Table 3.5: Mean gas yields from various types of agricultural biomass (Source: OEKOTOP,
compiled from various sources)
Substrate
Pig manure
Cow manure
Poultry manure
Horse manure
Sheep manure
Stable manure
Grain skew
Corn straw
Rice straw
Grass
Elephant grass
Bagasse
Vegetable residue
Water hyacinth
Algae
Sewage sludge
Gas-yield range (1/kg VS)
340-550
150-350
310-620
200-350
100-310
175-320
180-320
350-480
170-280
280-550
330-560
140-190
300-400
300-350
380-550
310-640
Average gas yield (1/kg VS)
450
250
460
250
200
225
250
410
220
410
445
160
350
325
460
450
Table 3.6: C/N-ratios of varios substrates (Source: Barnett 1978)
Substrate
Urine
Cattle dung
Pig dung
Chicken manure
Sheep/goat dung
Human excrements
Grain straw
Corn straw
Fresh grass
Water hyacinth
Vegetable residue
C/N
0.8
10-20
9-13
5-8
30
8
80-140
30-65
12
20-30
35
Digestion characteristics and gas yields
As long as the total solids content of the substrate does not substantially exceed 10%, simple
biogas plants can be expected to operate smoothly on a mixture of animal excrements and plant
material (straw, fodder waste).
Manure from ruminants, particularly cattle, is very useful for starting the fermentation process,
because it already contains the necessary methanogenic bacteria. On the other hand, the gas yield
from cattle dung is lower than that obtained from chickens or pigs, since cattle draw a higher
percentage of nutrients out of the fodder' and the leftover lignin complexes from high-fiber fodder
are very resistant to anaerobic fermentation. Urine, with its low organic content, contributes little to
the ultimate gas yield but substantially improves the fertilizing effect of the digested slurry and
serves in diluting the substrate.
19