In this paper the regulation of petrochemical wastewater from an activated sludge system is addressed via a simple robust feedback control approach. The control approach is based on simple step response models and a favorable choice of the discharge flow-rate from the settler as the manipulable variable. Based on simple first order model and first order model plus input delay to account for dead-times induced by the measurement of COD two controllers are derived. The proposed controllers are composed by two parts: (i) an uncertainty observer to compensate uncertainties and neglected terms in the input-output models, and (ii) an inverse dynamics feedback controller. Numerical simulations show good closed-loop performance and robustness properties.