Turning seawater into drinking water (video transcript)

This is a transcript for our video that explains how seawater is turned into drinking water at our Seawater Desalination Plant in Kwinana.

[music plays]

Oscar (voice-over): 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.

Oscar (to camera, walking along a beach): 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.

[Siobhan and Oscar say hello and shake hands]

Siobhan: Hi Oscar, how are you?

Oscar: Hi Siobhan, I'm good thank you. So Siobhan, tell us about seawater desalination.

Siobhan: Seawater desalination is where we take the salt out of seawater to make it suitable for drinking.

Oscar: It sounds pretty straightforward.  

Siobhan: Yeah, in theory, it is.

Oscar: I’m guessing seawater desalination is a good option because there’s so much of it?

Siobhan: It's also important because it's climate independent, which means it doesn’t rely on rainfall.

Siobhan (voice-over): Most of the earth’s water resource, 97% 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% 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.

[Walking outside the plant, both wearing Protective Personal Equipment (PPE)]

Oscar: I was on the beach earlier and couldn’t see any sign of seawater coming in. 

Siobhan: Yes, I do have to 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. 

[Seawater is animated in the pipe to illustrate where it is]

Oscar: So there’s a pipe running from the beach up to this point, underground?

Siobhan: Not only that, the same pipe goes out into Cockburn Sound for 200 metres.

Siobhan (voice-over)[Animated illustration shows how the intake tower works]
At the end, 10 metres below sea level, there is a  intake tower, 5 ½ 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.

[Back to Oscar and Siobhan]

Oscar: Back to the process. What's the first step?

Siobhan: The screening process is first up, and that happens in these 2 units.

Siobhan (voice-over): 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.

Oscar: What stage of the process is this?

Siobhan: After screening, we add substances to help with coagulation.

Oscar: Coagu… huh?

Siobhan: Coagulation. It helps smaller particles bind together to make larger particles. When it goes to the next stage, it’s easier for these larger particles to be filtered out.

Siobhan (voice over): The next part of the process occurs at the ‘dual media tanks’, which is a type of filtration process.

Oscar: So, what’s inside these tanks?

Siobhan: 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.

[Shows sample]

Inside, they’re like straws with very tiny holes. The water is pushed through the holes at high pressure.

Oscar: But haven’t all the particles been filtered out already?

Siobhan: Mostly. This is like a back-up. Even small particles can block up and possibly damage the reverse osmosis membranes, so we need to make sure they’re all removed.

Siobhan: In this building is the heart of the whole process – reverse osmosis, or RO. It’s a bit noisy in here so you'll need to put these in.

[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 2 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.

Oscar: Wow, that was impressive. But what happens to all the salt that’s now in the ‘RO’ membranes?

Siobhan: We have to keep the membranes clean by regularly backwashing, or flushing them out. 

Oscar: It must take a lot of energy to run the plant.

Siobhan: Yes, it does. But, to make seawater desalination plants environmentally sustainable, we offset their energy requirements against wind and solar farms. 

Oscar: So, can we drink the  water now that it’s been through reverse osmosis?

Siobhan: Probably not a good idea. Although we’ve taken the salt out of the water, it doesn’t yet have the minerals we need to make it suitable for drinking.

Oscar: So, how do you make it good enough to drink?

Siobhan: Ah, follow me.

[Head to dosing area]

Siobhan: Once the salt has been removed, we re-mineralise the water through these purple pipes. Inside are the dosing spears – if you imagine a hedgehog whose spikes or spears are putting  chlorine, lime water, carbon dioxide and fluoride to the water.

Oscar: 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?

Siobhan: Lime water helps maintain the necessary alkalinity levels 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.

Siobhan: Now, the water is stored in this tank here before being pumped offsite. It can hold around 12 million litres of water, or roughly 5 Olympic sized swimming pools.

Oscar: OK, So does this water go straight to our homes now?

Siobhan: Not just yet. In summer, first it will go to a reservoir where it mixes with groundwater, before being treated again and then being added to our drinking water supply. In winter, when we use less water, some of it is added to dams.

Oscar: Ok, so we’ve seen water happens to all the fresh water, but what about everything we’ve taken out of the seawater – where does that go? 

[Underwater footage of the pipe with a diver swimming around it]

Siobhan (voice-over): 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.

[They enter the control room at the plant]

Siobhan: And finally, we have the control room.

Siobhan (voice-over): 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.

Oscar: Thanks Siobhan, it’s good to know that we have such a large source of water we can use without relying on rainfall.

Siobhan: Yes, it is. But even though our 2 desalination plants can provide almost half of Perth’s water needs today, they still cost money to build and operate. So we still need to be careful with how much water we use.

Oscar: Well, after seeing what it takes to make seawater drinkable, I know I’ll be doing my bit. Thank you for the tour...

Siobhan: No worries - see you next time.