Modelling of hydrolysis controlled anaerobic digestion

Abstract

A mathematical model of anaerobic digestion has been developed which considers the process as occurring through hydrolysis followed by a sequence of steps which have been considered together as a pseudo-first order process. The Michaelis-Menten equation has been modified to incorporate variable enzyme concentration. An immobilized reactor has been designed, fabricated and operated on cowdung feed to obtain data for estimating the model parameters by non-linear regression. These model parameters were used to predict the performance of the reactor and good agreement between experimental and predicted values was observed. The model was further tested satisfactorily on data from the literature. From the parameters estimated it was concluded that hydrolysis was the slowest stage and that the mass transfer of hydrolytic enzyme from microbe to bulk was the rate controlling step in this stage. The hydrolysis reaction, represented by a modified Michaelis-Menten equation, was reduced to first order in bulk enzyme concentration and zero order in substrate concentration thereby emphasizing the significance of bulk enzyme concentration.