Anaerobic digestion


Today the job of waste pollutants has increased due to increasing figure of industries every bit good as urbanisation. Water is polluted in big sum due to these waste pollutants. These waste merchandises can be from different beginnings nutrient waste, industrial waste, agricultural waste, urban waste etc. Waste pollutants which are obtained from these beginnings are normally organic stuffs.

The procedure of anaerobiotic digestion purposes on dislocation of these waste stuffs. “ It is a complex biochemical procedure of biologically mediated reactions, by a pool of micro-organisms to change over organic acids to methane and carbon dioxide. ”

The procedure stabilizes the waste by decrease in its mass, decrease in smell and decrease in figure of pathogens present in it. The procedure helps in decrease of landfill gas in the ambiance. It produces energy in the signifier of biogas. Biogas contains methane and C dioxide. Digestate or waste produced after the procedure is over is rich in foods and can be used as fertilisers.

General equation of the whole procedure is

The procedure:

The procedure was divided into three phases viz. Hydrolysis, acerb formation and gas formation ( Lawrence and McCarty 1967 ) . Phase of acerb formation can be termed as non-methanogenic stage and gas- formation as methanogenic stage. A big figure of bacterial population is involved in these procedures.

Different species of micro-organisms which are used in anaerobiotic digestion vary harmonizing to the stuffs degraded by them. Microorganisms involved usage different foods as negatron acceptors. For illustration nitrate is used by Paracoccus denitrificans, Pseudomonas stutzerii ; Costridium aceticum and Acetobacteium woodi usage carbonate. Cellulolytic bugs, responsible for hydrolysing cellulose, are present in copiousness in anaerobiotic digesters.

It is estimated that in 1st and 2nd stage of debasement 128 ordersof 58 species and 18 genera are involved eg. Ruminococcus, Eubacterium. 3rd and 4th stage involve 81 different species from 23 genera. Eg. Methane bacterium. Some saccharides are besides degraded by homoacetogenic bacteriums which uses ethanoyl group CoA tract.

There are different specific genera responsible for peculiar stages.

Hydrolytic stage:

It is the stage in which undissolved organic compounds/ polymers are converted to simple monomers which are H2O soluble, by the action of facultative or obligate anaerobes. Compounds like amylum, cellulose, lignin, pectin are broken down into simpler compounds by the action of microbic enzymes. Different exoenzymes/hydrolases are responsible for this dislocation. Generally covalent bonds between these polymers are broken. Facultative anaerobes take O from water and bring forth a low oxidation-reduction potency.

There are many genera which participate in this stage.

  • Genus Bacterioides: B.uniformis, B. acidifaciens, B. ruminicola.Immobile Gram-negative rods, which convert saccharides, peptones, organic acids, aminic acids to succinate, acetate, formate etc.
  • Genus Lactobacillus: L. pentosus, L bird sanctuaries. Convert glucose to breastfeed or organic acids by the procedure of gay or heterofermentation
  • Propionibacteriawhich are gram positive, immobile, spore organizing rods use carbohydrate s as their substrate.
  • Sphingomonas, Sporobacterium, Bifidobacteriumare besides involved in this measure. Their function is really substrate particular.

Normally Acetate and H which is produced are used straight by methanogenic bacteriums, but sometimes merchandises with high concatenation lengths are formed. They are converted into simple compounds in following measure.


It can be said that acidogenesis procedure works parallel to the hydrolytic phase. It is the procedure in which complex molecules like amylum, cellulose are depolymerized by the action of enzymes like lipases, peptidases, amylases. After that these merchandises undergo agitation to organize volatile fatty acids ( propionate, butyrate ) , compounds like methyl alcohols along with gases like ammonium hydroxide, H and C dioxide.

About all the bacterium which are present in hydrolytic phase are present in this measure.

  • Clostridium- different species of clostridia act on different substrates. eg. C. butyricum – amylum, pectin ; C. acetolerans – Xylan and glucose
  • Ruminococcus – Carbohydrate agitation to organize ethanoate, formate, succinate, lactate, CO2, H2. eg. R.gnavus – transition of sugar to ethanol ; R. albus – Acts of the Apostless on cellulose and different sugars.
  • Paenibacillus – They produce lactate, formate, propionate as the merchandise of the reaction.


It is a really of import phase since all the metabolites which are formed so far are converted to acetate, H and C dioxide.

Compounds like volatile fatty acids are anaerobically oxidized by acetogenic bacteriums by the procedure of & A ; szlig ; – oxidization. Concluding merchandises of this phase are H and ethanoate. Homoacetogenic bacteriums act on H and C dioxide formed to bring forth ethanoate. Hydrogen inhibits the metamorphosis of acetogenic bacteriums.

There is an association between acetogenic bacteriums which produce H2 and Methanogenic bacteriums which consume H2, Which helps in keeping the thermodynamic balance. It can be concluded that acetate formation depends on the syntrophic association between acetogenic and methanogenic bacteriums.


Methanogenesis is the concluding measure of anaerobiotic digestion. Methane and CO2 are formed as the endproduct of this measure. There are two ways in which methanogenic bacteriums produce CH4. They either use C dioxide or acetic acid as terminal negatron acceptor.

Methanogenic bacteriums are really substrate particular. Some use formate or ethanoate as C beginning, Some autophytic bacteriums use CO or CO2 as C beginning. Some methanogens can obtain C from beginnings like formate, dimethyl sulphide etc. Some methanogens work in presence of certain cofactors for eg. Methanogenic archea nedd cofactor F420, and they act in presence of hydrogenase.


  • Anaerobic digestion of biomass by David p. Chyenoweth, Ron Isaacson.

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