N-Removal on Wastewater Treatment Plants: A Process Control Approach

Abstract

This paper is concerned with the design and development of simple but effective control structures for enhancing nitrogen (N) removal in activated sludge processes. Most of the approaches found in the literature lack a systematic method to tune the controller parameters and this is often carried out in an ad-hoc way, and hence suffers from a lack of repeatability, robustness and easy implementation. This fact makes it diffcult to implement and use these methods in practice. The novel control method and tuning of the controllers proposed here makes use of very simple and direct process information that can be obtained by application of realistic step tests on the control handles. The final goal of the control is to improve the effuent quality and to trade-off such quality with the energy consumption and operating costs. A multilayered control design approach is proposed here. Starting with the Dissolved Oxygen (DO) control loop in the last aerated tank, the control of the recirculating sludge is added next. Once the limitations of this two-loop control strategy are highlighted, a cascade control loop is proposed. This cascade control is further enhanced by a feed-forward control action that makes use of influent ammonia concentration. The resulting cascade+feedforward control configuration achieves satisfactory nitrogen removal for the three influent operating conditions (dry, rain and storm.)