Windermere is at the heart of the system, holding around 300bn litres of water. That is so big that it can take 9 months for water entering one end to reach the other. A 3.6bn pound proposal to stop sewage being discharged into Windermere, England's largest lake, was unveiled.
The project would see 12,800 people connected to the sewage network for the first time with up to 86 miles, or 138km, of new sewage infrastructure and a single wastewater treatment works built to serve them at the end of a 4m-wide sewage tunnel.
When treatment works are overwhelmed, such as during heavy rain, they spill raw sewage into the water. The rest of the time, they emit a continuous stream of treated effluent which, while cleaner than raw sewage, can contain high nutrient levels.
At the moment, both raw sewage and treated effluent are discharged into Windermere, and campaigners say that is why parts of the lake have been turning green each summer in what is known as algal blooming.
At present, the sewage network around Windermere is fragmented, with homes served by a mix of public wastewater treatment works and privately owned treatment facilities and septic tanks.
The plan, known as the ORiW proposal, would eliminate sewage spills into Windermere, which have been blamed for turning the water green.
On several occasions in the last five years, the high nutrient levels and warm temperatures have combined to cause algal blooming, turning parts of the lake a lurid green.