Anaerobic digestion is the break down of organic materials either occurring naturally or under controlled conditions in the absence of oxygen. The carbon content of the material is released as methane or biogas (also known as landfill gas), rather than carbon dioxide (R.Van Berkel, pers.comm; UNEP, 1998).
Anaerobic digestion converts organics into carbon dioxide, methane, hydrogen sulphide, other gases, and water. (C.E.C., 2000). The following useful by-products are generated (C.E.C., 2000):
- a solid soil conditioner;
- a liquid soil conditioner which can be used as agricultural fertiliser; and
- a gas (typically referred to as 'biogas'), which may be used for energy generation.
Waste Streams Handled
This technology is appropriate for the organic component of MSW but is usually utilised in combination with sewage sludge and commercial food waste. Anaerobic digestion is ideally suited to a clean organic input stream, which may include (C.E.C., 2000):
- sewage sludge;
- agricultural waste; and
- food waste.
With anaerobic digestion processes, temperature affects the rate of digestion and should be maintained in the mesophilic range (35 to 45 degrees Celsius) (C.E.C., 2000). It is possible to operate in the thermophilic range (40 to 70 degrees C), but the digestion process must be closely monitored (C.E.C., 2000) The overall process requires three to four stages involving mechanical processing, one or two distinct anaerobic decomposition phases, and an aerobic or other stabilizing process (UNEP 1998).
Anaerobic digestion is a mature but evolving technology. Traditionally, anaerobic digestion facilities have treated agricultural wastes and municipal and industrial wastewater (Waste Inquiry, 2000). Many technologies of this nature are commercially operating in Australia, Europe and North America (Waste Inquiry 2000). More recently, technologies of this nature are moving towards the processing of MSW.
- California Energy Commission (2000) www.energy.ca.gov/index.html
- UNEP (United Nations Environment Program) International Environment Technology Centre (1998) Solid Waste Management Sourcebook. www.unep.or.jp/ietc/estdir/pub/msw/
- Waste Inquiry (2000) Report of the Alternative Waste Management Technologies and Practices Inquiry.