Thermal solubilization treatment method for excess sewage sludge and use of treatment product in microalgae cultivation

Abstract

To provide a method for thermally solubilizing excess sludge generated during sewage treatment in a short period of time by treating the excess sludge in a thermal solubilization apparatus.SOLUTION: The excess sludge thermal solubilization apparatus used in the method of the present invention consists of a heat exchanger, a steam boiler that serves as the heat source for thermal solubilization, a steam supply valve, a steam pressure reducing valve set, a steam trap set, and an excess sludge supply pump. The product of thermal solubilization using the method of the present invention can be used as a nutrient source for the cultivation of heterotrophic and mixotrophic microalgae.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for thermally solubilizing flocculated excess sludge generated during sewage treatment in a short period of time by treating the excess sludge in a thermal solubilization apparatus, and to the use of the treated product as a nutrient source for microalgae cultivation. [Background technology]

[0002] Technologies for obtaining biomass, particularly biofuels, from products produced by biological cells have been attracting attention in recent years as a way to address issues such as global warming and the depletion of natural resources. Among these biomass sources, hydrocarbons, oils such as triacylglycerols, and polysaccharides produced by microalgae are highly anticipated for industrial use because they do not compete with food and can be mass-cultured, and the acquisition of biofuels and other useful components from microalgae is seen as promising.

[0003] Some microalgae grow by photosynthesis, but their growth is easily affected by changes in light intensity, air temperature, and the temperature of the culture solution in the culture tank during the cultivation process, making them unsuitable for industrial-scale mass cultivation. On the other hand, microalgae that grow heterotrophically or mixotrophically (photosynthesis + heterotrophy) are tolerant of changes in light intensity and temperature, making mass cultivation relatively easy, since their cultivation can be controlled by adding ions such as ammonia, phosphorus, and potassium, or organic substances, to the culture solution (medium) required for cultivation. For example, the present inventors discovered a novel strain belonging to the heterotrophic microalgae genus Aurantiochytrium (Non-Patent Document 1) and invented a method for producing squalene by culturing the novel Aurantiochytrium strain in a medium and harvesting the squalene produced by the grown strain (Patent Document 1).

[0004] Because the growth rate and production rate of desired products of heterotrophic and mixotrophic microalgae depend on the composition of the culture medium, it is important to secure a stable and low-cost culture medium with a suitable composition for cultivation in industrial-scale mass cultivation. The ions and organic matter required by these microalgae, such as ammonia, phosphorus, and potassium, are currently widely used as fertilizers in agriculture, raising concerns about competition for fertilizer. Furthermore, using fertilizers for mass cultivation of these microalgae is cost-effective.

[0005] Attempts have been made to use various waste materials as culture medium materials for culturing heterotrophic and mixotrophic microalgae. For example, it has been shown that microalgae can be cultured in a culture medium prepared using industrial wastewater containing phosphorus components and organic acids, such as plating wastewater (Patent Document 2 and Patent Document 3). Also shown is a method for culturing microalgae using awamori distillery wastewater or carbon dioxide gas produced by fermentation (Patent Document 4), and a method for highly efficient purification of organic wastewater, such as swine wastewater and sewage, using microalgae and activated sludge (Patent Document 5).

[0006] The large amount of excess sludge generated during sewage treatment becomes industrial waste if disposed of as is, resulting in high disposal costs, so sewage treatment facilities burn fossil fuels such as oil and gas to reduce the amount of sludge. The combustion heat generated in this process heats the inside of the methane fermentation tank that treats the sludge liquid to about 45°C, promoting methane fermentation, and some of the generated methane gas can be recycled and reused as fuel for boilers.

