Analysis of Granular Activated Carbon Removal Efficiency of Per- and Polyfluoroalkyl Substances in Groundwater
Per- and polyfluoroalkyl substances (PFAS) are a group of man-made contaminants that are highly persistent in the environment and bioaccumulate within humans. PFAS contaminants the environment through industries such as aviation, chemical manufacturers, and processing facilities. The increasing discovery of PFAS in groundwater sources for drinking water has raised concerns since studies have shown PFAS to be potentially carcinogenic. Routine exposure through drinking water could lead to long-term health effects depending on the concentration and duration of exposure. Due to this health concern, the Environmental Protection Agency (EPA) set an health advisory limit (HAL) of 70 parts per trillion (ng/L), thus requiring numerous contaminated water systems to begin treating for PFAS. The objective of this study is to review the granular activated carbon (GAC) removal efficiency for per- and polyfluoroalkyl substances (PFASs) at contaminated groundwater sites. PFAS treatment data were collected from three full scale granular activated carbon (GAC) systems that are currently treating PFAS. The data were graphed to analyze the behavior of PFAS with GAC treatment. The study looked at raw water and treatment characteristics that could be contributing to GAC’s effectiveness at removing PFAS from drinking water. Factors that contribute to the removal efficiency include water quality, design characteristics, adsorbent characteristics, and contaminate properties. Many pilot studies have shown GAC to be an effective form of treatment, but there are limited studies that show this on a full-scale over a long duration of time. This study showed that the GAC system was effective at reducing concentrations of PFOS and PFOA greater than 95% dependent on the quantity of water treated expressed as media bed volumes. The study also confirmed that GAC treatment is not as effective for short-chained PFAS such as PFBS and PFHxA. At one treatment site, PFHxA demonstrated an increase concentration leaving the lag vessel indicating that desorption was occurring. A comparison between the three different treatment sites media replacement cost showed that switching the lead and lag vessel prior to the replacement of media in the original lag vessel reduced the media replacement cost by nearly half. A theoretical PFAS breakthrough bed volume calculation was performed using Freundich’s equation to compare to the actual breakthrough observed at each treatment site. The theoretical calculation typically uses conservative values and applies a safety factor since there are several unaccounted factors that affect adsorption. These factors include water quality, competition for adsorption, total PFAS concentrations and more. Due to the number of variables that affect the adsorption behavior of GAC, a pilot study to develop specific treatment data is normally encouraged since the theoretical calculation cannot be solely relied on. Several factors that affect the adsorption of GAC were not examined in detail within this study. Other articles have shown that the type of TOC present in water affects the GAC adsorption efficiency more or less than other types. Other contaminants, such as VOC’s have different properties that could compete with PFAS for adsorption sites. The competition for adsorption sites has mainly been commented on among the different forms of PFAS, but not among other contaminants. The data presented provides an overall trend of PFAS treatment, but due to the numerous factors that affect adsorption it cannot always be directly correlated to other treatment sites believed to have similar water quality.
|Analysis of Granular Activated Carbon Removal Efficiency of Per- and Polyfluoroalkyl Substances in Groundwater
|CC BY-SA 4.0 (Attribution-ShareAlike)
|February 02, 2022