Biogas production in climates with long cold winters - SuSanA

2. Background on biogas production
2.1 Anaerobic digestion
Anaerobic Digestion (AD) is a biological process that happens naturally when bacteria breaks
down organic matter in environments with little or no oxygen. AD produces a biogas made up
of around 60 per cent methane and 40 per cent carbon dioxide (CO2). This can be burnt to
generate heat or electricity or can be used as a vehicle fuel. As well as biogas, AD produces a
residue called digestate which can be used as a soil conditioner to fertilize land.
This conversion of complex organic compounds into methane and carbon dioxide requires
different groups of micro organisms and is carried out in sequence of four stages: Hydrolysis,
Acidogenesis, Acetogenesis and Methanogenesis. During hydrolysis organic substrate is
converted into smaller components , then acidogenic bacteria use these smaller compounds and
produce volatile fatty acid, ethanol, CO 2 and H 2 . Acetogenic bacteria convert these
fermentation products in to acetic acid, CO 2 and H 2 . Finally methanogenic bacteria use
hydrogen and acetate (most important substrate) and produce methane and carbon dioxide. The
following scheme clearly shows sequence of anaerobic digestion;
Figure 1: Scheme of anaerobic digestion
A variety of factors affect the rate of digestion and biogas production. The most important is
temperature. Biogas production is carried out at different temperatures: temperature range 45–
60 °C is referred to as ‘thermophilic,’ whereas that carried out at temperature range 20–45 °C
is known as ‘mesophilic’ and at low temperatures (