in wastewater treatment, such as gravity filters, compression filters, vacuum filters, atmospheric flotation concentrates, centrifugal concentrates, belt-type filtration concentrates, immersion membrane separators, or external membrane separators, can be used. Furthermore, the solid-liquid separation means 11 and 13 may be equipped with a coagulant adding means for adding a coagulant to facilitate sludge sedimentation. Alternatively, a coagulation tank may be provided prior to the solid-liquid separation means 11 and 13 to coagulate the sludge by adding a coagulant.

[0043] As the biological treatment tank 12, activated sludge tanks, dripping filter beds, oxidation ditch tanks, etc., can be preferably used. An immersion-type membrane separator may be immersed inside the biological treatment tank 12. As the anaerobic digester 15, UASB-type anaerobic tanks, anaerobic membrane separator tanks, carrier-injected digester tanks, etc., can be preferably used. As the dewatering machine 16, dewatering machines commonly used in sludge treatment, such as gravity filtration dewatering machines, compression filtration dewatering machines, vacuum filtration dewatering machines, screw press type dewatering machines, centrifugal dewatering machines, filter press dewatering machines, belt press dewatering machines, etc., can be preferably used.

[0044] The pretreatment tank 14 can be any tank that has enough volume to ensure the necessary residence time to process the excess sludge from the biological treatment tank 12 at a pH of 5 or lower, preferably pH 2 to 3, at a temperature of 50°C or lower, preferably at room temperature (10°C to 25°C), and to subdivide the extracellular polymer substances of microorganisms in the excess sludge. It is not limited to a tank shape and may be a tubular shape such as a line mixer. The pretreatment process subdivides the polymer substances of the microorganisms but does not completely dissolve the cell walls, so it is preferable to process it for a short time, and the residence time should be 0.01 hours to 24 hours, preferably 12 hours or less, more preferably 3 hours or less, and even more preferably 0.01 hours to 2 hours, so the volume of the pretreatment tank can be small. In addition to the acid adding means 14a, the pretreatment tank 14 may also be equipped with a pH adjusting agent adding means (not shown). To ensure uniform contact between the sludge and the acid, it is preferable to have a stirring means (not shown). A line mixer, which can be installed in piping, is particularly preferable because it does not require heating and has a short residence time. Furthermore, although not shown, it is preferable that the pretreatment tank 14 is equipped with means for analyzing the sludge, such as a pH meter or zeta potential meter, means for adjusting the amount of inorganic acid added, and means for adjusting the amount of pH adjuster added, in order to appropriately control the amount of inorganic acid added. Since the decomposition reaction by inorganic acid is highly rapid, it is efficient to add the inorganic acid near the inlet of the pretreatment tank 14 where the sludge concentration is high. If an excess sludge thickening tank 19 is provided upstream of the pretreatment tank 14, the inorganic acid may also be added upstream of the excess sludge thickening tank 19. [Examples]

[0045] Excess sludge with the properties shown in Table 1 was treated to make it easily decomposable by adding hydrochloric acid at a temperature of 25°C to adjust the pH to 2 or 4. This was then added to a container containing digested sludge, as shown in Table 1, and subjected to anaerobic digestion treatment at 35°C under a load of approximately 0.3 g-VS / g-VSS. The properties of the excess sludge and digested sludge are shown in Table 1.

[0046] [Table 1]

[0047] Excess sludge (pH 6) with the properties shown in Table 1 was treated by adding hydrochloric acid or sodium hydroxide to adjust the pH to 2, 4, 6, 9, and 12. After shaking in a constant temperature bath at 25°C for 24 hours, the concentrations of each dissolved metal and EPS-COD were measured. Cr Dehydration and color were measured.

[0048] [Soluble metal concentration] Using an ICP emission spectrometer, each soluble metal (S-Mn, S-Fe, S-Mg, SC) is analyzed. (a) was analyzed. The relationship between pH and S-Fe is shown in Figure 11, the relationship between pH and S-Mn is shown in Figure 12, and the relationship between pH and S-Mg and S-Ca is shown in Figure 13. It can be seen that the concentrations of each dissolved metal are low at pH 9 or 12, but the concentrations of each dissolved metal are high at pH 4 and 2.

[0049] [EPS-COD Cr ] COD derived from extracellular polymers Cr (EPS-COD Cr The following procedure was used to measure the following: 50 mL of the measurement solution was centrifuged twice (3000 rpm x 10 min), the supernatant was discarded, approximately 40 mL of pure water was added to the precipitate and mixed, then autoclaved (105°C x 30 min) to make up 50 mL. After that, it was centrifuged again (15000 rpm x 10 min), the supernatant was collected, filtered through a 1 μm pore size filter, and the COD of the filtrate was measured. Cr We measured it.

