Yarra Valley Water


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Sewerage system

The sewerage system carries our sewage, or waste water, from homes and businesses to a sewage treatment plant.

Our sewage is all the waste water that leaves our kitchens, bathrooms, laundries and toilets.

We use two words to describe sewage leaving our homes:

  • blackwater is waste water that comes from the toilet;
  • greywater is waste water from our sinks, laundries, baths and showers.

There are lots of bugs in our sewage (and some can be really nasty), as well as chemicals called nitrates and phosphates, mixed in with the waste that people put down their drains.

Once the waste goes down the plug hole, it travels through an “S” bend, which makes sure no smells come drifting back into your house! There’s the same type of pipe on a toilet. From here it travels through a network of pipes, which make up the reticulated sewerage system. 

Every now and then, a sewer vent extends like a chimney from the sewerage system to release gases that build up in the pipes. All the pipes feed into bigger and bigger pipes, until they reach a trunk sewer, which finally takes all the waste to the sewage treatment plants. A trunk sewer is so big, you could drive a car through it! 

Melbourne has two big treatment plants – the Western Treatment Plant in Werribee and the Eastern Treatment Plant in Carrum. There are also lots of smaller treatment plants all around Melbourne. 

Once the sewage reaches the treatment plants it is cleaned up so that it can either be used for market gardens, or released back into the bay. Sometimes it can take up to 12 hours for the sewage to reach the treatment plants.

Did you know that for every 100 litres of sewage that goes to the treatment plant, over 99 litres are water droplets? The other litre is what we get rid of at the treatment plants. It contains all sorts of things which have gone down the drain, human wastes, soap, paper, food scraps and lots more.

There are lots of things that need to happen at the treatment plant to make sure the water is well cleaned before being released.

Firstly, it goes through a big screen, which is like a big sieve. It catches any larger things floating in the sewage. Things like cotton buds, plastics and even false teeth get caught on the screens, which is why it’s really important not to let anything go down the drain or toilet that shouldn’t be there! 

Did you know that toilet paper is a very special kind of paper?

Once it is flushed down the toilet and enters the sewerage system, it breaks down into tiny little fibres. Once it reaches the treatment plant, it is no longer paper, it just looks like part of the slushy brown water travelling to the treatment plant and doesn’t get caught on the screen! 

The water then travels into an aeration tank, where air bubbles are blown through it, like a big spa. There are some chemicals and bugs in the sewage that help to break down the organic matter. This eventually settles to the bottom of the tank as a sludge – like a thick, sloppy mud. This takes about one whole day to happen. 

The sludge is removed from the tank and the water then goes through a pit full of little pebbles to remove any bits and pieces still floating in it. The water droplets go through the holes between the small pebbles and filter out the bottom. The little bits and pieces get trapped. 

The waste water is pretty clean by this stage, but there may be some little bugs still left in it, so the last step in the process gets rid of them! The water travels under an ultra-violet or UV light, just like at a solarium, where any bugs are zapped. 

The water is now clean enough to leave the treatment plant to be released back into the natural water cycle, or used in large market gardens that produce all our fruit and vegetables!


  • Aeration – when air bubbles are blown through sewage like a big spa.
  • Blackwater – waste water that comes from our toilets.
  • Filter – allows water to travel through little pebbles, but catches little bits and pieces floating in the water.
  • Greywater – waste water that comes from our sinks, laundries, showers and baths.
  • Reticulated sewerage system – the network of pipes, pumps and equipment that transfers all our sewage (or waste water) from our homes and businesses to a treatment plant.
  • Sewage – all the waste water that goes down our sinks, out of the shower, from our washing machines and even from our toilets.
  • Sewer vent – like a chimney for the sewerage system. It prevents the build-up of dangerous and corrosive gasses that can damage the sewer and create an unsafe environment for our maintenance teams.
  • Sewage treatment plant – a large factory that cleans up all our sewage so that the water can be released back to nature.
  • Sewer vent – like a chimney for the sewerage system. It prevents the build-up of dangerous and corrosive gasses that can damage the sewer and create an unsafe environment for our maintenance teams.
  • Screen – like a big sieve that catches large items in our sewage.
  • Sludge – like a thick, sloppy mud which forms and settles at the bottom of the aeration tanks.
  • Trunk sewer – a very large pipe in the sewerage system that delivers our sewage to the treatment plants.
  • Ultra-violet or UV light treatment – zaps any bugs still left in the water before being released back to nature.