Online Monitoring of Disinfectants in Water

Online monitoring of disinfectants is essential to ensure a safe supply of water while at the same time limiting by-products and corrosion related issues. Oxidants are reactive and unstable, and intermittent testing on discrete samples can leave water temporarily without disinfectant. Continuous sensing captures the dynamics of disinfectant concentration variation and speciation due to a shift of parameters. We outline the chemistry that controls performance such as pH, temperature, ionic strength, dissolved oxygen, and oxidation–reduction potential (ORP); and link these to oxidant speciation, kinetics, and residual stability. We survey disinfectants across applications: free chlorine, monochloramine, chlorine dioxide, potassium permanganate, ozone, peroxene (O 3 /H 2 O 2 ), UV/H 2 O 2 , and HOBr/OBr − . We also highlight efficacy and constraints for online measurement. We assess sensing methods covered in this work: optical (traceable but reagent-dependent); electrochemical (fast, reagent-free, but reliant on stable reference electrodes and fouling control); and electrical (chemiresistors and solution-gated FETs), with challenges in selectivity, drift, and in-situ calibration. ORP is a rapid working alternative that must be interpreted with pH and oxidant concentration rather than used as a replacement for residual. Priorities for improvements in future methods include calibration, antifouling and self-verification with automated recalibration, and model-based control linking speciation and ORP to operational needs.