The Performance And Its Influencing Factors Of A Forward Osmosis-Membrane Bioreactor For Sludge Anaerobic Digestion

In recent years,with the improvement of wastewater treatment rate,the production of excess sludge also increased significantly.Anaerobic digestion of sludge can be implemented as the stabilization process.At the same time,biogas can be obtained as valuable resources,therefore,anaerobic digestion of sludge absorbs wide research interests.Traditional process of high solid anaerobic digestion of sludge is of small footprint,and low construction cost,making this process a very promising application.However,the traditional high-solid process often employs dewatered sludge as the feed,resulting in poor mass transfer,high energy consumption by sludge mixing,and low rate of organic matter degradation.A novel anaerobic digestion forward osmosis-membrane bioreactor(adFO-MBR)was proposed to overcome the problems stated above in this study.In adFO-MBR,concentrated sludge could be used as feed substrate,which is gradually dewatered by FO in order to augment the total solid in adFO-MBR reactor.The selection of FO membranes and draw solutions for the FO unit were studied firstly.The effects of reversed salt on the efficiency of anaerobic digestion was also studied.During the long-term running of adFO-MBR,total solid of anaerobic digestion sludge was increased gradually.The performance of adFO-MBR during the long-term operation was studied,and the mechanism of membrane fouling and cleaning strategies were also analyzed.The results showed that the tri-cellulose acetate FO membrane and magnesium chloride are more suitable for adFO-MBR.When magnesium chloride concentration in sludge anaerobic digestate was lower than 20 g/L,methane production was not affected;When the concentration of magnesium chloride reached 30 g/L,methane production was reduced to 10 mL/(L,d);when the concentration of magnesium chloride rose up to 50 g/L,methane production process was stopped.The main functional bacteria of hydrolysis acidification phase,clostridium and bacteroides,are very sensitive to the variation of salt concentration.High salinity environment has a strong inhibitory effect on hydrogen production of clostridium.The correlation between the live/died proportion of microbial and the methane production was significant.When the concentration of magnesium chloride was 50 g/L,the main cause of methane production being halted maybe due to that most of microbes could not survive the high salinity.The adFO-MBR reactor was operated with the 3%sludge being the feed substrate.The synchronized sludge dewatering process by FO could effectively increase the solid content in the reactor.From the 25th day of operation,the daily production of biogas was gradually stable.When the total solid content of sludge was 9%～11%。VS degradation rate of adFO-MBR was 30%,and the methane content in biogas was about 60%,which were both better than the control digester.The solid retention time of adFO-MBR was between 40～50 days due to the daily adjustment of sludge discharged,which was also higher than 20 days of the control digester.With the solid content of anaerobic digestion sludge increasing gradually,microbial community structure was changing to adapt to the higher solid content and higher SRT.The relative abundance of some microbial classes such as Deltaproteobacteria and Chloroflexi increased showing they could survive and thrive under the new conditions.Therefore,the degradation of organic matter rate increased.On the other hand,some of the acidogens such as Firmicutes and Clostridium could not adapt to the new environment,and as a result the relative abundance of these microbial declined.The effects of solid content on membrane fouling were significant.High solid content sludge could form a relatively dense fouling layer on the FO membrane surface;When solid content of sludge was kept the same,the bigger the sludge particle size,the better dewatering effect,but at the same time,the water flux declined more quickly,and the membrane fouling was severer.For physical cleaning methods,the in-situ cross-flow flushing can achieve a better cleaning effect,which could restore 60%of the water flux of the clean membrane.For chemical(oxidant)cleaning,high concentration of oxidant was likely to damage FO membrane structure.