A more realistic description of the energy potential from anaerobic digestion

GallonDaily has seen far too many highly exaggerated projections of the amount of energy produced from anaerobic digestion of municipal waste. That is not to say that we are opposed to anaerobic digestion (breakdown of biowastes in the absence of air) but even all the waste in the world is only going to produce a tiny fraction of our energy needs, not the huge fraction that some anaerobic digestion advocates have suggested.

For some time we have been looking for real data on how anaerobic digesters actually perform. The Norway-based power technology company Wärtsilä has provided some of the data we are seeking in a recent press release announcing a new aerobic digestion plant in Oslo.

So here are the numbers as provided by Wärtsilä and not independently verified by GallonDaily:

  • the plant will accept 50,000 tonnes of food waste a year [that is the food waste from roughly 125,000 North American homes]
  • processing of the food waste will make about 14,000 Nm3 (normal cubic metres) per day of biomethane
  • that is enough methane to power 135 city buses
  • which will reduce greenhouse gas emissions by about 10,000 tonnes per year
  • and which, according to a media report, will produce 90,000 cubic metres of organic waste material which can be applied to agricultural land as a fertilizer. [It is not clear how the 90,000 cubic metres output relates to the 50,000 tonnes input as the two streams likely have different moisture levels.]

GallonDaily suspects that Wärtsilä’s press people believe that we should be impressed that the waste to biogas plant produces enough fuel to keep 135 city buses on the road. However, GallonDaily is actually quite unimpressed. A city of 125,000 people [North America] or 590,000 [Oslo, Norway] needs more than 135 buses, so the waste to energy plant will not even keep all of the City’s buses on the road, let alone the cars and trucks and home heating and lights and everything else that takes energy to run. That is not to say that turning waste to fuel may not be a good idea – we see merit in it – but it will certainly be only a small part of our needed renewable energy future and, if we reduce food waste as we must if we are to feed the world’s rising population, then the amount of energy being derived from our food waste will inevitably decline. Energy from food waste has a role but it is by no means a panacea.

The Wärtsilä announcement concerning the Oslo food waste to energy plant can be found at http://www.wartsila.com/en/press-releases/biogas-liquefaction-plant-supplied-by-wartsila-to-produce-biofuel-for-buses-in-norway

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