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About your water

Wastewater treatment

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. Sewage is the water found in sewers. It can be a mixture of water which has been used for a variety of purposes in the home, at work or in leisure activities, 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 with over 99.9% being liquid and less than 0.1% solid.

The sewerage system

The sewerage system is the network of sewers, pipes and pumps that lie unseen beneath virtually every road and street that carry sewage from where it is produced to the sewage treatment works to be treated and cleaned.

There are two types of sewerage systems:

Combined sewers carry both sewage and rainwater in a single pipe.
Separate sewers use two pipes. One takes sewage to a sewage treatment works and the second carries rainwater straight to a nearby stream or river, as rainwater does not require treatment.

Why is sewage cleaned?

Sewage treatment works remove things from sewage that could harm the environment, so that the water can be returned to a river or the sea. If they weren’t removed they would pollute the river or sea and reduce oxygen levels which are vital for the health of the rivers and sea.

There are six stages in sewage treatment:

Preliminary: removes the large bits, sand and grit.
First settlement: removes the small solids.
Biological phase: removes things that are dissolved.
Second settlement: removes dead bacteria and their waste.
Tertiary treatment: removes any harmful germs.
Sludge drying: removes water so that it can be recycled as a fertilizer or a fuel.


At all points along the way the water is continuously tested and monitored to ensure the right amount of chemicals are being added and that the sewage is being treated so that it is clean enough to be returned to the environment and will be of no danger to plant and animal life.

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Northern Ireland Water is a trademark of Northern Ireland Water Ltd., incorporated in Northern Ireland,
Registered Number NI054463, Registered Office, Westland House, 40 Old Westland Road, Belfast, BT14 6TE,
Tel: +44 (0)3457 440088 | Fax: +44 (0)2890354798.

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