Sewage discharged into the River Tyne in Newcastle upon Tyne, UK
Leslie Garland Photos / Alamy
Untreated sewage may be responsible for around three-quarters of the bacteria found in a river in the UK during a rainstorm, despite claims by some water companies that storm discharges have minimal effect on water quality.
Water companies in England are allowed to discharge raw sewage into rivers and seas during unusually heavy rainfall, when there is a risk of the system becoming overloaded and the sewage flowing back into people’s homes. In 2022, raw sewage water ended up in English rivers and seas 825 times a day average.
Raw sewage water contains a high concentration of bacteria such as E coli and intestinal enterococci, which can cause diarrhea. But until now, no studies had directly linked river bacteria levels to raw sewage discharges. In England, bacteria levels are only regularly monitored on three river stretches designated as official bathing areas.
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To investigate the influence of sewage overflows on river bacteria, David Werner at the University of Newcastle, UK, and his colleagues took several water samples from the River Ouseburn in Newcastle upon Tyne in 2021.
Werner and his team collected one set of samples during a period of dry weather, another set during a downpour, and another set 24 hours after a period of heavy rain. They also collected a sample from a sewage overflow that flowed into the river.
The researchers then analyzed the DNA found in all samples. By comparing the composition of bacteria in the river samples with the sewage samples, they were able to estimate what percentage of the bacteria in the river was due to the release of sewage. “It can help us differentiate between bacteria found in human guts compared to animal guts,” says Werner.
They found that during a rainstorm, when sewage is actively discharged into the river, between 72 and 77 percent of the bacteria are released from the sewage downstream of the discharge. “I was surprised how high this figure was,” says Werner.
The researchers saw levels of sewage-related bacteria drop 24 hours after rainfall, but Werner says these levels are still too high to be considered safe for humans. “If you have a lot of discharges, this legacy builds over time,” he says.
The team also collected 16 separate samples from another stretch of the same river in a local park, where swimming is common. Analysis of these samples confirmed that bacteria levels rose dramatically during rainfall and returned to more typical levels soon after.
The amount of intestinal enterococci found in the river exceeded between seven and 611 times safe levels for human health. E coli levels reached about 33 times higher than it took to reach bathing state.
Some water companies have claimed that the impact of storm drains on river water is likely to be minimal because they are highly diluted by stormwater. But the team estimated that the discharge released during a sewer overflow is likely to be about 5 percent raw sewage.
“That may sound like a small amount, but the way I think about it is if you have a cup of tea that’s about 100 milliliters, 5 percent is a teaspoon of sewage – you wouldn’t be drinking any of it, would you?” says Werner. “And because sewage is so highly concentrated in fecal bacteria, a lot of it ends up in the river.”
Although this study only looked at one river, Werner says it is likely representative of small rivers in urban areas in England, which have less water to dilute contaminants than larger ones.
Data like this can help people make informed decisions about whether or not to go wild swimming, he says Sean Tyrrell at Cranfield University, UK.
“It’s also important to think about the discharges of treated sewage that happen 24/7, not just storm surges,” he says. “Long periods of dry weather mean lower dilution of treated wastewater discharges and may be times when the public is more likely to come into contact with river water, so a global, risk-based approach is a good thing.”
A spokesperson for Northumbrian Water, which manages wastewater in Newcastle, said in a statement: “The results of this latest study give us a very useful insight. It is an area that has not been explored before and we are happy to work with partners such as the university to reduce the frequency and impact of our activities on the environment.”
A spokesperson for Water UK, representing the industry as a whole, said in a statement: “Every water company firmly believes that urgent action is needed to address the damage to rivers caused by sewer overflows. Water companies plan to spend £10bn over the next seven years on an intensive program to improve wastewater overflows – a tripling of current investment levels – as part of the largest sewer modernization since the Victorian era.”
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