- The Anaerobic Digestion process begins when biomass is put inside a sealed tank or digester.
- Naturally occurring micro-organisms digest the biomass, which releases a methane-rich gas (biogas) that can be used to generate renewable heat and power; this helps cut fossil fuel use and reduce greenhouse gas emissions.
- The remaining material (digestate) is rich in nutrients, so it can be used as a fertiliser.
Many forms of biomass are suitable for Anaerobic Digestion; including food waste, slurry and manure, as well as crops and crop residues. However, woody biomass cannot be used in AD because the micro-organisms can’t breakdown the lignin, the compound that gives wood its strength.
Anaerobic Digestion is not a new technology, it has been used since the late 1800s, but now an increasing number of Anaerobic Digestion plants are being built in the US to generate clean renewable energy. Anaerobic Digestion is also used to treat the waste produced in homes, farms, supermarkets and industries across the US. This helps divert waste from landfill.
The products of Anaerobic Digestion are referred to as biogas and digestate. Biogas is a mixture of 60% methane, 40% carbon dioxide and traces of other contaminant gases. The exact composition of biogas depends on the type of feedstock being digested.
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.
Types of Anaerobic Digestion Systems
There are two general AD system configurations suitable for agri-food systems in Canada: completely mixed and plug flow.
Completely mixed systems, as the name implies, consist of a large tank where fresh material is mixed with partially digested material. These systems are suitable for manure or other agri-food inputs with lower dry matter content (4%–12%). Material with higher dry matter content will work in completely mixed systems by recirculating the liquid effluent.
Plug flow systems typically consist of long channels in which the manure and other inputs move along as a plug. These systems are suitable for thicker materials such as liquid manure with 11%–13% dry matter or higher.
Anaerobic Digestion Facts
- If 20 million tonnes of food waste were used to generate electricity through AD rather than being sent to landfill, the carbon saving would be around 3.4 million tonnes per year.
- Waste food sent to landfill is a major contributor to the production of greenhouse gases in the US. Once in landfill, food breakdown produces methane, a greenhouse gas 21 times more harmful to the environment than CO² (carbon dioxide). Anaerobic Digestion captures any methane produced and converts it to renewable energy and heat.
- At its full potential, Anaerobic Digestion could produce enough electricity to power 2 million homes.
Why Anaerobic Digestion?
There are essentially three main options for dealing with organic wastes:
- bury it – which means landfilling
- burn it – which means incineration, gasification or pyrolysis
- bio-digest it – which means either anaerobic digestion or composting