Although there are only a few wastewater stabilization lagoon plants in Portugal, this treatment system is particularly attractive because of the low energy requirements and because its operation is simple and the operating costs are small. Furthermore, the climatic factors are also favourable.

In this context, the main objective of the study, which was undertaken in the Frielas experimental complex, on the outskirts of Lisbon, was the development of an appropriate stabilization lagoon design procedure for application in Portugal, regarding the raw wastewater characteristics and the climatic factors.

The results obtained include the physical, chemical and biological characteristics and the flow rates of the influent and effluent wastewaters from the lagoons. The following main conclusions were reached:

  • the lagoons are normally over-designed, in particular the anaerobic and the facultative lagoons;

  • the soluble BOD concentration in the effluent, in general, meets the discharge requirements;

  • due to the presence of algae in the effluent, the TSS concentration is higher than the corresponding concentration in a secondary effluent.

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