Formation and Removal of Toxic Disinfection Byproducts from Cross-Linked Polyethylene (PEX) Pipes in Building Premise Plumbing System

Gaopin Cao, Purdue University

Abstract

Cross-linked polyethylene (PEX) pipes are widely installed in the US, and the usage is increasing due to several advantages that PEX pipes have over other alternatives (e.g. copper) that are related to cost, flexibility, and environmentally friendly. However, these pipes are well known to leach total organic carbon (TOC), and thus affect overall drinking water quality within premise plumbing systems. One area that has not been explored is the potential for such pipes to form disinfection by-products (DBPs) from these leached organics in the presence of chlorinated waters. This issue is of concern since DBPs are regulated within the water treatment plant and distribution system but are not regulated within premise plumbing systems. This study aims to identify how PEX pipes can alter the water quality by: (i) forming DBPs including trihalomethanes (THMs) and nitrosamines from the leached TOC and potentially leached total organic nitrogen (TON), following exposure to free chlorine or chloramines, respectively, (ii) do so following exposure to various pipe types and varying water quality conditions including pH (6 – 9), disinfectant dosage (0.5 – 3.5 mg/L-Cl2), bromide concentration (0 – 250 μg/L), and temperature (22 and 55 °C), and (iii) also remove DBP from the aqueous phase through PEX pipe adsorption. Assessing the formation of DBPs were carried out kinetic and dose-dependent experiments in which synthetic waters were exposed to PEX pipes for 3 days, removed from the pipe and chlorinated for up to 5 d. Adsorption experiments were conducted by cutting the pipe into pieces, exposing the pipe pieces to THM with varying initial concentration or pipe piece weight. For all tested PEX pipes, nitrosamines were not formed (< 10 ng/L) following pipe exposure and chloramination for up to 5 days. Alternatively, THMs were formed after chlorination, especially for the PEX-a pipe, where bromide addition, increasing temperature, high pH and increasing disinfectant dosage led to approximate 2× higher THM concentrations than the PEX-free controls. From the standpoint of total overall THM formation, temperature led to the greatest increase in the CHCl3 concentration which was 5-fold higher at 55 °C than 22 °C and well exceeded the 80 μg/L THM regulatory limit. In addition, these PEX pipes were not only a source of DBPs but were also verified to adsorb these DBPs as a sink. At 22 and 55 °C, the percentage of total THMs removed equaled 48% and 80%, respectively, for all three PEX pipe types after 3 d. The kinetic curve of THM adsorption onto these PEX pipes was found to fit second-order adsorption model, and the isotherm fits fairly well to the Langmuir and Freundlich equation. Thus when chlorinated waters are exposed to PEX pipes, THMs are predicted to be formed and also removed simultaneously from the water. Overall, the potential for PEX pipe to both form and treat THMs needs to be considered when assessing overall drinking water quality and the regulations that control it.^

Degree

M.S.C.E.

Advisors

Amisha D. Shah, Purdue University.

Subject Area

Environmental engineering

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