Silver Ion Biocide Delivery System for Water Disinfection

U.S. space exploration missions have long considered returning to the Moon and exploration of Mars that challenge life support systems. For these long duration missions, there is interest in replacing the iodine water treatment system with ionic silver, a proven biocide. Although iodine treated water has been used successfully with the International Space Station, its use requires that the iodine be removed before being consumed by astronauts due to its adverse effects on the thyroid. For long duration exploration missions, it is imperative that an effective biocide be used that prevents microbial growth, biofilm formation, and microbially induced corrosion in the water storage and distribution systems while minimizing logistical supply requirements associated with the biocide delivery system.

A biocide delivery system has been developed that electrochemically produces silver ions for disinfecting water throughout the water storage and distribution system. This system produces an abundance of silver ions at the 1000 ppb level upstream in the water distribution system to prevent biofilm growth that is followed by a downstream collection module that electrochemically removes these silver ions before the water is discharged. Although low levels of silver ions are compatible for human ingestion, maintaining a sufficiently high level of silver throughout the water storage and distribution system prevents biofilm formation. This overcomes present concerns where spurious silver deposition occurs on the container and flow line surfaces thus lowering the silver ion concentration to unsatisfactory disinfection levels.

The design of the silver ion biocide delivery system is discussed including the development of a new hybrid micro-filtration and ion-exchange membrane that minimizes ohmic losses in the electrochemical cell, performance data for the introduction and removal of silver ions in flowing water streams, mechanisms for minimizing and controlling deleterious silver oxide growth, and design options for a long duration silver ion biocide delivery system.