Effect of riboflavin and carbon black co-modified fillers coupled with alkaline pretreatment on anaerobic digestion of waste activated sludge

Additional various carbon and free riboflavin could improve anaerobic digestion of waste activated sludge (WAS). However, these substances were not reused. In this study, a reusable riboflavin and carbon black (RCB) co-modified filler was developed and combined with alkaline pretreatment for enhancing the production of volatile fatty acids (VFAs) and methane during anaerobic digestion of WAS. The results showed that RCB-modified fillers exhibited a promoting effect on the reduction of alkali-pretreated WAS. The amounts of the accumulated VFAs mainly containing acetate and the produced methane rose with the increased concentration of immobilized riboflavin (0-0.75 g/L) in the presence of 4 g/L carbon black. When the alkaline pretreatment time of WAS increased from 3 d to 8 d, the amount of methane production increased from 22.8% to 63.9% in the presence of 0.75 g/L riboflavin and 4 g/L carbon black compared with that without RCB-modified fillers. Moreover, 0.75 g/L riboflavin and 4 g/L carbon black had a synergetic effect on promoting methane production via broadening extracellular electron transfer pathways. During this process, microbial dehydrogenase activity, electron transport system activity and coenzyme F420 were enhanced. Microbial community analysis showed that RCB-modified filler addition promoted the enrichment of Syntrophomonas and Pseudomonas involved in direct interspecies electron transfer (DIET). These results indicated that DIET establishment was accelerated. Meanwhile, the populations of acetic acid-producing bacteria including Rikenellaceae_RC9_gut_group and Protei-niphilum, aceticlastic and acid-tolerant methanogenic archaea including Methanosarcina and Methanosaeta, RumEn_M2 were increased. These results indicate that RCB-modified fillers coupled with alkaline pretreatment is an effective method to promote the production of methane during anaerobic digestion of WAS.