I stand corrected on my own post.... it's not such a bad idea to put the half drained water in your pack after all. I got curious and found this at www.camelbak.com (transcribed for your reading pleasure)

All of Camelbak's reservoirs come equipped with HydroGuard(TM), our own specially designed weapon to fight bacteria and fungus groth. Our HydroGuard(TM) Anti-Microbial Technology, an embedded FDA-approved and EPA-registered silver ion compound, eliminates up to 99.99% of common bacteria and fungus on reservoir and tube surfaces.
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Experience is a hard teacher because she gives the test first, the lesson afterwards.