Kinetics-based evaluation of full-scale wastewater ozonation using fluorescent tracers and a non-ideal residual fraction model.

Wastewater ozonation is increasingly applied to reduce pathogenic microorganisms and micropollutants that are not sufficiently removed by conventional biological treatment. For full-scale operation, ozone dose should be managed according to water-quality variation; however, existing assessment approaches often involve a trade-off between simple bulk water-quality indicators and more direct but labor-intensive target analysis by liquid chromatography-tandem mass spectrometry. This study proposes a site-specific, kinetics-based framework for operational assessment of full-scale wastewater ozonation using fluorescent tracers (FTs) selected from non-spiked fluorescent compounds present in treated wastewater. The FTs were quantified by liquid chromatography with fluorescence detection without solid-phase extraction, and their residual fractions were linked to oxidant exposure through reaction rate constants with ozone and hydroxyl radicals. Among 12 candidate peaks detected at one full-scale wastewater treatment plant, six compounds were selected as FTs after excluding peaks that increased after ozonation, were potentially affected by particle or intracellular elution or formation reactions, or showed insufficient 24 h stability. A simple non-ideal residual fraction model developed for the full-scale ozone contactor reproduced the residual fractions of these six FTs and predicted the residual fraction of Escherichia coli, which was not used for model calibration, with absolute errors of 0.04-0.10. Conventional indicators, including absorbance at 254 nm and total organic carbon, weakly correlated with the estimated oxidant exposure parameters. These findings suggest that bulk water-quality indicators alone may be insufficient for quantitatively characterizing ozonation performance. The proposed FT-based framework may complement routine monitoring and support operational management of full-scale wastewater ozonation.