Inside a wastewater treatment plant (video transcript)

A transcript of our educational 'virtual tour' video, 'Inside a wastewater treatment plant'.

[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: Have you ever wondered where the water you use in your home goes? You know, the wastewater from your shower, sinks, washing machine and toilet.

[Oscar voice-over]: Every day, each of us produces about 200 litres of this wastewater — enough to fill a bathtub! Although it is 99.97 per cent water, it also contains oil, greases, detergents, nutrients and bacteria.

Oscar: So where does it go and what happens to it? Let’s find out.

[Oscar voice-over]: The wastewater from your home flows by gravity or is pumped through pipes and into a network of larger pipes called ‘sewer mains’. Its destination is a wastewater treatment plant. 

There, it goes through a series of treatment processes to make it safe enough to return to the environment.

Today, we’re going to take a look inside the Beenyup Wastewater Treatment Plant in Craigie.

It takes wastewater from homes and businesses in Perth’s northern suburbs.

But first, I need to change …
[Oscar changes into high-visibility personal protective equipment (PPE)]

Oscar: Hi Siobhan

Siobhan:  Hi Oscar 

Oscar: So Siobhan, is this a typical wastewater treatment plant?

Siobhan: Yes, most wastewater treatment plants use similar processes, but there can be slight differences depending on where they are located and how much wastewater they treat.
This plant can treat 135 million litres of wastewater every day.

Oscar: Wow, that sounds like a lot?

Siobhan: Yes That is enough to fill about 54 Olympic swimming pools.

Oscar: Not that we’d want to swim in it.

Siobhan: No, definitely not.

Oscar: So, where does it all start?

Siobhan: Well, lets have a look.

The first stage of the process is called ‘preliminary treatment’ but, to start with, we’re going to see where the raw wastewater comes in.

Siobhan: This is what we call an intake channel. It’s coming from more than 500,000 people in the northern suburbs. Once it’s here, it flows to the screenings building over there.

Oscar: It’s not in those pipes above us, is it?

Siobhan: No, this actually  part of the plant’s odour control – we’ll have a look at that later. The wastewater is actually flowing beneath us  using gravity.  In fact, the flow  from this plant to the ocean is all using gravity.

Oscar: That’s impressive – you’re saving energy and, I’m guessing, money.

Siobhan: Yeah, let’s go. 

Siobhan: The first thing we do is remove large objects, like rags, plastic and rubbish. This is done with specially designed filter screens and  this part of the process is called ‘screenings’.

The rubbish that’s collected here is taken away and disposed of at an approved landfill site.

Oscar: What kind of things get ‘screened’ out?

Siobhan: You’d be amazed at what we find — cotton buds, false teeth, even barbie dolls! They really clog up the system and can damage equipment, which costs a lot to replace.

Oscar: So we need to avoid putting things like that and other rubbish down the sink and toilet?

Siobhan: After screening, the wastewater goes into these grit removal tanks so that the inorganic material — they’re the bits made from rocks  or  minerals — can sink to the bottom. The water is drained  and flows on to the next stage.

The grit is washed and sent to an approved landfill site, together with the screenings.

Oscar: So it goes back over there to one of those bins?

Siobhan: Yes, that’s right.

Siobhan: Now it’s time for primary treatment.

These sedimentation, or settling, tanks help remove most of the solids in the wastewater. Inside each tank, the solids form a sludge at the bottom and mechanical scrapers push it to the end of the tank. The sludge is then pumped to the sludge treatment area.  and we will have a look that later too.

Oscar: So what is the wastewater looking like now?

Siobhan: Well Here’s a sample.  You can still see some  bits, but it’s a lot clearer than when it first came in.

Oscar: Wow you can really see the difference. So this wastewater is now going on to the next stage?

Siobhan: Yes, on to secondary treatment.

Siobhan: Under these covers are aeration tanks, where mechanical blowers pump air into the wastewater through fine membranes on the floor.

Oscar: But where does the air come from?

Siobhan: See that big grey pipe above us?  That’s taking air from the aeration building through to the tanks.

