Establishing Direct Interspecific Electron Transfer To Enhance Methane Production From Organic Compounds In Anaerobic Fermentation

In recent years,the short of energy is increasingly around the world,and the resource recycling from organic pollutants has been widely concerned by researchers.It was because that the organic pollutans have the characteristic of high organic matter content,and can produce a large amount of environmental friendly gas（methane）as energy during anaerobic biological treatment.However,in the process of methane production in anaerobic fermentation,there are often some disadvantages such as unstable hydrolysis,slow rate and low methane yield.Direct interspecific electron transfer（DIET）has been found as a potential pathway to solve the above problems.According to previous studies,the construction and enhancement of DIET are mainly based on the following ways:（1）Through the dissimilatory iron reduction of Fe（III）oxide,Fe（III）-reducing bacteria that may participate in the DIET process could be enriched specifically.In addition,the electrical conductivity of Fe（III）oxide could further accelerate the DIET between the electroactive microorganisms.（2）The ethanol-based fermentation or adding ethanol can also help enrich the DIET microorganisms by providing the most favorable substrate.Based on the above two pathways,DIET process was constructed to enhance the organics removal and methane production during anaerobic digestion in this research.The research results are as follow:Due to the difference of crystal structure,electric conductivity and reduction potential among different types of Fe（III）oxide,the effect on organics anaerobic fermentation are various.In this research,four types of Fe（III）oxides（ferric citrate,ferrihydrite,hematite and magnetite）were used to enhance the degradation of organic wastewater（in case of phenol）and methane production during anaerobic digestion.It was found that the best degradation of phenol and the fastest release of Fe²⁺occurred in the amorphous ferric citrate group,followed by the poor crystallization hydropyrite group.Because of the higher reduction potential of ferric citrate（+0.385 V,p H=7）and ferrihydrite（+0.1 to-0.1 V,p H=7）,Fe（III）-reducing bacteria were enriched during dissimilatory iron reduction of Fe（III）oxide.Then phenol was degraded to volatile fatty acids.Besides,ferric citrate could also stimulate the activity of enzymes that degrade phenol to acetate in DIET process.However,in the magnetite and hematite groups,although the better crystal structure of magnetite and hematite resulted in the low reduction potential（-0.314 V,p H=7.0 and-0.287 V,p H=7.0）,the higher electric conductivity helped increase the abundance of Enterococcus that is a good electroactive microorganism.The electron transfer were accelerated by Enterococcus attaching to magnetite or hematite together with Methanothrix.Therefore,the methane production was increased by 30.2%and 135.3%,respectively,in the magnetite and hematite groups.As a result,in ferric citrate group,the electron recovery rate in methane was only 58.9-76.5%from phenol anaerobic degradation,while it was 93.0-96.5%in magnetite group.Kitchen wastes（KW）have been used for bioethanol production,but they were not used to stimulate the growth and enrichment of electroactive microorganisms as ethanol source in researches.Furthermore,a large amount of volatile fatty acids would be produced from KW during biological ethanol-type fermentation pretreatment（BEEP）process,which could inhibit the methane production during anaerobic digestion.In this study,waste activated sludge（WAS）was used as a co-substrate to neutralize the acids produced by BEEP of KW,to realize the anaerobic co-digestion（Aco D）of WAS and KW to produce methane.The results showed that at the mixing ratio（KW:WAS）of 1:2 and 1:5,both methane production and organics removal efficiency were improved,and the best methane production rate reached to1200 m L/d.In addition,the potential DIET participators（Petrimonas,Bacillus,Sporanaerobacter,Sphaerochaeta and Pseudomonas）were specially enriched.DIET could occur between these electroactive microorganisms and methanogens（Methanosarcina and Methanospirillum）to make the syntrophic metabolism in Aco D stable and efficient.The analysis of electroactivity of microorganisms showed that at the mixing ratio（KW:WAS）of1:2 and 1:5,both the conductivity of the microorganisms（15.7±2.2μS/cm and 11.4±0.8μS/cm）and the electron transfer system（ETS）ability（21.4μg/m L/min and 29.5μg/m L/min）were high.The result further confirmed the better enrichment of DIET participators in the digesters at the mixing ratio（KW:WAS）of 1:2 and 1:5.