water Article Analysis of the Water–Energy Nexus of Treated Wastewater Reuse at a Municipal Scale Cristina Santos 1,2, * , Francisco Taveira-Pinto 1,3 , David Pereira 4 and Cristina Matos 2,5   Citation: Santos, C.; Taveira-Pinto, F.; Pereira, D.; Matos, C. Analysis of the Water–Energy Nexus of Treated Wastewater Reuse at a Municipal Scale. Water 2021, 13, 1911. https:// doi.org/10.3390/w13141911 Academic Editor: Christos S. Akratos Received: 15 June 2021 Accepted: 7 July 2021 Published: 9 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Civil Engineering, Faculty of Engineering, University of Porto, 4200-465 Porto, Portugal; fpinto@fe.up.pt 2 CIIMAR—Interdisciplinary Centre of Marine and Environmental Research, University of Porto, 4450-208 Matosinhos, Portugal; crismato@utad.pt 3 IHRH—Hydraulic and Water Resources Institute, 4200-465 Porto, Portugal 4 SIMDOURO—Multi-Municipal Drainage System of Porto Region, 4050-626 Porto, Portugal; d.pereira@adp.pt 5 ECT—School of Science and Technology, University of Trás-os-Montes e Alto Douro UTAD, Quinta de Prados, 5000-801 Vila Real, Portugal * Correspondence: csantos@fe.up.pt Abstract: Treated wastewater has the potential to be a feasible alternative to supply non-potable uses and avoid water scarcity in urban areas, but it is important to understand and compare the associated energy consumption and CO 2 emissions. This study presents a comparative analysis of the water–energy nexus associated with the traditional water supply and to the alternative reuse of treated wastewater, both for non-potable purposes. A case study of a Portuguese municipality was considered, regarding golf course irrigation and municipal gardens irrigation. A balance between production and demand was established, and the energy consumption and CO 2 emissions were calculated considering the supply with drinking water and with treated wastewater. Three scenarios were defined to analyze the water–energy nexus for different configurations of the potential end- uses: (1) golf course supply, (2) municipal irrigation supply and (3) simultaneous supply to the golf course and to municipal irrigation. A quality analysis was also carried out by comparing the records from discharged wastewater quality parameters with the limits presented in the legislation for each proposed non-potable use. The results show that all scenarios present significant annual savings from using treated wastewater instead of drinking water from the public network, especially scenarios 1 and 3, that consider the golf course irrigation (water costs decrease by about 60,000.00 EUR/year). Regarding the water–energy nexus, this study reveals that treated wastewater spends less energy on its production and supply and produces fewer CO 2 emissions. The energy savings can reach an average value of about kWh/year, with 5300 fewer kg of CO 2 emitted in the best scenario. Keywords: treated wastewater reuse; water–energy nexus; urban non-potable supply 1. Introduction According to the European Environmental Agency [1], about 80% of Europe’s fresh- water use (drinking and other uses) comes from rivers and groundwater, which makes these sources extremely vulnerable to threats stood by over-exploitation, pollution and climate change. For the Mediterranean region, climate change projections anticipate more extreme heat events and less water [1]. To improve the resilience of the cities and prepare them to this new climate reality, it is essential to provide alternative water sources, such as wastewater reuse. So far, only about 1 billion cubic meters of treated urban wastewater in Europe is reused annually, which corresponds to approximately 2.4% of the treated urban wastewater effluent, or less than 0.5% of annual EU freshwater withdrawals [1]. At a global scale, wastewater reuse has expanded through agriculture and garden irrigation, also reaching potable consumption through direct and indirect reuse. Treated Water 2021, 13, 1911. https://doi.org/10.3390/w13141911 https://www.mdpi.com/journal/water