Anaerobic Digestion of Soluble Organic Fraction of Municipal Solid Waste Under Inhibition Conditions: Analysis and Control

A nonlinear dynamical model for anaerobic digestion (AD) of the soluble organic fraction of municipal solid waste (OFMSW) is analyzed under inhibition conditions. The model incorporates both the acidogenic and methanogenic stages, accounting for substrate and product inhibition in both microbial consortia. Based on the dynamical properties, three stable equilibria are identified and linked to realistic operating scenarios: washout, acidification, and normal (stable) operation. Control schemes are proposed to regulate the organic substrate concentration and achieve the desired operating conditions. These strategies enhance operational stability under both normal and acidification conditions. A linear proportional-integral (PI) control scheme is designed, along with two tuning approaches to ensure closed-loop stability in the presence of external load disturbances, measurement noise, and substrate and product inhibition. The control schemes achieve, under normal operating conditions, 96% removal of soluble organic substrate and 60% removal of volatile fatty acids; under acidification conditions, 72% substrate removal is attained, with a 152% increase in volatile fatty acids relative to the inlet levels. Accordingly, the proposed model and control scheme offer a promising contribution toward improving the operational stability of AD processes treating OFMSW.