All farmers fear contaminated soil. New clean up processes help bring contaminated land back to its original state
I spent last winter, on my way to the rink, driving past a mountain of dirt being processed by various machines. It looked very labourious and complicated The grapevine reported that there had been an oil spill, and cleaning it up involved digging up the contaminated dirt, and “cooking” the oil out of it.
A I zipped past with kids in the backseat and skates in the trunk, the questions that kept coming to mind were, “What will be left of the dirt when this is through?” and, “Will this land ever grow anything again?”
Thermal desorption remediation
When a pipeline leaked on land owned by Griffin, Sask., farmers Stacey and Kara Lee Lund, the company that owns the pipeline arranged to have the spill cleaned up by Nelson Environmental Remediation Ltd. (NER).
NER, with headquarters at Spruce Grove, Alta., has 15 years experience with a process called “thermal desorption remediation.” This was the same process I had driven past last winter, and it’s also being used in the clean up on Lund’s land.
What is thermal desorption remediation? According to Tyrel Watchell, business development officer for NER, “It is the safest most economical way of treating contaminated soil on site, meeting the strictest of government standards.”
Thermal desorption remediation (TDR) is the process of removing harmful chemicals from soil, by using heat to change the chemicals into gasses. Special equipment is used to collect the gasses. Then dust and harmful chemicals are separated from the gasses and disposed of safely. The end result is clean dirt.
TDR is not the same as incineration, which uses heat to destroy chemicals in the soil. TDR does not sterilize the soil; the inorganic composition of the soil is left intact.
TDR can be used to clean up various chemical contaminations, however, across Western Canada the most common chemicals needing remediation are the petroleum hydrocarbons, such as crude oil, diesel and other chemicals found in the oil field industry. One of the advantages of TDR, Watchell points out, is that the contamination need not be recent to be successfully remediated. “Spills can be two months old or 20 years old. There are many historically contaminated sites that can be fixed using TDR, even after other methods have been tried.”
The process of TDR requires special equipment called a thermal desorption unit (TDU). A TDU is moved to the contamination site. The contaminated soil is excavated, brought to the TDU, dropped into a hopper, and then onto a conveyer belt. Once on the conveyor belt, the soil moves into the desorber at a controlled rate. There, it is heated to volatilize the hydrocarbons, which are removed from the soil as a gas stream. The temperature and processing time of the desorber are set according to the type and concentration of petroleum product in the soil, soil type and soil moisture content. The soil exits the desorber as clean dirt. It will be re-hydrated and cooled.
The gas stream of toxic chemicals separated from the soil pass through the baghouse — a filtration unit where dust and particulate are removed from the gas stream.
In the final stage of the process, the gas stream enters the oxidizer. Here, toxic hydrocarbon molecules are rendered harmless with the addition of oxygen and increased temperature.
Restoring the dirt
The clean dirt produced as the end result of the thermal desorption process is tested by an independent lab to ensure that petroleum toxins have been removed from the soil to specifications set by provincial and federal governments. This dirt is used as backfill at the excavation site. The thermal desorption process can remediate between 500 and 800 metric tonnes of contaminated soil in 12 hours.
When compared to alternative remediation methods, such as landfilling, TDR has environmental advantages. Had the pipeline leak on Lund’s land been landfilled, the contaminated soil would have had to have been excavated and hauled to a landfill site.
“There would have been a lot of truck traffic,” says Stacey Lund. “The on-site process saves the roads.” In this case, landfilling would have meant hauling away 16,000 tonnes of soil, then sourcing and hauling back in another 16,000 tonnes of uncontaminated soil to backfill the excavated site.
Landfilling does not eliminate contaminants from the soil, but simply moves the problem from one location to another. Because hydrocarbons could be released into the atmosphere or back into the ground at the new site, they continue to pose future environmental and liability issues.
TDR and landfilling cost about the same: about $50 per tonne of contaminated soil, but TDR has some clear advantages for farmland:
- At the end of the process, the soil is clean. Petroleum contaminates are eliminated to regulated standards.
- TDR leaves the inorganic integrity of the soil intact. The clean soil used to backfill the contamination site will have the same characteristics as the soil around the contamination site. Once organics are re-established, over time the soil has the potential to return to its original level of productivity. The process of re-establishing organics can be accelerated by adding straw and manure to the topsoil.
- With TDR, clean soil from the original location is used as backfill instead. This eliminates the need to bring in soil from another location, which carries the risk of introducing a new weed or insect pest to the land.
Tyrel Watchell of Nelson Environmental Remediation sums up the TDR finished product by saying, “At the end of the day we put that dirt back in, and contour it, and seed it, or leave it fallow to regenerate.”
The two acre piece of remediated ground owned by Stacey and Kara Lee Lund should be ready to be put back into production next growing season. I’ll be keeping an eye on it next spring, as I zip past with kids in the backseat, on my way to the soccer field. †