Flushing the human waste produced on space missions out an airlock isn't an option for astronauts. Currently its stored in containers before being loaded into cargo vehicles that burn up as they pass through Earth's atmosphere, but researchers at the University of Florida (UF) have found a better use for the material, by developing a process to turn it into rocket fuel.

Over the years, there have been different proposals from various countries and private companies to build a base on the Moon. While we're still waiting for construction of such a base to begin, in 2006 NASA announced that it planned to have a permanently staffed base on the Moon by 2024 and began exploring ways to reduce the weight of spacecraft leaving the Earth.

Because carrying back stored waste from such long-term missions would be impractical, and dumping the waste on the Moon's surface being ruled out as an option, the space agency turned to UF researchers to come up with some alternative ideas. Pratap Pullammanappallil, a UF associate professor of agricultural and biological engineering and then-graduate student Abhishek Dhoble (now a doctoral student at the University of Illinois) took up the challenge.

NASA supplied the scientists with a packaged form of chemically produced human waste that also included simulated food waste, towels, wash cloths, clothing and packaging materials, and they ran tests to see how much and how fast methane could be produced from the materials.

Using an anaerobic digester process, in which microorganisms break down the organic material and kill pathogens in the absence of oxygen to produce a mixture of methane and carbon dioxide, they found were able to produce the equivalent of 290 L (77 gal) of methane per crew member per day over the period of a week.

"We were trying to find out how much methane can be produced from uneaten food, food packaging and human waste," said Pullammanappallil. "The idea was to see whether we could make enough fuel to launch rockets and not carry all the fuel and its weight from Earth for the return journey. Methane can be used to fuel the rockets. Enough methane can be produced to come back from the Moon."

Additionally, the digestion process would also produce around 757 L (200 gal) or non-potable water, which could then be split into hydrogen and oxygen through electrolysis. Pullammanappallil says the oxygen could be used in a back-up breathing system, while the exhaled carbon dioxide and hydrogen could be converted to methane and water in the process.

The researchers' study was published in the journal Advances in Space Research.