Maintaining optimal temperature for the anaerobic digestion process is a quintessential aspect because varying
temperatures affect the overall rate of digestion process, the hydraulic retention time, and the composition of
the methanogenic bacteria. Research studies have shown that anaerobic digestion can be maintained at psychrophilic
(12˚C - 16˚C such in landfills), or mesophilic (35˚C - 37˚C such in animal rumen), or thermophilic environment
(55˚C - 60˚C such in artificially made biodigester). In thermophilic conditions, the optimal temperature
for maximum biogas production from animal manure was 39˚C - 43˚C [20]. Table 1 summarizes the optimal
conditions for AD process.
Another important element in anaerobic digestion process is the amount of carbon and nitrogen present in the
organic materials, which is represented by the carbon to nitrogen (C:N) ratio [24]. Unlike, for example, the lignocellulosic
crop residues with low nitrogen content and high C:N ratio ranging from 60 to 90, animal manure
has a low C:N ratio ranging from 10 - 20 (20 for cow manure) [4] [6] [17] [21]. A high C:N ratio is an indication
of rapid consumption of nitrogen by methanogens and results in lower biogas production (Table 2). The optimal
range of C:N ratio for anaerobic digestion is reported to be in the range of 25 to 35 [25]. See Table 2 for a comparison
between selected livestock manure.