[0050] [COD Cr ] EPS-COD at each pH Cr The measurement results are shown in Figure 14. EPS-COD of untreated sludge at pH 6. Cr EPS-COD at pH 2 and 4 Cr Because the EPS-COD is low, it is thought that at pH 2 and 4, some of the extracellular polymers are destroyed and eluted, and are removed as supernatant by centrifugation and are not included in the filtrate. On the other hand, EPS-COD of untreated sludge at pH 6 Cr EPS-COD at pH 9 and 12 Cr The pH is high, and at pH 9 and 12, not only extracellular polymers but also cell walls (cell membranes) are destroyed, releasing organic matter. The released organic matter is highly viscous and exists in a state of strong binding with the sludge. Autoclave treatment separates the sludge from the dissolved organic matter, and the filtrate contains the dissolved organic matter, resulting in EPS-COD Cr It is thought that this has increased.

[0051] [Dehydration] In accordance with the wastewater testing methods, Chapter 1, General Sludge Test, Section 3, CST test, the CST (capillary suction time) was measured to evaluate the sludge filtration characteristics. The results are shown in Figure 15. A shorter CST time indicates better dewatering performance. At pH 9 and 12, the CST was 3000 seconds or more, but when treated at pH 2 and 4, the CST was 400 seconds or less, indicating very good dewatering performance.

[0052] [Chromaticity] The color was measured according to Chapter 2, Water Quality Testing, Section 4, Color, of the wastewater testing method. The results are shown in Figure 16. At pH 2 and 4, the color was 80 degrees and 120 degrees, respectively, which is lower than the 120 degrees of the untreated pH 6 sample. However, it increased to 600 degrees at pH 9 and 1250 degrees at pH 12. It was confirmed that the increase in color could be suppressed by treatment in the acidic range.

[0053] Table 2 shows chromaticity, dehydration (CST), EPS-COD CrThe measurement results are summarized below. When treated in the acidic range of pH 2 and 4, the color and CST become lower than those of untreated sludge, indicating improved color and dewatering properties.

[0054] [Table 2]

[0055] [Solubilization degree] Using the surplus sludge shown in Table 3, the pH was adjusted to 2, 4, 6, 9, and 12 by adding hydrochloric acid or sodium hydroxide, and after shaking in a constant temperature bath at 25°C for 24 hours, the COD of the surplus sludge was measured. Cr (COD before processing) Cr ), soluble COD of excess sludge Cr (S-COD before processing) Cr ), solubility COD of sludge after treatment Cr (S-COD after processing) Cr The solubility of COD was measured and the degree of solubilization was calculated. Cr (S-COD Cr The solubility was measured using the filtrate filtered through filter paper with a pore size of 1 μm. The results are shown in Figure 17. The solubility of untreated sludge at pH 6 was the lowest at 0.48%, and the solubility increased as the pH decreased or increased. At pH 2, the solubility was 3.76%, and at pH 4, it was 1.10%. With alkaline treatment at pH 12, the solubility increased to over 13% because not only extracellular polymers but also the cell walls (cell membranes) were destroyed and intracellular organic matter was released. At pH 5 or below, extracellular polymers are subdivided, but it can be confirmed that a solubility of less than 10% is achieved, to the extent that the cell walls (cell membranes) are not destroyed and organic matter is not released.

number

[0056] [Table 3]

[0057] [Methane conversion rate] 400 mL of anaerobic digested sludge was mixed with 100 mL each of easily decomposable sludge treated at pH 2 and 4, untreated sludge at pH 6, and sludge treated with alkali at pH 9 and 12. The mixture was then operated in a 35°C constant temperature bath for approximately one month while stirring, and the COD (Chemical Oxygen Demand) was measured. Cr The amount of gas generated is measured and the COD is calculated using the following formula. Cr The methane conversion rate was calculated. In the formula, the methane gas conversion coefficient is a constant (0.35). The measurement results of the cumulative amount of methane gas generated are shown in Figure 18, and the calculation results of the methane conversion rate are shown in Figure 19. The cumulative amount of methane gas generated showed a similar trend for treatment at pH 2 and pH 12, and a similar trend for treatment at pH 4 and pH 9, with all showing an increase in the cumulative amount of methane gas generated compared to the untreated case. In sludge treated for easy decomposition at pH 2, a methane conversion rate of 35% was achieved after 150 hours, and a methane conversion rate of 45% was achieved after 400 hours. In sludge treated with alkali at pH 12, it took 200 hours to achieve a methane conversion rate of 35%, and the methane conversion rate was 40% after 400 hours. Acid treatment at pH 2 showed an increase in the methane conversion rate in a short time, confirming its rapid effect. There was no significant difference between the easily decomposable treatment at pH 4 and the alkaline treatment at pH 9. It took 300 hours to achieve a 35% methane conversion rate, and 38% after 400 hours. Compared to untreated sludge at pH 6, the methane conversion rate was significantly improved in both the easily decomposable treatment in the acidic range and the alkaline treatment in the alkaline range. However, as is clear from the color measurement results, the color deteriorated considerably with the alkaline treatment. Therefore, it can be said that acid treatment can improve the methane conversion rate in a shorter time than alkaline treatment.

