The microbiology of ex situ bioremediation of petroleum hydrocarbon-contaminated soil.

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.