The anaerobic digestion can be grouped into two categories depending upon the suspended and attached bacterial growth within the reactor.
a] Mobilized cell Process : In this process, bacterial cells are in suspension and are completely mixed with the reactor. The mixing is providing by artificial means.
b] Immobilized Cell Process In immobilized cell process, the bacteria are concentrated within the reactor by attaching themselves to an inherent supporting media or by way of attaching themselves together to form conglobe rates which settle rapidly. Three types of processes are characterized for immobilized cells.
i) Fixed – Medium System : The bacterial attachment medium is stationary and liquid flow regime is by plug flow.
ii) Completely mixed fixed film system : The bacterial attachment medium and the liquid regime are uniform throughout the system.
iii) Fluidized-Bed System : Fluidization offers mixing of the medium uniformly within the reactor. When little or no recycle is required for fluidization the liquid follows a plug flow pattern while bio-film approaches completely mixed state.
Anaerobic digestion involves the decomposition of organic and inorganic matter in absence of molecular oxygen. The anaerobic digestion of complex particulate organic matter has been described as a multi step process of series & parallel reactions. Complex material such as polysaccharides, proteins and lipids are first hydrolyzed to soluble products by extracelluar enzymes secreted by microorganisms, so as to facilitate their transport or diffusion across the cell membrane. These relatively simple settable compounds are anaerebically oxidized further to short chain fatty acids, alcohol, carbondioxide, hydrogen & ammonia. The short chain fatty acids are converted to acetate, hydrogen and carbon dioxide Methanognesis finally occurs from reduction of carbon dioxide and acetate.
The anaerobic digestion can be broadly grouped in two major stages. The initial stage of anaerobic degradation i.e. acid fermentation is essentially a constant BOD stage because the organic molecules are rearranged. Thus in general, the energy production by this step is very low and thus growth of the microorganism is also low. Since most of energy used by the bacteria for the growth, there is minimum energy liberated from the system. The first stage does stabilize the organic matter in the waste. However this step is essential for initiation of second step methane fermentation as it converts the organic material to a form usable by methane producing bacteria.
The second reaction is initiated when anaerobic methane forming bacteria act upon the short chain organic acid produced in the first stage. Hence these acid undergo methane fermentation with carbon dioxide acting as hydrogen acceptor and getting reduced to methane. The methane formed being insoluble in water, escapes from the system and can be tapped and used as energy source. The production & subsequent escape of methane causes the stabilization of the organic matter. Since methane has high energy content, most of the energy of the system goes into gas and not into production of large amount of cell mass and solids.
The acid fermentation stage is faster as compared to methane fermentation stage. This means that sudden increase in easily degradable organic matter will result in increased acid production with subsequent accumulation of acid. This inhibits methanognesis step. Parallel to the acid production, ammonia is released by degradation proteins and ammonia acids. The ammonia concentration thus established will generally not be of magnitude that will inhibit the methanogens except in case of nitrogen rich waste treated in highly loaded processes.
The methane producing bacteria consist of several different groups. Each group has the ability to ferment only specific compounds. Therefore, the bacterial consortia in a methane producing system should include number of different groups.
