River Nidd


New research suggests that approximately 11 billion litres of raw sewage were discharged from a sample of 30 water company treatment works in a single year.

The study aimed to uncover the volume of effluent released from storm overflows by water companies, as they are not currently required to disclose the volume of raw sewage discharged during such events. While regulators only demand data on the number and duration of discharges, recommendations from MPs on the environmental audit committee to install volume monitors have been rejected by ministers.

The research, conducted in 2020, analysed 30 treatment works operated by nine of the 10 water and sewerage companies in England and Wales. The estimated volume of raw sewage discharged from these works amounted to 11 billion litres, equivalent to 4,352 Olympic pools. The analysis was carried out by Prof Peter Hammond, a mathematician specialising in sewage discharge data. He has previously revealed to MPs that the scale of illegal raw sewage discharges by water companies is ten times higher than officially reported.

Prof Hammond emphasised the importance of establishing the volume of sewage discharges by water companies. He argued that the government’s target of reducing raw sewage releases to 20 per year by 2025 lacks robustness due to the absence of a requirement to disclose the volume of sewage discharged during each event.

“There is still no data readily available showing the volume of untreated sewage discharges,” he said. “Water companies have some idea, but the regulators [Ofwat and the environment agencies in England and Wales] and the government [the Department for Environment, Food and Rural Affairs] probably have no idea. Sewage detritus in rivers, on beaches and in seas offers clues but may not reflect the volume of discharges.

“So what is the potential discharge volume for 20 spills per overflow per year?”

Out of the 30 treatment works analysed, only one, the Mogden sewage treatment works in west London, had volume monitors installed. In 2020, Mogden, which serves over 2 million people, released a volume of raw sewage equivalent to 2,768 Olympic pools. The average volume of sewage discharged per spill from the remaining 29 treatment plants was approximately 1.3 Olympic pools. Thus, even if the government achieves its target of 20 spills per year by 2025, it would still result in significant volumes of untreated sewage being released, according to the analysis.

The River Nidd in North Yorkshire, one of the river catchments examined by Prof Hammond, receives untreated sewage discharges from at least seven treatment works. Calculations were possible for estimated volumes from four treatment works: Pateley Bridge, Harrogate North, Darley, and Kirk Hammerton. It was estimated that the river received an equivalent of 317 Olympic pools of raw sewage from these works in 2020.

“Even if the government’s storm overflow discharges reduction plan target of 20 spills per overflow per year were to be achieved, some treatment works could still discharge a volume equivalent to 26 Olympic pools of untreated sewage annually,” Hammond said.

Measuring the volume of discharges was vital to establish the environmental impact of them upon rivers, he said. “Individual rivers receive direct, simultaneous discharges of untreated sewage from multiple storm overflows on their journey from source to sea,” Hammond said. “So, during longer spills, the lower reaches of a river may already be polluted from upstream discharges when yet more overflows downstream discharge untreated sewage.”

David Clayden, a member of the Nidd Action Group, expressed the belief that water companies should disclose the volume of raw sewage being discharged into rivers. The group, in collaboration with the University of Leeds, is conducting testing on the river and working towards obtaining bathing water status for a section known as the Knaresborough Lido to drive efforts for a clean-up.

“There is a real buzz about this issue here as there is nationally,” he said. “It is extraordinary to see estimates of the volume being discharged. I am very disappointed that the government did not follow the recommendations of the environmental audit committee and make water companies fit volume monitors.”

The study also examined data from treatment works operated by Welsh Water that feed into the Conwy, a river known for its sea trout population in north Wales. Hammond estimated that the combined discharges from both inlets and storm tank overflows were equivalent to 34 Olympic swimming pools.

For the 29 treatment works without monitoring, volumes were estimated in one group by calculating the difference between flow meters at the inlet and outlet points. During spills of raw sewage, the estimated volume released was determined by the discrepancy between these meters. In another group of treatment works, Hammond had to rely on analysing data from overflow weirs, event duration monitors, and flow to treatment meters, which he admitted was the least reliable approach.

Apologising for past negligence regarding sewage pollution, the water industry has committed to tripling investment in the sewerage network to £10 billion during this decade. Clayden’s group is eagerly awaiting details from Yorkshire Water on how this increased investment will contribute to the clean-up of the Nidd.

A Yorkshire Water spokesperson said: “In the short time we’ve had to review this report we’ve identified several industry-wide claims we don’t recognise … The report focuses on potential impact of storm overflows at Pateley Bridge, Harrogate North, Darley and Kirk Hammerton. We are aware of an instrument issue that was identified in the 2020 Pateley Bridge return, which overinflated discharges from the 6x overflow and paints a false picture of the overflows.

“This monitoring is being replaced and was discounted in the 2021 and 2022 returns. Storm overflows are not identified by the Environment Agency as reasons for these sections of the Nidd not achieving good ecological status. The assessments suggests that where Yorkshire Water can make a difference is in reducing phosphorus from final effluent of wastewater. That is why we are investing £790m by 2025 in phosphorus removal.”



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