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ON-SITE BIOREMEDIATION OF OIL & GREASE CONTAMINATED SOILS

An On-Line Version of an Article First Published in:
The National Environmental Journal Sept./Oct. 1991 Vol. 1 No. 1 Pages 26-30

By: David B. Vance

INTRODUCTION

Contamination of soils with organic compounds is a problem often associated with the processing and distribution of crude and refined petroleum hydrocarbons. Retail outlets, bulk storage terminals and refineries are all facilities at which it is common to have spills or releases and associated soil contamination. In instances where hydrocarbon contamination has impacted near surface soils, removal by excavation is a reasonable remedial approach.

Broadly speaking, the excavated soils can be treated/disposed on or off-site. Off-site alternatives typically involve disposal in a landfill or treatment by incineration. On-site alternatives most often involve some form of destructive treatment system to remove the adsorbed hydrocarbons from the soils.

Soil roasting or incineration are examples of energy intensive (read expensive) means of destructive treatment that oxidize contaminating petroleum hydrocarbon to carbon dioxide and water.

Another alternative, that also oxidizes the hydrocarbon contaminants, is bioremediation. Rather than rely on an supplemental fuel and relatively sophisticated hardware, bioremediation takes advantage of the metabolic activity of microorganisms in a low energy environment (read inexpensive).

To use on-site bioremediation on excavated soils two important general criteria must first be met:

One, there must be enough space available on the site to accommodate the soil biotreatment area.
Two, there must be enough time available in the project to allow for the biodegradation process to occur.

If the above conditions can be met, there are three other criteria which are important in the implementation of aboveground bioremediation:

The presence of viable native bacteria in the impacted soils.
The ability of those bacteria to degrade the adsorbed hydrocarbons.
Determination of the level of nutrient supplementation required to stimulate optimum bacterial activity.

Taking a practical approach, what follows is a project case history to illustrate the design, construction, operation and closure of an aboveground bioremediation cell.

The project was performed at a petroleum strategic reserve facility in southeast Texas. Crude oil had leaked at a valve terminal area. The near surface soils were "gumbo type" clays. The 1350 cubic yards (1030 cubic meters) of treated contaminated soil was hand excavated due to the complexity of the surrounding piping network.

The project was conducted in four phases:

Completion of a feasibility study to determine whether the inorganic, organic, microbiological, and geological conditions were favorable for bioremediation of the hydrocarbon contaminated soils. An important part of this phase was the performance of a bench scale nutrient optimization study.
Construction of the aboveground biotreatment cell followed by the excavation and placement of the contaminated soils in the cell.
Operation of the induced draft aeration system with monitoring and maintenance of the facility.
Closure of the treatment unit, in compliance with the regulatory and client requirements.

FEASIBILITY AND DESIGN STUDY

The first step in designing an effective soil bioremediation program was to conduct feasibility and design studies on the contaminated soils. These studies:

Determined if there were viable native bacterial populations present in the soil.
Determined the appropriate nutrient loading levels needed to stimulate those bacteria.
Identified soil characteristics which may have adversely affected efforts to biotreat the contaminated soils.

During the collection of samples for the feasibility/design study sterile collection procedures and sterile sample containers were used to insure the microbiological integrity of the samples and subsequent laboratory work.

One of the most important factors in determining the feasibility of a biodegradation system is the confirmation of the presence in the soil of native bacteria capable of degrading the contaminants of concern. In some rare instances, where native bacteria are not present or incapable of degrading a specific organic contaminant, it may necessary to purchase and add to the soil bacteria specifically cultivated for this purpose. It must be emphasized that in most cases viable native bacteria are present in the soil, have adapted to utilize the contaminant as a carbon source and are likely to respond as well or better than any added strains.

For the Texas project, soil samples were plated on two types of media to enumerate background and hydrocarbon utilizing bacteria. Background bacteria use dead or decaying organic matter as a food source. Some have been found to utilize petroleum hydrocarbons as a food source, but not all. Hydrocarbon utilizers include some background bacteria and strains which exclusively use hydrocarbons as a food source.

Enumeration of background activity was accomplished using a nutrient media that included a supplemental carbon source. For enumeration of hydrocarbon utilizers, a mineral media was prepared using hydrocarbon contaminants obtained from the project site as the sole carbon source. Bacterial enumeration’s are reported as colony forming units (CFU) per unit weight or volume of dry soil. The term colony forming unit is used instead of bacteria because the discrete colonies seen in plate counts can represent the growth from a single bacteria or a group of bacteria that had adhered to a soil particle.

