Method for promoting granulation of anaerobic sludge

Abstract

To provide a method for promoting granulation of anaerobic sludge.SOLUTION: A method for promoting granulation of anaerobic sludge includes adding magnetite to the anaerobic sludge in wastewater at a weight ratio (concentration ratio) of 1 / 30 or more and 1 / 5 or less.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for promoting granulation of anaerobic microorganisms in anaerobic biological treatment. [Background technology]

[0002] The activated sludge process using aerobic bacteria is generally used for biological treatment of wastewater containing organic matter, excess sewage sludge, organic waste such as food waste, etc. The activated sludge process has a large impact on the environment because it requires high energy costs for aeration and drying, burning, and disposal of the excess sludge, and generates large amounts of carbon dioxide. Methane fermentation by anaerobic bacteria has attracted attention because it consumes little energy, produces little carbon dioxide, and can recover methane that can be used as fuel gas or a hydrogen source for fuel cells.

[0003] Anaerobic bacteria, including methanogens, grow slower than aerobic bacteria. It takes time to start up methane fermentation, so speeding up the start-up is an important point. After start-up, it is also important to prevent anaerobic bacteria from flowing out with the wastewater and to maintain a high concentration of anaerobic bacteria. Here, the bacteria aggregate to a high density and form self-granulated sludge called granules. Granules are easy to handle because they can be precipitated and filtered. For this reason, anaerobic sludge is granulated and the granules are added at the start of operation, and filtration is used to prevent the outflow of granules made of anaerobic bacteria.

[0004] For example, Patent Document 1 discloses an invention relating to a method for treating organic wastewater in which, when starting up a reaction tank, organic wastewater is started to flow into the reaction tank in a state in which non-biological carriers and methane bacteria granules are present in the reaction tank at a volume ratio of carrier to methane bacteria granules in the range of 100:5 to 100:500, and then the flow of the organic wastewater is continued to break down and disperse at least a portion of the methane bacteria granules in the reaction tank, promoting the attachment of microorganisms to the carriers at the start of operation and causing a highly active biofilm to be formed on the carrier surface at an early stage, thereby significantly shortening the time required to start up the apparatus and providing efficient treatment even after the apparatus has been started up.

[0005] It is known that granulation of anaerobic sludge is more likely to occur in high-concentration wastewater. In Patent Document 2, the applicant has proposed a wastewater treatment method for fermenting wastewater containing organic matter using anaerobic bacteria, the wastewater treatment method comprising a fermentation step in which low-concentration wastewater containing organic matter is used as a raw material and fermented by upward flow methane fermentation, a filtration step in which overflow water from the fermentation step is taken in, the sludge is separated by membrane and the filtrate is released, and a filtered sludge return step in which the sludge remaining after filtration is returned to the fermentation step, in which the generated biogas is diffused from the bottom and magnetite is added in the initial stage of the fermentation step to promote fermentation in the initial state when granular sludge is not generated. The wastewater treatment method described in Patent Document 2 relates to an initial state in which no granular sludge is generated, and in the examples, evaluation was only performed at the start-up stage, with no further experiments being performed thereafter. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2012-110821 A [Patent Document 2] Patent Publication No. 2022-84530 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a method for promoting the granulation of anaerobic sludge. [Means for solving the problem]

[0008] The means for solving the problems of the present invention are as follows. 1. A method for promoting the granulation of anaerobic sludge, comprising adding magnetite to the anaerobic sludge in wastewater in a weight ratio (concentration ratio) of 1 / 30 or more and 1 / 5 or less. 2. The method for promoting granulation of anaerobic sludge according to 1., wherein the total COD concentration of the wastewater is 100 mg / L or more and 1500 mg / L or less. Effect of the Invention

[0009] The present invention can promote granulation of anaerobic sludge. In particular, granulation can be promoted even in the treatment of low-concentration wastewater such as general sewage. The present invention can promote granulation and hasten normalization when anaerobic bacteria are reduced due to a problem or the like. [Brief description of the drawings]

[0010] [Figure 1] Graph showing granulated sludge concentration in an example. [Diagram 2] Graph showing the change over time in cumulative methane production amount in an example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] In the granulation promotion method of the present invention, magnetite is added to anaerobic sludge in wastewater so that the weight ratio (concentration ratio: magnetite / sludge) is 1 / 30 or more and 1 / 5 or less. In the present invention, the weight of anaerobic sludge means the dry weight. In addition, since magnetite and anaerobic sludge are contained in the same wastewater, the weight ratio value and the concentration ratio value are equal. By adding magnetite to the anaerobic sludge contained in wastewater to be subjected to anaerobic biological treatment by anaerobic microorganisms at this weight ratio (concentration ratio), the granulation of the anaerobic sludge can be promoted and the granulation can be performed more quickly. Note that anaerobic sludge is a collection of anaerobic microorganisms that can treat organic matter in wastewater under an anaerobic atmosphere containing anaerobic microorganisms. When the anaerobic sludge is granulated, the stirring efficiency is improved and outflow is suppressed, so that the amount of methane produced increases. The weight ratio (concentration ratio) of magnetite to the anaerobic sludge is preferably 1 / 25 or more, more preferably 1 / 20 or more, even more preferably 1 / 17 or more, even more preferably 1 / 14 or more, and even more preferably 1 / 6 or less, even more preferably 1 / 7 or less, and even more preferably 1 / 8 or less. From the viewpoint of promoting granulation, it is preferable that the median diameter (d50) of magnetite (magnetic iron ore, Fe3O4) be 0.8 μm or more and 3 μm or less, as measured by dynamic light scattering method.

