Anyone in Perth in the late 1970s couldn’t have missed the arrival of a new suburban artefact – the backyard bore. Today, there are an estimated 176,000 bores dotting suburban gardens in Perth.
It was the circuit breaker to drought that enabled all those living in the largest city on Australia’s western edge, in the long glare of the setting sun, to share in the green gardens that nurture city life and provide natural evaporative cooling.
Groundwater currently provides about 85 per cent of water consumption for the greater Perth-Peel region, a population of about 2 million people.[i]
It’s hard to imagine how liveable Perth would now be without it.
And it’s not just domestic bores – there are bores that draw enormously on groundwater for irrigation and industrial use as well.
But these bores rely on rainfall to recharge the groundwater. And as the term drought has been replaced by climate change with a persistent drying climate, and use of groundwater has accelerated, the watertable has been dropping.
Don McFarlane, a Perth-based researcher from CSIRO Land and Water, says the lowering of the water level has had a serious impact on wetlands not already drained in early development of the city. The falling water levels have also invited the gradual intrusion of sea water into the groundwater system, threatening this invaluable source.
But, of course, the water cycle doesn’t end there. Consider the wastewater produced by the city’s growing population.
Currently, only about 7 per cent of treated waste water is recycled and about 137 gigalitres (137 billion litres) is piped out to sea each year. And the amount of wastewater being produced in Perth is growing.
“Treated wastewater grows with urban development, even in a drying climate,” says Dr McFarlane.
And just as the shallow unconfined and deep confined aquifers underlying the sandy soils of coastal Perth provide this, in some cases ancient, groundwater[ii] – those same aquifers hold the possibility to store, transport and even further treat the wastewater that is currently being discarded.
Putting wastewater to use
Western Australia’s Water Corporation has developed a scheme to replenish groundwater by injecting highly treated wastewater into deep aquifers that can later be used for drinking. CSIRO has assisted in the trials for groundwater replenishment with more than a decade’s worth of research.[iii] The Water Corporation aims to recycle 30 per cent of Perth’s wastewater this way by 2030.[iv]
CSIRO is exploring ways in which treated wastewater might relatively cheaply be put to use via aquifers under the city, water that could be used in industry and for irrigation. It’s a system called Managed Aquifer Recharge, or MAR for short.
MAR was first trialled in Western Australia by CSIRO scientist Dr Simon Toze and his team from 2009 to 2012, supported by the WA Government Water Foundation. Their work looking into how to set up and operate a MAR scheme, the potential human and environmental health risks, and the social attitudes towards water recycling, paved the way for current trials and research into its applications.
According to the Australian Water Recycling Centre of Excellence (AWRCoE), across Australia “more than 1750 gigalitres of recycled water per year (three times the capacity of Sydney Harbour) could be available for aquifer recharge”.[v]
But there are still a number of major challenges to address. One is how to minimise clogging during operation of the infiltration system, so that water can infiltrate at the optimum rate into the soil through to the shallow aquifer. Another is how to ensure that the quality of the existing or ‘receiving’ groundwater is not negatively affected.[vi]
AWRCoE are funding and managing a series of projects to address these challenges and develop innovative approaches to increasing water recycling using aquifer storage. They have contributed $900,000 to this research.
Infiltrating the aquifers
CSIRO Land and Water’s Dr Joanne Vanderzalm, who researches MAR as part of AWRCoE’s suite of projects, has examined the mechanics of infiltrating treated wastewater into aquifers via a buried gallery system. The system uses a series of polypropylene crates, Atlantis Flo-Tank® modules, which are designed to enhance the percolation of water into the subsurface.
‘’Buried closed galleries are attractive in urban settings because they don’t need a vast amount of land and the potential for human contact with treated wastewater, loss through evaporation and algal growth are reduced,” says Dr Vanderzalm.
