Effect of different co-treatments of waste activated sludge on biogas production and shaping microbial community in subsequent anaerobic digestion

Different pretreatment approaches enhance hydrolysis and biogas production of waste activated sludge (WAS). However, the best WAS pretreatment method may vary for different purposes. The aim of this study was to compare different combined treatments using potassium ferrate, alkali, and ultrasonication treatment in terms of hydrolysis, biogas production, and microbial community structure during subsequent anaerobic digestion. The soluble chemical oxygen demand (SCOD) increased sharply during sludge pretreatment. Co-treatment with potassium ferrate and ultrasonication (PF + ULT) provided the highest concentration of SCOD (10,206 mg/L) and volatile fatty acids (VFAs) (9160 mg/L). Maximum cumulative methane production, 752.6 mL during 21 days of subsequent anaerobic digestion, was achieved for alkali and ultrasonication (ALK + ULT) co-treated sludge, a 39.3% improvement compared with raw sludge. Our results indicate that PF + ULT and ALK + ULT co-treatments are suitable for methanogenesis in subsequent anaerobic digestion. Co-treatment typically improves biogas production and sludge hydrolysis relative to a single pretreatment. Illumina sequencing of 16S rRNA genes showed that sludge subjected to ALK pretreatment and ALK + ULT co-treatment contained the same predominant populations, and the community structure of PF was similar to PF + ULT. Principal coordinate analysis (PCoA) based on weighted UniFrac distance indicated differences in the microbial communities of the pretreated and raw sludges. The predominant archaeal populations were Methanomassiliicoccus and Methanobacterium in the ALK + ULT and ALK pretreated sludges, and the majority were Methanomassiliicoccus (76.82%) in PF + ULT co-treated sludge, indicating that different pretreatment methods could substantially shape archaeal communities of pretreated sludge. These results reveal that sludge pretreatment not only impacts sludge hydrolysis and biogas production, but also significantly influences the microbial community in subsequent anaerobic digestion.