Study On Enhancing Methane Production From Anaerobic Digestion Of Domestic Wastewater By Exogenous Mediators

With the rapid development of economy,the rapid rise of industrial enterprises and the acceleration of urbanization process,the daily production of domestic sewage and industrial wastewater in China has increased dramatically.The energy consumption of wastewater treatment is huge,so it is urgent to treat wastewater with energy.Taking domestic sewage as the main research object,this paper explores the effect of adding exogenous mediators on anaerobic digestion efficiency and methane production of domestic sewage,and preliminarily explores the mechanism of exogenous mediators enhancing anaerobic digestion process.The main research contents are as follows:In order to screen the mediators which can promote anaerobic digestion,the screening experiments were carried out in a sequencing batch reactor.Fe₃O₄,Fe₂O₃,ZVI,PAC,CNT and GR were added to the anaerobic sludge,and the control group without any medium was set up.The concentration of COD in water,total gas production,methane gas production,surface morphology and sewage of the anaerobic granular sludge were detected.The electrochemical activity of sludge supernatant and the extracellular polymer of anaerobic sludge were screened out,and the intermediates with good strengthening effect were screened out.Compared with the control group,the removal rate of COD from effluent increased significantly in the experimental group with Fe₃O₄and CNT,from 63 mg/L and 55 mg/L to 33.9 mg/L and 33 mg/L,and the removal rate increased by 4.9%and 5.1%respectively.The methane production also increased significantly,from 5.3 mL and 5.33 mL to 6.55 mL and 6.35 mL,and the production increased by 25.8%and 18.7%.In order to obtain the optimum concentration of Fe₃O₄ and CNT in the previous experiment,the concentration optimization experiment was carried out in sequencing batch reactor.Fe₃O₄ and CNT with concentrations of 100 mg/L,300 mg/L,500 mg/L,700 mg/L and 1000 mg/L were added to anaerobic sludge respectively,and control group was set up.When the dosage of Fe₃O₄ was 500 mg/L,it had the best promotion effect.The COD removal rate of effluent increased by 8.2%,the gas production increased by1.84 mL and 31.4%.The effect of CNT on anaerobic digestion process was more obvious with the increase of the dosage of mediators.It had the best effect at 1000 mg/L.The COD removal rate of effluent increased by 7.1%,methane production increased by 1.67mL and 28.6%.The experimental effect of adding 500 mg/L Fe₃O₄ is better than that of adding 1000 mg/L CNT.The best selected intermediates and their optimum dosage concentration were applied to the continuous flow reactor,and the effects of exogenous intermediates on the anaerobic digestion process of the continuous flow reactor were explored.The experiment was carried out with AFBR.By adding 500 mg/L Fe₃O₄ to the anaerobic sludge of the reactor,the COD concentration,methane production and ORP in the effluent before and after the addition were compared.The COD in the effluent of the reactor decreased from161 mg/L to 110.8 mg/L,and the removal rate increased by 16.7%.The methane production increased from 240 mL/d to 323 mL/d,by 83 mL/d and by 34.6%.Before the addition of the mediator,the ORP in the system was basically stable,with an average ORP of-503 mV.After the addition of the mediator,the ORP in the reaction system increased sharply,from-503 mV to-491 mV,and remained about-495 mV in the subsequent reaction.The surface morphology of anaerobic granular sludge,the electrochemical activity of sludge supernatant and the change of extracellular polymer of anaerobic sludge were observed,and the possible mechanism of enhanced anaerobic digestion by exogenous mediators was obtained.After the addition of exogenous mediators,the surface morphology of anaerobic granular sludge changed significantly,the intercellular spacing decreased significantly,and the mediators filled with microorganisms;the cyclic voltammetry curve of anaerobic sludge supernatant after the addition of exogenous mediators had more obvious redox peaks;the content of extracellular polymer in sludge increased significantly after the addition of mediators.It can be concluded that exogenous mediators may enhance anaerobic digestion in two ways:one is that exogenous mediators enhance bacterial connectivity and promote direct electron transfer;the other is that exogenous mediators promote redox-active electronic shuttles and extracellular polymers enhance indirect electron transfer.