On the surface, using recycled water in green space makes sense, solving several problems at once – reducing the strain on fresh water supplies, improving garden health and decreasing the amount of water pumped from treatment plants to the coast and then out to sea.
Director of the Smart Green Cities Research Centre and Distinguished Professor at Macquarie University’s School of Natural Sciences, Michelle Leishman, explained however that in reality it’s not as simple as diverting treated wastewater straight onto gardens.
“Too much treated wastewater could raise the level of underlying groundwater, bringing salt-infused water too close to surface soil, hampering plant growth.
“Getting the formula right so soils stay healthy is important before water recycling is introduced on a broad scale,” said Professor Leishman, who heads up the new Western City Parklands Arboretum project, funded by Sydney Water in collaboration with Macquarie University’s Smart Green Cities Research Centre.
The project will test the impact of recycled water on different tree species, with a focus on western Sydney’s unique soil and landscape challenges.
The project site at the Australian Botanic Garden at Mount Annan will also feed data into the Which Plant Where online tool, a free resource to future-proof landscaping.
Fifteen different tree species – popularly used as street trees and in parks in the region – are being watered in the garden with either recycled or drinking-quality water and tested using different soil amendments.
A 1ha site has been planted with 600 young trees, with smart monitoring systems and hyper-local weather stations set to take measurements including soil moisture, salinity, temperature and humidity over the next few years.
Sydney’s west under the microscope
The core aim of the Western City Parklands Arboretum project is to assess whether recycled water can be used in urban green space in Sydney’s western suburbs.
“Not only would the use of recycled water in green infrastructure save significant amounts of drinking water every year, it would also turn the hot western suburbs of Sydney into more liveable, green and sustainable regions,” Prof Leishman said.
“Western Sydney is an important place to study when we’re looking to improve functional, healthy green space in cities to provide cooling under extreme heat conditions.”
Smart sensors
The sensor system at Western City Parklands Arboretum delivers real-time data on soil moisture, salinity, air temperature and humidity.
The region is already well acquainted with the challenges of climate change; Penrith was officially named the hottest place on the planet when the Bureau of Meteorology recorded an air temperature of 48.9°C on 4 January 2020.
Western Sydney routinely has the Sydney Basin’s hottest summer temperatures, recording 44 days over 35°C in both 2018 and 2019 – and the Bureau of Meteorology predicts the proportion of over-35°C days will keep rising.
“Extreme heatwaves and drought are challenging for plants and green spaces, and taxing on limited freshwater resources,” Prof Leishman said.
“The project is designed to have a long-term impact on improving the resilience of our urban environments in the face of accelerating warming.
“Using recycled water allows us to use our water resources sustainably to improve our green spaces, so they can provide benefits like cooling, improving air quality and supporting biodiversity.”
Already home to 2.6 million people, the New South Wales Department of Planning, Housing and Infrastructure projections suggest another 800,000 people will move to the region by 2041, raising demand for drinking water and straining wastewater infrastructure.
“The new developments needed to house western Sydney’s growing population will produce a large amount of wastewater and sewage that Sydney Water must manage.
“Treating it and returning it to the landscape for green space maintenance is the proposed solution, but we have to be sure the recycled water won’t cause problems with salinity or plant performance.”
Reducing urban heat islands
“The Western City Parklands Arboretum project could ease some of these challenges by providing solid data on the most suitable plants and shade trees for the region, and how best to use the recycled water produced by Sydney Water to reduce the urban heat island effect.”
Urban heat islands occur when built-up areas, roads, footpaths and rooftops absorb heat, becoming significantly warmer than those areas which have green spaces to absorb and dissipate the heat.
Rainfall on hard surfaces is often channelled into drains, rather than contributing to soil moisture in any surrounding gardens, which reduces the ability of those gardens to cool.
“There are two ways that trees cool urban areas; first through their shade which reduces the amount of sunlight hitting buildings and roads, keeping the shaded zone cooler.
“And second, through transpiration which is nature’s air-conditioner – where trees release water vapour into the air through their leaves, and as the vapour evaporates, it absorbs heat from the surrounding air, cooling it down.”
Trees are more effective at transpiration when well- watered and grown in healthy soils which hold moisture, Prof Leishman said.
“In drought or long dry spells, trees don’t provide as much cooling, unless they have been regularly watered, and that’s where recycled water can help.”
The right place at the right time
Prof Leishman said that undertaking a large-scale project like this inevitably has major challenges.
“Finding an appropriate site was the first challenge, and we have been very lucky that the Botanic Gardens of Sydney have been willing to partner with us and find a site at the Australian Botanic Garden at Mount Annan.
“They have been fantastic to work with, helping with site selection, preparation and maintenance.
Prof Leishman said that there have also been challenges with sourcing good quality plants in sufficient quantities at the right time – a challenge that council greenspace managers will be familiar with.
“And finally, there are a lot of hurdles to jump over to get permission to use recycled water – Sydney Water worked very hard to make sure that we had all the correct processes and infrastructure in place in order to obtain approval from NSW Health.”
Engaging with communities
Prof Leishman said that the project has engaged with the community at events such as the Sydney Science Trail, held at the Australian Botanic Garden during Science Week in 2023.
“We talked to hundreds of people at that event about water use and the importance of greenspace. We have had some media coverage, for example an interview on ABC 702 Sydney radio, and articles in magazines such as the upcoming winter issue of The Gardens.
“We also have funding from the New South Wales Greening our City program to develop educational and interpretive material for the project.”
Getting the recipe right
Prof Leishman said that the project will help develop a ‘recipe for recycling’ for water authorities, planners and landscape architects.
“We need to add enough water to the landscape for trees to perform well, but not add so much that we cause the water table to rise and therefore dissolve salts from the layers of soil it passes through, bringing the salt to the surface.
“We’ll also be looking at how efficient different tree species are in their water use, and because recycled water has different chemistry to potable (drinking quality) water, we will test interactions with the soil chemistry typical of the region.”
A range of educational material to help residents improve their water management practices will be produced as part of the project, supported by the New South Wales Government’s Greening our City program.
“We’re also developing on-site guides and education packages for schools and Botanic Garden visitors which are all informed by this research.
“The project is designed to have a long-term impact on improving the resilience of our urban environments in the face of accelerating warming.”
Project goals and outcomes
“The plants are in the ground and the smart sensing system is operational.
“We will continue to monitor plant performance for at least the next 12 months and will be able to report on the results then.
“In the meantime, the project has increased public awareness of the importance of tree canopy for cooling, particularly in hot urban areas such as in western Sydney, and the importance of careful water use including potential use of recycled water for keeping our green spaces cool.”
Prof Leishman explained that the project is set to provide the evidence base for the selection of plant species that will perform well when watered with recycled water including a diversity of different species, as well as provide an understanding of any potential interactions of recycled water with soil chemistry and how that may impact plant performance.
“It will help councils to diversify their plant selection by providing information on plant performance and water use. And it will help them to understand the benefits and any challenges associated with using recycled water for urban greenspace management.”
Prof Leishman said that climate change means that urban trees are increasingly under stress from hot and dry conditions.
“As well, urban populations are challenged by more intense and more frequent extreme heat events – extreme heat kills more people than all other weather-related events combined.
“We need functional green spaces with healthy trees and extensive canopy to provide relief from these extreme heat events.
“The use of recycled water offers great potential to enable healthy functional greenspace and tree canopy that can help mitigate the impacts of climate change and provide a more liveable urban environment.”
Featured image: The Western City Parklands Arboretum project. Image credit:
