Effects of volatile fatty acids, ammonium and agitation on thermophilic methane production from biogas plant sludge in lab-scale experiments

The effects of different volatile fatty acids (VFA, formate, acetate, propionate and butyrate), ammonium (NH (4) (+) ) and agitation on methane (CH4) production were determined in 120-mL serum bottles. We showed that the addition of formate did not lead to an inhibition of methanogenesis until a concentration of 120 mmol/L. A complete inhibition of methanogenesis was detected in variants containing 360 mmol/L formate or propionate until day 3 but the production started afterwards within next 2 days. This might indicate a kind of adaptation to the higher volatile fatty acid concentrations. Increasing NH (4) (+) concentrations led to higher initial CH4 production, with an optimum at 120 mmol/L. The addition of 720 mmol/L NH (4) (+) led to a complete inhibition until day 3; subsequently, CH4 production started again on day 5 though it was still significantly lower compared to the other variants. Finally, also the speed of agitation showed significant effects on methanogenesis. The CH4 production from complex carbon sources was most favourable at a moderate agitation of 150 rpm of the lab-scale serum bottles. A lower or higher speed brought about a distinct reduction of CH4 production.