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Treatment of Primary Influent Wastewater during Wet Weather Flow by Powdered Activated Carbon – Catalyzed Ozonation: From Fundamentals to Application
- Author / Creator
- Alameddine, Mirna
Cities equipped with combined sewers to accommodate domestic and industrial wastewater as well as storm water runoffs, encounter the need to prevent seasonal overflows into water bodies. An extensive literature review pertaining to the treatment of primary influent (PI) during wet weather conditions determined that a coagulation/ flocculation/ sedimentation process was required for at least the removal of suspended solids. Ozonation was deemed promising especially for the oxidation of micropollutants (MPs) and disinfection. Yet, although ozone (O3) is a renowned strong oxidant, several MPs are not sufficiently reactive with O3 which calls for a process upgrade. Powdered activated carbon (PAC) was favourable to catalyze the ozonation because of its adsorbing capacity. There has been limited and conflicting knowledge pertaining to PAC-catalyzed ozonation especially in the ambiguously described catalytic mechanisms. The objective of this study was to propose a complete treatment for PI during wet weather flow by optimizing the operational parameters in conventional primary treatment and exploring single or PAC-catalyzed ozonation as a supplementary step for MPs’ degradation and disinfection. This was addressed through a meticulous investigation of the catalytic mechanisms using a mixture of pharmaceuticals, personal care products, herbicides and perfluorinated compounds which have different affinities towards O3.
Initially, the enhanced primary treatment of municipal PI during wet weather conditions was studied through a comprehensive approach from bench to full scale. Three metal-based coagulants were tested in a series of jar tests. Aluminum sulfate (alum) outperformed other coagulants since 1 mg of aluminum added as alum with low mixing was able to remove 22 Nephelometric Turbidity Units (NTUs), 19 mg of oxygen as chemical oxygen demand (COD) and 0.8 mg of ortho-phosphate (ortho-P). The removal of total suspended solids (TSS) depended mostly on rapid mixing while COD and ortho-P removals depended on slow mixing and coagulant dose. In bench and full-scale operations, the addition of polymer did not lead to any pronounced improvements. Turbidity and percent ultraviolet transmittance showed good correlation with TSS and ortho-P which evoked their use as surrogates for online process control.
PAC was then used to catalyze the ozonation of a mixture of seventeen MPs at near environmental concentrations in clean water matrix. Thirteen of those MPs were studied for the first time by PAC-catalyzed ozonation. The latter improved the removal of O3-resistant MPs by up to 27% and promoted faster specific degradation rates. Radical probing experiments showed that scavenging hydroxyl radical (•OH) did not have a significant impact on MPs removals, while scavenging other reactive oxygen species was more influential. A detailed study by Electron Paramagnetic Resonance (EPR) spectroscopy ascertained that the decomposition of O3 in presence of PAC at neutral pH did not boost the generation of free •OH. Instead, it likely produced adsorbed •OH as PAC•–HO and other strong oxidizing species like [1O2+PAC]. Thus, free •OH did not play any significant role in PAC-catalyzed ozonation. The effect of varying the O3 specific dose as g O3/g DOC (dissolved organic carbon) on the removal of those MPs by single and PAC-catalyzed ozonation was also investigated. Most MPs were more prone for removal by catalytic ozonation where faster removal kinetics were obtained. Carbamazepine, an anticonvulsant, and Atrazine, an herbicide, were found to be good surrogates for fast and slow reacting compounds, respectively. The removal kinetics were strongly dependant on the level of DOC.
After establishing a better understanding of the catalytic mechanisms, single and PAC-catalyzed ozonation were explored for the first time during wet weather flow for organics removal, disinfection, MPs’ reduction and toxicity abatement. Overall, applying either ozonation process after the primary treatment was more efficient than applying them simultaneously. PAC-catalyzed ozonation was particularly favourable for the removal of organics and O3-resistant MPs. Both single and PAC-catalyzed ozonation achieved 4 log removal of E. coli, and reduced the acute and genetic toxicity and the estrogenic activity of PI. A detailed cost analysis revealed that in post treatment, single ozonation would cost 0.06 to 0.32 $/m3 while PAC-catalyzed ozonation would cost 0.32 to 0.63 $/m3 for a flow rate ranging between 100 and 600 MLD. A comprehensive performance assessment demonstrated that PAC-catalyzed ozonation was promising for PI treatment during wet weather flow with one drawback related to the disposal of PAC.
- Graduation date
- Fall 2020
- Type of Item
- Doctor of Philosophy
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