Extremum-Seeking Control for Methane Production Optimization in Low-Cost Anaerobic Digesters

This study proposes an extremum-seeking control strategy to optimize methane production in anaerobic digesters. The strategy is aimed at rural and low-cost plants, with scarce instrumentation, long sampling periods, and minimum knowledge of the process. The proposed scheme maximizes methane productivity by adjusting the dilution rate, relying only on weekly measurements of the methane production rate and the FOS/TAC ratio, which serves as a stability indicator. Stability issues due to process acidification are studied and addressed by including a pH control loop and slope generator acting as an auxiliary safety loop based on the FOS/TAC ratio. The control strategy is validated using numerical simulations with the ADM1 benchmark model. Results demonstrate robust convergence to the optimal methane production point, effective disturbance rejection under time-varying influent conditions, and the prevention of instability due to process acidification. The proposed method achieves near-optimal productivity with a low-cost implementation, making it well-suited for small-scale digesters and resource-limited settings, thus contributing to sustainable bioenergy generation and the efficient utilization of renewable resources.