Water pollution in wastewater treatment plants: An efficiency analysis with undesirable output

The environmental efficiency of 96 Tuscan (Italian) wastewater treatment plants (WWTPs) is investigated taking into account the quality of the outgoing water in terms of pollutant. In this regard, the presence of the residual nitrogen in the outgoing treated water is considered as undesirable output. The efficiency analysis is performed by applying a novel integrated Analytic Hierarchy Process/Non-radial Directional Distance Function (AHP/NDDF) approach, combining the benefits of the two techniques. Similarly to the standard NDDF approach, the suggested model allows to include simultaneously inputs, desirable and undesirable outputs and not to overestimate the efficiency scores. At the same time, the AHP inclusion gives the possibility to directly take into account the decision maker preferences in the weighting system and to encompass some existing directional distance function models as special cases.