Integrated Process Design Optimization Accounting for Co-Digestion of Sludge and Municipal Solid Waste

This work presents an optimization model for the integrated design of waste treatment facilities, accounting for co-digestion of sludge and municipal solid waste. The superstructure of alternatives includes anaerobic digestion under mesophilic or thermophilic conditions, composting, recycling, and final disposal in a landfill. Anaerobic digesters can be fed with different mixing ratios of sewage sludge (SS) and the organic fraction of municipal solid waste (OF). A mathematical formulation derived from disjunctive programming is proposed to find the optimal process design. It comprises nonlinear equations to estimate digestion yields according to substrate mixing ratios. Results show that joint treatment increases the profitability, especially in small cities. Co-digestion of SS and OF leads to an integrated waste-to-energy process that maximizes the economic value of waste by producing electricity, heat and fertilizer.