A mathematical model for the design of marine sewage disposal systems in accordance with the criteria of the EU Bathing Water Directive is developed and applied to the design of the outfall of the sewage from Athens, Greece. The model calculates the reduction of fecal coliforms due to initial and subsequent dilutions and die-off due to the effect of solar radiation and predicts the concentrations of fecal coliforms at the bathing area for specific hours and current velocities. A combination of these results with the probability distribution of the velocity of the sea currents results in a frequency distribution of fecal coliform concentrations at the bathing area, thus enabling evaluations with respect to conformity to the statistical criteria of the EU Directive.
The application of the model to the disposal system for the sewage of Athens shows that conformity, without the use of effluent disinfection, can be achieved for the nearest bathing area, located at a distance of 3.4 km from the outfall, irrespective of the definition of the bathing hours (8 am-8 pm or 24 hrs) and for a sufficiently conservative mortality rate. These results indicate that under Mediterranean or similar conditions, with typical wind induced currents, a sewage field travel distance of a few kilometres is sufficient to ensure bathing areas against infection, without the need for sewage disinfection. It should, however, be stressed that these conclusions are valid on the assumption that the sampling procedures outlined in the EU Directive refer to uniformly distributed sampling throughout the bathing period of the day or season.