Ensuring availability and sustainable management of water and sanitation for all is one of the 17 sustainable development goals (SDGs) set by the United Nations. Micropollutants increasingly detected in wastewater effluent have become an obstacle to achieve this SDG as they pose risks to water-related ecosystems and threaten the supply of high-quality drinking water. Although a series of advanced oxidation processes (AOPs, e.g., UV/H₂O₂) have been developed in the past decades, their full-scale applications are limited due to the high cost and low chemical utilization efficiency. Utilizing sunlight and water matrix components in-situ to reduce the chemical and energy demands would make treatment technologies more sustainable for micropollutant abatement in wastewater effluents. This the...[ Read more ]

Ensuring availability and sustainable management of water and sanitation for all is one of the 17 sustainable development goals (SDGs) set by the United Nations. Micropollutants increasingly detected in wastewater effluent have become an obstacle to achieve this SDG as they pose risks to water-related ecosystems and threaten the supply of high-quality drinking water. Although a series of advanced oxidation processes (AOPs, e.g., UV/H₂O₂) have been developed in the past decades, their full-scale applications are limited due to the high cost and low chemical utilization efficiency. Utilizing sunlight and water matrix components in-situ to reduce the chemical and energy demands would make treatment technologies more sustainable for micropollutant abatement in wastewater effluents. This thesis proposes a new strategy for micropollutant abatement in wastewater effluent through dissolved organic matter (DOM)-mediated photosensitized activation of monochloramine (NH₂Cl). Exposing the chlorinated wastewater effluent with residual NH₂Cl to solar irradiation (solar/DOM/NH₂Cl process) degrades six structurally diverse micropollutants at rate constants 1.26–34.2 times of those by the solar photolysis of the dechlorinated effluent (solar/DOM process). Notably, among the six micropollutants, the degradation rate constants of estradiol, acetaminophen, bisphenol A, and atenolol by the solar/DOM/NH₂Cl process are 1.13–4.32 times the summation of those by the solar/DOM and solar/NH₂Cl processes. The synergism in micropollutant degradation is attributed to the generation of reactive nitrogen species (RNS) and hydroxyl radicals (HO^•) from the photosensitized activation of NH₂Cl. Triplet state excited DOM (³DOM^*) dominates (i.e., ≥ 92.9%) the activation of NH₂Cl to generate RNS. While HO^• is produced via two pathways: (1) the interactions between RNS and other photochemically produced reactive intermediates (e.g., O₂^•– and DOM^•+/•–), and (2) the photolysis of chlorinated phenols. On the other hand, the solar/DOM/NH₂Cl process accelerated the NH₂Cl decay in wastewater effluent by 0.74–5.39 times compared to the dark/DOM/NH₂Cl process, therefore achieving photodechlorination and micropollutant degradation concurrently.

Based on the mechanism of the solar/DOM/NH₂Cl process, correlations (R² ≥ 0.7676) were established between the performance of the process and some easily measured physicochemical parameters of DOM (e.g., spectral slope (S₃₅₀)). Employing the correlations, the photosensitized decay rate constant of NH₂Cl, RNS concentration ([RNS]_ss), and HO^• concentration ([HO^•]_ss) can be predicted with < 24% differences against the experimentally determined values. The predictions facilitate understanding of the influences of operational conditions (e.g., influent quality) on the performance of the solar/DOM/NH₂Cl process.

DOM serves as a photosensitizer to activate NH₂Cl under irradiation at 297–405 nm and a light competitor to inhibit NH₂Cl photolysis at 254 nm. Therefore, DOM promotes the photo-decay of NH₂Cl and the degradation of micropollutants under solar irradiation but exhibits an inhibitory effect under irradiation by low -pressure ultraviolet lamps. While regardless of the irradiation wavelength, the DOM-mediated photosensitized activation of NH₂Cl accounts for >45% of the photodecay of NH₂Cl under conditions relevant to practical wastewater treatment, resulting in >1.9-fold higher [RNS]_ss in the radiated solutions containing DOM and NH₂Cl (UV/DOM/NH₂Cl process) compared to the UV/NH₂Cl process. Decreasing the irradiation wavelength monotonically increases the fluence rate-based rate constant of photosensitized activation of NH₂Cl. This is attributed to the greater generation and higher reactivity of ³DOM^* at lower irradiation wavelengths. In addition, the yield for HO^• (e.g., ∅_HO^•= 0.8%) and yield coefficient for ³DOM^* (e.g., f_TMP= 428 M⁻¹) from DOM at 222 nm were 15.2- and 1.6-fold higher than those at 254 nm, respectively. Accordingly, the DOM-mediated photosensitized activation of NH₂Cl is anticipated to play a more important role at 222 nm.

Overall, this thesis establishes a sustainable process for micropollutant abatement in wastewater effluents containing residual NH₂Cl. The influences of operational conditions (e.g., influent quality) and solar spectrum on the performance of the process were investigated. The findings advance the fundamental understanding of DOM-mediated photosensitization. The results also demonstrate the robustness of using artificial light sources (e.g., UV₂₅₄ and UV₂₂₂) to drive the DOM-mediated photosensitization process for micropollutant abatement in the engineered water treatment processes and facilities.