[0007] While the costs and labor required for sewage treatment place an excessive burden on sewage treatment businesses, the effective use of nutrients such as organic matter and minerals, which are abundant in sewage, for microalgae cultivation has been investigated. For example, a method for cultivating microalgae using activated sludge treatment water containing high concentrations of ammonia nitrogen has been shown (Patent Document 6). Also, it has been shown that sugars obtained by saccharifying polysaccharides in excess sludge through hot water treatment and acid or enzyme treatment, as well as liquid components produced during the treatment process, can be used for microalgae cultivation (Patent Document 7). [Prior art documents] [Patent documents]

[0008] [Patent Document 1] :International Publication 2012 / 077799 [Patent Document 2] :Patent Publication No. 2013-230091 [Patent Document 3] :Patent Publication No. 2023-169129 [Patent Document 4] :Patent Publication No. 2019-4807 [Patent Document 5] :Patent Publication No. 2018-183763 [Patent Document 6] :Patent Publication No. 2016-195586 [Patent Document 7] :Patent No. 5696310 [Patent Document 8] :Patent No. 5179650 [Patent Document 9] :US Patent 7987900B2 [Non-patent literature]

[0009] [Non-Patent Document 1] :BioScience, Biotechnology, and Biochemistry 75, 2246-2248 Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention aims to utilize the nutritional components of excess sludge generated during sewage treatment to reduce the volume of excess sludge and promote its treatment, and to increase the production efficiency of culture products by supplying nutrients suitable for the cultivation of heterotrophic and mixotrophic microalgae and promoting their growth. [Means for solving the problem]

[0011] To achieve the above objective, a heating means is used to solubilize flocculated excess sludge in a short time. The heating means is a thermal solubilization device that combines a heat exchanger and a steam boiler, which are designed to be less susceptible to clogging by flocs, and the excess sludge is thermally solubilized so that it can be used as a nutrient source for cultivating heterotrophic microalgae. [Effects of the Invention]

[0012] Conventionally, a batch process using an autoclave to maintain the temperature at approximately 120°C has been employed as a method for thermally solubilizing excess sludge, requiring at least two hours of processing time. In contrast, the method for thermally solubilizing excess sludge of the present invention uses a thermal solubilization apparatus of the first configuration example shown in FIG. 1 for continuous processing, enabling 1 L of excess sludge to be thermally solubilized in as little as one minute. Thermally solubilizing excess sludge through such extremely short continuous processing significantly reduces processing costs and labor. Furthermore, by utilizing the thermally solubilized product as a nutrient source for the mass cultivation of heterotrophic microalgae, it is possible to reduce the amount of fossil fuels used to reduce the volume of excess sludge at sewage treatment facilities.

[0013] Therefore, the present invention provides the following inventions. 1. A method for thermally solubilizing excess sludge generated during sewage treatment, comprising the following steps: i. A step of supplying excess sludge to a continuous treatment type thermal solubilization device, wherein the thermal solubilization device is composed of a heat exchanger, a steam boiler serving as a heat source for thermal solubilization, a steam supply valve, a steam pressure reducing valve set, a steam trap set, and an excess sludge supply pump; ii. solubilizing the excess sludge by heating it in a heat exchanger; iii. recovering the solubilized excess sludge from the heat exchanger, wherein the temperature of the solubilized excess sludge discharged from the heat exchanger is 85°C or higher; wherein the solubilized excess sludge can be used as a nutrient source for culturing heterotrophic and mixotrophic microalgae. 2. The method according to item 1, wherein the heat exchanger has heat transfer tubes without fins, and the heat transfer tubes are arranged at a distance such that the excess sludge introduced into the heat exchanger does not clog them. 3. The method according to item 1, wherein two or more heat exchangers are connected together. 4. The method according to item 1, wherein low-pressure steam is supplied from the upper part of the heat exchanger and liquid-phase steam drain water is discharged from the lower part of the heat exchanger. 5. A method for culturing heterotrophic and mixotrophic microalgae, comprising the step of adding excess sludge solubilized by the method described in Item 1 to a medium solvent containing purified water, tap water, primary treated water, secondary treated water, or seawater to prepare a culture medium for the heterotrophic and mixotrophic microalgae. 6. The method according to Item 1, wherein the heterotrophic microalgae are Labyrinthulida or colorless microalgae, and the mixotrophic microalgae are microalgae that have chloroplasts but grow by photosynthesis and organic matter assimilation, including Euglena, Botryococcus, Chlorella, Scutellaria, Monoraphidium, or indigenous mixotrophic algal communities. 7. The method according to item 5, wherein the culture medium is supplemented with one or more of carbohydrates, organic acids, inorganic acids, vitamins, amino acids, peptides, proteins, and inorganic salts in amounts appropriate for culture. [Brief explanation of the drawings]

[0014] [Figure 1] Fig. 1 shows a schematic diagram of a first example of the configuration of a thermal solubilization apparatus used in the method for thermal solubilization of excess sludge of the present invention. In this diagram, B represents a steam boiler, R represents a pressure reducing valve, PG represents a steam pressure gauge, TR represents a steam trap valve, and T represents a thermometer.

