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Bioventing can be effective for the remediation of soil contaminated with petroleum hydrocarbons. However, implementing laboratory results in field scenarios is difficult due to the lack of scale-up factors. Accordingly, laboratory bioventing experiments were undertaken at the meso-scale and then compared with previously completed micro-scale tests to evaluate the important scale-up factor. The developed meso-scale system holds 4 kg of soil, with bioventing conditions controlled from a nutrient, airflow, and water content perspective. Three soils were tested, and categorized as loamy sand, silt loam, and a mixture. Results over a 30-day period showed a two-stage degradation pattern that encompassed first-order degradation rates as compared with the single-stage first-order degradation rate determined in the micro-scale study. For the first stage (0-8 days), the degradation rate for loamy soil was 0.598 day −1 , with the silty soil at 0.460 day −1 , and mixed soil at 0.477 day −1 . After 8 days, the degradation rate constant for the loamy soil dropped to 0.123 day −1 , with the silty soil dropping to 0.075 day −1 , and the degradation rate for the mixed soil dropping to 0.093 day −1 . Comparison of the measured degradation rate values with the results from the micro-scale experiments gave scale-up factors varying from 1.9 to 2.7 for the types of soil considered in the current study. These differences in degradation rates between the two scales show the importance of scale-up factors when transferring feasibility study results to the field.

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Abstract:The ability of rhamnolipid biosurfactants produced by Pseudomonas aeruginosa UG2 to wash a model hydrocarbon mixture from unsaturated soil columns was studied. Both aliphatic and aromatic hydrocarbons were effectively removed without soil clogging with non-recirculating biosurfactant solutions. Recirculation of wash solutions did not substantially affect washing efficiency. Of the several chemical surfactants tested, only Triton X-100 provided comparable hydrocarbon washing efficiency without soil clogging. The results suggest that UG2 biosurfactants have the potential for remediation of hydrophobic pollutants in unsaturated soil.

Bioventing is a promising in situ remediation technology for hydrocarbon contaminated soil. Using low airflow rates to produce oxygen-rich conditions in the vadose zone, and nutrient addition, bioventing stimulates indigenous microorganisms that degrade the hydrocarbon contaminants. However, several questions about bioventing remain to be answered, including the optimum soil water content, type and amount of nutrients necessary, and contributions of different microbes. Experiments were conducted using small-scale respirometers containing gasoline-contaminated soil from an active remediation site to determine the effects of soil water content, nitrogen content, nitrogen form, and the composition of the microbial population on the gasoline biodegradation rate. Results indicate that optimum bioventing conditions were 18 wt.% soil water content, C:N = 10:1, using NH4+-N. A maximum first-order degradation rate constant of 0.12/d was observed. Biodegradation was limited at high C:N ratios by the availability of nitrogen and at low C:N ratios by acidification. It was also determined that aerobic bacteria were the dominant group responsible for biodegradation, with fungi playing a minor role. Key words: bioventing, degradation rate, nutrients, water content, scale-up, gasoline, microbial population.

Bioventing is a cutting edge, non-destructive treatment method that uses indigenous soil microorganisms in-situ to remediate petroleum hydrocarbons in the unsaturated soil zone.Transferring the application of this technology to a field environment still has some uncertainties due to scale-up challenges. In order to identify the scale-up factor, a 80 kg soil reactor system was developed, consisting of a custom made reactor, climate chamber, low flow venting system and an off gas capture device. Sandy and clayey soils were tested with known concentrations of spiked synthetic gasoline. Various environmental conditions were monitored which included: moisture levels, pH, microbial levels, nutrient and oxygen levels. Results show a second stage degradation rate similar to the degradation rate obtained from research conducted with a 4 kg reactor, giving an average scale-up factor of 2.3±0.4. The completed research shows that working with a 80 kg laboratory reactor is feasible, yet not always necessary for the development of scale-up factors. A complimentary study with aged soil contaminants was preformed and yielded degradation rates that were significantly reduced.

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