Rinse it out a few times then fill up with very very diluted bleach (ie. 1:200 bleach to water).
Next take some mild soapy water and clean it (ie. Dawn or Palmolive or your regular store brand (my favourite is the ASDA branded stuff, cheap and effective)).
Finally, you can either do water + vinegar or water + baking soda (but NOT BOTH). Let it sit overnight then rinse out.
Fill up your bladder with water and add a bit of lemon juice (or ginger, supposedly thats also something that kills bacteria and makes weird tastes go away or something).
The other thing you could do, but I don't suggest is add a bit of Vodka or similar to it. The Europeans in the old days knew about how ethanol makes water sterile or something (that's why they've developed the enzymes to break down alcohol as opposed to Asians that get the "asian flush" because they developed differently and found boiling water to sterilize water).
Also use boiled water as opposed to tap water (my mom used to make me do it all the time because it "dechlorinates the water" according to her (and tap water just tastes funny) and I've just picked up that habit).
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE