This stuff.
http://www.guardian.co.uk/environment/2011/sep/26/thames-water-sewage-flakes-electricity
We (humans), produce a lot of crap. Traditionally we’ve not done an awful lot with it. In the past it’s been sold to farmers after all the nasties have been removed. But more recently most of the sewage companies have started turning it into electricity, first to save money and then to make money and that’s a process (sic) that’ll only increase. It’s now very big business indeed.
Anyone driving over Barton Bridge by Manchester will see the two huge methane golf balls on Davyhulme treatment works. They’re part of a CHP process that generates 9MW of electricity. Even when the electricity used to run the plant is extracted, that’s still around 7MW that is being fed back into the grid. All from crap. And that’s just part of Manchester.