Sludge treatment method and sludge treatment system

Abstract

To provide a sludge treatment method using anaerobic methane fermentation with high methane gas yield, in which recovered sludge such as excess sludge is used as the fermentation raw material.SOLUTION: The sludge treatment method comprises a pretreatment step in which recovered sludge is treated with aerobic predatory bacteria, and a methane fermentation step in which the treated sludge obtained from the pretreatment step is used as a fermentation raw material.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a sludge treatment method and system using anaerobic methane fermentation. [Background technology]

[0002] The activated sludge process using aerobic bacteria is commonly used for the biological treatment of organic wastewater containing organic matter, excess sewage sludge, food waste, and other organic waste. The activated sludge process treats organic matter through the metabolism of aerobic bacteria, which causes the proliferation of aerobic bacteria. Therefore, in the activated sludge process, it is necessary to remove the excessively proliferated aerobic bacteria as excess sludge.

[0003] In 2023, the amount of excess sludge generated at sewage treatment plants and other facilities in Japan will reach 77,280,000 tons per year, accounting for approximately 48% of the sludge discharged as industrial waste (162,676,000 tons per year).Therefore, there is a need to curb the generation and reduce the amount of excess sludge.

[0004] Anaerobic wastewater treatment using anaerobic bacteria has attracted attention because it consumes little energy, produces less carbon dioxide, and can recover methane, which can be used as fuel gas or as a hydrogen source for fuel cells. However, anaerobic bacteria, including methanogens, grow more slowly than aerobic bacteria. Therefore, maintaining anaerobic bacteria is important for stable anaerobic wastewater treatment using methane fermentation and other methods.

[0005] In recent years, membrane separation, which combines anaerobic treatment with membrane separation technology, has attracted attention because it prevents the outflow of anaerobic bacteria and makes it easy to maintain bacterial mass. The membrane separation method is a wastewater treatment method that combines a reactor tank that performs anaerobic fermentation treatment with a membrane separation tank that discharges only liquid through membrane separation. For example, in Patent Document 1, the present applicant has proposed a wastewater treatment system that includes a reactor tank and a membrane separation tank that ferment low-concentration wastewater containing organic matter using upflow methane fermentation, in which the reactor tank and membrane separation tank are provided with an overflow pipe at the top and a sludge return pipe at the bottom. The reactor tank is provided with a wastewater supply pipe at the bottom and a gas circulation pipe that circulates biogas from the top to the bottom. The membrane separation tank is provided with a discharge pipe for filtered water.

[0006] Patent Document 2 proposes an anaerobic digestion method for recovered sewage sludge, in which recovered sewage sludge, consisting of excess sludge produced in an activated sludge process, is subjected to anaerobically digestion treatment in an anaerobic digestion tank, in which additives containing Fe, Cu, Zn, Mg, etc. are added to the anaerobic digestion tank when the temperature is lower than a predetermined temperature.

[0007] The method described in Patent Document 2 uses excess sludge as a raw material, which can reduce the cost of treating excess sludge as waste. However, because excess sludge contains a large amount of aerobic bacteria, and bacteria have cell membranes, biofilms, etc. to protect themselves, the anaerobic fermentation of excess sludge takes a long time, which is an issue.

[0008] Predatory bacteria are bacteria that can lyse (eat) other living bacteria and use them as a nutrient source for themselves. It has been shown that the lytic action of predatory bacteria promotes the solubilization and mineralization of excess sludge, potentially reducing the amount of excess sludge that needs to be treated (Non-Patent Document 1). However, in the investigations conducted by the present inventors, no reduction in sludge volume was observed due to predatory bacteria. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2022-084530 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-055216 [Non-patent literature]

[0010] [Non-Patent Document 1] Yu, R., Zhang, S., Chen, Z. et al. “Isolation and application of predatory Bdellovibrio-and-like organisms for municipal waste sludge biolysis and dewaterability enhancement.”, Front. Environ. Sci. Eng., 11, 10 (2017). Summary of the Invention [Problem to be solved by the invention]

[0011] An object of the present invention is to provide a sludge treatment method and system using anaerobic methane fermentation using recovered sludge such as excess sludge as a fermentation raw material. [Means for solving the problem]

