[DarronWyke]

Boiling is what did it. Boiling is the most sure-fire way to disinfect water.

[AstralStorm]

Unless contaminated chemically, e.g. by heavy metals. Most portable filters do not protect against these though.

[DarronWyke]

Most portable filters will, actually. They've got fine enough filters to catch protocysts; they'll easily catch any metal contamination. What they don't get are smaller bacteria and virii, which is where secondary purification in the form of boiling or purification tablets is necessary.