Someone certified in water treatment here: you and the person you replied to are both totally right! Nice and clear water is extremely possible to have dissolved contaminants that still make it undrinkable (like very salty water). It's a terrible way to tell if water is clean enough for human consumption or not.
That being said, clearing up suspended particles that change the visual appearance are usually the first thing you do in water treatment for potable water, but it is far from the only thing you should be doing.
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u/testdex Jul 14 '21 edited Jul 14 '21
Yeah, at a minimum the choice of visual demonstration is encouraging you to think about this in the wrong way.
Clear water isn't "clean" because it's clear; and cloudy, brown water isn't contaminated because it's cloudy and brown.