Effluent discharge into tidal waters: Optimal or economic strategy?

A variety of strategies, based on the use of holding tanks and variable discharge rates, are presented for reducing environmental impact and minimising economic cost of a continuous discharge of a conserved pollutant into a tidal estuary. The environmental objectives are to avoid intermittently high pollutant concentrations downstream of the outfall and to prevent upstream transport of pollutants on the flood tide. The first of these can be achieved by varying the discharge proportionally to the flow speed in the estuary, while the second is achieved by only discharging during the early part of the ebb tide. Operating costs can be reduced by pumping the discharge intermittently, especially if there is an overnight period of cheap energy costs. However, an intermittent discharge strategy will result in elevated pollution levels unless the discharge period is confined to the ebb tide. Intermittent discharges may also incur increased capital costs in terms of the need to provide a larger holding tank.