Sewage sludge fermentation raw material and sewage sludge treatment method

Abstract

To provide a fermentation raw material that can accelerate aerobic fermentation and to thereby contribute to effective use of resources, only by a simple operation of adding a specific amount of a specific material to sewage sludge.SOLUTION: A sewage sludge fermentation raw material for aerobic fermentation includes sewage sludge and oil cake. The oil cake has a pH of 5 or more and 9 or less. The raw material contains 10 or more and 130 or less parts by mass of oil cake per 100 parts by mass of sewage sludge. The oil cake is one or more selected from the group consisting of rapeseed oil cake, soybean oil cake, sesame oil cake, and cottonseed oil cake. The sewage sludge fermentation raw material of the invention further includes an aeration aid.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a raw material for aerobic fermentation of sewage sludge and a method for treating sewage sludge. [Background technology]

[0002] Sewage sludge is a muddy substance containing organic matter and water, and is inevitably discharged during the process of sewage treatment associated with daily activities. The amount of sewage sludge discharged is increasing as the amount of sewage treated increases, and its disposal, like that of urban solid waste, has become a problem. To treat sewage sludge, for example, attempts have been made to incinerate the sludge and use the heat generated during the process as an energy source, but it is desirable to reduce the moisture content of sewage sludge in order to perform incineration more efficiently.

[0003] A technique for aerobic fermentation of sewage sludge is known as a technique for inexpensively reducing the moisture content of sewage sludge. For example, Patent Documents 1 to 3 disclose methods for fermenting a mixture of organic sludge and organic materials in order to improve dewatering efficiency and reduce volume. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 11-079873 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-111374 [Patent Document 3] Japanese Patent Application Laid-Open No. 2009-274908 Summary of the Invention [Problem to be solved by the invention]

[0005] Generally, stable aerobic fermentation requires optimization of various conditions, such as the nutrients that serve as nutrient sources for the microorganisms contained in the material to be treated, as well as moisture content, pH, and aeration rate. When aerobic fermentation is performed on sewage sludge that contains little of the nutrients that serve as nutrient sources for the microorganisms, such as digested sludge that has undergone methane fermentation during sewage treatment, microbial activity can be sluggish due to a lack of nutrients, and aerobic fermentation of the material may not be sufficient.

[0006] As a method for improving the aerobic fermentation state of such a fermentation system in which microbial activity has become sluggish, Patent Documents 1 to 3 add a material rich in nutrients (hereinafter also referred to as a "nutrient source" or "nutrient source material"). However, when sewage sludge is used as the target of aerobic fermentation treatment, depending on the type of sewage sludge and the type and amount of nutrient source material added, there may be a shortage of nutrients suitable for the microorganisms contained in the sewage sludge, making it difficult to maintain a good aerobic fermentation state. Therefore, in order to ensure good aerobic fermentation, it has been important to design appropriate materials for each sewage sludge and material used.

[0007] In the past, material design for sewage sludge fermentation feedstocks has involved testing using actual facilities or test equipment consisting of large-capacity fermentation tanks, which required large amounts of sample material and required a long time. Furthermore, in recent years, sewage sludge fermentation treatment has begun to expand nationwide, and it is recommended to select sewage sludge and materials that are easily available in the local area, resulting in a wide variety of materials being used. Therefore, there was a need to select materials that are rich in nutrients necessary for microbial activity and promote microbial activity, and to design a material composition that would consistently promote aerobic fermentation.

[0008] Therefore, an object of the present invention is to provide a sewage sludge fermentation raw material that can promote fermentation simply and stably without having to adjust the composition each time depending on the fermentation state. [Means for solving the problem]

[0009] As a result of intensive studies to solve the above problems, the present inventors have found that oil cakes generated during the purification of vegetable oils contain a large amount of nutrients that promote the fermentation of sewage sludge. In addition, although oil cakes may be treated with chemicals such as acids in the vegetable oil purification process, it has been found that the fermentation of sewage sludge is promoted when the pH of the oil cake is within a specific range. Based on these findings, the present inventors have found that by adding oil cakes with a pH within a specific range to sewage sludge at a specific blending ratio, the fermentation of sewage sludge, which is the fermentation target, is stably promoted, and thus the present invention has been completed.

