Process for enhanced anaerobic digestion of sludge by alcoholization of food waste

Abstract

A process for enhanced anaerobic digestion of sludge by alcoholization of food waste, comprising steps of: (1) ethanol prefermentation of food waste—feeding the food waste having been crushed into a uniform grain size into an ethanol production reactor, where the food waste is subjected to rapid and efficient anaerobic fermentation activated by addition of yeast and pH adjustment to produce ethanol and obtain a fermentation broth and a fermentation residue; (2) sludge pretreatment—pretreating the sludge after mixing with the fermentation residue; and (3) methanogenesis of anaerobic digestion—subjecting products resulting from the sludge pretreatment to be mixed with the fermentation broth and then to methanogenesis of anaerobic digestion in a methane production reactor. With this process, the sludge and the food waste can be treated efficiently via multi-phase reactions, and the resource recovery and biogas quality can be enhanced.

Claims

1. A process for enhanced anaerobic digestion of sludge by alcoholization of food waste, comprising steps of: (1) ethanol prefermentation of food waste—feeding the food waste having been crushed into a uniform grain size into an ethanol production reactor, where the food waste is subjected to anaerobic fermentation activated by addition of yeast and pH adjustment to produce ethanol and obtain a fermentation broth and a fermentation residue; (2) pretreatment of sludge—feeding the sludge into a pretreatment reactor after mixing with the fermentation residue obtained in the step (1) for pretreatment so that dissolution and degradation properties of organic matter contained in the sludge and in the fermentation residue are improved; and (3) methanogenesis of anaerobic digestion—subjecting products resulting from the step (2) to be mixed with the fermentation broth obtained in the step (1) and then to methanogenesis of anaerobic digestion in a methane production reactor.

2. The process according to claim 1, wherein, in the step (1), the food waste is crushed into a grain size smaller than 10 mm before being subjected to the ethanol prefermentation, and mixed thoroughly; and, wherein, the yeast added is Saccharomyces cerevisae, and is inoculated into the food waste in the ethanol production reactor via a portion of the fermentation broth resulting from the ethanol production in a circular manner; and, wherein, the food waste in the ethanol production reactor is adjusted to a pH in the range of from 3.5 to 6.5 with a returned portion of the fermentation broth.

3. The process according to claim 1, wherein, in the step (1), the ethanol prefermentation of the food waste is carried out at 20 to 30° C. for 4 to 48 hours.

4. The process according to claim 1, wherein, in the step (2), a ratio of a total solids content in the fermentation residue of the food waste to a total solids content in the sludge is 10 to 0.1.

5. The process according to claim 1, wherein, the sludge in the step (2) is at least one of: primary sludge, secondary sludge, excess activated sludge, concentrated sludge, and dehydrated sludge from a sewage treatment plant; and, wherein, the pretreatment of the sludge comprises at least one of enzyme, ozone, ultrasonic, microwave treatments, and thermal hydrolysis.

6. The process according to claim 1, wherein, the methanogenesis in the step (3) is carried out with stirring at a stirring speed of 60 to 120 rpm for 10 to 30 days.

7. The process according to claim 1, wherein, the methanogenesis in the step (3) is carried out at a medium temperature of 30 to 40° C., or at a high temperature of 50 to 60° C., or at a medium high temperature of 40 to 50° C.; and, wherein, the mixture of the products resulting from the sludge pretreatment and the fermentation broth of the food waste has a total solid content within a low range of 2 to 10% or within a high range of 15 to 35%.

8. The process according to claim 1, wherein, in the step (3), an iron-based or carbon-based conductive material or an iron-carbon composite conductive material is added to the methane production reactor.

9. The process according to claim 8, wherein, the iron-based material comprises at least one of: magnetite, hematite, and goethite; and, wherein, the carbon-based material comprises at least one of: biochar, activated carbon, graphites, graphene, carbon cloth, and carbon nanotubes.

10. The process according to claim 1, wherein, in the step (3), the methane production reactor is coupled to an electrochemical device which is configured for applying a voltage within the range of 0.1 to 3.0 volts to a methanogenic phase.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawing shows a schematic diagram of the process for enhanced anaerobic digestion of sludge by alcoholization of food waste according to the present disclosure.

DETAILED DESCRIPTION

(2) Embodiments of the present disclosure will now be described in detail by way of examples.

Example 1

(3) A dehydrated sludge having a high solids content was subjected to the process for enhanced anaerobic digestion by alcoholization of food waste according to the present disclosure to show the effects thereof on improving degradation rate of the organic matter and the methane yield.

(4) The food waste used was obtained from the Xueyuan canteen of the Tongji University, and had a TS content of 16.78% and a volatile solid (VS) content of 16.33%, with the VS/TS ratio being 97.32%. The dehydrated sludge used was from a sewage treatment plant, and had a TS content of 18.1 to 25.5% and a VS/TS ratio of 46.2 to 68.8%.

