Why is the end product of anaerobic digestion methane-containing biogas and not, e.g., ethanol?

Anaerobic digestion is a natural process wherein microorganisms break down organic matter, such as animal waste, food waste, and plant material, in the absence of oxygen. This process results in the production of biogas, which mainly consists of methane (CH4) and carbon dioxide (CO2).

Anaerobic digestion occurs in four stages: Hydrolysis, Acidogenesis, Acetogenesis, and Methanogenesis. 1. Hydrolysis: In this stage, large organic molecules like carbohydrates, proteins, and fats are broken down into smaller soluble molecules such as sugars, amino acids, and fatty acids by enzymatic action. 2. Acidogenesis: In the second stage, the soluble molecules produced in hydrolysis are further converted into volatile fatty acids (VFAs), including acetic, propionic, and butyric acids, as well as alcohols, hydrogen, and carbon dioxide by fermentative bacteria. 3. Acetogenesis: During this stage, acetogenic bacteria convert other VFAs and alcohols into acetic acid, hydrogen, and carbon dioxide. 4. Methanogenesis: In the final stage, methanogenic archaea convert the end products of acetogenesis, mainly acetic acid and hydrogen, into methane and carbon dioxide.

Ethanol is produced from the fermentation of sugars by yeasts, which is an aerobic process. It results from the incomplete breakdown of glucose, producing smaller organic molecules such as ethanol and carbon dioxide. In anaerobic digestion, there is a much more complete breakdown of organic matter, reaching the final stages of Methanogenesis, where methane and carbon dioxide are produced. Although ethanol can be formed transiently during the Acidogenesis stage, it does not accumulate because it is further converted into acetic acid in the Acetogenesis stage, which then leads to the production of methane in Methanogenesis. Therefore, the end product of anaerobic digestion is methane and carbon dioxide, which constitute biogas, rather than ethanol, due to the more complete breakdown of organic matter in the absence of oxygen and the subsequent conversion of ethanol to methane through Acetogenesis and Methanogenesis.