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With widespread occurrence and increasing concern of emerging contaminants (CECs) in source water, biologically active filters (BAF) have been gaining acceptance in water treatment. Both BAFs and graphene oxide (GO) have been shown to be effective in treating CECs. However, studies to date have not addressed interactions between GO and microbial communities in water treatment processes such as BAFs. Therefore, in the present study, we investigated the effect of GO on the properties and microbial growth rate in a BAF system. Synthesized GO was characterized with a number of tools, including scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and Raman spectrometry. GO exhibited the characteristic surface functional groups (i.e., C-OH, C=O, C-O-C, and COOH), crystalline structure, and sheet-like morphology. To address the potential toxicity of GO on the microbial community, reactive oxygen species (ROS) generation was measured using nitro blue tetrazolium (NBT) assay. Results revealed that during the exponential growth phase, ROS generation was not observed in the presence of GO compared to the control batch. In fact, the adenosine triphosphate (ATP) concentrations increased in the presence of GO (25 μg/L - 1000 μg/L) compared to the control without GO. The growth rate in systems with GO exceeded the control by 20 % to 46 %. SEM images showed that GO sheets can form an effective scaffold to promote bacterial adhesion, proliferation, and biofilm formation, demonstrating its biocompatibility. Next-generation sequencing (Illumina MiSeq) was used to characterize the BAF microbial community, and high-throughput sequencing analysis confirmed the greater richness and more diverse microbial communities compared to systems without GO. This study is the first to report the effect of GO on the microbial community of BAF from a water treatment plant, which provides new insights into the potential of utilizing a bio-optimized BAF for advanced and sustainable water treatment or reuse strategies. 

Extensive building closures due to the unprecedented COVID-19 pandemic resulted in long-term water stagnation within the plumbing of large buildings. This study examined water chemical quality deterioration in ten large buildings after prolonged stagnation caused by the closure of a university campus in response to the COVID-19 pandemic. Volume-based and constant-duration flushing protocols were implemented to replace stagnant water with fresh drinking water. The effectiveness of the developed water flushing protocols was examined by monitoring the disinfectant residuals, heavy metal concentrations and temperature for water samples collected from the buildings' point of entry (POE) and select water fixtures. More than 14 m 3 of water were flushed in all ten large buildings. The results demonstrated a significantly greater average total chlorine residual concentration in POE water samples collected after flushing (1.1 mg L −1 ) compared to the stagnant condition (0.6 mg L −1 ). For water samples collected from fixtures during the extended stagnation, chlorine was absent in 71% of samples from academic buildings and 69% of samples from athletic buildings. The effectiveness of flushing practices is underscored by increasing the median total chlorine concentration from <0.1 to 1.0 mg L −1 in academic buildings and from <0.1 to 0.75 mg L −1 in athletic buildings. Furthermore, the concentrations of Pb, Zn, and Cu had decreased following the water flushing, but the concentration of Fe had increased in some buildings. This study could be beneficial to prepare for prolonged water stagnation events including but not limited to pandemics.