Emergy evaluation of water treatment processes

Abstract

The emergy evaluation (EmE) method is acknowledged to be a holistic approach to account for the primary (solar) energy that generates the renewable and non-renewable resource flows used up by human activities. This paper examines its application and robustness, using four water treatment plants (WTPs) as case studies. We obtained an average unit emergy value for potable water of 1.06 (±0.15) E12sej/m3, which is in accordance with existing literature. Chemicals and electricity were the most important man-made inputs; infrastructure, when accounted for, had a significant but lesser contribution. The application of several emergy-based indicators allowed comparing the ecological performance of water production with other types of resource extraction. These indices showed that WTPs are rather blind to economic markets and they exerted a low pressure on local non-renewable resources. A critical analysis of current EmE procedure highlighted the relative low accuracy of the method compared to Life-Cycle Assessment (LCA), when man-made inputs are predominant, as well as the complementary goals and scopes of the two methods. Methodological improvements in the classification and treatment of the emergy associated with man-made inputs are necessary to make EmE indicators more straightforward and robust.