How to feed astronauts on long space voyages is a major logistical problem, so researchers at Penn State are studying how to convert solid and liquid human waste into food. That may sound gross, but the idea is based on using a series of microbial reactors to break down the waste and convert it into an edible form with anaerobic digestion.

With costs of US$10,000/lb to send cargo into orbit, and the average person eating about 4 lb (1.8 kg) of food per day, supplying astronauts with food and water is already an incredibly expensive endeavor. However, because all space missions until now have been of either very short duration or confined to low Earth orbit, it's still feasible, though uneconomical, to provide crews with ready made meals.

The problem comes as manned missions of the future become much longer and travel much farther from Earth. With trips to Mars projected to take years, it simply isn't possible to bring along enough food or water to last the entire mission without a tremendous investment in storage space and propellant.

Aboard the International Space Station (ISS), experiments are being conducted on using hydroponics to grow vegetables in a weightless environment, but the bulky, water- and energy-hungry equipment, and the need for a crew to tend the garden are a bit of a problem. Worse, it takes a long time to grow things like tomatoes and cucumbers.

And then there is the matter of human waste. The usual way of handling it is to simply bag it up and stow it for later disposal, but that isn't practical on long missions – not to mention that such waste is actually a very valuable commodity from a supply point of view. Though the ISS has a very limited pilot plant to recycle urine, the system still leaves a lot to be desired and the solid wastes are stowed on used cargo ships that burn up in the Earth's atmosphere.

The Penn State approach is to try to turn a deep space ship into what is essentially a closed ecology by reproducing a simpler version of the biological cycles that turn wastes into food and fresh water on Earth. In this case, they're looking at how to use microbes in biological reactors to do the job while minimizing the dangers of pathogen contamination.

"We envisioned and tested the concept of simultaneously treating astronauts' waste with microbes while producing a biomass that is edible either directly or indirectly depending on safety concerns," says Christopher House, professor of geosciences, Penn State. "It's a little strange, but the concept would be a little bit like Marmite or Vegemite where you're eating a smear of microbial goo."

For their experiments, the Penn State team didn't use real human waste. Instead, they used specially made artificial solid and liquid waste that's produced commercially to test waste management systems. This pseudo-waste was placed in bioreactors, which were cylinders measuring 4 ft (122 cm) long and 4 in (10 cm) wide that used off-the-shelf aquarium waste management filters and other gear for compactness of design. However, this being an experimental set up, its purpose was to test the components in isolation rather than as an integrated system.

Into these were introduced microbes that could support anaerobic digestion similar to that used on Earth for waste recycling. The difference was that instead of turning the waste into fertilizer, the scientists exploited it more directly to grow food. In addition, the Penn State system was scalable, so it could be adapted to spacecraft of various sizes.

One example was using a microbe used to make animal feed called Methylococcus capsulatus. Colonies of these microbes not only grow quickly, but they're remarkably efficient at converting waste into food – producing something that is 52 percent protein and 36 percent fats. As to speed, they could manage up to 50 percent of the solid wastes inside of 13 hours as opposed to the several days needed for more conventional waste management systems.

But one major worry is the danger of pathogens getting into the mix. Unwelcome bacteria and the like can destroy any bioreactor system, so the Penn State team looked at ways to grow their desired microbes in environments that were high temperature or high alkaline – conditions that are hostile to most pathogens.

They found that a strain of Halomonas desiderata bacteria could withstand a pH level of 11, yet produced 15 percent protein and 7 percent fat, while Thermus aquaticus could handle temperatures of 158º F (70⁰ C). but produced 61 percent protein and 16 percent fats.

"Imagine if someone were to fine-tune our system so that you could get 85 percent of the carbon and nitrogen back from waste into protein without having to use hydroponics or artificial light," says House. "That would be a fantastic development for deep-space travel."

The research was published in Life Sciences in Space Research.

Source: Penn State