Award Date

December 2016

Degree Type


Degree Name

Doctor of Philosophy (PhD)


Civil and Environmental Engineering and Construction

First Committee Member

Daniel Gerrity

Second Committee Member

Dale Devitt

Third Committee Member

Donald Hayes

Fourth Committee Member

Jacimaria Batista

Fifth Committee Member

Eduardo Robleto

Number of Pages



Solids retention time (SRT) is one of the most important factors in designing and operating activated sludge systems for biological wastewater treatment. Longer SRTs have been shown to alter the structure and function of microbial communities, thereby leading to improved treatment efficacy with respect to bulk and trace organics, nutrient removal, and membrane fouling. However, research has also shown that longer SRTs lead to increased prevalence of antibiotic resistant bacteria, perhaps due to increased exposure to antibiotics present in influent wastewater. The purpose of this study was to characterize changes in microbial community structure in a laboratory-scale activated sludge system as a function of SRT (2-20 days) and influent concentrations (1x-100x ambient concentrations) of five antibiotics: ampicillin, sulfamethoxazole, tetracycline, trimethoprim, and vancomycin. Also, this research aimed to characterize the role of SRT and elevated antibiotic concentrations on AR proliferation in biological treatment processes. Changes in microbial community structure were evaluated based on traditional plating methods and 16s rDNA sequencing, and microbial community function was evaluated based on changes in effluent water quality, including bulk organic matter characterization and antibiotic concentrations. Spread plate technique was used to determine the number of Gram positive Staphylococcus/Streptococcus strains. The extent of AR was also determined based on minimum inhibitory concentrations (MICs) of resistant isolates. The results indicated that SRT—but not antibiotic loading—had a significant impact on microbial community structure (e.g., reduction in relative prevalence of Acinetobacter and Arcobacter) and effluent water quality. Therefore, spikes in influent antibiotics (at sub-therapeutic concentrations) are not expected to adversely impact biological wastewater treatment. The results revealed that longer SRTs and higher antibiotic concentrations select for antibiotic resistant bacteria (ARBs).The data obtained from this study suggests that longer SRTs may select for trimethoprim- resistant bacteria and/or result in false positives for trimethoprim resistance due to higher concentrations of free thymine or thymidine.


Antibiotic Resistance; Antibiotics; Biological Treatment; Soldis Retention Time; Wastewater


Civil Engineering | Environmental Engineering

File Format


Degree Grantor

University of Nevada, Las Vegas




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