[0012] The means for solving the problems of the present invention are as follows. 1. A pretreatment process in which recovered sludge is treated with aerobic predatory bacteria. a methane fermentation step using the treated sludge obtained in the pretreatment step as a fermentation raw material; A sludge treatment method comprising the steps of: 2. A pretreatment tank in which recovered sludge is treated with aerobic predatory bacteria; an anaerobic treatment tank for methane fermentation using the treated sludge discharged from the pretreatment tank as a fermentation raw material; A sludge treatment system comprising: [Effects of the Invention]

[0013] In the sludge treatment method and sludge treatment system of the present invention, recovered sludge is treated with aerobic predatory bacteria, and the treated sludge is used as a raw material for methane fermentation. The sludge treatment method and sludge treatment system of the present invention treat recovered sludge with aerobic predatory bacteria, and use the treated sludge as a raw material for methane fermentation, thereby reducing the amount of recovered sludge, such as excess sludge, treated as industrial waste. Treated sludge is converted into a form that is easily utilized (metabolized) by anaerobic predatory bacteria through treatment (preying) with the aerobic predatory bacteria, and can be made into a highly concentrated fermentation raw material for methane fermentation compared to untreated sludge, thereby enabling the treatment of recovered sludge more efficiently than conventional methods. Furthermore, the sludge treatment method and sludge treatment system of the present invention enable efficient methane fermentation immediately after the start of anaerobic treatment, thereby enabling the stable start-up of a sludge treatment system using anaerobic bacteria. The sludge treatment method and system of the present invention can produce methane gas, which can be used as an energy source, from excess sludge, which previously required large amounts of energy for drying, burning, and other processes, thereby significantly reducing adverse environmental impacts. [Brief explanation of the drawings]

[0014] [Figure 1] Graph showing cumulative methane gas production in Experiment 1. [Figure 2] Graph showing soluble COD concentrations after 24 hours of pretreatment in Experiment 2. [Figure 3] 1 is a graph showing the daily change in cumulative methane production during the 5-day methane fermentation process in Experiment 2. DETAILED DESCRIPTION OF THE INVENTION

[0015] The sludge treatment method of the present invention comprises a pretreatment step in which recovered sludge is treated with aerobic predatory bacteria, and a methane fermentation step in which the treated sludge obtained in the pretreatment step is used as a fermentation raw material. The sludge treatment method of the present invention can be carried out, for example, by a sludge treatment system having a pretreatment tank in which recovered sludge is treated with aerobic predatory bacteria, and an anaerobic treatment tank in which the treated sludge discharged from the pretreatment tank is used as a fermentation raw material for methane fermentation.

[0016] The recovered sludge is not particularly limited as long as it contains aerobic bacteria, and for example, one or more of the following can be used: primary sedimentation sludge produced in a primary sedimentation tank in sludge treatment using the activated sludge method; excess sludge mainly containing aerobic bacteria produced in an activated sludge tank; digested sludge mainly containing anaerobic bacteria produced by anaerobic treatment; etc. Among these, it is preferable to use excess sludge, which is produced in large quantities and requires high treatment costs for incineration, landfill, etc.

[0017] Recovered sludge contains many aerobic bacteria. Bacteria have cell membranes and cell walls, and may be surrounded by biofilms made of exopolysaccharides (EPS), which are their defense mechanisms. Therefore, bacteria are difficult to metabolize by anaerobic bacteria and are therefore a carbon source that is difficult to use as a fermentation feedstock for methane fermentation.

[0018] The sludge treatment method of the present invention treats (predates) recovered sludge containing a large amount of bacteria with aerobic predatory bacteria, converting the bacteria in the recovered sludge into lower molecular weight organic compounds that are easier to use as raw materials for methane fermentation, thereby producing treated sludge, which is then used as a fermentation raw material for methane fermentation.

