In Situ Biological Treatment for Soil, Sediment, and Sludge
The main advantage of in situ treatment is that it allows soil to be treated without being excavated and transported, resulting in potentially significant cost savings. However, in situ treatment generally requires longer time periods, and there is less certainty about the uniformity of treatment because of the variability in soil and aquifer characteristics and because the efficacy of the process is more difficult to verify.
Bioremediation techniques are destruction techniques directed toward stimulating the microorganisms to grow and use the contaminants as a food and energy source by creating a favorable environment for the microorganisms. Generally, this means providing some combination of oxygen, nutrients, and moisture, and controlling the temperature and pH. Sometimes, microorganisms adapted for degradation of the specific contaminants are applied to enhance the process.
Biological processes are typically implemented at low cost. Contaminants can be destroyed, and often little to no residual treatment is required. However, the process requires more time, and it is difficult to determine whether contaminants have been destroyed. Biological treatment of PAHs leaves less degradable PAHs (cPAHs) behind. These higher molecular weight cPAHs are classified as carcinogens. Also, an increase in chlorine concentration leads to a decrease in biodegradability. Some compounds, however, may be broken down into more toxic byproducts during the bioremediation process (e.g., TCE to vinyl chloride). For in situ applications, these byproducts may be mobilized to ground water or contacted directly if no control techniques are used. This type of treatment scheme requires soil, aquifer, and contaminant characterization, and may require extracted ground water treatment. Ground water with low level contamination may sometimes be recirculated through the treatment area to supply water to the treatment area.
Although not all organic compounds are amenable to biodegradation, bioremediation techniques have been successfully used to remediate soils, sludges, and ground water contaminated by petroleum hydrocarbons, solvents, pesticides, wood preservatives, and other organic chemicals. Bioremediation is not applicable for treatment of inorganic contaminants.
The rate at which microorganisms degrade contaminants is influenced by the specific contaminants present and their concentrations, oxygen supply, moisture, temperature, pH, nutrient supply, bioaugmentation, and cometabolism. In situ biological treatment technologies are sensitive to certain soil parameters. For example, the presence of clay or humic materials in soil cause variations in biological treatment process performance. Treatability studies are typically conducted to determine the effectiveness of bioremediation in a given situation. These parameters are discussed briefly in the following paragraphs.
Oxygen level in the soil is increased by avoiding saturation of the soil with water, the presence of sandy and loamy soil as opposed to clay soil, avoiding compaction, avoiding high redox potential, and low concentrations of degradable materials. To ensure that oxygen is supplied at a rate sufficient to maintain aerobic conditions, forced air or hydrogen peroxide injection can be used. The use of hydrogen peroxide is limited because at high concentrations (above 100 ppm, or 1,000 ppm with proper acclimation), it is toxic to microorganisms. Also, hydrogen peroxide tends to decompose into water and oxygen rapidly in the presence of some soil constituents.
Anaerobic conditions may be used to degrade highly chlorinated contaminants, although at a very slow rate. This can be followed by aerobic treatment to complete biodegradation of the partially dechlorinated compounds as well as the other contaminants.
Water serves as the transport medium through which nutrients and organic constituents pass into the microbial cell and metabolic waste products pass out of the cell. Too much water can be detrimental, however, because it may inhibit the passage of oxygen through the soil (unless anaerobic conditions are desired).
Nutrients required for cell growth are nitrogen, phosphorous, potassium, sulfur, magnesium, calcium, manganese, iron, zinc, copper, and trace elements. If nutrients are not available in sufficient amounts, microbial activity will become limited. Nitrogen and phosphorous are the nutrients most likely to be deficient in the contaminated environment. These are usually added to the bioremediation system in a useable form (e.g., as ammonium for nitrogen and as phosphate for phosphorous). Phosphates can cause soil plugging as a result of their reaction with minerals, such as iron and calcium, to form stable precipitates that fill the pores in the soil and aquifer.
pH affects the solubility, and consequently the availability, of many constituents of soil, which can affect biological activity. Many metals that are potentially toxic to microorganisms are insoluble at elevated pH; therefore, elevating the pH of the treatment system can reduce the risk of poisoning the microorganisms.
Temperature affects microbial activity in the environment. The biodegradation rate will slow with decreasing temperature; thus, in northern climates bioremediation may be ineffective during part of the year unless it is carried out in a climate-controlled facility. The microorganisms remain viable at temperatures below freezing and will resume activity when the temperature rises.
Heating the bioremediation site, such as by use of warm air injection, may speed up the remediation process. At Eielson AFB, Alaska, passive solar warming by incubation tanks (ex situ) or the application of heated water below the ground surface to the contaminated vadose zone is being investigated. Too high a temperature can be detrimental to some microorganisms, essentially sterilizing the soil.
Temperature also affects nonbiological losses of contaminants mainly through the increased volatilization of contaminants at high temperatures. The solubility of contaminants typically increases with increasing temperature; however, some hydrocarbons are more soluble at low temperatures than at high temperatures. Additionally, oxygen solubility decreases with increasing temperature.
Bioaugmentation involves the use of microbial cultures that have been specially bred for degradation of specific contaminants or contaminant groups and sometimes for survival under unusually severe environmental conditions. Sometimes microorganisms from the remediation site are collected, separately cultured, and returned to the site as a means of rapidly increasing the microorganism population at the site. Usually an attempt is made to isolate and accelerate the growth of the population of natural microorganisms that preferentially feed on the contaminants at the site. In some situations different microorganisms may be added at different stages of the remediation process because the contaminants in abundance change as the degradation proceeds. USAF research, however, has found no evidence that the use of non-native microorganisms is beneficial in the situations tested.
Cometabolism uses microorganisms growing on one compound to produce an enzyme that chemically transforms another compound on which they cannot grow.
Treatability or feasibility studies are used to determine whether bioremediation would be effective in a given situation. The extent of the study can vary depending on the nature of the contaminants and the characteristics of the site. For sites contaminated with common petroleum hydrocarbons (e.g., gasoline and/or other readily degradable compounds), it is usually sufficient to examine representative samples for the presence and level of an indigenous population of microbes, nutrient levels, presence of microbial toxicants, and soil characteristics such as pH, porosity, and moisture.
Statistical characterization techniques should be used to represent 'before' and 'after' situations to verify biological treatment effectiveness.
Available in situ biological treatment technologies include bioventing, enhanced biodegradation, landfarming, natural attenuation, and phytoremediation.