Potential and limitations of phosphate retention media in water protection: a process-based review of laboratory and field-scale tests

The application of phosphorus (P)-sorbing materials offers a possible solution for treating municipal wastewater and agricultural runoff. In this paper we discuss P retention and release mechanisms, and review studies on the P retention of different materials and their use as reactive media in filter beds. The main mechanisms for P retention are sorption on metal (mostly Fe or Al) hydroxide surfaces and, in alkaline conditions, the formation of Ca-P precipitates. The retention of P is strongly affected by the chemical composition of a material, its particle size and pH-related effects on sorption and precipitation both during testing and in practical operation. Laboratory tests are sensitive to solution chemistry (pH, alkalinity, ionic strength and composition, P concentration) and affected by material-to-solution ratio, contact time and agitation. Moreover, due to deviations from realistic field conditions, laboratory tests may produce imprecise estimates of the retention capacity and retention kinetics. In particular, materials that contain soluble substances (e.g., CaO) that elevate the pH of the ambient solution to high levels may in batch tests suggest a high capacity for P retention, but will most probably show much lower retention in field settings. On the other hand, materials containing metal oxides also retain P via slow reactions, and their retention capacity may be underestimated in short equilibrations. Appropriate laboratory test procedures will depend on their intended applications and material properties. Long-term field-scale tests are few in number, but some of them have shown promising results. Field-scale tests have, however, highlighted the design of the filters as a critical factor in their efficiency