Continuous detoxification, transformation, and degradation of nitrophenols in upflow anaerobic sludge blanket (UASB) reactors

Abstract

The anaerobic transformation and degradation of nitrophenols by granular sludge was investigated in upflow anaerobic sludge blanket (UASB) reactors continuously fed with a volatile fatty acid (VFA) mixture as the primary substrate. During the start-up, subtoxic concentrations of 2-nitrophenol (2-NP), 4-nitrophenol (4-NP), and 2, 4-dinitrophenol (2, 4-DNP) were utilized. 4-NP and 2, 4-DNP were readily converted to the corresponding aromatic amine; whereas 2-NP was converted to nonaromatic products via intermediate formation of 2-aminophenol (2-AP). These conversions led to a dramatic detoxification of the mononitrophenols because the reactors treated the nitrophenolics at the concentrations which were over 25 times higher than those that caused severe inhibition. VFA removal efficiencies greater than 99% were achieved in both reactors at loading rates greater than 11.4 g COD per liter of reactor volume per day even at volumetric loading of mononitrophenols up to 910 mg/L . d.The sludges obtained from each of the reactors at the end of the continuous experiments were assayed for their specific nitrophenol reducing activity in the presence of different primary substrates. Reduction rates of 45 and 26 mg/g volatile suspended solids per day were observed for 2-NP and 4-NP, respectively, when utilizing the VFA mixture as primary substrate. Hydrogen, an interspecies-reduced compound, and substrates that provide interspecies-reducing equivalents-such as butyrate, propionate, and ethanol stimulated nitrophenol reduction, whereas acetate and methanol did not. Anaerobic batch biodegradability tests with the 2-NP-adapted sludge revealed that its corresponding aromatic amine, 2-AP, was degraded to methane at a specific rate of 14.5 mg/g VSS . d. Acetate was observed to be the major intermediate during 2-AP degradation in the presence of a specific methanogenic inhibitor 2-bromoethanesulfonate. The results of this study indicate that UASB reactors can be applied to rapidly detoxify and, under certain circumstances, degrade nitroaromatic compounds. (c) 1996 John Wiley & Sons, Inc.