Remediation and Management of Contaminated Soil - Sustainable Practices in Geoenvironmental Engineering - page 420

removal eficiencies (DRE) for the contaminants were in the order of 99.9%. The produced
Ecomelt passed leachability tests and could be added for beneicial use in concrete, thus
replacing up to 40% Portland cement. Increased tipping fees could enhance the economics
of the process and could lead to break-even scenarios.
Mercury Recovery Services (MRS; http://www.hgremoval.com) developed and com-
mercialized a process that mixes a proprietary material and a mercury-contaminated
material at temperatures of 150°C-650°C (Weyand et al., 1994). The process can be mobile
or ixed, batch, continuous, or semicontinuous. Unit capacities ranged from 0.5 to 10 t/h.
The mercury can be as an oxide, chloride, and sulide. No liquid or solid secondary
products were generated. The treated material contained less than 1 ppm of mercury.
The process consisted of two stages, feed drying and mercury desorption, which was
then condensed as a 99% pure metallic form from the vapor phase. Air emissions did
not contain mercury. Costs were high, in the range of $650-1000/t. Soils from approxi-
mately 6000 metering sites along the natural gas pipeline system in the western United
States were treated using the MRS process. Over 18 months, a 12-tonne/day mobile unit
processed a total of 6000 tonnes of soil with 100-2000 mg/kg of mercury. The treated soil
contained less than 2 mg/kg of Hg, The results from the TCLP tests indicated that leach-
ing was minimal at less than 0.0025 mg/kg, which is substantially below the 0.2-mg/kg
EPA limit. In addition, more than 1590 kg of metallic mercury were recovered for sale
and recycling.
11.5 Biological Remediation
For heavily contaminated soils, various approaches can be used to enhance the rate of
bioremediation. Substances must be biodegradable and not toxic for treatment. Experience
to date shows that ex situ bioremediation has been more successful than in situ processes
due to easier control of environmental parameters such as mixing that allows uniform
nutrient and oxygen contents. Proprietary biological mixtures for bioaugmentation are
also available. Ex situ biotreatment systems include the use of slurry bioreactors, biopiles,
landfarming, and composting. In general, the more sophisticated the process, the more
expensive the treatment. Treatability studies are usually performed to determine the
eficiency of the bioremediation for the type of contaminants and sediments at the site.
Microbial population, nutrient levels, pH, moisture content, contaminant type and con-
centration, and sediment characteristics must be determined and followed. Bench, pilot,
and demonstration scale tests are needed to properly design the remediation technology.
Microorganisms have been effective in treating organic contaminated sediments such as
PAHs. Zhao et al. (2004) have demonstrated that anaerobic degradation of RDX was pos-
sible in a Halifax sediment. Degradation rates of TNT>RDX>HMX were found. Shewanella
and Halomonas bacterial isolates were found (Zhao and Hawari, 2008). Khodadoust et al.
(2009) showed that PCBs could be degraded anaerobically with the periodic addition of
iron (0.01 to 0.1 g/g).
11.5.1 Slurry Reactors
Slurry bioreactors use 5% to 30% solid content in a highly agitated treatment process. Mass
transfer, aeration, and environmental conditions such as pH can be optimized more easily