Date of Award
12-2023
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Environmental Engineering and Earth Science
Committee Chair/Advisor
Tanju Karanfil
Committee Member
Alex Chow
Committee Member
Cindy M. Lee
Committee Member
Onur G. Apul
Abstract
The presence of taste and odor (T&O) compounds in drinking water has been historically a major challenge for water utilities. They are difficult to remove using conventional water treatment processes, a combination of coagulation, flocculation, sedimentation, filtration, and chlorination. However, activated carbon (AC) adsorption, advanced oxidation processes, and biofiltration can be effective. The seasonal nature of T&O events makes it difficult to justify installing dedicated treatment technologies. Additionally, incorporating these technologies into existing water treatment plants can be expensive and require significant upgrades. Therefore, water utilities are always interested in exploring new approaches to minimize the occurrence and removal of T&O compounds in drinking water.
Nanobubbles (NBs) can provide innovative solutions to T&O problems in water supplies and water treatment. The main objectives of this dissertation were to conduct a comprehensive investigation to systematically examine (i) the characterization and stability of oxygen and ozone NBs in typical freshwater and drinking water treatment conditions and ii) the removal efficiency and mechanisms of geosmin and 2-methylisoborneol (MIB), the major and most common T&O compounds in freshwaters, by NBs.
First, the stability of oxygen NBs was investigated under the representative key natural water chemistry conditions and constituents, including pH, Ca2+, Na+, natural organic matter (NOM) and temperature. The half-lives of oxygen NBs followed the order Ca2+ < Na+ < pH 3 < high SUVA254 NOM < pH 5 < 30 ◦C. Calcium was the most influential parameter significantly decreasing NBs levels among all parameters investigated. The main disappearance pathway of the negatively charged oxygen NBs in water was found to be coalescence, which was promoted greatly by cations (i.e., Ca2+, Na+ and low pH) and adsorption of NOM with high aromaticity onto the surface of oxygen NBs. The impact of higher temperatures became more noticeable after longer storage periods, where higher temperatures increase the kinetic energy of oxygen NBs, making them more likely to collide and coalesce. Therefore, when oxygen NBs are released or used in freshwater, high calcium, high SUVA254 NOM, and low pH would significantly reduce their availability and residence times.
Second, removal of geosmin and MIB from water by oxygen NBs was investigated. Initially, comparisons of nitrogen, air, and oxygen NBs showed higher removal percentages of geosmin and MIB as the oxygen content in NBs increased. Using oxygen NBs, volatilization was the dominant mechanism for the removal of geosmin (~40%) and MIB (~20%), while oxidation by reactive oxygen species (ROS) brought additional removal of up to 15%. The formation of hydroxyl (•OH) radicals was promoted when NBs were mixed with microbubbles (MBs). Formation of singlet oxygen and superoxide radicals did not appear to play a role for removal of target compounds by oxygen NBs. Alkalinity decreased the removal percentages of both geosmin and MIB by scavenging •OH radicals and inhibiting the oxidative removal pathway, while pH in the range of 3 to 10 had no significant impact on the geosmin and MIB removal. Geosmin and MIB removals were higher at higher temperatures due to increased volatilization and oxidative processes, where they decreased in the presence of either NOM or hardness. Under all tested conditions, geosmin removal efficiency was consistently higher than MIB due to the difference in their physicochemical properties (i.e., hydrophobicity or Log Kow, Henry’s law constant, functional groups, and steric hindrance of the MIB structure). Overall, the use of oxygen NBs resulted in less than 15% enhancement in removal of geosmin and MIB through the oxidative pathway.
