Method for producing PHA (polyhydroxyalkanoate) by direct anaerobic digestion of residual activated sludge under condition of taking starch as carbon source

Abstract

The invention provides a method for producing PHA through direct anaerobic digestion of residual activated sludge under the condition that starch is used as a carbon source, and belongs to the technical field of residual activated sludge treatment.The method comprises the steps that sewage in an aerobic tank is taken to stand and be layered, then part of upper-layer clear liquid is removed, starch is added into obtained residual sludge turbid liquid, the mixture is mixed to be uniform, anaerobic fermentation is conducted, and the residual sludge turbid liquid is obtained; and separating the PHA to obtain the PHA. According to the method, the residual activated sludge from the aerobic tank is directly subjected to anaerobic digestion to produce the PHA, domestication culture of the residual activated sludge is not needed, the technological process is simple, the production cost of the PHA can be greatly reduced, and the method has a very good application prospect in the field of residual activated sludge recycling.

Description

Technical Field

[0001] The present invention relates to the technical field of excess activated sludge treatment, in particular to a method for synthesizing PHA by anaerobic digestion of excess activated sludge, and specifically to a method for producing PHA by directly anaerobic digestion of excess activated sludge under the condition that starch is used as a carbon source. Background Art

[0002] Polyhydroxyalkanoates (PHAs), a type of polyester polymer synthesized intracellularly by microorganisms, have physical and chemical properties similar to those of polyethylene and can be completely degraded into harmless monomers in the natural environment. Therefore, research into PHA production technology has become a hot topic.

[0003] At present, the main production method of PHA is microbial fermentation, but due to the limitation of carbon source cost, the cost of PHA is relatively high, which greatly restricts the promotion and application of PHA.

[0004] Excess activated sludge is a solid waste discharged from aerobic tanks at sewage treatment plants. It is primarily composed of aerobic and facultative microorganisms, including a high concentration of PHA-producing bacteria. This paves the way for utilizing excess activated sludge to produce PHA. However, the proportion of PHA-producing bacteria in excess activated sludge is low, resulting in low PHA content and a lack of commercial competitiveness. To date, two main methods have been used to increase the PHA content in excess activated sludge: One is to enrich PHA-synthesizing bacteria and increase the proportion of PHA-synthesizing bacteria in the residual activated sludge. Chinese invention patent CN101735440 B discloses the use of an anaerobic-good-sample and starvation-feast alternating method to treat residual activated sludge, enrich PHA-synthesizing bacteria in the residual activated sludge, and increase the proportion of PHA-synthesizing bacteria, thereby achieving the purpose of increasing the PHA content in the residual activated sludge. Veolia Group and Paques Biomaterials of the Netherlands used this method to establish a PHA pilot production plant in Europe, increasing the PHA content per gram of dry sludge to 470 mg. However, this method has a complex process and high PHA production costs.

[0005] The second approach involves isolating PHA-synthesizing bacteria from excess activated sludge, acclimating and culturing them, and then adding them to the excess activated sludge to increase the proportion of PHA-synthesizing bacteria. Chinese invention patent CN100445362C isolates PHA-synthesizing bacteria from excess activated sludge, acclimates them, and then adds them to the excess activated sludge for fermentation to produce PHA. However, the isolation and acclimation of PHA-synthesizing bacteria require strict sterilization, resulting in a complex process that increases PHA production costs.

[0006] Therefore, it is of great significance to develop a new method for producing PHA using excess activated sludge with simple process and high PHA yield. Summary of the Invention

[0007] The residual activated sludge discharged from the aerobic tank of the sewage treatment plant is mainly composed of various microorganisms and extracellular secretions, proteins and organic matter. During the anaerobic digestion process, it will be decomposed by acid-producing bacteria to produce short-chain volatile fatty acids such as acetic acid, propionic acid, butyric acid and valeric acid. These will be further converted into methane by methanogens, resulting in a low concentration of fatty acids in the digestate, insufficient carbon source required by PHA-synthesizing bacteria, and low PHA yield.

[0008] The present invention has discovered that by adding an appropriate amount of starch as a carbon source to excess activated sludge from aerobic tanks in sewage treatment plants, high yields of PHBV (PHA, a type of biodegradable polymer, is a copolymer of 3-hydroxybutyrate (PHB) and 3-hydroxyvalerate (PHV) and will be referred to as PHA) can be synthesized through anaerobic digestion. The PHBV content can reach up to 660 mg per gram of dry sludge, a finding not achieved with other carbon sources. The research and production of PHA synthesized through direct anaerobic digestion of excess activated sludge from aerobic tanks using starch as a carbon source has not been reported in China or abroad.

