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Water usage in cities tied to greenhouse gas emissions

In Changzhou, China 10% of the city’s energy footprint is related to water usage. Through strategic water conservation efforts, policy-makers can simultaneously conserve water and energy, save taxpayer money, and reduce climate change impacts.

Most people intuitively understand the importance of conserving water, especially those living in drought-prone areas. Less obvious is the connection between saving water and saving energy. A study recently published in Environmental Science and Technology shows that water saving actions in one Chinese city could cut energy use in the urban water system by 2-6%.
            Water and energy are inextricably linked to each other in modern communities. Energy is required to extract, treat, and transport water to make it safe and accessible to homes and businesses. Conversely, it takes water to produce energy, especially for cooling steam electric power plants and for fuel extraction and refining. Treating and pumping water through cities contributes an estimated 2-3% of global greenhouse gas emissions. Nor is energy cheap. Municipalities typically spend 25-60% of their budgets to supply energy for their city’s water infrastructure.
            Reducing greenhouse gas emissions through water conservation is under studied in the climate change mitigation field. A team of researchers from China’s Nanjing University devised a water flow analysis of one city’s entire water system. They investigated the relative climate impacts from urban water use in residential, industrial, agricultural and public sectors. The team’s goal was to measure the amount of energy used by each part of the water system and to compare water conservation options that could lead to the greatest energy and greenhouse gas reductions.
            The study area for the water flow analysis is Chanzhou, a city of 3.6 million people located in China’s Yangtze Delta region. Water, energy, and material flow analyses are common techniques to study urban metabolism, a field of research that aims to understand how resources move through a city. Water used within the city comes from ground water, surface water, and rainfall. Water then leaves the city through treated or untreated wastewater systems. Water also constantly moves about within the city through processes such as extraction, transportation, end uses, wastewater collection and treatment.
            The researchers gathered data from published literature and interviews with government officials to estimate the energy usage associated with every process in the city’s water system. The resulting water flow diagram shows an intricate web of boxes - representing processes such as water acquisition, treatment and distribution - connected by lines representing water transport through the city. 
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Figure 1. Estimated energy consumption of urban water flow of Changzhou (Zhou et al. 2013)
The researchers found that the whole water system represents 10% of Chanzhou’s total energy consumption. Industrial water usage constitutes 72% of energy usage within the water system, followed by residential water usage at 24%. These sectors require large amounts of energy primarily for water heating and wastewater treatment. The public sector and agricultural sectors only constitute 5% and 1% of water-related energy consumption because they use relatively small amounts of energy for water acquisition and distribution.
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Figure 2. Estimation of annual energy consumption of the urban water system (Zhou et al. 2013)
Next, the researchers measured the potential energy reductions that different water conservation activities could produce in Chanzhou. They constructed four different scenarios: (1) reuse 10% of wastewater for landscape irrigation; (2) harvest rainwater for toilet flushing; (3) reduce household water use 10% below national average; (4) reduce industrial water use to 15% below national average. The results show that water conservation in the industrial sector could reduce the total energy used for water by 7.2%, followed closely by rainwater harvesting at a 6% reduction. The scenarios that reduced quantity of water going to the energy-intensive wastewater treatment plants performed especially well in reducing energy consumption.
            Actually implementing such water conservation programs, however, is difficult due to China’s low water prices. The researchers suggest that Changzhou’s policy-makers adopt a tiered pricing structure so that heavy water consumers in industrial and residential sectors would have an incentive to conserve water. Likewise, policy-makers could update building codes so that new or renovated buildings include rainwater harvesting and wastewater reuse technologies. The municipality could also modernize public water infrastructure to reduce water leakages and aid in wastewater reuse.
            This study shows that saving water not only reduces stress on natural water systems, but can also produce energy savings and greenhouse gas reductions at the same time. Whereas typical water-energy studies focus on individual water systems (e.g. a single waste water treatment plant), this water flow analysis takes a bigger picture view.  Studying the entire city’s interconnected water systems provides information on each sector’s relative climate impact, thus helping urban planners and policy-makers weigh water and energy saving scenarios against each other. 
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Original Paper:
Zhou Y, Zhang B, Wang H, Bi J. (2013) “Drops of Energy: Conserving Urban Water to Reduce Greenhouse Gas Emissions.” Environmental Science & Technology. 47(19): 10753–10761.

DOI: 10.1021/es304816h


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