Long-term performance and microbial community characteristics of pilot-scale anaerobic reactors for thermal hydrolyzed sludge digestion under mesophilic and thermophilic conditions

Anaerobic digestion (AD) with thermal hydrolysis pretreatment has been attracted widespread attention in recent years due to its high efficiency. However, few studies focus on systematical comparison of the downstream AD processes for thermal hydrolyzed sludge and their corresponding microbial community compositions, especially on those at pilot scale and above. Thus, this study systematically compared the long-term performance and microbial communities of two pilot-scale anaerobic reactors for thermal hydrolyzed sludge digestion under mesophilic and thermophilic conditions. The results presented that mesophilic anaerobic digestion (MAD) showed a better performance of methane production than thermophilic anaerobic digestion (TAD). The hydraulic retention time (HRT) needed to be longer than 12 days in MAD while 20 days in TAD to achieve the relatively high methane production, which could be explained by that the ammonia nitrogen accumulation especially the free ammonia determined in TAD was higher than that in MAD at all HRTs, emerging an inhibition of methane yield in TAD. High-throughput Illumina sequencing results demonstrated a more diverse microbial community in MAD than that in TAD. TAD was mediated by a suite of thermophiles, such as Coprothermobacter and Methanothermobacter, while taxa harbored in MAD mostly belonged to Bacteroidetes and relatively broad types of methanogens. In addition, hydrogenotrophic methanogens were the predominant of archaea communities in both digesters probably due to the relatively high concentration of ammonia nitrogen. [Display omitted] •Pilot-scale anaerobic digesters for thermal hydrolyzed sludge were operated.•MAD showed a better performance of methane production than TAD.•Free ammonia accumulation could cause an inhibition of methane yield in TAD.•A more diverse microbial community was found in MAD than that in TAD.•Hydrogenotrophic methanogens were predominant in both digesters.