Suchergebnisse

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.

The sewage from the Greater Athens area is discharged without treatment to the nearby Saronicos Gulf. Recently the Greek government decided to implement an appropriate treatment and disposal scheme, on the basis of a study of the hydrodynamic behaviour of the Gulf and the fate of the discharged pollutants. The study was conducted with the aid of suitable hydrodynamic and water quality models for a number of alternative treatment schemes, ranging from primary treatment to biological treatment with seasonal nitrogen removal, combined with outfalls of different lengths. The results indicate that, although none of the examined schemes will lead to an immediate substantial impact, the scheme that includes biological treatment with seasonal nitrogen removal can eliminate the danger of serious ecological disturbances at a reasonable cost.

Industrial contributions constitute as much as 20% of the 700 000 m(3).d(-1) of sewage discharged to the Saronikos gulf through the sewerage system which serves the Greater Athens area. The main industrial discharges originate from food, textile, pulp and paper, chemical, tanning and metal processing and electrical industries. In the majority of the cases the pretreatment is inadequate and the existing emission standards are violated. This results in increased concentrations of toxic metals in the sewage. The impact of these toxic discharges on the marine environment and the sewage treatment works have been investigated. Increased concentrations in the water column and the sediments have been reported but there are no indications of accumulation of toxic metals in the body of benthic organisms. The impact on the imminent operation of the treatment works fur the sewage of Athens, have been investigated with the aid of pilot and bench scale studies. The results did not indicate a serious inhibition of the sensitive digestion and nitrification processes. However due to the high concentrations of the heavy metals, with average values approaching threshold values, it is important that appropriate action is taken to control industrial discharges through a revised and rational regulatory framework.