[0019] When bacteria are consumed, they are decomposed (metabolized) and converted into low molecular weight compounds, which are more easily used as fermentation raw materials. Therefore, the aerobic predatory bacteria used to treat recovered sludge in the present invention are not limited in any way, and any aerobic predatory bacteria can achieve the effects of the present invention. The aerobic predatory bacteria are not particularly limited, and examples thereof include Bacteriovorax sp. HI3 strain (deposit number: NBRC116486, hereinafter also referred to as HI3 strain), Bdellovibrio sp. BIS2 strain (currently undergoing biological resource deposit procedures at the National Institute of Technology and Evaluation Biotechnology Center, shipped on February 13, 2024, and received on February 14, 2024, hereinafter also referred to as BIS2 strain), Myxococcus sp. MH1 strain (National Institute of Technology and Evaluation Patent Microorganisms Depositary Center, received on February 14, 2024, accession number: NITE ABP-04079, hereinafter also referred to as MH1 strain), Bdellovibrio bacteriovorus ATCC 15356 (DSM 50701), Bacteriovorax stopii ATCC 27052 (DSM 12778), Pseudobdellovibrio exovorus Examples include ATCC BAA-2330 (DSM 25223), Halobacteriovorax marinus ATCC BAA-682 (DSM 15412), Myxococcus fluvus NBRC 100333 (ATCC 25199, DSM 16525), and Myxococcus xanthus NBRC 13542 (ATCC 25232, DSM 16526). It is known that the HI3, BIS2, and MH1 strains have different methods of preying on bacteria.

[0020] The pretreatment conditions are not particularly limited, but the concentration of aerobic predatory bacteria added should be 1 × 10 5 cells / mL or more is preferred, 5×10 5 cells / mL or more is preferable, and 1×10 6 The treatment conditions are preferably, for example, 25 to 32°C, pH 5.5 to 7.5, and about 1 to 72 hours. Furthermore, compared to untreated sludge that has not been pretreated with aerobic predatory bacteria, the treated sludge preferably has a cumulative methane production rate by methane fermentation over 15 days under similar conditions that is 10% or more higher, more preferably 12% or more higher, and even more preferably 14% or more higher.

[0021] The anaerobic treatment method in the sludge treatment method and sludge treatment system of the present invention is not particularly limited, and may be any of completely mixed methane fermentation, upflow methane fermentation, and others. However, completely mixed methane fermentation is preferred in the present invention because it allows the recovered sludge, which is the fermentation raw material, to be highly concentrated.

[0022] In the present invention, the sludge to be treated by methane fermentation (including treated sludge as a fermentation raw material) has an organic matter (VS: Volatile Solid) load of 1 kgVS / m 3 / day or more 20kgVS / m 3 / day or less. VS load is 3kgVS / m 3 / day or more is preferable, 5kgVS / m 3 / day or more is more preferable, and 10gVS / m 3 / day or more is even more preferable. VS load is 20kgVS / m 3 / day, the methane production rate reaches saturation and hardly increases any further. [Example]

[0023] Experiment 1 "Example 1" Pre-treatment process Excess sludge was collected from the sludge return line after the final settling tank of sewage treatment plant B. To eliminate the influence of organic matter other than bacteria in the excess sludge, the excess sludge was centrifuged (2,500 g, 10 min) to remove the supernatant, and then washed twice with the HM buffer shown in Table 1 to adjust the sludge concentration to 3,500 mg / L, which was then used as recovered sludge.

[0024] [Table 1] The pre-cultured HI3 strain was suspended in the HM buffer shown in Table 1, centrifuged (10,000 g, 10 min) and washed, and then diluted to a concentration of 1 × 10 8 The concentration was adjusted to cells / mL to obtain a culture medium for the HI3 strain.

[0025] 396 mL of recovered sludge was placed in a 500 mL Erlenmeyer flask and inoculated with 4 mL of HI3 culture solution. This was cultured for 72 hours with aeration at 28°C and 200 rpm, and pretreatment with the aerobic predatory bacteria HI3 strain was performed to obtain treated sludge (MLSS: 3,900 mg / L).

[0026] Methane fermentation process Excess sludge from sewage treatment plant B was washed twice with the inorganic salt medium shown in Table 2, and the final concentration was adjusted to 5 g-VSS / L.

[0027] [Table 2] A test system was created by adding 20 mL of excess sludge, 20 mL of treated sludge, and 10 mL of sodium bicarbonate solution (10 g / L, final concentration 2 g / L) to make a liquid volume of 50 mL. Although the excess sludge is aerobic, a small number of anaerobic bacteria coexist in the aerobic sludge, and methane fermentation can proceed simply by placing the aerobic sludge in an anaerobic environment.

[0028] "Comparative Example 1" A test system was prepared in the same manner as in Example 1, except that in the pretreatment step, an inorganic salt medium (MLSS: 3340 mg / L) was used instead of the HI3 strain culture medium. "Comparative Example 2" In the methane fermentation process, 20 mL of treated sludge was replaced with HI3 strain culture solution (1 × 10 8 A test system was prepared in the same manner as in Example 1, except that 20 mL of the culture medium (200 cells / mL) was added. "Reference example" A test system was prepared in the same manner as in Example 1, except that 20 mL of inorganic salt medium was added in place of 20 mL of treated sludge in the methane fermentation step.

