Improved Temperature Compensation for In Situ Humic-Like and Tryptophan-Like Fluorescence Acquisition in Diverse Water Types

The use of in situ fluorescence sensors is gaining popularity in water and wastewater treatment and water reuse industries. Tryptophan-like (TRP-like) fluorescence is particularly well suited to tracking fluorescent compounds derived from bacteria as well as a wide range of chemicals of concern for drinking water. Despite the merits of fluorescence sensors, they are affected by environmental factors such as temperature, which can influence photophysical properties of fluorescence. Although temperature correction factors have been well established for humic-like fluorescent dissolved organic matter (fDOM) sensors, there is a need to assess the suitability of existing temperature compensation models in diverse water types and derive new corrections that apply specifically to TRP-like fluorescence. Temperature experiments were conducted using a submersible fluorometer and a range of water types, including creek water, water from different treatment stages at a water reuse facility, synthetic wastewater, and prepared samples containing TRP and fulvic acids. Results from this study revealed that at low fluorescence intensities, no temperature corrections were needed for TRP-like and fDOM fluorescence sensors. For most water types, a published temperature compensation constant for fDOM fluorescence produced a fairly good fit to the reference temperature, but had limited applicability for TRP-like fluorescence. The best fit to the reference temperature for TRP-like fluorescence was achieved using a method to minimize the root mean square error (RMSE) between modeled and measured TRP-like fluorescence. The application of temperature corrections resulted in a difference of as much as 2500 RFU in water types with high DOC concentrations at high (>30 degrees C) and low (<10 degrees C) temperatures, which has important implications for the use of fluorescence sensors for source water protection and other monitoring applications in these conditions. For TRP-like fluorescence, empirical relationships and temperature compensation constants were also derived and recommended for samples with varying tryptophan and organic carbon concentrations.