Concerns have been raised after sewage was released into the River Wandle.
The high amount of rainwater seen in recent weeks means more water is entering the Beddington Sewage Works.
Cyclists spotted sewage in the river on the Mitcham/Hackbridge border
Water has filled up the works’ storm tanks and, due to the sheer volume of water, sewage has been released into the River Wandle which flows through Croydon, Sutton, Mitcham, Wimbledon and Wandsworth.
Sewage debris, sludge and fungus has been seen along the river.
A spokesperson for Thames Water said the matter being released into the river was ‘heavily diluted wastewater’ and normal in such exceptional weather conditions.
David Usher, 69, from Wallington, cycles next to the River Wandle once a week and noticed the appearance of sewage yesterday.
He said: “There’s a huge amount of toilet paper waste pouring into it. It is polluted. All the water is filled with toilet water – the water is grey.
“It is horrible. It’s a real shame. I cycle here once a week and I’ve not seen it like that before.
“We did report it to the National Trust people we saw in Morden Hall Park.”
Yesterday the Environment Agency informed the Wandle Trust, a charity dedicated to maintaining the River Wandle and its catchment, that no environmental damage had been detected.
However, sludge scoured from the sewage work tanks may be getting into the river and is described as looking like shredded toilet paper.
The Wandle Trust said they were taking the situation very seriously.
The river has been plagued by problems in the recent past. In 2007 the river’s fish population was decimated by an accidental spill of sodium hypochlorite by Thames Water and in 2012 toxic red diesel found its way into the river.
A trout in the River Wandle
A spokesperson for the Environment Agency said: "We are aware of this and are monitoring the situation and impacts.
"Currently there has only been a low environmental impact with minor impact on the quality of water and no impact on fish or wildlife.
"We are working with Thames Water and other partners, such as the Wandle Trust, to manage the situation."
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