Modeling the Response of Nonchlorinated, Chlorinated, and Chloraminated Water Distribution Systems toward Arsenic Contamination

A mechanistic simulation model predicting the response of water distribution systems (WDSs) operated with or without disinfectant residual toward accidental arsenic contamination is developed in this paper. The impacts of chlorination, chloramination, and organic loading to control the oxidation of arsenous acid [As(III)] and the adsorption/desorption of arsenic acid [As(V)] on/from iron pipe walls were simulated by applying the model to two real-world WDSs. The model predicted that during any As(III) contamination event, the arsenic spread in WDSs would engage conservatively in the absence of a residual disinfectant. Due to the swift reactions between chlorine and As(III), maintaining residual chlorine was recognized as an effective strategy to control the soluble As(III) levels. Chloramine was predicted to be less effective than chlorine in causing As(III) oxidation and subsequent As(V) adsorption onto the pipe wall. Besides, under the test conditions considered, the required chloramine dose in the source water had to be 10 times higher to produce equivalent effects in terms of As(III) depletion as the chlorine dose of $1\mathrm{mg}/\mathrm{L}$. The results presented that chlorine formation in chloraminated WDSs via the monochloramine hydrolysis mechanism contributes to $>99\%$ As(III) depletion inside the distribution pipes. Therefore, the paper recommends maintaining additional chloramine residual in chlorinated WDSs to control the As(III) spread during arsenic contamination events in the downstream sections.