Greenhouse gas emissions of water supply and demand management options (2008)

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This project by the Environment Agency examines differences in CO2 emissions associated with a variety of options for supplying and using water more efficiently. Based on a newly developed methodology, the study assesses new water supply options, working with an existing water supply network, plus methods and products to reduce and manage households’ water demand. It reveals that 89 % of CO2 emissions in the water supply system are caused by domestic water use, whereas public water supply and treatment account for 11 %. It concludes that simple demand management measures, particularly those which reduce hot water use, can have significant potential to reduce energy and CO2 emissions.

Science at the

Environment Agency

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Executive summary

Reducing greenhouse gas emissions is one of the key challenges of our generation. The UK Government has recognised the necessity for significant reductions. The Stern Report, Energy White Paper and Climate Change Bill provide the scientific and legislative impetus to mitigate and adapt to the effect of emissions across all sectors of the UK.

The water industry must play its part and reduce its greenhouse gas footprint. Water typically requires treatment prior to use and on its return to our environment. It is pumped and pressurised to reach our homes. All of these activities require energy and therefore result in greenhouse gas emissions.

The water industry contributes 0.8 per cent of annual UK greenhouse gas emissions. However, the emissions that result from heating water in the home increases this figure to 5.5 per cent.

This project examines the difference in greenhouse gas emissions associated with a variety of options for supplying water and using it more efficiently. We assess options for new supplies of water, working with an existing water supply network, plus methods and products to reduce and manage households’ water demand. This study does not include any assessment of other environmental, social or economic costs and benefits.

We provide an evidence base and framework to inform our understanding of water resource and carbon impacts, underpinning one of five modules of our new water resource strategy for England and Wales, planned to be published in December 2008. This is also one report in a wider two-year project looking at the potential for energy efficiency and carbon reduction across the entire water industry.

Throughout the report we refer to greenhouse gas emissions as carbon dioxide equivalent (CO2e). The cost of CO2e follows the Defra Shadow Price of Carbon guidance; £26 per tonne in 2008, thereafter rising by two per cent each year. This report does not include any assessment of other environmental, social or economic factors. We recognise that quantifying greenhouse gas emissions is just one factor to be considered in the overall decision-making process.

Our key findings

Science Report –– Greenhouse gas emissions of water supply and demand management options

89 per cent of carbon emissions in the water supply – use – disposal system is attributed to “water in the home” and includes the energy for heating water (excludes space heating), which compares with public water supply and treatment emissions of 11 per cent.

Simple demand management measures, particularly those which reduce hot water use, have significant potential to not only promote water and energy efficiency but also to reduce the carbon footprint of the water supply – use – disposal system. For example, moving to full water metering across England and Wales could reduce annual emissions by 1.1 – 1.6 million tonnes of carbon dioxide per year. Moving to full metering in areas of serious water stress could potentially reduce annual emissions by between 0.5 – 0.75 million tonnes CO2e per year.

All supply side measures result in an increase in carbon emissions (we assume new schemes are implemented to meet rising demand rather than replacing existing assets). There is often a wide range in carbon emissions associated with water supply schemes of a similar type, and therefore overlap between different types of schemes is common. For example, medium to large reservoirs and indirect effluent re-use can have similar carbon emissions per volume of water supplied, dependant on scheme design. To select the lowest carbon solution requires a scheme by scheme assessment.

Most demand management options, for example water metering, have low operational carbon emissions, the exception being retrofitting of household rainwater harvesting and greywater recycling to existing homes. Data concerning the energy use of these two techniques is scarce and requires further research.

Combinations of demand management options, even those including rainwater harvesting in new homes, offer larger water savings compared to individual water efficiency options and still compare favourably to supply side options in terms of overall lower carbon emissions.

Current legislation continues to require the sustainable management of rivers and groundwater. In some cases this will mean that water abstraction will need to be reduced to ensure a sustainable water environment, resulting in a reduction in the water available for supply. To offset this effect, companies are investigating alternative sources of water. Our work indicates this will increase carbon emissions overall. We believe that widespread implementation of demand management measures can offset or further reduce overall emissions, as well as reducing the need for some of these new supplies in the first place..

For example an initial assessment using South East data indicates that the lowest carbon cost is delivered by the scenario which includes both demand management for two million homes and 18 new supply schemes, delivering a 14 per cent carbon cost saving compared to business as usual.

We acknowledge that future technological developments may offer greater energy and carbon savings to both water supply and demand management options. The extent of these savings has not been looked at in this study due to the level of uncertainties involved.

In future, policies need to consider the greenhouse gas emissions across the whole of the water system, i.e. emissions arising from both the water industry and the use of water by consumers. Policy-makers also need to recognise the potential overlap with the aims of energy efficiency initiatives and ensure there is no double counting of carbon reductions.

Water Resource Management Plans require water companies to assess their carbon footprint related to water supply only and not the whole life cycle costs. Water companies planning future water resources options through the 25 year planning period are required to build-in the shadow price of carbon to the economic analysis. However, this typically relates to the direct energy costs of water production and embedded carbon for construction activities.

This current approach constrains the options appraisal as it fails to take full account of the life cycle costs of carbon and particularly the positive impact of demand management related to water use in the home as well as wastewater activities. This approach has therefore been unable, to date, to incorporate the largest and most significant aspects of carbon accounting within assessments between building new resources and managing demand.

The life-cycle emission model

We assessed new water supply options and demand management options working with an existing water supply network. The options considered included:

  • Supply options – storage reservoirs; regional water grids via transfer pipelines; desalination plant to make seawater and brackish water drinkable; effluent re- use; groundwater and river abstractions.
  • Demand management options – water saving devices for toilets, showers and baths; water meters; water efficient domestic appliances; rainwater collection systems; grey-water recycling (i.e. water from showers, baths and sinks used for toilet flushing); water mains leakage reduction.We developed our methodology in line with Defra guidance on the Shadow Price of Carbon and our guidance on water resources planning. Present value techniques are used to compare options in terms of their carbon cost as CO2e versus water delivered or saved over a planning horizon of 60 years.We model the life-cycle impact of individual options by calculating the greenhouse gas emissions associated with construction, manufacture, installation, maintenance and operation.
    • For a unit of water we evaluate the current carbon emissions and cost of carbon.
    • For water supply options we calculate the scheme carbon cost, e.g. new reservoir, new treatment facilities – clean and wastewater, and increased capacity in the distribution network.
    • For demand management options we calculate the carbon cost to introduce and operate the measure and the carbon savings from lower water demand.The model, built in MS Excel® for ease of use, is intended for high-level carbon cost appraisal in advance of more detailed study. It can be easily tailored to suit regional and scheme specific data as appropriate, and should help in our review of water company PR09 plans.