Special Issue “Advanced Research on Micropollutants in Water”

1. Introduction

The increasing occurrence of micropollutants in aquatic environments has become a global concern due to their persistence, potential toxicity, and resistance to conventional water treatment processes [1]. These include active pharmaceutical ingredients (APIs), personal care products (PCPs), pesticides, and microplastics, whose presence in water bodies can pose significant risks to ecosystems and human health, even at trace concentrations [2].

Their frequent detection in water systems is mostly due to the fact that current wastewater treatment plants (WWTPs) are not designed to specifically remove micropollutants [3]. Many of these compounds were intentionally developed to be resistant to biological degradation, ensuring stability during use. However, this same persistence hinders their removal during conventional treatment processes, as WWTP technologies have not evolved at the same pace as the complexity and diversity of emerging contaminants [4].

In response to the growing concern over micropollutants in aquatic environments, the European Union has revised its Urban Wastewater Treatment Directive. The updated directive (EU) 2024/3019 launched in November 2024 mandates the implementation of quaternary treatment processes in wastewater treatment plants serving agglomerations of ≥150,000 p.e. (population equivalent) by the end of 2045, requiring at least 80% removal of micropollutants [5].

As global environmental regulatory standards become more stringent, advanced research on micropollutants is required, aimed at improving detection methods and removal technologies and deepening our understanding of micropollutants’ environmental fate, transport, and impact. This Special Issue, titled “Advanced Research on Micropollutants in Water”, captures recent diverse approaches that advance research in micropollutants to meet environmental regulations and tackle emerging challenges in water treatment. This Issue comprises 10 articles, which are briefly described in the next paragraphs.

2. An Overview of Published Articles

The study by Dolatimehr et al. (contribution 1) focused on understanding the fate of Cyanotoxin Dihydroanatoxin-a (dhATX-a) in Drinking Water Treatment Processes, namely slow sand filtration, flocculation, adsorption onto activated carbon, ozonation, and chlorination. The authors reinforce the importance of understanding the behavior of dhATX-a under such processes because this micropollutant was recently detected in German surface waters that serve drinking water production at alarming concentrations. The study reveals that except flocculation and chlorination, all of the other water treatment processes studied are effective in removing dhATX-a.

The article by Elmorsi and Lai (contribution 2) is dedicated to optimizing a capillary electrophoresis method for the separation of four model pharmaceutical hydrochlorides, as a strategy to facilitate their monitoring and quantification in water quality testing. To this end, the authors used a statistical design tool, namely a Central Composite Design, to identify the concentration of sodium dibasic phosphate in the background electrolyte solution (BGE), pH, and applied voltage across the capillary (variables) that resulted in the best peak resolutions and total migration time (responses). The authors observed that the three variables had a significant impact on the separation, while their interactions had minimal effects. The best responses were achieved at a BGE concentration of 75 mM, pH 9, and applied voltage of 10 kV, with a good separation of all peaks under 5 min.

The third article from Mouthón-Bello et al. (contribution 3) studied the removal of nutrients from urban wastewater using three different configurations of a submerged membrane bioreactor. Configuration No. 1 comprises the anoxic/anaerobic/oxic sequence, with the concentrated sludge being returned from the submerged membrane bioreactor to the first stage.

Configuration No. 2 is a modified version of the three-stage Bradenpho biological nutrient removal system, where sludge is recycled from the submerged membrane bioreactor to both the anaerobic and anoxic zones. Configuration No. 3 employs a post-denitrification process. The authors verified that Configuration No. 1 demonstrated consistent ammonia removal with high nitrate effluent concentration due to the aerobic conditions in the last two reactor zones. Configuration No. 2 exhibited moderate nitrogen removal, with the advantage of a low bioreactor volume requirement, reduced oxygen demand, and simplified operation. Configuration No. 3 allowed for reducing the nitrate concentration before recycling it to the anaerobic zone, although it required a higher bioreactor volume and oxygen supply. These results indicate that is possible to reach the desirable nitrogen removal with all the configurations tested.