[0010] That is, the present invention includes sewage sludge and oil cake, the pH of the oil cake measured by the following method is 5 or more and 9 or less, and provides a sewage sludge fermentation raw material for aerobic fermentation treatment, which contains 10 parts by mass or more and 130 parts by mass or less of the oil cake with respect to 100 parts by mass of sewage sludge. <Method for Measuring pH> A mixed solution of 1 part by mass of oil cake and 10 parts by mass of water is stirred for 10 minutes, and the pH of the mixed solution is measured at 23°C.

[0011] The present invention also includes a step of subjecting a sewage sludge fermentation raw material containing sewage sludge and oil cake to aerobic fermentation to treat the sewage sludge, and provides a method for treating sewage sludge, which uses the sewage sludge fermentation raw material containing 10 parts by mass or more and 1,30 parts by mass or less of the oil cake with a pH of 5 or more and 9 or less with respect to 100 parts by mass of the sewage sludge.

Effects of the Invention

[0012] According to the present invention, aerobic fermentation can be rapidly advanced and sewage sludge can be stably fermented only by a simple operation of adding a specific material to sewage sludge in a specific amount. Furthermore, in industrial areas such as cement factories and sources of sewage sludge such as sewage treatment plants, a large amount of sewage sludge with different properties can be fermented and treated, leading to effective utilization of resources.

Brief Description of the Drawings

[0013] [Figure 1] FIG. 1 is a cross-sectional view schematically showing one embodiment of a sealed vertical fermenter. [Figure 2] FIG. 2(a) is a schematic perspective view showing the appearance and dimensions of the fermentation vessel used in the aerobic fermentation evaluation in the examples and comparative examples, and FIG. 2(b) is a cross-sectional view showing the arrangement positions of each component during temperature measurement. DETAILED DESCRIPTION OF THE INVENTION

[0014] Preferred embodiments of the present invention will be described below, but the present invention is not limited to the following embodiments.

[0015] The sewage sludge fermentation raw material of the present invention contains sewage sludge and oil cake as its ingredients. This sewage sludge fermentation raw material is suitable for use in aerobic fermentation treatment of sewage sludge.

[0016] The sewage sludge used in the present invention is a waste product generated during sewage treatment and is a muddy substance containing organic matter, inorganic matter, and water. Sewage sludge is typically obtained by dewatering excess sludge discharged from activated sludge-based wastewater treatment facilities. Examples of such sewage sludge include general sewage sludge generated in sewage treatment plants, sewage sludge generated in sewage treatment facilities, and septic tank sludge, which can be used alone or in combination. Sewage sludge may be used as is as undigested sludge, or a self-fermented product of sewage sludge, such as digested sludge, may be used. Digested sludge generally contains fewer nutrients necessary for fermentation than undigested sludge, and therefore, the effects of the present invention are more pronounced when digested sludge is used as the target of the treatment of the present invention. The moisture content of sewage sludge is not particularly limited, but is, for example, about 50% by mass to 90% by mass, and preferably 50% by mass to 85% by mass. The type of sewage sludge can be selected based on at least one of its moisture content, whether it has been digested, and the dehydration treatment method.

[0017] The oil cake used in the present invention is the residue left after squeezing oil from agricultural crops, etc. Oil cake contains crude fat, crude protein, etc., which can serve as a nutrient source for aerobic microorganisms, and therefore has the advantage that organic matter present in sewage sludge is decomposed by the microorganisms, allowing aerobic fermentation to proceed stably.

[0018] The oil cake used in the present invention is preferably used in an amount of 10 to 130 parts by mass, more preferably 20 to 100 parts by mass, and even more preferably 30 to 80 parts by mass, per 100 parts by mass of sewage sludge. Using 10 parts by mass or more of oil cake has the advantage of supplying a necessary and sufficient nutrient source for active aerobic fermentation of the sewage sludge, and allowing the aerobic fermentation to proceed stably, while using 130 parts by mass or less of oil cake has the advantage of significantly suppressing pH changes in the entire fermentation system due to the pH buffering action of the sewage sludge, and of supplying a necessary and sufficient nutrient source for active aerobic fermentation of the sewage sludge.

