Environmental Engineering Reference
In-Depth Information
soils. The amount of soil excavated in exceedance of regulatory limits was
27 000 tonnes. Because of this considerable amount of soil and the presence
of diesel hydrocarbons as the primary contaminant, it was decided to avoid
unnecessary landfill and follow the more sustainable option of ex situ bioreme-
diation using windrows, by particularly exploiting the activity of aerobic,
microbial degraders. In the past, the primary strategy for such sites was to
excavate and landfill the contaminated soil. Increasingly, however, bioreme-
diation offers a cost-effective and sustainable alternative to the traditional 'dig
and dump' approach (Alexander
1999
; Atlas & Philp
2005
).
Windrowing of hydrocarbon contaminated soil is, along with biopiling, an
ex situ bioremediation technique which relies on the action of aerobic micro-
organisms (mainly bacteria) to break down organic compounds (Semple et al.
2001
; Khan et al.
2004
;Liet al.
2004
). The methods involve the excavation
and piling of contaminated soils into piles, usually to a height of 2 4m, in
order to enhance aerobic microbial activity through aeration (if this is
forced, then biopile is the correct term; if aeration is achieved by turning with
passive diffusion of oxygen, then windrow is the appropriate term), the add-
ition of nutrients and the control of moisture and pH (Jørgensen et al.
2000
;
Khan et al.
2004
). Windrows and biopiles have been effectively used to remedi-
ate a wide range of contaminants such as petroleum hydrocarbons, pesticides,
PAHs and sewage sludge (Semple et al.
2001
; Thassitou & Arvanitoyannis
2001
;
Khan et al.
2004
).
The non-engineered windrows/biopiles rely mainly on wind-induced pres-
sure gradients (i.e., natural airflow) which are non-uniform and particularly
weak in the central part of the pile, which may lead to a local O
2
deficiency
(Eweis et al.
1998
;Liet al.
2004
). Engineered windrows and biopiles are often
covered and lined with waterproof plastic to control water infiltration, runoff
and volatilisation as well as to enhance solar heating (Fahnestock et al.
1998
; Khan et al.
2004
). Furthermore, an impermeable membrane or clay
layer may be used as a basis to reduce the risk of pollutant leaching into
uncontaminated soil.
The distribution of soil characteristics such as texture, permeability, water
content and bulk density/porosity which are critical to the activity of the
hydrocarbon degrading microbial population are often non-uniform, and
therefore turning the contaminated soil may be required to promote optimal
biodegradation conditions, including improved aeration (Khan et al.
2004
).
Windrows and biopiles containing organic matter (e.g., wood waste,
sewage sludge and food waste) are usually referred to as 'composting' systems
(Vidali
2001
). In this type of biopile/windrow, the degradation of organic
matter results in an increase in the biopile temperature leading to changes
in the microbial community structure during the course of bioremediation
(Semple et al.
2001
; Thassitou & Arvanitoyannis
2001
). The degradation process
