A swan swims through pollution and plant debris on the River Thames at Windsor (Picture: Maureen McLean/Shutterstock)

Water firms have been accused of a ‘scandalous cover-up’ after claiming they are unable to show how much sewage is being pumped into rivers and seas.

The Liberal Democrats said they used Environment Information Regulations to try and find out the volume of pollution but were told the data doesn’t exist.

Last year a parliamentary committee found that more than two billion litres of raw sewage had been dumped into the River Thames over two days following a storm.

Despite this, Thames Water told the Lib Dems it ‘does not record the volume of wastewater discharged into the environment via storm overflows’.

Other companies across England and Wales said the monitors they use only measure frequency and duration of spills, not volume.

Six of these companies now face legal action for allegedly misleading the Environment Agency and Ofwat about the number of times they have discharged sewage, meaning they may have overcharged customers.

Severn Trent Water, Thames Water, United Utilities, Anglian Water, Yorkshire Water and Northumbrian Water could end up paying over £800 million in compensation to more than 20 million customers if the cases are successful.

Water UK has said these claims are ‘entirely without merit’.

Scottish Water, which is publicly owned, does record and publish volume data, but only a small proportion of its discharges are monitored.

In Northern Ireland, sewage discharges are not routinely monitored and the last time information was released was in late 2021 in response to a question tabled in Stormont.

An aerial view shows the outfall into the River Thames, at Thames Water’s Crossness Sewage Treatment Works, in south east London (Picture: AFP via Getty)

A urinal on Sonning Bridge, residents suspect the mystery artist Impro has put the piece up in disgust at recent sewage levels released in to the Thames (Picture: Geoffrey Swaine/Shutterstock)

The Lib Dems believe the water firms in England and Wales are either lying about not holding volume data, or that the monitors they have installed are inadequate.

They said previous requests for information found nearly a quarter of monitors were faulty or had not been fitted throughout 2021 and 2022.

The party’s environment spokesman, Tim Farron, said: ‘These water firms could be guilty of a scandalous cover-up.

‘We have no idea how many billions of litres of sewage are being pumped into our precious rivers and lakes.

‘The true extent of environmental damage caused by this scandal is completely unknown. We don’t even know how much sewage is being pumped into bathing areas and shellfish water.’

A Water UK spokesman said the Lib Dem claims are ‘fabricated’ and ‘completely false’.

They added: ‘Water companies are investing billions of pounds to deploy around 50,000 monitors across overflows and in rivers.

‘These will take six different measurements defined by the Government, like spills, oxygen levels and different pollutants.

Water regulator Ofwat recently announced that from 2025 there will be new financial penalties for water companies when their sewage discharges breach targets (Picture: Maureen McLean/Shutterstock)

‘Unmatched anywhere in the world, this information will be published and open for all to see in virtually real time.

‘Only a tiny handful of monitors record volume. We could expand this, but bills would go up to pay for it.

‘We would prefer to get on and actually fix the problem rather than delaying progress by installing ever more monitors that would tell us nothing whatsoever about actual impact.’

A recent investigation by the Marine Conservation Society discovered around 6,000 emergency overflows – separate from the storm overflows water companies typically monitor – were not being recorded.

Emergency overflows are only supposed to be used in critical situations, such as if there is an electrical failure at a pumping station.

More than 90% of them are not monitored either for volume, duration or frequency.

Water companies are required to monitor only those that have the potential to discharge into shellfish waters.

The disgusting sight of raw sewage on the River Thames at Datchet in Berkshire (Picture: Maureen McLean/Shutterstock)

Mr Farron said: ‘Either these firms are withholding data from the public, which would be an extremely serious offence, or they just aren’t bothering to fit proper sewage monitors.

‘Ministers need to get tougher on these water firms by demanding they fit proper sewage monitors which are up to the job.

‘The Government is letting water firms get away with faulty and flawed monitors.’

Water UK and Thames Water have been contacted for comment.

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‘We have no idea how many billions of litres of sewage are being pumped into our precious rivers and lakes.’