[0019] The content is measured in dried oil cake. A "dry state" refers to an oil cake having a moisture content of 10% by mass or less, more preferably 7.5% by mass or less, and even more preferably 5% by mass or less. The moisture content of the oil cake can be appropriately adjusted by drying the oil cake using a common method, such as forced drying using a hot air convection dryer or natural drying in the sun. The moisture content of the oil cake is measured based on the weight loss after drying at 135°C for 2 hours in accordance with the Feed Analysis Method.

[0020] The inventors speculate that the reason why aerobic fermentation proceeds efficiently when a specific amount of oil cake having a pH within a specific range is added to the sewage sludge fermentation feedstock of the present invention is as follows: When sewage sludge is fermented by the action of microorganisms, acidic substances are produced, and these acidic substances lower the pH of the sewage sludge to the acidic side. When the pH drops, the activity of the microorganisms in the sewage sludge stagnates, inhibiting fermentation. In contrast, the present invention uses a specified amount of oil cake, which has a pH similar to that of the sewage sludge (pH 5 to 9), as a nutrient source. The inventors speculate that the pH is less likely to drop due to the pH buffering action of the sewage sludge, and the supply of the nutrient source promotes fermentation. If the pH of the oil cake is 5 or higher, the pH buffering action of the sewage sludge will have a significant effect in inhibiting a decrease in the pH of the entire fermentation system, and if the pH of the oil cake is 9 or lower, there is the advantage that the pH of the entire fermentation system is maintained within a range suitable for the activity of aerobic microorganisms.Therefore, the pH of the oil cake is preferably 5 or higher and 9 or lower, more preferably 5.5 or higher and 8.0 or lower, and even more preferably 5.7 or higher and 7.0 or lower.

[0021] The pH of the oil cake can be obtained by mixing 1 part by mass of the oil cake to be measured with 10 parts by mass of water, stirring the mixture for 10 minutes, and measuring the pH of the mixture at 23°C.

[0022] The oil cake can be the residue left after squeezing oil from plant seeds, such as rapeseed oil cake, soybean oil cake, sesame oil cake, cottonseed oil cake, castor oil cake, linseed oil cake, sunflower oil cake, safflower oil cake, etc. These oil cakes can be used alone or in combination of two or more. Among these, from the viewpoint of further enhancing the above-mentioned effects, it is preferable to use at least one selected from the group consisting of rapeseed oil cake, soybean oil cake, sesame oil cake, and cottonseed oil cake.

[0023] The oil cake used in the present invention preferably has an ignition loss of 90% by mass or more, more preferably 93% by mass or more. The ignition loss of oil cake reflects the amount of organic components contained in the oil cake. By using oil cake with such an ignition loss, aerobic fermentation can be further promoted, enabling efficient fermentation treatment of sewage sludge. The ignition loss of oil cake is measured according to JIS R5202:2015 "Methods for chemical analysis of cement."

[0024] Furthermore, the solid calorific value of the oil cake used in the present invention is preferably 4000 kcal / kg or more, more preferably 4300 kcal / kg or more, and even more preferably 4500 kcal / kg or more. This solid calorific value is the calorific value of the total amount of all types of oil cake used as the sewage sludge fermentation feedstock. Generally, a high solid calorific value indicates that the oil cake contains a large amount of organic matter, which is one of the nutrients useful for the progression of aerobic fermentation. Therefore, by using oil cake with such a calorific value, aerobic fermentation can be further promoted, allowing for efficient fermentation of sewage sludge.

[0025] The sewage sludge fermentation raw material of the present invention may consist of only sewage sludge and oil cake, or it may preferably further contain, in addition to these, materials other than sewage sludge and oil cake (hereinafter, these may also be simply referred to as "materials").

[0026] Examples of materials include those that promote stable aerobic fermentation when sewage sludge fermentation raw material is subjected to fermentation, such as those that reduce the moisture content of sewage sludge, improve the breathability of the sewage sludge fermentation raw material during fermentation, and supply a wide variety of microorganisms that contribute to aerobic fermentation.

