Turning seawater into drinking water | Education resources | Water Corporation
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Turning seawater into drinking water | Education resources | Water Corporation


Water Corporation is the main supplier of
water, wastewater and drainage services in Western Australia.
It helps to ensure we have enough drinking water for the future by developing new sources,
increasing water recycling and encouraging efficient water use
As our population grows, so too does the demand for water.
Our drying climate means we need to find other sources of water, now and into the future.
I’m in Kwinana, south of Perth, and this is Cockburn Sound.
Behind these sand dunes over there is the Perth Seawater Desalination Plant.
Built in 2006, it was Australia’s first large-scale desalination plant .
Today, we’re going to take a look inside. .
[Greeting] So Siobhan, tell us about seawater desalination. Seawater desalination is where we take the
salt out of seawater to make it suitable for drinking. It sounds pretty straightforward. Yeah In theory, it is. I’m guessing seawater desalination is a good
option because there’s so much of it? Yes That’s part of it But, more importantly,
it is climate independent, which means it doesn’t rely on rainfall. Most of the earth’s water resource, 97 per
cent in fact, is salty water found mainly in our oceans.
The rest is fresh water. But a lot of it is frozen in glaciers and
icecaps. That means there is less than 1 per cent of
freshwater in the world readily available to us.
So, it makes sense for us to use some of the seawater and turn it into fresh, drinking
water. I was on the beach earlier and couldn’t see
any sign of seawater coming in. Yes, I should warn you, you might not actually
see a lot of water around the plant. But, rest assured, there’s a lot happening
in pipes, tanks and even below ground. For example, right here is where the seawater
is coming in. So there’s a pipe running from the beach up
to this point, underground? Not only that, the same pipe goes out into
Cockburn Sound for 200 metres. At the end, 10 metres below sea level, there
is a intake tower, 5 metres high, which is where the seawater slowly filters in.
The top 2 metres have course screens that allow seawater to flow naturally through,
and fish to pass in and out. It then flows by gravity through a 2-metre
diameter pipe until it reaches this point. What’s the first step? The screening process is first up, and it’s
done in these two units. Before the raw seawater can be processed through
the plant, we need to remove some of the larger particles.
Inside the screening units are mesh screens, a bit like flyscreens, which trap things like
seaweed. Next, it’s time for some dosing. What does this stage of the process is this
? After screening, we add substances to help
with coagulation. Coagu? huh? Coagulation. It helps bind smaller particles
to form larger particles. When it goes to the next stage, it?s easier
for these larger particles to be filtered out. The next part of the process occurs at the
‘dual media tanks’, which is a type of filtration process. So, what’s inside these tanks? In each tank, there is a layer of hard coal
and a layer of sand. Both layers together take up about 1.15 metres, or one third of
the tank. The water filters through each layer, filtering
out the larger particles. It then goes through these cartridge filters. Inside, they’re like straws with very tiny
holes in them. The water is pushed through the holes at high
pressure. But haven’t all the particles been filtered
out already? Yes, mostly.
But these are our back-up system. Even small particles can block up and possibly
damage the reverse osmosis membranes, so we need to make sure they’re all removed. In this building is the heart of the whole
process, reverse osmosis, or RO. It’s a bit noisy in here so you might need
these.[Both put in earplugs and enter building.] Siobhan voice-over:
In here, we have 6 high pressure pumps that are pumping the water from the cartridge filters
to the RO racks. Then, the filtered water passes through the
RO membranes to remove the salt In each membrane, there is a tube in the centre
which is where the desalinated water flows through.
So, we end up with two streams, a desalinated stream, which has no salt; and a concentrate
stream that has all the salt, making it twice as salty as the original seawater.
The yellow cylinders are recovering energy from the high pressure salty water coming
out of the membranes. They use that energy to pressurise incoming seawater so that the
pumps don’t need to work as hard. The second stage of reverse osmosis further
polishes, or cleans, the water and reduces bromide levels. Wow, that was pretty impressive.
But what happens to all the salt that’s now in the ‘RO’ membranes? We have to keep the membranes clean by regularly
backwashing, or flushing them out. It must take a lot of energy to run the plant. It does.
But, to make seawater desalination plants environmentally sustainable, we offset their
energy requirements against wind and solar farms. So, can we drink the water now that it’s been
through reverse osmosis? Probably not a good idea.
Although we’ve taken the salt out of the water, it doesn’t have the minerals we need to make
it suitable for drinking. So, how do you make it good enough to drink? Ah,Follow me. Once the salt has been removed, we re-mineralise
the water through these purple pipes. Inside, there are dosing spears. if you imagine
a hedgehog whose spikes or spears are putting chlorine, lime water, carbon dioxide and fluoride
to the water. I understand chlorine disinfects the water,
and fluoride helps prevent tooth decay, but what about the lime water and carbon dioxide
what do they do? Lime water helps maintain the necessary alkalinity
level in our drinking water system and , carbon dioxide helps the lime water to dissolve into
the water. In this tank, we’re mixing lime powder with
water before adding it over there. Now, the water is stored in this tank here
before being pumped offsite. It can hold around 12 million litres, or roughly
5 Olympic sized swimming pools. OK, So does this water go straight to our
homes now? Not just yet.
In summer, first it will go es to a reservoir where it mixes with groundwater and before
being treated again and then being added to o our drinking water supply.
In winter, when we use less water, some of it is added to dams. Ok ,So we’ve seen water appens to all the
fresh water, but what about everthing we’ve taken out of the seawater, where does that
go? Okay, we know where the fresh water’s going.
But what about everything that’s been taken out of the seawater? Where’s that now?
Once the coagulated particles have been removed, all the concentrate goes through a pipe, out
to sea about 500 metres, and to a special diffuser system that mixes the concentrate
with seawater. Ocean tides and currents continue to mix the
water. Within 50 metres of the diffuser, the concentrate
is back to normal seawater salinity levels. As you would imagine, there is ongoing monitoring
of the ocean environment and reporting to the relevant environmental agencies.
What you might not imagine is that some ocean life can flourish around this area. And finally, we have the control room.
The processes are both manual and automated with multiple points of control.
The plant is monitored 24 hours a day all year round with stringent emergency procedures
in place. And, along with other plants, water quality
monitoring and testing is critical to ensure drinking water standards are maintained. Thanks Siobhan, it’s good to know that we
have such a large source of water we can use without relying on rainfall. Yes, it is.
But even though our two desalination plants can provide almost half of Perth’s water needs
today, they still cost money to build and operate.
So we still have to be careful with how much water we use. Well After seeing what it takes to make seawater
drinkable, I know I’ll be doing my bit. Thank you for the tour ..
No worries see you next time. Bye
.

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