It’s been a busy weekend for the astronauts on the International Space Station, as they continued work on both the outside and the inside of the station. But for Expedition 18 Commander Mike Fincke and Mission Specialist Don Petit, the toughest job was trying to set up the system that converts urine into potable water.

The Water Recovery System, which is part of NASA’s Regenerative Environmental Control and Life Support System (ECLSS), was designed to filtrate the astronauts’ urine, perspiration and hygiene water and turn it into water clean enough to drink.

The process involves the separation of liquids and gases in space. The water obtained is sent to the water processor, which removes free gas and solid material such as hair and lit, as well as all remaining organic contaminants and microorganisms. The water goes through several treatment processes, which turn it into water that meets the standards for human consumption.

But the Urine Processer Assembly (UPA) continues to have problems. According to NASA, the UPA experienced several shutdowns during testing, supposedly caused by vibration; this resulted in physical interference with its centrifuge, and further led to increased current draw and temperatures, and ultimately, shutdown.

Space.com reported that the unit experienced a shutdown after processing less than 4 liters of urine collected by the station’s crew. “That’s a third of a tank right there, so that sounds like a potential,” Expedition 18 Commander Mike Fincke said. “It looks like we made things better, but we’re still maybe not there yet.”

Despite all the trouble, the other system that processes the perspiration and hygiene water appears to be working better than the Urine Processer Assembly. The work on the systems will continue.