[0027] In particular, the sewage sludge fermentation raw material preferably further contains an aeration aid as a material for improving the aeration of the sewage sludge fermentation raw material and promoting aerobic fermentation of the sewage sludge. By including the aeration aid, the aeration of the sewage sludge fermentation raw material can be easily improved regardless of the state of consolidation of the sewage sludge fermentation raw material, and aerobic fermentation of the sewage sludge can be carried out stably. In particular, when aerobic fermentation is carried out using a vertical fermenter described below, for example, the sewage sludge in the fermenter can be consolidated due to the accumulation of the sewage sludge fermentation raw material, making it difficult for the aerobic fermentation of the sewage sludge to proceed. However, by including the aeration aid, it is possible to further suppress excessive consolidation while further ensuring aeration, which is advantageous in that it allows the aerobic fermentation of the sewage sludge to proceed stably and effectively.

[0028] Examples of ventilation aids include organic ventilation aids such as rice straw, rice husks, plants, and dried or crushed materials thereof, and inorganic ventilation aids such as perlite, zeolite, diatomaceous earth, and coal ash such as fly ash. These materials can be used alone or in combination.

[0029] When the sewage sludge fermentation raw material further contains an aeration aid, the content of the aeration aid in the sewage sludge fermentation raw material is preferably 5 to 50 parts by mass, more preferably 5 to 30 parts by mass, and even more preferably 5 to 15 parts by mass, per 100 parts by mass of sewage sludge. Here, the reference masses of the sewage sludge and the aeration aid are both based on the mass in a wet state. When multiple types of aeration aids are contained, the content of the aeration aids is based on the total amount. By ensuring a content within this range, it is possible to ensure uniform aeration while preventing the sewage sludge fermentation raw material from becoming excessively compacted over a long period from the start to the end of fermentation, and it is possible to proceed with aerobic fermentation of the sewage sludge stably and effectively.

[0030] From the viewpoint of ensuring a sufficient amount of moisture for stable aerobic fermentation from the initial stage of fermentation, the moisture content of the entire sewage sludge fermentation raw material is preferably 30% to 90% by mass, and more preferably 40% to 85% by mass. The moisture content can be measured, for example, using a commercially available halogen moisture meter based on the difference in mass before and after drying at a heating temperature of 120°C. Alternatively, it can be measured in accordance with JIS A1203 "Testing Method for Moisture Content of Soil." The moisture content of the sewage sludge fermentation raw material can be appropriately adjusted by selecting raw materials to achieve the desired moisture content, for example, by using sewage sludge with a high moisture content and dried oil cake, or by adding water to the raw materials or the sewage sludge fermentation raw material.

[0031] The sewage sludge fermentation feedstock of the present invention can be produced as a mixture or sediment by, for example, mixing or depositing sewage sludge and oil cake with various materials as needed. Specifically, examples include a method of obtaining the sewage sludge fermentation feedstock by mixing sewage sludge and oil cake with various materials as needed, or a method of obtaining the sewage sludge fermentation feedstock as a sediment by depositing the materials in any order indoors or outdoors. Alternatively, examples include a method of supplying one of the materials to a vessel such as a fermenter, and then supplying the other materials to the vessel in any order, thereby depositing the materials alternately or randomly in the vessel, and then mixing the deposit in a vessel such as a fermenter as is or in addition to the above to obtain a mixture.

[0032] The sewage sludge fermentation raw material described above, whether in the form of a sediment or a mixture, is suitable for use in the aerobic fermentation treatment of sewage sludge. The sewage sludge fermentation raw material can be placed outdoors or indoors as is, or can be placed in a container as a sediment or a mixture and subjected to the aerobic fermentation treatment of sewage sludge.

[0033] Specifically, the sewage sludge fermentation raw material can be aerobically fermented by piling it in a compost shed or by placing it in an open or closed fermenter. When the sewage sludge fermentation raw material is supplied to a fermenter for aerobic fermentation, aerobic fermentation can be carried out stably from the early stage of fermentation over a long period of time, regardless of the presence or absence of stirring equipment in the fermenter or the stirring method, and the sewage sludge can be efficiently treated. From the viewpoint of reducing adverse effects on the surrounding environment, such as odors, when the sewage sludge in the sewage sludge fermentation raw material is aerobically fermented preferably in a closed fermenter. A closed system refers to a reaction system in which the intrusion of air from the external environment and the outflow of fermentation gas generated from the fermenter to the outside during aerobic fermentation are controlled, while an open system refers to a reaction system in which the intrusion and outflow of gases such as air and gases are not controlled in any way.

