Iodine kills bacteria and disease-causing organisms. Iodine is, however, ineffective as an algicide.
Iodine has been used to disinfect water since the early 1900s. In its natural state, iodine is a solid black crystal. Iodine crystals will dissolve in water, dependent on the water temperature. The higher the temperature, the more will dissolve. The simplest method of disinfecting water with iodine is by dissolving iodine in water to form a saturated solution and then injecting the iodine solution into a water system.
Iodine does not kill bacteria on contact; a holding time of at least 20 minutes is needed depending on the iodine concentration. An iodine residual of 0.5 to 1.0 mg/l should be maintained and iodine at this level gives the water little or no iodide taste or odor. Iodine can be removed from water with a carbon filter just before drinking.
Iodine dosage is very temperature dependent because iodine crystals are more soluble at higher temperatures. Iodine remains effective over a wide range of pH and does not lose effectiveness until the pH of water reaches 10. Iodine residuals in water can easily be measured using a test kit that indicates a color change.
Iodine tablets were developed during World War II to disinfect small amounts of water for emergency or temporary use. A few drops of tincture of iodine or iodine tablets are popular with campers and the military for disinfecting water.
Iodine solutions are injected into a water system using bypass saturator systems or injection pumps. A holding tank or coil of pipe is used after iodine injection to provide the necessary holding time.
The most common type of iodinator is called a bypass saturator and consists of a solution tank containing iodine crystals. Bypass saturators do not require any electrical connections. The solution tank is connected to the water system and diverts a small amount of water through it and back into the water line. Valves are placed on either side of the iodinator to control the iodine dose. Fluctuation in water temperature affects the solubility of iodine. Adjustments in the bypass rate are needed if water temperature changes.
Chemical injection pumps can also be used to inject iodine solutions for individual water treatment. These are the same injection systems that are used for chlorine.
Iodinators are in-line systems that are sized to treat all the water used in a household.
The question of possible health effects of iodine is still unanswered. No adverse health effects have been shown, yet continuous consumption of iodine-treated water is not recommended. Carbon filters can be used to remove iodine just before drinking (note: not currently allowed under Ohio Department of Health for private water supplies). Iodine is also appropriate for occasional use in vacation homes, campgrounds, and restaurants.
Iodine treatment of drinking water supplies for dairy cattle is also a concern. Because dairy cattle can drink from 15 to 30 gallons of water a day, normal levels of iodine used for disinfection may cause iodine carryover into milk.