Persistent and mobile (PM) substances are suspected to pass drinking water treatment; however, the extent of the treatment gap remains uncertain. This study aimed to determine the elimination of 115 potentially persistent and mobile substances (PM candidates) for four established (biological filtration, adsorption onto activated carbon, ozonation, reverse osmosis (RO)) and one developing treatment method (reductive treatment represented by catalytic hydrogenation) for drinking water supply individually and in combinations. In systematic lab-scale experiments, less than half of all PM candidates were eliminated above 80% by adsorption or ozonation at typical doses (10 mg/L with 48 h contact time, 0.67 mg O3/mg DOC), whereby adsorption indicated slightly higher elimination. Overall, RO achieved the highest elimination, exceeding 80% removal for 96% of all PM candidates. In three common combined treatment sequences, 16 PM candidates remained below 50% elimination and were therefore identified as critical for drinking water treatment, among them six halomethanesulfonates and seven components of ionic liquids. For adsorption onto activated carbon, polarizability, logD and solubility showed strong correlations with PM candidate eliminations, whereas for RO, strong correlations were observed between PM candidate eliminations, and molar mass and polarizability. These correlations provide suitable and readily accessible molecular properties to feed predictive models for the assessment of PM candidate elimination.

Biotransformation in sand filters and natural subsurface barriers is a key process for eliminating organic micropollutants from drinking water. However, underlying mechanisms are complex and knowledge on influencing parameters is fragmentary. There is still a knowledge gap regarding contaminants of emerging concern, such as persistent and mobile substances, and their fate in those processes. This study investigated sorption and biotransformation of 18 polar and potentially persistent contaminants in oxic sand columns, evaluating the effect of biodegradable dissolved organic carbon (BDOC, drinking water vs. surface water), hydraulic retention time (9 h vs. 25 h) and sand type (microbial community, technical sand vs. sand from a groundwater recharge site). Substance and operation specific extent of biotransformation was observed. Overall, nine substances were biodegraded, while nine indicated persistence. Negligible sorption was observed, only the cationic 1,3-di-o-tolylguanidine was retarded by four bed volumes. The highest impact on biotransformation was observed for the parameter BDOC, followed by hydraulic retention time and finally sand type. The strongest effect was observed for 2-methyl-2-propene-1-sulfonic acid (MPSA), which was readily biodegradable in surface water and persistent under BDOC limiting conditions. Previous classifications as persistent, e.g. of 2-acrylamido-2-methylpropane sulfonate and adamantan-1-amine, were confirmed. Three substances previously categorized as persistent (acesulfame, MPSA and 4-hydroxy-1-(2-hydroxyethyl)-2,2,6,6-tetramethylpiperidine) showed biotransformation. These results emphasize the complexity of persistence assessments and the potential for some polar contaminants to be eliminated in treatment processes relying on biotransformation in the subsurface or sand filters under oxic conditions.

Agricultural water reuse is one approach to mitigate water stress. In addition to the minimum requirements, the European water-reuse regulation 2020/741 mandates a risk management approach for agricultural water reuse. In contrast to the microbiological monitoring, the extent of the chemical risk assessment and monitoring is not clearly defined. The resulting complexity of a typical agricultural water-reuse scheme was analyzed. Potentially relevant parameters were identified based on European and German regulatory frameworks, concerning key subjects of protection. An interdisciplinary assessment of efforts and challenges, regarding required analyses, was accomplished, using expertise from recent research investigating agricultural water reuse in Germany. Suggestions were provided for disinfection validation, microbiological monitoring parameters and analytical methods. Additionally, chemical indicator parameters were suggested to address relevant processes during monitoring. Both microbiological and chemical parameters presented analytical challenges, which were described with future needs to support water-reuse implementation. Costs for analyses were estimated using available price information, highlighting the high costs of certain analyses, especially for organic micropollutants. Therefore, analyses need to be further facilitated by the application of process indicators and the implementation of cost-effective, multi-target methods tailored to the requirements of risk management for agricultural water reuse.