[0034] In particular, the sewage sludge fermentation raw material of the present invention is suitable for fermenting sewage sludge in a sealed, vertical fermenter (hereinafter also referred to as a "sealed vertical fermenter"), because it allows the aerobic fermentation of sewage sludge to proceed stably over a long period of time without having to change environmental conditions such as the ingredient blend or aeration rate each time depending on the fermentation state. In other words, a method for fermenting sewage sludge preferably includes a step of supplying sewage sludge and oil cake, and optionally other materials, into a sealed vertical fermenter in any order, or supplying a mixture containing these raw materials into the sealed vertical fermenter and aerobically fermenting the mixture, and more preferably, this step is carried out in a sealed system. The sealed vertical fermenter may be equipped with stirring equipment for stirring the contents of the fermenter, so that the raw materials supplied to the fermenter are stirred continuously or intermittently.

[0035] Figure 1 shows one embodiment of a sealed vertical fermenter suitable for use in the fermentation treatment of sewage sludge fermentation material of the present invention. The sealed vertical fermenter 10 extends vertically relative to the installation surface and has a cylindrical tank section 20 capable of containing sewage sludge, oil cake, and, if necessary, a mixture of other materials. The tank section 20 is equipped with an inlet 30 at its top, through which the mixture can be introduced into the tank section 20, and an outlet 40 at its bottom, through which the aerobically fermented sewage sludge fermentation material can be discharged from the tank section 20. Both the inlet 30 and the outlet 40 are equipped with detachable or openable lid-like members (not shown), and the tank section 20 of the fermenter 10 can be sealed by attaching the lid-like members to the inlet 30 and the outlet 40. In other words, the sealed vertical fermenter 10 can treat sewage sludge by aerobic fermentation in a sealed system.

[0036] From the viewpoint of further improving the efficiency of aerobic fermentation, it is preferable that the sealed vertical fermenter 10 has a thermally insulated structure, for example, by providing a thermal insulator on the outer peripheral surface of the tank section 20. It is also preferable that the sealed vertical fermenter 10 is equipped with a stirring device 50 for mixing the raw materials in the fermenter to make the state of the components and the breathability uniform. 1 includes, for example, an agitator blade 51 provided in the tank section 20, an agitator shaft 52 connected to the agitator blade 51, and a motor (not shown) provided outside the tank section 20. The agitator blade 51 is connected to the motor provided outside the tank section 20 via the agitator shaft 52 and rotates in a fixed direction using the motor as a drive source. By further providing the agitator 50, the aeration within the reaction system can be increased, further improving the aerobic fermentation efficiency of the sewage sludge fermentation raw material.

[0037] The sealed vertical fermenter 10 also preferably includes an air circulation system 60 for supplying oxygen-containing gases such as air and oxygen into the fermenter, and an exhaust port 70 for exhausting gases from the tank section 20 to the outside of the tank section 20. This allows for appropriate control of the circulation of air and gases inside and outside the reaction system, thereby promoting aerobic fermentation more efficiently, particularly in a sealed system.

[0038] 1, oxygen-containing gas F can be supplied from air circulation equipment 60 provided outside the tank section 20 to the vertically lower side of the agitator blade 51, preferably via the hollow agitator shaft 52 and the interior of the agitator blade 51. The vertically lower side of the agitator blade 51 is also preferably provided with a plurality of gas circulation holes (not shown) through which the oxygen-containing gas F can flow. The oxygen-containing gas present in the tank section 20 and the gas generated by aerobic fermentation are exhausted from the top of the tank section 20 via exhaust port 70 as exhaust air.

[0039] From the viewpoint of facilitating the supply of oxygen-containing gas to the entire fermenter and increasing the efficiency of aerobic fermentation of sewage sludge, it is preferable that oxygen-containing gas F is supplied from the vertically lower side of tank section 20, and that oxygen-containing gas F and gas are exhausted from the vertically upper side of tank section 20. The sewage sludge fermentation raw material is continuously or intermittently introduced into tank section 20 of the fermenter through inlet 30, and the sewage sludge fermentation raw material is aerobically fermented in the fermenter for about two weeks, after which the fermented sewage sludge fermentation raw material is discharged from the outlet as fermented sludge.

