Passive irrigation system turns cities into ‘green sponges’

A South Australian startup is providing water-sensitive urban design solutions for local councils with a system designed to capture stormwater runoff using recycled wheelie bins.

A water sector conference in South Australia last week heard the company’s Treenet inlets replicate natural watershed functions to turn cities into ‘green sponges’, rather than directing precious water away from urban areas via traditional stormwater systems.

Dr Harsha Sapdhare, research and development manager with Space Down Under, says the Treenet system can be used alongside standard kerbside stormwater drains.

The system works by catching runoff as it flows down gutters in a shallow bowl, which works like a vortex to wash out debris and direct water into the inlet.

The inlet circulates the water into an infiltration zone made out of recycled wheelie bins, and it’s then dispersed over time into the surrounding soil.

Improved tree growth

A 2019 study found 65 per cent more growth over four years in trees using the system than in controls using other methods, Dr Sapdhare said.

The Treenet system (source: Space Down Under)

“It successfully provides sustainable, passive irrigation to street trees,” she told Government News after her presentation to the Australian Water Association State Conference in SA on Friday.

“It’s the idea of keeping water where it falls and redirecting it to the soil where trees can absorb it. All our existing systems as designed to take water away from the city but with this we are talking about shifting the flows.

“If a council uses the system, they only need to water a new tree just for one year to establish roots, and after the second year there is no need to water these plants again.”

The way the water is distributed in the soil also encourages tree roots to grow in a direction that minimises infrastructure damage to pavements and roads, she adds.

Thousands of litres of free water

City of Prospect is one council that is using water-sensitive urban design to promote the sustainable use of water, including Treenet inlets and permeable paving.

The City has installed over 80 inlet systems in its roadside verges, and says they capture more than 10,000 litres of free water for trees a year.

It also helps moderate urban heat island effects, with each inlet adding the equivalent of a 5kW evaporative air conditioning unit to the street based on extra cooling provided by the trees.

In addition, council has installed permeable paving that allows water to travel through a gravel base to the soil beneath.

Partnering with local government

Space Down Under has partnered with councils across Adelaide, including Mitchum City Council, City of Unley, Port Adelaide and the Legatus Group of regional SA councils to test the system across a variety of catchment areas.

It has sold 25,000 units, including 16,000 in Melbourne, at a cost of between $300 and $400 per unit depending on the model. Once installed, the system is there for good.

The company is currently looking to collect data from more sites, Dr Sapdhare says.

“If councils have a strategic plan to develop the canopy to reduce carbon, how they can achieve that?” Dr Sapdhare says.

“This is the solution.”