Influence of treatment processes and disinfectants on bacterial community compositions and opportunistic pathogens in a full-scale recycled water distribution system

In this study, for the first time, a recycled water distribution system was analysed in which the inorganic nitrogen content in the recycled water was substantially minimised (<0.2 mg/L) while the total chlorine residual was maintained above 0.1 mg/L (regulatory standard). The chemical parameters and bacterial community composition of the unique full-scale recycled water treatment and distribution system were comprehensively evaluated. The application of chlorine eliminated Escherichia coli (E. coli) and largely reduced the bacterial diversity, richness, and the number of species, despite the formation of organic chloramines (>85%) having minimal disinfection abilities. The majority of the bacterial communities were proteobacteria comprising alpha-, beta-, and gamma-proteobacteria, similar to the drinking water distribution system. The gene copy numbers of total bacterial 16S ribosomal ribonucleic acid (16S rRNA) and opportunistic pathogens (OPs) were significantly decreased after the chlorination, but their populations increased with the decrease of total chlorine residual level in the distribution system. The total bacterial 16S rRNA significantly correlated with Legionella spp., Mycobacterium spp., and Pseudomonas aeruginosa. Similarly, significant correlations existed between OPs (particularly Legionella spp. and Pseudomonas aeruginosa) and iron-oxidising, manganese-oxidising, and sulphate-reducing bacterial genera. The detection of several OPs in the absence of E. coli shows that the traditional indicator used for compliance monitoring may not accurately represent the microbial water quality. This study suggests monochloramine as an alternative secondary disinfectant.