Operation Conditions

As with other organics, biodegradation of explosive wastes is influenced by temperature, oxygen supply, nutrient supply, pH, the availability of the contaminant to the microorganism, the concentration of the wastes, and the presence of substances toxic to the microorganisms (e.g.,

Figure 3 Stepwise reduction of RDX through reduction of nitroso groups.

mercury) [3,12]. Therefore, operation conditions must be carefully controlled in order to reach optimal treatment results for explosive wastes.

Nutrients required for cell growth are nitrogen, phosphorus, potassium, sulfur, magnesium, calcium, manganese, iron, zinc, and copper. If nutrients are not available in sufficient amounts, microbial activity will stop. This is common in explosive wastes. Phosphates are suspected to cause soil plugging as a result of their reaction with minerals, such as iron and calcium. They form stable precipitates that fill the pores in the soil and aquifer. Nitrogen and phosphorus are the nutrients most likely to be deficient in the contaminated environment and thus are usually added to the bioremediation system in a useable form (e.g., as ammonium for nitrogen and as phosphate for phosphorus) [3]. pH affects the solution chemistry, for example, solubility of compounds, which can affect biological activity. Many metals 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. Oxygen level is very important in the biological remediation of explosive wastes.

Temperature affects microbial activity in the environment. The biodegradation rate would slow down 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. Provisions for heating the bioremediation site, such as use of warm air injection, may speed up the remediation process. Too high a temperature, however, can be detrimental to some microorganisms, essentially sterilizing the aquifer. It can also affect nonbiological losses of contaminants, mainly through the evaporation 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.

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