The research by Graça and Soares (contribution 4) focused on transforming biomass residues into biochars or activated carbons capable of removing pollutants resistant to ozone-driven water treatment, either by adsorption or catalytic ozonation. The authors verified that biochars generally outperformed activated carbons in adsorption tests, except for cork-derived materials. In this case, the biochar made from almond shell revealed the best cost/benefit ratio in terms of production cost versus pollutant uptake. Regarding catalytic ozonation experiments, activated carbon made from cork and coffee grounds exhibited the best performance, with the best cost/benefit ratio attributed to the later.

In the fifth article, Díaz-Gamboa et al. (contribution 5) investigated the potential of storage lagoons as a quaternary treatment step in wastewater treatment plants (WWTPs), aiming at aligning with the recent European Urban Wastewater Treatment Directive (UWWTD) which stipulates 80% reduction in a specific list of micropollutants. The methodology involves the storage of the secondary treatment effluent in two lagoons with an average hydraulic retention time of 34 days. Then, the effluent from the lagoons undergoes disinfection by chlorination. The authors verified that lagoons harbor a complex interplay of physical, chemical, and biological processes that enable the removal of the 12 substances listed in the new Urban Wastewater Treatment Directive and thus can serve as a quaternary treatment.

Valentukeviciene et al. (contribution 6) studied the potential of low-cost adsorbents and phytoremediation in removing chlorine from stormwater. The authors highlight the importance of this kind of research because the runoff of disinfected areas may transport residual chlorine present in stormwater to surface water bodies, posing a risk to aquatic flora and fauna. The results of this investigation indicate that the best chlorine retaining materials are sawdust and lightweight aggregates, and the plants used in phytoremediation tests (Tagetes patula or Pisum savitum) enable the removal of chlorine below the limit of detection of the analytical method and can this be considered effective processes.

The research of Aghel et al. (contribution 7) is aimed at developing a droplet-based microfluidic impedance flow cytometer for in situ detection of microplastics in water. This novel methodology for microplastics detection is based on conductivity and signal phase changes as microplastics move in water, enabling both their measurement and quantification. The developed system allowed for detecting microplastics with a sensitivity of 97.4%.

The article by Basapuram et al. (contribution 8) is aimed at understanding the distribution of 33 contaminants in the North Oconee River (Georgia) using Gas Chromatography–Mass Spectrometry in full scan mode for a non-targeted qualitative approach. The authors identified a prominent presence of pesticides in this river, which indicates contamination from agricultural and urban runoff.

Pronk et al. (contribution 9) grouped a total of 196 micropollutants in clusters according to emission sources, substance type, or type of use. Between Rhine and Meuse rivers (The Netherlands), the authors found 9 repeating clusters with similar composition, which enabled the identification of drivers. For instance, the authors linked a repeating cluster with polychlorinated biphenyls to high river discharge and attributed it to a potential release from sediment resuspension.

The work of Sariyildiz et al. (contribution 10) explored relationships between actively mined and reclaimed areas, vegetation change, and water quality parameters in Coal-Mine-Affected Watersheds in Kentucky, USA. This study indicated that conductivity is a predictable water quality indicator associated with Coal-Mine-Related Stream Chemistry in areas where agriculture and urban development are limited. Also, the authors verified that the area of reclaimed forests may be a predictor of the mining percentage and reclamation age. Such findings may help in understanding water quality attributes in watersheds affected by coal mining, as well as contributing to better land reclamation practices.

3. Conclusions

The articles presented in this Special Issue evidence the relevant research that has been conducted regarding the presence of micropollutants in water systems. Topics include the behavior of specific environmentally relevant micropollutants during conventional water treatment processes, the development of novel analytical methods for their quantification, and the advancement of new techniques aimed at their efficient removal. Some contributions also focus on the identification of micropollutants in river systems, exploring relationships in their chemical structure, sources of emission and use, as well as the environmental conditions of the monitored sites.

All of these investigations represent valuable contributions to the advancement of research in environmental remediation and reflect the global efforts that have been made in recent years to tackle environmental challenges related to water pollutants.

The investigations featured in this Special Issue are grounded in strong experimental components, ensuring that the results reflect current trends regarding the pollutants and/or locations under study. This aspect makes this Special Issue particularly relevant to the scientific community, especially for researchers working in environmental remediation and those seeking to stay up to date with the latest methodologies in this research field.