How much raw sewage is released into lakes, rivers and the sea?

United Utilities repeatedly dumped millions of litres of raw sewage illegally into Lake Windermere between 2021 and 2023 and failed to declare it, the BBC has revealed.

Water companies can release untreated sewage when it rains heavily to stop homes being flooded but, as experienced at Lake Windemere, there have been cases where this has been done prematurely.

Without full treatment sewage can cause more damage to the local environment and pose a health risk to swimmers.

How much sewage is released into the sea and rivers in England?

Sewage spills into England's lakes, rivers and seas by water companies more than doubled in 2023.

According to the Environment Agency, there were 3.6 million hours of spills, compared to 1.75 million hours in 2022.

Water UK, the industry body for sewerage companies, said this was "unacceptable", but argued that the record levels were due to heavy rain and increased data collection.

However, the Environment Agency said that increased rainfall did not override water companies' responsibility "to manage storm overflows in line with legal requirements".

Some environmental charities blame water companies for a failure to fix leaky pipes and other damaged infrastructure - and criticise the regulator for not forcing them to act.

Why is sewage released into the sea and rivers?

Most of the UK has a combined sewerage system, meaning that both rainwater and wastewater - from toilets, bathrooms and kitchens - are carried in the same pipes.

Wastewater is usually sent to a sewage treatment works.

But capacity can sometimes be exceeded during heavy rainfall, especially if dry ground cannot quickly absorb large quantities of water.

This could lead to inundation of sewage works and potential flooding of homes, roads and open spaces.

The system is therefore designed to overflow occasionally, and discharge excess wastewater into the sea and rivers from combined sewer overflows (CSOs)

However, according to industry regulator Ofwat, some water companies have been "routinely releasing sewage" outside times of heavy downpours as a result of failing to managing their wastewater treatment plants in breach of their permits - known as dry spilling.

Earlier this year, the BBC found 6,000 potential dry spills by England's water companies in 2022.

Water company permits also require them to treat a minimum amount of sewage before releasing it, even when it is raining.

BBC Panorama has found evidence that for the last three years United Utilities was discharging sewage into Lake Windemere before reaching this threshold.

• Sewage dumped illegally in Windermere over three years

What is being done to tackle sewage discharges?

In October 2023, Water UK, the industry body, announced plans on behalf of its members to almost double spending to pay for upgrades and cut sewage discharges.

It said this would be the "most ambitious modernisation of sewers since the Victorian era", but that customer bills would have to rise by £156 a year to cover the cost.

The exact increase for households over the next five year period is still being negotiated between the water companies and Ofwat. A final decision is expected in December.

• Water bills: How much will I have to pay?

Since taking office in July, the Labour government has announced a series of proposed reforms, promising consumers higher compensation for sewage failures and the power to hold executives to account.

It plans to introduce new measures to ensure that money earmarked for investment and improvements cannot be diverted to pay salaries or dividends.

Ofwat and the Environment Agency are still continuing with their investigations into all water companies in England and Wales.

In August, Ofwat announced the first tranche of fines in its investigation. It is proposing a combined £168m penalty for Thames Water, Yorkshire Water and Northumbrian Water for failure to adequately invest in their infrastructure, leading to repeated sewage spills.

What are the health risks of swimming in polluted water?

In May 2024, a team of the UK's top engineers and scientists warned of the growing public health risk from human faeces in the country's rivers, and called for more regular testing.

The presence of faeces can expose people to bacteria such as salmonella and E.coli, which cause diarrhoea and vomiting, or viruses like hepatitis A which can lead to liver infection.

Rowers taking part in the 2024 Oxford and Cambridge Boat Race were told not to swallow any splashed water after E.coli was found

Also in May, thousands of residents in Devon were told to drink bottled water after their supply was contaminated with the cryptosporidium parasite - possibly as a result of a faulty valve on private land. Consumption of the parasite can cause prolonged diarrhoea.

How can I check if the sea near me is clean?

Every week between May and September, the Environment Agency measures the water quality at "bathing sites" across England, and in some places it issues daily pollution risk forecasts.

You can search its website by location. There are similar websites where you can find out about beaches and bathing water in Scotland, Wales or Northern Ireland.

But outside of these times and locations regularly testing for organisms carrying faecal-oral disease is not carried.

By the end of this year all water companies are expected to have maps showing near real-time discharges at their storm overflows to keep the public better informed.

How clean are UK waters overall?

Sewage spills have only been routinely monitored for a few years, so it is difficult to tell exactly how they have changed over time.

However, overall water quality has been monitored for decades. This also accounts for other sources of pollution, such as agriculture.

In May 2023, the Environment Agency said that 16% of assessed surface waters in England achieved "good" ecological status, including:

• 14% of rivers and lakes

• 19% of estuaries

• 45% of coastal waters

• 93% of designated bathing waters

Water quality is generally higher in other parts of the UK.

In Scotland, around two-thirds of surface waters are classed as having "good" ecological status.

In Wales the figure is 40%, and in Northern Ireland 31%.