Hydraulic retention times as key parameter governing biomethanation of brewery spent grain and system stability in long-term continuously-feeding anaerobic digestion

Abstract

The feasibility of converting brewery spent grain (BSG) to biomethane in a mesophilic continuously-stirred tank reactor was demonstrated at various hydraulic retention times (HRTs) of 100, 60, 30, and 20 d. As HRT decreased to 30 d, the biogas and CH₄ production rates increased to 1.40 ± 0.05 and 0.89 ± 0.03 L/L/d, respectively. However, a shorter HRT of 20 d increased the instability of the system according to the ratio of total volatile fatty acid and total alkalinity (> 0.35). The modified first-order kinetic equation accurately predicted biogas and CH₄ production rates and organics degradation efficiencies. As HRT decreased from 100 to 30 d, the ratio of the conversion of organics based on chemical oxygen demand to CH₄ decreased from 80.8 ± 1.8 % to 40.8 ± 1.8 %. The results of the energy balance demonstrated the economic feasibility of anaerobic digestion (AD) of BSG. These finding provide valuable insights for industrial-scale AD of BSG.