Water–energy–greenhouse gas nexus of urban water systems: Review of concepts, state-of-art and methods

Highlights

• Water and energy are highly entwined.

• Direct energy use is relatively well accounted and studied than indirect or embodied energy use.

• Energy use of water end use is comparatively overlooked.

• The main assessment technique is Life Cycle Assessment (LCA).

• Lack of a holistic and systemic framework to capture the water–energy dynamics in urban water system.

Abstract

Water supply and wastewater services incur a large amount of energy and GHG emissions. It is therefore imperative to understand the link between water and energy as their availability and demand are closely interrelated. This paper presents a literature review and assessment of knowledge gaps related to water–energy–greenhouse gas (GHG) nexus studies in an urban context from an ‘energy for water’ perspective. The review comprehensively surveyed various studies undertaken in various regions of the world and focusing on individual or multiple subsystems of an urban water system. The paper also analyses the energy intensity of decentralized water systems and various water end-uses together with the major tools and models used. A major gap identified from this review is the lack of a holistic and systematic framework to capture the dynamics of multiple water–energy–GHG linkages in an integrated urban water system where centralized and decentralized water systems are combined to meet increased water demand. Other knowledge gaps identified are the absence of studies, peer reviewed papers, data and information on water–energy interactions while adopting a ‘fit for purpose water strategy’ for water supply. Finally, based on this review, we propose a water–energy nexus framework to investigate ‘fit-for-purpose’ water strategy.

Introduction

Drinking water scarcity is one of the main challenges faced worldwide as between 1.4 and 2.1 billion people lack adequate clean water supply (IPCC, 2008). The effect of water scarcity is more pronounced in urban areas where population is expected to increase to about 70% by 2050 (CSIRO, 2011). According to UNESCO (2012) estimates, about 75% of the world's population could face water scarcity in future as demand for good quality water in urban areas will increase around the world. In the USA, approximately 1.3 billion cubic meter of water which is equivalent to 30% of all runoff is extracted daily for feeding the entire population of which 30% is used for consumption (Webb and Johnson, 2009). The per-capita water demand during the last 25 years in US has decreased by adopting water efficient use measures. In Australia, an arid continent that has high rainfall variability make the problem of water scarcity more acute given additional concerns such as growing population, urbanization and climate change. However, meeting growing water demand is a significant only problem, but ensuring sustainability is of equal concern. The increasing demand for good quality water causes increased water extraction, conveyance, treatment and disposal which in turn demands more energy use and associated greenhouse gas emissions (GHG). Retrofitting of existing infrastructure is needed in developed countries to sustain the high quality of urban water supply while developing countries have to build new infrastructure to meet the growing water demand (Grant et al., 2012). This will further escalate the energy needs of urban water systems in the future.

In this review, we focused our attention on studies carried out in the USA, UK and Australia which account for the majority of references found in the literature. Adaptation strategies in urban water systems like implementing new and decentralized water systems in response to increased water demand and climate change should minimize the impact on the environment. In recent years, there is increased emphasis on the energy nexus of urban water systems and associated environmental impacts in Australia, where the energy supplied for the urban water systems is electricity which is produced by coal combustion with large amounts of GHG emissions. Around 5% (290 million tons) of total annual GHG emissions in the U.S.A. originate in the water sector (Sattenspiel and Wilson, 2009). In the UK, the annual GHG emission in 2006/07 from energy use of water sector is 5.03 million tons of CO_2e which is almost an additional one million ton over previous years (Environmental Agency, 2008). In this context, it is desired to look at the energy intensity and associated GHG emissions of urban water systems.

The objective of this paper is to conduct a comprehensive review of studies conducted in the area of water–energy–greenhouse gas (GHG) impacts of urban water systems and identify existing knowledge gaps in this relationship. This paper looks at existing urban water system and its drawbacks, the nexus between water and energy in water systems, studies pertaining to the water–energy nexus and methodology and tools used in these studies. It also draw attention to existing knowledge gaps in this field and proposes a new approach for assessing the water–energy linkage in urban water systems.