Influence of thermophilic aerobic digestion as a sludge pre-treatment and solids retention time ofmesophilic anaerobic digestion on the methane production, sludge digestion and microbial communities in a sequential digestion process

In this study, the changes in sludge reduction, methane production and microbial community structures in a process involving two-stage thermophilic aerobic digestion (TAD) and mesophilic anaerobic digestion (MAD) under different solid retention times (SRTs) between 10 and 40 days were investigated. The TAD reactor (R_TAD) was operated with a 1-day SRT and the MAD reactor (R_MAD) was operated at three different SRTs: 39, 19 and 9 days. For a comparison, control MAD (R_CONTROL) was operated at three different SRTs of 40, 20 and 10 days. Our results reveal that the sequential TAD-MAD process has about 42% higher methane production rate (MPR) and 15% higher TCOD removal than those of R_CONTROL when the SRT decreased from 40 to 20 days. Denaturing gradient gel electrophoresis (DGGE) and real-time PCR results indicate that R_MAD maintained a more diverse bacteria and archaea population compared to R_CONTROL, due to the application of the biological TAD pre-treatment process. In R_TAD, Ureibacillus thermophiles and Bacterium thermus were the major contributors to the increase in soluble organic matter. In contrast, Methanosaeta concilii, a strictly aceticlastic methanogen, showed the highest population during the operation of overall SRTs in R_MAD. Interestingly, as the SRT decreased to 20 days, syntrophic VFA oxidizing bacteria, Clostridium ultunense sp., and a hydrogenotrophic methanogen, Methanobacterium beijingense were detected in R_MAD and R_CONTROL. Meanwhile, the proportion of archaea to total microbe in R_MAD and R_CONTROL shows highest values of 10.5and 6.5% at 20-d SRT operation, respectively. Collectively, these results demonstrate that the increased COD removal and methane production at different SRTs in R_MAD might be attributed to the increased synergism among microbial species by improving the hydrolysis of the rate limiting step in sludge with the help of the biological TAD pre-treatment.