What is a Biodigester?
A Biodigester is an airtight chamber in which anaerobic digestion of manure, biosolids, food waste, other organic wastewater streams or a combination of these feed stocks occur. The digester is fed daily with organic material, which is broken down (decomposed) by micro-organisms (bacteria) in an oxygen-free (anaerobic) environment to produce a renewable energy called biogas (methane and carbon dioxide) and other material that can be used as a fertilizer (digestate). The biogas is collected and then used for energy in the form of power or heat. Some of the energy is required to heat the digesters to mesophilic and thermophilic temperatures. Excess energy can be used by the plant or returned to the grid for renewable energy credits.
Why use a Biodigester?
- Waste stabilization and reduction
- Odour and greenhouse gases reduced
- Energy Production
- Pathogen reduction
- Weed seed inactivation
- Nutrient conservation and mineralization
- Fertilizer by-product (digestate)
- Renewable energy certificates (RECs)
- Carbon credits
Soil benefits of using digestate can include:
- Increasing organic matter content;
- Reducing the need to apply chemical fertilizers and pesticides;
- Improving plant growth;
- Reducing soil erosion and nutrient runoff;
- Alleviating soil compaction; and
- Helping increase the soil’s water retention ability, which reduces the need for irrigation
- Biogas resulting by anaerobic digestion is a source of renewable energy because it replaces fossil energy.
- Reduction of pollution
- Sustainable management of organic waste
What is Anaerobic Digestion
Anaerobic digestion is a series of biological processes in which microorganisms break down biodegradable material in the absence of oxygen. One of the end products is biogas, which is combusted to generate electricity and heat, or can be processed into renewable natural gas and transportation fuels.
There are four stages to anaerobic digestion including Hydrolysis, Acidogenesis, Acetogenesis and Methanogenisis:
- Hydrolysis – The breakdown of complex organic molecules into simple sugars, amino acids, and fatty acids.
- Acidogenesis – Further breakdown of remaining components by Acidogenic (fermentative) bacteria. Volatile fatty acids (VFA’s) are created along with NH3, CO2, H2S as well as other by products.
- Acetogenesis – The third stage of anaerobic digestion. The above molecules are digested by Acetogenes to produce largely CH3COOH, CO2 and H.
- Methanogenesis – The terminal stage of anaerobic digestion. Methanogens use the above products and convert them to CH4, CO2 and H2O. These components make up the majority of biogas emitted from the system. Organisms capable of producing methane have been identified only from the domain Archaea, a group phylogenetically distinct from both eukaryotes and bacteria, although many live in close association with anaerobic bacteria.
Factors effecting the functionality of an Anaerobic Digester
- Contamination of feedstock with the following
- Plastic, glass, metals, chemicals & antibiotics
- Solids retention time
- High volatile fatty acids (VFA)
How do cows pollute the environment?
Cows contain one of the most well-known fermentation vats, the rumen, which is part of the stomach (in other animals as well). Fermentation takes place during digestion. A cow on average releases between 70 and 120 kg of Methane per year. Methane is a greenhouse gas like carbon dioxide (CO2).
Cows release methane as part of their digestion process. A significant portion of these emissions come from methane, which, in terms of its contribution to global warming, is 23 times more powerful than carbon dioxide.
The world’s 1.5 billion cows and billions of other grazing animals emit dozens of polluting gases, including lots of methane.