Phosphate adsorption to iron sludge from waterworks, ochre precipitation basins and commercial ferrihydrite at ambient freshwater phosphate concentrations.

Measures such as storm water ponds, constructed wetlands and buffer strips along streams are used to reduce diffuse phosphorus (P) loading to surface waters. These systems often retain particulate P well, whereas the retention of dissolved P is less efficient and might require addition of P adsorbents. In this study, we screened waterwork ochre sludge (WWS) originating from groundwater treatment and ochre sludge from ochre precipitation basins along streams for their applicability as P adsorbents at ambient P concentrations. We compared with a commercial ferric hydroxide (CFH 12™) for which adsorption properties is well described. The adsorption capacity of 9 products was measured over 24 h at different P concentrations (5-2000 µg L), a range that covers Danish drainage water and stormwater. WWS desorbed phosphate at concentrations below 50-200 µg P L and should only be considered for use in systems with a constantly high load of dissolved P. High affinity combined with little or no desorption characterized the commercial product and the ochre sludge from the precipitation basins, rendering these useful for treating drainage water and storm water. The study underlines that waste products may act as potentially effective P adsorbers at environmentally relevant P levels.