Anaerobic bacteria for treating waste-water will actually produce methane (http://en.wikipedia.org/wiki/Anaerobic_digestion http://en.wikipedia.org/wiki/Anaerob

Anaerobic bacteria for treating waste-water will actually produce methane (http://en.wikipedia.org/wiki/Anaerobic_digestion http://en.wikipedia.org/wiki/Anaerobic_digestion) - this can be harnessed and used as a low-cost energy source. For drinking water aeration is probably sufficient to deal with methane content (but I don't think it's necessary very often). Methane derivatives on the other-hand are known to be dangerous and can form as by products of disinfection http://en.wikipedia.org/wiki/Trihalomethane http://en.wikipedia.org/wiki/Trihalomethane.