A Practical Approach to Determine Chlorine Decay Rate from Naturally Occurring Tracer in Distribution Systems

Chlorine decay rate is one of the critical parameters in infrastructure design and operation optimization of drinking water distribution systems. Over estimation of decay rate could falsely justify the need for building costly booster stations; under estimation of decay rate on the other hand could lead to insufficient chlorine dosage and posses potential danger to the public health. Uncertainties in chlorine decay rate could also cast doubts on whether distribution models can be used as a tool to evaluate the design and operational alternatives. This paper presents a practical and unique approach to estimate site-specific chlorine decay rates. Analyses of operation records along a large distribution system revealed that there are discernible diurnal chlorine residual patterns traveling along the distribution system. Bach of these patterns can be identified clearly from the hourly operation records at monitoring locations. When operation records exist at two or more locations along the system, the travel time that a given pattern passes through these two locations and the chlorine residual changes during that the period can be calculated from the records. The chlorine decay rate can be derived with appropriate assumption of kinetics. The concept of the approach is the same as that of conventional tracer study, except the tracer signals happens naturally and continuously, and the required data to derive chlorine decay rate are conveniently available from the existing operation records.