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

The recalcitrance of lignin in cattle manure significantly limits energy recovery during anaerobic digestion. Mesophilic anaerobic digestion of cattle manure (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, HTH1and 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 CH 4 /g VSS.d for DCM and HTH2. However, at 55 °C, the MSMPR for HTH2 was roughly 3 times higher than at 37 °C but 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. • Coupling HTH with AD enhanced lignin-rich CM biodegradability from 42% to 46% • Recycling HTH digestate improved the digestibility of DCM by 28.8% at the same SRT. • The degree of solubilization of DCM after HTH ranged from 69-91 mg SCOD/PCOD. • HTH digestate recycling improved lignin, cellulose, and hemicellulose removal. • Firmicutes and Bacteroidota were enriched in the MR and HTH highlighting increased hydrolysis and fermentation. • Methanosarcina was enriched at 37 and 55 °C, while Methanothermobacter was only enriched at 55 °C.