Effects of ultraviolet irradiation on particles in secondary water supply systems: Interface changes, disinfection...

Loose deposit particles, primarily composed of iron corrosion products, ubiquitously exist in drinking water, continuously accumulating and migrating through drinking water distribution systems before entering secondary water supply systems (SWSSs). As ultraviolet (UV) disinfection becomes increasingly prevalent in SWSSs to ensure water safety, its potential to inadvertently influence toxic disinfection byproduct (DBP) formation and alter particle toxicity raises health concerns. This study investigated effects of UV irradiation (265 nm) on particles collected from field sampling. Results show that UV exposure progressively increased turbidity and facilitated dissolved organic matter (DOM) release from particles. Leaching and reabsorption of DOM inhibited particle agglomeration. Consequently, the particles exhibited a smaller size, an increased specific surface area, and reduced agglomeration. X-ray photoelectron spectroscopy revealed that UV exposure disrupted the association between iron oxides and organic matter. The presence of particles notably increased the formation potentials (FPs) of DBPs under UV irradiation. Following 30 min UV exposure, the FPs of trihalomethanes and haloacetic acids increased modestly (18.4 % and 38.6 %, respectively) as compared to particle-free controls, whereas that of the more toxic haloacetonitriles rose markedly, by 92.9 %. More importantly, UV irradiation intensified particle cytotoxicity, reducing liver cell viability from 93.2 % (0 min) to 86.0 % (30 min). Calculations based on elevated DBP levels in particle-containing groups showed more than 1.6-fold increases in cytotoxicity and genotoxicity indexes (compared to particle-free controls). This study reveals that UV irradiation during SWSSs significantly alters particle behavior, underscoring the imperative for targeted particle control strategies to safeguard public health.