The challenge of supplying water and energy required for food production and development while mitigating climate change and adapting to its consequences has been termed the Energy Water Nexus. Water, energy, greenhouse gas emissions and climate change are interlinked through a series of relationships. The water sector is a significant contributor to greenhouse gas (GHG) emissions through several routes including embedded emissions in capital equipment, energy consumption during drinking water treatment, water distribution and wastewater treatment, and direct emissions of methane and nitrous oxide from treatment. One effect of climate change in many parts of the world is likely to be a change in rainfall patterns, which may further restrict the supply of groundwater and surface water in areas where it is already limited and even reduce availability in comparatively water-rich countries, such as the UK. At the same time, many countries have increasing populations and demands for water. This could create positive feedback by driving the use of other sources, such as desalination of sea water, which may have higher energy demands and greenhouse gas emissions. Within Europe and elsewhere, the aims of different pieces of environmental legislation can come into contact. For example, the Water Framework Directive (WFD) requires higher standards for water that is discharged into surface water bodies, further raising the energy intensity of wastewater treatment. In the longer term, the WFD may result in higher quality source waters and reduce the energy requirement for drinking water treatment. However, the effectiveness of the catchment management measures currently being taken to deal with diffuse agricultural pollution is uncertain. This report will review and elaborate on these issues as they relate to municipal water supplies, and consider some of the options for sustainable water systems in the future.
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