Bottle Test Sampling Design to Control the Uncertainty of the Bulk Decay Coefficient

Free chlorine is commonly applied at treatment works as a disinfectant residual but decays throughout the water supply system. Chlorine decay consists of a bulk and a wall decay component. The bulk component depends on water composition (i.e., it does not vary across the network) and is often characterized at the entrance to the system by fitting a first-order decay model based on bottle test data. The bulk decay coefficient (kb) is known to vary widely depending on the water source characteristics and temperature. Recent studies have shown that the uncertainty of kb can be significant (>15%), and its quantification is essential to avoid misinterpreting imprecise kb values. This work aims to explore how bottle test sampling conditions affect the uncertainty of kb through State Estimation (SE) and Uncertainty Assessment (UA) techniques. First, a sensitivity analysis is presented to explore the effect of the number of measurement replications and sampling times. Then, an experimental comparison is carried out to highlight the relevance of kb uncertainty and assess the effect of different types of chlorination (laboratory vs. onsite chlorination). This two-stage approach is essential for deriving a set of practical recommendations for the appropriate design of bottle tests, controlling (i.e., limiting) the associated kb uncertainty for each application. This is key to better understanding and modeling free chlorine residual bulk decay and, thus, chlorine dynamics, through water supply systems.