Ammonium based aeration control in wastewater treatment plants : Modelling and controller design

Wastewater treatment involves many processes and methods which make a treatment plant a large-scaled and complex system. A fundamental challenge is how to maintain a high process efficiency while keeping the operational costs low. The variety in plant configurations, the nonlinear behaviour, the large time delays and saturations present in the system contribute to making automation and monitoring a demanding task. The biological part of a wastewater treatment process includes an aeration of the water and this process has been shown to often result in the highest energy consumption of the plant. Oxygen supply is a fundamental part of the activated sludge process used for aerobic microorganisms growing. The concentration of the dissolved oxygen should be high enough to maintain a sufficient level of biological oxidation. However, if the concentration is too high the process efficiency is significantly reduced leading to a too high energy consumption. Hence, there are two motivations behind the aeration control task: process efficiency and economy. One of the possible strategies to adjust the dissolved oxygen level in a nitrifying activated sludge process is to use ammonium feedback measurements. In this thesis, an activated sludge process is modelled and analysed in terms of dissolved oxygen to ammonium dynamics. First, the data obtained from a simplified Benchmark Simulation Model no.1 was used to identify the system. Both linear and nonlinear models were evaluated. A model with a Hammerstein structure where the nonlinearity was described by a Monod function was chosen for a more thorough study. Here, a feedback controller was designed to achieve L2-stability. The stability region was pre-computed to determine the maximum allowed time delay for the closed loop system. Finally, a feedforward controller was added to the system, and shown to significantly improve the disturbance rejection properties