Water is critical to human survival.
The water storage tank is resupplied on a regular basis by a water recycling system. The water is always checked for contaminants before being pumped into the holding tanks. Without water the mission is in jeopardy and the crew will have to return home within 15 days. You must come up with an emergency backup recycler that can clean the water and extend the crew's mission on the space station. The device must be able to purify at least 500 milliliters at a time.