The biological agents are used
to convert somewhat diffused and inconvenient sources of energy such as biomass
and sunlight into more convenient forms of energy such as methane, ethanol,
butanol, biodiesel and hydrogen. It is called as fuel biotechnology. Biofuels
are derived from biomass, while biomass is the organic matter or dry weight that
is produced by an organism. In general, the main goal of biofuel production is
to produce an alternative source of renewable energy, in place of nonrenewable
fossil fuels derived from petroleum.
In the beginning, the biomass was only used by man as a source of energy. But
with the industrial developments, it has been used for the production of biofuel,
due to the increased prices of fossil fuels (coal and oil) and also
environmental concern.
Renewable energy resources are replaced by natural processes and these are
inexhaustible. Biomass (from plants), sunlight, streams of water, wind, tide,
and draft animals are examples of renewable energy resources. On the other hand,
mineral oil, coal, natural gas, and even nuclear energy are non-renewable
resources of energy. These are present on the earth in a limited quantity. If
they are used up, then they become limited to a level where it is not
economically possible to use them. Therefore there is a dire need to develop the
renewable energy sources to fulfill the requirements.
As biofuels are produced from biomass that is low cost and locally available.
They do not contribute to environmental pollution. The substrate is often a
waste. Therefore, biofuel production from this waste helps in cleaning up the
environment.
The biomass may be utilized by following three processes:
● Direct burning to evolve heat energy for cooking food
● Thermo-chemical renovation by pyrolysis (to produce coal from wood)
● Biological Conversions
The biological conversion consists of anaerobic digestion of biomass to produce
methane and hydrogen. And by fermentation process ethanol and butanol are
produced.
There are several hundred species of bacteria that are involved in anaerobic
conversions and biofuels productions. These bacteria have been divided into
following four groups.
(1) Hydrolytic and Fermentative Bacteria:
This group consists of both obligate and facultative anaerobes. They remove the
small amount of oxygen available and produce anaerobic conditions. This group
hydrolyzes and ferments the organic materials such as cellulose, starch,
proteins, sugars, lipids etc and generate CO2, organic acids and H2.
(2) Syntrophic H2 Producing Bacteria:
This group is also known as obligate H2 producing or obligate proton reducing
bacteria because they oxidize NADH by reducing H+ to H2, and hence produce
hydrogen. These are present up to 4×106 cells/ml in sewage sludge digesters.
Syntrophomonas wolfei and S. wolinii are examples of this group.
(3) Methanogenic Bacteria:
These bacteria convert acetate and CO2+H2 into methane. The most familiar
anaerobes are the methanogenic bacteria. They may be found up to 106- 108
cells/ml of the slurry in digesters.
(4) Acetogenic Bacteria:
This group of bacteria oxidizes H2 by reducing CO2 to acetic acid, which is then
taken up by methanogens to produce methane, CO2 and H2.
Despite the fact that many naturally occurring microorganism have the ability to
degrade a number of wastes, but there are followings constraints to the
biological treatment of these wastes.
● Any single microorganism can’t degrade all the organic wastes.
● Due to high concentrations of some of the organic wastes, the growth and
activity of the microorganism is inhibited.
● Many contaminated sites contain mixtures of toxic chemicals. Microorganisms
degrade some of the chemicals, but these degraded chemicals may be inhibited by
other components.
● Microbial degradation of organic compounds is very slow process.
All these constraints can be overcome by the use of Recombinant DNA Technology.
The plasmids that carry genes for different degradative pathways can be
transferred into a recipient strains by conjugation process. Recombination can
occur between two resident plasmids if they have homologous regions of DNA. As a
result, a single larger “fusion” plasmid can be produced. Alternatively, if the
two plasmids belong to different compatibility groups i.e. do not have
homologous regions; they can be co-existed within a single bacterium.