Design and Operation of Ozone/BAC for Water Reuse - Experiences from Europe and USA

Ozone is well known as an effective disinfectant and oxidant and has been operated for drinking water treatment in combination with biofiltration or biologically active carbon filtration for more than 50 years. Its versatility makes ozone/BAC a powerful tool for non-potable and potable water reuse applications. It is frequently applied as major barrier for pathogens and chemicals in carbon-based indirect potable reuse projects, especially for inland applications with limited options for sustainable management of reverse osmosis concentrates. In addition, most recent regulation for California includes ozone/BAC as an important barrier for control of chemicals in direct potable reuse (DPR) projects. In Europe, pilot- and full scale ozonation systems are currently installed in several countries with the aim to mitigate the discharge of chemicals of emerging concern (CECs) into receiving surface waters. Recent regulatory changes are expected to accelerate the implementation within the European Union: In October 2022, the European Commission published a draft of the amended Urban Wastewater Treatment Directive, which for the first time includes requirements for mitigation of organic micropollutants. In addition, EU has defined minimum requirements for agricultural water reuse that came into force on 26th June 2023 and provoked pilot studies testing ozone/BAC also for water reuse applications. This presentation will discuss experiences with ozone/BAF design and operation in Europe and the US, identify key parameters for design and process control, and outline potential concepts for future design. Ozone/BAC for water reuse in the US is designed for simultaneous removal of chemicals and pathogens, often applying ozone to TOC ratios >1 g O3/g TOC. Efficient disinfection (> 6 log inactivation) was confirmed in various spiking tests with bacteriophages at pilot scale. In contrast, ozonation of secondary effluents is designed for CEC removal and recent applications often apply ozone/TOC ratios as low as 0.3 g O3/g TOC. However, even at higher doses around 1 g O3/g TOC only limited inactivation of abundant indicator bacteria was observed in different studies, which often results in the conclusion that disinfection of wastewater by ozone is not effective. While ozone alone is effective for the removal of many contaminants, its combination with post-treatment in BAC filters is typically recommended to remove easy biodegradable oxidation by-products generated during ozonation and thereby lower effluent TOC and adverse effects in downstream processes. Besides, adsorption in BAC filters may serve as additional barrier for CECs if the carbon is regenerated on a regular basis. In our presentation, we will highlight similarities and differences in process design, control and operation in the US and in Europe, discuss potential reasons for deviating results, and outline solutions for an optimized design of ozone-BAC including effective removal of CECs and pathogens as well as the control of by-product formation. The presented results are based on own research (pilot and full-scale studies in Europe and the US) and literature studies including previous work for the WRF project 4832.