Emerging advanced oxidation processes for water and wastewater treatment – guidance for systematic future research

Advanced oxidation processes (AOPs) for water treatment are a growing research field with a large variety of different concepts and materials being tested at laboratory scale. However, only few concepts have been translated into pilot- and full-scale operation recently. One major concern are the inconsistent experimental approaches applied across different studies that impede identification, comparison, and upscaling of the most promising concepts. The aim of this tutorial review is to streamline future studies on the development of new solutions and materials for advanced oxidation by providing guidance for comparable and scalable oxidation experiments. We discuss recent developments in catalytic, ozone-based, radiation-driven, and other mostly physical AOPs, and outline future perspectives and research needs. Suitable figures-of-merit for comparison and benchmarking of AOPs are reviewed. Since standardized experimental procedures are not available for the majority of AOPs, we propose basic rules and key parameters for lab-scale evaluation of new AOPs including selection of suitable probe compounds, model compounds, and scavengers for the measurement of (major) reactive species. A two-phased approach to assess new AOP concepts is proposed, consisting of (i) a feasibility-of-concept-study phase with validation of major radical species and comparison to suitable reference processes and materials, followed by (ii) a benchmarking phase conducted in the intended water matrix for the process, applying comparable and scalable parameters such as UV fluence or ozone consumption. Screening for transformation products should be based on chemical logic and combined with complementary tools (mass balance, chemical calculations) to advance mechanistic understanding of the process.