SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

Bioremediation - 2 - 10 September 1993 free oxygen then the reaction is anaerobic and the end products are methane, if the acceptor is nitrate, or hydrogen sulfide, if the acceptor is sulfate. The breakdown of 02 molecules produces energy. The breakdown of nitrate and sulfate molecules is not as energetically favorable as the breakdown of OZ. Therefore, anaerobic degradation rates are typically slower than aerobic rates. Under fermenting conditions organic compounds serve as both electron donors and acceptors. The by-products of the fermentation process are organic acids and alcohols. Because of water quality and aesthetic effects, the by-products of both anaerobic degradation and fermentation may pose greater remedial management problems than those associated with the aerobic decomposition pmdncts, CO2 and water. The indigenous microbial consortia at the site of a spill is distorted by contamination. Within the source area the toxicity limits for most microorganisms may have been exceeded and a sterile zone was at least temporarily created. With time, many species adapt to the contamination as a substrate (food source) and the composition of the consortia changes. Those species capable of degrading the contaminant compounds become dominant. With petroleum hydrocarbon contamination, the microbes have been observed to adapt within a few days to a few weeks. The more time that has elapsed since the contamination event, the more fully adapted the microbes will be, Both aerobic and anaerobic biodegradation at contaminated sites occurs naturally. However, the addition of large quantities of hydrocarbons creates a nutritional imbalance in which electron acceptors and micronutrients become limiting factors. The nutritional imbalance may be corrected by the addition of oxygen, nitrogen, phosphorus and trace minerals. Biodegradation natural processes may also he limited by temperature, moisture content, salinity, metal toxicity, and the toxicity of petroleum. For aerobic biodegradation, oxygen is usually the primary limiting compound, with most natural degradation occurring on the periphery of a source area or plume. The degradation rate is determined by the advection and diffusion rates of oxygen into the contaminant zone. Increasing the available oxygen may increase degradation rates and spread activity into the source area and plume. Oxygen may be added in the form of air, oxygen gas, or hydrogen peroxide (1120,). Because microbes will consume oxygen as the hydrocarbons are degraded, an aerobic ground water can quickly become anaerobic. This onset of anaerobic conditions is the most significant factor in limiting the rate of biodegradation in ground water. In addition, oxygen has a relatively low solubility in water and it can be difficult to deliver large amounts of % into an aquifer. The source area can often be anoxic because of its high levels of chemical oxygen demand (COD). Biodegradation in the source area can be exclusively anaerobic. For some compounds such as trichloroethylene (TCE), there is no aerobic degradation pathway. At low oxygen levels, denitrification will proceed if nitrate is available. The process also requires about 10 ,ug/l Fel` to maintain the reactions. Under controlled aerobic biodegradation, denitrification is to be avoided by maintaining sufficient oxygen supply. Denitrification has been studied as an alternative to aerobic biodegradation of hydrocarbons because nitrate is very soluble in water and can be easily and uniformly distributed throughout an aquifer. ZO ' d 800' oN ££.6 £6' 9T daS ON 131