In the sewer, there's a mixture of water that was used for a variety of purposes in the home, at work or in leisure activities. There's also rainwater from roads, footpaths and roofs, and water used for business and industrial purposes.
Sewage contains a wide range of waste products. It contains:
On average, each of us generates 135 to 180 litres of sewage a day. Over 99.9% of sewage is liquid, with less than 0.1% solid.
Sewage contains lots of materials, like
which must be removed before treatment can begin.
The sewage is passed through a screen to trap this material, and then the material is macerated (broken up into smaller bits!), and put into a skip.
The screened sewage passes through the detritor, which slows down the flow of the water. Grit and sand sinks to the bottom, before the second stage begins...
The sewage enters a tank where it sits for a couple of hours allowing smaller particles, like grit and sand, to sink to the bottom.
The sludge at the bottom of the tank is drawn off and treated in a separate process called Accelerated Anaerobic Digestion.
Things that are dissolved in the water cannot be removed by settling, so we use bacteria to eat them.
These bacteria live in either activated sludge tanks or in filter beds.
Once the sewage has been through the biological stage, we let it settle again to make sure it's clean.
At some sewage treatment works, the treated sewage is passed through ultra-violet lights before it finally re-enters the natural water cycle.
By passing the water through the ultra-violet lights, any disease causing micro-organisms left in the water are made harmless.
This treatment usually occurs at our coastal works. On completion of sewage treatment, the water is suitable for release into rivers and the sea.
Because the polluting matter has mostly been removed, it is not considered dangerous to any plant or animal life.