[0029] Table 3 shows the fermentation raw materials added during the methane fermentation process in the examples and comparative examples. [Table 3] After each test system was sealed, nitrogen was purged for 5 minutes, and an anaerobic digestion test was carried out at 37°C and 120 rpm for approximately 15 days. During the test period, the amount of biogas produced and the methane gas concentration were measured (GC-BID), and the amount of methane gas produced was calculated from these. Figure 1 shows the cumulative amount of methane gas produced over 15 days.

[0030] ·result In Comparative Example 2 and the Reference Example, no change was observed when the HI3 strain was added to a system where anaerobic methane fermentation was taking place. This is because the HI3 strain is an aerobic bacterium and therefore cannot function under anaerobic conditions. Example 1, in which treated sludge pretreated with the aerobic predatory bacteria strain HI3 was added, showed the highest cumulative methane gas production. Comparing Example 1 with Comparative Example 1, in which recovered sludge not pretreated with aerobic predatory bacteria was added, the cumulative methane gas production over 15 days increased by 12.8%. This confirmed that the aerobic predatory bacteria processed (preyed on) the bacteria in the recovered sludge, converting them into a form that was easily usable by methanogens.

[0031] Experiment 2 Pre-treatment process Excess sludge collected from sewage treatment plant B was concentrated to an MLSS concentration of 10 g / L, and then 200 mL of the sludge was dispensed into 300 mL Erlenmeyer flasks. The HI3, BIS2, and MH1 strains were added at a final concentration of 1.0 × 10 6 Two replicates were prepared: a test system inoculated with predatory bacteria at 0.1 cells / mL and a control system without predatory bacteria. The test system was cultured for 24 hours under aerobic conditions at 28°C, 500 rpm, and constant aeration. After the test, the soluble COD (sCOD) concentration was measured. The results are shown in Figure 2. After 24 hours of pretreatment, the sCOD concentration in the control system increased to 67.5 mg / L, whereas in the control system, the sCOD concentration after 24 hours of pretreatment increased to 80 mg / L or more in all systems where aerobic predatory bacteria were added. This confirmed that the bacteria in the excess sludge were converted into water-soluble organic compounds, i.e., organic compounds with low molecular weight, through predation (metabolism) by aerobic predatory bacteria.

[0032] Methane fermentation process 30 mL of excess sludge (16.7 g VSS / L) collected from sewage treatment plant B and 20 mL of treated sludge from each aerobic predatory bacteria in the pretreatment process described above were dispensed into a 100 mL vial, and the pH was adjusted to 7.8. After sealing and purging with nitrogen to create anaerobic conditions, the vial was subjected to rotary shaking culture at 37°C and 120 rpm for a 5-day anaerobic digestion test. An anaerobic digestion test was conducted in the same manner, except that 50 mL of excess sludge was used as a control. During the test period, the amount of biogas produced and the methane gas concentration were measured on days 1, 3, and 5, and the amount of methane produced was calculated. Figure 3 shows the daily change in cumulative methane production, and Table 4 shows the ratio of cumulative methane production to the control system.

[0033] [Table 4]

[0034] ·result The system using treated sludge pretreated with predatory bacteria as the fermentation feedstock produced more methane than the control system. One day after the start of the experiment, methane production was 16-35% higher than that of the control system, but by the fifth day, the difference had dropped to 4-9%. This confirmed that treated sludge contains many organic compounds that are easily processed (metabolized) by anaerobic bacteria. In actual sludge treatment, high methane production can be sustained by intermittently supplying treated sludge, which is the fermentation feedstock. These results demonstrate that the use of aerobic predatory bacteria can promote methane production.

Claims

1. a pretreatment process in which the recovered sludge is treated with aerobic predatory bacteria; a methane fermentation step using the treated sludge obtained in the pretreatment step as a fermentation raw material; A sludge treatment method comprising the steps of:

2. a pretreatment tank for treating recovered sludge with aerobic predatory bacteria; an anaerobic treatment tank for methane fermentation using the treated sludge discharged from the pretreatment tank as a fermentation raw material; A sludge treatment system comprising:

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