Alterations of Disinfection Byproduct Formation Following Exposures of Algae to Wildfire Ash Solutions and Copper Algaecide
Abstract
Previous studies demonstrated that wildfires can alter spectroscopic characteristics of terrestrial dissolved organic matter (DOM) and potentially affect disinfection byproduct (DBP) formation during drinking water disinfection processes (e.g., chlorination and chloramination). Elevated levels of nutrient due to wildfire ash input into stream waters will likely cause algal blooms. Thus, wildfires can also elevate DOM levels as a result of ash input and excessive algal growth in source waters, and consequently impacting DBP formation in finished water. However, it is unclear whether characteristics of thermally-altered DOM (TA-DOM) are altered by biogeochemical processes (e.g., transformed by growing algae) before entering water treatment facilities. In addition, little information is available regarding how quality and quantity of overall allochthonous and autochthonous DOM as well as associated DBP formation are changed during an entire algal life cycle. When resource water is impeded by algae and require immediate restoration, copper algaecides are usually applied to control their growth. Previous studies suggest that Cu2+ can promote reactivity of DOM in DBP formation. However, it is unclear that how DBP formation is changed after treatment of fire-induced algal bloom by copper algaecide. To answer these questions, three independent experiments were designed, and the results are reported as three chapters in this dissertation (CHAPTER TWO, THREE, and FOUR). In the first experiment, freshwater green algae Pseudokirchneriella subcapitata and blue-green algae Microcystis aeruginosa were separately incubated in the mixture of cultural medium and pine (Pinus palustris) litter-derived TA-DOMs (50 °C, 250 °C, and 400 °C) over 7 days to demonstrate the effects of algal growth on alterations in SDBP-FP. TA-DOM optical characteristics and SDBP-FP were quantified by absorption and fluorescence spectroscopy and chlorination-based DBP-FP experiments. After the inoculation with P. subcapitata, TA-DOM aromaticity (indicated by SUVA254) increased from 1.19 to 1.90 L/mg/m for 50 °C-extract but decreased from 4.95 to 3.75 L/mg/m for 400 °C-extract. The fraction of tyrosine-like components decreased from 25.9 to 9.3% for 50 °C-extract but increased from 0.9 to 1.3% for 400 °C-extract. Same patterns were also observed for M. aeruginosa. Growing algae generally increased chlorine reactivities and formations of trihalomethanes, haloacetonitriles, chloral hydrate, and haloketones. Our data suggest that the biodegradable dissolved organic carbon in TA-DOM decreases as fire intensity (i.e., temperature) increases. Post-fire algal blooms can increase chlorine reactivity of fire-affected terrestrial DOM for DBP formation. In the second experiment, Microcystis aeruginosa was cultured in the medium containing low and high concentrations [10% and 65% (v/v)] of black and white ash water extracts (BE and WE) to study dynamic changes of carbonaceous, nitrogenous, and oxygenated DBP precursors during algal growth. DOM was characterized by absorption and fluorescence spectroscopy and chlorination/chloramination-based DBP formation experiments. In the treatment with 10% BE, the amount of C-DBP precursors decreased from 6.8 to 3.0 mmol/mol-C at lag-exponential phase then increased to 4.2 mmol/mol-C at death phase. The same trend was observed for O-DBP precursors. However, these dynamic changes of C- and O-DBP precursors exhibited opposite patterns in 65% extracts. Similar patterns were also observed in the WE treatments. On the other hand, N-DBP precursors continuously declined in all treatments. These results indicate that postfire ash loading and algal bloom stage may significantly affect DBP formation in source water. In the third experiment, Microcystis aeruginosa was cultured in the medium containing black/white ash extracts (BE and WE) to study DBP concentrations before and after 4-days exposures to 0.5 and 1.0 mg-Cu/L. Algal population was indicated by optical density at 680 nm (OD680). DOM was characterized by absorption and fluorescence spectroscopy and chlorination/chloramination-based DBP formation experiments. In the end of experiment, OD680 and DOM in the treatments were lower than control. N-nitrosodimethylamine concentrations in both treatments were 4-6 times higher than control, but haloacetonitrile concentrations in the treatments and control revealed no significant difference, regardless of type of ash solution. The results may serve to support risk evaluations of algal population and DBP concentration when wildfire-induced M. aeruginosa bloom is left untreated and when it is treated by copper algaecide.