Energy optimisation from co-digested waste using a two-phase process to generate hydrogen and methane

The total energy produced from co-digested food waste and sewage sludge was compared for single phase mesophilic anaerobic digestion producing methane and two-phase hydrogen production followed by methane production. Both single and two-phase reactors were operated at close to optimum conditions. The single phase methaniser had a methane yield of 0.48 m³ methane/kg VS destroyed. The two-phase system had a hydrogen yield of 0.13 m³ hydrogen/kg VS destroyed, and a methane yield of 0.67 m³ methane/kg VS destroyed. Introduction of a hydrogen producing, pre-treatment phase increased the overall VS destruction 69-89%, however the total energy yield decreased by 13.4% due to the low hydrogen yield obtained in the first stage. The release of ammonia in the hydrogeniser was low and so with less alkalinity available, pH control was necessary. It was much higher in the methaniser and adequate to buffer any pH change. This also ensures more nitrogen in the digestate to enhance its value for recycling. The two-stage process is an attractive option where solids destruction is an important consideration but further optimisation of the hydrogen production stage is still required.