Biogas Formation Process
Methane is formed through complex chemical processes involving mixed populations of bacteria belonging to the group of archaea, known as methanogenic organisms.
The biomass decomposition process can be conventionally divided into four stages:
Stage I.
Aerobic hydrolytic bacteria break down high molecular weight organic compounds using specialised enzymes. These compounds include proteins, carbohydrates, fats, and cellulose. As a result, low molecular weight compounds are formed, including sugars, amino acids, fatty acids, and water.
The enzymes produced by hydrolytic bacteria are called exoenzymes because they are secreted outside the cell. They attach to the outer surface of bacterial cells and break down the components of the substrate. Polymers are converted into monomers, meaning individual simple molecules.
This process is called hydrolysis and occurs under the action of extracellular enzymes, including amylases, proteases, lipases, and others. The rate of hydrolysis depends on the pH level and the retention time of the substrate in the reactor. The optimal pH range for this process is 4.5 to 6.0.
Stage II.
Acid-forming bacteria then continue the breakdown process. Individual molecules enter the bacterial cells, where they undergo further decomposition.
Aerobic bacteria may partially participate in this process by consuming residual oxygen and thereby creating the anaerobic conditions required by methanogenic bacteria.
At a pH of 6.0 to 7.5, unstable fatty acids such as acetic, formic, butyric, and propionic acids; low-molecular-weight alcohols such as ethanol; carbon compounds; and gases such as carbon dioxide, hydrogen, hydrogen sulfide, and ammonia are primarily produced.
This stage is known as the acidification phase, during which the pH level decreases.
Stage III.
During this stage, hydrogen-producing bacteria convert organic fatty acids into precursors required for methane formation, including acetic acid, formic acid, carbon dioxide, and hydrogen.
These bacteria, which reduce the amount of carbon contained in organic acids, are highly sensitive to temperature fluctuations.
Stage IV.
During the final stage, methanogenic microorganisms convert acetic acid, formic acid, carbon dioxide, and hydrogen into methane, carbon dioxide, and water.
Approximately 90% of the total methane is produced during this stage, with around 70% of the methane formed from acetic acid. Therefore, the formation of acetic acid, primarily during Stage III and to a lesser extent during Stage II, is a key factor determining the rate of methane production.
Methanogenic microorganisms are strictly anaerobic and require oxygen-free conditions for their activity.
During the decomposition process, the metabolic products of each group of bacteria serve as nutrients for the next group of bacteria.
The breakdown of organic matter into individual components and its conversion into methane can only occur in a moist environment, as bacteria can process substances only in dissolved form. Therefore, water must be added when digesting solid substrates.
The step-by-step decomposition of organic matter occurs unevenly because different groups of bacteria work at different rates. Acid-forming bacteria work the fastest, breaking down organic matter within several hours to two days. Ideally, a dynamic equilibrium is established between the different stages of decomposition in terms of the concentration of substances, namely between the supply of nutrients and their breakdown.
The most common mistake is overfeeding bacteria with rapidly degradable substrate, which leads to the accumulation of acids as a result of the activity of acid-forming bacteria. This can cause a very sharp decrease in pH, which other types of bacteria cannot tolerate. In addition, an excessive concentration of the substance produced can inhibit the growth of the bacterial group responsible for its production.
Chemical Composition of Biogas
The dynamic equilibrium also depends on how easily the substrate can be broken down. For example, sugars and starches have a simple structure, so they decompose very quickly and require only a short retention time in the digester. The more complex the structure of the substrate, the longer the decomposition process takes.
Cellulose and hemicellulose have highly branched structures and decompose slowly. Lignin, a woody component of plants whose content increases as the plant matures, is poorly degraded by bacteria because it remains resistant even to acids. The rate of substrate decomposition directly affects the technically required retention time for the fermentation process.
Effective anaerobic digestion of organic matter requires four main conditions:
- an oxygen-free environment;
- an appropriate temperature of the digesting mass;
- a slightly alkaline reaction of the medium;
- the presence of methane-producing bacteria.
Methane is released from the material undergoing fermentation only under anaerobic conditions, meaning when there is no access to oxygen or air.
Therefore, fermentation must take place in specially designed tanks, closed fermentation chambers, and other similar equipment. The FOS/TAC indicator is used to analyse and monitor the fermentation process.
