Rethinking secondary disinfection in direct potable reuse: a risk perspective on opportunistic pathogens and antimicrobial resistance

Direct potable reuse (DPR) represents a technologically advanced and tightly controlled water supply strategy. However, legacy practices such as the emphasis on log-reduction credits for enteric pathogens and the routine application of secondary disinfection may not be appropriate nor effective for managing opportunistic pathogens (OPs) and antimicrobial resistance (AMR) in DPR systems. This paper critiques the use of disinfectant residuals from the perspective of microbial ecology, biofilm dynamics, growth and persistence of OPs, and AMR mechanisms. A case it presented that secondary disinfection can drive the selective pressure and subsequent enrichment of resistant taxa, destabilise advanced-treated water quality, and undermine the engineered principles underpinning DPR. Alternative approaches that prioritise infrastructure integrity, proactive monitoring, and ecological control may offer a more sustainable and resilient strategy for DPR, providing superior public health protection with a reduced AMR risk.