Biodegradation of Aromatic Organic Compounds in Alternating Aerobic and Denitrifying Environments

Clemson University, Clemson SC

Investigators

Abstract

9988086 Grady Over 60% of the large production chemicals in the USA are aromatic organic compounds. Consequently, they are common pollutants in wastewaters and the environment. Although their biodegradabilities depend on their individual structures, one key feature, the aromatic ring, defines the nature of the biodegradation process. This is because bacteria use different enzyme systems to degrade aromatic compounds in the presence and absence of oxygen. Under aerobic conditions, the enzymes catalyzing them are only synthesized and active when oxygen is present. Under anaerobic conditions, the ring cleavage reactions are reductive. Therefore, the enzymes catalyzing them are only synthesized and active when oxygen is absent. This raises questions about how bacterial that are cycled frequently between aerobic and anaerobic conditions degrade aromatic compounds. Because of the environmental impacts associated with the discharge of nitrogen, nitrogen removal processes are being applied with increasing frequency for the treatment of wastewaters. Such processes employ both aerobic and anaerobic zones, with nitrate as the electron acceptor in the latter. In all of these situations one must ask how the variation in redox state of the environment influences the biodegradation of aromatic compounds. The purpose of the proposed research is to answer that question. Two types of experiments will be run with mixed cultures of bacteria grown in chemostats (continuous cultures without biomass recycle), which will be used because they allow close control over both the bacterial growth rate and the environmental conditions. In one, the chemostats will be subjected to alternating aerobic and anaerobic (with nitrate as the electron acceptor) conditions of different frequency and duration to ascertain how the nature of the imposed cycle influences the ability of the culture to remove aromatic compounds. In the other, small quantities of oxygen will be continuously injected into otherwise anaerobic chemostats containing nitrate. The results obtained from this study will be used to derive guidelines to aid engineers faced with the task of designing wastewater treatment systems that are capable of removing both nitrogen and aromatic compounds to very low levels. They will also provide a better understanding of how bacteria control oxygen-sensitive enzymes when grown in environments that have fluctuations in redox state, thereby helping scientists make a better predictions of the fate of aromatic compounds discharged to such environments. Finally, the results will help engineers determine the efficacy of adding nitrate as an electron acceptor to contaminated aquifers containing low levels of dissolved oxygen. ***