Waste Water and Storm Discharges

In order to receive messages when sewage discharges occur you can sign up to the River Chess Association Facebook page, when they are alerted by Thames they publish a message.

Thames Water is responsible for managing the wastewater in Three Rivers and they are obligated to ensure that the water entering the rivers is of the best possible quality.

Chesham Sewage Treatment Works (STW) is designed to treat the sewage from the surrounding catchment and discharge this treated effluent into the river to the standards required set out in the site permit (administered by the Environment Agency).  During heavy rainfall the capacity of these works can be exceeded, which means possible inundation of the sewage works and the potential to back up and flood peoples’ homes, roads and open spaces, unless it is allowed to spill elsewhere.

In addition to the standard permit Thames Water have a  storm discharge permit, issued and enforced and regulated  by the  Environment Agency, which allows diluted sewage from Chesham STW to be discharged into the Chess if the storm tank’s capacity is exceeded.

Of late, these discharges have occurred because of the inability of the works to fully treat all incoming flows during exceptionally wet weather. While rainfall is the most visible contributing factor, high levels of the water table (groundwater) throughout the area also have a significant impact on the flow levels within the sewer network.

In Chesham, Thames Water’s investigations have found that ground water ingress into the sewage system is considerable. It enters  through cracks and leaks in the sewer pipes, surface water runoff from saturated fields, highways, surface water misconnections into the foul sewage system (such as down pipes from roofs), and sometimes flood water from the River Chess entering sewers directly through manholes. Together this additional groundwater greatly exceeds the capacity of the plant which was just designed for sewage, and so Thames Water have to discharge this excess diluted sewage into the Chess to prevent sewage flooding on land.

The Environment Agency works closely with Thames Water to ensure they are closely monitoring and reporting back on the storm discharge activity. This leads to investigations of the cause of the discharge and directs investment to find solutions.

Extreme weather events are becoming more frequent which results in more storm discharge, so Thames Water are taking a twin-track approach to resolve the issues by increasing treatment capacity of the sewage treatment works, whilst dealing with known sources of ground water infiltration and undertaking further investigations in the sewer network.

There is a huge programme of work underway at present, which includes temporary fixes to increase treatment capacity and reduce discharges, repairing pipes to prevent ground water infiltration (as of June 2021 this project was 67% complete),  and a project to increase the capacity at the plant by 39% which will be complete in 2023.


How to Report an Environmental Incident

Please call the Environment Agency hotline to report

  • damage or danger to the natural environment
  • pollution to water or land
  • poaching or illegal fishing
  • dead fish or fish gasping for air
  • main rivers blocked by a vehicle or fallen tree causing risk of flooding
  • flooding from any river, stream, canal, natural spring or the sea
  • incidents at Environment Agency-regulated waste sites
  • illegal removals from watercourses
  • unusual changes in river flow
  • collapsed or badly damaged river or canal bank

Incident hotline
Telephone: 0800 80 70 60
24-hour service



Under the Bathing Water Directive (BWD) the River Chess is not a Designated Bathing Water. Members of public and businesses should be aware that rivers are not sterile environments and bacteria are widespread both upstream and downstream of the sewage works at all times. All water users should take relevant health and safety measures.

If you do choose to swim in rivers or canals you do so at your own risk.

There are many factors to consider when interacting with rivers. Safety advice can be sought from the Rivers Trust or Royal Life Saving Society