Many organisms adhere to matter. The most commonly used chemical disinfectants are halogens - iodine and chlorine. These kill organisms by oxidizing essential cellular structures. Halogens are inexpensive, relatively safe, not bulky, and can disinfect any quantity of water. Moreover, disinfection can be accomplished overnight or while you carry the water.
Most experts prefer iodine over chlorine.
Iodine is less affected by pH or organic wastes and the taste is less offensive. After extensive testing, the U.S. Army now uses iodine in the field. Increasing one allows a reduction in the other. Other relevant factors are the temperature of the water and the presence of organic contaminants. Low temperature slows reaction time requiring longer contact, especially to kill giardia.