Anaerobic mesophilic co-digestion of ensiled sorghum, cheese whey and liquid cow manure in a two-stage CSTR system: Effect of hydraulic retention time

•Co-digestion using a mixture of agro-wastes was tested in a two-stage system.•The effect of HRT on biohydrogen and methane production was investigated.•In acidogenesis, the highest H2 production and yield were observed at HRT 0.5 d.•In methanogenesis, the highest CH4 productivity was observed at HRT 16 d.•Methanogenic reactor instability occurred during operation at HRT 12 d.

The aim of this study was to investigate the effect of hydraulic retention time (HRT) on hydrogen and methane production using a two-stage anaerobic process. Two continuously stirred tank reactors (CSTRs) were used under mesophilic conditions (37 °C) in order to enhance acidogenesis and methanogenesis. A mixture of pretreated ensiled sorghum, cheese whey and liquid cow manure (55:40:5, v/v/v) was used. The acidogenic reactor was operated at six different HRTs of 5, 3, 2, 1, 0.75 and 0.5 d, under controlled pH 5.5, whereas the methanogenic reactor was operated at three HRTs of 24, 16 and 12 d. The maximum H2 productivity (2.14 L/LR d) and maximum H2 yield (0.70 mol H2/mol carbohydrates consumed) were observed at 0.5 d HRT. On the other hand, the maximum CH4 production rate of 0.90 L/LR d was achieved at HRT of 16 d, whereas at lower HRT the process appeared to be inhibited and/or overloaded.