Formation and sorption of trihalomethanes from cross-linked polyethylene pipes following chlorinated water exposure†
In recent years, cross-linked polyethylene (PEX) pipes have become more commonplace in building plumbing. However, their presence can strongly impact water quality by leaching organic carbon and sorbing contaminants, to and from the water supply, respectively. This study assessed how these processes could further impact concentrations of disinfection by-products (DBPs) and specifically trihalomethanes (THMs) when exposed to chlorinated water. One brand of three different PEX types (PEX-a, PEX-b, and PEX-c) were exposed to synthetic water, which was subsequently chlorinated over 120 h under varying water quality conditions (e.g. temperature, pH, bromide concentration, and free chlorine dose). Results indicated that THM formation was fairly modest for all three PEX types at 22 °C but increased by a factor of ∼2 at 55 °C for the PEX-a pipe. Other water quality conditions exhibited more limited effects. Pipe-storage time strongly affected leached organic carbon levels but did not affect THM formation. THM sorption also occurred for all three PEX types and was similarly controlled by temperature. Sorption data fit well to a kinetic adsorption model. Combined effects of THM formation and sorption at 22 °C generated no aqueous phase THMs, although this effect was not similarly examined at 55 °C. These results suggest that sorption may mitigate the effects of THM formation if PEX pipes continue to reside downstream of where formation occurred, but this may not be the case if other pipe types (e.g. metal pipes) are present. Overall, these findings have important regulatory consequences since current THM monitoring may or may not account for building PEX plumbing.