1 ACHIEVING WATER QUALITY SYSTEM RELIABILITY USING GENETIC ALGORITHMS José A. Vasquez 1 , Holger R. Maier 2 , Barbara J. Lence 3 , Bryan A. Tolson 4 and Ricardo O. Foschi 5 ABSTRACT: This paper presents an efficient approach for obtaining wasteload allocation solutions that provide the optimal tradeoff between treatment cost and reliability. This approach links a Genetic Algorithm (GA) with the First-Order Reliability Method (FORM) for estimating the probability of system failure under a given wasteload allocation. The GA-FORM optimization approach is demonstrated for the case study of managing water quality in the Willamette River, Oregon, USA. The objective function minimizes the sum of the treatment cost and the penalty associated with breaching a reliability target for meeting a water quality standard. The random variables used to generate the reliability estimates include streamflow, temperature and reaeration coefficient values. The results obtained indicate that the GA-FORM approach is nearly as accurate as an approach that links the GA with Monte Carlo Simulation (MCS) and is far more efficient. The tradeoff between total treatment cost and reliability becomes more pronounced at higher water quality standards, and is most sensitive to the uncertainty in the reaeration coefficient. The sensitivity to the reaeration coefficient also increases at increased reliability levels. INTRODUCTION In recent years, Genetic Algorithms (GAs) have been shown to have advantages over classical optimization methods (Goldberg 1989; Holland 1975) and have become one of the most widely used techniques for solving a number of hydrology and water resources problems. Since GAs are not based on objective function gradient estimates, they may be used for complex, discontinuous and nonlinear problems. Variations of GAs have also been 1. Graduate Student, Department of Civil Engineering, University of British Columbia, Vancouver B.C., V6T-1Z4, CANADA, Associate Professor on leave from the Universidad de Piura, Piura, PERU. 2. Post-Doctoral Fellow, Department of Civil Engineering, University of British Columbia, Vancouver B.C., and Lecturer at Department of Civil and Environmental Engineering, University of Adelaide, Adelaide, S. A., 5005, AUSTRALIA. 3, 4, 5. Associate Professor, Graduate Student and Professor, respectively, Department of Civil Engineering, University of British Columbia, Vancouver B.C. Citation: Vasquez, J.A., Maier H.R., Lence B.J. and Tolson B.A. (2000) Achieving water quality system reliability using genetic algorithms. Journal of Environmental Engineering, ASCE, 126(10), 954-962, DOI: 10.1061/(ASCE)0733-9372(2000)126:10(954)