| Resumo: | SANEST is a public sanitation company that manages a wastewater treatment plant located at Guia, on the west coast of Lisbon, Portugal. This company collects and treats the sewage of four municipalities with an estimated 750 000 population equivalent, thus being one of the biggest sanitation companies in Portugal. A Decision of the Commission 2001/720/CE conceded SANEST derogation, exempting it to apply less than secondary treatment to wastewaters discharged into the Atlantic Ocean from the four agglomerations. This decision was supported on a large monitoring program, presented to the EU, and set up by SANEST. It surveys the impact of the effluent disposal and includes measurements of physical, chemical, biological and microbiological properties in the effluent and in the receiving waters. This paper presents methods and results for the effluent chemical and microbiological quality as well as for the receiving waters and an ichthyofauna survey, and resumes an eleven year situation, with the preliminary wastewater treatment before effluent disposal. The WWTP results correspond to medium load urban effluents without treatment with temporal variability related to flood fluctuations. In the receiving waters almost legal values are respected and the plume of the outfall is only identifiable by faecal bacteria in the vicinity of the discharge. The fish community, in particular benthic species, has revealed a slight degradation probably due to the fact that pollutants tend preferentially to accumulate on sediment. The treatment plant is being upgraded to fulfil, by May 2009, an advanced primary treatment level that includes disinfection during the bathing season to fully observe the European Commission Decision 2001/720/EC.
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