(5) After having been crushed into a uniform grain size, the food waste was fed into an ethanol production reactor with an effective volume of 1 liter, and was subjected to ethanol production therein at 25° C. and a pH of 5 for 8 hours. The resulting fermentation residue was mixed with the dehydrated sludge at a VS mass ratio of 2:1, and was then fed into a pretreatment reactor. Thermal hydrolysis was performed in the reactor at 160° C. for 30 minutes to complete the pretreatment of the sludge. The products resulting from the sludge pretreatment were mixed with the fermentation broth resulting from the ethanol production and were then fed into a methane production reactor after pH adjustment to a pH of 6.8 to 7.2.

(6) The methane production reactor had an effective volume of 2 liters. The methane production or methanogenesis was carried out in a water bath at 42° C. with stirring at a stirring speed of 100 rpm in a batch manner. During this, contents of volatile fatty acids (VFAs) were monitored. The biogas production and components were measured every day. The TS and VS contents were also measured before and after the AD process.

Comparative Example 1

(7) Comparative Example 1 followed the process of Example 1 except that the alcoholization of the food waste was not performed. The food waste was mixed with an equal amount of the dehydrated sludge and was subjected to pretreatment and then the synergistic anaerobic co-digestion.

Comparative Example 2

(8) Comparative Example 2 followed the process of Example 1 except that the pretreatment of the sludge was not performed. The food waste having been subjected to the alcoholization was mixed with an equal amount of the dehydrated sludge and was subjected to the synergistic anaerobic co-digestion.

(9) By comparing Comparative Example 1 with Comparative Example 2, it was found that the degradation rate of the organic matter and the total methane production were increased, but the methane production rate and the methane proportion in the biogas were decreased. This showed that the processes in Comparative Examples 1 and 2 each had its own advantages and disadvantages. By comparing Example 1 with Comparative Examples 1 and 2, it was found that the maximum methane production rate was increased from 20.87 ml/(g VS.sub.add.d) in Comparative Example 1 and 23.28 ml/(g VS.sub.add.d) in Comparative Example 2 to 30.26 ml/(g VS.sub.add.d) in Example 1, with the methane production rate increased by 45% and 30% over Comparative Examples 1 and 2, respectively; and the degradation rate of the organic matter was increased from 51.7% in Comparative Example 1 and 49.9% in Comparative Example 2 to 59.5% in Example 1, with the degradation rate of the organic matter increased by 15% and 19% over Comparative Examples 1 and 2, respectively.

Example 2

(10) An excess activated sludge having a low solids content was subjected to the process for enhanced anaerobic digestion by alcoholization of food waste according to the present disclosure to show the effects thereof on improving enrichment of the electroactive microorganisms and the AD efficiency.

(11) The food waste used was obtained from the Xueyuan canteen of the Tongji University, and had a TS content of 14.33% and a VS/TS ratio of 92.15%. The excess activated sludge used was from a sewage treatment plant, and had a TS content of 3.9 to 5.6% and a VS/TS ratio of 50.1 to 65.7%.

(12) After having been crushed into a uniform grain size, the food waste was fed into an ethanol production reactor with an effective volume of 0.5 liters, and was subjected to ethanol production therein at 30° C. and a pH of 4.5 for 4 hours. The resulting fermentation residue was mixed with the excess activated sludge at a VS mass ratio of 1:1, and was then fed into a pretreatment reactor. Ultrasonic processing was performed in the reactor at an output of 250 W and a frequency of 24 kHz for 15 minutes to complete the pretreatment of the sludge. The products resulting from the sludge pretreatment were mixed with the fermentation broth resulting from the ethanol production and were then fed into a methane production reactor after pH adjustment to a pH of 6.8 to 7.2.

(13) Biochar was additionally introduced into the methane production reactor as a conductive material, which can facilitate the establishment and enhancement of the DIET in the AD process. The methane production or methanogenesis was carried out in a water bath at 37° C. with stirring at a stirring speed of 80 rpm in a batch manner. During this, contents of volatile fatty acids (VFAs) in the feed and in the discharge were monitored, and the TS and VS contents therein were measured. The biogas production and components were also measured.

Comparative Example 3

(14) Comparative Example 3 followed the process of Example 2 except that the alcoholization of the food waste was not performed. The food waste was mixed with an equal amount of the excess activated sludge and was subjected to pretreatment and then the synergistic anaerobic co-digestion. The same amount of biochar as in Example 2 was added to the methanogenesis phase.

Comparative Example 4

(15) Comparative Example 4 followed the process of Example 2 except that the pretreatment of the sludge was not performed. The food waste having been subjected to the alcoholization was mixed with an equal amount of the excess activated sludge and was subjected to the synergistic anaerobic co-digestion. The same amount of biochar as in Example 2 was added to the methanogenesis phase.

