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theses

Microbial pollution is one of the leading causes of surface water impairments. It is associated with wet weather events expected to increase due to climate change in many cities with outdated infrastructure. Of particular concern is the release of emerging microbial contaminants. Antibiotic resistance genes (ARGs) have been linked to elevated rates of antibiotic resistant infections, but significant gaps in understanding the environmental fate of ARG limit the ability to comprehensively assess threats to human health. The objective of this work was to investigate the treatment and fate of microbial agents in sewage, sewers, and wet weather flows towards a better understanding of the risk associated with their release into the environment.
A series of field and bench-scale studies were performed towards better understanding the water quality impact of wet weather flows and the performance of end-of-pipe treatment for combined sewer overflow effluent. (1) First, a field study was conducted to characterize the intra- and inter-storm variability in ARG and fecal marker genes concentrations and the microbial community structures during CSOs. The partitioning of gene targets onto settleable particles during CSO events was monitored to provide insight into end-of-pipe treatment and fate upon release to surface water. ARG, fecal indicator and wastewater signature at the outfall varied both during and between storms and timing of peak concentrations targets did not necessarily coincide. The majority of ARG and fecal indicator concentrations were attached to particles rather than in the free phase, suggesting promise for treatment by enhanced sedimentation i.e. hydrodynamic separation. (2) Then, further insight into potential treatment for ARG and fecal indicator marker genes was obtained by investigating peracetic acid (PAA) disinfection kinetics in simulated CSO effluent. Using viability-based qPCR, PAA was found to be an effective disinfectant for reducing concentrations of ARG originating from viable cells but was unsuccessful in destroying the DNA. PAA disinfection resulted in significant shifts in the microbial community. However, further treatment would be needed to remove or destroy ARG. (3) Finally, to complement the first two studies, the effectiveness of end-of-pipe treatment with removal of settleable particles and PAA disinfection was investigated with traditional cultivation-based methods. The potential for the treatment train to select for sul1 gene carrying E. coli and to promote regrowth of indicator organisms upon release to estuarine waters was investigated. The treatment train achieved >2 log removal of total coliform and E.coli with significant removal attributable to disinfection but not hydrodynamic separation, TSS removal by hydrodynamic separation may have enhanced disinfection. Incubation of surface water inoculated with treated CSO did not result in regrowth of fecal indicators. Although the proportion of E.coli carrying sul1was greater after disinfection, the concentration of E.coli CFU per 100 ml carrying sul1 decreased significantly in disinfected samples and these targets were not observed after a seven-day incubation period.
Towards understanding the availability of ARG for proliferation via different mechanisms, a field study was conducted to investigate the relative proportion of ARGs in cells with intact membranes to total ARGs observed in wastewater treatment plant (WWTP) effluent and receiving waters. ARGs in the effluent from three municipal wastewater treatment plants and the receiving surface waters was investigated using a viability-based qPCR technique (vPCR) with propidium monoazide (PMA). ARGs, fecal indicator marker genes BacHum, and 16S rRNA gene copies were found to be significantly lower in viable-cells than in total concentrations for WWTP effluent. Viable-cell and total gene copy concentrations were similar in downstream samples except for tet(G). Differences with respect to season in the prevalence of nonviable ARG in surface water or WWTP effluent were not observed. The results of this study indicate that qPCR may overestimate viable-cell ARG and fecal indicator genes in WWTP effluent but not necessarily in the surface water.
A final field study was conducted to characterize the factors that drive the loading of microbial agents in sewers. Sewer sediments represent an important source of contaminants released during overflow events. The amount of attenuation, growth and/or selection for antibiotic resistant microbes and other pathogens in this matrix is poorly understood. Sewer sediment and wastewater influent samples were collected from five wastewater collection systems over two seasons. ARG were more abundance in sewer sediments compared to wastewater. Differences in ARG concentrations between season and sewer type (separate vs. combined) were observed, but correlations between ARG and heavy metals were generally not observed.
Overall the results presented provide new insights into the fate of microbial contaminants in sewers, sewage, wet weather flows, and end-of-pipe treatment systems. The results of these studies will help inform future treatment for ARG from urban water sources and future risk assessments for these emerging contaminants.
 

ETD_9261

doi:10.7282/t3-781m-1f64

Ph.D.

Includes bibliographical references

by Alessia Eramo

ETD doctoral

The author owns the copyright to this work.