Ammonia concentration and pH control in pilot scale two-phase anaerobic digestion of food waste for hydrogen production: Focus on start-up

This paper deals with the start-up strategy of a two-phases thermophilic anaerobic digestion process treating food waste, optimized for the production of hydrogen and methane. In order to keep the pH of the dark fermentation reactor in the optimal H2 production range (5-6), recirculation of anaerobic digested sludge was used. A drawback of such approach is the accumulation of ammonia into the system with the risk of hydrogenogenic and methanogenic processes inhibition. Therefore this study was focused on the investigation of the recirculation ratio that allows to carry out the start-up phase control approach at pilot scale. Two pilot scale stirred reactors were used; the experiment was set-up considering two recirculation ratio of 0.4 and 1, maintaining the organic loading rate in the first and second phase of 18-20 kgtotal volatile solids/m3d and 4.5 kgtotal volatile solids/m3d respectively. The hydraulic retention time applied was of 3.3 days in the first and of 12.7 days in the second phase. A promising start-up strategy was obtained applying a recirculation ratio of 1; the partial alkalinity content in the second reactor of about 2,800 mgCaCO3/L allowed to reach a pH value in the first reactor of 5.5 after 10 days