Optimal expansion of a coastal wastewater treatment and ocean outfall system under uncertainty (II): Optimisation analysis

This paper presents an integrated simulation-optimisation assessment of the various physical, chemical, and microbiological processes that determine the transport and fate of key constitutes associated with coastal wastewater treatment and disposal effluents in coastal environments. The well-calibrated and validated ocean mixing simulation model - Cornell mixing zone expert system (CORMIX) - was employed in the first stage as an indispensable tool to simulate the initial dilution factor with respect to two major water quality indicators, biochemical oxygen demand (BOD) and Escherichia coli. The CORMIX outputs, which were obtained in a companion study, were used as the inputs in both deterministic and chanceconstraint based stochastic programming models to evaluate a few alternatives for system planning and design. Such integration allowed the identification of the optimal expansion strategies for a large-scale coastal wastewater treatment and ocean outfall system in the city of Kaohsiung, South Taiwan. Research findings clearly indicate that the original alternative proposed by the city government, which had planned on upgrading the wastewater treatment process directly from the primary to the secondary level, is economically undesirable. Given the technology options of primary or enhanced primary treatment, in the most cost-effective expansion strategy, the optimal length of a newly constructed second ocean outfall pipe is no less than 1260m in order to gain sufficient assimilative capacity in the ocean environment.With this strategy, the BOD standards in the ocean environment are met for both Classes B and A types of ocean waters. The reliability of this system design may be assessed by taking the stochastic characteristics of sewerage inflow and diffusion and dispersion processes into consideration. Such an optimisation analysis could eventually lead to considerable cost savings of up to 75% when compared to the original, officially favoured alternative.