## 2 Edge-of-Field Technologies
### Filter Materials  
The edge-of-field technologies described above are especially effective in the retention of PP from agricultural drainage discharge. The retention of DRP, on the other hand, can be rather poor or highly variable between systems and under different periods, thus being generally less predictable. Loads of dissolved P can be harmful to surface waters as these largely include P reactive forms readily available for biological uptake, potentially causing eutrophication. Therefore, it is crucial that alternative technologies are tested in relation to retaining DRP at acceptable levels. These include a range of filters containing P sorption materials with varying sorption capacities and hydraulic conductivities. These systems are advantageous in relation to the previously described edge-of-field technologies in the sense that little or no agricultural land is needed for implementation, making them less costly. A large part of the literature describes the potential of FMs for P retention from various types of effluents through batch and column experiments. This section, however, is restricted to those filters implemented at the outlet of tile drains or tested with simulated agricultural drainage discharge under laboratory conditions aiming at P retention. Filters include natural, synthetic and industrial by-product materials, which contain metal cations and are able to retain P through sorption, thus forming insoluble P compounds. They are built so as to allow the discharge to flow through the system at a sufficient rate and without the risk of immediate clogging, taking into account average local precipitation. The materials are porous and rich in P sorbents such as Fe, Al, Mg and/or Ca, thus with a high affinity for P. They are expected to retain P even at low inflow P concentrations and present low P desorption rate, allowing a gradual decrease in the concentration of DRP from the inlet to the outlet [137]. The retention mechanisms for DRP in FMs include chemisorption or ligand exchange onto the surface of sorption sites in Fe and Al-based filters, and precipitation in Mg and Ca-based filters [138]. Batch experiments are important to test the P sorption potential of FMs prior to implementation in the field. A series of materials with different compositions were assessed in relation to solutions containing P and simulating agricultural drainage discharge. The studies generally considered a number of FMs tested for a variety of particle size intervals, P concentrations and HRTs. These have shown that Fe and Al-based filters are usually superior to those based on Mg and Ca [139; 140; 141]. [PERSON] et al. [139], for example, demonstrated that the Fe-oxide based filter CFH presented higher P sorption capacity, reactivity and stability than the Ca-based filters limestone, calcined diatomaceous earth and shell-sand. CFH was also superior to Filtralite-P when the P concentration of the solution was low (16 \(\upmu\)M). The study found that the finest CFH (particle size interval of 0.05-0.5 mm) was the only FM recommended for extreme field conditions, i.e., high and low water flows and P concentrations. The superiority of CFH for P retention to the FMs mentioned above was also demonstrated in [PERSON] et al. [140], where Filtralite-P reached an approximate efficiency. [PERSON] et al. [141] reported similar results to the above and related the higher efficiency of CFH to fast and strong bonds of P to Fe on outer surfaces of the material followed by migration of P to interior sorption sites, while retention of P in Filtralite-P and limestone by precipitation depended on pH \(\geq\) 10 and long HRT, respectively. This study also found CFH suitable for application at the outlet of tile drains, while Filtralite-P and limestone presented limitations. [PERSON] and [PERSON] [142] demonstrated that other Fe-based FMs, including zero-valent Fe, porous Fe composite, sulfur modified Fe and Fe oxide/hydroxide were also highly effective in retaining P (\(\geq\)95%) for P concentrations in solution up to 100 mg L\({}^{-1}\).