Next-generation sequencing (NGS) and metagenomics were used to identify microbial communities in biofilms of microfiltration (MF) and reverse osmosis (RO) membranes in an advanced water purification facility (AWPF) that treats municipal wastewater to produce potable quality water. Secondary treated wastewater effluent is the source of influent to the AWPF treatment train and was also characterized by NGS. Results show low bacterial diversity in biofilms obtained from the feed-side surfaces of MF and RO membranes. Microbial communities and antibiotic resistance genes (ARGs) in the RO biofilm were compared with those of the MF and influent, revealing lower abundance of bacterial species and ARGs in the RO biofilm than in the other samples. Opportunistic pathogens were detected in all samples; however, indicator bacteria, viruses, and bacteriophages were not detected in the RO biofilm. It is concluded that NGS has great potential for improved detection and characterization of microbial communities in biofilms that form on AWPF MF and RO membranes.