Sewage has been pumped into the River Chess non-stop for the whole of March - sparking a safety warning.

The Chesham treatment works has been discharging since 6.45am on February 29, totalling over 438 hours and counting, according to a live map run by Thames Water. 

It means sewage could be present in the chalk stream which runs through the Chess Valley from Chesham to Rickmansworth, where it then joins the River Colne.

River Chess Association chair Paul Jennings said he was “disappointed” that recent works to increase the facility’s operating capacity had failed to stem the latest discharge.

He raised concerns that high groundwater levels meant that it may leak into the sewer system, meaning the treatment facility run by Thames Water is overwhelmed by the “significant increase in volume”.

Watford Observer: The sewage treatment works has been discharging continuously since February 29.The sewage treatment works has been discharging continuously since February 29. (Image: Thames Water)

In a statement to the association dated March 11, Thames Water admitted that the Chesham treatment plant had “a history of prolonged storm discharges” and that “high volumes of unwanted surface water” can enter the sewage system in areas like the Chess Valley.

It argued it had invested over £4 million in increasing the facility’s capacity by 40 per cent, meaning it can treat at least 353 litres per second of wastewater before it needs to use the storm tank.

Thames Water added that it had lined 144m of sewers and sealed 256 manholes to keep surface water out.

“We are clear that such discharges of storm sewage are unacceptable, even when they are permitted, and we are working to reduce them in the Thames region, in line with government targets.”

It added: “We absolutely recognise that more needs to be done and have more work planned that will both reduce further the amount of unwanted flow entering the sewer system and increase the resilience of the sewage treatment works.”

Mr Jennings said the extended discharge means residents should take care near the river and keep their pets away. If they do come in contact with the water, they should wash their hands thoroughly and use antibacterial gels.

The chair also warned that the leakages will change the river’s chemistry, reducing oxygen levels which could harm the plant life and fish that live in the waterway.

Recently released data on sewagemap.co.uk revealed which Hertfordshire rivers have been worst affected by sewage discharges this year.

Among the worst was the Grand Union Canal, with waste pumped into the waterway near Hemel Hempstead for a total of 612 hours since the start of this year.