Oscar: Why do you need aeration?

Siobhan: Well, the microbes — which are  the good microscopic bacteria  — need oxygen to survive. 

[Siobhan voice-over]: While the microbes are in the aeration tanks, they break down and gobble up all the remaining organic matter and nutrients in the wastewater.

By the time they’ve done their job, the wastewater is almost ready to return to the environment.

Oscar: So you’re not using lots of chemicals to clean the wastewater?

Siobhan: No, it’s mostly natural. If we used chemicals, we’d have to have another process to remove them before returning the water to the ocean.

Siobhan: After the aeration tanks, the water flows through to the  secondary  sedimentation tanks. These  tanks don’t need to be covered because the water is pretty clear and doesn’t smell.

[Oscar sniffs air, smiles and nods] Siobhan: The water overflows at the perimeter of the tank and now, it is safe enough to return to the ocean.

Oscar: Can we take a look?

Siobhan: Sure.

[Go to one that has lid off.]

Siobhan: The algae you can see is just a sign of nature at work.

Oscar: Where does the water go now?

Siobhan: Over to that yellow concrete block.

[Point. Walk toward ocean outfall block]

Oscar: So this is where the treated wastewater begins its journey back to the ocean environment.

Siobhan: Yes, the big pipe you can see is taking water from here out to the ocean – all by gravity.

So this is the end of the treatment process at this plant, but... 

Oscar: It’s not the end of the water cycle — that keeps going.

Siobhan: Correct.

[Oscar voice-over]: Once the treated wastewater reaches the ocean, a long pipe takes it out to sea.

The pipe has small holes towards the end to help disperse it evenly into the sea. 

Salt, sunlight, oxygen and ocean currents all combine to continue its treatment.

To make sure it’s not harming the marine environment, there is regular monitoring and testing.

Ocean outlets are the most cost effective way to manage treated wastewater because they use very little energy.

Oscar: Okay, we’ve seen where the treated wastewater goes, but what about all that sludge? Where did that end up?

Siobhan: The sludge comes to this area and into those heated digestion tanks, where it’s broken down by bacteria. The tanks are 8 meters high and go down another 8 meters into the ground, each one holds 4 million litres of ...

Oscar: Sludge!  Eeewww.

Siobhan: We then remove the water from the digested sludge, and we end up with something called biosolids.  This is trucked off site to be made into fertilizer or  used for agriculture.

Oscar: So what’s the weird looking tank over there?

Siobhan: That’s for the methane gas  produced in the digesters. Some of it is used as fuel for heating and mixing inside the digesters.

Oscar: What about the odours?  I’ve noticed it doesn’t smell too bad around here, considering what comes in.

Siobhan: The smell could be pretty  unpleasant. But we do everything we can to reduce odours.

We cover those parts of the plant where smells could develop, and take odours from those  areas, through the overhead ducting, into chemical scrubbing stacks.

Inside stacks,  hydrogen sulphide and other odorous gases are removed, and the treated air is released to the atmosphere through those two stacks.

Oscar: They look pretty high?

Siobhan: They’re 50 metres — high enough to  ensure any remaining odours are dispersed into the air. 

Siobhan:  Finally we have the control room.

Oscar: There’s quite a lot going on here. How do you know it’s all working properly? 

Siobhan: We have  monitoring points throughout the plant and they all feed into our control systems.

In the control room, we can keep track of what’s happening — how the equipment is working, the quality of the water, and generally making sure everything is running smoothly.

Oscar: So, sounds like you’ve got it under control.

[Siobhan nods]

Oscar: Okay, I guess we can leave you to it.

Siobhan: Great thanks Oscar

Oscar: see you Siobhan

Oscar [to camera]: Well, now you know what happens to wastewater and why it’s so important to be careful with what you put down sinks, drains and toilets.

By putting rubbish in the bin, where it belongs, you’ll help keep the system running efficiently.

Speaking of systems running efficiently …

Little boy (to Oscar): Oscar can I please go to the toilet.

Oscar (to camera): And so the cycle continues.