Anammox bacteria, which were discovered just 20 years ago, may provide free energy while cleaning our sewage:
In conventional sewage plants, micro-organisms digest solid waste in “activated sludge”. They convert the organic matter into methane but leave liquid waste containing ammonium and phosphates, which must be removed before the water can be poured into rivers.
Existing treatment plants use a lot of energy to get rid of the ammonium. The process uses bacteria that convert ammonium into nitrate, and the bugs that do this need oxygen, which must be constantly supplied to the treatment tanks by electric pumps. The nitrate is then converted into nitrogen gas by still more bugs, known as denitrifying bacteria. These require methanol, which must also be added to the mix.
This process consumes an average of 44 watt-hours per day for each person who adds waste to the sewage system. This can add up to megawatts in a big city.
But now Gijs Kuenen at Delft University of Technology in the Netherlands and colleagues are developing a technique that cuts out the energy-consuming processes. The key is a recently discovered type of bacteria that can munch ammonia without oxygen. So-called anammox bacteria short-cut the nitrogen cycle by converting ammonium directly into nitrogen gas.
One by-product of this process is methane, which Kuenen proposes to harvest and use as fuel. The team calculates that, far from consuming energy, the process could generate 24 watt-hours per person per day.
“44 watt-hours per day for each person,” which is enough energy to run one 100-watt light bulb for about half an hour, ignoring generation & transmission losses. Or enough energy to run a conventional over (assuming 3 kW) for a little less than a minute.