Marc St-Cyr
Shell Canada Products Ltd.
Montreal
Christopher H. Nelson
Groundwater Technology Inc.
Englewood, Colo.
C. Tony Hawke
Groundwater Technology Canada Ltd.
Montreal
A scientific assessment of soil conditions at Shell Canada Products Ltd.'s Montreal-East refinery gave the company the confidence to implement an on site engineered biotreatment complex for contaminated soil.
This solution allowed Shell Canada to dispose of the soils on site, at a fraction of the original landfill estimate, and eliminate ongoing liability.
In spite of poor soil conditions, higher than anticipated volumes of contaminated soil, and delays that caused treatment to begin in winter, the treatment goals were achieved ahead of schedule and within the scope of the adjusted work plan.
BACKGROUND
Low soil permeability and low temperatures have been known to inhibit the biological treatment of petroleum-contaminated soils and tank sludges. Low soil permeability restricts the air flow necessary to promote bacterial growth. Cold retards, and may even stop, bacterial activity.
Low soil permeability and frigid temperatures, however, are merely constraints; they do not necessarily prevent effective bioremediation.
This was learned recently at Shell Canada's Montreal-East refinery, where the first permanent aboveground engineered biotreatment complex was used to effectively treat petroleum-contaminated soils containing over 50% clay (Fig. 1).
The Shell Canada refinery, located on the St. Lawrence River, needed to dispose of petroleum-contaminated soils excavated during the replacement of petroleum distribution lines from the refinery to the dock facility. Sending the estimated volume of soils to a landfill was going to cost approximately $2 million, and Shell Canada could retain liability for the soil permanently.
Shell Canada requested that Groundwater Technology Inc. propose an alternative solution which consisted of treating the soils using an aboveground biotreatment complex.
After the project began, it was discovered that the amount of contaminated soils was nearly twice the volume originally estimated, approximately doubling disposal cost estimates. The total cost of bioremediating the actual volume of soils on site was considerably less than the estimated $3 million for off-site disposal.
Liability concerns with off site disposal were also eliminated.
BIOTREATMENT
The traditional method of treating biosludges, tank sludges, and contaminated soils at refineries has been landfarming-literally tilling the contaminated soils and sludges and letting nature take its course.
Landfarming, however, has several drawbacks:
- It requires considerable amounts of open space.
- Precipitation and runoff control can be difficult.
- Treatment efficiencies with landfarming are somewhat variable.
- In northern climates, the practice of landfarming is restricted by the seasons.
These drawbacks, however, do not mean that biotreatment should be abandoned. It just has to be made more effective.
Bioremediation (in situ or in an engineered aboveground treatment cell) is a proven method of remediating petroleum-contaminated soils, as well as soils contaminated by many other organic chemicals. It simply provides optimal environmental conditions for common aerobic soil bacteria to degrade organic contaminants at maximum rates.
Bioremediation involves the stimulation of indigenous bacteria through the addition of essential nutrients and oxygen. The bacteria used in the process naturally inhabit the soils.
As long as the design and operation of the oxygenation and nutrient system provide an adequate supply of key metabolic factors to promote bacterial growth, biodegradation proceeds and the treatment is successful. Biological treatment of contaminated soils is therefore less dependent on the biological process itself than on the engineering of the system.
Bioremediation of soils and tank sludges at refineries using highly controlled aboveground engineered biotreatment cells generally cuts treatment time in half, as compared to landfarming. It uses one third to one tenth the amount of land.
Construction costs are slightly higher while operating costs are substantially less. The cost per cubic meter for engineered biotreatment is far lower than that of either incineration or secured landfill disposal. Costs typically range from $50/cu m to $200/cu m.
PROCESS REQUIREMENTS
The effectiveness of an engineered on site biotreatment complex for various sludges and petroleum-contaminated soils is based on three principles:
- Stimulation of natural biodegradation
- Controlled and uniform application
- Containment of the reaction.
Proven technology exists to fulfill the requirements of each of these principles.
NATURAL STIMULATION
Biological treatment of a wide range of petroleum hydrocarbons can be achieved by assuring proper oxygenation, maintaining proper moisture conditions, adding essential nutrients at required levels, and controlling soil temperature. Of these factors, oxygenation is the most critical.
The rate of biodegradation is proportional to the rate of oxygenation. Moisture content and the supply of nutrients (e.g., nitrogen and phosphorus) are not as critical to the rate of biodegradation as is oxygen supply.
With moisture and nutrients, it is important to provide a threshold level; the key is to maintain a sufficient quantity. Too low a level will generally slow degradation. An overabundance, on the other hand, will be wasted, or can even be detrimental.
Soil temperature can be controlled by regulating the rate of negative-pressure aeration and incorporating heated, positive-pressure aeration is necessary.
CONTROLLED APPLICATION
As the bioremediation process is dependent on the transport of oxygen and nutrients through the contaminated soil, uniformity and control of application are necessary for the successful treatment of the excavated soils. This requires that the soil be uniformly permeable and well-mixed with nutrients.
Heterogeneity can lead to incomplete treatment and isolated zones of untreated soil. Low-permeability soils will decrease the flow of oxygen, significantly extending the time required for treatment.
