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Distribution System Residuals – Is “Detectable” Still Acceptable for Chloramines?
Citation:
Wahman, D. G. AND J. G. Pressman. Distribution System Residuals – Is “Detectable” Still Acceptable for Chloramines? JOURNAL OF THE AMERICAN WATER WORKS ASSOCIATION. American Water Works Association, Denver, CO, 107(8):53-63, (2015).
Impact/Purpose:
This article will be useful for drinking water utilities that use chloramines as a secondary disinfectant. It should help them think about and better manage the minimum residuals they allow throughout their systems.
Description:
Recently, Roberson (2014) noted that one of the distribution system issues currently on the regulatory radar is, “Should disinfectant residual requirements be a specific number as opposed to the current detectable residual?” As our title suggests, we seek to assess what it means to maintain a “detectable” disinfectant residual in drinking water distribution systems in the United States, particularly with respect to the use of inorganic chloramines as a secondary disinfectant. Under typical drinking water conditions (i.e., pH > 7 and chlorine to nitrogen mass ratios < 5:1), inorganic chloramines primarily consist of monochloramine with some dichloramine (Vikesland et al, 2001; Jafvert & Valentine, 1992).While we recognize the need to assess maintaining free chlorine residual as well, the unique factors involved with inorganic chloramines (e.g., organic chloramines, nitrification, mode of action) along with the fact that inorganic chloramines were not addressed separately in the Surface Water Treatment Rule (SWTR), result in the need for an evaluation specific to inorganic chloramine use. The focus of the discussion herein will ultimately center on what “detectable” means for chloraminated drinking water distribution systems (CDWDSs) and whether merely holding a “detectable” residual is meaningful in these systems in light of what we currently know versus when the original SWTR was implemented in 1989 (USEPA, 1989a). In framing this discussion, we will walk through the original reasoning behind requiring disinfectant residual maintenance, how States currently implement the disinfection residual requirement, and whether a “detectable” residual reasonably achieves these intentions for inorganic chloramines because of issues centered on organic chloramine formation and nitrification. Our focus at this time is not on reviewing whether or not a distribution system disinfectant residual should be required for pathogen disinfection, distribution system integrity, contamination detection, or the associated implications for disinfectant byproduct formation. Rather, our focus is discussing whether a “detectable” concentration provides any confidence that one indeed has an effective disinfectant residual when required and the best practices for maintaining the distribution system when the disinfectant also provides a food source (ammonia) for biological growth. Ultimately, we hope to elucidate whether a “detectable” residual is an acceptable goal, especially for CDWDSs, given what we now know about organic chloramines and distribution system nitrification.
