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Impacts of tuberculated iron and surface biofilm on trihalomethane formation in chlorinated drinking water

University of British Columbia

This research investigated the interactions between chlorine residual in drinking water and the inner surface of distribution system pipes. Specifically, tuberculated cast-iron pipes were targeted, and their effect on the evolution of trihalomethane (THM, i.e. chloroform) byproducts was the essential focus. The project was carried out in two major phases, following a preliminary feasibility stage. A field-monitoring program, spanning a one-year period, collected fundamental data on chloroform occurrence in a real distribution system containing a significant inventory of old, tuberculated cast-iron water mains. A laboratory experimental phase complimented these observations under more controllable conditions. Several novel techniques and original procedures were developed during the implementation of both phases. Preliminary studies revealed the definite potential of iron tubercles to form and release chloroform by-product to waters in the presence of a chlorine residual. Incubation of chlorinated, organic-free water in resurrected cast-iron pipes showed significant contribution of THM precursors associated with the inner pipe surface. In the field study, an attempt was made to delineate chloroform formation change as a result of exposure to distribution system piping, in the absence of accurate measures of residence times. This included comparisons of both preformed trihalomethanes and trihalomethane formation potentials, at various locations within the system chosen to represent a range of exposure times. Only the most distant sampling station, corresponding to the greatest pipe residence time, revealed a significant reduction in overall chloroform formation. The many confounding factors (initial chlorine application variability, mixing of water ages, temperature change due to pipe friction, demand variability, uncertainty in residence times) necessitated the inclusion of hydraulic modelling as an important aspect of the data interpretation. The "split plug flow" model was developed specifically to ascertain average water age attributable to the sampling locations, as well as to estimate total exposure time to specific pipe material and condition. Statistical correlation revealed a strong relationship between chloroform formation efficiency and exposure to tuberculated cast-iron pipes. A modification to the current "simulated distribution system" (SDS) test was developed for the laboratory experimental phase. This "material specific" (MS-SDS) procedure allowed disinfection by-product formation to be investigated in the same environment encountered in actual distribution systems. MS-SDS testing showed that pipe-generated chloroform (i.e. "benthic" THM) is dependent on initial chlorine dose, and that release to the water phase is dependent on aqueous chloroform concentration. A portion of the chlorine demand associated with the pipe wall is nonTHM-yielding, likely inorganic in nature. At lower chlorine applications, this component of overall demand dominates, resulting in bulk water chloroform levels lower than those attributable to the water phase alone (i.e. "planktonic" THMs). A residual threshold (~5 mg/L free chlorine) exists where, if exceeded, the nonTHM-yielding benthic demand is satisfied and benthic formation and release of chloroform nets an overall increase in THM concentration in the bulk water. A "black box" macromodel of this phenomenon summarises the major findings of this work.

Thesis/Dissertation

application/pdf

2009-06-19

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http://hdl.handle.net/2429/9490

Civil Engineering

University of British Columbia

UBCV

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