Modelling MSW decomposition under landfill conditions considering hydrolytic and methanogenic inhibition

A landfill typically progresses through a series of microbial degradation phases, in which hydrolysis, production and consumption of fermentation products, such as fatty acids, and methane formation play important roles. For ultimate degradation of the waste, stable methanogenic conditions have to be attained, and maintained for sufficient time. Using experimental data from 100-L landfill simulation reactors containing municipal solid waste from a residential area, a distributed model, which accounts for vertical water flow, was developed. As a first step, the waste was divided into two fractions: readily degradable and recalcitrant waste. Secondly, the general hydrolysis of the recalcitrant waste was accounted for by including a specific, well-defined chemical substance in the model that generally occurs in Municipal Solid Waste (MSW) and is hydrolysed before its further degradation to methane. For this purpose we chose diethyl phthalate and its hydrolysis product monoethyl phthalate, for which leachate data are available from the reactors. The model indicated that inhibition of the hydrolytic and methanogenic processes occurred during the acidogenic phase and that it could be overcome either by improving the chemical environment or by the complete oxidation of the inhibiting, i.e. the easily degraded, fraction of the waste. The generality of the model was confirmed by the patterns of the phthalate di- and monoester transformations obtained. The validity of the model was further confirmed using experimental data from parallel reactors, which were subjected to either leachate exchange with an already methanogenic reactor or to initial aeration to force the reactor into stable methanogenic conditions.