COGNIS, Inc.'s, biological/chemical treatment is a two-stage process that treats soils, sediments, and other media contaminated with both metals and organics. Metals are first removed from the contaminated matrix by a chemical leaching process. The organics are then removed by bioremediation.
Although metals removal usually occurs in the first stage, bioremediation may be performed first if organic contamination levels are found to inhibit the metals extraction process. Bioremediation is more effective if the metal concentrations in the soil are sufficiently low so as not to inhibit the microbial population. However, even in the presence of inhibitory metal concentrations, a microbe population may be enriched to perform the necessary bioremediation.
The soil handling requirements for both stages are similar, so the unit operations are fully reversible. The final treatment products are a recovered metal or metal salt, biodegraded organic compounds, and clean soil.
The contaminated soil is first exposed to a leachant solution and classified by particle size (see figure below). Size classification allows oversized rock, gravel, and sand to be quickly cleaned and separated from the sediment fines (silt, clay, and humus), which require longer leaching times. Typically, organic pollutants are also attached to the fines.
After dissolution of the metal compounds, metal ions such as zinc, lead, and cadmium are removed from the aqueous leachate by liquid ion exchange, resin ion exchange, or reduction. At this point, the aqueous leaching solution is freed of metals and can be reused to leach additional metal from the contaminated soil. If an extraction agent is used, it is later stripped of the bound metal under conditions in which the agent is fully regenerated and recycled. The heavy metals are recovered in a saleable, concentrated form as solid metal or a metal salt. The method of metals recovery depends on the metals present and their concentrations.
After metals extraction is complete, the "mud" slurry settles and is neutralized. Liquids are returned to the classifier, and the partially treated soil is transferred to a slurry bioreactor, a slurry-phase treatment lagoon, or a closed land treatment cell for bioremediation. This soil and the residual leachate solution are treated to maximize contaminant biodegradation. Micronutrients are added to support microbial growth, and the most readily biodegradable organic compounds are aerobically degraded.
Bench-scale tests indicate that this process can remediate a variety of heavy metals and organic pollutants. The combined process is less expensive than separate metal removal and organic remediation.
This remediation process is intended to treat combined-waste soils contaminated by heavy metals and organic pollutants. The process can treat contaminants including lead, cadmium, zinc, and copper, as well as petroleum hydrocarbons and polynuclear aromatic hydrocarbons that are subject to aerobic microbial degradation. The combined process can also be modified to extract mercury and other metals, and to degrade more recalcitrant halogenated hydrocarbons.
This remediation process was accepted into the SITE Emerging Technology Program in August 1992. Bench- and pilot-scale testing of the bioremediation process is complete. A full-scale field test of the metals extraction process was completed under the Demonstration Program. For further information on the full-scale process, refer to the profile in the Demonstration Program section (completed projects).
This remediation process is no longer available through COGNIS, Inc. For further information about the process, contact the EPA Project Manager.
EPA PROJECT MANAGER:
Steven Rock
U.S. EPA
National Risk Management Research Laboratory
26 West Martin Luther King Drive
Cincinnati, OH 45208
513-569-7149
Fax: 513-569-7105