Two outfalls in an estuary: Optimal wasteload allocation

When two outfalls are discharging wastewater into a narrow (rapidly mixed) estuary within a tidal excursion of each other, the pollutant concentrations experienced at the two outfall sites are strongly inter-dependent. It is shown how a given total tidally integrated effluent load can be allocated optimally between the two outfalls so that the peak concentration (in time and position) of the principal contaminant species is minimized. Graphical results show that dependence of the wasteload allocation and of the peak concentration upon the pollutant decay rate, the separation between the outfalls and the fresh water flow along the estuary. Optimization with respect to any one of a mixture of pollutants is close to optimal for a wide range of other pollutants.