Mathematical prediction of methane formation in an anaerobic co-digestion CSTR.

This paper presents a dynamic mathematical model able to predict the methane production in an anaerobic completely stirred tank reactor (CSTR), performing the co-digestion of the organic fraction of municipal solid waste (OFMSW) and sewage sludge. The model mass balances, kinetics and stoichiometry are based on the approach proposed by the IWA Anaerobic Digestion Model n°1 (ADM1), which has been modified to take into account the peculiarities of a co-digestion system. The main distinctiveness of the proposed model is the capability to consider two separate influent substrates (i.e. sewage sludge and OFMSW), which are modelled with different biodegradation kinetics. In particular, the sewage sludge degradation is modelled according to the ADM1 while a surface based kinetics is used to simulate the OFMSW disintegration process, which depends on the surface area, i.e. on the particle size distribution (PSD), of the solid waste to be disintegrated. Further peculiarities of the model are the OFMSW fractionation into a readily and a slowly biodegradable components and the pH determination, which is based on a ten-order polynomial equation derived by acid-base equilibria. The methane production of a full scale municipal wastewater treatment plant (MWWTP) digester has been evaluated to assess the model capability to estimate the potential energy production under different process conditions. In particular, model simulations show the model suitability to predict the effect of the OFMSW particle size and the organic loading rate (OLR) on the methane production of a digester where the OFMSW is co-digested with sewage sludge.