Microbial Growth Control in the International Space Station Potable Water Dispenser

The International Space Station (ISS) United States On-orbit Segment currently provides potable water for crew consumption via the Potable Water Dispenser (PWD). The PWD receives iodinated water from the Potable Water Bus, removes the iodine biocide and filters the water for particulates and bacteria, and then dispenses the potable water into an attached food or drink package for consumption. The user can choose to dispense water at either ambient temperature (65°F-123°F [18°C-51°C]) or hot temperature (150°F-200°F [66°C-93°C]). The first PWD unit commenced on-orbit operation in 2009 and has been continually operated since. The second PWD unit completed certification in September 2015 and is to be launched on need when the first PWD unit requires replacement. Until then, the second PWD unit is undergoing periodic maintenance to prevent microbial growth during storage. Each PWD unit poses unique challenges in reducing microbial counts to acceptable levels. The first PWD unit’s effluent is currently sampled on-orbit monthly for coliform bacteria and quarterly for total number of bacterial colony forming units to validate that the system is providing potable water within the limits defined by the NASA ISS Medical Operations Requirements Document. This PWD unit also utilizes operational constraints in the form of Flight Rules to manage off-nominal scenarios (such as prolonged stagnation) requiring the use of high biocide concentrations to “shock” the system to return the unit to its fully serviceable state. The second PWD unit is currently disinfected every 6 months using an iodinated water solution to maintain the bacterial counts within acceptable levels. The disinfection process and timeline has been chosen to reduce the likelihood of unacceptable levels of bacterial growth, minimize the potential for biofilm formation, decrease the potential for corrosion caused by repeated disinfections, and lessen the overall cycling of the PWD unit to preserve hardware life.