Key emerging contaminants and their implications for water quality: A critical review of occurrence, regulation, and treatment performance

Emerging contaminants (ECs) have gained increasing scientific and societal attention in recent decades due to their environmental persistence, potential toxicity, and limited regulatory control. This review synthesizes current knowledge on their occurrence in aquatic systems, the evolving legislative framework, and the effectiveness of available removal technologies. Unlike previous papers that typically isolate specific pollutant classes, the novelty of this review lies in providing a unified framework that simultaneously integrates the environmental occurrence, regulatory evolution, and treatment performance of seven representative substances (diclofenac, azithromycin, 17β-estradiol, bisphenol-A, glyphosate, imidacloprid, and thiamethoxam). Analysis of 92 scientific publications and regulatory documents revealed a marked rise in research activity between 2012 and 2025 confirming alarming concentrations across aquatic systems. A critical challenge identified is the widespread absence of binding concentration limits despite recent legislative progress (e.g., Directive (EU) 2024/3019), which severely hinders effective risk management. Another major challenge is the persistent failure of conventional wastewater treatments, coupled with significant knowledge gaps regarding full-scale removal of specific compounds like glyphosate. Spain's Royal Decree 1085/2024 requires monitoring of emerging contaminants in reclaimed water, but it does not establish binding concentration limits for many substances. High levels of these contaminants have been reported in surface water, wastewater, and groundwater. Twenty‑two removal technologies were identified, with multibarrier systems, ozonation combined with activated carbon, membrane bioreactors, and biochar adsorption achieving the highest efficiencies (typically >80%, and in some cases >90%). Conversely, conventional treatments such as activated sludge showed limited performance, and a significant knowledge gap regarding glyphosate removal under full-scale conditions and standardized monitoring frameworks. Ultimately, this review concludes that mitigating the threat of ECs will require the progressive adoption of quaternary treatments, supported by the development of harmonized and legally binding environmental thresholds.