[0012] The wastewater to be treated in the present invention is not particularly limited, but the total COD concentration is preferably 100 mg / L or more and 1500 mg / L or less. In this specification, the COD concentration means the CODCr concentration measured using potassium dichromate. By the granulation promotion method of the present invention, even low-concentration wastewater that is difficult to granulate can be granulated more quickly. The total COD concentration of the wastewater is more preferably 1400 mg / L or less, and even more preferably 1200 mg / L or less.

[0013] The granulation promotion method of the present invention can be carried out in an independent tank for the purpose of producing granules, or in a reaction tank of an anaerobic wastewater treatment device. When carried out in a reaction tank, the reaction tank may be one in the start-up stage or one in the middle of wastewater treatment operation. When the anaerobic bacteria are reduced due to a problem with the wastewater treatment device, the amount of anaerobic bacteria can be more quickly restored to normal by adding magnetite to the reaction tank in operation. The wastewater treatment device may be any of a complete mixing type methane fermentation system, an upflow type methane fermentation system, and others, but the upflow type methane fermentation system is preferred because it is easier to form granules. EXAMPLES

[0014] Anaerobic sludge (sludge concentration: 100 mg / L), inorganic salt medium (magnesium chloride hexahydrate: 1000 mg / L, calcium chloride dihydrate: 375 mg / L, ammonium chloride: 1250 mg / L, dipotassium hydrogen phosphate: 2180 mg / L, potassium dihydrogen phosphate: 1700 mg / L, sodium bicarbonate: 2500 mg / L), trace metals (iron (II) sulfate heptahydrate: 50 mg / L, nickel chloride hexahydrate: 4 mg / L, cobalt chloride hexahydrate: 4 mg / L, copper chloride dihydrate: 3 mg / L, zinc sulfate heptahydrate: 4 mg / L, sodium molybdate (IV) dihydrate: 3 mg / L, manganese chloride IV hydrate: 4 mg / L), distilled water, and glucose solution (final concentration: 1.5 g-COD / L) were added to a 150 mL pressure-resistant vial to make a total of 80 mL. Magnetite (Fujifilm Wako Pure Chemical Industries, product code 093-01035) was added at concentrations of 5 mg / L, 10 mg / L, and 50 mg / L so that the weight ratio (concentration ratio) to the anaerobic sludge was 1 / 20, 1 / 10, and 1 / 2, respectively. As a control, a group without magnetite was prepared.

[0015] The flask was purged with nitrogen, sealed with a butyl rubber stopper and an aluminum cap, and the test was started. The test was carried out at 30°C with shaking (80 rpm). The amount of gas generated and the gas composition were measured periodically to calculate the amount of methane produced. At the same time, the vial was opened and the pH was measured. If the pH had dropped below 6.8, inorganic salt medium and glucose solution were added to bring the pH to around 7.0. At this time, an equal amount of supernatant was withdrawn from the vial that had been left to stand beforehand, and then inorganic salt medium and glucose solution were added, adjusting the amount of liquid in the vial to a constant level. If the pH had not fallen below 6.8, only glucose solution was added. The study was conducted for 63 days.

[0016] ·result After the test, the liquid in the vial was passed through a 0.425 mm sieve to measure the amount of sludge trapped on the sieve. The amount of sludge was converted into sludge concentration. The results are shown in Figure 1. In the present invention, under conditions where the weight ratio (concentration ratio) of magnetite to sludge was 1 / 20 and 1 / 10, the concentration of the granulated sludge was more than double that of the control, and granulation was promoted. When the weight ratio (concentration ratio) of magnetite to sludge was 1 / 2, the concentration of granulated sludge was equivalent to that of the control.

[0017] No difference was observed in the cumulative methane production under each condition up to the 39th day after the start of the test. Therefore, this period was considered the acclimation period, and the change over time in the cumulative methane production under each condition, with the 39th day after the start of the test considered to be day 0, is shown in Figure 2. In all conditions where magnetite was added, cumulative methane production increased compared to the control.

Claims

1. A method for promoting granulation of anaerobic sludge, comprising adding magnetite to anaerobic sludge in wastewater in a weight ratio (concentration ratio) of 1 / 30 or more and 1 / 5 or less.

2. 2. The method for promoting granulation of anaerobic sludge according to claim 1, wherein the total COD concentration of the wastewater is 100 mg / L or more and 1500 mg / L or less.

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