A biochemical process in which particular kinds of bacteria digest biomass in an oxygen-free environment. Several different types of bacteria work together to break down complex organic wastes in stages, resulting in the production of biogas.

TECHNOLOGY
Many different technologies are available for anaerobic digestion:

• Enzymatic Hydrolysis Digestion: Insoluble organic polymers such as carbohydrates, cellulose, proteins and fats are broken down and liquefied by enzymes produced by hydrolytic bacteria.

• Acidogenesis Digestion: Organic acids formed in the hydrolysis and fermentation stage are converted by acetogenic micro-organisms to acetic acid. At the end of this stage carbon dioxide and hydrogen concentrations begin to decrease.

• Methanogenesis Digestion: Methane (60%) and carbon dioxide (40%) are produced from the organic acids and their derivatives produced in the acidogenic phase. The methane is a useful fuel source and methanogenic bacteria play a further role in maintaining wider breakdown processes. 1,2

TYPES

• Upflow Anaerobic Sludge Blanket (UASB) is known as a "high-rate" system that treats wastewater. Wastewater is fed in from the bottom of an empty tank and passes upward over microorganisms - bacteria - that degrade the wastewater into biogas. The upward motion of released gas bubbles causes hydraulic turbulence that provides reactor mixing without any mechanical parts.

• Expanded Granular Sludge Bed (EGSB) is a variant of the upflow anaerobic concept. The distinguishing feature of the EGSB is the faster upward-flow velocity of the wastewater that passes over the microorganisms. The increased rate of velocity expands the sludge bed of bacteria, increasing the contact time between the wastewater and bacteria. Increased velocity is achieved from taller reactor tanks than UASB systems use.

• Internal Circulation Reactor (IC reactor) is a very complex evolution of the UASB and EGSB. Even taller than the EGSB, the IC reactor processes wastewater in a lower compartment into biogas and then processes the biogas in an upper compartment. The second process creates a better quality biogas that is more consistent and clean.3

ENVIRONMENTAL IMPACT

• Since biogas could potentially supplant fossil fuels as source of energy, anaerobic digesters are quintessential in the arena of renewable energy and energy independence.

• Environmentally friendly waste management strategy.

• The process captures methane gas which would otherwise be emitted into the atmosphere.

BENEFITS

• Avoids transport of waste.

• Reduces the emissions of landfill gas into the atmosphere.

• Produces biogas, providing an energy source with possible revenue both from sale of the energy, and benefit from government tax credits.

DISADVANTAGES
The start-up costs of anaerobic digester systems are high, making the trade-off of biogas production for fossil fuel production not yet cost-effective. Heating of the tank, using petroleum gas, may be required for achieving reactions between bacteria and wastewater.

VALLEY ACCESS POTENTIAL
Most anaerobic digestion technologies are commercially available. Where unprocessed wastes cause odor and water pollution such as large dairies, wastewater treatment plants, or landfills, anaerobic digestion reduces the odor and liquid waste disposal problems and produces a biogas fuel that can be used for process heating and/or electricity generation.4

ANAEROBIC DIGESTION IN THE GREAT CENTRAL VALLEY
Fiscalini Farms in Modesto has installed an anaerobic digester as a waste solution for the dairy. The methane captured through the digestion process is used to power an engine that feeds electricity into the grid. For more information on Fiscalini Farms visit www.fiscalinifarms.com.

For more information on anaerobic digestion visit the U.S. Dept. of Energy at www.energysavers.gov.

1 Monsal Limited: www.monsal.com/pages.asp?ContentID=1 (last accessed April 22, 2009)
2 WASTE: www.waste.nl/content/download/472/3779/file/WB89-InfoSheet(Anaerobic%20Digestion).pdf (last accessed April 22, 2009)
3 Jim Field, Anaerobic Granular Sludge Bed Reactor Technology, www.uasb.org/discover/agsb.htm (last accessed April 22, 2009)
4 California Energy Commission: www.energy.ca.gov/biomass/anaerobic.html (last accessed April 22, 2009)