The objective of this research is to understand the NIMBY phenomenon, through the study of the social acceptability of three waste water treatment sites in the suburbs of Paris. The request emanates from a public organization, who fear NIMBY reactions among local residents, and wish to prevent the phenomenon by establishing communication with the local population. The NIMBY (Not In My Back Yard) refers to a phenomenon usually observed around environmental facilities, when residents recognize the utility of them, perceive the benefits for the environment, but do not wish to see them established in their close environment. From an individual point of view, this can be easily understood, but from a global point of view, the problem appears insolvable, when no one, nowhere, seems ready to accept them in their neighbourhood. The case of waste water treatment plants is specific, because their localization depends on urban demographic development, and on the configurations of the land. Therefore, the choice of their location is limited. Nevertheless that does not constitute a valid argument for most refractory people. This applied study is carried out on three different sites, one is a waste water treatment plant that is being extended, another one is about to be extended, and a third one has yet to be constructed. The sites also differ by their environmental characteristics: residential or industrial, with a lot of greenery or multi-harmful. The method consisted in carrying out individual semi-structured interviews amongst the bordering populations, on the topics of perception of quality of life at the scale of the neighbourhood, knowledge of and experience with the site, perception of harmful effects, as well as personal implication towards waste water treatment and, and on reduction of harmful effects. A last part takes into account personal factors which can modulate environmental assessment. Ongoing data analysis allows for comparisons between the three sites, on environmental and temporal levels. The results are put in relation with objective factors such as the view of the site and microclimatic aspects. They will be discussed in light of theoretical contributions in social and environmental psychology.