How do sewage works work?


A sewage treatment works, also known as a wastewater treatment plant, is a facility that is designed to remove pollutants from raw sewage before it is released into the environment. The core function of a sewage treatment works is to purify and clean the water, making it safe for release into the environment and for human use. In the UK approximately 11 billion liters of waste water is treated everyday by approximately 9 thousand wastewater treatment works.

The stages of treatment

The process of treating sewage typically involves several stages, including:

  • Screening: The first step in the treatment process is to remove large debris, such as sticks and leaves, from the raw sewage. This is done by passing the sewage through a series of screens that remove the larger particles.
  • Primary treatment: The next step is to remove the solid matter from the sewage. This is done by allowing the sewage to settle in large tanks, called primary clarifiers. The solid matter sinks to the bottom of the tank and is removed, while the liquid is passed on to the next stage of treatment.
  • Secondary treatment: The liquid that is passed on from the primary clarifiers is then treated in a secondary treatment stage. This is typically done by using bacteria to break down the organic matter in the sewage. The bacteria consume the organic matter, releasing carbon dioxide and water, which leaves behind a sludge that is removed and sent to a sludge treatment process.
  • Tertiary treatment: The liquid that is passed on from the secondary treatment stage is then treated in a tertiary treatment stage. This is typically done by using physical, chemical, and biological processes to remove any remaining pollutants from the sewage. The purified water is then released into the environment.


As a sewage treatment works is designed to handle a maximum volume of wastewater its lagoons, pumps, pipes and supporting infrastructure are designed with this in mind. it simply doesn’t have the physical storage capacity to treat volumes beyond this design limit. It also requires power to operate the numerous pumps and aeration systems to move these liquids around the facility and provide the optimised conditions for microorganisms to break down the organic matter. This is why during storms (where the volume of waste water is too great) or power cuts, the treatment works is unable to operate and untreated sewage often enters the environment.
23rd January 2023
Tom McNamara


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