In a May 2024 report by the Royal Academy of Engineering, a group of top scientists in the UK issued a warning about the growing public health risk from human faeces in the country’s rivers. The contents of raw sewage include bacteria, viruses, harmful chemicals such as PFAS and microplastics. Meanwhile, associated health risks include bacterial infections and viruses such as hepatitis A.

That same month, a town in Devon saw an outbreak of a diarrhoea-type illness caused by drinking water contaminated by cryptosporidium, a water parasite found in human and animal faeces. Around 40,000 residents were issued a ‘boil water’ warning. In 2023 sewage spills into England’s lakes, rivers and seas by water companies more than doubled compared with the previous year. The Environment Agency, a government department, reported that only 16% of surface waters in England qualified for ‘good’ ecological status.

The consequences of sewage pollution extend beyond human health. Pollutants in rivers will inevitably end up in the ocean, where they trigger a cascade of environmental impacts. These contaminated waters create hypoxic (oxygen-depleted) areas through a phenomenon called eutrophication, where excessive inputs of nutrients such as nitrogen lead to an overgrowth of algae that suck up oxygen and damage ecosystems. In one hypoxic area of the River Thames estuary, around a quarter of the excessive nitrogen load is estimated to come from sewage discharges. This is happening all around the UK’s coasts. In Cornwall’s Widemouth Bay, sewage outfall is linked to hypoxic events that have killed two local mollusc populations. In Pagham Harbour, on the south coast, excessive algal blooms caused by both municipal and agricultural pollution have cut off food supplies for bird, fish and crustaceans.

Sewage is also a source of microplastic pollution. A study in greater Manchester found that untreated wastewater in UK rivers is resulting in clusters of microplastics in riverbeds, with levels of contamination exceeding 130 plastic particles per gram of sediment. Wet wipes, condoms and similar products that are frequently flushed down toilets release microscopic plastic particles into wastewater systems, further contributing to the global plastic pollution problem.

Many of the country’s sewage pipes simultaneously collect rainwater and sewage. They are connected to safety valves called Combined Sewer Overflows (CSOs), which are intended to prevent sewers from flooding by reducing pressure on the system in extreme weather conditions—but also mean that untreated sewage spills out of the sewers and into waterways whenever there is heavy rainfall. In 2021 there were 1,651 CSOs located within a kilometre of a marine protected area, spaces that are intended to protect fragile ecosystems like salt meadows, mudflats and reefs.

But the problem with the UK’s infrastructure goes deeper than CSOs, says David Johnson, technical director at the Rivers Trust, a network of local river trusts working to protect and restore rivers and ecosystems across the country. “CSOs are the canary in the coal mine,” he says. “They’re a pressure relief valve for a sewage system that’s not functioning well, due to a mixture of underinvestment, climate change and other factors that have made the system increasingly overloaded.”

The Rivers Trust put together the UK’s first national interactive map of sewage overspills, tracking how often CSOs discharge sewage into the rivers. “The logic behind CSOs has always been that they are only discharged during exceptional flooding circumstances, when the only alternative is flooding someone’s house with sewage,” explains Mr Johnson. “But the reason our maps have had so much impact is because these CSOs are not going off exceptionally. Some of them are going off all the time, which shows us that the treatment works are not working very well. The CSOs aren’t the biggest problem, they’re a sign that the system isn’t working.”

The UK’s sewer systems are some of the oldest in Europe. London’s historic sewer network was constructed in Victorian times following the infamous ‘great stink’. A hot summer in 1858 caused raw sewage along the Thames to bake in the sun, creating an unbearable stench that forced Parliament to cease operations.

The network of brick-lined tunnels built in the 1860s by Joseph Bazalgette was revolutionary at the time but has since fallen behind. Modern European sewers typically employ a dual system that separates rainwater from sewage, preventing treatment plants from becoming overwhelmed during heavy rainfall, while British systems continue to combine the two streams. “Our systems are the oldest, the most tired, so we’ve got more of a historical legacy to deal with than Europe,” says Mr Johnson. “I’ve been swimming in places in Switzerland and France where I wouldn’t dare swim in England.”

One piece of the puzzle will be improving data on pollution incidents. The Rivers Trust’s map documents spills that are reported, but unreported incidents may be even more widespread. A BBC Panorama investigation found that some water companies have been illegally dumping sewage into rivers in England and Wales, regularly breaching the conditions of sewage treatment permits. Sewage legally released into rivers must be screened and passed through storage tanks before being discharged; despite this, the investigation identified one company dumping sewage without a permit and another reporting incorrect information about the contents of the sewage.

To try and detect unreported spills of untreated sewage, a study by researchers at the UK Centre for Ecology and Hydrology used machine learning to identify spills through pattern recognition. Using data from two treatment plants, the study recognised 926 cases where untreated sewage was being released for at least three hours at a time. In response, Water UK, the national water industry body, announced that it would invest £1.1bn to improve storm overflows and wastewater treatment works, noting that companies are already beginning to use AI techniques for water management.

Another crucial area for improvement is catchment management, says Mr Johnson. “Effectively, the whole catchment is connected to the sewage treatment system, so you want to store as much water in the catchment as possible, whether in ponds or through planting trees—this prevents the water from rapidly entering the sewage system and overwhelming the sewage treatment works,” he says. While engineering updates to the sewage system will be required to make sure that it is big enough for increasing inputs, these sorts of nature-based solutions for catchment management can make an important contribution. “Engineering will always do the heavy lifting, but green solutions can solve 10-20% of the problems—and if it gets even 10% of the budget, we can do massive things with that kind of funding,” says Mr Johnson.

Nature-based solutions are also important for another reason: getting communities engaged with the sewage problem at the grassroots level. Years of horror stories about polluted rivers and beaches have brought public outrage to a boiling point. In November 2024 15,000 people marched along the Thames to Parliament, holding signs with slogans like “species not faeces”.

While the government has promised tougher regulations, Mr Johnson argues that lasting change will require a twin-track approach: strengthening oversight while supporting partnerships for local initiatives that tackle the problem at its source. “Some of the communities we work with are experiencing flooding, and have no control over why this is happening to them,” he says. “When they go out and start digging ponds and planting trees, they feel like they’ve got some control. Nature-based solutions can make a small but significant contribution, but the way they encourage behaviour change and engagement with the environment is invaluable.”