Page 46 - Biofuels Refining and Performance

Energy and Its Biological Resources 29

Organic matter (35 C, pH 5–6) Acetic acid
Organic matter → ⎯⎯⎯⎯→
Alcohols, H , CO 2 H ,CO
2
2
Methane (45 C, pH 4–6) 2 →
←⎯⎯⎯⎯⎯
CO 2
An oversimplified mass balance may be written as

C H O → 3CH 3CO 2
12
6
6
4
The technical values of yield coefficient, biological efficiency, chemical/
biological oxygen demand (COD/BOD), biological efficiency in productivity/
ecologic efficiency rate (BEP/EER) ratios, and so forth are yet to be estab-
lished for each setup or system. Mostly obligate anaerobes and a few fac-
ultative microbes contributing to these conversions belong to different
genera. A few may be mentioned: Actinomyces, Aerobacter, Aeromonas,
Arthrobacter, Bacillus, Bacteroides, Cellulomonas, Citrobacter, Clostridium,
Corynebacterium, Enterobacter, Escherichia, Klebsiella, Lactobacillus,
Laptospira, Micrococcus, Nocardia, Peptococeus, Proteus, Pseudomonas,
Ruminococcus, Sarcina, Staphylococcus, Streptococcus, Streptomyces, and
many others. A few methanogenic species are also known: Methano-
bacterium bryantii, Methanococcus vanniellii, Methano-genum aggre-
gans, Methanomicro-bium mobile, Methanosarcina barkeri, Methano-
thrix concillii, usually eukaryotic organisms, and blue-green algae are
incapable of performing such bioconversions [8].
Morphologically, the organisms belong to wide groups: coccus, sarcina
(flower-like), rod, filamentous, and other shapes. G C (guanine-cytosine)
values of DNA of these organisms also suggest that they all have varied
origin and hence are likely to have different metabolic patterns. Khan
(1980) found that Acetivibrio cellulolyticus producing acetic acid and
hydrogen from cellulose are readily utilized by M. Barkeri to produce
methane and carbon dioxide. It has been established beyond doubt that
the process is chemolithotrophic metabolism, favored by strict anaero-
bic condition, and facilitated by the absence of sulfates, abundance of mois-
ture, approximate temperature range of 25–40 C (37 C), and pH 6.2–8.0
(pH 6.8). The organic materials on which these organisms survive and
grow are usually cellulose in nature. Crop residues, agricultural residues,
animal excreta, municipal sewage, and other organic materials derived
from terrestrial and aquatic origin are also considered as good sub-
strates. Plant materials with high lignin content are an inferior type of
feed for such reactions.
A pretreatment or partial putrefaction or degradation makes the
process easy. In this respect, animal excreta appear to be a ready-made
substrate. The art of producing gaseous fuel out of cattle excreta is well