Land Treatment is a full-scale bioremediation technology in which contaminated soils, sediments, or sludges are turned over (i.e., tilled) and allowed to interact with the soil and climate at the site. The waste, soil, climate, and biological activity interact dynamically as a system to degrade, transform, and immobilize waste constitutes. Wastes are periodically tilled to aerate the waste.
Soil conditions are often controlled to optimize the rate of contaminant degradation. Conditions normally controlled include:
Moisture content (usually by irrigation or spraying).
Aeration (by tilling the soil with a predetermined frequency, the soil is mixed and aerated).
pH (buffered near neutral pH by adding crushed limestone or agricultural lime).
Other amendments (e.g., Soil bulking agents, nutrients, etc.).
A Land Treatment site must be managed properly to prevent both on-site and off-site problems with ground water, surface water, air, or food chain contamination. Adequate monitoring and environmental safeguards are required.
Land Treatment is a medium- to long-term technology.
Typical Land Treatment Unit
Soil bioremediation has been proven most successful in treating petroleum hydrocarbons and other less volatile, biodegradable contaminants. Because lighter, more volatile hydrocarbons such as gasoline are treated very successfully by processes that use their volatility [i.e., soil vapor (vacuum) extraction and bioventing], the use of aboveground bioremediation is usually limited to heavier hydrocarbons. As a rule of thumb, the higher the molecular weight (and the more rings with a PAH), the slower the degradation rate. Also, the more chlorinated or nitrated the compound, the more difficult it is to degrade. (Note: Many mixed products and wastes include some volatile components that transfer to the atmosphere before they can be degraded.)
Contaminants that have been successfully treated include diesel fuel, No. 2 and No. 6 fuel oils, JP-5, oily sludge, wood-preserving wastes (PCP, PAHs, and creosote), coke wastes, and certain pesticides.
Factors that may limit the applicability and effectiveness of the process include:
A large amount of space is required.
Conditions affecting biological degradation of contaminants (e.g., temperature, rain fall) are largely uncontrolled, which increases the length of time to complete remediation.
Inorganic contaminants will not be biodegraded.
Volatile contaminants, such as solvents, must be pretreated because they would evaporate into the atmosphere, causing air pollution.
Dust control is an important consideration, especially during tilling and other material handling operations.
Presence of metal ions may be toxic to the microbes and possibly leach from the contaminated soil into the ground.
Runoff collection facilities must be constructed and monitored.
Topography, erosion, climate, soil stratigraphy, and permeability of the soil at the site must be evaluated to determine the optimum design of facility.
Waste constitutes may be subject to 'Land-ban' regulation and thus may not be applied to soil for treatment by land treatment (e.g., some petroleum sludges).
The depth of treatment is limited to the depth of achievable tilling (normally 18 inches).
The following contaminant considerations should be addressed prior to implementation: types and concentrations of contaminants, depth profile and distribution of contaminants, presence of toxic contaminants, presence of VOCs, and presence of inorganic contaminants (e.g., metals).
The following site and soil considerations should be addressed prior to implementation: surface geological features (e.g., topography and vegetative cover), subsurface geological and hydrogeological features, climate, precipitation, wind velocity and direction, water availability, soil type and texture, soil moisture content, soil organic matter content, cation exchange capacity, water-holding capacity, nutrient content, pH, permeability, and microorganisms (degradative populations present at site).
In some cases, the presence of co-contaminants, that is contaminants which are not the primary contaminants of concern, may have appreciable effects on land treatment. Ecotoxicity tests may also be helpful in characterizing the performance of land treatment operations.
Numerous full-scale operations have been used, particularly for sludges produced by the petroleum industry. As with other biological treatments, under proper conditions, land treatment can transform contaminants into nonhazardous substances. Removal efficiencies, however, are a function of contaminant type and concentrations, soil type, temperature, moisture, waste loading rates, application frequency, aeration, volatilization, and other factors.
Ranges of costs likely to be encountered are:
Costs prior to treatment (assumed to be independent of volume to be treated): $25,000 to $50,000 for laboratory studies; about $100,000 for pilot tests or field demonstrations.
Cost of land treatment: $30 to $70 per cubic meter ($25 to $50 per cubic yard).