Wastes may be the most important source of sustainable renewable energy in a post fossil fuel economy. Treating wastes to recover energy prevents them from polluting the environment, and harvesting energy from organic wastes saves carbon emissions twice over, by preventing carbon emissions that would otherwise have gone into the atmosphere, and by substituting for fossil fuels. Biogas is efficiently generated by anaerobic digestion of organic wastes, and can be used for combined heat and power generation in buildings and as fuel for mobile vehicles when carbon dioxide and hydrogen sulphide are removed from the methane. Fuel-efficient super-clean cars that run on biogas methane are already on the roads in Sweden, Switzerland and Germany, which have no fuel duty on renewable natural gas. Sweden is the world leader in gaspowered
vehicles, and has around 4 500 natural gas vehicles, 40 percent run on biogas produced in community biogas plants. Hydrogen can be readily produced from organic wastes, but
compact hydrogen storage remains a major obstacle for using it as fuel in small vehicles.
A new fully contained, low temperature thermal conversion process shows considerable promise in recovering biodiesel from food-processing and slaughterhouse wastes and also in making diesel from waste plastics. Recycling plastics save substantially on energy and carbon emissions, but the best way to save on both is to use less. Diesel produced from plastic wastes that are difficult to recycle into plastics is not renewable, but it prevents toxic pollutants from landfills and incinerators and generates extra non-renewable fuels.
Green algae can potentially combine low-cost and energy efficient capture of carbon dioxide from power plant exhausts with sustainable biodiesel production.

  1. Common bacteria, naturally found in organic wastes when confined in anaerobic digesters, ferment the wastes to produce ‘biogas’ as by-product, which typically consists of about 60 percent or more of methane (CH4) and a small amount of hydrogen (H2), both of which can be burnt as smokeless fuel.
  2. Hydrogen can be produced by anaerobic digestion in a twostage process, with the first stage optimised to produce hydrogen, followed by methane in the second stage. The key appears to be a slightly acidic pH of 5.5 in the hydrogen reactor, instead of pH 7 in the methane reactor, with both reactors run at 35 C. In the pilot lab experiment, the two stages together removed 68 percent of chemical oxygen demand in the waste.
  3. A bioelectrochemically-assisted reactor at bench-top scale was able to produce hydrogen from any biodegradable organic matter. A combined fermentation and bioelectrochemically assisted anaerobic microbial fuel cell has the potential to produce as much as 8 to 9 molecules of hydrogen starting from a molecule of glucose (the theoretical maximum is 12).
  4. A conservative estimate suggests that if all the wastewater sites in large urban areas of Ontario, Canada, were to use anaerobic digesters and simply recover the methane to generate electricity, this would produce 1.51 GWh/day and save 432 tonnes of CO2.
  5. Methane mitigation will slow global warming and benefit public health by reducing the growing global background concentration of ozone. Ozone damages agriculture and ecosystems, and is associated with premature deaths in humans. It is estimated that reducing methane emissions 20 percent beginning 2010 will decrease ozone levels in the atmosphere sufficiently to prevent 370 000 premature death by 2030.
  6. The United Nations Development Programme (UNDP) 1997 Report, Energy After Rio: Prospects and Challenges identified community biogas plants as one of the most useful decentralized sources of energy supply.
  7. The many benefits of biogas in the Third World are now generally recognized. It has resulted in a smoke- and ash-free kitchen, so women and their children are no longer prone to respiratory infections. Women are spared the burden of gathering firewood, a load of 60-80 lb per week, which can take up to one day a week. That, and the practice of containing livestock for manure collection, which might otherwise graze in the forest, both contribute to protecting the remaining forests and allowing the forests to regenerate. The sludge remaining after anaerobic digestion is richer in valuable nutrients than the animal manure, providing vegetables, fruits and cereals with a top quality fertiliser that guarantees better crops.
  8. Nepal has overtaken China and India in the number of biogas plants per capita. Each of its 125 000 functioning digesters prevents five tonnes of carbon dioxide equivalents from being pumped into the atmosphere every year. This ’saved’ greenhouse gas is worth US$5 million. This money can be invested back into clean energy that would make Nepal eligible to trade even more carbon offset to rich polluters.
  9. Producing carrier bags from recycled rather than virginpolythene reduces energy consumption by two-thirds, produces only a third of the sulphur dioxide and half of the nitrous oxide; it reduces water use by nearly 90 percent, and carbon dioxide emission two and a half times. For every tonne of recycled polythene produced, 1.8 tonnes of oil are saved.
  10. Although all types of plastics could be recycled, only 7 percent actually were in 2001. The rest were buried in landfills (80 percent) or incinerated (8 percent). Recycling is done mechanically or chemically. In mechanical recycling, the waste plastics are sorted, then melted, shredded or turned into granules and moulded into new shapes. In chemical recycling, the plastic polymers are broken down into their constituent monomers by heat treatment (thermal depolymerization), which can then be used again in refineries or petrochemical and chemical production.
  11. PVC, polyvinyl chloride, is the second most commonly used plastic in the world, and causes the most problems for health and the environment. It is the largest source of dioxin when burnt in incinerators and in accidental fires in buildings. Dioxin is also created during the manufacture process, and toxic chemical additives are incorporated in PVC products. PVC is difficult to recycle and contaminates other plastics.
  12. A relatively new low temperature thermal conversion process (TCP) - which can be carried out in adapted oil refineries - offers a completely contained and highly efficient way of turning food-wastes into biodiesel. One claimed advantage of TCP in treating foodprocessing and slaughterhouse wastes is that it breaks down the prion proteins associated with mad cow disease, which survive normal boiling or autoclaving. However, no evidence was presented for this claim.
  13. TCP looks promising also for recycling mixed plastics wastes chemically that cannot easily be recycled back into plastics.
  14. Green algae could offer a cost-effective and environmentally benign way to capture carbon dioxide on-site with no need for transport or storage, and at the same time, provides renewable biodiesel fuel much more effectively and sustainably than energy crops. The algae proliferate in the exhaust from power plants, removing up to 40 percent of the carbon dioxide for photosynthesis, and also 86 percent of the nitrous oxide. Algae are prolific and can produce 15 000 gallons of biodiesel per acre, compared to just 60 gallons from soybean.
  15. Fuel-efficient super-clean cars are now available, which run on biogas methane. Compared with petrol or diesel, renewable methane (from biogas) considerably reduces exhaust noise levels, lowers emissions of nitrogen oxides, and has almost zero emissions of particles or dust. Sweden, Switzerland and Germany have no fuel duty on renewable natural gas, and Sweden is world leader in gaspowered vehicles. There are around 4 500 natural gas vehicles in Sweden, 40 percent run on biogas. The Swedish Association of Green Motorists has ranked biogas methane driven cars the best environmental car for 2005.

Extracted from the article: “Which Energy“, By Mae-Wan Ho

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