Practice question

Ex situ bioremediation entails physical removal of contaminated matrix via excavation, dredging or pumping from its original location and subsequent treatment at same site but in engineered containment within treatment units or at off-site licensed facility, allowing tighter control over conditions that frequently limit in situ biodegradation. Techniques encompass landfarming where excavated soil spread in 30 centimeter layers and periodically tilled, composting windrows mixing soil with organic amendments, and slurry phase bioreactors where soil mixed with water to create 10 to 30 percent solids suspension agitated mechanically achieving high oxygen transfer kLa 200 per hour and uniform contact between microbes and pollutants. Highly polluted soils containing total petroleum hydrocarbons above 10,000 mg per kg or chlorinated pesticides that require addition of surfactants to enhance bioavailability benefit because permeability restrictions and heterogeneous contaminant distribution severely impede in situ delivery. Excavation entails costs, worker exposure risk requiring personal protective equipment and manifesting under Resource Conservation and Recovery Act, but ensures predictability, capability to achieve stringent cleanup thresholds below 100 mg per kg within months through optimized adjustments of pH, moisture, C:N:P ratio and temperature, with effluent treatment and volatile emission controls via biofilters.