Sewage treatment

Sewage treatment

Sewage is the water found in sewers.

Once we have used water, we pull the plug from the sink, flush the toilet or pour it down the drain where it enters the sewerage system. In the sewer there is a mixture of water used for a variety of purposes in the home, at work or in leisure activities, plus rainwater from roads, footpaths and roofs and water used for business and industrial purposes.

Sewage contains a wide range of waste products. It contains
• solids suspended in the water
• things dissolved in the water
• bacteria and other sewage micro-organisms living in the water.

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.

There are six stages in sewage treatment: -


Removes the large bits, sand and grit.
• Sewage contains lots of materials, such as paper, rubbish, plastics, cotton and grit, which must be removed before treatment can begin. The sewage is passed through a screen which traps this material which is broken up into smaller bits (macerated) and put into a skip.
• The screened sewage passes through the detritor which slows
down the flow of the water. Grit and sand which are heavier than
water separate out and sink to the bottom.

First settlement

Removes the small solids.
• The sewage enters a tank where it sits for a couple of hours allowing smaller particles to sink to the bottom.
• The water at the top of the tank flows to the next process.
• The sludge at the bottom of the tank is drawn off and treated
in a separate process called Accelerated Anaerobic Digestion

Biological phase

Removes things that are dissolved.
• Things that are dissolved in the water cannot be removed by settling so we use helpful bacteria to eat them.
• These bacteria live in either activated sludge tanks or in filter beds.

Second settlement

Removes dead bacteria and their waste.
• Once the sewage has been through the biological stage we let it settle again to make sure it is really clean.

Tertiary treatment

Removes any harmful germs.
• 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 of no danger to any plant or animal life.

Advanced Anaerobic digestion

Downloadable content

  1. thumb

    Sewage treatment

    Download the PDF version of the sewage treatment process.




With your help, WaterAid transforms lives by improving access to safe water and sanitation. Find out more

We use cookies on our website. By continuing to browse our website, you are agreeing to use our cookies. Terms and Conditions