Previous research has established that dense membranes are very good at blocking passage of several PFAS [1]. Even very low molecular weight PFAS compounds show high rejections (>98%) in reverse osmosis and some nanofiltration processes[1]. Although permeate water can be produced with PFAS concentrations below regulatory limits, PFAS remains in the concentrate stream meaning none is necessarily removed from the water cycle if the concentrate is discharged to the environment. The concentration of PFAS in the concetrate stream is higher than in the feed stream however, making removal from the concentrate stream more feasible.

Recent research (2021) by Franke et. al. has shown the potential of combining membrane filtration and adsorption processes for PFAS removal from water streams [2]. Depending on the PFAS species being removed, either activated carbon or anion exchange resins showed better performance as adsorptive materials. Long-chain (>C6) PFAS compounds adsorb well on both activated carbon and anion exchange resins. Short-chain (<C7) PFAS however adsorb poorly to activated carbon due to repulsive electrostatic interactions dominating for these compounds.

Furthermore, the wide array of sources and locations where PFAS remediation is required means there is no one-size-fits-all solution. To address this, we investigate the possibility of using filter cakes as adsorption media, since these can be tailored to the specific needs of the feedstock to be treated. Besides being flexible in operation, this approach is also more future proof, since equipment will not have to be retrofitted when new concerning pollutants are discovered, which can require new adsorbent materials. We also found that whilst adsorption in cake layers is commonplace in industry (e.g. for the bleaching of edible oils), we could not find any studies on the effect of the buildup of the filtration cake on its subsequent use as an adsorption layer. Therefore, we decided to investigate the effect precoat and filter aid filtration on the subsequent adsorption of various PFAS’ over time.

In this work we combine ‘closed-cycle reverse osmosis’ (CCRO) with simultaneous adsorption in a cake layer for PFAS removal at environmently relevant concentrations. By doing...