A Perspective on Quaternary Treatments for Micropollutant Elimination in Urban Wastewater Treatment Plants

The recent Urban Wastewater Treatment Directive (EU2024/3019) by European Union requires implementation of quaternary treatment in specific urban wastewater treatment plants (WWTPs), which marks a transformative shift in water policy and environmental protection. This directive aims to eliminate micropollutants, e.g., pharmaceuticals and pesticides, which are resistant to conventional WWTPs. Despite their biologically active behaviour and toxicity to aquatic ecosystems and humans, micropollutants in WWTP effluent remain largely unregulated worldwide. This perspective presents a critical roadmap for integrating quaternary treatment in existing WWTPs by focusing on three leading technologies: activated carbon (AC) adsorption, high retention nanofiltration/reverse osmosis (NF/RO) membranes, and ozonation. Each technology is critically evaluated using a decision matrix by considering their technical performance, cost effectiveness, operational complexity, and environmental footprint. We recommend hybrid and modular treatment systems designed for site-specific conditions and micropollutant profiles. Finally, we provide strategic recommendations for research, policy, and wastewater treatment industry to advance the science and scalability of technologies for micropollutant elimination.