This work develops a methodology for the determination of costs associated to products generated in a small wastewater treatment station. The methodology begins with plant units identification, relating their fluid and thermodynamics features to each point indicated in its process diagram. Following, a functional diagram and a formulation are developed in exergetic basis, describing all equations for these points, which are the constraints for optimisation and are used to determine costs associated to products generated in a Small Wastewater Treatment Station - SWTS. The methodology is applied to a hypothetical system based on SWTS plants and presents consistent results when compared to values based on previous experiments and evaluations. (c) 2010 Elsevier Ltd. All rights reserved.