Taking into account the legal regulations it is intended to requalify the sewage treatment plant of S. Sebastião. This sewage plant is intended to receive tributaries of 3900 inhabitants on the project horizon (2005). Construction work was completed in 1980 and the sewage treatment plant operated up to the horizon, during which serious structural problems were detected which led to its decommissioning. It is intended to obtain a treated effluent compatible with the use in watering pot in a system