Land Farming

Background
Land farming is a bioremediation treatment process that is performed in the upper soil zone or in biotreatment cells. Contaminated soils, sediments, or sludges are incorporated into the soil surface and periodically turned over or tilled to aerate the mixture.

Applicability
This technique has been successfully used for years in the management and disposal of oily sludge and other petroleum refinery wastes. In situ systems have been used to treat near surface soil contamination for hydrocarbons and pesticides. The equipment employed in land farming is typical of that used in agricultural operations. These land farming activities cultivate and enhance microbial degradation of hazardous compounds. As a rule of thumb, the higher the molecular weight (i.e., the more rings within a polycyclic aromatic hydrocarbon), the slower the degradation rate. Also, the more chlorinated or nitrated the compound, the more difficult it is to degrade [1,2].

Limitations
Factors that may limit the applicability and effectiveness of the process include: (1) large space requirements; (2) the conditions advantageous for biological degradation of contaminants are largely uncontrolled, which increases the length of time to complete remediation, particularly for recalcitrant compounds; (3) inorganic contaminants are not biodegraded; (4) the potential of large amounts of particulate matter released by operations; and (5) the presence of metal ions may be toxic to microbes and may leach from the contaminated soil into the ground [2].

Hydrocarbon compounds that have been identified as being not readily degraded by land farming include creosote, pentachlorophenol (PCP), and bunker C oil [3].

Performance
The effectiveness of land treatment of petroleum products has been confirmed in controlled experiments in both laboratory and field experiments. It has been demonstrated in the laboratory that gasoline, jet fuel, and heating oil were extensively degraded when affected soils were treated with fertilizer, lime, and simulated tilling [4].

In a field experiment, land farming was investigated at a site contaminated with about 1.9 million liters of kerosene. Initial oil concentrations of 0.87% in the top 30 cm and 0.7% at a depth of 30-45 cm (after initial emergency cleanup) was reduced to <0.1% and 0.3% in the former and latter depths, respectively, after 200 kg of nitrogen, 20 kg of phosphorus, and lime were added to the soil [5].

Performance data for several land-farming systems is summarized in Table 1 [2].

Table 1. Performance data of land-farming systems.

Site Characteristics	Beginning Levels	Levels Attained
PCP at wood preservation site	100 mg/kg of soil	<5 mg/kg in 4 months
Pesticide storage facility	86 ppm	5 ppm
Fuel oil spill	6,000 ppm TPH	100 ppm in 120 treatment days

PCP: pentachlorophenol
TPH: total petroleum hydrocarbons

Data Requirements
Land treatment is designed to optimize degradation in an aerobic biological process using soil as the inoculum and support medium for biological growth. Moisture content, oxygen level, nutrients, pH, and bulking of the soil are normally controlled during the process.

The general procedure for land farming is as follows: (1) apply additives using mixing techniques to achieve at least 90% contact between soils and additives; (2) add inorganic nutrients to obtain a mass ratio of 100:10:1 hydrocarbon:nitrogen: phosphorus (trace amounts of potassium may be beneficial; (3) increase the porosity of the soil as much as possible (mixing during earth-moving activities or adding a bulking agent can be used; soils with high clay content usually benefit from a bulking agent such sand, sawdust, or wood chips); and (4) adjust soil pH with either lime, alum, or phosphoric acid as required [2].

Cost
Typical costs for land farming range from $50 to $115 per cubic yard [1].

Status of Technology
Numerous full-scale operations have been used, particularly for sludges produced by the petroleum industry. Land farming, combined with other biological treatments, can convert contaminants into nonhazardous substances. This technology is widely used and has been applied to many waste types, especially for disposal of oily sludge and other petroleum refinery wastes.

References
1. Cookson, J.T., Jr., 1995, Bioremediation Engineering Design and Application, McGraw-Hill, Inc., New York, NY.

2. Office of Research and Development, EPA. ATTIC Downloadable Documents, available at http://www.epa.gov/bbsnrmrl/attic/documents.html.

3. Alexander, M., 1994, Biodegradation and Bioremediation, Academic Press, San Diego, CA.

4. Song, H.G., X. Wang, and R. Bartha, 1990, Bioremediation Potential of Terrestrial Fuel Spills, Applied and Environmental Microbiology, 56 (3), pp. 652-656.

5. Dibble, J.T. and R. Bartha, 1979, Rehabilitation of Oil-Inundated Agricultural land: A Case History, Soil Science 128 (1), pp. 56-60.

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