[0015] [Figure 2] FIG. 2 is a schematic diagram showing a second example of the configuration of heat exchangers arranged in series in a solubilization apparatus used in the method for thermal solubilization of excess sludge of the present invention.

[0016] [Figure 3]FIG. 3 shows the growth promotion effect of heterotrophic microalgae in a medium containing 70% thermally solubilized sludge liquid treated batchwise using a conventional autoclave and adjusted to a salinity of 1.6%.

[0017] [Figure 4A] FIG. 4A shows the growth promoting effect of the mixotrophic microalgae Monoraphidium in a medium prepared by mixing heat-solubilized sludge obtained by similar treatment with primary treated water. [Figure 4B] FIG. 4B shows the growth-promoting effect of an indigenous mixotrophic algal community in a medium in which heat-solubilized sludge obtained by similar treatment was mixed with primary treated water.

[0018] [Figure 5] FIG. 5 shows the conditions for thermal solubilization of excess sludge of the present invention used in a test to compare the water quality between thermally solubilized sludge liquid treated batchwise in an autoclave (FIG. 3) and thermally solubilized sludge liquid obtained by the method of the present invention.

[0019] [Figure 6] FIG. 6 shows the results of a comparison of water quality between the heat-solubilized sludge liquid treated batchwise by autoclave (FIG. 3) and the heat-solubilized sludge liquid obtained by the method of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0020] Generally, sewage is a general term for aqueous waste generated in the course of human life, such as cooking, laundry, bathing, washing, cleaning, and excretion, which are collected together through sewerage systems and subjected to appropriate sewage treatment.Aqueous waste generated in the course of industrial activities, such as manufacturing, agriculture, livestock farming, and food production, is treated as industrial waste and is the responsibility of the generator to treat, and is therefore usually distinguished from sewage.

[0021] The muddy solid material containing organic matter and microorganisms separated from water during sewage treatment is called excess sludge, and is the subject of treatment in this invention. Excess sludge originates from the source of sewage and is rich in various substances such as carbohydrates, organic acids, vitamins, amino acids, peptides, proteins, minerals, and microorganisms.

[0022] Excess sludge forms flocs (aggregates) due to polysaccharides produced by microorganisms, which hinder the treatment of excess sludge. Therefore, to facilitate the treatment of excess sludge, a process is required to destroy the flocs and microbial cells in the excess sludge, and this process is called solubilization. Solubilizing excess sludge by heat treatment, as in the present invention, is called thermal solubilization.

[0023] The excess sludge treatment method of the present invention relates to an improvement in the process of thermally solubilizing this sludge, and enables thermal solubilization with significantly higher efficiency than conventional techniques. The treated product is suitable for use as a nutrient source in the cultivation of heterotrophic microalgae, just as suitable as sludge solubilized over a longer period of time using conventional methods.

[0024] Excess sludge thermal solubilization equipment FIG. 1 shows a first example of the configuration of a thermal solubilization apparatus used in the method for treating excess sludge of the present invention.

[0025] As shown in FIG. 1, the thermal solubilization apparatus used in the excess sludge treatment method of the present invention comprises a steam boiler, a steam supply valve, a steam pressure reducing valve set, a steam trap set, an excess sludge supply pump, and a heat exchanger.

[0026] The steam boiler can be a high-pressure steam boiler fired by heavy oil, kerosene, or gas, which are fossil fuels; a biomass boiler fired by fermentation methane gas containing moisture; or an environmentally friendly low-pressure, small electric boiler. In the first configuration example, an electric boiler with a gauge pressure of 0.5 MPa is used for the steam supplied to the steam boiler.

[0027] The temperature of the steam supplied to the heat exchanger is determined by the absolute pressure of the steam. In the first configuration example, the gauge pressure of the supplied steam is 0.5 MPa, which corresponds to an absolute pressure of 0.6 MPa-A, and the steam temperature is approximately 159°C.