Dr Vanderzalm’s team conducted a five-month field experiment at CSIRO’s Floreat laboratories in Perth, taking advantage of the medium-grained, sand deposits typical of the coastal plain of Perth that sit above the Tamala Limestone aquifer.[vii]
This experiment produced guidelines on how infiltration galleries can work. The research found that the buried galleries need to be kept clear of plant roots that block water flow, for example. It also found that total suspended solids in the treated wastewater has to be kept to a maximum of 5mg/L to avoid physical clogging from particulates.
Something they want to investigate further is how to construct a system of galleries that could be rotated allowing some to lie unused, so they dry out and the biological material that forms at the base of the gallery breaks down.
The team also looked at the resultant water quality and showed that phosphate had been completely removed from the wastewater by the time it reached the aquifer, and 69 per cent of organic carbon had been removed. This shows that infiltration can assist in further treating wastewater. However, the experiment showed limited potential for nitrogen removal in the aerobic aquifer.
“Perth is particularly important in the development of these new technologies because of the water shortages, the need to diversify water sources and interest in participating in nationally relevant demonstration projects,” says Dr Vanderzalm.
Real world wastewater infiltration
Meanwhile, Don McFarlane has had the advantage of having a real life example of wastewater infiltration to examine. Kwinana town, near Perth’s Kwinana Industrial Area, has treated its waste water on site and added it to the shallow aquifer through infiltration basins in the Cockburn Sound Catchment for the past 40 years.
“It’s a great test of what we’re trying to look at more generally,” he says.
Dr McFarlane’s team of researchers at the Floreat laboratories have analysed monitoring data and developed a groundwater model to simulate the impact of climate, MAR and water extraction on the aquifer.
The research calculated the travel time, for example, of recycled treated water through the aquifer and established that at the highest travel rate, water would reach Cockburn Sound within 20 years. But if managed well, MAR-added water could be intercepted by pumping bores for industrial use before it gets to the Sound, as a lot of the existing infiltrated water is already.
Importantly, there is some evidence it has helped to raise the water table in that area, possibly saving two lakes as well as reducing the rate of sea water intrusion, he says.
The Kwinana Industries Council contributed $100,000 to this research. The industrial area for some years has had a water reclamation plant which is now near capacity, according to council director Chris Oughton.
Running right past the industrial zone is a pipeline that takes secondary waste water out to sea.
“It’s the ‘blind Freddy’ solution to the industrial water needs of the future – to take some of that water, clean it up and put it into the ground so that it can be stored and taken later,” he says.
“CSIRO’s work has proven that it can be done. What’s needed now is a pilot project.”
There are already recommended sites for infiltration. The issue is who might fund a trial and who might be interested if a commercial MAR project were put into place to recycle water for industry.
“Everyone assumes that current industry will pay for it but why would they when they have existing licenses [to extract groundwater]. There has to be room for a third party to get in and make a profit from recycling the secondary waste water and selling it to industry.”
Is it feasible?
The financial argument for MAR is something both Dr Vanderzalm and Dr McFarlane have worked on.
Dr Vanderzalm recently undertook an economic assessment for a number of recycled water managed aquifer recharge case studies[viii]. One hypothetical case study of MAR infiltration basins reported a benefit to cost ratio of over three, due to the avoided costs of covered surface storage.
Dr McFarlane’s team is also looking at the advantage to housing prices of being located near viable wetlands.
“Perth has abundant aquifers compared with most cities that could be recharged with wastewater and stormwater but they haven’t filled naturally because of the drying climate,” he says.
“It’s a matter of when, not if, we can safely use those aquifers to further purify and store the treated wastewater. And if we raise the groundwater level we might be able to save the drying wetlands of Perth and increase the reliability of a very water supply for irrigation and heavy industry.”
Dr Vanderzalm’s technical reports are now published on the Australian Water Recycling Centre of Excellence Centre’s website under ‘Managed aquifer recharge and recycling options’.
Dr McFarlane’s research results will be available there soon under ‘Recycled water for heavy industry’.
[vi] “Recharging Australia’s aquifers with recycled water” published by the Australian Water Recycling Centre of Excellence
[vii] “Recharging Australia’s aquifers with recycled water” published by the Australian Water Recycling Centre of Excellence