Sewage-polluted water flowed into the Hogsmill River near Ewell this week after recent heavy rainfall saw water levels rise.

In footage posted to Twitter, employees of the South East Rivers Trust (SERT) captured the scene at one Thames Water "storm tank" site near Ewell, where heavy rainfall had caused the tank to overflow into a spillway that drains directly into the Hogsmill River.

Accompanying the footage on Twitter, a spokesperson for the Trust said:

"Sewers are overloaded from rainwater running off hard surfaces.

"They have a 'safety valve' to stop sewage backing up called a Combined Sewer Overflow and discharge into rivers.

"The TidewayLondon Super Sewer will help in central London but further out this is still the norm."

With so much of the region built on with tarmac and concrete that doesn't let water drain naturally, floods become more common and storm tanks like these fill up faster and can overflow.

Indeed, this existing method of avoiding sewage back up during heavy rains — by draining it off into rivers — seems to be standard practice in many areas.

Responding to a request for comment from the Comet, a spokesperson for Thames Water confirmed that the storm tank in Ewell was operating as normal and sought to reassure residents.

"The storm tanks are designed to capture excess wastewater and release it to the river during periods of heavy rainfall to stop the sewer network filling up and backing up into homes and businesses, and this is what happened due to the torrential rain this week.

"The tanks are fitted with alarms which alert us to any discharges into the river so we can mitigate the impact and clean up as much as we possibly can, and the clean-up from this week’s rain is already underway," the spokesperson said.

As the footage showed, storm tanks like the one in Ewell have special grates on them that filter out "larger solid items" such as sanitary products and wet wipes before discharging into nearby rivers.

Nevertheless, his current state of affairs is far from ideal, as SERT's Director Bella Davies told the Comet.

"Many people don't know that this is part of the system," she pointed out.

"Our drainage and sewage network is largely built on these old designs and old values...our rivers are such an important resource for our communities and our wildlife, and the Hogsmill in particular is this fantastic community asset.

"So many people enjoy it and I think a lot of people don't know that it, as all rivers do, gets sewage emptied into it like this," she said.

Surrey Comet: US government graphic showing the tiny amount of fresh water available on Earth. Credit: Howard Perlman, USGS; globe illustration by Jack Cook, Woods Hole Oceanographic Institution (©); and Adam NiemanUS government graphic showing the tiny amount of fresh water available on Earth. Credit: Howard Perlman, USGS; globe illustration by Jack Cook, Woods Hole Oceanographic Institution (©); and Adam Nieman

Ms Davies emphasized that SERT were aiming to raise awareness about this as being the "norm" in the hope of speeding up more environmentally friendly changes.

The current rate of change is slow, she said, despite SERT working directly with water companies like Thames Water in order to promote the protection of rivers in the South East.

"If people know about these problems they can start asking the powers that be whether it's water companies or MPs or central government to do something about it. To say, 'We love our rivers and we don't want them to be polluted in this way.'"

The SERT director pointed out that our reliance on fresh water in the world generally and the South East region in particular is underestimated.

Just 2-2.5 per cent of Earth's water is fresh, with most of that locked away in glaciers of ice in colder regions that are fast melting away due to climate change.

When fresh groundwater and soil moisture are also subtracted, that leaves around just 0.01 per cent of fresh water that makes up all Earth's rivers, swamps and lakes.

"In the South East of England we are one of the driest places per head in terms of water availability.

"We have one of the lowest water availability levels in Europe, on a par with Cyprus," she said. "We have to value our fresh water and there has not been enough action yet. We all have to work together to fix this," she said.