Question

Why is some water harmful even after treatment?

Answer 1

i think it depends on the treatment. some microbes could survive boiling. toxic chemicals will most likely remain toxic after heat treatment, and they could pass through membranes (used for filtration) because they can definitely be small enough to pass through the pores.

if you combine a bunch of treatments, then i can't think of why the water could still be harmful, unless it's a chemical that's highly toxic even at very low concentrations.

perhaps more likely is the scenario in which the water becomes infused with harmful stuff after treatment--during the transportation or storage stage.