Water Quality Monitor for Recovered Spacecraft Water 851347
Currently, Space Shuttle flights rely upon stored expendables for the generation of potable water (via fuel cells) and waste fluids such as himidity condensate and urine are stored for return to earth and subsequent disposal. Water is not presently recycled due to the relatively short nature of each mission.
On future long duration Space Station flights (i.e., 90-day resupply) most of the water from urine, humidity condensate, hygeine water (shower and hand wash) and ultimately clothes and dishwashers will be recycled to eliminate the launch weight penalties associated with large amounts of water. Consider that each crewperson requires about 17 liters (4.5 gallons) of water daily. On a 90-day mission with a six member crew, a water supply of 9159 kilograms (20,242 pounds) would be required.
Presently proposed water recovery systems include the following basic steps: 1) collection, 2) treatment to stabilize, 3) a phase change reclamation process, 4) post-treatment to improve taste and remove trace organic impurities, 5) quality monitoring, 6) biocide addition, and 7) storage for reuse.
The water quality monitoring step used to determine potability necessitates a rapid, accurate, and low maintenance form of analysis to allow sufficient warning so that the water can be reprocessed should break through of post-treatment beds occur. One of the more difficult parameters to be monitored is total organic carbon (TOC). Previous attempts to measure low levels of TOC involve the use of expendable and potentially dangerous chemical reagents.
This paper describes a TOC analysis system base upon ultra-violet absorption. Although commercial units currently exist, the unit developed and currently under evaluation can handle extremely low levels (about 2 parts per million full scale) on a continuous basis. The sample is totally unchanged after exiting the unit and can be safely returned to the fresh water system. In addition, varying system flowrates do not affect reading or stability, so all recovered water can be put through the analyzer for analysis. This state-of-the-art prototype/breadboard shows extremely promising results in the sense that it is particularly sensitive to some of the organic profile of human waste water. Of the two candidate systems originally examined, UV absorption proved to be the best available solution with currently available technology uniquely customized to meet the conditions described above.