M14.5 Anaerobic Sludge Digester

Anaerobic sludge digesters are used to degrade compounds (in waste water that is high in suspended solids) to carbon dioxide and water. A schematic of an anaerobic digester is shown in Figure M14-5.

The time unit is days, and the inputs and outputs have been scaled to cover the same ranges. The outputs and inputs are y₁ = substrate concentration, y₂ = temperature, u₁ = inlet flow rate, and u₂ = heat addition rate (°C/day). The steady-state inlet flow rate is 300 m³/day, the heat addition rate is 10°C /day. The steady-state temperature is 55°C and substrate concentration is 300 mg/liter.

Besides considering setpoint responses, assume that disturbances in the inlet flow rate occur.