Enhanced energy recovery from lignin-rich cattle manure: impact of coupling mesophilic anaerobic digestion with hyperthermophilic hydrolysis

The recalcitrance of lignin in cattle manure (CM) significantly limits energy recovery during anaerobic digestion. Mesophilic anaerobic digestion of CM was investigated in continuously stirred tank reactors (CSTRs). In phase 1, the control mother reactor (MR) was operated at an SRT of 30 d, while in phase 2 the MR was followed by a hyper-thermophilic hydrolysis reactor (HTH) at 75 °C and SRT of 2 d (HTH2), with recirculation back to the MR which operated at an SRT of 22.4 d. The average steady-state biodegradability based on methane yields, in the MR, after recirculation, was 46 % ± 3 % compared to 42 % ± 5 %, without recirculation, primarily due to enhanced lignin removal of 20 % (12 % without recirculation). HTH1 (1d SRT) was tested at 75 °C to investigate the impact of SRT on solubilization of digested cattle manure (DCM). Biomethanation potential tests (BMP) conducted at 37 °C on DCM, HTH1 and HTH2 achieved biodegradabilities of 17 %, 19 %, and 26 %, respectively. Specific methanogenic activity tests (SMA) at mesophilic conditions for DCM and HTH2 showed comparable maximum specific methane production rate (MSMPR) of 14.6 and 14.1 mL CH4/g VSS.d for DCM and HTH2, respectively. However, at 55 °C, the MSMPR for HTH2 was roughly three times higher than at 37 °C but it was comparable for DCM at both temperatures. Firmicutes and Bacteroidota were the main phyla in MR effluent, HTH2 and all SMA tests at different temperatures (37 °C and 55 °C). Methanosarcina was the most abundant methanogen at mesophilic and thermophilic temperatures. The predominant mechanism for the enhancement of methane production by the HTH recirculation was not solubilization but the enhanced biodegradation kinetics of particulate organics, including lignin, cellulose, and hemicellulose.