Enhanced bioremediation is a process in which indigenous or inoculated micro-organisms (e.g., fungi, bacteria, and other microbes) degrade (metabolize) organic contaminants found in soil and/or ground water, converting them to innocuous end products. Nutrients, oxygen, or other amendments may be used to enhance bioremediation and contaminant desorption from subsurface materials.
In the presence of sufficient oxygen (aerobic conditions), and other nutrient elements, microorganisms will ultimately convert many organic contaminants to carbon dioxide, water, and microbial cell mass.
In the absence of oxygen (anaerobic conditions), the organic contaminants will be ultimately metabolized to methane, limited amounts of carbon dioxide, and trace amounts of hydrogen gas. Under sulfate-reduction conditions, sulfate is converted to sulfide or elemental sulfur, and under nitrate-reduction conditions, dinitrogen gas is ultimately produced.
Sometimes contaminants may be degraded to intermediate or final products that may be less, equally, or more hazardous than the original contaminant. For example, TCE anaerobically biodegrades to the persistent and more toxic vinyl chloride. To avoid such problems, most bioremediation projects are conducted in situ. Vinyl chloride can easily be broken down further if aerobic conditions are created.
Enhanced bioremediation of soil typically involves the percolation or injection of ground water or uncontaminated water mixed with nutrients and saturated with dissolved oxygen. Sometimes acclimated microorganisms (bioaugmentation) and/or another oxygen source such as hydrogen peroxide are also added. An infiltration gallery or spray irrigation is typically used for shallow contaminated soils, and injection wells are used for deeper contaminated soils.
Although successful in situ bioremediation has been demonstrated in cold weather climate, low temperature slows the remediation process. For contaminated sites with low soil temperature, heat blankets may be used to cover the soil surface to increase the soil temperature and the degradation rate.
Enhanced bioremediation may be classified as a long-term technology which may take several years for cleanup of a plume.
Typical Enhanced Bioremediation System
Bioremediation techniques have been successfully used to remediate soils, sludges, and ground water contaminated with petroleum hydrocarbons, solvents, pesticides, wood preservatives, and other organic chemicals. Bench- and pilot-scale studies have demonstrated the effectiveness of anaerobic microbial degradation of nitrotoluenes in soils contaminated with munitions wastes. Bioremediation is especially effective for remediating low level residual contamination in conjunction with source removal.
While bioremediation (nor any other remediation technology) cannot degrade inorganic contaminants, bioremediation can be used to change the valence state of inorganics and cause adsorption, immobilization onto soil particulates, precipitation, uptake, accumulation, and concentration of inorganics in micro or macroorganisms. These techniques, while still largely experimental, show considerable promise of stabilizing or removing inorganics from soil.
Factors that may limit the applicability and effectiveness of the process include:
- Cleanup goals may not be attained if the soil matrix prohibits contaminant-microorganism contact.
- The circulation of water-based solutions through the soil may increase contaminant mobility and necessitate treatment of underlying ground water.
- Preferential colonization by microbes may occur causing clogging of nutrient and water injection wells.
- Preferential flow paths may severely decrease contact between injected fluids and contaminants throughout the contaminated zones. The system should not be used for clay, highly layered, or heterogeneous subsurface environments because of oxygen (or other electron acceptor) transfer limitations.
- High concentrations of heavy metals, highly chlorinated organics, long chain hydrocarbons, or inorganic salts are likely to be toxic to microorganisms.
- Bioremediation slows at low temperatures.
- Many of the above factors can be controlled with proper attention to good engineering practice.
Important contaminant characteristics that need to be identified in an enhanced bioremediation feasibility investigation are their potential to leach (e.g., water solubility and soil sorption coefficient); their chemical reactivity (e.g., tendency toward nonbiological reactions, such as hydrolysis, oxidation, and polymerization); and, most importantly, their biodegradability.
Soil characteristics that need to be determined include the depth and areal extent of contamination; the concentration of the contaminants; soil type and properties (e.g., organic content, texture, pH, permeability, water-holding capacity, moisture content, and nutrient level); the competition for oxygen (e.g., redox potential); the presence or absence of substances that are toxic to microorganisms; concentration of other electron acceptors, nutrients; and the ability of microorganisms in the soil to degrade contaminants.
Treatability or feasibility tests are performed to determine whether enhanced bioremediation is feasible in a given situation, and to define the remediation time frame and parameters. Field testing can be performed to determine the radius of influence and well spacing and to obtain preliminary cost estimates.
The main advantage of the in situ process is that it allows soil to be treated without being excavated and transported, resulting in less disturbance of site activities. If enhanced bioremediation can reach the cleanup goal in a compatible time frame, it can save significant costs over methods involving excavation and transportation. Also, both contaminated ground water and soil can be treated simultaneously, providing additional cost advantages. In situ processes generally require longer time periods, however, and there is less certainty about the uniformity of treatment because of the inherent variability in soil and aquifer characteristics and difficulty in monitoring progress.
Remediation times are often years, depending mainly on the degradation rates of specific contaminants, site characteristics, and climate. Less than 1 year may be required to clean up some contaminants, but higher molecular weight compounds take longer to degrade.
There is a risk of increasing contaminant mobility and leaching of contaminants into ground water. Regulators often do not accept the addition of nitrates or non-native microorganisms to contaminated soils. Enhanced bioremediation has been selected for remedial and emergency response actions at an increasing number of Superfund sites. Generally, petroleum hydrocarbons can be readily bioremediated, at relatively low cost, by stimulating indigenous microorganisms with nutrients.
Typical costs for enhanced bioremediation range from $30 to $100 per cubic meter ($20 to $80 per cubic yard) of soil. Variables affecting the cost are the nature and depth of the contaminants, and use of bioaugmentation.
Additional cost information can be found in the Hazardous, Toxic, and Radioactive Wastes (HTRW) Historical Cost Analysis System (HCAS) developed by Environmental Historical Cost Committee of Interagency Cost Estimation Group.