Perth’s population is growing, but drinkable water is becoming harder to find. To keep the city quenched, this utility’s engineers are sending kilometres of pipelines out to sea.
Off the coast of the northern suburbs of Perth, buried 15-60 m beneath the ocean floor, two pipelines that will help secure the city’s future for decades to come are being laid.
The shorter of the two, which extends 1.6 km from the shore, will pull seawater from deep below the ocean’s surface and transfer it to the Alkimos Water Precinct, where a new desalination plant will filter and treat it, then remove salt, bacteria, viruses and other impurities via reverse osmosis.
The resulting drinkable water will then be delivered to Perth’s water network through a 33.5 km, 1.6 m diameter underground pipeline – the largest ever built by Western Australian water utility Water Corporation – that runs to Wanneroo Reservoir.
The wastewater from the process – a brine that is twice as salty as seawater – is returned to the ocean through the longer of the offshore pipes, which extends three kilometres from the coast.
The enterprise is vital for keeping the residents of the growing – and increasingly parched – Perth region supplied with a clean and reliable water supply. The city’s population is expected to reach 3.5 million by 2050, yet rainfall in the region has reduced by a fifth since 1970.
Today, 40 per cent of Perth’s water supply comes from groundwater, but that allocation is set to drop by 27 per cent in 2028 – when the Alkimos Desalination Plant is scheduled to come online.
- Average annual rainfall runoff in Perth dams in 1975: 420 billion L
- Average annual rainfall runoff in 2026: <70 billion L
- Projected Perth population in 2050: 3.5 million
- Alkimos desalination plant annual capacity: 50 billion L
- Energy sourced from renewables (annually): 400 MW
- Pipeline from plant to reservoir: 33.5 km long
- Intake pipe: 5.9 m below sea level
- Outfall pipe: 20 m below sea level
- Estimated delivery date: 2028
Seawater, not groundwater
“We’re ready for full production by mid-2028 when some of those groundwater allocation reductions come our way,” said Daniel Rossi, Water Corporation’s Manager of Major Source Projects. “We’re well into our civil and structural works on the plant itself, and we’re building our pipelines out to the ocean. Right now, we’ve got two large jack-up barges installing our marine work.”
These pipes have to extend kilometres out to sea so that they can collect the best quality water to use in the desalination process – and then to dispose of the resulting brine in the safest environment.
“There are a lot of investigations and environmental studies so we’re able to best locate where that water goes,” Rossi said. “Ultimately, what we’re trying to do is bring the best quality water in and then disperse the saltier water out in the high-energy environment along the coast, where it can mix well with the existing environment and within a regulated area.”
Installed beneath the seabed, these pipelines funnel water in, bring it back to shore and deliver it to the Alkimos plant through a series of vertical risers. Selecting the best depth and locations requires significant engineering expertise.
“That’s all about the soil conditions that we need to construct in and trying to find the best conditions for tunnel boring.
“There’s a lot of technical work that’s done to get optimum outcomes in terms of how deep these tunnels go, where they’re finally located, and then the type of equipment that you select for their installation.”
As well as drawing on the global expertise of partners such as Acciona, which brings substantial tunnelling experience to the project, Water Corporation also builds on the knowledge it has accumulated in constructing two previous desalination plants for the Perth area.
“We have two existing facilities, and we learn from how those plants perform,” Rossi said. “We learn from other facilities around Australia and the desalination network is quite a tight-knit network. Then globally, we’re looking at the latest practice and best technology around energy efficiency and around the performance of the key equipment that goes into these desal plants.”
Eye on energy
One advantage the Alkimos plant will have over its predecessors is the size of the pipe delivering the treated water to the Wanneroo Reservoir – 1.6 m in diameter.
“Having a larger pipe means that we get quite substantial energy savings that offset the cost of buying and installing that larger pipe,” Rossi said.
A larger diameter of pipe uses less energy to pump the same amount of water, and those savings will add up. And to ensure Water Corporation could access a pipeline big enough for this project, it worked with long-term supplier Steel Mains to upgrade its equipment so it would be able to produce the parts needed.
“They’ve upgraded their facility here so we can make these larger pipes, and so we then save further on transportation and those [factors] that we’re interested in around sustainability and carbon footprint.
“These are long-term assets that will be operating for 100-plus years and so having a solution that gives us as low as possible operating cost over time is really critical.”
While Rossi acknowledges that desalination can be energy intensive, he said the plant incorporates processes to reduce energy use, including such behind-the-meter solutions as solar cells.
“Energy recovery devices convert the high pressure that’s used in the desalination process into energy and put it back into the system to get benefit.
“We’ve been able to reduce that power consumption by up to 40 per cent, and that brings down our overall operating cost, which is really important. This is a long term, 100-year asset, so we’re trying to have the lowest operating cost solution.”
Water Corporation is also working closely with state utilities to connect to the power grid’s sustainable energy production solutions, as well as pursuing other carbon offsets to ensure the project achieves net-zero carbon emissions during construction and operation.
Desalination plants are subjected to complex hydrodynamic forces, making accurate load prediction essential. Discover an alternative approach to this process.
Reverse Osmosis removes even the good minerals, which is harmful to health. Will you be injecting those good minerals back into the water?
Could you not incorperate easily spun turbines in this pipeline to creat electricity.
Or ever place them in the ocean to use the unstopable currents.