Auckland drought: Why floating desalination plants got the thumbs down

One or even two huge floating desalination plants, to turn seawater into fresh water, were briefly considered as solutions to Auckland’s drought-driven water shortage.

Each of the barges could have supplied 6 per cent of Auckland’s water needs, from a location on Manukau Harbour, but would have come with a big financial and environmental cost.

The desalination barge idea was canvassed in a confidential report to the June meeting of the board of Auckland Council subsidiary Watercare, obtained by Stuff.

Watercare is pressing on with $224 million of more traditional upgrades over the next year to boost supplies, with dams currently just 60 per cent full, compared with normal levels of 89 per cent.

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Each barge could have delivered 25 million litres of water a day, feeding the Lower Nihotupu Dam in the Waitākere Ranges. They would have needed six metres of water for berthing, and a connection to shore-based services.

However, one big minus was the generation plant for the project, which would have consumed 50,000 litres of diesel fuel daily.

“The diesel load from one desalination barge would add 140 per cent to Watercare’s total (annual) operating carbon emission with an operating cost of $18m,” said the board report.

“Due to consenting, logistics and the overall carbon footprint, this option is not being taken further,” it concluded.

Large land-based desalination plants have been built in Australia, but they are costly to build and used a lot of energy.

The technology is being considered as a long-term solution to Auckland, but both Watercare and Mayor Phil Goff have noted that cost and environmental factors might make it unsuitable.

In addition to the $224m of additional work, Watercare is looking at a further layer of local sources that could contribute during winter months, amounting to 5 per cent or so of winter needs.

These include expanding the capacity of the Waitākere treatment plant, upping the take from the Onehunga aquifer, taking water from the Lower Mangatawhiri Dam, and tapping into the Wairoa River.

The Waikato River is currently supplying 40 per cent of Auckland’s needs through the treatment plant at Tuakau, which is being upgraded to deliver 175 million litres a day from late August.

Smaller upgrades of local sources over the next year include recommissioning the Hays Creek Dam at Papakura and a bore in Pukekohe by December.

The possibility of heavier than forecast rain through spring, averting tighter water supplies in summer, remains uncertain.