Third, the characterization of ozone NBs, their stability and •OH radical formation from ozone decomposition were examined under freshwater conditions. Ozone NBs were more stable than oxygen NBs because of their higher negative surface charge (i.e., −32.0 mV and −23.6 mV, respectively). Ozone NBs generated at a higher dissolved ozone concentration (12.5 mg/L) exhibited greater stability (and higher negative surface charge) than those generated at a lower dissolved ozone concentration (1 mg/L) during long-term storage, which showed that ozone NBs generation conditions affect their stability and physicochemical properties. The stability of ozone NBs (generated at 12.5 mg/L dissolved ozone) were investigated under different pH, NOM, alkalinity, calcium, and temperature of freshwater conditions, for an extended storage time (i.e., 255 days). The half-lives of ozone NBs followed the order of 3 mM Ca2+ < pH 3 < high SUVA254 NOM (4.1 L/mg.m) < pH 7 < pH 9, while the effects of carbonate (or alkalinity) and temperature on the stability of ozone NBs were insignificant. Thus, ozone NBs would be stable for up to several months in natural waters depending on the water hardness and aromaticity of NOM. The formation of •OH radicals in ozone NBs solutions was 2 – 3 times higher than conventional ozonation during the same reaction time. A rapid disappearance of ozone NBs in the presence of 3 mM Ca2+ led to almost no additional •OH radical formation and the overall concentration of •OH radicals in that solution was comparable to conventional ozonation. The presence of carbonate ions lowered the formation of •OH radicals, but it was not enough to stop the continuous generation of radicals. However, NBs concentrations were not affected by the presence of carbonate in the background water.
Fourth, the removal efficiencies of geosmin and MIB were investigated by ozone NBs and compared side-by-side with conventional ozonation. The primary mechanism of geosmin and MIB removal by both conventional ozonation and ozone NBs was oxidative degradation via •OH radicals. Ozone NBs were more effective at removing geosmin and MIB (i.e., 80% and 73%, respectively) than conventional ozonation (i.e., 69% and 54%, respectively) in a 10-minute contact time at 20°C in distilled and deionized (DDI) water, which was due to the higher •OH radical formation in ozone NB water. Furthermore, in natural waters, ozone NBs maintained its performance with 85% and 74% removal of geosmin and MIB, respectively, which is more than 10% higher than conventional ozone at 20 ᵒC. Increasing temperature from 20 to 30 ᵒC enhanced the removal efficiencies of geosmin and MIB for ozone NBs (i.e., up to 15 %) and conventional ozonation (up to 6%) in DDI and natural waters. Reducing the initial ozone dose from 1.0 (O3/DOC= 0.43 mg/mg) to 0.5 mg/L (O3/DOC= 0.22 mg/mg) at 20 ᵒC widened the gap in removal efficiencies (i.e., up to 20%) between ozone NBs and conventional ozonation in natural water, demonstrating that the effectiveness of ozone NBs was less impacted by lowering the ozone dose. This can be explained by the presence of the same amount of ozone NBs in solutions for 0.5 and 1.0 mg/L ozone even though ozone level was reduced. While the addition of calcium (300 mg/L as CaCO3) reduced the ozone NBs concentration, it did not impact the geosmin and MIB removal by ozone NBs. On the other hand, the presence of background alkalinity (250 mg/L as CaCO3) decreased the removal efficiencies of geosmin and MIB, but its impact on ozone NBs (i.e., 7-10%) was less than conventional ozonation (11-13%). Lastly, bromate formation in the presence of 250 µg/L bromide was not significantly different between ozone NBs and conventional ozone, and the bromate level was below the USEPA regulatory limit of 10 µg/L. The results showed that the use of ozone NBs is more efficient and performs better than conventional ozonation during water treatment, which will reduce the cost and the environmental impact of the treatment process.
Overall, this research showed that oxygen NBs will be more effective for oxygen transfer applications (e.g., aeration) especially in low hardness waters with long term stability of oxygen NB, while the oxidative capabilities of oxygen NBs are rather much less important due to lower amount of ROS (mainly •OH) formation. On the other hand, ozone NBs, with their •OH formation, are more effective in the abiotic degradation of organic compounds than conventional ozonation. Ozone NBs, depending on the generation conditions, carry a higher negative surface charge than oxygen NBs, giving them longer stability in water, except increasing calcium levels significantly reduce both NBs stability.
Recommended Citation
Soyluoglu, Meryem, "Nanobubble Technology for the Removal of MIB and Geosmin From Drinking Water" (2023). All Dissertations. 3473.
https://tigerprints.clemson.edu/all_dissertations/3473
Author ORCID Identifier
0000-0002-5474-1627