number

[0058] [Methane gas production amount] 300 mL of anaerobic digested sludge was dispensed with 60 mL each of sludge treated at pH 6.5 at 25°C, 50°C, and 75°C. The mixture was then operated in a 35°C constant temperature bath for approximately one month with stirring, and the amount of gas generated was measured. The results are shown in Figure 20. No significant difference in methane gas generation was observed even at different temperatures. Therefore, at temperatures below 50°C, it can be said that the amount of methane gas generated is equivalent, but the heating energy is lower, thus reducing operating costs.

[0059] [Color, dehydration, viscosity, and odor] Table 5 shows the results of evaluating the color, CST, viscosity, and odor of excess sludge (pH 6) with the properties shown in Table 4. The pH was adjusted to 2 or 12 by adding hydrochloric acid or sodium hydroxide, and the mixture was shaken in a constant temperature bath at 25°C for 24 hours. Color and CST were measured by the method described above, viscosity was measured using a Type B viscometer (RB80L: Toki Sangyo), and odor was evaluated by sensory testing.

[0060] [Table 4]

[0061] [Table 5]

[0062] Table 5 shows that at 25°C and pH 2, the chromaticity, CST, and viscosity were lowest, exhibiting excellent dewatering and handling properties, resolving the chromaticity problem, and preventing the generation of unpleasant odors. On the other hand, if either the pH or temperature deviates from the range of the present invention, the chromaticity, CST, and viscosity increase, and unpleasant odors are also observed.

[0063] As shown in Table 6, the wastewater treatment method using the pretreatment of the present invention uses less energy, reduces operating costs, offers good operability, produces clearer treated water, and provides good sludge dewatering compared to conventional wastewater treatment using solubilization treatment.

[0064] [Table 6]

[0065] As described above, the wastewater treatment method of the present invention yields anaerobic digested sludge with excellent dewatering properties, low color and viscosity, and can achieve a methane conversion rate equivalent to or higher than that of conventional alkaline treatment in a short time, despite requiring less energy consumption and operating costs than conventional solubilization methods. [Explanation of Symbols]

[0066] 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, immersion-type membrane separator) 14. Pre-treatment process (pre-treatment tank, line mixer) 15: Anaerobic digestion process (anaerobic digestion tank) 16: Dehydration process (dehydrator) 17: Separated sludge concentration process (gravity concentration device) 18: Sludge mixing process (sludge mixing storage tank) 19: Excess sludge thickening process (mechanical thickening equipment) 20: Acid generation process (biological desulfurization equipment)

Claims

1. A wastewater treatment method comprising an anaerobic digestion step for anaerobic digestion of excess sludge generated after biological treatment, and a dewatering step for dewatering the sludge after anaerobic digestion, Prior to the anaerobic digestion process, the process includes a pretreatment step in which an inorganic acid is added to the excess sludge to adjust the pH to 5 or less, and the extracellular polymer substances of microorganisms in the excess sludge are broken down at 50°C or below to obtain easily decomposable sludge. A wastewater treatment method characterized in that the amount of inorganic acid added in the pretreatment step is controlled based on one or more values ​​of the alkalinity of the sludge in the anaerobic digestion treatment step, the organic acid concentration, the amount of methane gas generated, or the zeta potential.

2. The wastewater treatment method according to claim 1, characterized in that the amount of inorganic acid added in the anaerobic digestion step is controlled based on the zeta potential value of the sludge.

3. A wastewater treatment device, A pretreatment tank is used to obtain easily decomposable sludge by adding inorganic acid to excess sludge from a biological treatment tank to adjust the pH to 5 or less, and by breaking down extracellular polymers of microorganisms at a temperature of 50°C or less. An anaerobic digester for anaerobic digestion of the easily decomposable sludge from the pretreatment tank, A dewatering machine for dewatering the sludge from the anaerobic digester, A control device that controls the amount of inorganic acid added based on one or more values ​​of the alkalinity of the sludge in the anaerobic digestion process, organic acid concentration, methane gas generation amount, or zeta potential, An anaerobic digestion apparatus characterized by comprising the following:

4. The anaerobic digestion apparatus according to claim 3, characterized in that the control device controls the amount of inorganic acid added based on the zeta potential value of the sludge in the pretreatment tank.

5. The anaerobic digestion apparatus according to claim 4, characterized in that the control device comprises a zeta potential meter, an acid addition amount adjustment means for adjusting the amount of inorganic acid added, and a pH adjusting agent addition amount adjustment means.

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