Contaminated Land and Bioremediation 107Figure 5.8 Schematic diagram of land farming
of indigenous micro-organisms to bring about the remediation, though specialist
bacteria or fungi can be added if required. The soil to be treated is laid on a sand
layer, which itself stands on a gravel bed, through which a series of drainage
pipes have been laid. An impermeable clay or polymer lining isolates the whole
system from direct contact with the underlying ground. Water and nutrients are
added to stimulate biological activity and soil aeration is maintained by means
of turning or ploughing.
The inherent simplicity of the process, however, makes its effectiveness highly
dependent on soil characteristics and climatic conditions. For example, heavy
clay soils, make attaining adequate oxygenation difficult and uniform nutrient
distribution is almost impossible to achieve. In colder climes, it may be necessary
to cover the soil to overcome the worst effects of the weather.
Throughout the treatment itself, a process of sampling and monitoring helps to
assess progress and compliance with required standards and, at completion, the
treated soil can be removed either for return to original site or use elsewhere.
Soil banking
In some respects, soil banking is an inverted version of the previous system,
ranging from a long row of soil at its simplest, to a more engineered approach,
with aeration pipes, a drainage layer, impermeable liner and a reservoir to col-
lect leachate.
Just as with the previous approach, soil is excavated and screened, often also
being stored prior to treatment. As the name suggests, the soil to be processed
is formed into banks, sometimes with the addition of filler material like chaff,
wood chips or shredded organic matter, if the character of the contaminated soil
requires it to improve the overall texture, ease of aeration, water-holding capacity
or organic matter content. This technique is sometimes termed ‘soil composting’