Slurry Bioreactor System For Soil Decontamination

The first ex situ bioremediation process to be introduced here is the Eimco Biolift Slurry Reactor, developed by Eimco Process Equipment Company, Salt Lake City, Utah, United States. The process (Fig. 8) has successfully removed toxic polynuclear aromatic hydrocarbons (PAH) in soil. Traditional biological treatments, such as landfarming and in situ bioremediation, may not reduce PAHs in soil to target levels in a timely manner. Slurry reactors are more efficient for bioremediation and more economical than thermal desorption and incineration.

In a typical onsite bioremediation project, a mixing bioreactor, a first slurry bioreactor, a second fermentation slurry bioreactor in semicontinuous plug-flow mode, and a third slurry bioreactor may be operated in the following manner for contaminants removal:

1. The mixing bioreactor receives and mixes the contaminated soil, makeup process water and supplements of salicylate and succinate as nutrients. Salicylate induces the naphthalene degradation operon on PAN plasmids. This system has been shown to degrade phenanthrene and anthracene. The naphthalene pathway may also play a role in carcinogenic PAH (CPAH) metabolism. Succinate is a byproduct of naphthalene metabolism and serves as a general carbon source. The first reactor in series will remove easily degradable carbon and increase biological activity against more recalcitrant PAHs (i.e., three-ring compounds and higher).

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Figure 8 Biological slurry reactor system for soil decontamination. (Courtesy of USEPA.)

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Figure 8 Biological slurry reactor system for soil decontamination. (Courtesy of USEPA.)

2. The fresh contaminated soil slurry and nutrients in the mixing bioreactor are pumped to the first slurry bioreactor (60 L) for treatment.

3. The effluent from the first slurry bioreactor overflows to the second fermentation slurry bioreactor (10 L) in series, where Fenton's reagent is added to accelerate oxidation for four-to six-ring PAHs. Fenton's reagent (hydrogen peroxide in the presence of iron salts) produces a free radical that can effectively oxidize multiring aromatic hydrocarbons.

4. The third slurry bioreactor (60 L) in series is used as a polishing reactor to remove any partially oxidized contaminants remaining after Fenton's reagent is added.

5. Slurry is removed from this third soil slurry bioreactor and clarified using gravity settling techniques The treated or reclaimed soil is settled at the bottom of the clarifier, ready to be returned to the site.

6. The effluent from the settling clarifier is pumped to a final effluent container, from where the effluent can be partially discharged and partially returned to the very first mixing bioreactor.

Operation of the slurry bioreactor system for soil decontamination will increase the rate and extent of PAH biodegradation, making bioslurry treatment of impacted soils and sludges a more effective and economically attractive remediation option. This technology is applicable to PAH-contaminated soils and sludges that can be readily excavated for slurry reactor treatment. Soils from coal gasification sites, wood-treating facilities, petrochemical facilities, and coke plants are typically contaminated with PAHs.

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