When dealing with heterogeneous or low-permeability soil such as clays, Groundwater Technology uses soil conditioning techniques adapted from the mineral industry to obtain a stable, permeable soil matrix that is uniformly mixed with nutrients. The basic approach is to mechanically shred the soil. The shredded soil may be stabilized by mixing in a small amount of a soil-conditioning material such as gypsum. In cases of very high organic loading or clay content, an inert support material such as sand or wood chips may be added to improve stability and flow characteristics.
The need for soil modification must be evaluated with each batch of soil treated.
REACTION CONTAINMENT
Containment of the reaction is necessary from three standpoints: optimal space efficiency, circulation efficiency, and control of potential discharges. To accomplish these ends, treatment is done on a diked pad.
The pad itself can be constructed as a permanent facility, using an asphalt or concrete base, or as a batch facility, using a soil or clay base with replaceable impervious liners. In both cases, a monitoring system can be installed under the pad to assure containment integrity.
In most applications a geosynthetic, or cross-linked polyethylene, covering is used to control soil moisture and possible contaminated runoff (Fig. 2). A properly designed facility can also accommodate chemical pretreatment capabilities for highly contaminated soil.
CONSTRAINTS
The prospects for on site bioremediation at the Shell Canada facility were constrained by poor soil conditions in addition to cold weather. Soils at the site contained over 50% clay, high moisture content, unevenly distributed contamination, and large amounts of rock. In addition, the harsh Canadian winter limited the full-scale operation of many on site treatment technologies for up to half the ear.
However, based on previous experience at numerous bioremediation sites, Groundwater Technology believed that these constraints would not prevent effective biotreatment. Getting the most out of a biotreatment system during the winter would involve providing enough air to the soil to promote microbial degradation of the contaminants while maintaining the heat produced by the hydrocarbon degradation process.
The poor soil conditions were overcome by aggressively conditioning the soils prior to loading them into the treatment cells. The treatability of the conditioned soils, as proposed by Groundwater Technology, was ultimately confirmed by independent studies performed at McGill University, and later through Canada's National Research Council and its Biotechnology Institute in Montreal.
Groundwater Technology's project team had enough confidence in the solution to guarantee system performance, and Shell authorized the project.
The engineered biotreatment complex designed for Shell is supported by a large cement pad which serves as a base for soil conditioning and provides secondary containment. The pad has ample space for stockpiling, conditioning, and treating the soils in one of two treatment cells.
Each of the cells in the complex has a minimum capacity of 1,200 cu m soil. Vent systems provide aeration to the contaminated soils. These consist of two tiers of slotted pipes running the width of each cell and connected to negative-pressure blowers via a header piping system. Because the treatment complex was designed for multiple, long-term use, the aeration blower system is permanent.
WINTER TREATMENT
Ideally, treatment should have begun early in the summer, but results from the independent treatability study were not available until August of 1991. Groundwater Technology engineered and constructed an automated soil-conditioning system on site and began operating it in the fall.
Soil conditioning involved:
- Modifying the soil matrix by adding gypsum
- Mixing soils to reduce the particle size and homogenize the contamination
- Significantly increasing soil permeability by introducing a natural filler
- Uniformly adding nutrients to speed bioremediation.
At the same time, a large amount of rock was removed. Conditioning was completed and the soils were loaded into the treatment cells by December. The aboveground biotreatment complex achieved high levels of microbial activity over the winter months by careful control of the air flow into and out of the biotreatment cells.
Optimal temperatures for bioremediation range between 10 C. and 40 C., while winter temperatures in Quebec average - 10 C. By limiting the escape of warm air and the introduction of frigid air to brief aeration periods, temperatures inside the treatment cells were held at an average of 15 C. over the winter months.
Carbon dioxide levels (another indicator of biological activity) were elevated throughout the winter. During the spring and summer months, the permanent aeration system was operated continuously, via the embedded network of piping, for maximal treatment efficiency.
Initial soil concentrations averaged 6,500 mg/kg total petroleum hydrocarbons. The treatment goal was 1,000 mg/kg or better within 12 months. By late spring, the cells were within 200 mg/kg of the treatment goals.
By July, average contaminant levels in the soils had been reduced to 790 mg/kg - below the targeted treatment goals and 2 months ahead of schedule.
The original estimate of the volume of soil to be excavated was approximately 1,500 cu m. Because soil volumes typically increase during excavation and soil conditioning, Groundwater Technology designed the facility to accommodate a large increase in soil volume. This proved to be invaluable because the total volume of excavated soil from the pipeline renovation project was more than 3,000 cu m.
RESEARCH COMPONENT
While the conditioned soils were being loaded into the cells, Shell Canada, with assistance from Groundwater Technology, applied to Environment Canada, Montreal, for a grant to study the microbial populations within the biotreatment complex. These funds were used to study field results using respirometry, tracer mineralization, and gene probe techniques. In the process, the companies developed a greater understanding of the microbial processes involved in bioremediation.
Studies confirmed that the strains of bacteria most active in degrading contaminants vary with changes in temperature.
Such studies provide remediation scientists with a more accurate window to the bioremediation process, making it possible to fine-tune system performance according to soil conditions and seasonal temperatures for better treatment efficiencies.
The permanent biotreatment cells constructed at Shell Canada's facility remain a cost-effective, ongoing means of treating contaminated soils and recurring tank sludges.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
