۱٫ Introduction

۲٫ Insights on contaminants of emerging concern (CECs)

۳٫ Treatment technologies

۴٫ Process intensification

۵٫ Conclusions

Acknowledgments

References

Abstract

Urban wastewater treatment plants (WWTPs) are among the major sources of contaminants of emerging concern (CECs) release to surface water. This is mainly associated with the low performance of conventional technologies (e.g. activated sludge, sand filtration and UV disinfection processes) for the removal of residual amounts (ng L-1 – μg L-1) of CECs. In this review, a list of CECs commonly detected in the aquatic environments is presented. Light-driven chemical/catalytic processes for CECs removal, namely UV/oxidant and photocatalysis, are presented with discussion of their process, advantages, drawbacks, and the main attainments. In this context, process intensification has been gaining high relevance for wastewater treatment purposes due to its potential to overcome the main treatment limitations. Accordingly, an overview of advances to overcome the limitations in light-driven chemical/catalytic processes for CECs mitigation was carried out, focusing mainly on the design of novel reactors and devices. Some aspects related to reactor configuration, removal mechanism, devices applied, illumination system, comparison criteria, and catalyst immobilization through implementation of novel reactors designs are also discussed. Regarding the reactor design, several devices exhibit a satisfactory mass transfer due to a large surface-tovolume ratio or enhanced mixing conditions. However, improvements in photon transfer are still the biggest challenge to be overcome. The irradiation of the entire reaction solution and/or catalyst surface has shown to be challenging in many reactor configurations. Nevertheless, the reactor design and treatment process to be applied for CECs removal must be selected according to the site-specific conditions, in order to accomplish CECs mitigation and attain the best reactor performance.

Introduction

About 71 percent of the Earth’s surface is water-covered, however less than 3 percent of this amount can be considered as freshwater [1]. In addition, this limited resource is under threat from the  pollution, mainly caused by human factors. Power generation, industrial, agriculture and mining activities among others are some of the contributors to the aquatic compartments contamination, 6 which affects directly life on Earth [2]. Approximately two-thirds of the world’s population experiences water shortage for at least one month in a year [3]. Additionally, the World Health 8 Organization (WHO) reported that, in the year of 2017, 2 billion people living in various regions all over the world used drinking-water source contaminated with faeces [4]. In fact, it is estimated that almost 1.8 billion people worldwide might face moderate or severe water scarcity by 2025 [5].  Commonly, materials and chemicals used in daily human activities are continuously introduced into the environment. These so-called Contaminants of Emerging Concern (CECs) are one of the main causes of water pollution and, hence, being often related to the risks to human and biota health [6]. Therefore, as water safety and quality are fundamental to human development and well-being, the scientific community has been applying great efforts to the development of efficient technologies for water purification.