A novel control strategy for improved nitrogen removal in an alternating activated sludge process—part I. Process analysis

Abstract

Increasing demands on discharged water quality have led to the development of activated sludge processes which incorporate the biologically mediated removal of nitrogen and phosphorus. A major obstacle in the development of new control strategies for such processes is the lack of variables which can effectively alter process behavior and can feasibly be manipulated. This two part paper deals with a novel means to improve the nitrogen removal in an alternating type nutrient removal activated sludge process through control of the cycle length. In this first part, an analysis of process dynamics is undertaken. Using a simple model to describe the nitrogen dynamics in the alternating process, the existence of an optimal cycle length as a function of process conditions is demonstrated and explained. A graphical technique is developed which allows quick visualization of nitrogen dynamics under constant process conditions. This also serves as a means to assess whether a selected cycle length is optimal, too long, or too short for a given set of conditions. Based on the findings of this first part, the second part of the paper develops and demonstrates control strategies which serve to automatically adjust the cycle length to compensate for changing process conditions.