CAPE CANAVERAL, Florida (Reuters) - NASA is finding it is not just mechanical glitches that make the International Space Station a tough place to operate.

Engineers trouble-shooting a problem with the station's $250 million water recycling system, which processes urine into clean water for drinking, believe the cause is a high concentration of calcium in the astronauts' urine, which clogs the system.

Scientists do not yet know if the high calcium concentration is due to bone loss, a consequence of living in a zero-gravity environment, or other factors.

"We've learned a lot more about urine than we ever needed or wanted to know -- some of us anyway," said station flight director David Korth.

The $100 billion space station project involving 16 nations has been under construction 220 miles above the Earth for more than a decade.

Before the urine recycler was started up in November 2008, it was fully tested by NASA.

"Folks had good knowledge of the content of the urine going in, but the chemistry changes as it works through the processor are not always understood," said program scientist Julie Robinson. "There are a lot of parameters including urine calcium and pH (acidity) that everyone is looking at."

Engineers at the Marshall Space Flight Center in Huntsville, Alabama, are hoping to come up with a fix in time to fly replacement parts out on the shuttle Endeavour, which is scheduled for launch on February 7 on a construction mission.