Smart design is leading the way in solving the problem of water shortages and pollution in our impermeable cityscapes. By Robyn Deed.
Tucked away in the back streets of East Melbourne is a world-leading project, patent pending. There’s not much to see—there’s a nature strip planted with wetland grasses in the middle of Darling Street, and an intriguing-looking steelwork shed in the park across the road. If you peered into the shed, you might see a poster that describes what’s going on. But most likely you’d walk on by, perhaps remarking that the grasses are thriving or the trees in the park look healthy. Which is exactly what’s intended.
What’s hidden beneath the bed of grasses is an innovative system for stormwater treatment and harvesting. With four years of research behind it, this City of Melbourne initiative uses existing technologies—water storage tanks, a natural biofiltration ‘rain garden’ and filters—but combines them in a smart way . The result is a system that’s more robust than other systems, takes up a tenth of the space and costs less to both install and maintain.
The system aims to solve two problems: contaminated stormwater flowing into our waterways and the shortage of water for irrigation in our warming climate.
Contaminated stormwater is a major problem in our impermeable cityscapes. In a natural landscape about 15% of rainwater runs off into waterways, with the remaining 85% soaking into the soil. In a built-up area that gets flipped: about 85% is now entering our waterways.
Ralf Pfleiderer from the City of Melbourne says it’s getting worse. “We’re improving our road surfaces but making them less permeable in the process, concreting over the bluestone gutters and replacing asphalt with bluestone over concrete,” he says.
The water running off the streets into nearby rivers is contaminated by lots of things: by ‘gross pollutants’ (larger things such as plastic bags, other litter and leaves); by silts and sands washed out of the soil; and by oils from cars contributing toxic heavy metals.
The extent of water runoff into our waterways, combined with the extended drought and water restrictions, means that water for irrigation is at a premium. Many of our urban trees are struggling. Even after a recent year of good rain, soil moisture levels haven’t returned to normal. So, as well as treating the water to remove pollutants, this project provides a source of water that can be harvested for irrigation.
Central to the system is the separation of stormwater capture from treatment. Other gravity-feed systems use rain gardens for both capture and treatment: the water from road gutters runs directly, by gravity, into the rain garden as it rains. This means the rain garden has to be below the level of the road—a design and safety issue in an urban streetscape (imagine a garden bed below the level of the road). In addition, the rain garden has to be much larger to enable it to capture and treat all the stormwater as it’s flowing.
What the Darling Street system does instead is to first capture the stormwater into a large (300 kL) underground tank, which allows the water to be slowly filtered through the garden bed after the rain event has passed. Using this approach, the system is able to treat the water from a 37 hectare catchment area—this would normally require 1200 m2 of rain garden, but this system needs just 120 m2 to do the same job. In addition, it’s able to supply 21 million litres of water annually (or 18 Olympic swimming pools) for irrigation.
Rain garden as filter
The rain garden itself is a natural biofiltration system, made of sand and grasses. Most of the filtering is performed by the bacteria coating the sand particles and plant roots—a natural biofilm, not by added bacteria. The plant stems help to break the surface tension and so maintain the flow of water through the filtering sand. They look great, too. Brendan Condon from Biofilta, one of the companies involved in the system’s research and design, says that the plants in the Darling Street rain garden are already fully mature whereas ones planted at the same time in a typical gravity-feed rain garden would still be establishing.
The system has been robustly designed to avoid many of the problems that plague other rain garden systems. Although rain gardens are good at filtering out contaminants, they get clogged by litter, sediment and silts, requiring expensive manual maintenance. To avoid this issue, before capturing the water into the holding tank, there are two filters applied to the water—a gross pollutant trap, to remove leaves and litter, and a sedimentation chamber, to settle out a lot of the silts. Both these filters fill up too, of course, but they’re easier to maintain as they can be vacuumed clean mechanically, rather than manually.
The cleaned water is either stored in a reuse tank (also beneath the ground) for irrigation use, or returned to the stormwater drains as cleaned stormwater—much better for our waterways than the original polluted stormwater. The final step is a UV filter to ensure that any residual bacteria are killed.
Harvesting the water
Come summer, the system will switch to using water from the reuse tank for irrigation as needed. Irrigation could also be applied in winter, but Ralf Pfleiderer says that this is something for down the track. In winter the water needs to be applied below the surface as the surface layer is often already wet, leading to run off. But irrigation is still needed in winter: beneath the surface the soil moisture remains low, after so many years of drought.
Unlike gravity-feed systems, this system does require electricity to pump the water through the rain garden for filtering. But Ralf notes the pumps run just two hours out of 24, under low load, with a total electricity cost of about $200/year, offset using carbon credits.
Future water supply?
The most significant current application is to clean stormwater, thus reducing pollution in our waterways. When used for irrigation, such systems can also help keep parks and street plantings healthy, and so help alleviate the heat island effect in cities as our climate warms. Brendan Condon notes that he’s been involved with installing similar systems for apartments to provide irrigation for their gardens.
Looking further ahead, Ralf speculates that such systems could become part of a decentralised water storage approach. We could be using harvested stormwater on our gardens, and in our toilets and washing machines, while our reservoirs are used purely for drinking water.
Stormwater could even cover all our water needs. Melbourne currently uses 400 GL of water a year, compared to available stormwater of about 463 GL. We only use around 10 GL of that at the moment, so at the very least stormwater could be a much bigger part of our future water supply.
If we can switch to thinking of reservoirs as just one part of our storage system, rather than the full system—as super-sized rainwater tanks in effect—maybe we can start to reduce our dependence on them.
The system was researched and developed by Biofilta and Cardno, in conjunction with the City of Melbourne. A patent for the BiofiltaTM bioretention system has been applied for.
This entry was posted on Wednesday, February 11th, 2015 at 2:16 pm