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this is the model of my Old Steripen ; in Australia water tends to be either : Clear or Salt - and if it is Clear then the Steripen can kill it. [Inland you need an Osmosis set-up]
I have once needed to "filter" water from a lake containing decomposing vegetable matter [a suspended dune-lake in the sub-tropics , but fresh] and I used my Silk Sleeping-bag liner.
But other than than that : Kill it with [UV] Fire and drink.