Anaerobic Digestion

Anaerobic digestion is a natural biological process that occurs when bacteria break down biodegradable material in the absence of oxygen, which creates biogas, a mixture of mainly methane and carbon dioxide with small quantities of other gases such as hydrogen sulphide. The biogas can be burned to create heat, used to generate electricity, or used as automobile fuel.

A by-product of anaerobic digestion is a solid/liquid residue called digestate. Digestate is not harmful and can be used as fertilizer.

Most organic material can be used including sewage, agricultural residues, animal wastes, grass, paper products, and food waste. Many of these sources are usually disposed of at local landfill sites and left to degrade naturally; they release greenhouse gases into the atmosphere. Anaerobic digestion technology shows promise because it can transform low-grade organic wastes that do not have a meaningful end use into useful energy.

Large-scale anaerobic facilities can process reactions in a controlled environment under optimum conditions. The different processes that make up anaerobic digestion are:

• Hydrolysis – materials break down insoluble organic polymers, such as carbohydrates, and make them available for other bacteria.
• Acidogenesis – acidogenic bacteria convert sugars and amino acids into carbon dioxide, hydrogen, ammonia and organic acids.
• Acetogenesis – acetogenic bacteria convert the resulting organic acids into acetic acid, with additional ammonia, hydrogen, & carbon dioxide.
• Methanogensis – Methanogens convert these products into methane and carbon dioxide.

There are many different configurations of anaerobic digesters including Batch Digester, Plug-Flow Digester, Covered Lagoon, Completely Stirred Tank Reactor, Upflow Anaerobic Sludge Blanket and Anaerobic Sequencing Batch Reactor.

Click here to see an animation that explains the anaerobic digestion technique.

Additional Resources:

Anaerobic Digestion BIC Factsheet
Anaerobic Digestion Basics
Biogas and Anaerobic Digestion