The Russian ASU (Ассенизационно-Санитарная Установка, or "Waste Management System") aboard the ISS has pretty well clapped out following the failure on 21 May of the unit's air/water separator heralded by a "loud noise", according to NASA.
The agency explained last week: "The crew then replaced the separator with a spare unit but reported afterwards that the ASU lacked suction. The crew next replaced the F-V filter insert, which provided good suction for a while but again exhibited weak suction. TsUP/Moscow instructed the crew to deactivate the ASU and use the toilet facility in the Soyuz spacecraft."
The ASU is a pretty belt-and-braces affair, sucking in a urine/air mix through a funnel, separating the air for recycling into the ISS's atmosphere via a deodorising filter while storing the waste liquid in a tank. Solid waste is likewise sucked into the system via a nozzle for bagging and later disposal.
Women astronauts wishing to take a leak are not, of course, suitably equipped to avail themselves of the funnel, and are provided with sanitary-napkin-type pads.
Despite the ISS crew's best attempts to fix the cantankerous bog, it was still on the blink yesterday, as NASA admitted: "Troubleshooting continues on the Russian ASU toilet facility. Almost all system components have been changed out at this time, including the separator with no improvement in function. Specialists feel the problem is with the separator pump, though they have never before seen this failure signature."
Spare parts are, mercifully, being "last-minute manifested" for Space Shuttle Discovery's jaunt to the ISS due to blast off on Saturday, with NASA hopefully finding space aboard for "a new ASU separator pump".
Discovery's main task on the 14-day STS-124 mission is to deliver the second consignment of the Japan Aerospace Exploration Agency's Kibo laboratory, constisting of the large Japanese Pressurized Module and robotic arm system.
The spacecraft will also drop off new ISS crew member Greg Chamitoff and bring back Flight Engineer Garrett Reisman at the end of a three-month stint aboard the outpost. NASA has a mission summary here (pdf). ®