Tracer Studies to Measure Water Residence Time in a Distribution System Supplied by TwoWater Treatment Plants

摘要

The residence time of water in the distribution system (DS) is an important factor indetermination of the extent of reactions. For example, disinfectant residual decreases and thedisinfection by-products increase with increasing residence time. Loss of disinfectant residualcan lead to serious bacterial regrowth problems. While chemical tracer studies are becomingmore familiar to water utility personnel, a methodology has not been adopted for quantitativeanalysis of the resulting data from the DS. The alternative is water age calculations fromhydraulic modeling but this requires development of the network model and its calibration.Moreover, the extent of skeletonization of the network may limit its utility for estimating waterage in smaller pipes that are not included in the model. This study presents a method to calculatea mean constituent residence time (MCRT); the constituent is a conservative chemical tracer.The method is applied in the DS of Durham, N.C. where the analysis was made more complexby inclusion of two water treatment plants (WTPs). A unique tracer study was developed todetermine simultaneously the percent contribution to flow from each WTP at each samplingstation and the flowrate weighted average of the MCRT. This involved a switch in coagulants atone WTP and a change in fluoride feed concentration to the other.