Sydney's desalination plant has been flicked on after much anticipation.

Its activation tells us Sydney's dam levels have dipped below 60 per cent and we need a backup source of drinking water.

There has been a renewed focus on desalination recently due to a prolonged drought putting metropolitan and regional water supplies under stress.

It may surprise some people to know the desalination plant is the first line of defence, with water restrictions not imposed until dams reach 50 per cent.

But what exactly is desalination and is it the answer to all our problems?

Where's the on button?

A simple email acts as the green light for turning the plant on.

"There is no big lever or button that is activated," plant chief executive Keith Davies said.

Instead, Mr Davies would have received an email this morning from Water NSW telling him that the dams have dropped.

"I will write back confirming I have received it and confirming I'm taking the plant into a restart mode," he said.

The plant contains over 36,000 reverse osmosis membranes which, under pressure, filter out particles. ( Supplied: Sydney Desalination Plant )

Mr Davies said although the plant sat idle most of the time, starting it up did not exactly fill him with excitement.

"I just want to be sure we don't let the people of Sydney down and we actually deliver on the promise … to have a climate-independent source of water," he said.

The plant is now given eight months to restart, which seems like a long time right?

But over that time 20 extra staff have to be hired, chemicals need to be brought onsite and the pipeline has to be flushed through and disinfected.

The water that will come out of your taps will travel through this pipeline.

However, Mr Davies told the ABC he thought the plant could be firing earlier than expected, perhaps in three to four months.

This is because the plant was tested quite recently — in 2015 — after it had to undergo a rebuild as a result of a freak tornado that hit the suburb of Kurnell.

Once fully operational, the plant will provide 15 per cent of Sydney's drinking water.

Desalination is happening right across Australia with plants in Victoria, South Australia, Queensland and Western Australia.

Costly to turn on but costly to keep it off

Predictably, some people are not pleased the plant is being turned on because it means an increase to water bills.

When the plant is not operational, and just sitting at the ready, it costs $90 a year per house.

Now the trigger has been pulled, Sydney residents can expect their water bill to climb by a further $25 to $30 a year.

So how do you get salt out of water?

Desalination is all about making use of ocean water. Afterall, it counts for 96.5 per cent of the Earth's water.



History shows humans have been finding ways to take salt out of ocean water since ancient Greece.

Desalination is, in fact, one of mankind's earliest forms of water treatment.

To do it on a large scale, the key is simple: pressure.

You may have heard the term reverse osmosis — this involves high pressure water being pumped to filters where very fine membranes only allow water through.

The remaining sea water, which carries all the salt, is returned back to the ocean.

At the Sydney desalination plant, 42 per cent of the sea water becomes drinking water.

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Isn't this bad for the ocean?

The extra salty water that is released back into the ocean from a desalination plant has long been considered dangerous to the environment.

But, it turns out, there is little to worry about.

The returned water has minimal ecological impacts, according to University of New South Wales research.

There is only a small 100-metre impact zone and the high salt levels are not toxic, study lead Graeme Clark found.

Instead it was changes in water flow due to the returned water that were of most concern, and while some invertebrates in the affected area declined in numbers, others increased.

The returned sea water has about twice the salinity than regular ocean water and it is one degree warmer, but it returns to normal about 50-75 metres from the outlet.

The plant's chief executive says it should take three to four months for drinking water to be ready. ( Supplied: Sydney Desalination Plant )

How is it powered?

Desalination is an energy-intensive industry and relies heavily on fossil fuels.

This makes it expensive and a huge producer of greenhouse gases.

This is particularly the case at Kurnell as it is one of the largest desalination plants in the world.

While the Kurnell plant is powered by the national grid, it is 100 per cent offset by wind at the Capital Wind Farm near Bungendore in NSW.

Most importantly, how will it taste?

The fresh water produced by the plant is supposed to be so clean it is actually tasteless.

But it must meet Australian Drinking Water Guidelines, so like water from the dams it has fluoride and chlorine added.

Between 2010 and 2012 the plant operated to prove its reliability and capacity, and then-premier Kristina Keneally described the water as "delicious".