Using Bleach for Water Disinfection

10-28-13 Update: I've read more about the efficacy of bleach to sanitize water.  While powerful against bacteria and viruses, it is less effective against Giardia cysts.  In addition, Cryptosporidium cysts are highly resistant to bleach.  For this reason I have switched to Aquamira chlorine dioxide treatment.  Chlorine dioxide is much more aggressive to cysts and leaves less byproducts compared to bleach or iodine.

(From Greg Gressel @ BackpackingLight)
Here is some helpful information on using bleach as a water disinfectant against giardia. It comes from an leading expert in the field. Keep in mind that this applies to Regular bleach that has 5.25% sodium hypo.

1) Is bleach effective against giardia? Yes, but it takes a long time. (See below)
2) Effect of water temperature. Colder water will take longer than warmer water. He recommends 1.5x the time for very cold water to be on the safe side.
3) Water hardness and organic material will negatively impact effectiveness.
4) Bleach will destablize over time. Don't expect three year old bleach to be as effective as fresh. Light and heat will also negatively effect the bleach.
5) Organic or muddy water should be filtered as much as possible. They recommend in emergency situations to filter using a sock or other material before using bleach.
6) Recommended Usage.
    a) Filter water with a cloth to remove organics.
    b) Apply bleach 2-4 drops per qt.
    c) Let stand for 20 minutes (30 if cold)

Two drops of bleach in a clothes washing ziploc will kill odor causing bacteria as well.

A 0.25 oz eye dropper is more than enough bleach for most hiking.