[0028] The temperature and flow rate of the steam supplied to the heat exchanger is regulated by a steam pressure reducing set consisting of a steam supply valve that adjusts the flow rate and a steam pressure reducing valve that adjusts the steam temperature to the level required by the heat exchanger. After the steam supplied to the heat exchanger transfers latent heat to the excess sludge, the steam undergoes a phase change under 1 atmosphere and becomes steam drain water at 100°C. Because the generated steam drain water hinders heat transfer in the heat exchanger, a steam trap set is installed as a means of separating the gaseous vapor from the liquid steam drain.

[0029] Since the excess sludge has a high viscosity, an electric roller pump, for example, can be used to regulate the flow rate of the sludge supplied to the heat exchanger at a constant value. Alternatively, an electric centrifugal pump or a bellows pump can also be used as the supply pump.

[0030] Common heat exchangers are either fin-and-tube types, which use fins made of thin metal plates welded at close intervals to heat transfer tubes to promote heat transfer capacity, or plate types, which have closely spaced plates (Patent Document 8, Patent Document 9). However, when highly viscous excess sludge containing flocs is fed into a fin-and-tube heat exchanger, the closely spaced fin structure inhibits the flow of the sludge, instantly blocking the flow path. Similarly, plate-type heat exchangers are also prone to flow path blockage due to the narrow spacing between the plates.

[0031] In a preferred embodiment, the heat exchanger of the present invention has no fins and no protrusions on the surface of the heat transfer tubes. Furthermore, due to the impingement jet mechanism in which the heating medium vapor collides with an impingement plate built into the heat transfer tube to promote heat transfer, and the effect of forming a swirling flow within the heat transfer tube to promote heat transfer, there is no need to narrow the spacing between the plates as in a plate-type heat exchanger. Furthermore, excess sludge flows smoothly around the heat transfer tubes, preventing clogging inside the heat exchanger.

[0032] Heat exchanger connection arrangement Figure 2 is a schematic diagram of a second configuration example of a solubilization apparatus used in the thermal solubilization method of excess sludge of the present invention, in which identical heat exchangers are connected so that the temperature of the treated product (thermally solubilized sludge liquid) at the heat exchanger outlet can be heated and maintained at 85°C or higher in a single operation. If the single-stage heat exchanger shown in the first configuration example cannot maintain a temperature of 85°C or higher due to the viscosity of the excess sludge introduced through the heat exchanger inlet or changes in the temperature environment between summer and winter, a mechanism is required to return the thermally solubilized sludge liquid after heat treatment to the heat exchanger inlet and repeatedly heat it. To solve this problem, the second configuration example shown in Figure 2 ensures the ability to heat the excess sludge to the desired outlet temperature by connecting multiple heat exchangers in series (overlapping).

[0033] It is desirable to use a gravity-based structure in which low-pressure steam is supplied from the top of the heat exchanger and liquid-phase steam drain water is discharged from the bottom of the heat exchanger.

[0034] The heat-solubilized sludge liquid treated by the heat-solubilization method of the present invention is suitable for use as a nutrient source in the cultivation of heterotrophic and mixotrophic microalgae. When culturing these microalgae, an appropriate amount of this heat-solubilized sludge liquid is added to a medium solvent (pure water, tap water, primary treated water, secondary treated water, seawater, etc.) to prepare a culture medium. In a preferred embodiment, the heat-solubilized sludge liquid or the prepared culture medium may be sterilized by known means, such as filtration sterilization, autoclave sterilization, boiling sterilization, or radiation sterilization.

[0035] In the present invention, various additives may be added to the culture medium to adjust the composition of the culture medium to be suitable for culturing heterotrophic and mixotrophic microalgae. Examples of such additives include carbohydrates, organic acids, inorganic acids, organic bases, inorganic bases, vitamins, amino acids, peptides, proteins, and minerals. Furthermore, if necessary, the pH can be adjusted appropriately by adding an appropriate acid or base. The appropriate pH of the culture medium will depend on the type of heterotrophic and mixotrophic microalgae being cultured.

[0036] If necessary, appropriate amounts of carbohydrates, organic acids, inorganic acids, vitamins, amino acids, peptides, proteins, inorganic salts, etc. may be added to the culture medium.

