Energy consumption for water use cycles in different countries: A review Muhammad Wakeel a , Bin Chen a,b,⇑ , Tasawar Hayat b,c , Ahmed Alsaedi b , Bashir Ahmad b a State Key Joint Laboratory of Environmental Simulation and Pollution Control, School of Environment, Beijing Normal University, Beijing 100875, China b NAAM Group, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia c Department of Mathematics, Quaid-i-Azam University, 45320 Islamabad, Pakistan highlights  Energy consumptions for water use recycles in different countries were investigated.  Different case studies in context of energy-water nexus were reviewed.  Various models for energy consumption in water sector were examined. article info Article history: Received 20 February 2016 Received in revised form 24 May 2016 Accepted 22 June 2016 Keywords: Water use cycle Energy-water nexus Energy intensity abstract Energy and water are essential, inseparable, and inextricably linked resources that are recognized as indispensable inputs to modern economic and national security. Understanding the nexus of energy and water may help minimize energy and water consumption and reduce environmental emissions. This paper presents a literature review of the current status, research gaps, dependence factors, and possible improvement measures to reduce energy consumption in the water sector. The energy use in the extraction, desalination, pumping, collection, and wastewater treatment stages of the water sector from a global perspective in various developed and developing countries is investigated. The results show that water supply and wastewater services are energy intensive globally owing to old infrastructures and technologies. Energy intensities in the water sector vary with technology, geographical distances, and demographic conditions. The methodologies for energy accounting for various water uses are also summarized. Finally, knowledge gaps, challenges and policy implications are identified by synthesizing previous research. Ó 2016 Elsevier Ltd. All rights reserved. Contents 1. Introduction ......................................................................................................... 869 2. Energy-water relationship .............................................................................................. 869 2.1. Water in energy................................................................................................. 869 2.2. Energy in water ................................................................................................. 870 2.2.1. Abstraction and conveyance of water ........................................................................ 870 2.2.2. Raw water treatment and supply ........................................................................... 871 2.2.3. End use of water ......................................................................................... 872 2.2.4. Wastewater collection and treatment ........................................................................ 873 2.2.5. Effluent discharge, reuse, and recycling ...................................................................... 873 3. Methodologies for energy accounting of water use cycles .................................................................... 874 3.1. Bottom-up models............................................................................................... 874 3.2. Top-down models ............................................................................................... 874 3.3. Hybrid models .................................................................................................. 876 4. Discussion and perspective ............................................................................................. 876 5. Policy implications .................................................................................................... 880 http://dx.doi.org/10.1016/j.apenergy.2016.06.114 0306-2619/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: No 19, Xinjiekouwai Street, Beijing 100875, China. E-mail address: chenb@bnu.edu.cn (B. Chen). Applied Energy 178 (2016) 868–885 Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy