The relevent section from the link pasted below:
Iodine has been used to disinfect water for nearly a century. It has advantages over chlorine in convenience and probably efficacy; many travellers find the taste less offensive as well. It appears safe for short and intermediate length use (3-6 months), but questions remain about its safety in long-term usage. It should not be used by persons with allergy to iodine, persons with active thyroid disease, or pregnant women.
Note that Iodine and other halogens appear to be relatively ineffective at killing cyclospora, a troublesome diarrhea-causing bacteria seen in Nepal only in the late Spring and Summer months. At these times it may be reasonable to pre-filter water to remove the large cyclospora (about the size of Giardia cysts), and then treating with iodine. (See the comment from Dr. Backer)
Iodine is available in numerous forms, which can be confusing to the traveller. A simplified table is presented below; for more complete information refer to (Backer 1995).
Iodine Topical Solution 2%
Iodine Tincture 2%
Lugol's Solution 5%
Povidone-Iodine (Betadine®) 10%
(Globaline®, Potable Aqua®, EDWGT®) 8 mg
Disinfecting Contact Times
WATER CLARITY WATER TEMPERATURE
5 °C 15 °C
final drinking concentrations calculated at 8 mg iodine/liter
measure with a tuberculin syringe or dropper: 1 drop = 0.05 ml
In general, if you are in a hurry double the chemical dose and halve the contact time; if you want better flavor halve the dose and double the contact time.
If you believe the water may be heavily contaminated, double the chemical dose or double the contact time.
Iodine Topical Solution and Iodine Tincture also contain 2.4% sodium iodide, Lugol's Solution also contains 10% potassium iodide, increasing the dose of iodine ingested.
Povidone is a non-toxic polymer that binds the iodine and allows higher concentrations in a water-based solution. This complex system provides a sustained-release reservoir of free iodine, and makes calculation of the "strength" of the solution difficult.
A system comprising iodine crystals in plain water is available, and works well. It lasts an extremely long time. I have not shown it here because the amount of iodine dissolved in the water is highly temperature-dependent, and this is problematic in the universally cold environment of the Himalaya.
Addition of a small amount of vitamin C (50 mg) to your water after the contact time with the iodine will render the water nearly flavorless!
Discussion and Personal Preferences
Trekkers visit Nepal in huge numbers, and have a significant impact on the environment here. There are some important steps they can take to minimize this negative impact. Using bottled water in remote areas is unsupportable. It results in a terrible waste problem with non-reusable, non-biodegradable plastic bottles. Boiling water requires heat, which may be from a kerosene stove or dried yak dung, but is often in the form of a wood fire. Burning wood = deforestation in many of these fragile mountainous areas. For this reason I encourage trekkers to enjoy hot drinks in the tea houses, but not ask for boiled water to fill their water bottles. Filters do not do an adequate job of disinfection, and so I see no point in spending money on them or taking up valuable backpack space carrying them. I recommend iodine as the treatment of choice, unless there are contraindications to its use.
My own preference is to use Betadine® (10% povidone-iodine) in a small dropper bottle, and a one liter nalgene bottle. I fill up the bottle from a stream, drop in 4 drops of Betadine®, screw on the lid, and put it in my pack. Half an hour later, it is ready to drink. With minimal planning ahead, I rarely need water faster than this. I like this system because I carry Betadine® in my first aid kit anyway, and I personally think the flavor is less noxious than some other forms of iodine. I don't usually feel the need to add flavor crystals to my water, rather I joke that iodine is "the taste of safety." A little goes a long way: 30 ml of Betadine® is enough to disinfect 150 liters of water, or drinking water for about 50 days of active trekking (I am presuming that some additional fluid intake exists from tea, soup, meals, etc.).
I encourage trekkers to try a "taste test" at home with the various forms of iodine to determine which form tastes least bad, what strength they can tolerate (balanced against the necessary contact time), and whether they need to plan on carrying bulky flavoring crystals. Try the vitamin C trick!