A eutrophication-dissolved oxygen water quality model was developed and applied in order to predict the environmental impact from the discharge of the sewage of some 700,000 inhabitants of Attiki, Greece to the nearby Evoikos Gulf. Three alternative treatment and disposal schemes were examined, including primary treatment, biological treatment and biological treatment combined with seasonal nitrogen removal. The critical water quality parameters were found to be chlorophyll-a and transparency and it was concluded, on the basis of the model results, that, unless a stable and sufficiently long thermal stratification of the water body is sustained, biological treatment with seasonal nitrogen removal should be adopted as the minimum degree of treatment prior to discharge.

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