Investigation on influence factors affecting the formation of bromate and mitigation measures for toxicologically relevant transformation products during oxidative wastewater treatment

Micropollutants such as pharmaceuticals, personal care products, and industrial chemicals are increasingly detected in aquatic environments. Even at trace concentrations, these substances can harm ecosystems, bioaccumulate in food webs, and pose risks to aquatic organisms and human health. Conventional wastewater treatment plants typically are not designed to fully eliminate these compounds, prompting the need for advanced treatment methods. Among these, ozonation has emerged as a promising technology due to its ability to remove a wide range of organic trace compounds. However, ozonation can lead to the formation of potentially harmful transformation products. A major concern is bromate, a toxic and persistent transformation product formed by the oxidation of bromide, a naturally occurring and anthropogenic ion commonly present in wastewater. Due to its health relevance, bromate is often a key focus in feasibility studies and can become a major obstacle during the early planning of ozonation systems. In wastewater ozonation, bromate formation depends on several factors, including ozone dose, pH, background organic matter, and bromide concentration. These variables make bromate formation difficult to predict and control under variable wastewater conditions. Consequently, understanding the mechanisms behind bromate formation and identifying effective mitigation strategies is essential for the safe and scalable use of ozonation. This is especially relevant given the upcoming requirements of the European Union's Urban Wastewater Directive. This thesis aimed to elucidate the conditions driving bromate formation during ozonation of wastewater and to explore practical mitigation approaches that safeguard both environmental and regulatory outcomes. The work combined full-scale plant studies, laboratory experiments, and computational modeling. Applying a multilevel methodological approach, the main objectives of this PhD thesis were: (i) to evaluate bromate formation potential under varying operational conditions and scales, (ii) to assess mechanical and chemical mitigation strategies, (iii) to identify key influencing parameters, and (iv) to compare the performance of innovative ozone injection technologies with conventional systems. Hypotheses centered on the predictive reliability of lab-scale data, the effectiveness of mitigation techniques, the influence of wastewater composition, and the role of immediate ozone reactions on both micropollutant removal and bromate formation. Key findings highlight the need for a multiscale and multidisciplinary approach to optimize ozonation operation. Laboratory-scale experiments were effective for assessing micropollutant elimination but could not reliably predict bromate formation at larger scales. This gap was mainly due to unrealistic hydraulic conditions in small-scale setups. Computational fluid dynamics modeling addressed this gap by revealing how mixing behavior and local ozone concentrations influence bromate hotspots, underscoring the importance of standardized and hydraulically representative lab protocols. The results confirmed that integrating mechanical (e.g., serial dosing) and chemical (e.g., hydrogen peroxide addition) strategies can significantly reduce bromate formation without influencing micropollutant elimination efficiency. Data-driven modeling identified ozone dose and contact dynamics as primary drivers of bromate formation. While the overall influence of wastewater composition was less pronounced, bromide concentration remained a critical factor. The research further demonstrated that immediate ozone reactions, which occur within seconds, play a key role in both micropollutant degradation and bromate generation. Finally, full-scale applications of advanced ozone injection systems demonstrated improved ozone distribution and rapid reaction rates, resulting in enhanced micropollutant elimination and effective bromate mitigation. This thesis shows that bromate risks can be managed effectively through targeted operational strategies. With the right design and controls, ozonation can be both efficient and safe. These insights support the broader implementation of ozonation as a viable solution for advanced wastewater treatment in compliance with future regulatory demands.