Determination of the hydrolysis constant using Anaerobic Batch Tests

Hydrolysis is the rate limiting step for particulate/solid substrates under anaerobic digestion. The first order kinetic model is commonly applied to describe this step and it can be determined by anaerobic batch test. In this paper we assess three issues with respect to defining the hydrolysis rate constant; source of inoculum, influence of substrate to inoculum ratio and the pre-incubation time of the inoculum. Tests were carried out with five different inoculums that showed different hydrolytic activity under the same test conditions. Sludge from MWWTP showed the highest activity (0.95 d) and sludge from co-digestion plant the lowest (0.29 d). 7 days pre-incubation of sludge reduced the hydrolytic activity by ca. 30%. Increasing the substrate to inoculum ratio lead to an increase in hydrolytic activity until the optimum ratio, and then it decreased. Under real substrate to inoculum ratio conditions maintained in large scale biogas plants, the measured hydrolysis rate constants were 0.32, 0.49, 0.41 and 0.17 d for dog food, cacao shells, CCM and rye respectively. The parameter can be used for anaerobic reactor design and as initial values for modelling.