[0040] The sludge fermentation product produced by subjecting sewage sludge fermentation raw material to aerobic fermentation can be used for purposes such as fertilizer, soil improvement material, green farmland materials such as horticultural soil, cement clinker raw material, solid fuel, etc., enabling effective utilization of resources. In particular, it is preferable to mix the sludge fermentation product with raw materials such as limestone and use it as a cement clinker raw material, as this increases the effective use amount of the produced sludge fermentation product and further contributes to effective utilization of resources. Furthermore, this sludge fermentation product can be reused as the nutritional supplement of the present invention when preparing sewage sludge fermentation raw material, which also contributes to effective utilization of resources.

[0041] As is clear from the above explanation, this specification discloses not only a sewage sludge fermentation raw material, but also a method for producing the sewage sludge fermentation raw material and a method for treating sewage sludge using the sewage sludge fermentation raw material. The method for treating sewage sludge comprises a step of aerobically fermenting a sewage sludge fermentation feedstock containing sewage sludge and oil cake to treat the sewage sludge. The above-mentioned explanations apply appropriately to the sewage sludge and oil cake contained in the sewage sludge fermentation feedstock used in this treatment method, as well as the types and amounts of various materials contained as needed. [Example]

[0042] The present invention will be described in more detail below with reference to examples. The scope of the present invention is not limited to these examples. The pH of the oil cakes shown below was measured at 23°C using a pH meter (MM-60R multi-water quality meter, manufactured by DKK-TOA Corporation) to obtain a stirred solution obtained by mixing 1 part by mass of the oil cake to be measured with 10 parts by mass of water and stirring for 10 minutes with a magnetic stirrer (600 rpm, manufactured by Advantec Toyo Co., Ltd.). The moisture content of the sewage sludge and sewage sludge fermentation raw material shown below was calculated from the mass difference when dried at a heating temperature of 120°C using a halogen moisture meter (HM1105, manufactured by AS ONE Corporation). Furthermore, the calorific values ​​shown below indicate the calorific values ​​of solid content.

[0043] [Examples 1 to 4 and Comparative Examples 1 and 2] [Preparation of sewage sludge fermentation raw material] The sewage sludge and oil cake shown in (1) to (6) below were mixed in the content ratios shown in Table 1 below to prepare sewage sludge fermentation raw materials with moisture contents of 65.2% to 66.8% by mass.

[0044] (1) Sewage sludge: Digested sludge obtained from a sewage treatment plant (moisture content: 84.4% by mass) (2) Rapeseed oil cake: Rapeseed oil extraction residue (pH 5.8, moisture content 8.0% by mass, loss on ignition 93.6% by mass, calorific value 4821 kcal / kg) (3) Soybean oil cake: soybean oil extraction residue (pH 6.7, moisture content 6.9% by mass, loss on ignition 93.8% by mass, calorific value 4520 kcal / kg) (4) Sesame oil cake: sesame oil extraction residue (pH 5.9, moisture content 1.5% by mass, loss on ignition 92.1% by mass, calorific value 5550 kcal / kg) (5) Cottonseed oil cake: cottonseed oil extraction residue (pH 6.3, moisture content 6.2% by mass, loss on ignition 94.0% by mass, calorific value 4450 kcal / kg) (6) Corn oil cake: corn oil extraction residue (pH 4.0, moisture content 1.4% by mass, loss on ignition 92.6% by mass, calorific value 4520 kcal / kg)

[0045] [Aerobic fermentation test] The sewage sludge fermentation raw materials of the Examples and Comparative Examples were subjected to aerobic fermentation treatment, and the progress of the aerobic fermentation of the sewage sludge was evaluated as a change in the sample temperature. A 500 mL plastic beaker and a simple insulated container covering the sides and bottom of the beaker were used as fermentation vessels. The positions and dimensions of these were as shown in Figure 2(a). Approximately 400 mL of the sewage sludge fermentation raw material of each Example and Comparative Example was placed in each plastic beaker to prepare the samples.

[0046] Next, the plastic beakers containing each sample were placed in an insulated container as shown in Figure 2(b), and a T-type thermocouple (manufactured by Chino Corporation) was inserted into the center of the sample inside the plastic beaker. A data logger was connected to the thermocouple, and the sample was subjected to aerobic fermentation while continuously measuring its temperature. These experiments were carried out for 8 days in a room set at 20°C. The progress of aerobic fermentation was evaluated based on the maximum temperature measured. A higher maximum temperature indicates a more stable aerobic fermentation of the sewage sludge. From the perspective of stable aerobic fermentation of sewage sludge, a sample was deemed to have passed the test if its peak temperature was 40°C or higher and reached 40°C within 48 hours. The results are shown in Table 1 below.

