For many people, the idea of having a device like a home water filtration system is based on concerns such as having hard water, often because they live in the southern half of Britain where much of the supply comes from aquifers and contains lots of calcium.

While that may all be true and can be tasted, felt on the skin and seen by the limescale building up in the kettle, there are other issues that are faced by Britain’s water, even that which some might expect to be among the cleanest and purest around.

For those living further north, the situation might seem different as so much water comes from reservoirs up in the hills. For example, Manchester gets its water mostly from Thirlmere, Haweswater and Wet Sleddale reservoirs in the Lake District, while neighbouring areas in Tameside and Stockport are served by reservoirs in the Peak District. Usually, this water is softer than average in a lot of cities in the UK.

Many of these reservoirs have long been established. For instance, Thirlmere reservoir was created in 1894 when a dam was built to fill up a valley occupied by two small lakes, Leathes Water and Wythburn Lake, with a 78-mile aqueduct taking the water by gravity to Manchester. Haweswater was added in 1940 after a dam was built to enlarge the existing lake.

However, it is worth noting that when it gets dry for prolonged spells these reservoirs can run low and they are supplemented by water extracted from other rivers and lakes. In the case of the Lake District, this includes Windermere, England’s largest lake.

The problem with this is that Windermere is suffering from a growing pollution problem that has manifested itself in growing blooms of algae. Speaking to the BBC recently, zoologist and campaigner Matt Staniek warned that an “environmental catastrophe” is looming unless more is done to stop the phosphate that encourages algae seeping into the lake from septic tanks and local agriculture.

He painted a grim picture of a future when the lake will be too toxic to swim in and dead fish will litter the shoreline. Suffice to say, the thought of this water being used to supplement supplies heading down the Thirlmere Aqueduct towards Manchester is not a pleasant one.

Windermere is not the only lake to be suffering from algae; Loweswater, which lies in the north-east corner of the Lake District, also has a problem with it, which will take years of work to fix.

Most significantly, algae is mainly seen in summer when the hot sunshine encourages the growth of bloom. This, of course, is precisely the kind of weather that can prompt drought through low rainfall and increased water use.

Algae can make humans ill and can kill dogs, all of which means going in the water can be perilous for man and furry friend alike. But while any water extracted from lakes will be treated before it ends up in the pipes, absolute purity is not something that can always be taken for granted, especially if the problem gets worse, as Mr Staniek has warned.