Analysis and control of an up-flow anaerobic sludge blanket reactor

Abstract

Up-flow Anaerobic Sludge Blanket (UASB) reactors are widely used in wastewater treatment and biogas generation through anaerobic digestion. In Brazil, this technology is commonly adopted due to its low construction and operational costs, as well as the favorable climatic conditions of the country, particularly the warm climate. In Ceará, approximately 40% of Wastewater Treatment Plants (WWTPs) use UASB reactors. However, these reactors lack automated operational solutions, and the absence of sensors in the deployed systems complicates the monitoring of the internal processes of the plant. In this context, the use of a mathematical model becomes essential to infer the behavior of the reactors and assist in defining optimal strategies to ensure compliance with health regulations autonomously. In this study, a mathematical model of a UASB reactor is implemented and analysed in terms of stability, controllability, and observability. The modeled system has 71 state variables, 1 input, and 7 outputs. The results indicate that the system is stable, structurally controllable and observable, although not in the classical sense. Based on these results, a control system for the sludge volume of the reactor is proposed by manipulating the sludge discharge flow rate, also considering the necessary instrumentation for monitoring internal processes. Additionally, a cascade control strategy is presented to regulate the Chemical Oxygen Demand (COD) levels of the treated effluent. The control of the sludge volume showed satisfactory results in both tracking the reference value and control actions. However, the cascade control approach did not yield significant results, highlighting the need for more advanced control strategies.