An Optimum Design for Activated Sludge Systems

Abstract

The high costs of construction, maintenance and operation of waste water treatment systems exert economic pressure, even in developed countries. Therefore, engineers look for creative, cost-effective environmentally sound ways to control pollution. In this study, how an optimization algorithm can be used to determine the design parameters of an activated sludge system which make the cost minimum is presented. The cost of an investment can be analyzed in two categories: fixed capital investment and operational costs. For an activated sludge system, the former consists of an aeration tank, settling tank, pumps, piping system, aeration system and scraper constructions, whereas the latter mainly involves energy costs for aeration and sludge removal when labor or non-operational expenditures are ignored. For a defined inlet flow rate and the characteristics of wastewater and for the desired outlet characteristics, optimum design parameters can be determined using that optimization algorithm. The Activated Sludge Model No.3 and Takács settling model have been used in modeling the activated sludge system. Simulations are executed in Matlab® and fmincon (active-set algorithm) is used as optimization algorithm with some improvements. The proposed algorithm can be a good choice for the design of activated sludge systems. The recycle ratio, the waste sludge ratio and the oxygen transfer rate in the aeration pond are the most important parameters in the energy cost and so, in the operational cost. The sizes of aeration and settling tanks are the most important parameters of fixed capital investment. The results show that the volume of the aeration tank, settler area, recycle ratio and the waste ratio can be lowered by 26.79 %, 49.21 %, 20.90 %, and 44.42 % respectively by the application of the algorithm. The investment and operational costs of an activated sludge system can be lowered consequently.