'Tis a great question and one very worth digging into. First off, I suggest getting The Big Thirst from the library. It’s a great book describing the challenge of water for several cities across the world and the processes used to make your water safe.

Second, I’d also suggest checking out some YouTube videos like this Animagraffs video: www.youtube.com/watch?v=KsVfshmK0Ak. As The Big Thirst and others describe, there are a plethora of engineering techniques now to purify water on a citywide scale, and it is very much up to local utilities to decide how they do it.

Third, someone has already pointed out that there are water quality standards in N. Ireland. As others have noted, it needs to be “safe,” not “sterile.” Indeed, you actually do want some minerals in your water—otherwise it can be detrimental to your health. Drinking straight distilled water continuously, for example, is problematic because “pure” water will actually leach the minerals it encounters.

Fourth, the purity of water is ultimately about how much money is invested in purifying it. Chip companies and scientific endeavors need higher levels of water purity for some of their processes. This can be achieved through the engineering processes mentioned above, producing incredibly pure water—which is actually dangerous to drink.

Lastly, the purification of water from your swamp is a function of how much money the city is willing to spend, but it is feasible to take nearly any water and make it safe to drink with enough investment in infrastructure. As part of this, the pipes that deliver water to your house are crucial: they must remain full and pressurized (pushing clean water out, not allowing contamination in). I mention this because it’s important to understand that the infrastructure around water delivery is nearly as important as the treatment itself. (Similarly true for wastewater.)