From waste to energy
Basic principles of anaerobic digestion

What is biogas?
Biogas is produced through the anaerobic digestion (fermentation) of decaying plant or animal matter. It is the naturally occurring emission of bacteria that thrive without oxygen, and occurs in three steps. First is the decomposition, or hydrolysis, of the biodegradable material into molecules such as sugars. Next, these molecules are converted into acids. Lastly, the acids are converted into biogas. Anaerobic digesters harness the bacteria's natural processes to capture and utilize the biogas, all in a safe, controlled environment.
Biogas produced in anaerobic digesters is burned to generate clean, renewable energy. The main components of biogas are carbon dioxide (approx. 40%) and methane (approx.60%), which, if released in uncombusted form, is harmful to the environment as a particularly potent greenhouse gas. Anaerobic digestion helps manage two of the biggest sources of manmade methane: livestock manure and landfills. Livestock manure, since it is the main ingredient for the digesters, and landfills, since food waste can be digested rather than dumped. Certainly, methane can be extracted from landfills, but anaerobic digestion is far more efficient for the same waste source.
By preventing the emission of methane while producing clean energy, anaerobic digesters make a twofold contribution to climate protection: The usual unchecked discharge of methane into the atmosphere is prevented, and the burning of fossil fuels is replaced with an unlimited supply of clean, renewable energy (biogas).

Process temperature
A rise in temperature to a certain point increases the biological activity of bacteria, evidenced by an increase in growth rate and the specific nutritional requirement of the bacteria. Methanogenic (biogas producing) bacteria are divided into psychrophilic, mesophilic and thermophilic strains according to their optimum temperature ranges. Both mesophilic and thermophilic strains are used for biogas generation. However, thermophilic digesters are highly sensitive to changes in feed materials and temperature, and therefore do not have the same reliability as mesophilic digesters. The optimum range for the mesophilic strains is 30°C to 35°C, while for the thermophilic strains it reaches 50°C to 60°C. At temperatures falling below the corresponding optimum range, metabolic activity - i.e. biogas production - decreases significantly until it stops altogether. Conversely, excessively high temperatures may harm the bacteria, and ultimately kill off the biogas-producing bacteria in the digester. Biogas Energy's anaerobic digesters sustain a beneficial temperature in the substrate to maximize biological activity and optimize biogas production. And since these digesters are able to tolerate fluctuations in temperature and feed source, downtime is minimized.

pH Value
In general, the pH value of the substrate during digestion varies between slightly acidic (ca. pH 6) and slightly alkaline (ca. pH 8). Variations from that range must be monitored, and appropriate measures taken to regain balance.

Inhibitors
The maintenance of the biogas process, or the activity of the microorganisms, is mainly affected by the concentration of substances added. The effects of adding substances ranges from a slight inhibition of growth to the complete, yet reversible inhibition of metabolic activity, and ultimately to the killing off of all microorganisms in the entire biological process. Antibiotics, disinfectants or other chemicals would have an inhibiting effect, and are avoided.

Digester Load
Digester load tracks how much organic material is added to the digester per cubic meter of tank volume. The optimum digester load depends on the substrate and process temperature, and is calculated using organic matter content, retention time and digester volume.

Steady Loading of the Digesters
In order to avoid shocking the biological processes within the digester, an even loading of material is advised. It is better to add a steady supply than a big load at once. This applies to the basic substrate (slurry or manure) and, more importantly, to energy-rich substances such as food waste, grease and fats.