Microbial Ecology and Water Chemistry Impact Regrowth of Opportunistic Pathogens in Full-Scale Reclaimed Water Distribution Systems

Need for global water security has spurred growing interest in wastewater reuse to offset demand for municipal water. While reclaimed (i.e., nonpotable) microbial water quality regulations target fecal indicator bacteria, opportunistic pathogens (OPs), which are subject to regrowth in distribution systems and spread via aerosol inhalation and other noningestion routes, may be more relevant. This study compares the occurrences of five OP gene markers (Acanthamoeba spp., Legionella spp., Mycobacterium spp., Naegleria fowleri, Pseudomonas aeruginosa) in reclaimed versus potable water distribution systems and characterizes factors potentially contributing to their regrowth. Samples were collected over four sampling events at the point of compliance for water exiting treatment plants and at five points of use at four U.S. utilities bearing both reclaimed and potable water distribution systems. Reclaimed water systems harbored unique water chemistry (e.g., elevated nutrients), microbial community composition, and OP occurrence patterns compared to potable systems examined here and reported in the literature. Legionella spp. genes, Mycobacterium spp. genes, and total bacteria, represented by 16S rRNA genes, were more abundant in reclaimed than potable water distribution system samples (p ≤ 0.0001). This work suggests that further consideration should be given to managing reclaimed water distribution systems with respect to nonpotable exposures to OPs.