Citation: Castro Oliveira, M.; Iten, M.; Matos, H.A. Review on Water and Energy Integration in Process Industry: Water-Heat Nexus. Sustainability 2022, 14, 7954. https:// doi.org/10.3390/su14137954 Academic Editor: Sergio Nardini Received: 12 May 2022 Accepted: 20 June 2022 Published: 29 June 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 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/). sustainability Review Review on Water and Energy Integration in Process Industry: Water-Heat Nexus Miguel Castro Oliveira 1,2, * , Muriel Iten 1 and Henrique A. Matos 2 1 Low Carbon and Resource Efficiency, R&Di, Instituto de Soldadura e Qualidade, 4415-491 Grijó, Portugal; mciten@isq.pt 2 Centro Recursos Naturais e Ambiente (CERENA), Departamento de Engenharia Química, Instituto Superior Técnico, Universidade de Lisboa, Avenida Rovisco Pais 1, 1049-001 Lisboa, Portugal; henrimatos@tecnico.ulisboa.pt * Correspondence: dmoliveira@isq.pt; Tel.: +351-22-747-1950 Abstract: The improvement of water and energy use is an important concern in the scope of im- proving the overall performance of industrial process plants. The investment in energy efficiency comprehended by the most recent sustainability policies may prove to be an effective response to the fall of energy intensity rates associated with the economic crisis brought by the COVID-19 pandemic. The improvement in water efficiency may also prove to be a potential approach due to its interdependencies to energy use, whose exploitation comprises part of the study of the water-energy nexus. Waste heat recovery and water reclamation practices have been exploited to improve water and energy efficiency. A specific method designated “Combined Water and Energy Integration” has been applied to water recycling as both an additional water source and a heat recovery source in a set of water-using processes. In scientific and industrial domains, there is still a need for integrated approaches of water-using and combustion-based processes for overall water and energy efficiency improvements in industrial plants. In this work, an innovative approach for a simultaneous im- provement of water and energy use is proposed based on process integration and system retrofitting principles. This proposal is based on the delineation of two innovative concepts: Water and Energy Integration Systems (WEIS) and Water-Heat Nexus (WHN). A review on existing technologies for waste heat recovery, thermal energy storage and heat-driven wastewater treatment is performed, following a conceptualisation design. Keywords: heat recovery; thermal energy storage; wastewater treatment; Water and Energy Integration Systems; Water-Heat Nexus 1. Introduction Water and energy efficiency are prominent concerns for industry and the improvement of the use of both of these resources is prominent in sustainability policies established by each country worldwide [1]. The adoption and planning of water and energy manage- ment strategies is essential to securing an efficient and sustainable operation of plants [2]. Considering the inevitable use of such resources in industries, water and energy use have obligatory interdependencies. Such interdependencies are encompassed in the concept of the water-energy nexus [3–6]. The improvement of energy efficiency in industries has been a subject of study by several authors (namely regarding the identification and analysis of improvement measures [7–10], their application into industrial processes factories [11] and the optimization of single equipment [12]). Within the scope of energy efficiency improvement, the authors of the present study have assessed potential water efficiency improvements, namely measures which simultaneously improve water and energy use [13], through simulation models [14]. Water efficiency and energy efficiency are concepts that have been gradually intro- duced in the sustainability policies adopted by each country in the world. In the European Sustainability 2022, 14, 7954. https://doi.org/10.3390/su14137954 https://www.mdpi.com/journal/sustainability