BIOGAS RECOVERY

In industrialized as well as developing countries the possibility of regaining energy, has prompted an interest in applying biogasification to waste treatment, in the form of the combustible gas methane. It serves a two fold function –

a) waste treatment

b) energy production

which have drawn attraction of this concept.

Process of Biogas Generation:

It is decomposition of organic matter of biological origin under anaerobic conditions with an accompanying production primarily of methane (CH₄) and secondarily other gases, most of which is carbon dioxide (CO₂).

In the anaerobic digestion, wastes are decomposed without oxygen at relatively high moisture contents ( 90 ~ 99.5 %). The wastes undergo decomposition and produce

- firstly organic acids

- then biogas from organic acids

The process also involves the breakdown of proteinaceous materials into amines and such fertilizers as nitrites and ammonia. These products are more easily available as nutrients for plants, than are the complex proteins in the original waste.

Stages in Conversion of Organic Substances:

In the conversion of organic substances into methane by biogas microbes, some stages are considered. These are –

a) fermentive bacteria

b) hydrogen-producing acetogenic bacteria

c) methane-producing bacteria

The stages are described below with related reactions:

a) Fermentive bacteria

In first stage of the biogas formation process a mixed group of bacteria involved which hydrolyze various complex organic substances and then ferment them to yield various volatile acids, hydrogen and carbon dioxide according to fallowing typical reactions:

C₆H₁₂O₆+H₂O ---» 2CH₃COOH (acetic acid) + 2CO₂ +4H₂

C₆H₁₂O₆ ---» CH₃CH₂CH₂COOH (butyric acid) + 2CO₂ +2 H₂O

b) Hydrogen producing Acetogenic Bacteria

The substances produced in the first stage(such as propionic acid, aromatic acid, alcohol etc. which methane producing bacteria can not use) into acetic acid, hydrogen, carbon dioxide is decomposed by these bacteria. The reactions involved are:

CH₃CH₂COOH + 2H₂O ---» CH₃COOH + CO₂ + 3H₂

CH₃CH₂CH₂COOH + 2H₂O ---» 2CH₃COOH + 2H₂

c) Methane-producing bacteria:

Conversion of the substances produced in the First and Second stages (acetic acid, hydrogen, carbon dioxide, formic acid etc.) into methane and carbon dioxide is done these bacteria.

The bacterias are namely acetoclastic methane bacteria (acetophilic) and hydrogen-utilizing methane bacteria. The reaction involved are: