This paper deals with the optimization of a two-phase anaerobic process treating biowaste for hydrogen and methane production. Neither physical nor chemical pre-treatments were used to optimize the process. The work was carried out at pilot scale, using two CSTRs (200 and 380 L working volume respectively) both maintained at thermophilic temperature (55 °C) and fed semi-continuously with biowaste. The experiment was divided into three periods; during the first two periods the organic loading rate was maintained at 20 kg TVS/m3 d and the hydraulic retention time was changed from 6.6 to 3.3 days, while in the last period the digestate of the second reactor was recirculated to the first reactor in order to buffer the system and control pH at levels around 5. The HRT was maintained at 3.3 days and the OLR was decreased at 16.5 kg TVS/m3 d. The best yield was obtained in the last period where a specific hydrogen production of 50.9 L/kg VSfed was reached, with a H2 content in biogas from the first reactor of 36%. The methanogenic stage after the hydrogen conversion reached a specific biogas production of 0.62 m3/kg VSfed and an overall organic removal above 70%, without any stability problem. The overall biogas production was some 1.5 m3 per day with a gas composition of 10% H2 and 50% CH4.

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