Effect of solid retention time (SRT) on protein hydrolysis and acidogenesis at pH 5 and pH 7 using gelatine as a model protein

Anaerobic conversion can be used to recover volatile fatty acids (VFA) from high-strength wastewaters and organic wastes. However, many waste(waters) contain considerable concentrations of proteins and knowledge about anaerobic conversion of protein into VFAs is limited. In this study the effect of the solids retention time (SRT) and pH on dissolved protein conversion into VFAs was investigated in completely stirred tank reactors operated at 35 °C. At pH 5 and with an SRT of 12 and 30 days, hydrolysis was the rate-limiting step of protein degradation. At pH 7 and at SRT ≤ 8 days, the system shifted from being limited by hydrolysis to being limited by the conversion of amino acids to VFA. Even after a long-term exposure of the biomass to pH 5 (480 days), the hydrolysis rate constant for protein (0.05 L g⁻¹VSS day⁻¹) was still much lower than at pH 7 (0.62 L g⁻¹VSS day⁻¹). The difference between pH 5 and pH 7 is explained by the inhibitory effect of the large fraction of undissociated VFA at pH 5, which was confirmed in batch experiments. The highest volumetric VFA productivity of 2.3 g COD L⁻¹ day⁻¹ was obtained at pH 7 and at an SRT of 10 days. For complete removal of protein a longer SRT is required.