The background bacteria ranged up to 6.8 x 10⁵ CFU per gram of dry soil and the hydrocarbon utilizers ranged from 3.2 x 10⁴ to 9.7 x 10⁵ CFU per gram. These were levels indicative of healthy and potentially useful native bacterial populations.

In the nutrient optimization study, multiple microcosms (suspensions of collected soil and water in sterile beakers) were set up to evaluate the effect of oxygenation and nutrient addition. The microcosms included:

One, with a biocide added to serve as a killed control.
A second, to serve as a live control. Nutrients were not added, but the system was oxygenated.
The remaining three systems were set up with different levels of nutrient (ammonium and phosphate salts) addition.

The nutrient optimization studies indicated a progressive numerical response to increased nutrient supplies. In addition, increased microbial growth was also observed in response to aeration of the control system. However, significant nutrient adsorption to the site soils was noted, and bacterial response was most significant once the adsorption threshold was exceeded. The high adsorption rate observed during this study was attributed to the high clay content of the soil.

Based on the results of the laboratory studies, it was determined that aeration, together with the addition of 3.4 pounds of ammonium chloride and 1.0 pounds of dipotassium phosphate per cubic yard (4.5 and 1.3 pounds respectively per cubic meter), would result in degradation of the contamination in less than six months.

During the design phase of the project soil samples were collected and analyzed for oil and grease from a series of locations on and around non-impacted areas of the clients site. These results gave a background oil and grease concentration for the native soils of 75 mg/Kg. This value was then used as the target clean-up level for the remedial program.

CONSTRUCTION OF THE BIOREMEDIATION CELL

The proximity of piping in the contaminated zone required that soil removal could only be by hand excavation. By this method 1350 cubic yards (1030 cubic meters) of contaminated soils found were excavated.

The aboveground bioremediation cell was constructed to interior dimensions of 115 feet by 116 feet (35 x 35 meters). The cell was lined with a 12 ply high density polyethylene cross laminate sheet. Three foot (1 meter) berms were constructed around the perimeter of the cell.

The soils to be treated were mounded on the liner to a thickness of 3 feet to 4 feet (0.9 to 1.2 meters). A mechanical soil mixer was used to ensure that the soils were adequately porous.

A series of two inch, slotted PVC air extraction pipes were placed along the bottom of the mound, on 15 foot (13.7 meter) centers. Due to the very fine sediment size, all air extraction lines were packed with gravel. The vent pipes were connected to a 2 HP high vacuum blower capable of inducing an air flow rate of 60 CFM at vacuum of 60 inches of water (1.7 cubic meters/minute at a vacuum of 112mm of Hg).

A one inch, porous piping system was installed on top of the mound. This system was connected to a nutrient mix tank and pump to provide inorganic nutrients and moisture to the biodegradation cell.

A french drain was constructed along the length of the biodegradation cell to collect leachate. The cell floor was sloped, so that all liquids could be collected in a single drain. The leachate was used as make-up water for the nutrient addition system.

To insure that rain water was not a concern during the operation of the cell, the mound was covered with polyethylene sheeting. The cover was vented to allow the circulation of air, but was installed so that rain water was diverted outside of the bermed area.

BIODEGRADATION CELL OPERATIONS, MONITORING AND MAINTENANCE

During the life of the project there were a total of six rounds of sampling at 0, 22, 53, 73, 109 and 156 days. During each sampling event at least five soil samples were collected and analyzed for:

Soil moisture
Nutrient levels
Bacteria counts
Oil and grease concentration

During the period following the first week of system start-up the addition of nutrient salts was suspended to lower the moisture content of the soils. Subsequently, analyzed levels of nutrients dropped to the undetectable level. For this reason the addition of nutrients was resumed in after 60 days of operation, even though the moisture content had not dropped as desired. Throughout the remainder of the project analyzed nutrient levels were at or below the limit of detection. However, analysis of contaminant degradation indicated that no detrimental effects resulted from the low nutrient levels. Proper aeration to provide an adequate oxygen supply was the dominant factor in the stimulation of bacterial activity in the soils.

