Optimization of the Operating Variables of an Industrial Wastewater Treatment Process

Abstract

The Complex Method of optimization has been applied to minimize the operating costs of a large (1200 m3/hr effluent with an average of 1500 gCOD/m3) chemical effluent processing plant. The process, an oxygen enriched activated sludge process, consists of 3 parallel trains of UNOX bioreactors, each of four compartments, followed by 3 settling tanks.The five most important operating variables for a given recycle and waste sludge flowrate are the amount of gas vented to the atmosphere from the last compartment of each bioreactor train and the submersion depths of the electrically driven aerators in each bioreactor compartment. Using the cost prices of oxygen and electricity these five operating variables have been incorporated into a cost function which, subject to various constraints, must be minimized. The constraints are described by a steady-state, nonlinear model in which the concentrations of the dissolved oxygen in each compartment of each reactor are expressed in terms of the operating variables. The model parameters have been estimated from steady-state measurements.The result of the optimization is a simple strategy, independent of any of the model parameters, which fixes the depths of submersion of the aerators. The amount of gas vented to the atmosphere can be calculated regularly offline by means of a numerical search procedure. Savings in the order of Hfl. 15, 000 to Hfl. 30, 000 per year can be predicted from this study.