Stages Of Anaerobic Digestion Part 1

Stages of anaerobic digestion

As with aerobic systems, the bacteria in anaerobic systems, and the growing and reproducing microorganisms within them require a source of elemental oxygen to survive.  In an anaerobic system there is an absence of gaseous oxygen.  Gaseous oxygen is prevented from entering the system through physical containment in sealed tanks.  Anaerobes access oxygen from sources other than the surrounding air.  The oxygen source for these microorganisms can be the organic material itself or alternatively may be supplied by inorganic oxides from within the input material.  When the oxygen source in an anaerobic system is derived from the organic material itself, then the ‘intermediate’ end products are primarily alcohols, aldehydes, and organic acids plus carbon dioxide.  In the presence of specialized methanogens, the intermediates are converted to the ‘final’ end products of methane, carbon dioxide with trace levels of hydrogen sulfide.  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 microorganisms typically take a significant period of time to establish themselves to be fully effective.  It is therefore common practice to introduce anaerobic microorganisms from materials with existing populations, a process known as “seeding” the digesters, and typically takes place with the addition of sewage sludge or cattle slurry.

The key process stages of anaerobic digestion

There are four key biological and chemical stages of anaerobic digestion:

  1. Hydrolysis
  2. Acidogenesis
  3. Acetogenesis
  4. Methanogenesis

In most cases biomass is made up of large organic polymers.  In order for the 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 by 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 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 (VFA’s) with a chain length that is greater than acetate must first be catabolised into compounds that can be directly utilized by methanogens.

The biological process of acidogenesis is where there is further breakdown of the remaining components by acidogenic (fermentative) bacteria. Here VFAs are created along with ammonia, carbon dioxide and hydrogen sulfide as well as other by-products.   The process of acidogenesis is similar to the way that milk sours.

The third stage 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 methanogens utilize the intermediate products of the preceding stages and convert them into methane, carbon dioxide and water.  It is these components that makes up the majority of the biogas emitted from the system.  Methanogenesis is sensitive to both high and low pHs and occurs between pH 6.5 and pH 8.  The remaining, non-digestible material which the microbes cannot feed upon, along with any dead bacterial remains constitutes the digestate.

A simplified generic chemical equation for the overall processes outlined above is as follows:

C6H12O6 → 3CO2 + 3CH4

refer to “what is anaerobic digestion” article and “what do floating covers do?”


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