Pollution and Pollution Control - Environmental Biotechnology: Theory and Application - page 79

to play. With the ability to give almost instantaneous detection of specific con-
taminants, they may prove of value in giving early warning of potential pollution
problems by monitoring production processes as they occur.
Desulphurisation of coal and oil
Microbial desulphurisation of coal and oil represents a further potential example
of pollution control by the use of clean technology. The sulphur content of these
fossil fuels is of environmental concern principally as a result of its having been
implicated in the production of acid rain, since it produces sulphur dioxide (SO 2 )
on combustion. Most of the work done to date has tended to focus on coal, largely
as a result of its widespread use in power stations, though similar worries equally
surround the use of high-sulphur oils, particularly as the reserves of low-sulphur
fuels dwindle. The sulphurous component of coal typically constitutes between
1 and 5%; the content for oil is much more variable, dependent on its type and
original source.
There are two main ways to reduce SO 2 emissions. The first is to lessen the
sulphur content of the fuel in the first place, while the second involves removing
it from the flue gas. There are a number of conventional methods for achieving
the latter, the most commonly encountered being wet scrubbing, though a dry
absorbent injection process is under development. At present, the alternative
approach of reducing the sulphur present in the initial fuel, works out around
five times more expensive than removing the pollutant from the flue gas, though
as stock depletion forces higher sulphur coals and oils to be burnt, the economics
of this will start to swing the other way. Methods for achieving a sulphur content
reduction include washing pulverised coal and the use of fluidised bed technology
in the actual combustion itself, to maximise clean burn efficiency.
Sulphur is present in coal in a variety of different forms, both organic and inor-
ganic and biological methods for its removal have been suggested as alternatives
to the physical means mentioned above. Aerobic, acidophilic chemolithotrophs
like certain of the Thiobacillus species, have been studied in relation to the
desulphurisation of the inorganic sulphur in coal (Rai, 1985). Microbes of this
genus have long been known to oxidise sulphur during the leaching of metals
like copper, nickel, zinc and uranium from low grade sulphide ores. Accord-
ingly, one possible application which has been suggested would be the use of a
heap-leaching approach to microbial desulphurisation at the mine itself, which is
a technique commonly employed for metals. However, though this is, clearly, a
cheap and simple solution, in practice it is difficult to maintain optimum condi-
tions for the process. The micro-organisms which have most commonly been used
to investigate this possible approach are mesophiles and the rapid temperature
increases experienced coupled with the lengthy period of contact time required,
at around four to five days form major limiting factors. The use of extreme
thermophile microbes, like Sulfolobus sp . may offer the way ahead, giving a
faster rate of reaction, though demanding the more sophisticated and engineered
environment of a bioreactor if they are to achieve their full process efficiency.