Researchers at Cranfield University’s Water Science Institute are conducting research to determine how different treatment processes can remove a range of potentially harmful per- and polyfluoroalkyl (PFAS) compounds in drinking water.
As the prevalence of PFAS substances in drinking water sources continues to raise public health concerns, understanding the potential health impacts and effective mitigation strategies has become imperative. Over 4,500 PFAS compounds have been reported to be commercially available, having various applications including as anti-foaming agents, propellants, non-stick coatings, lubricants and for use in fire-fighting materials.
PFAS compounds are typically highly stable and persistent molecules due to their C-F bond that are recognised as the strongest bond in organic chemistry. Among those, Perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) stand out as the most extensively examined PFAS compounds in the context of water source pollution, due to their widespread application. Both perflurooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) have been discontinued globally, however their continued persistence and historical application have resulted in their continued high prevalence in the environment. Because of the risks and several reported cases of PFAS contamination in source waters across the UK, monitoring raw waters (such as water from rivers and lakes) is now a requirement for them to be used as drinking water source in England and Wales. The revised Drinking Water Directive has established a parametric value of 0.1 µg/L for a combination of 20 PFAS, along with a value of 0.5 µg/L for the total PFAS concentration (European Union, 2020).
A recent report surveyed the concentrations of PFAS in England and Wales sources and identified 44 PFAS compounds that may be present in source waters; this included 16 PFAS compounds that were present in a small number of water sources at concentrations that would exceed the guidance limit of 0.1 µg/L for PFAS in drinking water (WRC, 2020) (DWI (2021). Although a significant amount of work has already been undertaken evaluating PFAS removal by various treatment processes, the majority of these studies have not specifically addressed water sources in the UK.
Funded by the Drinking Water Inspectorate for England and Wales (DWI) in collaboration with the Drinking Water Quality Regulator for Scotland and UKWIR, experimental work at Cranfield University includes bench-scale testing and pilot scale evaluation of both conventional and novel treatment techniques to determine the mechanisms of removal and the operational conditions required to achieve sustained PFAS removal.
Testing different water sources is crucial as the removal efficiency is strongly dependent on the background water environment as well as the concentration, characteristics, and types of PFAS present in the water. A poorly understood aspect is the influence that water quality plays on PFAS removal (e.g. natural organic matter type and concentration, pH, ionic strength, alongside the presence of other contaminants), as these constituents may affect the removal mechanisms of PFAS. In addition, given the varied characteristics of the PFAS compounds with respect to molecular weight, degree of charge and hydrophobicity, there is a need to understand the specific removal mechanisms.
To ensure the results of the project are relevant to UK water utilities, researchers are testing different surface and groundwater sources for each treatment technology. The team is currently conducting tests on adsorption, ion exchange, membrane filtration, advanced oxidation/reduction, foam separation, and coagulation for the removal of 15 selected PFAS, representing a diverse range of physicochemical properties. In the second phase of the work, selected treatment processes will be taken forward for pilot scale evaluation. Here we are utilising the UKCRIC National Research Facility for Water and Wastewater Treatment and the Point-of-use Water Treatment Development Laboratory at Cranfield University.
By the end of the study together with the long-term pilot scale trials, the research aims to have assessed the efficacy of each treatment process considering the background water characteristics and the physicochemical properties of the PFAS. To date this work has found that different mechanisms control PFAS removal depending on molecular properties such as carbon chain length, charge, terminal functional group, and hydrophobicity. Among the tested bench technologies, membrane filtration and ion exchange have yielded promising results, even for the most difficult-to-remove short chained PFAS.
Future research should focus on developing models that can predict how new and emerging PFAS compounds can be removed by different treatment processes based on their chemical structure. This will enable effective risk assessments to be developed as new.
References
European Union (2020). Interinstitutional File: 2017/0332(COD). Directive of the European Parliament and of the Council on the quality of water intended for human consumption pdf (europa.eu).
DWI (2022). DWI Information Letter 03/2022. DWI guidance document on PFAS.
Water Research Centre Limited (WRC) (2020). Poly and perfluorinated alkyl substances in drinking
44 water. Drinking Water Inspectorate Report Reference: DWI14078.02/16964-0.