## 2 Edge-of-Field Technologies
### Filter Materials
If this is often the case, placing the FM upstream the bioreactor may optimize the retention of P.  
The variability in P retention between FMs is largely caused by differences in HRT and inflow P concentration [153; 154]. It is generally observed that higher mass P retention is found under increasing water flows, while percentage P retention enhances under longer HRTs, i.e., lower water flows [145; 147; 149; 155]. This is because increasing water flows implies higher P loads for cumulative retention in a fixed volume and time period, although this reduces the contact time for a fraction of the P load to be retained [154]. The short response of P retention to these hydrological factors, considering the relatively small volume of filters, can result in marked variability in P retention when receiving event-driven drainage discharge. The inflow P concentration, on the other hand, normally contributes positively to both mass and percentage P retention, as it supports sorption reactions with Porbents [146; 154; 156]. The material of the filter reflects a specific P sorption capacity and hydraulic conductivity (Equation (11)), which also affect the P retention performance. An ideal material would have a high capacity for P sorption as well as rapid hydraulic conductivity. However, it is common for these parameters to correlate inversely [137]. This is because FMs tend to increase the P sorption capacity with smaller particle sizes, as these reflect on larger surface areas for P sorption [139]. Smaller particle sizes, on the other hand, decrease the filter porosity and hinder the water flow, decreasing consequently the hydraulic conductivity. \[\text{hydraulic conductivity}=\frac{\text{water flow (m${}^{3}$ yr${}^{-1}$)}}{\text{cross - sectional area (m${}^{2}$)}\times\text{hydraulic gradient}} \tag{11}\]  
The content and forms of specific P sorbents in the material affect the P sorption capacity and retention [138]. Filters based on Fe and Al are recommended when the HRT in the system is relatively short, i.e., high water flows, due to fast ligand exchange with P in comparison to those based on Mg and Ca, in which precipitation reactions are comparatively slower [138; 153; 154]. Thus, the former filters are generally more efficient and can retain more P, whereas the latter may be suitable in situations where the HRT is relatively long and P concentrations are lower (e.g., <2 mg L\({}^{-1}\)[138]) [154]. This was clearly demonstrated in a flow-through setting by [PERSON] et al. [156], where the Fe-based filter CFH presented a P sorption capacity and affinity 10 times higher than the Ca-based filter Filtralite-P. The study also demonstrated that P release from previously sorbed P was lower in CFH (<10%) than in Filtralite-P (>35%). Finally, it was observed that CFH was able to retain P even at high water flows, while Filtralite-P depended on sufficiently long HRTs for proper retention. The study concluded that CFH was preferred in catchments with varying P concentrations and HRTs. Moreover, Fe and Al-based filters with a higher fraction of amorphous than crystalline oxides enhance P retention due to higher reactivity of the former forms, while retention in Mg and Ca-based filters depends on the water solubility of the different forms--commonly controlled by the discharge pH--so as to allow precipitation with P [138]. Therefore, a faster precipitation of P in Mg and Ca-based filters can be observed when the material has a high and buffered pH (>8), which enhances the sorbents solubility [154]. The complex inherent composition of FMs ultimately contributes to the large variability in P retention. This consequently hinders or impedes direct comparisons between filters. [PERSON] et al. [154] discussed that normalizing P retention as a function of P addition per unit mass of the FM facilitates comparisons between filters as the cumulative P load is taken into account. However, the study indicates that variations in HRT and inflow P concentration can still make proper comparisons difficult.