# Approaching Breakthrough: Resource-Efficient Micropolitan Removal with MBR-GAC Configuration
## 4 Discussion  
The efficient and long-lasting removal of persistent pharmaceuticals in the investigated GAC filters can be attributed to several factors. The MBR process, used as a pretreatment to the quaternary treatment, produces high-quality effluent with no particles and low  
Figure 4: Removal efficiencies in the full-scale Himmerfäardsverket WWTP for (**a**) all analyzed pharmaceuticals and the sixteen most persistent pharmaceuticals; and (**b**) all analyzed PFASs (\(\underline{\mathbb{C}}\)PFAS11) and only PFOS for sample occasions (calendar week). levels of nutrients and easily degradable organic pollutants. This creates good conditions for any filter-adsorbent, as it prevents filter clogging in several ways. Clogging due to particles is avoided, and the formation of a thick biofilm is prevented due to the limited substrate available for bacterial growth. With fewer traditional pollutants in the water passing through the GAC filters, there is less competition for adsorption sites, thereby increasing the adsorption of OMPs. High concentrations of OMPs on the filter material and the lack of other more readily available substrates may also facilitate biofilm formation, with OMPs serving as a substrate. This would allow for biological regeneration of the filter materials, as adsorbed OMPs and other organic molecules are biologically degraded, liberating adsorption sites and increasing further OMP adsorption. This phenomenon has been indicated by studies, such as those by [39] and [40]. Additionally, the high oxygen concentrations in the MBR effluent, caused by continuous air scouring of the membranes, may further aid in the biological degradation of OMPs. The high removal rates of PFOS and other PFASs in the MBR process, compared to the CAS process in the full-scale WWTP, are more challenging to explain. A new research project has recently been initiated to investigate the underlying mechanisms. One potential explanation could be the removal of PFOS by foam, which commonly forms in MBR processes due to the enrichment of extracellular polymeric substances (EPSs). This foaming can be likened to foam fractionation for removing PFASs from aqueous matrices, as suggested by studies like [41]. Due to intensive foaming in the MBR pilot, the system was modified after the first year of operation to remove excess sludge by surface sludge extraction from the biological basins only. This may have favored PFAS removal by foam, as a foam destruction and mixing with process water, which would reintroduce PFASs into the water phase, is avoided. In comparison with ozonation as the most relevant other quaternary treatment techniques, similar removal efficiency for investigated pharmaceuticals can be expected, depending on plied ozone doses. For such an evaluation, actual tests have to be performed. Considering PFAS removal, however, no removal can be expected by ozonation [21; 22; 23]. Aspects such as the risk of formation of toxic degradation and by-products by ozonation that may require a post treatment e.g., by GAC filtering [37], significant increased onsite energy demands at the WWTP for ozone production, and potential for future regulations on other OMPs, such as PFASs, made the utility focus on the investigate combination of MBR-GAC as the main quaternary treatment. However, at the time of process selection, a potential significant removal of PFOS in the MBR process, as observed in this study, was not considered. The resource and cost reduction potential presented here provides only an indication based on the current state of the pilot trials. A replacement of the filter material has not yet been performed. If only the adsorbent in the first filter in series is replaced, while the adsorbent in the second filter is moved to the first position to receive a high load of OMPs for maximum capacity utilization, the expected lifetime of the filter material will be extended until a complete adsorbent exchange is necessary. Consequently, resource consumption and associated operation costs will be lower than currently presented.