Robust Multivariable Controller Design for an Activated Sludge Process

Abstract

The importance of freshwater supply and safely treated wastewater return cannot be overemphasized. The human race is still a long way from the most efficient, economi- cal, and reliable ways to ensure our cities with a properly equipped treatment system. It demands the treatment of polluted/used water without discharging it to receiving water bodies. Principally, a sudden drop of dissolved oxygen concentration is observed in receiving water bodies when the organic pollutants are discharged along with the un- treated wastewater. This reduces the self-purification character of the water body, which involves the breakdown of complex organic molecules similar to biological treatment systems. The organic effluent generally contains a large quantity of suspended and set- tleable solids that will obstruct sunlight to reach the bottom surface of water bodies which then gives rise to water pollution causing eutrophication. Particularly, this can be solved by optimizing the aeration rate for better treatment, which intern reduces energy consumption and any additional chemical dosage in biological Wastewater Treatment plants (WWTP). One tool that has been successfully implemented to achieve such goals is the WWTP process model. Model development is necessary for simulating a sys- tem’s behavior, and optimizing or controlling its performance. The main motive behind controlling any WWTP is, primarily to abide by the effluent discharge standards and secondarily to maintain the operational costs as low as possible. The robust controller designs are plant-specific, but the principle and goal remain the same. Hence in this research, a mathematical model is identified using two approaches namely the system identification technique and the Process Reaction curve method for an Activated Sludge Process (ASP). By keeping this as a benchmark, the controllers are designed that aimed to control effluent dissolved oxygen or biomass concentration and substrate concentration by manipulating the aeration rate and recycle sludge flow- rate. Two types of controllers are designed to govern the ASP system: Centralized and Decentralized controllers. Each type has its respective pros and cons which are discussed in the upcoming chapters. To overcome the challenge of the grey box model for the ASP system, a data-driven approach was selected to fit a model class for the ASP unit. This technique will reduce the effort, complex tasks, and time for the process and control engineer to develop a mathematical model of the plant. Subsequently, it is then utilized to design a centralized control system for an ASP unit.