I gotta think that no backwash is a big part of it. Maybe the liner helps inhibit growth by providing an inhospitible medium? Either way, I gotta think it's not smart to put backwash laden half-bottles of water in your camelbak if the goal is to not consume old possibly contaminated water. You then contaminate your reusable container.
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Experience is a hard teacher because she gives the test first, the lesson afterwards.