Bioremediation and Contaminates

Classification of Bacteria
Bacteria are classified in several different ways, all of which are based on different aspects of the metabolic process. In terms of carbon sources, bacteria are divided into autotrophs, which use carbon dioxide as their carbon source, and heterotrophs, which derive carbon from organic compounds. Additionally, bacteria are grouped according to their source of external energy: phototrophs, which derive energy from photosynthesis, and chemotrophs, which obtain energy from the oxidation of chemical substances.

Based on these two classifications, there can be four basic types of bacteria based on their external sources of energy and carbon. Bacteria that play a key role in bioremediation derive their carbon and energy from organic compounds.

In the wastewater industry, bacteria have traditionally been classified based on their oxygen requirements: aerobic bacteria, which convert their food source (organic compounds) to energy by transferring electrons from the compounds to oxygen (electron acceptor): anaerobic bacteria, which metabolize their food in the absence of oxygen and instead utilize inorganic chemicals such as nitrates, sulfates, carbon dioxide, or metals such as iron as substitute electron acceptors; and facultative bacteria, which can function in both aerobic and anaerobic environments. United-Tech formulas use facultative bacteria that are heterotrophs.

In a report published by the National Research Council (NRC), three metabolic processes were identified as playing the most significant roles in bioremediation: aerobic, anaerobic, and cometabolism (National Research Council, 1993). In cometabolism reactions, transformation of a specific contaminant occurs indirectly, i.e., as the result of the metabolism of another substance, which otherwise could not be metabolized by the bacteria themselves.

Factors and Conditions Affecting Bioremediation
In-situ bioremediation is highly dependent on site conditions and soil properties, more so than Soil Vapor Extraction/Air Sparging. Factors that play a significant role in the design and successful operation of a bioremediation system include contaminant characteristics, natural supplies of macronutrients and micronutrients, availability of electron acceptors, and subsurface characteristics.

Contaminant Characteristics
The biodegradability of petroleum products is dependent on the chemical structure of its various components. In general, the lighter, more soluble petroleum hydrocarbons are more biodegradable than the heavier, less soluble members of the group. A compound's resistance to biodegradation increases with increasing molecular weight. Additionally, highly viscous hydrocarbons are less successfully biodegraded because of the inherent physical difficulty in establishing contact among the contaminate and the microorganisms, nutrients, and electron acceptors. For example, gasoline, which is considered more easily biodegradable than diesel fuel, has a solubility of 50 to 100 ppm and a viscosity of 0.5 to 0.6 centistokes as compared to a solubility of less than 1 ppm and a viscosity of 2 to 4 centistokes for diesel (Table, Cole, 1994). Similarly, diesel is more biodegradable than used oil, which has a solubility of less than 1 ppb and a viscosity of 40 to 600 centistokes.

Solubility and Viscosity of Data of
Representative Petroleum Products

From Cole, M.G., Assessment and Remediation of Petroleum Contaminated Sites, CRC Press, Boca Raton, FL, 1994, 63. With permission.

Simpler chemical structures are also easier to degrade. Branched structures degrade at a slower rate than the corresponding straight-chain hydrocarbons. Alkanes are degraded more rapidly than aromatic compounds. Monoaromatic compounds such as BTEX are broken down faster than the two-ring compounds such as naphthalene.

Some chemicals may be toxic to the microbes. In some cases, compounds that are readily biodegradable in low concentrations may exhibit toxicity characteristics to the microorganisms at high levels.

United-Tech's all natural bacteria and enzyme formulas perform in the broadest possible physical and chemical treatment parameters to achieve the fastest breakdown of contaminates. United-Tech's formulas have proven in the field to not only breakdown the complete BTEX DRO-GRO ranges but a large number of toxic organic chemicals in the unheard of time of just three to five weeks or less.