I use the Aquamira ones
Aquamira Water Treatment Drops - Bivouac Online Store
as the water is treated faster which helps when you are thirsty
I've got some of these too
Potable Aqua with PA+Plus Drinking Water Germicidal Tablets - Bivouac Online Store
and they don't have an expiry date (from memory) but are slower to act
A lot of guys swear by their life straws and whatnot, I've not used them myself
Not endorsing bivouac as a store, just the first image in search
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