[0009] Based on the above research results, the present invention provides a method for producing PHA by direct anaerobic digestion of excess activated sludge using starch as a carbon source.

[0010] To achieve the above object, the technical solution adopted by the present invention is: A method for producing PHA by direct anaerobic digestion of excess activated sludge using starch as a carbon source comprises the following steps: taking sewage from an aerobic tank, allowing it to stand and separate into layers, removing a portion of the upper clear liquid, adding starch to the resulting excess sludge suspension, mixing the mixture uniformly, performing anaerobic fermentation, and then separating PHA to obtain the PHA.

[0011] Furthermore, the solid-liquid volume ratio of the excess sludge suspension is 0.5-10, preferably 4-8.

[0012] Furthermore, the pH value of the excess sludge suspension is 6.1-6.8.

[0013] Furthermore, the volume-to-weight ratio of the excess sludge suspension to the starch is 1 L: 0.1-10 g, preferably 1 L: 0.5-5 g.

[0014] Furthermore, the starch is at least one of cassava starch, corn starch, potato starch, and sweet potato starch.

[0015] Furthermore, the temperature of the anaerobic fermentation is 10-40°C, preferably 15-35°C.

[0016] Furthermore, the anaerobic fermentation time is 3 to 20 days, preferably 6 to 15 days.

[0017] Furthermore, the sewage in the aerobic tank is allowed to stand for 4 to 24 hours.

[0018] The beneficial effects of the method for producing PHA by direct anaerobic digestion of excess activated sludge using starch as a carbon source are as follows: Since starch is cheap and readily available, PHA can be produced by direct anaerobic fermentation of excess activated sludge from aerobic tanks. This process is simple, significantly reducing the production cost of PHA and offering great commercial value and application prospects in the field of excess sludge resource utilization. The present invention does not require starch to be pre-treated by hydrolysis, acidification and hydrolysis, and the starch can be directly added to a fermentation tank to be utilized by microorganisms and converted into PHA, which can greatly improve the yield of PHA. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is a mass spectrum of PHBV prepared in Example 1 of the present invention; Figure 2 This is the H NMR spectrum of PHBV prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0020] The technical solutions in the embodiments of the present invention are described clearly and completely below. Many specific details are set forth in the following description to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the scope of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0021] Example 1 A method for producing PHA by direct anaerobic digestion of excess activated sludge using starch as a carbon source This embodiment is a method for producing PHA by direct anaerobic digestion of excess activated sludge with starch as the carbon source. The specific preparation process includes the following steps: S1. Let the sewage from the aerobic tank of the sewage treatment plant stand for 4 hours and separate into layers. The upper clear liquid is called the liquid layer, and the lower dark microbial layer is called the solid layer.

[0022] S2. Remove part of the upper clear liquid to obtain a residual sludge suspension with a solid-liquid volume ratio of 0.5 and a pH of 6.1; S3. Add 0.1 g of cassava starch per liter of excess sludge suspension and mix well to obtain a mixed solution; S4. The mixed solution was subjected to anaerobic fermentation at 10°C for 20 days, after which samples were taken for analysis. When the PHA content in 1 gram of dry bacterial cells reached 540 mg, the anaerobic fermentation was stopped, and the resulting digestate was centrifuged to obtain bacterial cells. PHA was extracted using a sodium hypochlorite cell wall-broken chloroform extraction method, yielding 35 g of PHA per liter of digestate.

[0023] The obtained PHA is a copolymer of 3-hydroxybutyrate (PHB) and 3-hydroxyvalerate (PHV), referred to as PHBV. Its mass spectrum and nuclear magnetic resonance hydrogen spectrum are as follows: Figures 1 to 2 shown.

[0024] Example 2 A method for producing PHA by direct anaerobic digestion of excess activated sludge using starch as a carbon source This embodiment is a method for producing PHA by direct anaerobic digestion of excess activated sludge with starch as the carbon source. The specific preparation process includes the following steps: S1. Let the sewage from the aerobic tank of the sewage treatment plant stand for 24 hours and separate into layers. The upper clear liquid is called the liquid layer, and the lower dark microbial layer is called the solid layer.

