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Oceans provide, in general, extensive assimilation capacity and are commonly used for wastewater disposal by a lot of communities (Metcalf & Eddy, 1991). In Portugal, the wastewater of some crowded coastal areas is carried to offshore discharge points by a pipe land on/or buried in the ocean floor. The Costa do Estoril is one of those areas. Costa do Estoril is a large zone located near the end of the Tagus estuary, on the west side of Lisbon region.

The effluent from the Estoril Coast Wastewater System is discharged in the zone of Guia, at a location considered as less sensitive zone by the Portuguese Decree Law 152/97.

The discharge location is situated nearly 2750 m from the coastline, and the effluent is discharged through two diffuser stretches of 400 m in length, installed at nearly 40 m depth. At present, the system serves more than 700,000 population equivalents.

In order to study and understand the effects of the wastewater discharges, an environmental program was implemented, even before the operation of the system. Field results included measurements of concentrations of nitrates, chlorophyla-a, dissolved oxygen, bacteria and, also, transparency and sediments, and are presented with detail in SANEST (1999). Mathematical simulations of the water quality have also been performed.

The variation in the concentration of substances discharged into the receiving waters is, in general, the result of high initial dispersion and subsequent dispersion conditions.

For the simulations carried out, the evolution of Biochemical Oxygen Demand (BOD5) and the concentrations of dissolved oxygen, nitrates, ammonia, phytoplankton and coliforms were assessed, for several different discharge scenarios, corresponding to conditions envisaged in the project and to effluents subjected to primary treatment or secondary treatment. The results obtained by mathematical modelling were confirmed, in general, by the results obtained in the field campaigns, and have revealed much reduced increases in concentrations of discharged substances, generally less than 1%. These increases are, in fact and in general, significantly lower than the very spatial and temporal variations of substances in the receiving waters and the degree or limit of uncertainty of the measurements themselves.

The results of the mathematical simulations also lead to the conclusion that the choice of secondary treatment of the effluent, instead of primary treatment, would not lead to any visible benefits for the receiving waters.

These circumstances have justified a derogation application to the European Commission, under article 8.5 of the directive 91/271/EEC, for the discharges of the Estoril Coast agglomerations.

In fact, this directive mandate secondary treatment before discharge, unless it can be concluded, by technical studies, that discharges will not deteriorate the receiving waters and the environment, and that a more advanced treatment would not present environmental advantages for the receiving waters.

The European Commission and the European Committee have considered the Costa do Estoril case as absolutely exceptional, and have agreed on a solution of enhanced primary treatment.

The authors consider the Costa do Estoril wastewater system, that includes a long sea outfall, an example that may be reproduced in other situations, namely along large stretches of American and African coasts.
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Document Type: Research Article

Publication date: 2001-01-01

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