Modeling the mixing of wastewater effluent discharged from ocean outfalls using a hybrid model

Abstract

A hybrid model in which line plume equations are used for the computation of the initial mixing in the near field and a random-walk particle-tracking model is used for the simulation of the far field transport was developed to predict the mixing characteristics of wastewater effluent plumes discharged from ocean outfalls. In the particle-tracking model, the governing equation is transformed to the σ coordinate to use grid velocities computed by the σ-layered hydrodynamic model. In addition, the particles introduced at the end of the near field are distributed with the normal distribution to represent the vertical concentration distribution of the wastewater plume. The results of model applications to the Masan sea outfall in Korea show that there is no large difference between the minimum dilutions for the summer and winter seasons even though the effluent plume is trapped at mid-depth during the summer season due to density stratifications. The periodic variation of the ambient current does not have a significant effect on near field dilutions, since the ambient velocities at the outfall site are quite low. The simulation results of the far field transport with settling processes reveal that concentrations at the bottom layer in the vicinity of the outfall site are higher than those in the remote sites from the outfall. This simulated result agrees with the depositional trend of the contaminated sediment that was obtained from the field survey.