American Society of Civil Engineers


An Approach to Assessing Potential Compliance with the Proposed Stage 2 Disinfection By-Products Rule


by Philip C. Singer, (Department of Environmental Sciences and Engineering, School of Public Health, University of North Carolina, CB 7431, Chapel Hill, NC 27599-7431 E-mail: phil_singer@unc.edu) and Nora J. Han, (Department of Environmental Sciences and Engineering, School of Public Health, University of North Carolina, CB 7431, Chapel Hill, NC 27599-7431)
Section: Applied Research in Water, Wastewater, and Stormwater, pp. 1-12, (doi:  http://dx.doi.org/10.1061/40792(173)99)

     Access full text
     Purchase Subscription
     Permissions for Reuse  

Document type: Conference Proceeding Paper
Part of: Impacts of Global Climate Change
Abstract: The objective of this study was to assess the degree to which eleven utilities in North Carolina would be able to comply with the impending Stage 2 Disinfection By-Products Rule. The Stage 2 Rule will require that all utilities meet the 80 μg/L maximum contaminant level (MCL) for trihalomethanes (THMs) and the 60 μg/L MCL for haloacetic acids (HAAs) on a locational running annual average (LRAA) basis. This represents a major departure from past disinfection by-product (DBP) regulations for which compliance was based on a system-wide running annual average (RAA). Relevant historical data were obtained from the eleven participating utilities and were transferred into Excel files. The data included quarterly THM and HAA concentrations at each of the current compliance monitoring stations in the utility’s distribution system for the past 3 years, mean hydraulic residence times (water ages) associated with each of the stations, and water temperature. For systems using a combined chlorine residual, we assumed that the DBP concentrations would be essentially the same system-wide. This was verified by observing that the standard deviations of the measured THM and HAA concentrations for systems with a combined chlorine residual were very small relative to the standard deviations for systems using free chlorine. Hence, we assumed that the LRAAs would be the same for all sampling sites, even for new remote sampling stations that might be incorporated into the monitoring program in accordance with Stage 2 requirements. Because chlorine dose and corresponding THM and HAA formation are highly temperature dependent, we developed a record of monthly system-wide THM and HAA concentrations using the quarterly measured values and monthly average temperature data. We used EPA’s Water Treatment Plant (WTP) model to predict changes in THM and HAA levels with changing temperature. The resulting monthly DBP concentrations were then used to calculate LRAAs for THMs and HAAs for the system each month, using the values for the preceding 12 months. For systems using a free chlorine residual, we expected there to be significant spatial as well as temporal variation in DBP levels in the distribution system. For these utilities, we used the measured THM and HAA concentrations from the existing compliance monitoring sites along with the EPA WTP model to predict THM and HAA levels at remote sites with residence times (water ages) of 7–14 days, using knowledge of the water ages associated with each of the DBP monitoring stations. We then used the WTP model to predict monthly THM and HAA levels for these remote sites using monthly average temperature data. The resulting monthly concentrations were then used to calculate maximum LRAAs for THMs and HAAs for the system each month.


ASCE Subject Headings:
North Carolina
Disinfection
Waste management
Water pollution