Raw sewage has spilled into seawater in parts of England and Wales over the past week after heavy rains flooded sewage systems. The polluted stream triggered new pollution risk warnings and anti-swimming advice from the Environment Agency at dozens of swimming spots. Popular beaches in places like Sussex and Devon had to be closed when waste washed ashore.

Britain’s top health official called waste disposal a “growing problem” and a “serious public health concern for governments and regulators”, saying water companies needed to do more. Swimming or boating in water containing human waste can put people at risk for gastrointestinal disease, as well as respiratory, skin, eye, or ear infections. Waste spills can also contain microplastics, which are increasingly being found in the marine food chain.

“This is only the tip of the iceberg we are seeing now and there will be many more health challenges in the future due to waste pollution,” said Hugo Tagholm, executive director of environmental campaign group Surfers Against Sewage.

Why do waste spills occur?

The most common and least controversial of sewer discharges occurs when it rains on the road. The water flows into storm sewers and is then discharged directly into local rivers.

Problems can arise in areas with a combined sewer system, which conveys drained surface water, as well as wastewater discharged from homes and businesses, in one pipe. This type of infrastructure is common in Britain’s older cities, including London. Strong storms can overwhelm this mixed system. So, to prevent raw sewage from flooding roads or houses, the system is designed to release additional raw water, which contains some human waste, into rivers.

While discharge to local waterways is intended as a last resort, there are concerns that it has become a more common practice for some companies. “We have made it clear that water companies’ reliance on overflow is unacceptable and they should significantly reduce the amount of wastewater they discharge as a priority,” said UK Water Secretary Steve Double in a recent blog post.