Phosphate Removal from Effluent of Secondary Wastewater Treatment: Characterization of Recovered Precipitates and Potential Re-use as Fertilizer

Abstract

Phosphorus is a non-renewable essential element for plant’s nutrition, while it is considered also as the primary nutrient responsible for the eutrophication of inland surface waters. Therefore, the removal and recovery of phosphorus for potential reuse as fertilizer from wastewaters is currently considered as an issue of high priority. This study examines the content and the bioavailability of phosphorus in the phosphate precipitates/solids produced by the treatment of secondary effluent at the wastewater treatment plant “AINEIA”, near Thessaloniki (N. Greece). Initially, batch precipitation tests were conducted for collecting the respective ferric phosphate precipitates, after the direct addition of dissolved Fe(III) solution. Subsequently, the application of “Rapid Small Scale Column Tests” was performed by implementing iron oxy-hydroxides (FeOOH) as adsorbent medium. When the residual concentration of the column reached the (expected by legislation) disposal limit of 1 mg P- $$ {\text{PO}}_{4}^{3 - } $$ L−1, the column was regenerated and phosphates were recovered from the regeneration solution either as calcium, or magnesium insoluble salts. The phosphorus bioavailability tests revealed that phosphorus release was satisfactory from calcium (35 mg P kg−1 soil increment in acidic samples), or magnesium (16 mg P kg−1 soil increment in alkaline samples) precipitates. On the contrary, the content of phosphorus in soil after the addition of ferric phosphate precipitates was found to increase insignificantly (7 mg P kg−1 soil) and only in the alkaline samples. The respective phytotoxicity tests showed that all examined precipitates can be considered as non-toxic for higher plants, as they did not inhibit their seed germination, neither root elongation, with the only exception of alkaline soil samples with Lepidium sativum seeds, where a 20 % decrease was observed.