Samenvatting

Eutrophication because of heightened phosphorus and nitrogen input into freshwater ecosystems is an increasing problem, while at the same time phosphorus availability for agricultural purposes is decreasing on a high pace. B-Ware Research Centre and Radboud University Nijmegen developed a novel biocascade water purification system to contribute to the solution of both problems. In this system, most phosphorus is immobilized via sorption with oxidized iron, in contrast to conventional helophyte filters, which often utilize phosphorus immobilization via sorption with aluminium in clay-rich soil. This research investigated if immobilized phosphorus can be recovered by means of growing the floating fern Azolla filiculoides on both iron-rich sediment and clay-rich sediment, and if adding glucose could be a suitable management tool to enhance oxygen depletion by increasing the biological oxygen demand in the water layer. Addition of sulphate and sufficient plant density proved to be key factors in facilitating phos-phorus mobilization in the iron-rich treatment, resulting in increased phosphate concentrations in the water column after 40 days. Optimum biomass production rates of 0.465 g dw m-2 day-1 and a concomitant phosphorus yield of 1.85 kg ha-1 year-1 were found. No phosphorus mobilization in the clay-rich treatment was observed, resulting in decreasing biomass due to phosphorus deficiency. The glucose pilot experiment showed that addition of glucose to the water layer resulted in very low dissolved oxygen concentrations and increased phosphorus and iron mobilization. Glucose concentrations of 0.5 and 1 g/L resulted in extreme-ly high iron concentrations in the pore- and surface water. 0.2 g/L resulted in the most stable anaerobic conditions and was considered the most suitable option in large scale application. The results of this study show that recovering phosphorus from iron-rich soil by using A. filicu-loides has great potential to meet the need of sustainable solutions to eutrophication and phos-phorus mining.