On-line integrated control system for an industrial activated sludge process

An on-line integrated control system was devised for an activated sludge process. The process treats wastewater from a food processing plant where the disposal rate and composition of wastewater fluctuate drastically. The major objectives of the control system were to maintain the process effluent chemical oxygen demand (COD) below 50% of the regional regulation limit and to minimize the cost of aeration. The integrated control system consists of two layers, supervisory control and process control layers. In the process control layer, six independent control loops were configured for four kinds of process variables; influent flow rate, pH, DO, and mixed liquor suspended solids of the series of aeration tanks. In the supervisory control layer, a rule-based expert system was used to provide optimum set points for the process control layer. In addition, a valve position control loop was designed to minimize the throttling loss through aeration valves and a rule-based load distribution system was devised to prevent some of the blowers from being excessively worn out. The developed control system has been successfully operated for over 2 yr. After installation of the integrated control system, the average effluent COD was decreased by more than 50% compared with the COD before installation, and at the same time the cost of aeration was reduced by about 50%.