In this paper, a robust feedback control improving the performance of a wastewater treatment is presented. In operational terms, the main control objective is to keep the concentration of the biomass proportional to the influent flow rate with the various disturbance compensation, noisy measurements and varying process model parameters. The H-inf control design is based on a simplified Activated Sludge Model No. 1 (ASM1) with the description of the nonlinearities by bounded structured uncertainty. It is found that the proposed controller yields excellent responses in the face of parameter uncertainties, load disturbances and set-point changes.