Anaerobic digestion
•The digestion process begins
with bacterial hydrolysis of the input materials in order to break down
insoluble organic polymers such as carbohydrates and make them available for
other bacteria. Acidogenic bacteria then convert the sugars and amino acids into
carbon dioxide, hydrogen, ammonia, and organic acids.
Acetogenic bacteria then
convert these resulting organic acids into acetic acid, along with additional
ammonia, hydrogen, and carbon dioxide.
Methanogenic bacteriafinally are able to
convert these products to methane and carbon dioxide.
Biogas
•Chemical composition of
biogas%: Methane, CH4 50-75
•Carbon dioxide, CO2 25-50
•Nitrogen, N2 0-10
•Hydrogen, H2 0-1
•Hydrogen sulfide, H2S 0-3
•Oxygen, O2 0-2
Biogas may require treatment or
'scrubbing' to refine it for use as a fuel.
Advantages of Biogas
•The methane in biogas can be
burned to produce heat and electricity.
•Biogas does not contribute to
increasing atmospheric carbon dioxide concentrations because the gas is not
released directly into the atmosphere and the carbon dioxide comes from an
organic source with a short carbon cycle .
•the nutrient-rich solids left
after digestion (digestate) can be used as fertiliser.
Microorganisms and the recovery of metals
•Accumulation of metals by
microorganisms
•Biotechnology and mining
Accumulation of metals by microors
•microorganisms accumulate
metals by biosorptionor bioaccumulation processes. Biosorption involves metal
adsorption around the cell envelope and is a metabolism-independent process.
Bioaccumulationis strictly dependent on cell metabolism and involves active
translocation of metals into the cell.
•This phenomenon can probably
make the most significant impact for removing toxic heavy metals from
industrial effluents or for recovery of metals of high commercial value such as
silver from industrial solutions.
Biosorption
•Biosorption phenomena occur as
a result of metal ion interactions with functional groups in organic polymers
on the cell surface.
It is believed that phosphate, carboxyl, amine and amide
groups found in carbohydrates, lipids, proteins, and other biopolymers of the
microbial cell envelope represent the main sites for metal adsorption.
The
charge distribution of these binding sites may vary with the composition of the
cell envelope of each microorganism, resulting in markedly different
metal-binding affinities( algae, bacteria, cyanobacteriaand fungi).
Bioaccumulation
•Bioaccumulation is a much
slower process of metal uptake than biosorption and may be inhibited by the absence
of nutrients such as glucose, nitrogen, and phosphate, by low temperatures and
by the action of metabolic inhibitors.
Biotechnology and mining
•Bacterial leaching or
Bioleaching is the extraction of metals from their ores using microorganisms.
Why bioleaching:
•Worldwide reserves of
high-grade ores are diminishing at an alarming rate due to the rapid increase
in the demand for metals.
•However there exist large
stockpiles of low grade ores yet to be mined.
The problem is that the recovery
of metals from low and grade ores using conventional techniques is very
expensive due to high energy and capital inputs required.
Another major problem
is environmental costs due to the high level of pollution from conventional
techniques.
Biotechnology is regarded as one of the most promising solution to these
problems, It reduces the capital costs. It also
offers the opportunity to reduce environmental pollution. Biological processes
are carried out under mild conditions, usually without addition of toxic
chemicals.