[0037] In the present invention, heterotrophic and mixotrophic microalgae are cultured using a treatment product obtained by thermally solubilizing excess sludge as a nutrient source. Heterotrophic microalgae refer to microalgae that cannot synthesize the organic carbon sources necessary for growth themselves but instead absorb and acquire them from extracellular organic carbon sources. Mixotrophic microalgae refer to microalgae that have the ability to synthesize organic carbon sources through photosynthesis, absorb and acquire them from extracellular organic carbon sources, and can grow even under weak light or at night. Therefore, heterotrophic microalgae cultured in the present invention include Labyrinthula species such as Aurantiochytrium and Schizochytrium, and colorless microalgae. Mixotrophic microalgae include microalgae that have chloroplasts, can perform photosynthesis, and can grow by assimilating organic matter, such as Euglena, Botryococcus, Chlorella, Castanea gracilis, Monoraphidium, or indigenous mixotrophic algal communities. When culturing heat-solubilized sludge liquor with added sugars such as glucose as a culture medium, the preferred heterotrophic microalgae is Aurantiochytrium; for example, the culturing of Aurantiochytrium limacinum is tested in the Examples of this application. When culturing heat-solubilized sludge liquor with added sugars such as glucose as a culture medium, the preferred mixotrophic microalgae are the microalgae species Ikasugamo and Monoraphidium, and in the field, indigenous mixotrophic algal communities; for example, the culturing of Monoraphidium and indigenous mixotrophic algal communities is tested in the Examples of this application.

[0038] In the present invention, heterotrophic and mixotrophic microalgae are cultured by seeding the microalgae in the culture medium prepared as described above and culturing them according to standard methods. The culture conditions depend on the type of microalgae being cultured, and the culture is typically performed at a temperature of 5 to 40°C, preferably 10 to 35°C, and more preferably 10 to 30°C, for 1 to 10 days, preferably 3 to 7 days. Culture can be performed by aeration or anaerobic agitation culture, shaking culture, or static culture. Outdoor cultivation of indigenous mixotrophic algal communities can be performed without temperature control.

[0039] The heterotrophic and mixotrophic microalgae used in the present invention can be cultured in a culture device equipped with an appropriate cell culture means. "Cell culture means" refers to any means having any function for culturing cells, such as a culture tank. The culture tank may have one or more devices selected from a stirring device, a vibration device, a temperature control device, a pH adjustment device, a turbidity measurement device, a light control device, a device for measuring the concentration of a specific gas such as CO2, and a pressure measurement device. The culture tank may be the same tank as the concentration / separation tank, or may be a separate tank from the concentration / separation tank. If the culture tank is a separate tank from the concentration / separation tank, the two tanks may be connected by an appropriate means, such as a flow path. [Example]

[0040] The results of verification of the effectiveness of the method for thermal solubilization of excess sludge of the present invention are shown below.

[0041] Example 1: Growth promotion effect of heterotrophic microalgae Aurantiochytrium limacinum (hereinafter referred to as Aurantiochytrium) cultured by adding heat-solubilized sludge obtained by batch-type heat treatment using a conventional autoclave to a culture medium

[0042] Culture and testing procedures Aurantiochytrium precultured in GTY medium (2% glucose, 1% tryptone, 0.5% yeast extract, 50% seawater) was heat-treated with 20 g / L sludge. The resulting heat-solubilized sludge was mixed with purified water to a 70% concentration, and sodium chloride was added to adjust the salinity to 1.6%. The culture was inoculated into a 300 mL Erlenmeyer flask containing 100 mL of culture medium. The culture was then cultured at 25°C and 110 rpm for 4 days with shaking. 2 g of glucose was added on days 1-3. Aurantiochytrium cultures using GTY medium were similarly performed as a control.

[0043] The results of this test are shown in Figure 3. The growth of Aurantiochytrium in the culture medium containing heat-solubilized sludge liquid was faster than that in the control (GTY medium), with the growth rate reaching approximately three times that of the control by the fourth day.

