This paper consists on a global environmental analysis of a waste water treatment (Conventional Activate Sludge System, CAS, designed for 13,200 population equivalent) and some possible additional tertiary treatments allowing water reuse to that purified waters (UF and immersed and external Membrane Biological Reactors, MBR). The environmental assessment of these water treatment technologies has been realized by means of the Life Cycle Assessment (LCA) technique, in order to establish with a broad perspective and in a rigorous and objective way the technology that provokes the lowest environmental load. The software SimaPro 5.1, developed by Dutch PRe Consultants, has been used as the LCA analysis tool, and three different evaluation methods-CML 2 baseline 2000, Eco-Points 97 and Eco-Indicator 99-have been applied. The results show that tertiary treatment does not increase significantly the environmental loads but provide new uses for that purified water, thus justifying the intensive use of water reuse techniques in water scarce areas.