Kinetics of the Chemistry and Photochemistry across Different Reaction Stages of UV/chlorine and UV/H2O2 in Water Treatment and Water Reuse
Bibliographic record
Abstract
This thesis addressed four distinct (photo)chemical phenomena that are related by their importance to the design of UV-based advanced oxidation processes (AOPs) for water treatment. The first phenomenon was the impact of breakpoint chlorination chemistry on UV/chlorine AOPs in the context of potable reuse. In reverse osmosis-based treatment, chloramines are often applied to control fouling of the reverse osmosis (RO) membrane. Chloramine rejection by the RO is incomplete, so when free chlorine is applied for the UV/chlorine AOP, chlorine-chloramine breakpoint reactions will occur. These rapid reactions can affect the oxidant speciation and concentrations entering the UV reactor, and hence, the UV/chlorine performance. A model validated in this study showed the impracticality of eliminating the residual chloramines in RO permeate by dosing chlorine beyond the breakpoint Cl/N ratio. Operating parameters that limit monochloramine and favour dichloramine may slightly improve UV/chlorine performance, such as by increasing the water travel time prior to the UV reactor or increasing the applied Cl/N ratio. The second factor explored was related to radical scavenging capacity (Sc), which is a water quality parameter that directly affects the radical concentration and therefore the target pollutant decay kinetics in UV/AOP. To date, there have been very few studies of the variability of Sc within a water source, or across treatment trains. In this work, Sc was tracked at 5 surface and 1 ground drinking water treatment plants in Ontario over approximately one year. The variation in Sc was observed to range within 15–30%. The impact of this variation on pollutant removal efficiency in a UV/AOP was estimated to be comparatively smaller (±10% in pollutant removal rate) since pollutant removal is a function of the scavenging of not only the background water matrix (Sc), but also the oxidant (H2O2 or chlorine). Sc was not strongly correlated with total organic carbon, UV absorbance at 254 nm, or fluorescence excitation-emission matrix components. The third phenomenon examined relates to the design of UV/AOPs, and specifically how UV/chlorine is compared to UV/H2O2 in terms of predicted performance. Past studies have made such comparisons based on UV collimated beam testing or testing using small (pilot)-scale UV reactors. In this research, however, it was demonstrated that such small-scale tests are biased against UV/chlorine since UV/chlorine efficiency increases with longer UV path lengths (i.e., when using more powerful lamps that are spaced further apart, such as at full-scale). Modelling and experiments were conducted to examine mono- and polychromatic UV/H2O2 and UV/chlorine performance at 2–30 cm path lengths. For monochromatic (254 nm) UV light, the path length effect was not significant, but for medium pressure (polychromatic) lamps, the difference in such lamp spacing makes the pollutant removal efficiency of UV/chlorine relative to UV/H2O2 increase by 40%. The effects of natural water matrix absorbance, oxidant dose, and water pH were also discussed. The fourth phenomenon examined was a detailed study of H2O2 quenching kinetics using thiosulfate, bisulfite, and chlorine. When applying UV/H2O2 advanced oxidation, the majority of the H2O2 survives and need to be quenched since H2O2 exerts a significant downstream chlorine demand. It was determined that over the normal pH range of drinking water (7–8.5), chlorine is the most rapid quenching agent. The form of chlorine (hypochlorite vs. Cl2 gas) can impact H2O2 quenching rate, with gaseous chlorine slowing the reaction and hypochlorite having the opposite effect. These impacts diminish when water alkalinity increases to 80 mg/L as CaCO3.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".