Unit 1.4: Environmental Benefits and Concerns: Placing Anaerobic Digestion into Context

Background and Guiding Questions
Anaerobic digestion and biogas are generally judged by many organizations, government agencies to be a sustainable method for reducing the strength and environmental impacts of waste and a way to produce renewable energy. Evidence for the “green” credentials of anaerobic digestion is found in the EPA’s AgSTAR initiative for manure management, the diverse availability of carbon and green energy credits for anaerobic digestion from both public (e.g. California’s climate reserve program, state renewable energy credit standards, the EPA’s Renewable Fuel Standard) and private (Terrapass) programs, and the efforts of many sustainable development efforts around the world. Detailed scientific analyses have compared the full life cycle carbon emissions of biogas from anaerobic digestion to alternative sources of fuels with positive outcomes.

Nevertheless, anaerobic digestion has come under environmental scrutiny in two major ways. First, the demonstrated “green” credentials of anaerobic digestion, as with any other green technology, cannot be viewed in isolation. If other methods are available to meet the same positive environmental attributes, but at a lower cost, there may be an environmental “opportunity cost” to using anaerobic digestion. Second, anaerobic digestion has been scrutinized for some environmental impacts, such as the potential for increased ammonia emissions and the creation of air contaminants associated with biogas combustion when compared to alternative methods of livestock waste management. While anaerobic digesters may reduce some environmental effects of concentrated animal livestock operations (CAFOs) – such as odor reduction – they do not mitigate all environmental impacts on large scale operations (e.g., water quality, air emissions). As such, they are not a panacea and should not be construed as a ‘silver bullet’ remedy for all the potential environmental impacts of CAFOs. The full environmental footprint of anaerobic digestion is complex and will vary depending on the application, management and design specifics of each project.

Guiding Questions

  • What are the main environmental attributes of anaerobic digestion?
  • What alternative waste management or energy systems should be compared to anaerobic digestion and biogas?
  • What environmental impacts and criticisms of large commercial livestock farms do anaerobic digesters address and what impacts do these systems not address?
  • What is an “indirect” environmental impact and how should society weigh these impacts?

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