Effects of Organic Carbon on the Biodegradation of Estrone in Multiple Substrate, Mixed-Culture Systems

Abstract

This dissertation describes the study of the effect of organic carbon on the biodegradation of estrone (E1) in multiple substrate, mixed-culture systems. In exploring this topic, important degradation mechanisms related to organic carbon were tested to determine which, if any, play an important role. Additionally, the effects of organic carbon concentrations, loads, and quality on E1 degrading activity of cultures from a wastewater treatment system were determined. Catabolic repression effects on E1 degradation was studied by adding synthetic septage to an E1 degrading culture to determine if degradation rates were affected. No differences in first-order E1 degradation rates between test and control reactors were observed in the 2 h or 8 h period following the addition of synthetic septage, ruling out catabolic repression as an important mechanism in E1 degradation in wastewater treatment-like conditions. Cultures were grown in membrane bioreactors (MBRs) with and without exposure to E1 to determine if (i) E1 exposure is necessary for E1 degrading ability, and if so (ii) whether multiple substrate utilization and/or cometabolism play an important role in the degradation of E1. These cultures were capable of degrading E1 regardless of prior exposure. Higher rates of E1 degradation were observed in cultures with prior E1 exposure, and a lag phase of 6 h was observed in cultures without prior E1 exposure. These results indicate that E1 was degraded metabolically, demonstrating that multiple substrate utilization is the key mechanism for E1 degradation. Longer term effects of organic carbon concentrations on E1 degrading activity were explored by comparing cultures operating under starvation conditions and cultures operating on a daily feeding cycle. Cultures fed daily showed a large initial increase in E1 degradation activity, attributable to a corresponding increase in biomass. Subsequently, however, E1 degradation activity dropped substantially even though biomass continued to increase, suggesting that E1 degraders were outcompeted when subjected to repeated exposure to high organic carbon concentrations. Conversely, starvation cultures had moderate but sustained increases in E1 degradation rates. Another experiment using MBRs to distinguish organic loads from organic concentrations confirmed the positive effect of organic carbon loads on E1 degradation via biomass growth, indicating that high organic carbon concentrations rather than loads were responsible for the drop in E1 degradation rates. A follow-up study was carried out to determine if altering the duration between feeding cycles could mitigate the negative effects of high organic carbon concentrations on E1 degradation. When cultures were exposed to high organic carbon concentrations (600 mg COD/L over a 6 d period), increasing the duration between feeding cycles improved performance. Conversely, at lower organic carbon concentrations (180 mg COD/L over a 6 d period), no differences in E1 degrading activity was observed. Effects of organic carbon quality on E1 degradation were explored using aged synthetic septage and waters from various treatment and natural sources to culture mixed communities. In these experiments, spectrophotometric methods (specific UV absorbance, spectral slope ratios, excitation-emission matrices, and fluorescence index) were used to characterize organic carbon. Additional analyses and experiments were conducted to rule out organic carbon, nitrogen species, and trace element concentrations as complicating factors. These experiments showed that microbially-derived organic carbon was associated with E1 degrading ability, while organic carbon from natural water sources (river and lake) was not. Furthermore, the experiments with aged synthetic septage suggest that products from cell lysis and/or microbial products under stress by starvation may be important for E1 degradation. Overall, this work shows that multiple substrate utilizing bacteria are important for E1 degradation in wastewater treatment-like systems and indicates various organic carbon parameters that are vital for the selection of these bacteria.