Environmental Engineering Reference
In-Depth Information
in the composting method is initiated by mesophilic bacteria which grow
optimally between 20 and 40 degrees (Thassitou & Arvanitoyannis
2001
).
However, the increase in the biodegradation rate results in heat production
leading to temperature increases of up to 65
C (Semple et al.
2001
; Thassitou &
Arvanitoyannis
2001
). The increase in the pile temperature results in an
increase in thermophilic bacteria and a decline in the mesophilic microbial
population (Semple et al.
2001
; Thassitou & Arvanitoyannis
2001
).
The advantages of windrow/biopile strategies include their cost efficiency
(approximately £15 35/tonne) (Semple et al.
2001
), they are easy to design and
they can also be applied on site (Khan et al.
2004
). In addition, the area and time
required for biopile treatments are less than those required for land farming.
Furthermore, vapour emissions can be controlled using a closed system, and
they can be designed to fit a range of products and site conditions (Khan et al.
2004
). The limitations include space requirements and the evaporation of
volatile compounds which are required to be treated prior to discharge to the
atmosphere (Mueller et al.
1996
; Khan et al.
2004
). Contamination at the former
railway marshalling yard in Aberdeen required intensive management such as
covering during heavy rainfall to not only prevent slumping, but also to ensure
a soil matric potential and aeration/oxygen diffusion commensurate with
optimal hydrocarbon degradation activity.
On the study site, an algorithm (1) was used which, coupled to optimal
management of water, pH, nutrients and aeration, enabled optimisation of
the hydrocarbon degradation by the soil microbial community.
Bioremediation factor
a
(f ) [TPH] (f ) availability (f ) degraders (f ) constraints (1)
The terms in the algorithm represent a combination of contamination
factors which drive the bioremediation. 'TPH' is the total petroleum hydro-
carbon concentration in the soil and 'availability' refers to the fraction of these
hydrocarbons that is available for bioremediation. This is measured using
hydrocarbon catabolic bacterial biosensors where lux genes are downstream
of hydrocarbon catabolic promoters and so the induction of light output from
the sensors indicates hydrocarbon bioavailability (Killham & Paton
2003
) and
microbial factors ('degraders' refers to the population density of hydrocarbon
degraders in the soil measured by most probable number techniques, and
'constraints' refers to the toxicity imposed on the microbial community by
the chemical toxicity of the contaminated soil environment and is measured
using the lux-based toxicity biosensors previously described).
Using the algorithm to predict the likely success of bioremediation, the
windrows of contaminated soil were carefully managed and the hydrocarbon
contamination degraded down to compliance concentrations over 6 8 weeks
(
Fig. 12.7
). The windrows were monitored during this period to ensure high
rates of hydrocarbon degrader activity (in the sandy soils under remediation,