Have you ever wondered what happens to turn the brownish water in the River Murray into the clear water that pours out of our taps?
SA Water recently invited Fairfax Media to visit one of its treatment plants to find out.
This particular plant supplies potable water to Murray Bridge and surrounding districts; there are 29 others, big and small, old and new, across South Australia.
Our guides are Aaron Marshall, the controls the plant’s processes every day; Adam Medlock, an operations manager for Trility, the company which manages it; and SA Water contracts manager Dan Bonini.
Water drawn from the river is pre-treated with powdered and activated carbon, which removes some of its dirty colour and unpleasant taste.
"Aesthetics is an important part of SA Water water," Mr Bonini says.
"It can be okay from a visual point of view, but when you get River Murray water, the taste can be a bit nasty. We work with Trility to add carbon to the water if required and make it taste a bit more palatable."
Alum and a coagulant agent are added to the water, which is then mixed at a high speed to make them bond with any dirt or other impurities.
Dirt is negatively charged and the chemicals positively charged, Mr Medlock says, so they are naturally attracted to each other – "it looks quite simple but the chemistry behind it is pretty complex".
Lime may also be added to correct the water's pH level, or acidity.
In flocculation bays – your new word of the day – the water is then agitated more slowly as clumps of greenish-brown dirt float to the surface.
"It starts as a sort of pin-flock and eventually turns into something like that," Mr Marshall says, pointing.
Contrary to expectations, there is no unpleasant smell.
The sludgy clumps settle on the bottom of a series of clarifiers where they are sucked away, letting the clean water float to the top.
Any remaining dirt settles in a honeycomb of hexagonal tubes set at a 60-degree angle, accompanied by a sound like a trickling creek.
Four or five megalitres of water is kept in the plant's tanks at any given time.
It can process up to 38ML per day in summer, though it averages only 11 over a year – “there is plenty of spare capacity,” says Mr Bonini.
The clarified water passes through four filters containing sand and coal.
This is what River Murray water might have looked like in centuries past, Mr Marshall says.
The filters are cleaned every 72 hours.
Into two channels it goes, to be exposed to ultraviolet light.
"It doesn't actually kill anything, but it inactivates any organisms that might be in there," Mr Medlock says: "it stops anything from being infectious.”
In go traces of chlorine, which kills any bacteria which might have survived the UV treatment, and remains in the water to serve the same purpose.
The gas is kept in bright yellow drums marked "dangerous poison", surrounded by leak detectors and alarms; but only 3.1 milligrams is added to a litre of water, Mr Marshall says, "a very small amount".
Without it, we would have to boil water for human consumption.
In goes fluoride at the request of SA Health to strengthen people's teeth; and a caustic agent, sodium hydroxide, to keep the chlorine from decaying and bring the water's pH level up again.
"You only need to go out of specification a small amount before you cause an issue, so we have to keep it pretty well controlled," Mr Medlock says.
The finished product is rigorously tested, both at the plant and through samples collected from customers' homes and businesses.
If anything is out of balance, the doses of chemicals added at the plant are adjusted accordingly.
"We have testing of our testing every month to make sure our technique is foolproof," Mr Medlock says.
"We have back-ups to our back-ups."
Waste water from the plant flows into a settling pond where it is left to dry for 12 months.
The resulting sludge is dug up and disposed of at a waste depot eight kilometres away, or used to rehabilitate vacant land elsewhere.
As a pond fills up, some of the water is sucked up and fed back into the start of the cycle.
The drinkable water from this plant flows into a large, covered tank before being pumped to tanks at White Hill – beside the main freeway exit to Murray Bridge – in closed pipes, to remove any chance of contamination.
From that point it is gravity-fed to everyone's taps.