Fluid-mechanics aspects in the design of sewage outfalls into the sea

Abstract

Sewage outfalls into the sea have much in common with chimneys for emissions into the atmosphere: both are intended to release pollutants in such a way that they are rapidly diffused and reduced to harmless concentrations. Chimney design aims at restricting the concentrations of pollutants near the ground to a tolerable level whilst, in outfalls, the aim is to afford the upper strata of water and the coastal regions in particular the maximum possible protection against pollution. Yet an essential difference lies in the fact that the atmosphere into which chimneys discharge extends upwards for large distances and, generally speaking, is subject to a high degree of vertical mixing while the coastal waters into which waste is released are comparatively shallow. Waste outfalls and chimneys can have only a localized influence upon the reduction of concentrations of pollutants. At any great distance of discharge, it hardly matters what method is used to release pollutants. The sole decisive factor is how quickly they are decomposed by physical, chemical and biological processes. The continual release of substances which do not decompose is bound to cause an increase in the concentration. No discharge structure, however well designed, can remedy this. With decomposable substances this increase does not occur! Hence sea outfalls are acceptable for the harmless removal of pollutants such as domestic sewage but for nondecomposable substances they are just as unsuitable as any other form of dispersion.