Anaerobic digestion is widely used as a source of renewable energy. The produces a biogas, consist of methane, carbon dioxide and traces of other ‘contaminant’ gases. The biogas can be used directly as fuel, in combined heat and power gas engines or upgraded to natural gas-quality biomethane. The nutrient-rich digestate also produced can be used as fertilizer.
Many microorganisms affect anaerobic digestion, including acetic acid-forming bacteria (acetogens) and methane-forming archaea (methanogens). These organisms promote a number of chemical processes in converting the biomass to biogas.
Gaseous oxygen is excluded from the reactions by physical contaminant. Anaerobes utilise electron acceptors from sources other than oxygen gas. These acceptors can be the organic material itself or may be supplied by inorganic oxides from within input material. When the oxygen source in an anaerobic system is derived from the organic material itself, the ‘intermediate’ end products are primarily alcohols, aldehydes, and organic acids, plus carbon dioxide. In the presence of specialised methanogens, the intermediates are converted to the ‘final’ end products of methane, carbon dioxide, nitrogen and trace levels of hydrogen sulphide. In an anaerobic system, the majority of the chemical energy contained within the starting material is released by methanogenic bacteria as methane.
Populations of anaerobic microrganisms typically take a significant period of time to establish themselves to be fully effective. Therefore, common practice is to introduce anaerobic microorganisms from materials with existing populations, a process known as “seeding” the digesters, typically accomplished with the addition sewage sludge or cattle slurry.
Process stages:
There are four biological and chemical stages of Anaerobic digestion:
Hydrolysis
Acidogenesis
Acetogenesis
Methanogenesis
In most cases, biomass is made up of large organic polymers. For bacteria in anaerobic digesters to access the energy potential of the material, these chains must first be broken down into their smaller constituent parts. These constituent parts, or monomers, such as sugars, are readily available to other bacteria. The process of breaking these chains and dissolving the smaller molecules into solution is called hydrolysis. Therefore, hydrolysis of these high-molecular-weight polymeric components is the necessary first step in anaerobic digestion. Through the hydrolysis the complex organic molecules are broken down into simple sugars, amino acids, and fatty acids. Acetate and hydrogen produced in the first stages can be used directly by methanogens. Other molecules, such as volatile fatty acids (VFAs) with a chain length greater than that of acetate must first be catabolized into compounds that can be directly used by methanogens.
The biological process of acidogenesis results in further breakdown of the remaining components by acidogenic (fermentative) bacteria. Have, VFAs are created, along with ammonia, carbon dioxide, and hydrogen sulphide, as well as other by-products. The process of acidogenesis is similar to the way milk sours.
The third stage of anaerobic digestion is acetogenesis. Here, simple molecules created through the acidogenesis phase are further digested by acetogens to produce largely acetic acid, as well as carbon dioxide and hydrogen.
The terminal stage of anaerobic digestion is the biological process of methanogenesis. Here, the methanogens use the intermediate products of the preceding stages and convert them into largely methane, carbon dioxide, and water. These components make up the majority of the biogas emitted from the system. Methanogenesis is sensitive to both high and low pH and occurs between pH 6.5 and pH 8. The remaining, indigestible material the microbes cannot use any dead bacterial remains constitute the digestate.
Anaerobic digestion can be performed as a batch process or a continuous process. In a batch system biomass is added to the reactor at the start of the process. The reactor is then sealed for the duration of the process. In its simplest form, batch processing needs inoculation (Seeding) with already processed material to start the anaerobic digestion. The process provided in this instance id therefore a semi continuous one, where one daily feed will continue the process of digestion and biogas production.
