New criteria to determine the destabilization of the acidogenic anaerobic co-digestion of organic fraction of municipal solid waste (OFMSW) with mixed sludge (MS)

•HRT of 1.2days led to the maximum hydrogen production of 3.67L H2/L reactor/day.•Maximum specific hydrogen production was 33.8mL H2/gVSadded (1.2-days HRT).•Two indirect parameters NSC and ASC determine the balance in hydrolytic phase.•At HRTs between 2.9 and 1.2-days the system operates stable.•Failure of the hydrolytic phase occurs at HRTs lower than 1.2days. Effect of hydraulic retention time (HRT) on bio-hydrogen production from co-digestion of organic fraction of municipal solid waste (OFMSW) and mixed sludge (MS) in dry thermophilic conditions (55°C and 20% total solids) was investigated. A decreasing sequence of six HRTs, from 2.9 to 0.8-days, was performed to evaluate the stability of the system and the influence of HRT on the organic matter solubilization, the daily hydrogen production (HP) and the specific hydrogen production (SHP). Best results were obtained operating at 1.2-days HRT: HP of 3.67L H2/Lreactor/day, SHP of 33.8mL H2/gVSadded and hydrogen percentage in biogas of 52.4%. However, HRTs lower than 1.2-days induce failure in the system due to an unbalance of the hydrolytic phase. This fact was corroborated through the evaluation of two indirect parameters, “non-solubilized carbon” (NSC) and “acidogenic substrate as carbon” (ASC), and the relationships of NSC/TOC and ASC/TOC.