CONTROL STRATEGIES FOR ACTIVATED SLUDGE TREATMENT SYSTEM By Bhakta

The activated sludge treatment system for treating municipal wastewater presents an interesting application of system dynamics modeling. This paper presents such a modeling approach to the strategy formulation of the treatment system in order to economically control effluent quality. First, factorial designs are carried out on the simulation results to identify factors that significantly affect effluent quality. Thereafter, open-loop control (both constant and timevarying), output feedback control, and output-integrated feedback control strategies have been applied. Statistical tests of significance indicate that the strategy of output feedback control has the maximum potential, in both summer and winter, to achieve the dual objectives of maintaining effluent quality within acceptable limits and minimizing aerator energy. Principles Underlying the Activated Sludge Treatment System The process of Activated Sludge Treatment System (Fig. 1) is the most widely employed technique today it for treatment of municipal wastewater. The influent stream of municipal wastewater, rich in soluble organic compounds (known as substrate or food for the bacteria), enters the reactor (known as aeration tank). Bacteria feed on the organic waste present in the water. Aeration is done by mechanical means providing the much needed oxygen required for the growth of bacteria in the tank. Figure 1: Schematic Diagram of Activated Sludge Treatment System. The sewage stream loaded with biomass goes out of the aeration tank and reaches the clarifier. The clarifier allows the biomass to flocculate and settle down, due to gravity, as sludge. The sludge contains live biomass. It is re-circulated, in a controlled manner, to the aeration tank so as to maintain the biomass concentration in the tank at a desired level. The excess sludge, which is not re-circulated, is withdrawn from the clarifier and put to sludge drying beds for future use as manures. The clarified supernatant stream goes out of the system as treated effluent. Researchers have developed a number of models for studying and understanding the wastewater treatment process. These models can be broadly categorized as (1) Component level models and (2) Comprehensive models. Component-level models pertain to sub-areas such as ‘substrate removal process’ (Novak 1974, Lawrence and McCarty 1974), ‘biomass growth process’ (Grady and Roper, Jr. 1974, Gaudy, Jr., et al. 1974, Grady et al. 1986) and Monod 1949), ‘oxygen transfer, dissolved oxygen consumption, and oxygen assimilation process’ (Bliss and Barnas 1986, Picionreanu, et al. 1997), and ‘clarification process’ (Ford and Eckenfelder 1967). Comprehensive models pertain to the functioning of the whole treatment system and can be classified as analytical models (Ford and Eckenfelder 1967, Roper, Jr. and Grady, Jr. 1978, Smeers and Tyteca 1984, Uber, et al. 1985, Tang, et al. 1987, Zhao, et al. 1999, and Anderson, et al. 2000), simulation models (Busby and Andrews 1975, Barton and Mckeown 1986, and Anderson, et al. (2000)), and system dynamics models (Das, et al. 1995, 1997 and Clemson, et al. 1995). But for Anderson, et al. (2000), none of the studies reported above has designed any control strategy for the output quality exceedences. Clemson, et al. (1995) have developed a system dynamics model for wastewater treatment plant and have used Taguchi methods in conducting sensitivity analysis. But their model considers neither the biological process of growth of microorganism nor the process of oxygenation. It also does not try to design a strategy for effluent quality control. The present work presents a comprehensive system dynamics model for wastewater treatment plant by using the tools, techniques and concepts of design of experiments, statistical quality control, and modern control theory in order to decide the number of aerators to use and design the sludge re-circulation policy while maintaining the effluent quality within acceptable limits. A Dynamic Model for the Treatment System Four physical flows can be distinguished in an activated sludge plant: (1) Flow of Liquid, (2) Flow of Biomass, (3) Flow of Substrate or Pollutants, and (4) Flow of Dissolved Oxygen.