Addition of HCl as a means to improve biogas production from protein-rich food industry waste

The effect of pH on the production of biogas during anaerobic digestion of a protein-rich substrate, containing mainly slaughter house waste, was investigated. Four laboratory scale reactors (4 L liquid volume) with an organic load of 3.5 g volatile solids (VS) L−1 reactor volume day−1, and a hydraulic retention time 24 days were run under mesophilic conditions in semi-continuous mode for 64 days. Two of the reactors were pH-regulated (target pH was 7.6 and 7.8, respectively) by adding HCl, while the other two reactors were operated as controls (pH 8.0). By the end of the experiment the pH-controlled reactors produced 0.6 L of methane g VSadded−1 day−1, while the controls produced 0.4 L. The gas produced did in all cases have a CH4 – content of about 65%. The improvement in process performance in the pH-regulated reactors, compared to the controls, was also reflected in lower final levels of acetate, propionate, isobutyrate and 2-metylbutyrate. The laboratory-scale study showed that lowering the pH by 0.2–0.4 units by adding HCl to the reactors increased the methane yield with about 50%, indicating a considerable increase of the microbial ability to utilise the organic material for biogas production.

► The pH of anaerobic digestion process was reduced from 8.0 to 7.8.
► A ∼50% increase in methane yield was achieved.
► Levels of acetate, propionate and iso-butyrate were reduced.
► The positive effects are likely related to decreased concentration of NH3.