Under the most ideal circumstances, how 'clean' is drinkable tap water by the time it reaches our taps?

I've always been under the childlike impression that my tap water is clean clean, but when thinking about it today I realised that it's unlikely that tap water is completely sterile, certainly not by the time it reaches my house through miles of pipes. So, just how unsterile can it be and still pass muster with the local government?

If we accept a certain number of rodent hairs or cockroach shells in each helping of our processed foods, I can only imagine what's considered acceptable when it comes to tap water.

For reference, I'm in N. Ireland, which is, regrettably, the UK. But obviously the island of Ireland is where my water comes from. From this nightmarish swamp, to be precise.

Stay moist, hydrohomos.