What is Biodigestion?
Biodigestion or anaerobic digestion (AD) is a biological process that occurs when organic matter is decomposed by bacteria in the absence of oxygen (i.e., anaerobic). As the bacteria decompose the organic matter, biogas is released and captured. Biogas consists of approximately 60% methane and 40% carbon dioxide. The remaining byproduct is called Digestate and is low in odor and rich in nutrients1.
Biogas can be combusted to provide heat, electricity or both. Alternatively, the biogas can be ‘upgraded’ to pure methane, often called biomethane, by removing other gases. This pure stream of biomethane can be used as a substitute for natural gas. Digestate is the left over material in the AD process. It contains valuable plant nutrients like nitrogen and potassium. Digestate can be used as a fertilizer and soil conditioner2.
There are two basic anaerobic digestion processes, which take place over different temperature ranges:
- Mesophilic digestion: the most commonly used process for anaerobic digestion. It takes place between 20ºC and 40ºC and can take a month or two to complete, produces less biogas and requires additional sanitation.
- Thermophilic digestion: less common and more expensive technology. Typically takes place from 50-65ºC, it requires additional energy to maintain the higher operating temperature. It produces more biogas in a shorter period faster but the bacteria are more sensitive34
Anaerobic digestion has a defined process flow that consists of four distinct phases.
- Pre-Treatment: In pre-treatment, wastes may be processed, separated, or mixed to ensure that they will decompose in the digester.
- Digestion: During digestion, waste products are broken down by bacteria and biogas is produced and collected.
- Biogas Processing: Biogas produced is either combusted or upgraded and then used to displace fossil fuels. During upgrading, scrubbers, membranes or other means are used to remove impurities and carbon dioxide (CO2) from biogas.
- Reuse or disposal of solid digested waste: Digested waste has a high nutrient content and can be used as fertilizer so long as it is free of pathogens or toxins, or it can be composted to further enhance nutrient content5.
Anaerobic digestion systems are often referred to as “anaerobic digesters”, “biodigesters” or “biogas recovery systems”.
Anaerobic digestion is a proven technology that can be applied to a variety of waste streams. It has several positive impacts. From an environmental standpoint, AD reduces the volume of waste going to landfills and reduces methane and carbon dioxide emissions. Additionally, the AD process removes pathogens, phosphorous and metals that could otherwise end up contaminating water supplies.
From an economic standpoint, AD technology converts a source of inexpensive energy and creates a valuable fertilizer.
The most significant challenge facing AD is the price of the technology. Despite this challenge, AD is rapidly expanding in developing countries and seeing moderate growth in developed countries.
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