Understanding Windermere’s Decline: The Role of Treated Sewage
For the last few years, public attention has focused on the tens of thousands of hours of untreated sewage discharged into Windermere and waterbodies around the country. The media’s interest in these spills is understandable and justified. The number of spill hours is staggering, there are risks to human health, and it points to decades of underinvestment.
But this is only one part of the story about sewage and England’s largest lake.
In Windermere, untreated sewage discharged for 33,410 hours between 2020 and 2024, all of it ending up in the lake. In the same period, our work with Professor Peter Hammond from Windrush Against Sewage Pollution (WASP) identified more than 600 days when spilling occurred illegally.
When we talk about ecological decline in Windermere, we are referencing persistently elevated phosphorus levels, falling deep-water dissolved oxygen, and rising algal biomass, particularly in the North Basin. But untreated sewage is not the only contributor to these trends.
Treated sewage is also playing a major, and often overlooked, role in the degradation of water quality. And it has been flowing into our lake quietly, legally, and constantly for years.
A few years ago, researchers at the University of Oxford’s Department of Biology investigated the impacts of treated sewage, agriculture and urban runoff on river systems. Studying four rivers above and below sewage treatment works, they found that treated sewage discharge was the strongest predictor of high nutrient levels, bottom-dwelling algae and sewage fungus, regardless of surrounding land use.
We wanted to understand how this applies to Windermere. So we requested data on the volume of treated sewage discharged directly into the lake, its inflowing rivers, and smaller feeder lakes. We approached United Utilities for this information because several sewage outfalls are inaccessible. For example, the Grasmere wastewater treatment works outfall sits around seven metres below the lake surface, making independent monitoring virtually impossible.
As ever with United Utilities, the results were astonishing. Between 2017 and 2024, data provided to and analysed by our colleague Geoff Tombs at WASP shows that just six United Utilities sites discharged more than fifty billion litres of treated sewage into the catchment’s rivers, streams and lakes, including Windermere itself.
The permits for these sites only require annual averages to be taken when measuring the quality of this effluent, which fails to capture short-term failures and smooths out peak loading periods, but the volumes alone speak for themselves when considering the impact on Windermere.
This is where Windermere’s unique vulnerability becomes clear. It is a still body of water, effectively an enclosed ecosystem. A drop of water takes, on average, nine months to travel from the top of the lake to the bottom, meaning much of what enters the lake remains there. Windermere is also naturally oligotrophic, meaning nutrient levels would typically be so low that algal blooms are rare or absent. The vast quantities of treated effluent entering the lake, carrying the nutrients algae depend on, are pushing Windermere away from its natural state and making it increasingly productive.
These conditions are ideal for the growth of potentially toxic blue-green algae, which threaten both the ecological integrity of the lake and its recreational users. We saw the consequences in the summer of 2022, when Windermere experienced what was likely the largest blue-green algal bloom on record. The entire North Basin turned vivid green, so striking that it was visible from space.
People often ask us, “Don’t septic tanks also contribute to this?” Well, yes, but the reality is that the volume from United Utilities’ assets far exceeds all other sources in the catchment, even when combined. One of the unusual features of Windermere is that many towns and villages sit directly on the lake shore. United Utilities is responsible for collecting and treating almost all of the resulting sewage, so it is no surprise that their contribution dominates. In addition, we have already carried out a detailed review of the legislation around septic tanks, and there is a clear solution and legislative route to consolidate private systems into the mains network.
Work on the ecological history of the lake supports this. A research paper published in 2018 by Heather Moorhouse and colleagues concluded that:
“The major driver of (algal) assemblage change appeared to be the installation and upgrade of wastewater treatment works. Influx of nutrients, particularly phosphorus from point sources, commonly accelerates the eutrophication of oligotrophic and mesotrophic lakes. The observation that the timing of treatment works operation is more important for altering production than population growth may reflect the fact that major tourist towns are located within subcatchments of lowland lakes and that the >15 million annual visitors are excluded from population estimates yet drive a need for WwTW development.
Changes in primary producer assemblages were greatest in lowland lakes receiving influx of human wastewater. In particular, the terminal basins of Lake Windermere integrated pollutants throughout the catchment and displayed the highest magnitude of algal modifications. Strong effects of nutrients on lake production are noted regularly elsewhere and have been suggested to override the effects of climate change in other lake catchments.
Based on these findings, it is clear that management of Lake Windermere requires [...] strategies to reduce emissions from wastewater treatment works.”
We felt this evidence was so central to explaining the drivers of algal blooms in Windermere that we featured these volume statistics prominently in our new film, Origin – A Journey to Find the Source of Windermere, to help people grasp the true scale of the issue and why Windermere must be treated differently from rivers and the sea.
Other countries have already taken the impact of sewage on lakes seriously. They have protected their freshwater ecosystems by removing both treated and untreated sewage discharges entirely, preventing them from drifting further from their natural state. Lake Washington is one of the leading examples. Diversion began in 1963, and by 1968 all sewage inputs had been eliminated. The project took nine years and cost $125 million, but the transformation was immediate. Water clarity, which had fallen to thirty inches in 1964, reached ten feet once the diversion was complete.
Lake Annecy in France took the same approach.
The only way to safeguard Windermere for future generations is to eliminate both treated and untreated sewage from the lake, because these inputs are the primary reason it is drifting away from its natural state. Every year we delay, more algae grows, more oxygen is stripped from the depths, and this iconic lake slips further from what it once was. What is being done now is simply not enough in the face of climate change and decades of underinvestment. The choice is stark: remove the sewage, or accept the death of Windermere.
