DISINFECTION OF WATER


Elemental iodine (I2) is used for disinfection of reclaimed waters in all American designed systems. The Russian space program utilizes silver ion (Ag+). The two species are mutually incompatible. Each of the biocides, and the problems which must be overcome in joint Russian-American missions are discussed below:


IODINE

 

Disinfection of the drinking water supplies aboard spacecraft is a requirement for the maintenance of flight crew health. Current NASA specifications for potability limit the maximum microbial population to less than 100 Colony Forming Units (CFU)/100mL, with total coliforms less than 1 CFU/100mL. Elemental iodine (I2) was first applied as a drinking water disinfectant on U.S. spacecraft in the Lunar Module during the Apollo missions which began in 1969. Prior to each mission, Lunar Module drinking water storage tanks were prefilled with water containing an I2 concentration of 12 mg/L. This level was sufficient to maintain a residual disinfectant concentration of < 0.5 mg/L. During the Skylab mission potable water tanks were iodinated to 12 mg/L I2 prior to launch. Afterward aqueous elemental iodine concentrations were monitored and maintained in the range between 0.5-6.0 mg/L by direct additions to the water storage tanks during flight using a 30g/L stock solution containing potassium iodide (KI) and I2 in a 2:1 molar ratio.

 

For the Space Shuttle program, a new device for the controlled release of I2 termed the Microbial Check Valve (MCV) was introduced. The MCV is a canister containing iodinated strong base ion exchange resin. In aqueous solution in the presence of excess iodide (I-), polyiodide anions are formed.

MCV resin consists of polyiodide anions bound to the quaternary amine fixed positive charges of a polystyrene-divinylbenzene copolymer anion exchange resin. The bound polyiodide anions release I2 to water.

 

Onboard the Space Shuttle, high purity water produced by the fuel cells flows through an MCV canister which provides both a contact microbial kill and imparts an elemental iodine residual ranging between 0.5-4.0 mg/L. Similar devices are installed at the Galley Auxiliary Port and the Extravehicular Mobility Unit Service and Cooling Umbilical for microbial control. The MCV has proven effective as a means for maintaining drinking water potability, and in ground-based tests has demonstrated a remarkable ability to inhibit the growth of biofilm.

 

Aboard the Shuttle Orbiter all MCV canisters require periodic replacement. To simplify resupply logistics for future longer duration missions, regenerative MCV (RMCV) hardware has been developed. This technology utilizes a packed bed of crystalline elemental iodine to produce a saturated aqueous solution ([I2] = 300 mg/L) which is used to replenish depleted MCV resin.



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Author: Tugrul Sezen
sezen@nas.nasa.gov

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