New insight into the effect of thermal hydrolysis on high solid sludge anaerobic digestion: Conversion pathway of volatile sulphur compounds.

Untreated sludge (total solids = 10%) and thermally hydrolysed sludge (total solids = 10%) were subjected to high-solid anaerobic digestion (HSAD) to study the effect of thermal hydrolysis pre-treatment (THP) on the conversion pathways of volatile sulphur compounds (VSCs). Typical VSCs were detected in the gas produced by THP at 160 °C for 30 min, including HS, methyl mercaptan (MM), dimethyl sulphide (DMS) and dimethyl disulphide (DMDS). After THP, the organic sulphide ratio in the treated sludge had decreased from 96% to 90%, and inorganic sulphide had increased from 4% to 10%. In the THS (THP + HSAD) group, the productivity and total volume of VSCs were significantly increased. These results suggest that THP directly promotes converting organic sulphur (OS) into VSCs. Further tests revealed that THP increased the activity of reductases (adenine phosphate sulphate reductase and sulphite reductase), OS hydrolysis was the main source of VSCs in biogas, and MM could be converted into HS (78%), DMS (18%) and DMDS (4%). These findings are used to elucidate the conversion pathway of sulphides in HSAD.