Thames Water has been fined £125,000 for polluting the River Wandle in what the court described as 'a 5km tragedy' for the water course.
The Environment Agency has not held back in its condemnation of the water company, claiming the chlorine spill has wiped out 20 years of painstaking river restoration work in a single day.
Croydon Crown Court heard how in September 2007 a large quantity of industrial strength chlorine leaked from a sewage treatment plant during a cleaning operation in Mitcham, South London.
A scientist working for Thames Water noticed the strong smell of chlorine in the outlet channel and instructed the staff cleaning the plant to stop their work, before informing the site manager.
The site manager decided that 'very limited damage' would occur to the environment and they would deal with the incident internally.
The Environment Agency was not informed.
More than two tonnes of dead fish were removed from the river over the following three days, with the pollution also bleaching and killing large amounts of vegetation on the river bed.
It took three days for Environment Agency officers, helped by members of the Wandle Trust and local angling clubs, to remove some two tonnes of dead fish from the river. An experienced Environment Agency officer at the scene said that it was the worst pollution incident he had ever dealt with.
Historically the river has suffered extreme pollution and was officially declared a sewer in the 1960s.
But over the last 20 years it has become a vibrant rich habitat due to better environmental regulation, a fish stocking programme and huge local enthusiasm for the river which has resulted in a vast improvement of water quality.
Environment Officer Peter Ehmann - who was one of the first on the scene, said: "This pollution effectively wiped out 20 years of painstaking restoration work on the River Wandle.
"For many years individuals and organisations, including the Environment Agency and the Wandle Trust, have achieved great improvements to water quality and aquatic life in the Wandle. This incident is a major set back to all their hard work."
In sentencing Thames Water the Court took into account the company's early guilty plea and its work with the Environment Agency on the River Wandle since the incident.
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