Environmental Engineering Reference
In-Depth Information
runs the first one, methane yield will suffer. Therefore special care has to be taken
while designing the process to achieve minimal loss (Karakashev et al.
2005
).
The next step, digestion, takes place under both mesophilic and thermophilic
conditions. A constant temperature throughout the entire process is necessary for a
fruitful yield of biogas. Numerous studies have shown that methanogenic bacteria
work best at thermophilic temperatures and tolerate a temperature change of ± 3
°
C
(Karakashev et al.
2005
). In terms of pH, methane gas is released most efficiently
at pH between 6-8. However if the pH goes down below 6 or rises above 8, the
production of methane gas is severely inhibited. The pH alters or reaches above 8
mostly due to accumulation of ammonia gas released upon degradation of protein
substrates (Abdoun and Weiland
2009
).
3.3.2
Growth Parameters for Microorganisms
All living microorganisms require certain macro and micronutrients for growth
and reproduction. Carbon, phosphorus and sulphur are added as macronutrients
for the culture medium. Micronutrients are added in considerably lower amounts
(0.05-0.06 mg/l), since their need is not particularly essential (Bischoff
2009
).
Trace metals are added in significant amounts, as they are needed as co-factor for
enzyme functions. Nickel, cobalt, selenium, molybdenum, tungsten and iron are
the common trace metals added (Bischoff
2009
). Nickel is utilized by the methano-
genic bacteria for the synthesis of an essential cell component called F
430,
involved
in the formation of methane. Iron is normally added in the concentration ranging
from 1 to 10 mg/l.
3.3.3 Feedstock
The criteria for use of biomass as substrates for biogas production is the presence of
carbohydrates, cellulose, proteins, lipids, fats and hemicelluloses as main compo-
nents. According to researchers, the biogas yield depends upon the content of car-
bon, proteins and fats. Wood, a strong lignified organic biomass is not encouraged
for use as substrates in biogas production mainly because its anaerobic decomposi-
tion is extremely slow. Different biomasses produce different amounts of biogas
concentrations, depending on the content of their organic substances. For instance
fats yield the most biogas amount but require long retention time, whereas proteins
and carbohydrates have faster rates of conversion but low amount of gas yields.
Crop plants are the most paramount, biogas producing substrate candidates.
Figure
2.2
shows the potential of crop plants and organic wastes for usage as bio-
mass for biogas production in Germany. About 50 % of biogas energy is synthesized
from using crop plants as co-substrates. The net energy yield per hectare of crop
plants is phenomenal. Maize, forage beets, perennial grass and cereal crops provide
high gross energy per hectare. Certain parameters of crop plants such as harvesting
and the frequency of harvesting are to be kept in consideration while choosing the