[0044] Example 2: Growth promotion effect of the mixotrophic microalgae Monoraphidium and indigenous mixotrophic algal community when heat-solubilized sludge obtained by batch-type heat treatment using a conventional autoclave was added to a primary effluent culture medium

[0045] (Cultivation and testing procedures) Sludge water at 10 g / L and 20 g / L of sludge was heat-treated, and the resulting heat-solubilized sludge was mixed with primary treated water at 5%. Monoraphidium was inoculated into a culture tube (100 mL) containing 50 mL of heat-solubilized sludge liquid-strengthened primary treated water medium, and incubated at a temperature of 25°C and a light intensity of 140 μmol / m 2 Cultures were conducted under conditions of 25 mL / min of air mixed with 1% CO2, with constant light and constant light, for 2 seconds. A control culture of Monoraphidium was conducted in the same manner using a solvent medium containing only primary effluent. A native mixotrophic algal community, which had emerged after two weeks of aeration in a culture tank containing primary effluent, was inoculated into a culture tank containing 100 L of a thermally solubilized sludge-enriched primary effluent medium, which was prepared by blending 5% thermally solubilized sludge obtained from the same sludge treatment. Cultures were conducted in an outdoor environment with fluctuating temperatures and light levels from February to March under conditions of aeration at 3 L / min for agitation and oxygen supply, and 100% CO2 at 20 mL / min for photosynthesis. A control culture of a native mixotrophic algal community was conducted in the same manner using a solvent medium containing only primary effluent.

[0046] The results of this study are shown in Figures 4A and 4B. Figure 4A shows the increase in dry weight of the mixotrophic microalgae Monoraphidium as a function of culture time (days). In this study, the dry weight of mixotrophic microalgae cultured with the addition of heat-solubilized sludge liquor was significantly greater than that cultured with primary effluent alone after 5 days of culture. The greatest growth promotion was observed in the culture with 20 g / L of heat-solubilized sludge, demonstrating that adding heat-solubilized sludge liquor to the mixotrophic microalgae culture medium enhances algal growth. These results demonstrate that the addition of heat-solubilized sludge liquor enables efficient and large-scale cultivation of mixotrophic microalgae. Figure 4B shows the daily productivity of an indigenous mixotrophic algal community that emerged after two weeks of aeration in a culture tank containing primary effluent. The community was then cultured in the field from February to March in either primary effluent alone or in a medium containing heat-solubilized sludge liquor added to primary effluent. The addition of heat-solubilized sludge liquor was found to increase the daily productivity of the indigenous mixotrophic algal community.

[0047] Example 3: Comparison of water quality between the heat-solubilized sludge liquid treated batchwise by autoclave (FIG. 4) and the heat-solubilized sludge liquid obtained by the method of the present invention

[0048] The prepared sewage was concentrated and reduced in volume by natural settling separation to obtain excess sludge, which was then fed into a heat exchanger at the design flow rate. Steam generated by a steam boiler was introduced into the heat exchanger as a heating medium, maintaining a predetermined pressure (steam temperature). The introduced steam passed through specially shaped heat transfer tubes without fins, and the steam's latent heat energy heated the excess sludge flowing outside the tubes. The steam passing through the heat transfer tubes transferred its latent heat energy to the excess sludge flowing outside the tubes, consuming the latent heat energy and releasing it from the heat exchanger as high-temperature water (steam drain) at approximately 130°C. Gaseous water vapor and steam condensate were automatically discharged from the steam trap. The heated excess sludge was discharged from the outlet of the heat exchanger and processed as thermally solubilized sludge liquid. The temperature of the excess sludge was measured at the inlet and outlet of the heat exchanger.

[0049] The thermal solubilization treatment conditions for each test specimen are shown in Figure 5. The number of heating cycles indicates the number of times the excess sludge passed through the heat exchanger. The flow rate of the heat exchanger used in the test was adjusted to a design flow rate of 1.2 L / min. The flow rate is a value designed according to the amount of excess sludge to be treated, and indicates the amount treated per unit time (minute). The sludge temperatures at the inlet and outlet of the heat exchanger were measured using a thermometer.

[0050] This test was conducted in February during the winter, and the inlet liquid temperature of the heat exchanger during the first heating operation was measured to be 16°C, which is equal to room temperature, and the outlet liquid temperature was measured to be 80°C. Because steam is used as the heating medium, the activated sludge solution was able to reach a maximum liquid temperature of 95°C, within the range where it does not undergo a phase change.