Figure one illustrates the results of bacterial enumeration’s over the course of the project. Hydrocarbon utilizing bacterial populations after 22 days of operation had risen to 1 x 10⁷ CFU per gram of dry soil. The bacterial numbers fell to 1 x 10⁴ CFU in the day 53 sample. Counts then increased to 8 x 10⁵ by day 73 and 1 x 10⁶ at day 109. On day 156 of system operation the bacterial counts had fallen to 3 x 10³, perhaps an indication of suppressed activity due to the complete degradation of the hydrocarbon source.

Figure two illustrates the average trend of the progressive degradation of contaminants adsorbed to the soil in the biodegradation cell. In addition, the high and low contaminant concentrations for each sampling event are also plotted on figure two.

The average of the initial contaminant concentration before system start up (from two samples) was 960 mg/kg as oil and grease. After 22 days the average contaminant level had dropped to 290 mg/kg. The sample set collected after 53 days of cell operation gave anomalous appearing results, with an average value for oil and grease of 588 mg/kg.

Results of this type can be initially disturbing, however, it is not unusual to have a degradation trend exhibiting this pattern occur at sites undergoing bioremediation. The sudden increase in contaminant concentration was probably an effect induced by the high levels of bacterial activity stimulated in the hydrocarbon rich soil. The bacteria active in the biodegradation of the contaminants excrete extracellular surfactant-like polymers during their metabolic cycle. These surfactant polymers mobilized the hydrocarbon contaminants to levels physically lower in the soil pile, from which the samples had been collected.

Nevertheless, these results prompted an early round of additional sampling, which was done after a total of 73 days of activity. These new data indicated that contaminant in the soils had been degraded to a level below the limit of detection (50 mg/kg oil and grease) for the analytical method used (Mod. Std. Method 503D).

Consequently, after an additional month of operation (for a total of 109 days), a total of ten samples were taken from across the cell. Six of the samples had levels of oil and grease below or near the detection limit of the test (50 mg/kg). The four remaining samples indicated residual hot spots in the soils within the cell, with oil and grease values of 75, 100, 120 and 180 mg/kg.

At this point the cover was partially removed to improve aeration of the cell and the "hot spots" were sampled again after 156 days of operation. Four of the five samples yielded oil and grease levels below the limit of detection. The fifth sample had an oil and grease concentration of 110 mg/kg. After the 156 day sampling event it was determined that the soils had been remediated to the required level and the site was ready for closure. However, aeration did continue until closure was officially approved.

PROJECT CLOSURE

Between day 53 and day 156 of the biosystem operation a total of 20 soil samples were taken from the biodegradation cell and analyzed for oil and grease. This represented one sample for every 70 cubic yards (54 cubic meters) of soil in the cell. The average oil and grease concentration for the 20 samples was 59 mg/kg. The average of background soil samples near the site was 75 mg/kg.

It was determined that this was sufficient proof for an adequate level of remediation. At this point a report of activities and request for closure was submitted to the Texas Railroad Commission (the governing regulatory agency). This request was granted within 30 days of submittal.

Much of the material used in the construction of the cell was removed and stored at the site for future use. After the recovery of ancillary equipment, earth moving equipment was used for the final stages of the closure. First the liner beneath the soil pile was removed. The site was then leveled to grade using the treated soils as fill for low lying areas.

CONCLUSIONS

Gumbo clay type soils contaminated with oil and grease at concentrations near 1000 mg/Kg were successfully treated in a induced draft aboveground biodegradation cell. Samples of contaminated site soils were used in bench scale tests to generate data for the design of the system. Bacterial evaluations included enumeration’s of native bacterial populations and optimization of nutrient salt addition.

Monitored contaminant concentrations during the biocell operation rapidly declined. An anomalous (although not unusual) increase in oil and grease concentrations was seen after 50 days of cell operation. Thereafter, oil and grease concentrations decreased to levels near or below detection limits, except in localized "hot spots". With 60 days of additional operation, the oil and grease concentrations in the "hot spots" had in turn been reduced to low levels. After six months of operation, the biodegradation process had reduced contaminant concentrations to the point where closure for the site was requested and subsequently granted.

The cost for hand excavation of the contaminated soils and backfilling was $ 88 per cubic yard ($ 115 per cubic meter). The cost for the construction, operation, maintenance and closure of the aboveground bioremediation cell was $ 48 per cubic yard ($ 63 per cubic meter).

If you have comments or suggestions, e-mail me at dbv7@mindspring.com