State Water Resources Research Institute Program

Project ID: 2012AZ478B
Title: Does Increasing Solids Retention Time in the Wastewater Treatment Process Affect the Persistence of Antibiotic Resistance Genes?
Project Type: Research
Start Date: 3/01/2012
End Date: 2/28/2013
Congressional District: 7th
Focus Categories: Wastewater, Treatment, Toxic Substances
Keywords: Water Reuse, Antibiotic Resistance, Wastewater Treatment
Principal Investigators: Rock, Channah (University of Arizona); Abrell, Leif (University of Arizona)
Federal Funds: $ 7,300
Non-Federal Matching Funds: $ 14,662
Abstract: The conventional activated sludge (CAS) process exposes bacteria to both ideal growth conditions and relatively high concentrations of trace chemical pollutants. Though increased solids retention time (SRT) has been correlated with reductions in trace antibiotics, higher SRTs also provide prolonged exposure of bacteria to influent antibiotic levels, potentially increasing the development of antibiotic resistance (AR). The proposed study will assess the effects of varying SRT in full-scale activated sludge processes on the degradation of trace antibiotics and microbial selection for AR. A detailed assessment of rates in AR development and identification of bacterial processes contributing to AR will aid in technological advances to decrease the prevalence of AR in recycled water, alleviating environmental and public health concerns.

The proposed study will expand upon our preliminary results to provide a comprehensive evaluation of temporal variability in loadings of antibiotic concentrations, genes conferring AR to bacteria, and relative proportion of AR E. coli (Gram negative) and Enterococcus (Gram positive) in raw wastewater, activated sludge solids, and finished effluent.

Many investigators have recognized that wastewater treatment plants (WWTPs) are the principal recipients of enteric bacteria with multiple AR. However, few studies have examined the effects of different WWT strategies, including SRT optimization, on the prevalence of AR bacteria and/or AR genes in treated effluent. Researchers have examined antibiotic degradation through WWT processes and have reported fairly high effluent concentrations in the µg L-1 range. Thus, though increasing SRTs will increase microbial diversity and achieve more extensive degradation of antibiotics, the combination of rapid bacterial growth and high antibiotic concentrations may provide ideal conditions for the development of AR. Additional processes (e.g., secondary clarification and tertiary filtration) may further increase the proportion of AR bacteria in effluent water.

Though the related work indicates the potential for development of AR with increasing SRT, to date this phenomenon has not been studied in controlled experiments. A mass balance of AR bacteria, genes coding resistance, and the total microbial population is necessary to determine relative changes during WWT processes. There is also a need to identify factors (e.g., antibiotic concentrations, SRT, disinfection processes) most significantly impacting levels of AR genes leaving WWTPs. To date, no standard methods for evaluation of AR genes have been established for wastewater. Preliminary work performed by our research team has focused on the development of statistically robust and reproducible laboratory methods proposed for this study.

By monitoring several locations within the WWT train, this study will allow the project team to characterize the impact of WWT on AR prevalence and the downstream impacts on end-users and the environment. Ultimately, this study will provide utilities with new knowledge and tools for treatment process optimization and AR mitigation.

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