Nicholas Apostolidis, Director, GHD
Introducing water and energy footprint considerations into decision making provides water managers an opportunity to drive better water and energy efficiency outcomes. In Australia severe water shortages, combined with community concerns with climate change, have driven water managers to look closer at the nexus between water and energy use. The two are closely related and will play an important role in the shape of the water industry in future in Australia and around the world.
The following paper provides a snapshot of the energy and water footprints associated with different elements of the water cycle in Australia. The objective is to provide a more holistic view of the nexus between energy and water. It is important to recognize however that focusing on energy and water efficiency alone is not enough. Environmental, economic and social issues must also be considered, otherwise it will lead to sub optimal outcomes and will fail to gain appropriate community support.
The severe water shortages that have been experienced in Australia over the past decade have driven water utilities to consider and implement a range of higher energy sources of water such advanced recycling and desalination. One of the challenges for water utilities in adopting higher energy sources of water is gaining community acceptance for such schemes as they are seen by the community as adding to the energy footprint of the country and thus contributing to the climate change problem.
The question some managers are now asking is; have we solved one problem and unintentionally created another one? There is a strong correlation between energy and water use and it important that in delivering solutions in a climate change world proper consideration is given to relationship between water and energy use. In the last decade there has been some excellent work done in this area in Australia.
This paper provides a perspective of the energy use across the water cycle for different technologies to aid water planning decision making.
Water and energy footprints in perspective
To give readers some perspective the world water footprint is estimated to be about 7500 Gm3/yr. Australia’s water footprint is about 25 Gm3/yr. The countries with the three biggest water footprints are India, China and US with 1000, 900 and 700 Gm3/yr respectively. The majority of the water is used by agriculture in most countries accounting for about 70% of their entire water consumption. Figure 1 summarises the water footprints of the top 20 countries.
Figure 2 provides a break up of water is used in Australia. As can be seen the majority of the water is used in agriculture. Urban use, the focus of this conference represents about 15%. This is not unique to Australia, a similar pattern is observed in most of the developed countries around the world. Australia is recognised as one of the driest continents on earth. Despite this Australia uses less than 10% of the freshwater runoff from the continent. There are regions within Australia with abundant sources of water especially the northern tropical areas that are located thousands of kilometres from major urban and irrigation areas. Being a major exporter of agricultural products, Australia is in fact a net exporter of water, the third largest after USA and Canada.
Water uses within urban areas vary considerably depending on land use, age of system and management practices. In general residential use makes up about 60-70% of the water use in most major Australian cities. Figure 3 shows how water is used within a typical household.
The great story in Australia over the past decade has been the ability of most of its water utilities to achieve significant reduction in their water footprint through a comprehensive suite of demand management practices and supply side initiatives. Some of the best practices in the world can be found in Australia.
Table 1 compares the domestic water use per property for the major urban cities over the past five years. As can be seen significant reductions have been achieved in most of Australia’s major cities. In the case of Brisbane the reduction is almost 50%.
The world energy footprint is about 500,000 petajoules (PJ, 1015 joules). Australia’s footprint is about 6,500 PJ. The countries with the three largest energy footprints are the US, China and Russia consuming 110,000, 85,000 and 33,000 PJ respectively. In fact the top 20 countries contribute 80% of the world’s entire energy footprint. Australia is the 18th highest energy consumer in the world.
Figure 4 summarises the energy footprints of the top 20 countries.
Contrary to popular perceptions the water industry is a relatively modest user of energy. (An often misquoted statistic is that in California 25% of the energy is used for transporting water. The true figure is closer to 7%. The 25% figure includes energy associated with other end uses such as water heating, food processing, washing clothes, etc. Even allowing for these types of uses, it is hard to find reliable statistics that justify the 25% figure. The only reference to the 25% in the reference paper by Wolff 2004, is the energy use in transporting water in California is equivalent to 25% of the energy use in the state of New Mexico!)
