A comparative study on Mesophilic and thermophilic anaerobic digestion of different total solid content sludges produced in a long sludge-retention-time system

Most waste sludge belongs to long sludge-retention-time (SRT) sludge in China, but its outlet is becoming a severe environmental problem nowadays. In this study, the characteristics of mesophilic and thermophilic anaerobic digestion of long SRT sludge was investigated under different total solid content (TSC) of 4%, 6%, and 8% in batch reactors. The results showed that the methane production potential (Po) and methane production rate (Rmax) of thermophilic anaerobic digestion (TAD) were increased by about 60% and 200% compared with that of the mesophilic anaerobic digestion (MAD) at 4% TSC. In TAD groups, the Po of 240.95 LCH4 kgVS-1added and Rmax of 22.17 LCH4 kgVS-1added day-1 were achieved in the reactor of 4% TSC while the inhibition concentration of propionate and free ammonia reached 1022 mg L-1 and 603 mg L-1 in the reactor of 8% TSC. Although the diversity of microbial communities were poor in TAD, Coprothermobacter was enriched in TAD, which could degrade protein efficiently. In addition, the dewaterability of digested sludge in TAD was poorer than that in MAD, and both got worse with the increased TSC. Therefore, it is appropriate for the long SRT sludge to anaerobic digestion under thermophilic condition and lower TSC. So, thermophilic anaerobic digestion with lower TSC might be an alternative approach for the treatment and resource utilization of long SRT sludge.