Psychrophilic anaerobic co-digestion of highland barley straw with two animal manures at high altitude for enhancing biogas production

•High I/S ratio (>2/1) was favorable to both sole digestion and co-digestion.•Biogas production from BS was feasible at low temperature and low air pressure condition.•Long SRT (>80 days) is needed for biogas production at low temperature and low air pressure condition.•BS to manure ratio of 1/1 could increase biogas production.•IVS removal efficiency was correlated with biogas production.

Biogas production from the co-digestion of highland barley straw (BS) with Tibet pig manure (TPM) and cow manure (CM) was investigated at Tibet plateau under low temperature (15 °C) condition. The effect of inoculum to substrate (I/S) ratio and BS to manure ratio on the biogas production was studied using a series of batch digesters performed at substrate concentration of 20%, based on total solid (TS). The results showed that biogas production from BS was feasible at low temperature and low air pressure condition. High I/S ratio (>2/1) and BS to manure ratio of 1/1 could increase the biogas production. Long solid retention time (SRT) (>80 days) was needed for biogas production at low temperature and low air pressure condition. The highest cumulative biogas production obtained from the co-digestion of BS with TPM and CM was 233.4 ml/gVS and 192.0 ml/gVS, respectively. Removal efficiencies of substrate showed that biogas production was correlated with the removal efficiency of water-insoluble volatile solids (IVS) but not with the change rate of soluble chemical oxygen demand (SCOD).