[0025] S2. Remove part of the upper clear liquid to obtain a residual sludge suspension with a solid-liquid volume ratio of 10 and a pH of 6.4; S3. Add 10 g corn starch per liter of excess sludge suspension and mix well to obtain a mixed solution; S4. The mixed solution was subjected to anaerobic fermentation at 35°C for 3 days, after which samples were taken for analysis. When the PHA content in 1 gram of dry bacterial cells reached 510 mg, the anaerobic fermentation was stopped, and the resulting digestate was centrifuged to obtain bacterial cells. PHA was extracted using a sodium hypochlorite cell wall-broken chloroform extraction method, yielding 86 g of PHA per liter of digestate.

[0026] The obtained PHA is a copolymer of 3-hydroxybutyrate (PHB) and 3-hydroxyvalerate (PHV), referred to as PHBV. Its mass spectrum and nuclear magnetic hydrogen spectrum are similar to those of the PHBV obtained in Example 1. Figure 1 To.

[0027] Example 3 A method for producing PHA by direct anaerobic digestion of excess activated sludge using starch as a carbon source This embodiment is a method for producing PHA by direct anaerobic digestion of excess activated sludge with starch as the carbon source. The specific preparation process includes the following steps: S1. Let the sewage from the aerobic tank of the sewage treatment plant stand for 24 hours and separate into layers. The upper clear liquid is called the liquid layer, and the lower dark microbial layer is called the solid layer.

[0028] S2. Remove part of the upper clear liquid to obtain a residual sludge suspension with a solid-liquid volume ratio of 5 and a pH of 6.5; S3, adding 5g of potato starch per liter of excess sludge suspension and mixing evenly to obtain a mixed solution; S4. The mixed solution was subjected to anaerobic fermentation at 25°C for 10 days, after which samples were taken for analysis. When the PHA content in 1 gram of dry cells reached 591 mg, the anaerobic fermentation was stopped, and PHA was extracted from the resulting digestate using a sodium hypochlorite cell wall-broken chloroform extraction method, yielding 67 g of PHA per liter of digestate.

[0029] The obtained PHA is a copolymer of 3-hydroxybutyrate (PHB) and 3-hydroxyvalerate (PHV), referred to as PHBV. Its mass spectrum and nuclear magnetic hydrogen spectrum are similar to those of the PHBV obtained in Example 1. Figure 1 To.

[0030] Example 4 A method for producing PHA by direct anaerobic digestion of excess activated sludge using starch as a carbon source This embodiment is a method for producing PHA by direct anaerobic digestion of excess activated sludge with starch as the carbon source. The specific preparation process includes the following steps: S1. Let the sewage from the aerobic tank of the sewage treatment plant stand for 24 hours and separate into layers. The upper clear liquid is called the liquid layer, and the lower dark microbial layer is called the solid layer.

[0031] S2. Remove part of the upper clear liquid to obtain a residual sludge suspension with a solid-liquid volume ratio of 8 and a pH of 6.8; S3. Add 5 g of sweet potato starch per liter of excess sludge suspension and mix well to obtain a mixed solution; S4. The mixed solution was subjected to anaerobic fermentation at 30°C for 6 days, after which samples were taken for analysis. When the PHA content in 1 gram of dry bacterial cells reached 620 mg, the anaerobic fermentation was stopped, and the resulting digestate was centrifuged to obtain bacterial cells. PHA was extracted using a sodium hypochlorite cell wall-broken chloroform extraction method, yielding 74 g of PHA per liter of digestate.

[0032] The obtained PHA is a copolymer of 3-hydroxybutyrate (PHB) and 3-hydroxyvalerate (PHV), referred to as PHBV. Its mass spectrum and nuclear magnetic hydrogen spectrum are similar to those of the PHBV obtained in Example 1. Figure 1 To.

[0033] Example 5 A method for producing PHA by direct anaerobic digestion of excess activated sludge using starch as a carbon source This embodiment is a method for producing PHA by direct anaerobic digestion of excess activated sludge with starch as the carbon source. The specific preparation process includes the following steps: S1. Let the sewage from the aerobic tank of the sewage treatment plant stand for 24 hours and separate into layers. The upper clear liquid is called the liquid layer, and the lower dark microbial layer is called the solid layer.