[0047] [Table 1]

[0048] As shown in Table 1, the sewage sludge fermentation feedstocks of Examples 1 to 4, in which oil cakes with a pH within a predetermined range were added to sewage sludge at a predetermined content, had higher peak exothermic temperatures and shorter times to reach 40°C compared to the respective comparative examples. The reason for this is believed to be that rapeseed oil cake, soybean oil cake, sesame oil cake, and cottonseed oil cake have pHs of 5.8 to 6.7, which are close to the pH of sewage sludge, so adding them to sewage sludge did not delay fermentation, and the temperature rose to 40°C or higher within two days. On the other hand, the sewage sludge fermentation feedstock of Comparative Example 2, which used corn oil cake with a low pH, reached a peak temperature of 40°C, but it took 55 hours to reach 40°C. The reason for this is believed to be that the low pH of corn oil cake acidified the fermentation feedstock when added to sewage sludge, causing a delay in fermentation due to a deviation from the pH suitable for fermentation.

[0049] [Examples 5 to 9 and Comparative Examples 3 and 4] [Preparation of sewage sludge fermentation raw material] The above-mentioned sewage sludge (1) was mixed with the above-mentioned rapeseed oil cake (2) in the content ratios shown in Table 2 below to prepare sewage sludge fermentation raw materials with moisture contents of 46.2 to 83.3 mass%.

[0050] [Aerobic fermentation test] The degree of equilibrium in aerobic fermentation of each sewage sludge fermentation raw material was evaluated using the same methods as in Examples 1 to 4 and Comparative Examples 1 and 2. From the perspective of stably aerobic fermentation treatment of sewage sludge, a test was deemed to pass if the peak temperature was 40°C or higher and reached 40°C within 48 hours. The results are shown in Table 2 below, which also lists the results of Comparative Example 1.

[0051] [Table 2]

[0052] As shown in Table 2, the sewage sludge fermentation raw materials of Examples 5 to 9, in which rapeseed oil cake with a pH in a predetermined range was added to sewage sludge to a predetermined content, had a higher peak temperature than each of the comparative examples and reached 40°C within 48 hours. This is thought to be because mixing a predetermined amount of rapeseed oil cake with sewage sludge provided a sufficient supply of nutrients necessary for fermentation. [Explanation of symbols]

[0053] 10. Closed vertical fermentation tank 20 Tank section 30 Inlet 40 Outlet 51 Mixing blade 52 Agitator shaft 60 Air circulation equipment 70 exhaust port

Claims

1. A method for treating digested sludge and oil cake, The pH of the oil cake measured by the following method is 5 or more and 9 or less, A sewage sludge fermentation raw material for aerobic fermentation treatment, comprising 10 parts by mass or more and 130 parts by mass or less of the oil cake per 100 parts by mass of digested sludge. <pH measurement method> A mixture of 1 part by mass of oil cake and 10 parts by mass of water is stirred for 10 minutes, and the pH of the mixture is measured at 23°C.

2. 2. The sewage sludge fermentation raw material according to claim 1, wherein the oil cake is at least one selected from the group consisting of rapeseed oil cake, soybean oil cake, sesame oil cake and cottonseed oil cake.

3. The sewage sludge fermentation raw material according to claim 1 or 2, further comprising an aeration aid.

4. A method for treating sewage sludge by aerobic fermentation of a sewage sludge fermentation raw material containing digested sludge and oil cake, A method for treating sewage sludge, comprising using the sewage sludge fermentation raw material containing 10 to 130 parts by mass of oil cake having a pH of 5 to 9, as measured by the following method, per 100 parts by mass of the digested sludge. <pH measurement method> A mixture of 1 part by mass of oil cake and 10 parts by mass of water is stirred for 10 minutes, and the pH of the mixture is measured at 23°C.

5. 5. The method according to claim 4, wherein the sewage sludge fermentation raw material is aerobically fermented in a sealed, vertical fermenter.

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