Stormwater helping to tackle Australia's water crisis
This topic is sponsored by the Australian Research Council Linkage Learned Academies Special Project Grant.
With reduced water supplies and a growing population, should Australians be letting stormwater go down the drain?
Water supplies in Australia
Water is a big issue in Australia. There are water restrictions in most major cities, and climate change and recurrent droughts affect our agriculture industry. And it's easy to see why water is a big issue here. The average Australian uses 320 litres of water per day for domestic purposes. For overall per capita water usage, we are the third highest consumer in the OECD, only behind Canada and the United States. Irrigated agriculture uses nearly eight times more water than total household consumption.
There are ever increasing pressures on our water supplies. Our population is growing – it's predicted to reach 25 million by 2032. And we will be very thirsty indeed, because the world will be a hotter place by then. Global warming is expected to decrease rainfall in Australia's 10 largest cities by an average of 15 percent by 2030. Perth will be worst affected, with a predicted 20 percent drop in the drop, and only Hobart can continue to expect healthy rainfalls. It's pretty clear that we need to make some changes to the way we use water.
For Australia to effectively address the water crisis, a range of water management strategies need to be used. By signing the National Water Initiative (2004) Australian states and territories committed to more efficient water use, better water management and reuse of water. Desalination plants, recycled water and stormwater can all play a part in helping to tackle the water crisis. Already, the largest recycled water scheme in the southern hemisphere has been announced in Brisbane; desalination plants have been built or are currently being built in Perth, the Gold Coast, Sydney and Melbourne (Box 1: Australia's desalination situation); proposals for dam enlargements and the introduction of recycled water are being considered for Canberra, and a package of options which include desalination and stormwater reuse have been implemented in Adelaide. Every one of these cities is implementing water reuse in some form – a sign that the old model of letting our waste and stormwater run out to sea is no longer feasible.
Tapping into stormwater
Until recently, little attention has been paid to the reuse of stormwater. In most towns and cities, stormwater runs directly into a dedicated drainage system designed to carry the water away as quickly as possible into natural waterways or the ocean.
In Australia, a significant volume of rainwater ends up as stormwater runoff. It is estimated in Sydney alone, 420 gigalitres of stormwater goes straight out to sea every year. That's the equivalent of almost the entire contents of Sydney Harbour.
While it's tempting to think of all the ways wasted stormwater could be used, it's not as simple as directing stormwater drains into a dam. Firstly, suitable storage areas need to be established. In addition, stormwater runoff picks up all kinds of pollutants from our roads, driveways and footpaths – everything from garden clippings to detergents, sediment to paint, plastic bags and cigarette butts. These pollutants, in addition to the unnaturally high flows, can damage creeks and wetlands. Before stormwater can be safely used, the pollutants have to be removed (Box 2: Removal of pollutants from stormwater).
The biofiltration approach
One solution to the problem of pollutants in stormwater is to make stormwater systems more like natural waterways. When water flows through a creek or wetland, particulates and chemicals are filtered out by the aquatic ecosystem, cleansing the water. By retaining natural creeks in urban developments or creating vegetated waterways instead of large concrete stormwater channels, stormwater can be stripped of most of its offending pollutants.
This approach is being adopted in several places around Australia. Cabbage Tree Creek, which flows into Moreton Bay in south-east Queensland, is one example of a waterway that has been rehabilitated through this approach. By installing a sedimentation pond and stormwater wetland to reduce nutrient and pollutant loads from stormwater runoff, the health of the creek's downstream ecosystem has been significantly improved.
At Homebush, the Sydney Olympic Park was built to use recycled stormwater, which is filtered through the Millennium Wetlands, stored, and then used for water features, irrigation and toilet flushing, saving significant quantities of water.
While in Melbourne, 'rain gardens' have been installed in new housing developments to filter stormwater runoff. These gardens are five metre square garden beds, filled with a sandy loam soil and water-loving plants, which filter the runoff as it passes through. The Wungong redevelopment in Western Australia will incorporate eucalypt-lined park avenues that serve as biofilters for its stormwater runoff, protecting the Wungong River from pollution.
In Salisbury near Adelaide there is also research into treating stormwater to provide water which meets drinking quality requirements. CSIRO scientists have developed a method in which stormwater is treated by passing it through a reed bed; the water is then injected into a limestone aquifer via a well, where it is stored for twelve months. The water is then tested for its compliance with drinking water criteria and bottled.
As the drought situation in Australia worsens, there is increasing interest in tapping into existing sources of water, such as stormwater, which at the moment is just going down the drain – literally. There is ongoing research to make better use of the water that we have, in the hope of alleviating the ever-increasing strain on this precious natural resource.
Posted June 2008.