When ozonation is employed in drinking water treatment plants, dissolved natural organic matter (NOM) is oxidized by ozone and/or hydroxyl radicals (•OH). Depending on water quality, ozone and •OH react with NOM at different proportions which are quantified through Rct (•OH /O3 exposure). Rct is determined from the decay of both ozone and p-chlorobenzoic acid (an •OH probe compound). Water utilities, however, may find it difficult to adapt this method for routine characterization of their ozonation process. It may be more beneficial for them to assess Rct using commonly monitored parameters like inorganic nitrogen.
In this study, we investigated the correlation between •OH and O3 exposures and NH4+-N and NO3--N concentrations after ozonation. Furthermore, disinfection by-product (DBP) formation potential and NOM properties were determined before and after ozonation.
•OH and ozone exposures were varied in batch experiments at different transferred ozone dose (0.4-1 mg/mg DOC), pH (6-8), alkalinity (0-2.5 mM bicarbonate) and in the presence of H2O2 (1 mg/mg O3) and •OH scavenger tert-butanol (10 mM). The levels of inorganic nitrogen in coagulated water samples were affected by changes in exposures of these oxidants. Higher NO3--N concentrations were measured for O3 dominant reactions (high O3 dose, low pH, high alkalinity, O3/tert-butanol), whereas higher NH4+-N were detected for •OH dominant reactions (high pH, low alkalinity, O3/H2O2). These changes were observed presumably because of different reaction mechanisms of the oxidants – direct N oxidation by O3 and H abstraction from the carbon backbone by •OH. When comparing ratios of NH4+-N/NO3--N with Rct values at different ozone doses, a direct relation between the two ratios was revealed. Thus, measuring NH4+-N/NO3--N after ozonation may be an alternative approach to evaluate the contribution of ozone and •OH on NOM oxidation.
The variations in levels of inorganic nitrogen may also be used to track changes in DBP formation potentials. The DBPs analysed in this study include trihalomethanes, haloacetic acids, haloacetonitriles, halonitromethanes, haloacetamides, chloral hydrate and haloketones. It was observed that when ozone exposure is low (high NH4+-N/NO3--N), DBP formation potentials were higher than at high ozone exposure (low NH4+-N/NO3--N). Regarding NOM properties, specific UV absorbance at 254 nm (SUVA), electron donating capacity (EDC), and fluorescence excitation-emission matrix (EEM) were analysed. Both SUVA and the electrochemically determined EDC of the oxidized NOM were observed to decrease with increasing ozone exposure. EEM results showed that •OH dominant reactions caused better removal of protein- and soluble microbial products-like peaks while direct ozone reactions decreased fulvic and humic acid-like peaks.