Evaluation of two-stage GAC filtration for PFAS reduction in drinking water

Sammanfattning: PFAS, or per- and polyfluoroalkyl substances, are a group of chemicals found in a wide range of industrial and consumer products. Due to their persistent nature and widespread use, concerns have been raised about their impact on human health and the environment. Humans are exposed to PFAS through the air and dust (both indoor and outdoor) as well as consumption (water and diet). As a result, various jurisdictions around the world are taking steps to regulate these substances. The new regulation limits the sum of PFAS 4 to 4 ng/L in Sweden with a transitional period that will end in 2026. This thesis, performed in collaboration with Uppsala Water and Waste Ltd, aimed to evaluate the removal efficiency of PFAS with two-stage granular active carbon (GAC) filtration and determine the number of bed volumes before individual PFAS breakthroughs and how the flow rate and empty bed contact time (EBCT) affects the time until GAC regeneration, as well as the selective removal of long versus short PFAS. The hypothesis of this study was that a two-stage GAC filtration would enhance removal efficiency and extend the time until GAC regeneration compared to a single-stage GAC filtration. The assumption is that a two-stage process would double the number of bed volumes before PFAS 4 breakthrough. A reduction in flow rate would lead to higher removal efficiency by extending the empty bed contact time (EBCT), ultimately prolonging the time until breakthrough for both stages due to increased adsorption. This thesis is based on a pilot experiment that consisted of eleven plastic columns divided into two stages. Nine of the columns were filled with three different granular activated carbon (GAC) – new Aquasorb® 6300 and both new and regenerated (“Old”) Filtrasorb® 400-E. The experiment started in June 2022 and continued until January 2023. Approximately 40 000 bed volumes of water were treated during this time. The analysis included a total of 34 PFASs. The results showed that a two-stage process was more effective in removing PFAS and extending the time until GAC regeneration compared to a single-stage process. The study also found that long-chained PFASs have a higher level of adsorption than short-chained ones, which is consistent with previous research. The lower flow rate gave a lower removal efficiency than the higher one, contradicting the hypothesis, but this is probably due to the amount of water already treated by the columns before the flow rate was reduced. Thus, the effects of the flow rates could not be directly compared. Based on previous research, a lower flow rate should provide better removal efficiency. The study concluded that a two-staged GAC filtration can approximately treat three and a half times as many bed volumes as a single stage before the limit of PFAS 4, 4 ng/L were reached. These results are unique for Bäcklösa drinking water treatment plant during this time period, and PFAS concentration and removal efficiency could vary elsewhere. The evaluation of removal techniques in drinking water production contributes to protecting human health from harmful water pollution.