Australia

Sydney water quality issues keep desal plant running

Concerns about water quality in Sydney's main reservoir following last summer's bushfires have prompted the government to continue to operate the city's desalination plant even as dams near capacity.

Warragamba Dam on Friday reached 99 per cent capacity after recent rains lifted water levels by one-eighth in a week, WaterNSW data shows.

Across the network, Sydney's dams were 97.5 per cent full having doubled since February rains effectively ended the region's drought. Nepean reservoir continues to spill, while most others are more than 94 per cent full.

Despite the high dam levels, the government has also left in place level 1 water restrictions. Sydney residents are only permitted to water their gardens with a hand-held hose fitted with a trigger nozzle, a watering can or a bucket before 10am and after 4pm.

Part of the heavily burnt catchment of Lake Burragorang and Warragamba Dam, west of Sydney.

Part of the heavily burnt catchment of Lake Burragorang and Warragamba Dam, west of Sydney. Credit:James Brickwood

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A spokeswoman for the Sydney Desalination Plant said SydneyWater has asked the operators to provide "additional back-up to their system until the end of September 2020".

"This means [the desal plant] has been producing small quantities of water, which are the minimum required to ensure the plant is in working order and available to ramp up production immediately, as and when required," she said.

However, the government says the plant is also needed to run in case SydneyWater has to respond quickly to water quality issues related to last summer's fires.

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To shut it entirely would cost $15 million to restart and take eight months to get to full capacity, which amounts to about 15 per cent of Sydney's water supplies.

A spokeswoman for Water Minister Melinda Pavey said the government is scheduled to review existing water management actions in December 2020.

"The desalination plant is currently in ‘hot standby’ running at 20 per cent capacity," she said.

"This enables the government to quickly increase the plant’s output should we see water quality issues arise from the recent rainfall or from bushfire debris entering the catchment."

Warragamba Dam is 99 per cent full and could spill within days.

Warragamba Dam is 99 per cent full and could spill within days.Credit:James Brickwood

The bushfires burnt through 80 per cent of the catchment of Lake Burragorang, which sits behind Warragamba Dam. The storage supplies about 80 per cent of Sydney's water.

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Bushfire ash contains organic material and concentrated nutrients such as phosphorous and nitrogen, which can trigger challenges for treating water for human consumption.

Contamination risks range from mild discolouration and turbidity if the organic material gets washed into the reservoir, to the sudden growth of algae and cyanobacteria – or blue-green algae – triggered by phosphorus or other nutrients.

Stuart Khan, a professor in the University of NSW's School of Civil & Environmental Engineering Research, said it is a prudent move to continue to run the desal plant even as the dams fill up.

"It's about having a diversity of supply," Professor Khan said, adding that the operators of Warragamba have been "very concerned about the water quality" after the bushfires.

Apart from running the desal plant, WaterNSW had also been channelling from the Nepean Dam to Prospect Dam, the main treatment site for Sydney's water.

Ms Pavey's spokeswoman said SydneyWater would reassess this need to continue operating the plant beyond September based on water quality issues following the significant inflows.

Photography for this article was partly supported by the Colong Foundation for Wilderness.

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