(16) After the methane production system in Comparative Example 3 became stable, the system was subjected to 16S microbial sequencing and no enrichment of the electroactive microorganisms was found. This showed that the enrichment of the electroactive microorganisms and the establishment of the DIET pathway cannot be realized by simple mixing of the food waste and the sludge. Although the electroactive microorganisms were found in the methane production system in Comparative Example 4, they were in a low relative abundance of only 0.03%. The composition and abundance of the microorganisms in the methane production system in Example 2 showed significant difference compared to Comparative Examples 3 and 4, and the relative abundance of the microorganisms was increased to 20% or greater. With regard to the AD performances, the degradation rate of the organic matter in Example 2 was 62.7%, with an increase of 11% over Comparative Example 4; and the average daily methane production was 113.91 ml/g VS.sub.add, with an increase of 20% or greater over Comparative Example 4.

Example 3

(17) A mixed sludge having a high solids content was subjected to the process for enhanced anaerobic digestion by alcoholization of food waste according to the present disclosure to show the effects thereof on enhancing methanogenesis in anaerobic sludge digestion and improving the methane proportion in the biogas.

(18) The food waste used was obtained from the Nanyuan canteen of the Tongji University, and had a TS content of 13.71% and a VS/TS ratio of 82.99%. The mixed sludge used was a mixture of secondary sludge and dehydrated sludge from a sewage treatment plant, and had a TS content of 17.3 to 25.2% and a VS/TS ratio of 47.7 to 62.1%.

(19) After having been crushed into a uniform grain size, the food waste was fed into an ethanol production reactor with an effective volume of 4 liters, and was subjected to ethanol production therein at 23° C. and a pH of 4.3 for 10 hours. The resulting fermentation residue was mixed with the mixed sludge at a VS mass ratio of 2:1, and was then fed into a pretreatment reactor. Enzyme (protease and carbohydrase) treatment was performed in the reactor to complete the pretreatment of the sludge. The products resulting from the sludge pretreatment were mixed with the fermentation broth resulting from the ethanol production and were then fed into a methane production reactor after pH adjustment to a pH of 6.8 to 7.2.

(20) A microbial electrolysis cell, composed of iron and carbon electrodes, was coupled to the methane production reactor, and was used for applying a micro-voltage of 0.6 volts to the methanogenesis phase, which can facilitate hydrolysis and acidogenesis of the organic waste, enhance the DIET and improve the methane proportion in the biogas. The methane production or methanogenesis was carried out in a water bath at 47° C. with stirring at a stirring speed of 90 rpm in a batch manner. During this, contents of volatile fatty acids (VFAs) in the feed and the discharge were monitored, and the TS and VS contents therein were measured. The biogas production and components were also measured.

Comparative Example 5

(21) Comparative Example 5 followed the process of Example 3 except that the alcoholization of the food waste was not performed. The food waste was mixed with an equal amount of the mixed sludge and was subjected to pretreatment and then the synergistic anaerobic co-digestion. The same electrochemcial device as in Example 3 was used and the same voltage was applied to the methanogenesis phase.

Comparative Example 6

(22) Comparative Example 6 followed the process of Example 3 except that the pretreatment of the sludge was not performed. The food waste having been subjected to the alcoholization was mixed with an equal amount of the mixed sludge and was subjected to the synergistic anaerobic co-digestion. The same electrochemcial device as in Example 3 was used and the same voltage was applied to the methanogenesis phase.

(23) Results of Comparative Examples 5 and 6 showed that, when the micro-voltage (0.6 volts) was applied to the methanogenesis phase, the biogas production rate was increased, but the methane proportion in the biogas had no appreciable change. As a comparison, results of Example 3 showed that, when the micro-voltage (0.6 volts) was applied to the methanogenesis phase, the biogas production rate was increased substantially, and the methane proportion in the biogas was increased gradually. After the methane production systems in Example 3, and in Comparative Examples 5 and 6 became stable, the system in Example 3 provided a methane production rate of 152.15 mL/(g VS.sub.add.d), with an increase of 18% and 22% compared to the methane production rate of 128.91 mL/(g VS.sub.add.d) in Comparative Example 5 and the methane production rate of 124.71 mL/(g VS.sub.add.d) in Comparative Example 6, respectively. It was also found that the biogas produced in Example 3 had a methane proportion of 87.1%, with an increase of 19% and 12% compared to the methane proportion of 73.2% in Comparative Example 5 and the methane proportion of 77.8% in Comparative Example 6, respectively.

(24) The above description of exemplary embodiments of the present disclosure is provided to enable those skilled in the art to understand and practice the present disclosure. It will be apparent to those skilled in the art that various modifications can be made to these exemplary embodiments and the general principles described herein may be applied to other embodiments without creative efforts. Therefore, the present disclosure is not intended to be limited to the above embodiments, and various modifications and improvements made without departing from the spirit of the present disclosure are also included in the scope as defined by the appended claims.