A great number of industries, such as textiles, tanneries, chemicals, food processing, paper, among others, generate liquid waste (wastewater) which requires treatment prior to disposal. Whenever organic compounds must be removed by microorganisms, the most suitable treatment is activated sludge. During the treatment, there is an increase in the mass of activated microorganism, also known as sludge, which needs to be removed. The daily operation in industrial wastewater treatment plants is subject to disturbances in the flowrate as well as in the concentrations of the effluent streams. The operation of these plants usually requires manipulation of the inflow of the aeration tank, sludge recycle flowrate and sludge discharge flowrate. The purpose of this paper is to develop an optimization model to minimize the sludge discharge by manipulating the feed stream distribution in the aeration tank. The resulting model is a Nonlinear Programming (NLP) problem that generates the best operational strategy for the aeration tank feed streams so as to minimize the sludge growth with a high substrate removal. Results show that reductions of up to 30 % can be achieved with the proposed strategy.