Occurrence and proliferation of antibiotics and antibiotic resistance during wastewater treatment

Wastewater treatment plants play an important role in protecting human and environmental health from pollution in wastewater, but they are also considered one of the “leading reservoirs” of antibiotic resistance (AR) in the environment. The main objective of this research was to characterize the impact of solids retention time (SRT) in activated sludge systems on the quantity and extent of AR in treated effluent. The quantity of AR was determined based on the number of culturable Gram positive bacteria that were resistant to a suite of single or combined antibiotics at standard clinical concentrations. The extent of AR was determined based on minimum inhibitory concentrations (MICs) of Gram positive isolates. The results showed that longer solids retention times appear to select for antibiotic resistant bacteria, presumably because of the microbial community’s extended exposure to antibiotics in raw wastewater and the greater potential for horizontal gene transfer. In addition, it was observed that longer SRTs potentially promote bacteria that are increasingly resistant to antibiotics. Finally, the research confirms that longer SRTs achieve lower effluent concentrations of trace organic compounds (TOrCs). The reason for this trend appears to be a combination of changes in the microbial community and higher mixed liquor suspended solids concentrations.