Evaluation of two bacterial delivery systems for in-situ remediation of PAH contaminated sediments

Intention, Goal and Background. Contaminated sediments represent a significant, worldwide environmental problem since they contain a mixture of different xenobiotics and heavy metals. The presence of mixed contamination presents a unique set of obstacles for remediation efforts. Often sediment remediation occurs as an ex-situ application (i.e., after dredging) in an attempt to minimize some of the problems. However, dredging poses it's own issues. It does not address contaminated water and often material is not completely removed thereby leaving a long-term residual contamination source in the waterway. Objective. The potential of bioremediation to treat sediments contaminated with polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls, and heavy metals was addressed. The primary objective was to assess two delivery mechanisms for microbial inoculation to facilitate in-situ remediation of PAH contaminated sediments. Methods. Simulated river beds were constructed to mimic the Mahoning River. Contaminated sediment from the river was added to each reactor at a uniform depth, followed by the addition of river water. Fifteen inoculation points were used in each simulated river bed to ensure adequate microbial populations. One tank was inoculated with an acclimated bacteria solution as a free suspension. The other tank was inoculated with an attached growth biofilm system. Sediment samples were taken throughout the experiment and the percent PAH degradation determined. Water characteristics (DO, pH, bacterial activity, etc.) were also tracked as corroborating evidence. Results and Discussion. The monitoring sites indicated that an attached growth system was more effective, and achieved a 99% PAH degradation efficiency at some of the sampling sites. Tracking individual PAH compounds also indicated a higher overall microbial activity with the attached growth system. This activity was evident by the formation and subsequent biodegradation of lower molecular weight degradation byproducts. However, more of the sediment area was treated by the free suspension inoculum due to the ease of microbial migration. Conclusions. The applicability of using an aerobic microbial consortium composed of Mycobacterium sp., Pseudomonas aeruginosa, and Pseudomonas flourescens to treat contaminated sediment was demonstrated. In addition, it was found that introducing the consortium as an attached growth was more effective than when delivered as a free suspension. Recommendation and Outlook. The results demonstrated that the consortium was effective at treating the PAHs present in the contaminated soil. An additional study to evaluate the consortium's effectiveness at remediating the PCB present in the sediment is warranted. Optimization of the consortium-nutrient combination could enable a treatment approach to effective for all the organic contaminants present. Although this would not address the heavy metals present in the sediment, it would afford a great opportunity at remediating a severely contaminated sediment system.