Wastewater systems are a structural part of public utilities and as such are vital to the general wellbeing, public health, economic activities and environment protection. These services, with the primary
objective of protecting the interests of users, shall be governed by principles of universality of access,
continuity and quality of service, efficiency, equity and sustainability of price.
The aim of this paper is to contribute for the improvement of the Portuguese performance assessment system,
through the development and application of a complementary methodology to define sectoral and global indices of sanitation service performance in order to evaluate the sustainability of a given wastewater utility. This methodology allows a truly quantitative evaluation in which each performance indicator represents a criterion to be considered and judiciously weighted. The sectoralindices are calculated as a weighted linear combination of the normalized scores of each performance indicator, which is one of the most common aggregation procedures available in the context of multicriteria evaluation. The criteria normalization process essentially based on fuzzy sets defined for each indicator. A sensitivity analysis of indices values to different weighting methods was also performed.