Field evidence for the impact of aged cast iron pipes on emerging DBPs in drinking water distribution systems

Aged cast iron pipes are widely known to cause aesthetic problems such as "yellow water" in drinking water distribution systems (DWDS), but their influence on disinfection by-product (DBP) formation has been less studied, especially for emerging DBPs. Field-based evidence on how aged pipes affect DBPs under realistic distribution conditions is still limited. In this study, a representative urban DWDS in northern China was investigated to evaluate the impact of aged cast iron pipes on emerging DBP occurrence, compared with lined ductile iron pipes. High-resolution Fourier transform ion cyclotron resonance mass spectrometry was applied as a non-target analytical tool to characterize molecular-level changes in dissolved organic matter and to identify unknown halogenated and nitrogen-containing DBPs in finished and distribution water, while regulated DBPs were quantified using GC-based methods. Along aged cast iron pipe sections, the number of Cl-containing DBPs increased from 60 to 113 in one pipe and from 72 to 229 in another, while multiple subclasses of nitrogen-containing DBPs also showed clear increases. By contrast, regulated DBPs, including haloacetic acids and trihalomethanes, showed relatively minor changes during distribution. At the same time, higher iron concentrations and dissolved organic carbon were observed at locations with more DBPs, suggesting a linkage between corrosion-related processes and DBP formation. Meanwhile, in the lined ductile iron pipes with the same finished water, the abundances of emerging DBPs were much lower. Overall, the results indicate that aged cast iron pipes are associated with enhanced formation of emerging DBPs in DWDS and highlight the usefulness of high-resolution mass spectrometry for more reliable DBP assessment in distribution systems.