Anaerobic hydrolysis of primary sludge: influence of sludge concentration and temperature

Anaerobic hydrolysis of primary sludge from urban wastewater treatment was first studied as part of anaerobic sludge stabilisation and later as a process for readily biodegradable matter (volatile fatty acids) production in order to improve biological nutrient removal. Furthermore, hydrolysis was found to be the rate limiting step in both single- or two-step anaerobic treatment of domestic wastewater. However, the influence on the process of several factors such as sludge concentration and temperature is still barely understood, as are kinetic aspects. The aim of this work was to study the influence of sludge concentration and temperature on the hydrolytic and acidogenic conversion of primary sludge and to determine the hydrolysis kinetic coefficients at different temperatures. Specific VFA production varied from 0.17 to 0.34 gVFA_COD/gVSS of raw primary sludge, depending mainly on the assay sludge concentration and, to a lesser extent, on the process temperature. The first-order hydrolysis constants were 0.038, 0.095 and 0.169 d⁻¹ for 10, 20 and 35°C, respectively.