Anaerobic cultivation of waste activated sludge to inoculate solid state anaerobic co-digestion of agricultural wastes: Effects of different cultivated periods

This study investigated effects of waste activated sludge (WAS) after anaerobic cultivation as inoculum on solid-state anaerobic digestion (SSAD) of agricultural wastes. WAS was anaerobically cultivated for 0, 20 and 50 days and then used as inoculum for co-digestion at substrate/inoculum (S/I) ratios of 2 and 4, respectively. Results showed that treatments inoculated with cultivated WAS exhibited better buffering capability. The highest cumulative methane production (218 L/kg VS) was achieved when inoculating WAS after 50 days of cultivation at the S/I ratio of 2. Fresh WAS without any anaerobic cultivation as inoculum led to digestion instability with significant acidification and limited biogas production, particularly at the S/I ratio of 4. Microbial analysis deciphered that Methanosarcina was the dominant archaea in all treatments and its relative abundance increased with the extension of WAS cultivation time. Hence, WAS after prolonged anaerobic cultivation could be a favorable inoculum for SSAD of agricultural wastes.