Anaerobic Digestion of the Soluble Organic Fraction of Municipal Solid Waste Under Inhibition Conditions: Biomass State Observer and Control

Background: The anaerobic digestion (AD) of the organic fraction of municipal solid waste (OFMSW) is a widely used waste-to-energy technology. However, inhibition effects can drive the process toward acidification or washout, with a consequent loss of treatment efficiency. Feedback control can prevent these undesirable conditions, but the biomass concentration is not measurable online. Methods: This work proposes an observer-based control scheme for the AD of the soluble fraction of OFMSW near an inhibited nominal operating condition. An extended Kalman filter (EKF) estimates the acidogenic biomass from the online measurement of the soluble organic substrate. A proportional–integral (PI) controller regulates the estimated biomass by manipulating the dilution rate. Three tuning methods are compared: approximate M-constrained integral gain optimization (AMIGO), Skogestad internal model control (SIMC), and frequency-domain auto-tuning. The convergence of the scheme is analyzed through Lyapunov and input-to-state stability arguments. The evaluation is simulation-based. Results: In an open loop, the observer converged under initial offsets of up to 35%, with steady-state error norms below 0.31 mg/L. The convergence time ranged from 45.3 to 81.3 d, depending on the initial estimation error. In a closed loop, nine controller designs tracked the reference and rejected feed perturbations over a 1000 d horizon. Under a stoichiometric mismatch of ±5%, the true-state tracking error of design S2 increased from 459.5 to 1783.5 mg d/L. The estimated-state error remained at 558.4 mg d/L. Estimated-state tracking can therefore conceal substantial true-state errors. The fastest design achieved the lowest estimated-state IAE, 501.1 mg d/L, but exhibited the lowest gain margin, 5.8 dB. The lowest true-state IAE, 450.2 mg d/L, corresponded to a design with a phase margin of 60∘. Conversely, the most conservative designs paired the largest IAE values with the largest gain margins, up to 15.2 dB. Conclusions: The proposed scheme maintains the anaerobic process around its inhibited nominal operating condition under the tested feed disturbances. The scheme uses a standard analytical measurement. The results support its feasibility as a soft sensor-based control strategy for anaerobic reactors near this operating condition. The prevention of acidification or washout under increased inhibition disturbances is beyond the tested scope.