[0034] S2. Remove part of the upper clear liquid to obtain a residual sludge suspension with a solid-liquid volume ratio of 6 and a pH of 6.6; S3. Add 7g (mass ratio of cassava starch to corn starch = 1:1) per liter of excess sludge suspension and mix well to obtain a mixed solution; S4. The mixed solution was subjected to anaerobic fermentation at 30°C for 10 days, after which samples were taken for analysis. When the PHA content in 1 gram of dry bacterial cells reached 660 mg, the anaerobic fermentation was stopped, and the resulting digestate was centrifuged to obtain bacterial cells. PHA was extracted using a sodium hypochlorite cell wall-broken chloroform extraction method, yielding 81 g of PHA per liter of digestate.

[0035] The obtained PHA is a copolymer of 3-hydroxybutyrate (PHB) and 3-hydroxyvalerate (PHV), referred to as PHBV. Its mass spectrum and nuclear magnetic hydrogen spectrum are similar to those of the PHBV obtained in Example 1. Figure 1 To.

[0036] Example 6 A method for producing PHA by direct anaerobic digestion of excess activated sludge using starch as a carbon source This embodiment is a method for producing PHA by direct anaerobic digestion of excess activated sludge with starch as the carbon source. The specific preparation process includes the following steps: S1. Let the sewage from the aerobic tank of the sewage treatment plant stand for 24 hours and separate into layers. The upper clear liquid is called the liquid layer, and the lower dark microbial layer is called the solid layer.

[0037] S2. Remove part of the upper clear liquid to obtain a residual sludge suspension with a solid-liquid volume ratio of 8 and a pH of 6.8; S3. After the suspension was anaerobically fermented at 30°C for 6 days, samples were taken for analysis. The PHA content per gram of dry bacterial cells reached a maximum of 320 mg. The anaerobic fermentation was stopped, and the resulting digestate was centrifuged to obtain bacterial cells. PHA was extracted using the sodium hypochlorite cell wall-chloroform extraction method, yielding 14 g of PHA per liter of digestate.

[0038] The obtained PHA is a copolymer of 3-hydroxybutyrate (PHB) and 3-hydroxyvalerate (PHV), referred to as PHBV. Its mass spectrum and nuclear magnetic hydrogen spectrum are similar to those of the PHBV obtained in Example 1. Figure 1 To.

[0039] By comparing Example 4 and Example 6, it can be seen that directly performing anaerobic fermentation without adding starch will lead to a significant decrease in PHA yield, indicating that the present invention can effectively increase the PHA yield by adding starch for fermentation.

[0040] Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

Claims

1. A method for producing PHA by direct anaerobic digestion of excess activated sludge using starch as a carbon source, characterized in that: The method comprises the following steps: taking sewage from an aerobic tank, allowing it to stand and separate into layers, removing part of the upper clear liquid, adding starch to the remaining sludge suspension, mixing them evenly, performing anaerobic fermentation, and then separating PHA to obtain the PHA.

2. The method for producing PHA by direct anaerobic digestion of excess activated sludge using starch as a carbon source according to claim 1, characterized in that: The solid-to-liquid volume ratio of the excess sludge suspension is 0.5-10.

3. The method for producing PHA by direct anaerobic digestion of excess activated sludge using starch as a carbon source according to claim 1 or 2, characterized in that: The pH value of the excess sludge suspension is 6.1-6.

8.

4. The method for producing PHA by direct anaerobic digestion of excess activated sludge using starch as a carbon source according to claim 1 or 2, characterized in that: The volume-to-weight ratio of the excess sludge suspension to the starch is 1 L: 0.1-10 g.

5. The method for producing PHA by direct anaerobic digestion of excess activated sludge using starch as a carbon source according to claim 1 or 2, characterized in that: The starch is at least one of cassava starch, corn starch, potato starch and sweet potato starch.

6. The method for producing PHA by direct anaerobic digestion of excess activated sludge using starch as a carbon source according to claim 1 or 2, characterized in that: The temperature of the anaerobic fermentation is 10-40°C.

7. The method for producing PHA by direct anaerobic digestion of excess activated sludge using starch as a carbon source according to claim 1 or 2, characterized in that: The anaerobic fermentation time is 3 to 20 days.

8. The method for producing PHA by direct anaerobic digestion of excess activated sludge using starch as a carbon source according to claim 1 or 2, characterized in that: The sewage in the aerobic tank is left to stand for 4 to 24 hours.

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