Research funded by the European Space Agency into ways of feeding
future astronauts on missions to Mars is about to find a very
down-to-earth application — how to dispose of the sewage sludge
left over after wastewater treatment.

The MELISSA (Micro-Ecological Life Support Alternative) project,
which ESA is funding in companies and research institutes throughout
Europe, is developing a system of recycling as much of the waste as
possible produced by astronauts on long-duration space missions into
food and other consumables. EPAS, a Belgian company participating in
the project, is using some of the research results to devise methods
of substantially reducing the amount of solid material left over
after sewage treatment on Earth.

Presently-available technologies reduce the amount of solid waste
left in effluent such as pig waste, vegetable waste or sewage by
40-60% at most, according to Dries Demey from EPAS. In space, it’s
essential to find ways of using the remainder. On Earth, it’s not
essential, but would be highly desirable. At present, this undigested
fraction is disposed of in landfill sites or, when suitable, by
spreading on agricultural land. "In Flanders, there’s not a lot of
land and the tax on sludge disposal at landfill sites is getting
higher," says Demey.

Whether in space or on Earth, waste initially enters a fermentation
chamber where carefully chosen bacteria break down the solids. As
this process is unable to biodegrade recalcitrant fractions, EPAS
began investigating additional treatments to reduce the waste

The method that worked best involves using hydrogen peroxide, a
reactive but harmless oxidant, to break down the resistant fraction
which can then be reintroduced to the original fermentation chamber
after removal of the peroxide. "The results have been quite positive,"
says Demey. "We’ve been able to remove 85% of the solid waste and
convert it into water and methane gas, which can be used to generate
electricity. We’ve tested the method on sludge waste from a Flemish
food company. The only obstacle is that the cost is higher than that
of current disposal methods, but this will change in future".

Flammable methane may be a desirable end product on Earth, but in
space it could be a disaster. "By adapting the process conditions,
we can slow down the fermentation process and stop it before methane
is produced, " says Christophe Lasseur, MELISSA project manager at
ESTEC, ESA’s technical centre in the Netherlands. This could
involve maintaining a high ammonium (urea) concentration, high
acidity, or by washing methane-producing bacteria out of the
fermentation chamber.

During fermentation, long organic (carbon-based) molecules are
gradually broken down until ultimately carbon dioxide and methane,
molecules containing just one carbon atom, are produced. "In the
MELISSA system, we stop when the long molecules have broken down
into fatty acids," says Lasseur. The fatty acids are then used to
feed bacteria which also consume some of the ammonium. The remaining
ammonium is fed into the third MELISSA compartment where it is
converted into nitrates which are used to feed the plants that
astronauts eat for dinner.

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[Image 1:]
Classical activated sludge process for waste water treatment. Credits:

[Image 2:]
A lab-scale anaerobic reactor used for performing sludge digestion tests.
Credits: EPAS