Practical Applications to Pollution Control
Contaminated land and bioremediation, which typically form a wider area of concern for environmental biotechnology, will be considered in some detail. To give a practical context with which to close this section, however, a brief discussion of air pollution and odour control follows.
Bacteria normally live in an aqueous environment which clearly presents a problem for air remediation. Frequently the resolution is to dissolve the con-taminant in water, which is then subjected to bioremediation by bacteria, as in the following descriptions. However, there is scope for future development of a complementary solution utilising the fact that many species of yeast produce aerial hyphae which may be able to metabolise material directly from the air.
A variety of substances can be treated, including volatile organic carbon containing compounds (VOCs) like alcohols, ketones or aldehydes and odorous substances like ammonia and hydrogen sulphide (H2S). While biotechnology is often thought of as something of a new science, the history of its application to air-borne contamination is relatively long. The removal of H2 S by biological means was first discussed as long ago as 1920 and the first patent for a truly biotech-based method of odour control was applied for in 1934. It was not until the 1960s that the real modern upsurge began, with the use of mineral soil fil-ter media and the first true biofilters were developed in the succeeding decade. This technology, though refined, remains in current use. The latest state-of-the-art developments have seen the advent of the utilisation of mixed microbial cultures to degrade xenobiotics, including chlorinated hydrocarbons like dichloromethane and chlorobenzene.
A number of general features characterise the various approaches applied to air contamination. Typically systems run at an operational temperature within a range of 15 – 30 ◦ C, in conditions of abundant moisture, at a pH between 6 – 9 and with high oxygen and nutrient availability. In addition, most of the substances which are commonly treated by these systems are water soluble.
The available technologies fall naturally into three main types, namely biofil-ters, biotrickling filters and bioscrubbers. To understand these approaches, it is probably most convenient to adopt a view of them as biological systems for the purification of waste or exhaust gases. All three can treat a wide range of flow rates, ranging from 1000 – 100 000 m3 /h, hence the selection of the most appropriate technology for a given situation is based on other criteria. The concen-tration of the contaminant, its solubility, the ease of process control and the land requirement are, then, principal factors and they interact as shown in Table 4.4 to indicate the likely best approach.
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