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The Efect of Operational Parameters on the Fate of Tetracycline Resistant Bacteria in Biological Wastewater Treatment Plants
A laboratory study was conducted to examine the influence of key operating conditions on the fate of tetracycline resistant organisms in the activated sludge processes. Four lab-scale sequencing batch reactors (SBRs) were operated to simulate the activated sludge process in three phases. To evaluate two hypotheses (the effects of antibiotic presence and wastewater feed pattern simultaneously), four lab-scale biological suspended growth sequencing batch reactors (SBRs) were employed. To address the issue of how influent tetracycline concentration influences tetracycline resistant organism survival, two of the reactors were operated with antibiotic added to the influent wastewater (A types) and two served as controls with no antibiotic added (B types). To test the feed pattern influence on tetracycline resistant organism survival, each subset of two SBRs (based on antibiotic addition) was further subdivided by the rate of influent addition. For two reactors, influent was added nearly instantaneously (over a two minute period) simulating a pulse load, called ÒslugÓ. The other two reactors had influent added slowly and continuously over the appropriate feed cycle time and were considered to more closely mimic a continuously fed reactor, called ÒcontÓ. In Phase 1, the overall growth rate was fixed in all reactors by controlling the mean cell residence time (MCRT) at 10 days. A hydraulic residence time (as defined by volume/flowrate) of 24 hrs was applied during Phase 1 experiments. To evaluate the effects of a higher organic loading rate, the hydraulic detention time in Phase 2 studies was decreased to 7.4 hrs which increased the organic loading rate by over three-fold. All other conditions, including sampling, from Phase 1 were maintained. To evaluate the importance of growth rate, SBRs were operated in Phase 3 at a MCRT of 3 days and a hydraulic residence time of 7.4 hrs. These data were compared to that collected in Phase 2 to evaluate the effect of growth rate changes. In addition, the fate of tetracycline was assessed in each of the studied bioreactors. For both the continuous and slug operated SBRs, one SBR received an influent of 250 µg/L while the other SBR only received the background tetracycline concentration that was present in the Amherst, NY wastewater. To elucidate tetracycline removal mechanisms, adsorption experiments were conducted and these data are presented also. From this research, several conclusions were delineated. Domestic wastewater contains significant numbers of tetracycline intermediate resistant and resistant heterotrophic, enteric, and lactose-fermenting bacteria. Amherst WWTP, primary effluent, used in this study as the influent, contained intermediate resistant and resistant heterotrophic bacteria. Cultured bacteria numbers showed similar ranges with the investigated bacterial counts which were previously reported. Background total tetracycline concentrations in the influent were below 1 µg/L throughout the study. Average tetracycline removal in excess of 75 percent was observed in all tetracycline fed SBRs. Sorption of tetracycline on biomass was a significant contributor to the observed removal. Tetracycline increased the concentrations, net specific growth rate and percentages of tetracycline intermediate and resistant bacteria under higher organic loading and increased growth rate conditions. There was no conclusive evidence that tetracycline resistance was promoted under either slug or continuous feed patterns. Increased concentrations, percentages and EC50 and EC90 concentrations of tetracycline intermediate and resistant bacteria were observed under increased organic loading rate conditions. Higher growth rate increased the net specific growth rate of tetracycline intermediate and resistant bacteria, the percentages of heterotrophic resistance, and EC50 and EC90 concentrations of heterotrophic bacteria under tetracycline fed conditions.