Design and implementation of the SRT control at the Ljubljana WWTP

The solid retention time (SRT) is one of the most important parameters for the operation of a wastewater treatment plant (WWTP). Maintaining the SRT at the prescribed set-point is necessary to achieve an appropriate food-to-microorganism (F/M) ratio. This article describes the design and implementation of SRT control at the Ljubljana WWTP. The designed SRT control wastes a defined proportion of the mixed liquor suspended solid (MLSS) mass in the aerobic reactors. The sludge wasted from the secondary settlers is determined based on measurements of the MLSS concentration, waste sludge concentration, and waste sludge flow rate. The SRT control is simple and does not require an SRT estimation or an SRT feedback controller. It includes a constraining controller that keeps the MLSS in the aerobic reactors within the prescribed minimum and maximum values. Validation of the designed SRT control on a mathematical model of the Ljubljana WWTP shows better set-point tracking than the proportional integral (PI) SRT control, as well as reduced SRT variations and lower ammonia nitrogen concentrations compared to the initial MLSS control. The testing of the designed SRT control at the Ljubljana WWTP confirmed the set-point tracking and reduced SRT variations despite large influent variations and operational disturbances.