Micropollutant degradation, product formation and mass transfer in ozonation water treatment

: (Alternative Format Thesis)

Abstract

Ozone is a strong oxidant used in water and wastewater treatment for disinfection, removal of taste, colour and odour and abatement of trace organic contaminants (TrOCs). TrOCs, such as pharmaceuticals, have been attracting growing attention in the last decades due to their widespread presence in the environment and their ecotoxicological effects. The ozone-induced oxidation of water constituents generates a very large number of known and unknown by-products, including bromate formed from bromide and structurally diverse transformation products of TrOCs. The mass transfer of ozone is important for both process efficiency and reaction pathways and is conventionally achieved via bubble-based systems. In order to address the major issues surrounding ozonation treatment, this PhD thesis investigated the abatement of TrOCs, the formation of ozonation products and the bubble-less transfer of ozone.A multi-compound ozonation study was performed by utilising liquid chromatography-mass spectrometry to provide a large dataset on the ozone reactivity of environmentally relevant TrOCs. The ozonation of 90 compounds with diverse chemical structures was studied in pure buffered water, tap water and wastewater effluent at three specific ozone doses and three pH levels. A review of the literature revealed that little information is known on the ozonation kinetics of illicit drugs and their metabolites. The experiments showed that most illicit drugs, such as cocainics, amphetamines and ecstasy-group compounds, are ozone-resistant.
In addition to the reactivity of the parent compounds, investigating the biodegradation of ozonation products of TrOCs is important to assess the efficiency of advanced treatment schemes involving ozonation and a subsequent biofiltration step. A Continuous Ozonation merged with Biofiltration (COMBI) laboratory system was developed to perform investigations that were previously only feasible at large-scale or pilot-scale plants. After an equilibration time of three weeks, biodegradable ozonation products, for example the main product of carbamazepine, were removed in the sand filtration columns. In contrast, other compounds, such as trifluoroacetic acid formed from fluoxetine, passed through the columns at unchanged concentrations.
The abatement of TrOCs using ozone requires the design of efficient ozonation processes. The use of membrane contactors for the bubble-less transfer of ozone into water and wastewater is a promising alternative to conventional bubble-based methods. Polymeric membranes made of polydimethylsiloxane (PDMS) and polytetrafluoroethylene (PTFE) were tested in a single tube membrane contactor and in a multi-tube hollow fibre module. High removals of TrOCs at their inherent concentrations in wastewater effluent were achieved using membrane ozonation. However, the analysis of bromate formation in bromide-containing groundwater indicated that the non-uniform distribution of ozone inside a membrane contactor can lead to elevated bromate concentrations that exceed the regulatory limit of 10 μg L⁻¹.
Finally, a case study for the ozonation of a specific group of substances was conducted. The study focused on the ozonation kinetics and transformation products of substituted furans. Despite being a widespread moiety in natural and synthetic chemicals, the aqueous ozonation of furan rings was previously poorly understood. The analysis of transformation products targeted α,β-unsaturated dicarbonyl compounds, which are well known toxicophores. The formation of 2-butene-1,4-dial and other α,β-unsaturated dicarbonyls was demonstrated in aqueous ozonation for the first time. Despite the low yield of these substances, which reached maximum values of 7%, their high toxicity raises concern about their presence in treated water.
Overall, this thesis achieved a better understanding of the ozone reactivity and transformation products of TrOCs, including compound classes such as illicit drugs and substituted furans that had not been studied comprehensively with ozone before. In addition, the developed experimental setups can facilitate future research on ozonation-biofiltration treatment and on bubble-less transfer of ozone. The results of this thesis have led to three publications in peer-reviewed journals, while two further manuscripts are currently being prepared.

Date of Award	16 Jun 2021
Original language	English
Awarding Institution	University of Bath
Supervisor	Jannis Wenk (Supervisor), Barbara Kasprzyk-Hordern (Supervisor) & John Chew (Supervisor)