BIOREMEDIATION TECHNOLOGY
ADVANCES IN COMPOSTING CONTAMINATED SOILS
Projects demonstrate the effectiveness of composting as
a bioremediation tool.

THE ABILITY of the composting process to degrade soil contaminants has been demonstrated in projects and research trials for a number of years. Recently, the U.S. Environmental Protection Agency’s Office of Solid Waste and Emergency Response released a report, “Use of Bioremediation At Superfund Sites.” The inclusion of composting as one of four “ex situ” bioremediation technologies further validates its role as an acceptable treatment process.

The report cites eight composting projects at Superfund sites (between FY 1982 and FY 1999). Contaminants treated include PAHs (polyaromatic hydrocarbons), other nonchlorinated and chlorinated semi-volatile and volatile organic compounds, BTEX (benzene, toluene, ethylbenzene and xylenes), pesticides and herbicides, and explosives and propellants.

One of the largest composting-based bioremediation projects in the United States got underway in May 1999 at the Joliet Army Ammunition Plant in Illinois, where there were 200,000 dry tons of soil contaminated by TNT, DNT, Tetryl and a small amount of RDX. The contaminated soil is composted in windrows. Composting amendments include corn processing by-products, wood chips and stable bedding.

Sevenson Environmental Services of Niagara Falls, New York oversees the composting project as a subcontractor to Montgomery Watson. Three steel buildings were constructed at the site (each about 80 feet wide by 400 feet long) to house the composting operation. “We treat 1,500 to 1,600 tons on a dry weight basis, per batch,” says Pat Faessler, Sevenson’s site treatment manager. “Contaminant concentration varies with each batch. The remediation standard we must reach is 190 mg/kg for TNT and 8.4 mg/kg for each isomer of DNT.”

The composting cycle time — from when the feedstocks are mixed and the windrows built through collecting analytical samples and waiting for confirmation data is 36 to 40 days. “Approximately 20 days of that time period is what we consider active treatment with fairly regular turning and monitoring,” adds Faessler. Composting takes place year round.

Feedstocks are mixed outside the building with an Allu AS38 turner; the mix then is brought inside with a front-end loader and the windrows are constructed. The same turner is used to agitate the piles, typically every one to three days during the active composting time.

A mix that includes soil is significantly more dense than one consisting primarily of leaves, for example, which means that the volume capacity of the turner decreases. “If specifications say a machine can turn five cubic yards/linear foot, it will not have that same capacity with this material,” says Faessler. “Equipment capacity can be limited by density.” In addition, a turner with lower RPMs may be more suitable to soil remediation projects because there tends to be debris in the soil, such as rocks, and a lower RPM machine helps to minimize projectiles created by the turner.

Sevenson expects the project will last another four to five years. So far, about 50,000 dry tons of soil have been remediated. At this point, the treated soil is being stockpiled until an end use is determined.

Faessler, who worked on one of the original composting bioremediation projects at the Umatilla Army Depot in Oregon, believes composting is becoming more popular for soil remediation. “You need to have a contaminant that is a little more difficult to degrade, as you do end up with more treated product than you start out with. But composting is seen as an approach that can be applied to a few different types of contaminants.” He adds that cost comparisons with other technologies is difficult because it is rare to find an “apples to apples” remediation situation. “However, all things being equal, composting is probably less than $80/ton, depending on the size of the project and the specific requirements.”

TACKLING HYDROCARBONS

Before getting into soil remediation via composting, Glen Brown of Brown’s Compost Company, Inc. in Lloydminster, Saskatchewan worked for a company that used a thermal degradation technology to treat hydrocarbon contaminated soil. “The process heats up contaminated soil until contaminants evaporate and then the vapors are drawn off and put through an afterburner at 1850 degrees,” recalls Brown. “That would result in zero emissions, but the only problem is, you end up with a soil that was sterilized and nothing would grow in it.”

As a soil technician, Brown always had an interest in composting, but it wasn’t until 1999 that he decided to pursue it full-time, including its application to remediating soils contaminated with hydrocarbons. “I found that at the right temperature, the microbes degrade the hydrocarbons,” he says. “Success depends in part on how fine the contaminated particle is divided to expose it to microbes. For example, it is hard for microbes to get to the center of a lump of dirt that is six inches in diameter. But if that is broken into one-quarter inch pieces, and compost and/or compost tea is added, there is a lot better chance to get right to the source of the contamination.”

While Brown has experience using the composting process to remediate soil, he currently works with a finished compost product. His company composts agricultural residuals, including alfalfa, liquid pig manure, straw and leaves, at four sites it manages. He follows the composting methodology developed by Midwest Bio-Systems and utilizes the Aeromaster composter turners. Over 20,000 cubic yards of material are produced. A variety of compost teas are brewed using the compost produced.

Brown’s Compost Company works with oil contaminated sites in Alberta and Saskatchewan. Typically, the sites are at oil wells (versus soil contaminated with refined petroleum products). Brown uses a recipe of about 25 percent finished compost and 75 percent contaminated soil, but that varies depending on the degree of contamination. The soil and compost are mixed together and built into a windrow, and then compost tea is applied. The piles are turned, oftentimes with equipment already located at the oil well. “The first project in central Saskatchewan started at 140,000 ppm contamination and was brought down to about 4,000 ppm in three turnings,” notes Brown. “The second project was in Alberta, where the level of contamination was 55,000 ppm and it was reduced to Alberta’s Tier 1 criteria in 20 days.”

Combining feedstocks such as raw manure with contaminated soil is effective as a bioremediation method, but lengthens degradation time and costs, he adds. “Bacterial microbes in the manure are beneficial. Using finished compost plus compost tea as an inoculant, however, increases the microbial activity as much as 100 times that of raw manure and in return, yields a much more cost and time effective solution. Using fresh manure requires additional structuring agents, leading to higher processing costs.”

Brown’s Compost Company performs bioassays with compost and contaminated soil to determine the degradation rates. These samples are then used as feedstock to make compost tea. “The end product is an inoculant that can be introduced into the remediation project,” says Brown. “The bacteria are familiar with the contaminant and this in turn speeds up the degradation rate.”

RESEARCH WITH DRILLING MUD

An upcoming article will review recent and current research on composting and bioremediation. Among the new research initiatives is work with invert drilling mud from oil well sites, being done by Donna Chaw of Olds College in Alberta. Drilling holes create a lot of silty mud that is contaminated with salt and diesel, explains Chaw. “It’s lighter in hydrocarbons – the contamination in the soil gets up to about 13 to 14 percent.”

The research project will involve combining the mud with fresh manures and blending in a bulking agent like wood chips or sawdust, then building static piles. The goal is to get the level of contamination down to 2.5 percent. “We will be experimenting with different aeration methods,” adds Chaw, “and will be trying to limit labor as much as possible, because access to these sites is difficult.” The project is scheduled to get underway by the end of 2001. — N.G.

The U.S. EPA report mentioned in this article, “Use of Bioremediation At Superfund Sites,” is available at EPA’s web site: http://www.epa.gov/tio/ or http://clu-in.org.