Performance And Optimization Of A Forward Osmosis Membrane Bioreactor For Sludge Anaerobic Digestion

A novel forward osmotic membrane bioreactor for sludge anaerobic digestion?adFO-MBR?was established in this study.As a novel process,the performance of adFO-MBR in several aspects such as agitation energy consumption,heavy metal safety and organic load stayed vague.Owing to this,?1?the variation of sludge rheology during the operation of adFO-MBR;?2?the accumulation and speciation variation of different metals in digested sludge;?3?the increase of organic load based on the optimization of pretreatment conditions and?4?the membrane fouling of adFO-MBR were analyzed in this study.Experimental results showed that the flow curves fit Herschel-Bulkley model and Casson model well.Comparing with conventional anaerobic digestion reactor,less negatively charged sludge particles,more EPS content in the adFO-MBR,as well as higher conductivity brought by the reverse salt flux result in larger particle size.Thus,the shear stress and viscosity of digested sludge in adFO-MBR was reduced.At relatively high TS,comparing with the conventional anaerobic digestion reactor,in adFO-MBR,the calculated agitation energy consumption could be reduced by up to 39%.The metal concentration in the digested sludge of adFO-MBR gradually accumulated during the operation and the amount of accumulation was higher than that of the conventional anaerobic digestion reactor.This was probably related to the longer sludge retention time of adFO-MBR.During the operation of adFO-MBR,the speciation of other five kinds of metals Fe,Mg,Zn,Cu and As was becoming more stable and the bioavailability of them all decreased.The improvement of the metal speciation in adFO-MBR was probably due to a higher humic substance concentration in adFO-MBR since humic substance has the ability to stabilize metals.Different pretreatment conditions were optimized by the orthogonal test according to the effects of sludge anaerobic digestion.The comprehensive optimum pretreatment condition selected was using Na OH to adjust p H to 9,and then heated for 60 min at 90?.This optimal pretreatment condition was then applied to the long-term operation of adFO-MBR and then several anaerobic digestion performance indexes were improved.The volumetric methane production of the reactor increased to200-300 m L/L/d,which was 2-3 times of that under the pretreatment of ultrasound.Based on this,the organic load of adFO-MBR was increased from 1.50 g VS/?L·d?to 2.25 g VS/?L·d?and the volumetric methane production and degradation rate of volatile solid raised to 400 m L/L/d and approximately 60%,which were 2 and 1.7 times of that of the lower organic load,respectively.Cl and Mg were main inorganic fouling elements.The sludge layer on the FO membrane has high concentration of Mg²⁺.As a result,bacteria which can survive in the unfavorable environment such as Bacteroidetes,Firmicutes and the classes of them had a phenomenon of enrichment in the sludge layer on the FO membrane.Furthermore,WSA2 or WCHA1-57,which was relatively sensitive to salt ions also had a phenomenon of enrichment.