The microbiology of ex situ bioremediation of petroleum hydrocarbon-contaminated soil.
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Date
1996
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Abstract
Bioremediation is the process whereby the degradation of organic polluting compounds
occurs as a result of biochemical activity of macro- and microorganisms. Bioremediation of
hydrocarbon contaminated soils can be practised in situ or ex situ by either stimulating the
indigenous microorganisms (biostimulation) or introducing adapted microorganisms which
specifically degrade a contaminant (bioaugmentation).
This investigation focused on ex situ remediation processes with special attention to the
processes and microbiology of landfarming and thermal bioventing. Landfarming was
investigated at pilot-scale and full-scale, and thermal bioventing at laboratory and pilot-scale.
This study indicated that pilot-scale bioremediation by landfarming was capable of effecting
a total petroleum hydrocarbon concentration (TPHC) reduction of 94% (m1m) from an
initial concentration of 320 gkg-I soil to 18 gkg-I soil over a period of 10 weeks. Reactors
receiving biosupplements showed greater rates of bioremediation than those receiving
nutrients. Promotion of TPHC catabolism by addition of a commercial or a site-specific
microbial biosupplement was similar. Seedling experiments proved that bioremediation did
not necessarily leave the soil in an optimal condition for plant growth.
The full-scale landfarming operation reduced the TPHC concentrations from 5 260 -
23 000 mgkg- I to 820 - 2335 mgkg- I soil over a period of 169 days. At full-scale, the larger fraction of more recalcitrant and weathered petroleums. and the less intensive treatment
resulted in a slower rate of TPHC reduction than was found in the pilot-scale study. Three
distinct decreases in the TPHC were observed during the full-scale treatment. These
presented an ideal opportunity to investigate the microbiology of the soil undergoing
treatment. The dominant culturable microorganisms were isolated and identified. The
bioremediation process was dominated by Bacillus and Pseudomonas species. The method
used to study the population was, however, biased to culturable, fast growing
microorganisms which represent a small portion of the total microbial population. For this
reason, a method to study the total eubacterial population in situ with rRNA targeted
oligonucleotide probes was adapted and found to be a valuable technique.
Soil microorganisms respiratory activity was investigated at different times in the full-scale
treatment. A clear correlation between activity and degradation was recorded. The effect of
a supplement. anaerobically digested sludge, was also assessed by this method.
Thermal bioventing was investigated as an ex situ in-vessel treatment technology for small
volumes of highly contaminated soils. This proved to be a viable technique for the
bioremediation of petroleum hydrocarbons at laboratory-scale. Volatilisation contributed to
at least 40% of the reduction. Of the two supplements evaluated. dried sludge promoted
degradation to a greater extent than chicken manure.
The pilot-scale study proved that a chemical contaminant reduction of at least 50% could be
achieved in 13 weeks by thermal bioventing. Of the supplemented reactors. the presence of dried sludge and commercial biosupplement etfected the largest contaminant decrease. As a
possible supplement to increase the rate of bioremediation. dried anaerobically digested
sludge was more effective than chicken manure. A parallel laboratory-scale experiment
gave similar results. Gravimetric analyses were found to be conservative indications of the
remediation process.
The results of this study shed some light on our. still. limited understanding of
bioremediation. The gap between the technology in the laboratory and field was narrowed
and a better understanding of the soil microbiology was achieved. Due to the limited
control of environmental parameters in the case of landfarming. thermal bioventing was
investigated and proved to be an effective alternative. The latter technology is novel in
Southern Africa.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1996.
Keywords
Bioremediation., Petroleum--Biodegradation., Soil pollution., Compost., Hazardous waste--Biodegradation., Theses--Microbiology.