Contaminated Land and Bioremediation - Environmental Biotechnology: Theory and Application - page 108

because of the similarity it has with the windrow method of treatment for biowaste
material, which is described in Chapter 8. It is not a true example of the compost
process, though there are many functional parallels between these procedures
and the same windrow turning equipment may be used in either. Often these
rows are covered, either with straw or synthetic blanketing materials, to conserve
heat and reduce wash-out. Accordingly, this method is generally better suited to
colder and wetter climates and is typically faster than land farming. Indigenous
micro-organisms are again the principal agents of remediation, though specialised
bacterial or fungal cultures can be introduced as required, and nutrients added to
optimise and enhance their activities.
To further boost the speed and efficiency of this treatment approach, partic-
ularly when space is limited, a more sophisticated version, often termed 'engi-
neered biopiling', is sometimes used to ensure greater process control. Leachate
is collected in a reservoir and recirculated through the pile to keep the soil moist
and return the microbes it contains and a series of pipes within the pile or the
underlying drainage layer forces air through the biopile itself. The increased air
flow also permits VOCs to be managed more efficiently and having the whole
system above an impermeable geotextile liner prevents leachate migration to the
underlying ground.
In both versions of soil banking, a regime of sampling and monitoring is
established which again aids process assessment and control. After treatment is
concluded, the soil may be returned to the original site for use, or taken elsewhere.
Both land farming and soil banking are relatively unsophisticated approaches,
effectively utilising the mechanisms of natural attenuation to bring about the
necessary clean-up, though enhancing and accelerating the process, having first
isolated, concentrated and contained the material to be treated. The final com-
monly encountered technology to be described in this section is a more engineered
approach, which works by increasing the levels of water, nutrients and dissolved
oxygen available to the microbes.
Soil slurry reactor
In most respects, this system shares essentially similar operating principles to the
activated sludge system described in the next chapter, which is used in treating
effluents. Figure 5.9 shows a schematic representation of this method.
After excavation, the soil is introduced into a mixing tank, where a slurry
is produced by combining it with water. Nutrients are then added to stimulate
microbial growth. The suspension formed is transferred to a linked series of well-
aerated slurry reactors, and micro-organisms within them progressively treat the
contaminants. Clarifiers and presses thicken the treated slurry and dewater it, the
recovered liquid component being recirculated to the mixing tank to act as the
wetting agent for the next incoming batch of soil, while the separated solids are
removed for further drying followed by reuse or disposal.