Variations in phosphorus retention by a solid material while scaling up its application

• We scaled up the application of a potential P-retaining material.
• The laboratory, meso- and large-scale filters retained 19, 3.2 and 0.06 mg P/g, respectively.
• Decreases in influent P concentration and presence of organic carbon impeded P retention.
• Preferential and high flows are critical factors in a filter’s efficiency.
• P-filters should target critical sources to achieve a significant P sorption.

Significant reductions in phosphorus (P) inputs from point sources into bodies of water have been achieved in many countries, but curbing losses from non-point P sources still poses a challenge. One potential solution is to use solid P-retaining filters. In this study, we scaled up the application of a promising material (Sachtofer PR®) and observed the effects of variable flows, influent P concentrations, the presence of organic matter, and material alteration on P retention. The test set-ups included 1) a 6-g filter that received influents with a dissolved P concentration of 50 mg/l, 2) a 20-kg filter that received variable flow rates of tap and river water containing dissolved P amounts of up to 6 mg/l, and 3) a seven-ton filter constructed in a ditch that collected water (P concentration of up to 0.25 mg /l) from 17-ha of cropland. All filters achieved similar initial discrete P removals, but increasing the scale finally reduced cumulative P retention. The laboratory filter retained 19 mg P/g vs. 3.2 mg P/g for the meso-scale filter vs. 0.06 mg P/g for the large-scale filter, a result due largely to the decrease in the influent P concentration and the presence of humic substances. In addition, low retention times due to preferential flows and high-flow conditions greatly compromised the efficiency of the large filter. Filters should target critical sources and, consequently, the loading with significant amounts of P would improve their economic value while facilitating the future recovery and reuse of P.