Development of an Electrolytic Silver Biocide Dosing System for Use in a Spacecraft Potable Water Bus

Maintaining microbial control in spacecraft potable water storage and delivery systems is a significant challenge for long-duration human space exploration. Simple, robust biocide systems are needed that are capable of continuous dosing while offering low-equivalent system mass. Use of silver as a biocide is of particular interest. Silver has been successfully implemented as a biocide in spacecraft systems and is an effective microbial control agent at concentrations suitable for direct human consumption. In addition, silver lends itself to strategies for on-orbit dosing. This is a critical need for long-duration missions, where silver must be added to purified water recycled back through a closed-loop water recovery system. In addition, on-orbit dosing may be needed to add back silver that may be lost through chemical reactions with materials found/used in spacecraft potable water systems. This paper reports on the development of an in-line electrolytic-based silver biocide dosing system employing closed-loop feedback control. The overall design of the system is small, lightweight, and can be optimized for low power consumption. A prototype of the system has been built and tested, demonstrating control of ionic silver at 300 ± 40 ppb for 7 weeks in a simulated sub-scale metallic water bus system. The test system volume was approximately 10 liters and incorporated about 1.5 sq. meters of wetted surface area. Results and observations from the development and test of the prototype dosing system are provided herein.