Earth’s phosphorus resources are being depleted at an alarming rate, while at the same time eutrophication caused by its uncontrolled disposal in surface waters is considered as a significant environmental problem. In order to achieve phosphate recovery from the secondary effluents of an urban wastewater (biological) treatment plant, the adsorption onto single iron (GFH, Bayoxide and FeOOH) and onto binary iron-manganese (AquAsZero) oxy-hydroxides, as well as the ion exchange by using Purolite A200EMBCL resin, were investigated as post-treatment methods. Among them, laboratory batch experiments and dynamic Rapid Small Scale Column Tests (RSSCTs) evaluated AquAsZero, as the relatively better qualified material, presenting the higher efficiency. Based on these experimental results a pilot-plant, utilizing AquAsZero, was constructed and operated, treating 200 L/h. The breakthrough curves of RSSCTs for AquAsZero showed an adsorption capacity of 33.6 mg PO43−/gads at the equilibrium concentration of 3 mg PO43−/L, whereas at pilot-scale application the respective breakthrough curve indicated a similar adsorption capacity (31.5 mg PO43−/gads). The regeneration process, by applying a NaOH solution at pH range 12.6–13, resulted in the efficient (>80 wt.%) phosphate desorption, which in turn allows the multiple reuse of adsorbent media. Subsequently, phosphate was recovered from the alkaline regeneration (concentrate) solution by precipitation with the appropriate Ca2+ addition, as the respective calcium salt (hydroxy-apatite, HAP). Phosphate concentration in the finally collected amorphous (precipitated) solids from the laboratory scale experiments was around 51 wt.% and that of calcium was around 19 wt.%, while the corresponding concentrations in the precipitated solids collected from the pilot-scale experiments were around 36 wt.% for phosphate and 33 wt.% for calcium. This high phosphate content of finally recovered solids indicates their potential utilization as efficient (alternative) fertilizers.
Phosphate recovery Secondary effluents Adsorption Desorption Iron oxy-hydroxides HAP Precipitation
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The financial support through the co-Financed by the European Union and the Greek State Program PAVET, Project (PhoReSE)-“Recovery of Phosphorus from the Secondary Effluent of Municipal Wastewater Treatment”, is gratefully appreciated. This research project was supported by the approval of EYATh’s S.A.-Department of Plants’ Operation, Maintenance & Environmental Monitoring, which is gratefully appreciated. An initial version of this paper has been presented in the WasteNet Program International Conference, “Sustainable Solutions to Wastewater Management: Maximizing the Impact of Territorial Co-operation”, Kavala, Greece, June 19–21, 2015.
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