Biodegradation of 2,4-Dinitrotoluene in the Waste Streams of a Munitions Plant
Wastewater from the manufacture of propellants typically contains 2,4-dinitrotoluene (DNT), a suspected animal carcinogen. Previous studies have indicated that DNT is aerobically biodegradable. However, inconsistent removal of DNT during aerobic treatment has been observed at a munitions wastewater treatment plant, necessitating the use of activated carbon pre-treatment. The objective of this study was to evaluate the effect of nutrient and cosubstrate amendments on the rate and extent of DNT removal. Addition of ethanol (100-500 mg/l) and phosphate (0.8-3.3 mg/l) significantly accelerated the rate of aerobic DNT (0.3-5.6 mg/l) biodegradation. Addition of phosphate alone also increased the rate of DNT degradation, but to a lesser degree. The presence of ethyl ether, another substrate commonly found in munitions plant wastewater, had comparatively little effect on the rate of DNT removal. Interruptions in the DNT manufacturing process can result in DNT being absent from the munitions plant wastewater for extended periods. The effect of such interruptions was evaluated in semi-continuously operated reactors, fed daily with phosphate-amended wastewater, at a hydraulic residence time of 3 days. DNT removal resumed without a lag even after it was absent from the feed for periods up to 15 days. During aerobic biodegradation of DNT, reduction to 4-amino-2-nitrotoluene and 2-amino-4-nitrotoluene was consistently observed, with reduction at the para position predominating. The highest level of aminonitrotoluene formation was 23% of the total DNT degraded. Aminonitrotoluene isomers were consumed shortly after they formed in the semi-continuously operated reactors, confirming the potential for degradation of these metabolites. Although the aminonitrotoluene isomers are not currently regulated, their presence in treated munitions wastewater is a concern due to possible toxicity.