| Excess sludge is an auxiliary product in sewage disposal process.With the remarkable improvement of sewage treatment capacity,the production of excess sludge also increases rapidly.Anaerobic digestion technology has a series of advantages including reducing sludge volume,stabilizing sludge properties,improving sludge dewatering performance and recycling clean energy(methane),which is widely used for the disposal of excess sludge.Electron transfer efficiency is one of the important factors that determine the methane production during the anaerobic digestion of excess sludge.Although the addition of solid conductive materials can effectively promote the direct interspecies electron transfer,it is not suitable for large-scale promotion and application because of the high cost and secondary pollution.Extracellular polymer substances(EPS)is a generic term of the substances secreted by microorganisms.Because of the redox active substances such as C-cyts and humic substances,EPS have the potential to promote interspecies electron transfer.At present,there is no related study to explore the influence of EPS on methane production during the anaerobic digestion of sludge.Taking excess sludge as the study object and selecting EPS as the exogenous redox active material,this paper was focused on exploring the influence of EPS addition on methane production,and the role and function of EPS was clarified.Meanwhile,the effect of EPS on the performance of the electro-fermentation process was also investigated.The main research contents and results were as follows:(1)The addition of EPS significantly improved the performance of methane production during the anaerobic digestion of sludge,and the methane yield was 82.9% higher than that in the reactor without EPS addition.Electro-chemical experiments showed that the addition of EPS can increased the concentration of riboflavin and C-cyts in the reaction system and reduced the charge transfer resistance,which was beneficial for the electron transfer.In addition,the addition of EPS increased the relative abundance of Hydrogenophag and Methanosaeta,and improved the hydrogen utilization for methane production.(2)The addition of EPS promoted the solubilization and hydrolysis during the anaerobic digestion,which resulted in an increase in the activity of soluble proteins and proteases by 42.8% and 8.9%,respectively.In the acidification phase,EPS promoted the conversion of different substrates(amino acids and glucose)to acetic and propionic acid.In addition,it could affect the conversion of acetate to acetyl phosphate,acetate to acetyl coenzyme A,and acetyl coenzyme A to 5-methyl-THMPT during the acetotrophic methanogenesis.Moreover,the abundance of functional gene mcrA was significantly increased in the process of acetotrophic methanogenesis and hydrogenotrophic methanogenesis after the EPS addition.(3)In the electro-fermentation system,the addition of EPS also effectively promoted the methane production.The optimal potential and EPS dosage are-0.8 V and 0.5 g/L EPS,respectively.Electro-chemical experiments showed that the addition of EPS resulted in a higher concentration and a larger capacitance of redox active substances,which was conducive to promote interspecies electron transfer.The analysis of the microbial community indicated that the thickness of the anode biofilm and the relative abundance of the Methanosarcina,Clostridium and Syntrophobacter increased after the EPS addition.Moreover,the addition of EPS led to the increase of methane energy efficiency and economic benefit of the electro-fermentation system by 1.55 times and 1.68 times,respectively.In conclusion,adding EPS effectively enhanced the electro-chemical activity of the anaerobic digestion system and significantly promoted methane production during the anaerobic fermentation of sludge.The results in this study will provide a new idea for improving methanogenesis with waste sludge,which is beneficial for the popularization and application of anaerobic digestion technology. |
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