## 2 Edge-of-Field Technologies
### Filter Materials
However, most of the studies with FMs targeting agricultural drainage discharge were conducted in the laboratory. More field studies covering a number of locally available FMs are therefore essential to ascertain their successful application. Woodchip bioreactors are implemented in agricultural catchments to remove nitrogen through denitrification due to their large carbon contents and anaerobic medium. [PERSON] et al. [147], however, tested the effect of this filter medium on the retention of P as well. It was found that these systems retained on average 0.21 (21%) and 0.30 (19%) g m\({}^{-3}\) d\({}^{-1}\) of the total P and DRP loads, respectively, which may be attributed to P immobilization into organic forms. The percentage P retention more than tripled when the bioreactors were amended with album-based drinking water treatment plant residuals (10% of the bioreactor volume). This finding highlighted the applicability of materials for P retention to systems originally designed to remove nitrogen. Alternatively, other studies proposed pairing woodchip bioreactors with FMs. [PERSON] et al. [148], for example, found that although a woodchip bioreactor mixed with crushed mussel shells (25-50% of the bioreactor volume) retained from 29 to 33% of the total P load--largely in the form of PP--the retention of phosphate varied from net annual release to retention. Thus, the study suggested pairing the bioreactor with a FM to increase the phosphate retention efficiency. In general, this approach is likely more beneficial, as there would be no need for homogeneous mixing of two or more different mediums, which can be difficult to achieve at the field scale, so as to prevent preferential flow. Moreover, the FM could be easily replaced or rejuvenated upon saturation [149]. [PERSON] et al. [149], for example, tested filters comprised of either acid mine drainage treatment residuals (Fe-based) or steel slag (Ca-based) and paired downstream of woodchip bioreactors under HRTs from 7.2 to 51 h. On average, the study found that the former filter alone retained DRP from 25 to 133 g m\({}^{-3}\) d\({}^{-1}\) (corresponding to 98% and 58%, respectively, in the paired system), while the latter filter alone retained from 8.8 to 48 g m\({}^{-3}\) d\({}^{-1}\) (corresponding to 26% and 36%, respectively, in the paired system). [PERSON] et al. [150] also found significant P retention values when testing the pairing of an Fe-based filter comprised of steel turnings downstream a woodchip bioreactor. In this study, near complete orthophosphate retention was achieved in the paired system. A comparable effective retention of phosphate by steel by-products treating the effluent of a woodchip bioreactor (65% retention) was reported by [PERSON] et al. [151]. Here, the retention of phosphate in the bioreactor alone was relatively high and ranged on average from 0.25 to 0.88 g m\({}^{-3}\) d\({}^{-1}\) (corresponding to 56% and 17%, respectively). In contrast, a large variability in the retention of DRP by a filter comprised of steel shavings and placed downstream a woodchip bioreactor was observed in Thapa [152]. This study reported P retention in the filter alone ranging from 2.2 to 184 g m\({}^{-3}\) d\({}^{-1}\) with an average 49 g m\({}^{-3}\) d\({}^{-1}\) (10-90% and average 45%). The studies above suggested placing the FM downstream the woodchip bioreactor for optimal P retention. They demonstrated that an opposite configuration could lead to the release and export of P from the paired system--potentially increased under longer HRTs [150]--due to the emergence of reducing conditions in the effluents originated from the bioreactor as a result of denitrification, which support the release of Fe-bound P. An exception to the optimal configuration could be when the HRT in the woodchip bioreactor is sufficiently long to export highly reduced effluents to the filter, which could inhibit P sorption in the material and/or favor P desorption and release [149].