San Antonio in the United States could become the first city to draw all its energy requirements from methane gas generated from the city's water treatment system through recycling 14,000 tonnes of biosolids in sewage annually. The methane source includes human waste that, if left untreated and unutilised, would only pollute soil and water. Treating bio-waste, however, could generate an average of 1.5 million cubic feet of gas a day - enough to fill 1,250 tanker trucks daily - according to the system's chief operating officer. A by-product of human and organic waste, methane is the chief component of natural gas that can fuel generators, power plants and furnaces. Closer home, gobar gas - natural gas obtained from methane released by cattle waste - as a green alternative to diesel and other fossil fuels has been taken up seriously, particularly in rural households. However, a lack of adequate hygiene is a constraint because the gas formation - in the large containers filled with gobar - makes the drum's lid rise, and there is spillage all around the plant. So, in India gobar gas plants are fertile breeding grounds for mosquitoes and other pests. But this is not an insurmountable problem. Gobar gas plants could be expanded and diversified to include energy extraction from all kinds of biomass and the gas so produced could fuel power stations - as San Antonio proposes to do - and with improved sanitation, the experiment could yield good results for several Indian cities. As a renewable resource, biomass - either from plants, agriculture and forestry residues, animal or human waste - is biodegradable and so is far more eco-friendly than petroleum-derived fuels. And they are relatively easier to source and process, unlike the sophisticated instruments and know-how required to extract oil or refine coal. Ethanol derived from biofuels has a very high octane rating. It might deliver less energy than gasoline, but by blending about 10 per cent ethanol and petrol or diesel together, a feasible balance is achieved with no perceptible effect on fuel economy. America's space agency NASA is sponsoring a joint project to turn human waste into a power source for spaceships using a process that could also produce other chemicals that can be used on board. Instead of turning up our noses at the idea of recycling human waste and other biosolids in sewage, it would be worthwhile to explore fully and exploit the immense potential hidden in what we routinely regard as being useless.