Process analysis applied to water reuse for a “closed water cycle” approach

Abstract The reuse of wastewater is a key factor in a closed water cycle approach, in which wastewater is treated and then reused. This approach is both mandatory for the development of dry areas and necessary for the sustainability of industrialized countries in terms of environmental impacts and resource preservation. Although there are some virtuous examples of water reuse projects in the world, there is still much to be done, especially in terms of incentives and economic viability. Aim of the present paper is to give thermodynamic and engineering elements in order to develop an economic incentive to promote wastewater reuse and to adopt the closed water cycle approach. At this scope a techno-economic analysis of the civil wastewater depuration and reverse osmosis treatment of the secondary effluent is presented, by using the typical approach of the chemical engineering. The cost of the treated water in relation to the fundamental parameters of the plant is calculated together with an “energy based” incentive, evaluated through the efficiency of the state-of-the-art desalination process. This last can make a reuse project economically feasible on the basis of rigorous thermodynamic considerations. These latter give a universal character to the incentive calculation and also reward the process optimization towards the goal of lowering the carbon emissions. The validity of the proposed method is evaluated through the analysis of three wastewater treatment and reuse projects at different scale. The results show how it is possible to obtain a positive Earnings Before Interests and Taxes (EBIT) for plant productivity above the 200 m3/day, by including the proposed incentive in the Business Plan of the integrated plant of Water Treatment and Reuse.