Australia has long been prone to inconsistent rainfall, imposing severe water shortages on its residents, industry and agriculture. In Victoria a huge desalination plant is taking shape as part of the state’s solution to the problem. Martina Booth reports.
Australians call it the Big Dry. They are not referring to a dearth of the national drink but the parching droughts that can afflict the country.
For the past three years large parts of Australia have seen dramatic reductions in rainfall that have left states such as Victoria with dangerously low water reserves.
The Victoria state government’s solution to this, and future droughts, has been to commission Australia’s largest seawater desalination plant. Work started at the site near Wonthaggi − 80km from Melbourne on the Bass Strait − in September last year.
The desalination solution, first proposed in the state government’s 2007 water plan Our Water Our Future: The Next Stage of the Government’s Water Plan, will supply 150 gigalitres (GL) of water a year to Melbourne, Geelong and via other connections to South Gippsland and Western Port towns.
“The objective of our water plan is to move away from relying on one major source of supply to a portfolio of resources”
Other initiatives intended to secure Victoria’s water supplies for the next 50 years and beyond include modernising irrigation infrastructure in northern Victoria, expanding the Victorian water grid, plus increasing water recycling and water conservation measures.
“The objective of the water plan is to move away from relying on one major source of supply from our reservoirs, to a portfolio of diverse water sources,” says a Victorian Desalination Plant spokeswoman.
“Desalination is the fastest growing for of water provision throughout the world,” she adds. “It creates ‘new’ drinking water supplies that are not rainfall dependent, therefore being a drought proof source of water.”
The site at Wonthaggi plant was chosen after a feasibility study determined the area had the best mix of features, including ready access to open ocean water for intake, freely circulating water for rapid dispersal of saline concentrate and suitable connections for existing water distribution infrastructure.
It is being constructed and operated through a Public Private Partnership, with AquaSure − a consortium consisting of Degremont, Theiss and Macquarie Capital − winning a contract from the state government to design, build and operate the A$3.5bn (£2.2bn) plant in July 2009.
The plant will remove dissolved salts from seawater by reverse osmosis. This technique uses pressure to force seawater through membranes that filter suspended particles and dissolved salts to produce fresh water. Once converted, the water will be treated and blended with current drinking water supplies.
When complete, the plant will provide 150GL of additional water per year − about one-third of Melbourne’s water needs − with the capacity to supply up to 200GL in the future. Producing 150GL of water requires the plant to take in 360GL of seawater a year and pump back 210GL of saline concentrate into the sea a year.
Two tunnels − one intake tunnel, with a diameter of 4m and a length of 1.25km, and one outlet tunnel, with a diameter of 3.2m and a length of 1.5km − will be built 20m under the coastal reserve (dunes and beach) and seafloor.
Only the shafts and risers holding the intake and outlet heads will rise through the seafloor. The distance between the tunnels varies, but it will be about 500m at the sea ends.
Tunnel boring machines will be used to construct the intake and outlet structures, being driven from the bottom of the shaft below the coastal strip, beach and seafloor.
The intake structure has been designed with four mushroomshaped heads, each with a diameter of 6m. Each head will have a large cross-sectional flow area at the point of entry, which narrows as water progresses towards the riser to the intake tunnel.
When complete the plant will provide 150GL of additional water per year − about one third of Melbourne’s water needs − with capacity of 200GL in future
This will make the flow at the intake reference relatively slow, reducing the risk of mobile organisms becoming drawn into the intake stream and enabling free-swimming marine animals to swim away from it. The intake heads will be situated in a seawater depth of at least 15m.
At the end of the process, saline concentrate will be expelled through rosette-style diffusers on the outlet structure, which will be elevated 2m above the seafloor to prevent content reaching the seabed. The diffusers will be situated in a seawater depth of at least 10m.
Two onshore shafts will link the desalination plant to the tunnels. The shafts − 10m in diameter − will be sunk to a depth of around 65m-75m below ground at the plant site so that the tunnels, as they extend seawards, will slope gradually upwards but with sufficient clearance below the seafloor to remain in the rock strata.
An 84km water transfer pipeline − comprising around 6,200 sections of pipe, each 13.5m long, 1.93m in diameter and weighing 13t − will carry water from the plant to Melbourne.
It is currently under construction with two-thirds of the easement already cleared in preparation for pipe-laying with 1,800 pipes on site and more than 10km of pipe laid.
The pipeline has been designed as a ‘two-way’ pipe, so that when it is not running, water from the local Cardinia reservoir can flow to supply offtakes along the pipeline.
Construction is on track to deliver water by the end of next year. Earthworks on the Wonthaggi plant site are almost complete, with all soil kept on site to create new dunes that will assist in integrating the plant into the coastal landscape.
Foundations and footings for the reverse osmosis and dual media pressure filter buildings are also nearing completion, and AquaSure has begun erecting structural steel for the reverse osmosis building.
Despite positive progress onsite, the project has faced intense criticism from local environment groups, who are concerned about the 1.2Mt of greenhouse gases that the plant will emit.
However, the state government and AquaSure have pledged to offset 100% of the energy used by the plant and pipeline through the purchase of Renewable Energy Certificates, which will support investment in new wind farms in Victoria.
“The government and AquaSure have committed to fully offset the energy used by the plant and pipeline through the purchase of renewable energy,” says a project spokesperson.
“This will mean that the same amount of renewable energy will be injected into the electricity grid as will be taken by the plant and pipeline during operations.”