## 2 Edge-of-Field Technologies
### Filter Materials
The pH of the discharge is also important to determine the proper FM so that P sorption and retention can be optimized. Phosphorus retention increases in Fe and Al-based filters when the discharge pH < 7.5, which supports positive charge sites in the material and decreases the competition of phosphate with hydroxides for sorption sites [153]. Filters based on Mg and Ca, on the other hand, optimize P retention when the discharge pH > 6 [153], which enhances the sorbents solubility and precipitation with P. In general, the pH of a FM is recommended to be between 5.5 and 7.5 for optimal P sorption and retention as well as to protect downstream aquatic ecosystems from effluents with detrimental pH [138]. Design parameters, including the mass and volume of the material used and the configuration of the filter (e.g., water flow direction), also affect the retention of P [137]. Therefore, it is important to understand the catchment conditions (e.g., load of DRP) prior to filter implementation in order to determine the proper material and design for acceptable P retention within a time period (i.e., the filter lifetime). In other words, a successful design will ultimately have the appropriate sizing to target a retention goal based on the P inputs into the system. This can be achieved with the so-called design curve, which estimates P retention based on the amount of P added per unit mass of the FM, and therefore calculates the required mass of the material [154]. The efficiency of FMs will eventually decrease over time due to saturation of P sorption sites until a moment when it is no longer able to retain P. The lifetime of a filter will primarily depend on the P sorption capacity, P load and water flow dynamics. Therefore, the configuration of the filter shall allow replacement or rejuvenation of the material after it becomes spent. Moreover, the use of FMs may be limited to agricultural catchments with marked concentrations of DRP (e.g., >0.2 mg L\({}^{-1}\)) [137; 154]. This is because low P concentrations during long periods will likely reduce the ability of filters to sorb P due to equilibrium reactions between the amounts of P sorbed and the low P concentration in the flow discharge. Therefore, the implementation of these systems may be restricted to catchments characterized by high loads of DRP. The use of filters is also restricted if the material contains considerable amounts of heavy metals prone to leaching, which could harm downstream aquatic ecosystems. This is particularly the case when the discharge pH and redox conditions favor the solubility of these elements [138]. A few studies carried out cost-efficiency assessments to determine the feasibility of treating DRP with FMs. Common costs include the mass of the material used and the area needed, being these regulated by the P load, retention goal and expected lifetime of the filter [154]. [PERSON] [152], for example, found an estimated cost of $209 kg\({}^{-1}\) DRP in a filter of steel shavings, which the author considered expensive. Additional construction costs (e.g., large amount of material mass and area) may incur if the design of a certain FM shall enable long HRTs in catchments characterized by high water flows. As discussed above, FMs based on Fe and Al are less sensitive to variations in HRT, and may thus lower the costs under high water flow conditions. This highlights the impact of a specific material on the costs. Moreover, industrial by-products are generally considered cheap and readily available in comparison to naturally occurring materials, which may depend on mining and transportation costs. The lifetime of the FM will also influence the costs. It is considered that rejuvenating spent materials for reuse instead of replacing them will contribute to improve the cost-efficiency [154]. However, this needs to be investigated, where results in this direction would support the use of filters.