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Anaerobic digestion produces two main products: (i) digestate and (ii) biogas.

The digestate is the material remaining after the anaerobic digestion of a biodegradable feedstock which is then used as a source of renewable energy through the participation of a variety of chemical reactions (Table A-4).

Acidogenic digestate is fibrous and comprises structural plant material which includes lignin and cellulose. It is the acidogenic digestate that possesses the high moisture retention properties and the raw digestate usually also contains minerals and the remains of the micro-organisms (mainly bacteria) which were active during the digestion process. On the other hand, methanogenic digestate is a liquid or sludge (sometimes referred to as a liquor) which is often high in nutrients such as ammonium derivatives and phosphate derivatives.

Table A-4 Common reactions in anaerobic digestion process.

Key: acetate CH₃CO₂₋ , propionate C₃H₉CO₂₋ , butyrate C₄H₉CO₂₋

There are four stages to the digestion process, each of which has different characteristics: (i) hydrolysis, which is a chemical reaction where particulate matter (solid matter, such as carbohydrate derivatives) is solubilized and high molecular weight chemicals are converted into lower molecular weight products, (ii) acidogenesis, which is a biological reaction in which lower molecular weight products are converted into volatile fatty acid derivatives, (iii) acetogenesis, which is a biological reaction in which the volatile fatty acid derivatives are converted into acetic acid where volatile fatty acids are converted into acetic acid, carbon dioxide, and hydrogen, and (iv) methanogenesis, which is a biological reaction in which acetate derivatives are converted into methane and carbon dioxide, while hydrogen is consumed. Thus, using carbohydrate derivatives as the example of the digester feedstock:

 (i) Hydrolysis

 (ii) Acidogenesis

 (iii) Acetogenesis

 (iv) Methanogenesis

The acidogenic digestate is a stable organic material comprised largely of lignin and chitin, but also of a variety of mineral components in a matrix of dead bacterial cells and some plastic may be present. This resembles domestic compost and can be used as compost or to make low-grade building products such as fiberboard. Another byproduct is a liquid (methanogenic digestate) that is rich in nutrients and can be an excellent fertilizer dependent on the properties of the digester feedstock (Table A-4).

If the feedstock sent to the digester includes low levels of toxic heavy metals (i.e., metals with relatively high density, atomic weight, or atomic number) or synthetic organic chemicals such as pesticides or polychlorobiphenyls, the effect of digestion is to concentrate such materials in the product (i.e., the digester liquor). In such cases further treatment will be required in order to dispose of this liquid properly. In extreme cases, the disposal costs and the environmental risks posed by such materials can offset any environmental gains provided by the use of biogas. This is a significant risk when treating sewage from industrialized catchments.

See also: Anaerobic Digestion, Digester, Digestion.