Contaminated Land and Bio-Remediation - Environmental Biotechnology: Theory and Application - page 110

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 landfarming. 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,
particularly when space is limited, a more sophisticated version, often termed
'engineered 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 landfarming and soil banking are relatively un-sophisticated 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 re-use or disposal.
Process selection and integration
However, when complex mixtures of compounds are required to be treated, com-
bining a series of different individual process stages within a series of inter-linked