[0051] Figure 6 shows a comparison of water quality between the thermally solubilized sludge liquor processed batchwise using an autoclave (Figure 4), which promotes the growth of mixotrophic microalgae, and the thermally solubilized sludge liquor obtained by the method of the present invention, processed under the above conditions. The thermally solubilized sludge liquor obtained by the continuous thermal solubilization apparatus of the present invention was added to and mixed with primary effluent at a concentration of 5%. The results of water quality analysis are shown for the following comparison items: BOD (biochemical oxygen demand) and COD (chemical oxygen demand) as indicators of organic matter, SS (suspended solids) as indicator of solids, TN as indicator of total nitrogen, TP as indicator of total phosphorus, and pH (hydrogen ion concentration). BOD was measured and calculated in accordance with the Japanese Industrial Standards (JIS) K0102.21, a method for testing industrial wastewater. COD was measured and calculated in accordance with JIS K0102.17. The SS concentration (amount of suspended solids) was calculated by filtering the sample through a weighed 1 μm glass fiber filter, drying the used glass fiber filter at 105-110°C for approximately 2 hours, and measuring the weight. TN (total nitrogen) was measured and calculated in accordance with JIS K0102.45.6, TP (total phosphorus) in accordance with JIS K0102.46.3.4, and pH in accordance with JIS K0102.12.1. The values ​​in parentheses indicate the ratio of that item when TP is set to 1.

[0052] Based on the ratios of other parameters to TP, the treatment in Test No. 4 (samples 10-12), which was repeated three times, was most similar to the culture medium (primary effluent + solubilized liquid) containing excess sludge that had been batch-type heat-solubilized in an autoclave at 120°C for 2 hours. However, no significant differences were observed in Test No. 3 (samples 7-9) and Test No. 5 (samples 13-15) compared to heat-solubilized sludge obtained by autoclave treatment. These results demonstrate that the continuous heat-solubilization method of the present invention, which maintains the heat exchanger outlet temperature at 85°C or higher, can produce heat-solubilized excess sludge that promotes the growth of heterotrophic and mixotrophic microalgae.

Claims

1. A method for thermally solubilizing excess sludge generated during sewage treatment, comprising the following steps: i) A step of supplying excess sludge to a continuous treatment type thermal solubilization apparatus, wherein the thermal solubilization apparatus is composed of a heat exchanger, a steam boiler serving as a heat source for thermal solubilization, a steam supply valve, a steam pressure reducing valve set, a steam trap set, and an excess sludge supply pump; ii. solubilizing the excess sludge by heating it in a heat exchanger; iii. Recovering the solubilized excess sludge from the heat exchanger, wherein the temperature of the solubilized excess sludge discharged from the heat exchanger is 85°C or higher; wherein the solubilized excess sludge can be used as a nutrient source for culturing heterotrophic and mixotrophic microalgae.

2. 2. The method of claim 1, wherein the heat exchanger has heat transfer tubes without fins and the heat transfer tubes are spaced apart so as not to clog with excess sludge introduced into the heat exchanger.

3. The method of claim 1 , wherein two or more of the heat exchangers are connected together.

4. 2. The method according to claim 1, wherein low-pressure steam is supplied from an upper portion of the heat exchanger and liquid-phase steam drain water is discharged from a lower portion of the heat exchanger.

5. A method for culturing heterotrophic and mixotrophic microalgae, comprising the step of adding excess sludge solubilized by the method of claim 1 to a medium solvent containing pure water, tap water, primary treatment water, secondary treatment water, or seawater to prepare a culture medium for the heterotrophic and mixotrophic microalgae.

6. 2. The method of claim 1, wherein the heterotrophic microalgae are Labyrinthulida or colorless microalgae, and the mixotrophic microalgae are microalgae that have chloroplasts and grow by photosynthesis and organic matter assimilation, including Euglena, Botryococcus, Chlorella, Castor oil, Monoraphidium, or indigenous mixotrophic algal communities.

7. The method according to claim 5, wherein the culture medium is supplemented with one or more of carbohydrates, organic acids, inorganic acids, vitamins, amino acids, peptides, proteins, and inorganic salts in amounts appropriate for culture.

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