In Australia the water sector uses about 45 PJ for transporting water representing about 1.4% of all energy use. This covers the entire water cycle form source to final release to the environment and all uses of water including urban, agriculture, industrial and mining. Whist the subject of most attention, the amount of energy used to supply water to the urban water sector in Australia is only about 12 PJ. This represents less than 0.5% of the total urban energy use. In a recent study (Kenway et al. 2008) noted that if all the additional water supplies were sourced from desalination over the next 20 years the energy use will still be less than 1% of the total urban energy use.
Figure 5 provides a break up of energy use in Australia across different sectors. As can be seen, residential energy use is much greater than the energy used to supply water. Figure 6 provides a break up of energy end uses for a typical household in a moderate climate location in Australia. Whilst energy end uses can vary considerably for different climatic regions, in general, energy used in heating water, heating and air conditioning dominate all other household uses.
In the study by Kenway et al, they concluded that a 15% savings in hot water heating across Australia will offset the entire energy use to supply water to the urban water sector.
Climate, topography urban form, technology and management practices all play a significant role in the amount of energy used across the water cycle. Figure 7 shows a profile of energy use across the water cycle in urban Australia. The bar graphs under the profile show the range of energy use for different applications.
As can be seen the energy use can vary significantly across the water cycle. Excluding energy use within the household the energy use across the complete water cycle can be as little as 0.5 kWh/m3 and as high as 15 kWh/m3.
For example water stored in the mountains gravitating to an urban centre with a predominantly gravity based collection system that discharges its effluent to the ocean with minimal treatment will have a relatively small energy footprint close to the lower value referred to above. Conversely water sourced from the ocean that is desalinated, pumped to the urban area over a long pipeline to the users who then discharge to a sewage collection that pumps to a treatment plant which further treats the wastewater to an advanced level and then recycles the product to the urban area via further pumping will have a much greater energy footprint approaching the upper value.
The following sections discuss the potential variability in energy use for different components of the water cycle.
Increasing population, urbanization and climate change have exhausted most of the readily available source of fresh water. (The world’s freshwater resources are less than 3% of all the water on earth. Of this about 2% is stored in icecaps and 1% is underground. The earth’s surface fresh water is only 0.3% of all water.) To meet future water demands communities are looking further afield for their water supplies and increasingly using new technology such desalination or on-site solutions such as rainwater tanks to secure water closer to the urban centers. This trend is certainly occurring in Australia and increasingly throughout the world.
The past approach to securing water has been to transport it from water rich regions to the demand centre. Schemes such as the Kalgoorlie Pipeline (one of the longest pressure pipelines in the world) in Western Australia, North-South Canal in China, and the California State Water project are good examples of major water transportation schemes. With technologies in desalination and advanced recycling improving and becoming more reliable and cost effective the need to transport water over long distances is being challenged and we now have a much wider range of opportunities that what was possible 20 years ago.
Table 2 outlines several different water supply schemes and their energy use. (The figures quoted in this and subsequent tables for energy end use are average figures to give the reader an appreciation of the order of magnitude. The actual data showed significant variability.) The table also includes some more recent applications such desalination, rainwater harvesting and grey water recycling. Contrary to popular perceptions the energy use associated with some of the on-site schemes are much greater than traditional centralized schemes relying on surface sources close to the urban areas. A South East Water report found that a key factor in the high energy use for water sourced from rainwater tanks is the small pumps used to pump water from the tank for household use. The pumps are very inefficient (less than 20% in some cases) resulting in high energy use per unit of water supply. With better pump design it is conceivable that in future as the demand for rainwater harvesting increases so will the pump efficiency.
The energy use for Melbourne, Sydney and Adelaide includes the harvesting, transportation, treatment and distribution. As can be seen Adelaide uses more energy to supply water because it has to pump its water much longer distance from its primary source (Murray River) to it customers.
The energy use for the treatment of water is relatively low in comparison with the rest of the water cycle.
Table 3 provides some typical values for different types and level of treatment. As can be seen increasing use of membranes to meet more stringent water quality standards has increased the energy use in treating water.
