Anaerobic digestion (AD) is the process by which organic residues of all types are broken down to produce biogas (methane) which can be used for the generation of electricity. The decomposition of organic matter under anaerobic conditions produces amino acids, carbon dioxide, hydrogen sulfide, and methane. All these gases are either very toxic (hydrogen sulfide) or  lack oxygen (carbon and methane). Biogas under most circumstances will burn from the direct gas output of the digester. For applications in some internal combustion engines, the carbon dioxide and hydrogen sulfide must be removed.  American Great Lakes AD System includes methods for biogas capture, scrubbing and usage.

The aim of anaerobic digestion is to maximize the decomposition of as much organic matter as possible plus as much biogas as possible. This requires a high quantity of degradable starch, and a little cellulose. Grain straw, grass, and citrus residues are not ideal for organic materials. Animal manure, on the other hand, contains much degradable carbohydrates, has little cellulose, and has a relatively high nutrient level. More carbohydrates may be desired depending upon the type of animal manure being used. Poultry manure is lower in organic material than crop residues and is higher in organic material than manure from swine or ruminants (cattle, sheep, and goats). Thus, crop residue alone is not desirable for the production of biogas; a mixture of animal manure and crop residue is most desirable.

There are 3 basic types of AD systems. These are based on the operating temperature;

• Psychrophilic
  The term used for “cold” digestion at approximately 25 degrees Celsius.
  This type is rarely used.
• Mesophilic
  The term used for “warm” digestion at approximately 25 to 40 degrees Celsius.
  This is the most common for European digesters.
• Thermophilic
  The term used for “hot” digestion at approximately 52 to 55 degrees Celsius.
  The high temperature guarantees pathogen kill.

Manure as a Valuable Resource

Nitrogen is a necessary nutrient for both soil conditioning and animal feed. Nitrogen also plays an important role in aerobic (open-air, or “in the presence of air”) and anaerobic (closed, or in the absence of air) digestion processes; however, these processes change the form of nitrogen, which may influence its availability to plants, its volatility, or its leachability. “Nightsoil” and manure constitute more productive inputs for anaerobic digestion when combined with crop residues. Even with the rigorous decomposition that occurs in the digestion process, some pathogens and parasites, in particular E-coli, can survive and enter the soil. As was witnessed in the Walkerton, Ontario water contamination tragedy in early 2000, where 2,300 people fell ill, and seven died from E-Coli poisoning of the town water supply from cattle manure run-off, this can be extremely dangerous as these pathogens and parasites can eventually re-enter the human body.

The proprietary American Great Lakes AD System operates in the thermophilic temperature range (55º C) ensuring the highest possible level of process pathogen-kill combined with the highest possible output of methane. Other methods include aerobically drying the effluent for an extended period of time (i.e. composting). However, since ammonia is quite volatile, nitrogen loss is substantial during composting, and the value of the manure as a fertilizer is lost. Crop residues and animal manure not only fertilize the soil, but also go along way to maintaining healthy soil, while providing nutrients to crops without chemical fertilizers.

Digester Solution Diagram