## 2 Edge-of-Field Technologies
### Filter Materials
Iron oxide/hydroxide, however, proved to be the most efficient, with P retention reaching 99% when the solution contained 1000 mg P L\({}^{-1}\). This study indicated that P was not readily desorbed once it bound to these materials, and that the hydraulic conductivity of the filters was high in most cases (\(>\)0.01 cm s\({}^{-1}\)), supporting their potential use in the field. Additional experiments confirmed the viability of porous Fe composite in the treatment of agricultural drainage discharge both in terms of P retention and hydraulic conductivity [143]. Filters comprised by surfactant-modified zeolite and high Ca oxide-high carbon fly ash also presented effective P retention (\(\geq\)50%) for solutions ranging from 0.1 to 1 mg P L\({}^{-1}\) during 1 to 24 h of HRT [144]. Batch experiments are valid to give an indication of the potential of FMs for field application. However, these may sometimes overestimate the performance, as hydrological factors are not taken into account. Column experiments, on the other hand, are designed to test the potential of FMs for P retention under varying water flow conditions. [PERSON] et al. [140], for example, demonstrated that FMs with non-equilibrium transport (CFH and seashells) were capable of higher P retention and lower P release (P desorption) compared to those where an equilibrium flow was reached (Filtralite-P and limestone). The study showed that P retention could be further increased in the filters with non-equilibrium transport by increasing the water flow, as it supported P concentration gradients between mobile and immobile pore regions and ultimately the diffusion and sorption of P. This way, the study characterized intragranular P diffusion as an important P retention mechanism. Other FMs, including a mixture of activated carbon, zeolite and activated alumina also demonstrated potential in the treatment of P from agricultural drainage discharge, with average P retention of 52% in a laboratory experiment [145]. Column studies were also important to attest the extreme efficiency (\(\geq\)90% P retention) of the Fe-based filters zero-valent Fe, porous Fe composite, sulfur modified Fe and Fe oxide/hydroxide, regardless of water flow and HRT (down to 2.5-4.0 min), highlighting how quickly P sorption reactions occur in these materials [142]. Although column studies investigate the feasibility of FMs for field application, it is often recommended that these are further tested in small and/or large-scale experimental setups in the long-term and under varying conditions (e.g., P load and HRT) prior to field implementation. [PERSON] et al. [146] tested the retention performance of a sand filter enhanced with a layer of the P binding material Fosfilt-s and receiving agricultural drainage discharge for 3.5 years. The filter retained on average 0.03 g m\({}^{-3}\) d\({}^{-1}\) (37%) of total P and 0.04 g m\({}^{-3}\) d\({}^{-1}\) (45%) of DRP, as a result of proper water distribution. The study concluded that incorporation of Fosfilt-s to sand filters can be recommended for P retention from agricultural drainage discharge, where the concentration of suspended particles--a precursor to clogging--is normally low. Moreover, [PERSON] et al. [49] demonstrated that a filter comprised of a mixture of melter slag (90%) and basic slag (10%), and inserted at the outlet of tile drains reduced the loads of total P and DRP in relation to control drains backfilled with greywacke aggregate by 63 and 69%, respectively. Finally, [PERSON] and [PERSON] [111] reported that not only slag, but also limestone, seashells, shell-sand and tree bark were the most promising FMs in retaining P from the effluent of SFCWs receiving agricultural drainage discharge, when considering the retention potential, availability in New Zealand, cost and potential for reuse upon saturation. The studies above were important to investigate the applicability and limitations of FMs under field conditions for future considerations.