A 40-year-old, 500,000-gallon waste containment area located in the middle of a crude oil refinery in Mississippi was bio-remediated of all contaminants such as gasoline, diesel, oil, asphalt and debris in 30 days. An 80-percent reduction of solids was achieved, and the final analytics on the water were within EPA permit discharge limits for the wastewater treatment plant. The estimated cost savings of remediation versus the disposal of hazardous waste was over $1 million. Here’s how the refinery did this: The refinery initiated a multiyear project to modernize its wastewater treatment system. The first phase of this project included the remediation and cleaning of an existing concrete basin that was 10 feet deep and contained an average of 17 inches of oily sludge material across the center and approximately 72 inches of water. Probing showed solids and sludge levels as high as 24 inches, with an average of 14 inches in each corner. Hydrocarbon vapors from the pond prevented any hot work from being done in the area.
The refinery planned to clean up the pond and dispose of the hazardous waste using normal industry methods, such as muck and suck, solidification and disposal. It decided to try an alternative bio-remediation option instead, and a proprietary “in-situ microbial remediation process” was used by the refinery.
Sludge pumps were used in the pond for maximum agitation and sludge movement. A propriety “wetting agent” was added to the pond to promote emulsification and loosen any solidified hydrocarbons and inorganic materials that had settled on the bottom. After circulating the pond’s contents, the genetically advanced microorganisms were added to the pond. Constant circulation was then maintained on a 24/7 basis. Analytical sampling was conducted twice each week for chemical oxygen demand (COD) and total petroleum hydrocarbon test results.
Two days after the first inoculation, the volatile vapors ceased to be a problem in and around the pond area, even with constant circulation. The second batch of microorganisms was added three days after the first inoculation, and circulation was maintained for the next three weeks. Ammonia levels soon began to increase in the pond, indicating the degradation of nonhydrocarbon materials on the bottom of the pond. It was observed the COD levels varied up and down as the layers of hydrocarbons on the bottom of the pond were being softened, released and integrated into the pumpable slurry. These hydrocarbon layers were being released sporadically, and the pond would intermittently become covered with a floating layer of heavy oil for several days and then dissolve. This “peeling of the onion” effect would be repeated over and over every 2-3 days.
The weekly sample results indicated fluctuations of COD levels and were not indicating a trend toward total hydrocarbon reduction. By the end of the third week, it was decided another physical probe of the pond was warranted. The physical sludge probe was performed, and the center sludge level was measured to be between 2-4 inches across the center of the pond. Subsequent probing found the average sludge level of the entire pond was 2-4 inches, except for in the hard-to-circulate corners of the pond. The initial probe level of 24 inches had been reduced by at least 75-80 percent.
Because of the layered hydrocarbon material on the bottom of the pond and no effective way to get these hardened layers of asphalt into emulsion and circulation, the COD level of the pond never declined significantly during the initial 30-day schedule. However, it was discovered after settling the pond for a period of 24 hours or longer, the separated water samples showed significant COD reduction. The analytical results on the water portion of the pond were within permit limits for plant discharge, thus allowing for removal of the water phase from the pond. As evidenced by and decided based on the test results, the COD level from the mixture of water and sludge was not an accurate measure of the hydrocarbon degradation within the pond. As the hardened contents on the bottom of the pond softened and were released into the circulated slurry, the COD levels were constantly being replenished with new hydrocarbons. However, the final settled water tested out with a COD of 561 milligrams per liter, which was below the refinery’s discharge permit level.
The final cost of this remediation project was over 50-percent less than budgeted, and a significant amount of water was safely processed out of the pond. It was also noted, although the contents of the wastewater pond were classified as “hazardous” by regulatory classifications, the lab analysis showed the pond’s contents tested “nonhazardous.” The majority of benzene, toluene, and xylene in the sludge had been removed by the microremediation process. This process resulted in an overall 80-percent reduction of solids, and hazardous water and hydrocarbons were converted to dischargeable, nonhazardous water.
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