Biodigestion of easily-acidifying cheese whey in a sequential fermentative-methanogenic process: Strategies for boosting energy production and minimizing alkalinization costs

Abstract

The biodigestion of cheese whey towards enhanced bioenergy production still challenges researchers, considering incipient biohydrogen (bioH2) production during fermentation and the great dependence on an efficient (usually high-cost) alkalinization strategy to achieve high and stable methane production. Using a sequential fermentative-methanogenic process in fixed-film reactors, this study assessed strategies to boost bioH2 production (pH control at ca. 5.5 + prevention of biomass accumulation) and achieve a low-cost stable methanogenic process (replacing NaHCO3 dosing by effluent recirculation and further eliminating any alkalinization approaches). Maintaining fermentation pH values between 5.0 and 5.5 favored bioH2 and butyrate production, but the enhanced establishment of homoacetogenesis (accounting for 70–100 % of the acetate produced) associated with the selective removal of hydrogen-producing bacteria in biomass discharges resulted in a marked unstable hydrogenogenic activity. Processing fermented cheese whey without applying any alkalinization strategy in methanogenesis was further demonstrated to be technically feasible at an OLR of 5.0 kg COD m−3 d−1. COD removal (70–75 %) and the methane yield (150–170 NmL CH4 g−1COD) were equivalent regardless of the alkalinization (NaHCO3 dosing or effluent recirculation) or non-alkalinization approach. However, the alkalinization-derived production cost of methane was reduced from 1.55 USD Nm−3CH4 to zero when fully removing NaHCO3.