Anaerobic Treatment of Dairy Wastewater: Impact of Substrate-to-Inoculum Ratio on Biomethane Production

Abstract

Anaerobic digestion (AD) of synthetic and real dairy wastewater was the focus of the study for parametric estimation of kinetic models with examining biochemical methane potential (BMP), treatment efficiency, and energy effectiveness of the process. The effect of substrate-to-inoculum ratio (S/I) on batch reactor performance was investigated under mesophilic (35°C±2°C) conditions for 30 days. Based on the volatile solid (VS) content, three different S/I ratios were considered: 1∶1 (Treatment A), 1∶3 (Treatment B), and 1∶6 (Treatment C). Organic matter removal in VS units was as high as 56%. Almost 90% of the biogas was produced during the first 14 days of digestion, with a biogas yield of 337±23, 305±48, and 356±37 mL/g VS for Treatments A, B, and C, respectively. Methane content increased with increasing inoculum amounts in the medium. The experimental cumulative methane yield from both synthetic and real dairy wastewater was contoured for parametric estimation in four different kinetic models, namely first-order (FO), modified Gompertz equation (GE), modified logistic (LM), and modified transference (TM) models. The modified TM model was the best fit for overall treatment, with predicted methane production potential of 279.8 and 235.9 mL/g VS compared with the experimental value of 272 and 229 mL/g VS in treatment C for synthetic and real dairy wastewater, respectively. Preliminary studies showed the potential of energy generation, which may supplement 32% of the overall energy use. These results demonstrate the importance of understanding the effect of the S/I ratio involved in AD to treat synthetic and real dairy wastewater.