(Soil Recycling)
The Toronto Harbour Commission's (THC) soil recycling process removes inorganic and organic contaminants from soil to produce a reusable fill material (see photograph below). The process consists of three technologies operating in series: a soil washing technology; a technology that removes inorganic contamination by chelation; and a technology that uses chemical and biological treatment to reduce organic contaminants.
Soil Washing Plant (Metal Extraction Screwtubes in Foreground
and Bioslurry Reactors in Background)
The process uses an attrition soil wash plant to remove relatively uncontaminated coarse soil fractions using mineral processing equipment while concentrating the contaminants in a fine slurry which is routed to the appropriate process for further treatment. The wash process includes a trommel washer to remove clean gravel, hydrocyclones to separate the contaminated fines, an attrition scrubber to free fines from sand particles, and a density separator to remove coal and peat from the sand fraction.
If only inorganic contaminants are present, the slurry can be treated in the inorganic chelator unit. This process uses an acid leach to free the inorganic contaminant from the fine slurry and then removes the metal using solid chelating agent pellets in a patented countercurrent contactor. The metals are recovered by electrowinning from the chelation agent regenerating liquid.
Organic removal is accomplished by first chemically pretreating the slurry from the wash plant or the metal removal process. Next, biological treatment is applied in upflow slurry reactors using the bacteria which have developed naturally in the soils. The treated soil is dewatered using hydrocyclones and returned to the site from which it was excavated.
The technology is designed to reduce organic and inorganic contaminants in soils. The process train approach is most useful when sites have been contaminated as a result of multiple uses over a period of time. Typical sites where the process train might be used include refinery and petroleum storage facilities, sites with metal processing and metal recycling histories, and manufactured gas and coal or coke processing and storage sites. The process is less suited to soils with undesirable high inorganic constituents which result from the inherent mineralogy of the soils.
The THC soil recycling process was accepted into the SITE Demonstration Program in 1991. The soil recycling process was demonstrated at a site within the Toronto Port Industrial District that had been used for metals finishing and refinery products and petroleum storage. Demonstration sampling took place in April and May 1992.
Results have been published in the Demonstration Bulletin (EPA/520-MR-92/015), the Applications Analysis Report (EPA/540-AR-93/517), the Technology Evaluation Report (EPA/540/R-93/517), and the Technology Demonstration Summary (EPA/540/SR-93/517). These reports are available from EPA.
This technology is no longer available through a vendor. For further information on the technology, contact the EPA Project Manager.
The demonstration results showed that soil washing produced clean coarse soil fractions and concentrated the contaminants in the fine slurry.
The chemical treatment process and biological slurry reactors, when operated on a batch basis with a nominal 35-day retention time, achieved at least a 90 percent reduction in simple polyaromatic hydrocarbon compounds such as naphthalene, but did not meet the approximately 75 percent reduction in benzo(a)pyrene required to achieve the cleanup criteria.
The biological process discharge did not meet the cleanup criteria for oil and grease, and the process exhibited virtually no removal of this parameter. THC believes that the high outlet oil and grease values are the result of the analytical extraction of the biomass developed during the process.
The hydrocyclone dewatering device did not achieve significant dewatering. Final process slurries were returned to the excavation site in liquid form.
The metals removal process achieved a removal efficiency for toxic heavy metals such as copper, lead, mercury, and nickel of approximately 70 percent.
The metals removal process equipment and chelating agent were fouled by free oil and grease contamination, forcing sampling to end prematurely. Biological treatment or physical separation of oil and grease will be required to avoid such fouling.
EPA PROJECT MANAGER:
Teri Richardson
U.S. EPA
National Risk Management Research Laboratory
26 West Martin Luther King Drive
Cincinnati, OH 45268
513-569-7949
Fax: 513-569-7105