The laboratory experiments indicate that the Ca-rich materials such as Filtra P, steel slag and Sachtofer as well as Fe-rich MDR, possess high P retention capacities. Two distinct P retention mechanisms, precipitation of Ca-phosphates and sorption onto Fe-hydroxide surfaces, were identified after the weathering process and in the pH-manipulation tests. The weathering of the Ca-rich materials significantly reduced their P retention potential, but also revealed, in the case of Sachtofer, the conversion of precipitation-controlled to sorption-controlled P retention. Laboratory protocols for identifying promising P-filters should also entail P release tests involving extractions with large volumes of water.
The upscaled application of Sachtofer implied that cumulative P retention declined by three orders of magnitude; the decline in the influent P concentration was of the same order of magnitude as the scale of the application increased. This emphasizes the need to use influents with similar characteristics in the laboratory as in the field. However, the use of influents with relatively high P concentrations is still important when estimating the capacity of a material to retain significant amounts of P. Moreover, the meso-scale experiment confirmed the suppressive effect of DOC on P retention. The accumulation of organic matter and the formation of preferential flows stemming from the disintegration of the material further compromised the performance of the large filter over time.
According to the results from the meso- and large scale experiments, maintaining a certain effluent limit was rather difficult, even at lower P influent concentrations. An achievable goal would be to retain a significant part of the total P input, which might lead to better-quality of bodies of water.
Most of the P input into the large filter was delivered under high flow conditions resulting from fast snowmelt in spring and heavy rainfall in autumn. Thus, the peak flows should be incorporated as a design parameter into the construction of a P removal structure. Long winters at high northern latitudes and the persistence of frost obstacle the proper performance of P removal structures. In the present study, the annual effective period of the large filter lasted for about eight months, from April to November.
Industrial by-products such as steel slags and mine drainage residuals are competitive retainers due to their low costs. However, to justify the cost of building a P removal structure, the material should be able to achieve a significant P saturation, thus enabling potential P recovery from the spent P-filter. Moreover, such methods should be coupled with other best management practices in order to curb P losses from agriculture.
The application of P-retaining materials offers an additional potential solution for treating agricultural runoff. Although the need for standard laboratory protocols is great, protocols should be modified to suit the properties of the material. In addition, the design of a P removal structure clearly depends on site-specific parameters such as the quality characteristics of the influents and flow conditions.
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