Applications of natural organic matter optical properties for assessing drinking water disinfection and distribution

Doctoral thesis, 2019

The task of providing safe drinking water requires proper monitoring of water quality and treatment performance from source to tap. Accordingly, the demand for online monitoring is increasing both at treatment plants a nd within distribution networks. Some of the available techniques use correlations between the optical properties of dissolved organic matter (DOM), mainly absorbance, and other water quality parameters. Fluorescence spectroscopy is significantly more sensitive than absorbance spectroscopy and gives comprehensive information about the composition and concentration of organic matter, so it has a strong potential for online monitoring applications. In this thesis, the application of fluorescence spectroscopy was investigated for two locations in the drinking water treatment system: the ultraviolet (UV) disinfection chambers and the distribution network. With respect to UV254disinfection, fluorescence spectroscopy was investigated as a proxy of potential unwanted by-products (assimilable organic carbon (AOC) and disinfection by products (DBPs), in addition to the administered UV dose. The UV254 irradiation increased assimilable organic carbon (AOC) concentration, with the estimated AOC production induced by UV254irradiation at a dose of 40 mJ cm-2being 0.4 % of the dissolved organic carbon. The concentration of adsorbable organic chlorine (AOCl), a subset of adsorbable organic halogens (AOX), generally increased following UV254 irradiation at the typical disinfection dose. Weak but statistically-significant correlations were found between the UV-induced fluorescence reduction and increases in both AOC and AOCl concentrations. Compared to absorbance, greater reductions were observed in fluorescence intensity following sequential UV irradiation and chlorination and these correlated more strongly with AOCl production. Following UV254disinfection, a linear relationship was observed between UV254dose and changes in long-wavelength fluorescence (> 400 nm) intensities at doses up to 200 mJ cm-2. However, the application of fluorescence as a proxy of the UV254dose is limited due to the relatively small and unpredictable direction of change of fluorescence intensity. In the distribution network, the sensitivity of fluorescence to detect contamination caused by entrainment was compared to the sensitivity of other common water quality parameters including several trace elements and microbial indicator species abundances. Of these, fluorescence was the most sensitive tracer for distinguishing contamination from natural variation, followed by absorbance. The relationship between fluorescence and microbial regrowth was also examined; however, no correlation was observed. The results of this thesis imply that although fluorescence might not always correlate with the chemical and microbial water parameters, its prompt response to treatment-induced modifications and fluctuations, together with high analytical precision and sensitivity of fluorescence measurements, make it a useful parameter for real-time monitoring of water quality changes in drinking water treatment plants and distribution systems.