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

An ocean outfall system is an integrated engineering system in which a coastal wastewater treatment plant (WWTP) is connected with a pipeline that discharges wastewater effluent into a body of water. Safety factors to accommodate for uncertainty in modelling of such systems often result in a degree of wastewater treatment that is not cost-effective. This paper presents the first part of an integrated simulation and optimisation analysis to resolve some such uncertainties in the Cornell mixing zone expert system (CORMIX) model by testing it against a laboratory flume. CORMIX is a United States Environmental Protection Agency (USEPA)-supported mixing zone model for environmental impact assessment of regulatory mixing zones resulting from continuous point source discharges. This model especially emphasises prediction of plume geometry, flow classification, and dilution characteristics within an initial mixing zone of a receiving water body so that compliance with regulatory constraints can be evaluated. With the aid of a flume test, such information may be used as stochastic inputs to an optimisation model to assess the cost-effectiveness of discharge treatment.A total of 19 dye-study scenarios were designed and carried out for single-port (co-flow or counter-flow) and multi-port experiments according to the physical features of the outfall pipe to be expanded in the coastal environment of Taiwan Strait. The flume test showed a good agreement between CORMIX1/CORMIX2 models and the dye-study data. This approach is a stepwise, flexible technique allowing for continuous feedback of the varying design and operating conditions and the observed impacts on coastal waters and may be used to identify the optimal expansion strategy of an integrated coastal WWTP and ocean outfall system such as the case study presented for Kaohsiung, South Taiwan.