Mathematical Modeling of Batch, Single Stage, Leach Bed Anaerobic Digestion of Organic Fraction of Municipal Solid Waste

Abstract

Energy recovery can play an important role in municipal solid waste (MSW) management strategies by providing a saleable by-product and mitigating the environmental effects of the residue that requires disposal. Incineration, in-vessel anaerobic digestion processes and bioreactor landfills can all produce energy and can be used for pretreatment of MSW prior to eventual disposal. Organic fraction of municipal solid waste (OFMSW) is biochemically converted to methane and carbon dioxide in anaerobic digesters and bioreactor landfills. A lumped parameter mathematical model that describes this conversion process in a batch, single-stage, leach bed anaerobic digester under flooded conditions is developed and validated in this chapter. The model uses information such as mass of organic matter loaded in the vessel, amount of water used to flood the waste bed, headspace volume, alkalinity, pH and initial microbial concentrations to predict methane (or biogas) production rate, composition of biogas, residual concentration of organic matter, intermediate metabolites and alkalinity, and pH variations when the digester is operated at mesophilic (38°C) temperature. Most parameters of the model were obtained from literature and a sensitivity analysis used to identify those that required further refinement for improving model predictions. To improve numerical stability and rapid convergence of simulations, a novel solution procedure was developed to solve the charge balance equations in the differential algebraic equations set. Parameter estimation and model validation was carried out using data obtained from three pilot scale experiments conducted in 200 l vessels with 30 kg of OFMSW. Whereas parameter estimation was carried out using the results of one experiment, the model was validated using the results of the other two. The model was found to satisfactorily predict the experimental results and revealed that sufficient concentrations of microbial populations are present naturally in OFMSW and these can be activated rapidly by providing adequate alkalinity to prevent acidification. Such a start up procedure guarantees sustained and stable operation of the digester. Additional simulations determined that alkalinity and pH buffering capabilities provided by an initial concentration of 11 g l−1 of sodium bicarbonate (NaHCO3) was sufficient to accomplish this.