Order to resume - by email At 8am each day, WaterNSW calculates the water storage levels. If the average reservoir level sinks below 60 per cent, the agency will send an email to Sydney Water and to Keith Davies, chief executive of the desal plant, to order the plant's start. "Then it's basically game on," Davies tells the Herald. Loading But don't expect an immediate flurry of engineers scurrying between the 20 trains - or blocks - that will filter out the salt molecules from sea water sucked from intake sites located about 300 metres offshore from the Kurnell plant. Nor will there by any drinking water produced from day one - or for quite a few days.

Rather, the plant's operator Veolia will begin a meticulous process of priming the plant, not least the disinfecting of the18 kilometre-long pipeline that connects the facility to a major Sydney Water supply line at Erskineville. The 17-strong crew that has overseen the plant's "care and maintenance" since 2012 will gradually increase to double that number, as more mechanical, chemical and other engineers and technicians are hired. Drip feed Desal plants take time to crank up - particularly when they've been offline for more than six years, making this weekend's likely restart effectively a recommissioning. The "high science" involved in desalinating sea water, as Davies calls it, means the plant will take as much as four months to produce usable water, and twice that long to reach maximum output of 250 million litres a day. That's about one-sixth of Greater Sydney's needs.

A primary process has the seawater sifted by gravity through a layer of filter coal and sand. But to get rid of most of the salt and other impurities, you need pressure - and a lot of it. So after primary treatment, water is then pushed through membranes that filter out salt molecules at 50-60 times atmospheric pressure at sealevel, in a process called reverse osmosis. Sydney's plant has 36,000 reverse osmosis polymers in those 20 trains, contained in 4500 vessels. Loading For every litre of seawater extracted, half a litre of drinkable water is produced. The waste water, at twice the salinity of seawater, is pumped back into the sea. Energy recovery systems extract as much as 90 per cent of the heat from the seawater concentrate, helping to cut electricity demand. All of the plant's power is supplied from wind energy because of early worries about its energy hunger.

Retesting of all components may also mean replacing some of them. The membranes have a life of about eight years and there's $30 million earmarked for their replacement. Even when the main pipeline is disinfected, raising the water to meet Sydney Water standards requires adding fluoride and other minerals - and monitoring quality throughout the process. Hence, the lengthy start-up. When it last operated - between mid-2010 and June 2012- the plant supplied about158 billion litres of drinking water. At full tilt, it can produce 91 billion litres a year. Troubled history

Desalination plants have always had their critics, not least because alternatives, such as water recycling or reducing waste, are ignored. For Victoria, then-Liberal Opposition leader Ted Baillieu proposed a $400 million plant at his party's election launch in November 2006, only to be attacked by his Labor incumbents for making policy on the run. After retaining office, Premier Steve Bracks grabbed the idea - and expanded it to a $3.1 billion, 400 megalitre-a-year giant as dams levels slid below 30 per cent. Loading In NSW, Carl Scully - a Labor minister whose ambition to replace Bob Carr as premier after his retirement in 2005 was stymied - said the plan for a Sydney desal plant was made in haste in August of that year by Morris Iemma's government. The call did not come from Sydney Water or even after cabinet consideration but was prompted by focus group feedback, Mr Scully said in a memoir published in 2017.

Opposition to Sydney's plant included then Liberal prime minister John Howard, who thought the operation to be too energy-intensive. "I do worry that the NSW government has been a little too ready to dismiss, almost out of hand, the options of recycling, and I'm not convinced that the case for preferring desalination has been strongly enough made," he told the Herald in July 2005. Andrew Stoner, the NSW Liberals' then spokesman on energy and utilities, also took a stance more in line with the Greens and environmental groups. "The water produced by a desalination plant is up to twice as expensive as recycling and up to four times as expensive as stormwater harvesting," he said at the time. "It doesn't make sense to allow hundreds of billions of litres of water to flow out to sea and to then draw in seawater to turn it into fresh drinking water at huge expense." Stuart Khan, a water expert at the University of NSW, says having a plant built at such expense, '"heavily influences the economics of choosing to use it", particularly as the margin cost of running the plant is dwarfed by the capital repayment costs. "If it hadn’t already been constructed, we’d probably be again waiting for a trigger of around 30 per cent of water storage capacity to be reached, before initiating construction," Professor Khan says.

When dormant, the plant costs average households $90 a year, and when operating only about $40 more, he says. Tornado strike and dam dive The plant's dormancy has not been incident-free. In December 2015, a tornado ripped through the 45-hectare site, leaving "earthquake-like" damage, a plant spokeswoman told the Herald last September. The main building's roof - some two hectares in size - was torn off, wrapping around other buildings. The array of membranes, though, was "tucked away nicely" and largely avoided damage, Davies says. He downplays any differences with the plant insurers that some have blamed for a stretching out the three-year repair job. John Holland, the plant's original builder, "had the edge" and eventually secured the contract.

Urgency was dulled, however, by a couple of wet years that had Sydney's largest reservoir - Warragamba - with 80 per cent of the city's storage capacity- spilling surplus flows in 2016 and coming close a year later. Then the skies dried up. Over the past 20 months, "the almost straight-line decline [in dam levels] is even more severe than the millennium drought". Davies says. "We've never seen anything like it." Inflows in the Sydney's dams were 143 billion litres in 2018, compared with usage of about 587 billion litres, according to WaterNSW and Sydney Water. Those inflows were barely above the nadir inflows of 136 billion litres in 1944, and a fraction of the long-run average of 1,396 billion litres.

What next? According to Sydney Water's Metropolitan Water Plan, if reservoir levels continue to sink to below 50 per cent full, preliminary design work will begin on stage two of the desal plant. Water-use restrictions would also start. At 40 per cent, those plans would become "detailed" and at 30 per cent full, plant construction would begin. "We could expand it fairly easily," to double the current size, Davies says. The main pipelines - whether for ocean intake or pumping drinking water to Erskineville - have already been scaled for a 500-megalitre a day plant. The existing owners - Utilities Trust of Australia and The Infrastructure Fund, and Canada's Ontario Teachers Fund - have "first right of refusal" if the government decides to build any additional plant at the Kurnell site.

Those three funds secured a 50-year lease over the plant in May 2012 for $2.3 billion, helping the then O'Farrell government retire about $2 billion in debt. More optimistically, should rainfall totals rebound and the dams start to fill, the desal plant will operate for a minimum of 14 months (including eight months to get up to full output). The plant can keep desalinating water until dam levels reach 70 per cent, or halt earlier if the operators decide, Davies says.