Cost-effective Remediation of PCB Contamination
Northern Telecom (Nortel) suspected polychlorinated biphenyl (PCB) contamination at its telecommunica-tions manufacturing facility at Paignton in South West England. Seeking remediation, it retained Golder Associates to conduct site inves-tigations and determine the most cost-effective remedial solution.
Golder's investigations found PCBs at two discrete areas within a-20 hectare area. At one of those locations, the average concen-tration measured in the soil was 120 mg/kg, with hotspots of up to 1300 mg/kg. Other constituents present included trichloroethene (TCE), tetrachloroethene (PCE), trichloroethane (TCA), and 1,1,2-trichloro-l,2,2-trifluoroethane (Freon 113) in a high-clay-content soil.
Golder determined that the levels could pose a risk to human health and would inhibit future construction or development activities at the site.
Constituent levels were such that no licensed landfill in the UK would accept the soil. Also, the volume of earth involved made the cost of high-temperature inciner-ation prohibitive.
This situation led Golder to identify a number of alternative strate-gies.
- Bioremediation was initially consid-ered but discounted because of the rel-atively high chlorine content of the PCBs.
- Cost and time constraints precluded solvent washing.
Golder ultimately chose low-temperature thermal desorption (LTTD)--a process that involves heating the soil to temperatures around 450°C.
Following excavation and temporary on-site lined and capped storage, the contami-nated soil was taken offsite to the LTTD unit. During the heating process, the con-stituents were desorbed from the soil and removed as a vapor stream. The vapors were subsequently incinerated at temperatures in excess of 950 ºC. This destroyed the constituents and mitigated any possibility that such toxic products as dioxins would form.
Initially, the clayey nature of the soil inhibited its breakdown to less than the 50-millimeter-size clods required for the process, inhibiting air passage through the material. The process operator, British Aerospace Defence Royal Ordnances Environmental Services Group, however, by trial and error overcame the problem. It experimented with various mixtures of contami-nated soil and such non-cohesive materials as sand and crushed concrete. It thereby identified the optimum mixture to ensure the requi-site soil-size material.
Once these initial issues were addressed, the remediation program reduced contamination levels to less than 0.5mg/kg
The costs associated with traditional high-temperature incineration of this quantity of soil would have been about £77 (U.S.$125) per ton (total cost £1.8 million). The total LTTD treatment costs were £470,000, representing a savings of some 75%.