BOOK OF ABSTRACTS 67 IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 Tuesday, 25 August 2026 / Hall B+C Research Frontiers SL27 High Molecular Weight Disinfection Byproducts in Nitrifying Drinking Water Distribution Systems Marlena Marie Cheshire1, William A. Mitch1 1 Department of Civil and Environmental Engineering, Stanford University, 473 Via Ortega, Stanford, California 94305, United States Recent research indicates that the poorly characterized high-molecular weight disinfection byproduct (DBP) fraction (>2 carbons) contributes more to cytotoxicity than the 1-2 carbon DBPs of current interest. Given the aromatic nature of NOM, haloaromatics represent a critical component of this nonvolatile high molecular weight DBP fraction. However, little research has been done on the quantification of haloaromatics alongside conventional aliphatic DBPs in authentic drinking water distribution systems. Nitrification in chloraminated distribution system storage facilities represents a common operational condition that decreases disinfectant residuals and promotes the growth of nitrifying biofilms that actively release DBP precursors, including nitrogen- and aromatic-rich moieties. Given that DBP formation and toxicity are strongly influenced by the characteristics of natural organic matter, the contribution of nitrifying biofilm may lead to preferential formation of high molecular weight DBP classes. This study compares the concentrations of haloaromatic DBPs (6 halonitrophenols, 8 halosalicylic acids, and 12 halophenols) to 28 conventional 1–2 carbon DBPs across five chloraminated drinking water distribution storage tanks under ongoing nitrifying and non-nitrifying conditions. We also quantified biomolecule-derived DBPs (chlorinated tyrosine and chlorinated fatty acids) to highlight the contribution of peptides and lipids as DBP precursors. To evaluate the impacts of nitrification control strategies, we tested DBP formation under chloramine residual boosting and breakpoint chlorination. Results demonstrate that nitrifying biofilms act as sources of DBP precursors and preferentially form nitrogenous DBPs (halonitrophenols). Specifically, we found that total halonitrophenols concentration in nitrifying tanks were significantly higher than the halonitrophenols concentration in non-nitrifying tanks of the same source water. Under breakpoint chlorination conditions, total halonitrophenols concentrations in nitrifying tanks were up to 8 times greater than the halonitrophenol concentrations in comparative non-nitrifying waters (concentration of 170ng/L in nitrifying tank compared to 20ng/L in nonnitrifying tank). Thus, the organic matter contribution of nitrifying biofilm in storage tanks poses unique challenges to DBP formation and associated drinking water toxicity within distribution systems.
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