Credit: JACEY

In his entertaining Futures story 'The brown revolution' (Nature 455, 564; 2008), Norman Spinrad waxes eloquent about turning faeces into energy. Some, however, are taking the idea more seriously. Swedish Biogas International is collaborating with Kettering University in Flint, Michigan (hometown of the US ambassador to Sweden, Michael Wood), to create a waste-energy plant that will recycle human faeces and turn them into renewable energy. The project will cost about $78 million, with $4 million coming from the Michigan Strategic Fund (see

However, the value of this technology has been questioned. Taking human faeces, for example, a daily diet of 2,000 calories (8,372 kilojoules) produces an energy residue in the faeces of about 7% (586 kilojoules) — roughly equivalent to the amount of solar energy shining on one square metre for just over seven minutes (see B. B. Desai Handbook of Nutrition and Diet, Dekker, 2000).

Compare this with the energy in a litre of petrol: 32,000 kilojoules. One bowel movement yields the equivalent of 1.8% of a litre of petrol. We are not going to motor very far on that. Neither does this calculation consider the energy it would take to convert faeces into energy. What is the net energy of the conversion? Is the energy ratio greater than unity, indicating that we are getting more energy out than we are putting in?

Faecal matter should be returned to the soil, from which the food that produced it originated. That closes the cycle, replenishes soil nutrients, and allows us to sustain our ecosystems and lives. Burning it, in whatever form, is akin to flaring off natural gas at the wellhead.