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Digestion Process

The digestions process to produce biogas has four phases.

Phase 1: Hydrolysis

During Hydrolysis long chain organic compounds such as proteins, fats, carbohydrates, etc. are split into more simple organic compounds such as amino acids, fatty acids, and sugars through a bacterial action. The most common of these long chain organic compounds include animal proteins and waste from cattle, pigs, chicken and fish husbandry, and production food waste including that from canneries and bakeries, byproducts of beer, and alcohol manufacturing, human and animal sewage waste, and any other process that releases methane gas.

Phase 2: Acidogenesis

During phase 2 (Acidogenesis), the products of Hydrolysis (phase 1), are metabolized by acidogenic bacteria and broken down into short chain fatty acids such as acetic acid, propionic acid, butyric acid, valeric acid, and alcohol. Acetic acid, hydrogen and carbon dioxide are created and serve as the initial products for the formation of methane. A key component during this phase is the concentration of hydrogen also known as the hydrogen partial pressure.

Phase 3: Acetogenesis

Through the Acetogenesis phase, the organic acids and alcohols break down from acetogenic bacteria into carbon dioxide, hydrogen, and acetic acid which are the main components for the production of biogas.

Phase 4: Methanogenesis

During this final phase biogas is created. Biogas is a combustible gas that occurs when methane and carbon dioxide are converted into methanogenic microorganisms or archaea.

System Configuration:

Digestion systems can be configured in either a single stage digestion system or a two-stage digestion system formation. There are advantages and disadvantages to both systems. For example, using a single stage system has a smaller initial construction and equipment cost but leads to less control of the reactions occurring in the system which can greatly reduce output. The biological reaction of the various species in a single stage reactor can be in direct competition with one another. The two bacteria reaction types are known as acidogenic bacteria, which through the production of acids reduces the pH of the tank, and methanogenic bacteria, which operates strictly in a defined pH range. The anaerobic reactions of these two bacteria occur in a pond and are contained within a anaerobic lagoon or pool with natural anaerobic sludge.

In a two-stage digestion system, also known as a multistage digestion system, multiple digestion containers are optimized to bring maximum control over the bacterial communities developing within the digester. As with single stage digestion, there are multiple types of bacterial processes. In the first reaction vessel, hydrolysis, acetogenesis and acidogenesis occur simultaneously. During this reaction feedstock is added and controlled. That organic material is then heated to the ideal operating temperature (either mesophilic or thermophilic) and then pumped into a methanogenic reactor. During this process, a sterilization process may also occur heating the matter to a temperature that kills the harmful bacteria found in waste.

Once these bacteria reactions have taken place, a residence time occurs which allows the feed bacterial to begin degrading and developing gases. This time ranges in most cases from 14 to 40 days depending on the type of feedstock and the digestion system utilized. In some cases, in an up flow anaerobic sludge blanket digestion system, a hydraulic residence time can be as short as even an hour with solid retention times can be up to 90 days. Continuous digesters utilize mechanical and hydraulic devices which continually mix the contents enabling the bacteria and the feed to be in contact with one another. This also allows for a more consistent volume within the digestion tanks as the excess material is continuously extracted.