Ehanced Anaerobic Digestio Of Waste Activated Sludge By Electrical Stimulation Technology

Anaerobic digestion of waste activated sludge?WAS?is a cost-effective and sustainable technology to realize sludge stabilization,mass reduction and methane production simultaneously.However,the application of conventional anaerobic digestion is often limited by its long retention time,low removal efficiencies of organic compounds and low biogas production rate.Electrical stimulation refers to a microbial process performed in the presence of electrolysis by low direct current.The operation of electrical stimulation is easy and energy conservation.Therefore,the potential for practical application of electrical stimulation to microbial processes is high.Meanwhile electrical stimulation is a green and environment friendly technology.Electrical stimulation has been applied in sewage treatment,groundwater purification and soil remediation.However,applying electrical stimulation in sludge anaerobic digestion under practical conditions was still limited,and the relationship between stimulating effects and applied voltage was not established to date.In this study,different voltages?0.3 V-1.5 V?were applied to investigate the influence of electrical stimulation on anaerobic digestion of WAS.Firstly,the best performance in terms of methane production and sludge reduction was obtained with the applied voltage of 0.6 V.Secondly,the effect of WAS concentration?15 g/L,25 g/L,35 g/L,45 g/L?and initial pH?3,5,7,9,11?on anaerobic digestion performance under 0.6 V direct electrical stimulation were investigated.The main conclusions are as follows:?1?Different voltages?0.3 V-1.5 V?were applied to investigate the influence of electrical stimulation on anaerobic digestion of WAS.The results revealed that applied voltages could accelerate sludge hydrolysis and acidification process.The best performance in terms of methane production and sludge reduction was obtained with the applied voltage of0.6 V.In this case,methane production increased by 76.2%with an enhanced VS removal rate?26.6%?compared to the control group.The energy consumption at 0.6 V could be neglected compared to the incremental energy generated from the methane.However,methane production decreased and hydrogen was produced when the applied voltage increased to 0.9 V.At higher voltages?1.2 V and 1.5 V?,more soluble organic matters were released.In particular,the VFAs concentration peaked at 640 mg/L and 1001 mg/L,respectively.?2?Pyrosequencing revealed that hydrogenotrophic methanogens consisted majority of methanogen population when the applied voltages was over 0.6 V,while acetoclastic methanogens showed overwhelming dominance at 0.3 V.Moreover,0.6 V enriched Pseudomonas for protein degradation and Methanoregula for methane generation with species richness of 19.1%and 53.3%,respectively.?3?Batch tests were operated to investigate effects of WAS concentration?15 g/L,25 g/L,35 g/L,45 g/L?on anaerobic digestion of waste activated sludge under 0.6V direct electrical stimulation.The results showed that WAS concentration had a great influence on biogas production and VS removal efficiency.At the optimal concentration of 35 g/L,maximum methane yield of 139.4 mLCH₄/g-VS and VS removal efficiency of 46.7%were achieved after 35 days digestion.At higher WAS concentration?35 g/L,45 g/L?,the SCOD,VFAs and ammonia from WAS were increased in quantities.In particular,at concentration of 45 g/L,the VFAs concentration peaked at 12,000 mg-COD/L and the methanogenesis was inhibited.The bacteria communities were enriched at concentration of45 g/L,which was in agreement with the result of SCOD and VFAs concentrations.Methanobacterium,Methanosaeta,Methanoculleus,Methanospirillum and Methanobrevibacter were dominated methanogens in these four samples.?4?Batch tests were operated to investigate effects of initial pH?3,5,7,9,11?on anaerobic digestion of waste activated sludge under 0.6 V direct electrical stimulation.The results showed that initial pH had a great influence on biogas production and VS removal efficiency.At the optimal initial pH of 9.0,maximum methane yield of 224 mLCH₄/g-VS and VS removal efficiency of 38.1%were achieved after 32 days digestion.Initial pH could accelerate hydrolysis and acidification process of waste activated sludge.Alkaline environment?pH 9,11?enhanced SCOD and VFAs concentrations in suspensions,especially at pH 11.Acetic acid and butyric acid were dominant at pH 3 and 11 while acetic acid and propionic acid were main products at pH 5,7 and 9.Initial pH facilitates release of ammonium from waste activated sludge and in acidic environment ammonium was released in excess.