Household End Use
The amount of energy used in handling water within the household is significantly greater than energy used outside the household. Table 4 provides some estimates of energy use for different uses within a household.
Wastewater Collection & Treatment
The energy use for the collection of wastewater is very much topography dependent. In the case of wastewater treatment the desired water quality regulations and final use dictate the level of treatment. Many of the early wastewater collection systems were gravity based involving extensive networks of large diameter sewer tunnels and gravity pipes. The advent of more efficient and cost effective sewage pumping technology combined with urbanization saw much greater use of sewerage pump stations. Previously difficult to service areas such as flat areas with high groundwater tables became more feasible to service using shallower trunk sewers and more pump stations. This trend continues today where the most difficult terrain can now be serviced using either vacuum or pressure systems. Table 5 provides typical energy use values associate with different types of collection systems
Figure 8 illustrates the variability in energy use associated with different levels of treatment processes. As can be seen the increasing use of membranes has increased the energy use.
Effluent Discharge, Reuse & Recycling
The energy associated with different effluent management practices is stark. Clearly the lowest energy use is to discharge treated effluent to waterways be they rivers, estuaries or the ocean. The energy needed is related to the level of treatment that is required to meet environmental and social requirements and possibly some pumping to the outfall location. Community and environmental concerns with waterway discharges have forced many utilities in Australia and most of the developed countries in the world to consider alternatives such as re-use for irrigation of public space, agriculture, cooling right through to potable re-use. As can be seen in Table 6, the community’s desire for more recycling is pushing higher energy use. In response to the drought and the need to protect water quality there have been many well meaning initiatives to save water and recycle. Whilst many of these schemes have delivered water savings and improved water quality many have resulted in higher energy use and greenhouse gas emissions. In Australia there is widespread community support for recycling within the household using on-site technologies. However given the level of treatment needed to produce an acceptable water quality for safe use many of the on-site technologies require much higher energy to produce water than more traditional regional schemes.
Other Water Management Considerations
The above sections have illustrated the variability in energy footprints across the water cycle and the importance in understanding the total water cycle impacts rather than focus on a specific element of the water cycle.
In a period of severe drought there has be justifiable focus on lower water footprint solutions. Including the energy footprint in the mix adds another level of sophistication in the decision making process that should yield even better outcomes. However, it is also important to recognise that water and energy footprints alone will not result in the optimum outcome. Environmental, social and economic factors also play a vital role they need to be considered to gain community support.
To illustrate the above point consider the simple comparison of a water security project involving the construction of a new dam and a desalination plant. Focusing on water and energy footprints alone will most likely support the construction of the dam. However if we consider the social dislocation costs of having to acquire the land from existing owners, the loss of habitat and impact on downstream ecology then the difference between the two options become closer. If the dam is located a long distance form the demand centre and requires major pumping the difference is even closer and possibly favouring the desalination plant option.
Another example is the use of rainwater tanks compared to a desalination plant. During the recent drought a very large number of households installed rainwater tanks believing they were being more ecologically responsible. They were even given rebates by governments to encourage them to do so. Yet a life cycle assessment of such initiatives will show that in some cases the rainwater tank solution with a typical pump installation uses more energy and costs much more per unit of water supplied than a desalination plant.
It is not proposed to cover all these factors in this paper rather to highlight their importance. Several of the Life Cycle Analysis tools and some Sustainability Rating tools are currently trying to take this broader perspective into the decision making process but this is still work in progress.
The following conclusions result from this analysis of water and energy footprints:
- The Water industry is a minor contributor to the energy footprint in Australia representing less than 0.5% of total energy use.
- In an urban environment energy use within the house is much greater than the energy required to supply and manage water.
- There is considerable variability in energy use across the water cycle. It is very much site and use specific. It is important to understand the use across the total water cycle rather than a specific element.
- Many of the on-site technologies currently marketed as more sustainable alternatives to traditional schemes will need to reduce their energy use to fulfill this claim.
- Considering energy, and water footprints together allows for improved decision making on water management. To achieve more sustainable outcomes, environmental, economic and social issues must also be considered.
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