Disinfection Residual Behaviour within Drinking Water Distribution Systems

Abstract

Understanding the persistence of different disinfection residuals in drinking water distribution systems (DWDS) is critical to water quality safety and public health. Chloramines are expected to persist, due to their chemical composition, further into the extremities of DWDS providing reduced risks to water quality, however there is limited evidence of this from operational systems. Total chlorine data from regulatory sampling at WTW outlets and customer taps from multiple DWDS was collected and analysed. The first data set compares long term performance from equivalent systems at the same time. Results showed the decay of residuals was similar across all seasons in the DWDS analysed, showing little correlation with residual type. The second data set compares six systems that experienced a disinfection switch from chlorine to chloramine. Decay of total chlorine residuals was clearly reduced for three and marginally reduced for a fourth following the switch, suggesting these DWDS experienced increased persistence. However, two sites showed little change. This analysis highlights the uncertainties around residual persistence under different disinfection types, indicating the assumption that chloramine persists for longer than chlorine in every distribution system is not a given.