OP
Imagine a motorway, you've driven on those before right?
The left three lanes go north, the right three go south.
There's a barrier in the middle right?
The lanes can't mix.
Right. Hold that thought.
Now imagine waste water is the left lanes, all flowing one way...
Clean water is the right lanes.. all flowing the opposite way..
They're separate like motorway lanes.
Does that make sense?
Now.
If the left lanes of a motorway gets blocked..
More often than not, it doesn't affect the right lanes that much. The blockage gets cleared and the traffic flows again.
Same with waste water. If the lane gets blocked, it may back up a little bit its still separated from the other 'lanes'
There would have to be a truly truly catastrophic issue for any waste to flow all the way back into a sink, bath or shower. The overflow outlet would have to be blocked the main lines would have to be blocked and it would have to have not been emptied since Jesus Was Kid..
And for it to get into drinking water? Not a chance. Clean drinking water is a separate system linked to the water lines and doesnt connect to waste lines.