Thames Water, the UK's largest water company, has been ordered to pay £61,049 after a serious sewage spill in Hampshire last July left at least 20,000 of fish dead, and the aquatic life in two brooks devastated.

The company pleaded guilty at Winchester Crown Court yesterday (8th) of causing sewage sludge to enter the Silchester Brook (Hampshire) and Foudry Brook (Berkshire) in July 2010. On July 21, 2010, the Environment Agency’s hotline received numerous calls from local residents about a strong smell of sewage and grey-coloured water in the Silchester Brook. Agency officers attended the scene and were greeted with a strong stench of sewage and thousands of dead or dying fish, including brown trout, perch and pike.

Officers dosed the river with hydrogen peroxide and deployed aeration equipment to maintain oxygen levels but the sewage continued to spread, affecting the total length of Silchester and Foudry brooks through Stratfield, Mortimer, Grazely, Three Mile Cross and Green Park in Reading. Residents were upset at the state of the brook following the incident as the brooks are used widely by the local community for agriculture and recreation. Over the course of the incident 17 members of the public raised concern about the colour and smell of the brook, and the large number of dead or dying fish within it. A local farmer rescued around 100 fish including chub, brown trout, perch and minnows in a makeshift aquarium on his property.

The court heard that Thames Water operates Silchester Sewage Treatment Works in Hampshire. In May 2010 two storage tanks used to store sewage sludge failed. Two redundant storm tanks were used to store sewage sludge – these were isolated from the sewage treatment process to prevent stored sewage sludge being pumped back into the works. However, on the morning of 20 July 2010 the two storm tanks were full and needed to be emptied.

In an interview under caution, Thames Water admitted that instead of waiting for a tanker already scheduled to remove the sludge, a Thames Water employee manually opened the valves on the storm tanks and pumped the entire quantity of stored sewage sludge to the inlet of the works. The highly concentrated material completely overwhelmed the system and a thick, sludgy effluent was discharged into the brook. Thames Water stopped discharging from the sewage treatment works and were able to remove some of the pollution from the watercourse with tankers. The company also imported clean effluent from Basingstoke sewage treatment works to dilute the pollution in the watercourse, but the impact of the incident was already catastrophic.