LAUSR

Phosphorus (P) is an essential element to produce feed and fertilizers but also a non-renewable resource. Both the predicted exhaustion of phosphatic rocks and the risk of eutrophication lead to an increasing necessity of P recovery methodologies to be applied in municipal wastewater treatment plants (WWTPs). One of the most promising solutions involves the precipitation of P-based minerals reusable as slow-release fertilizers. In this study, P recovery as struvite and hydroxyapatite from a municipal WWTP digestate liquid fraction (centrate) was investigated at varying pH (8-10), reagent typologies (MgCl2 , NaOH, Ca(OH)2 , CaCl2 ) and concentrations under limiting magnesium doses through liquid- and solid-phase analyses and thermodynamical modelling. A maximum P recovery of 87.3% was achieved at pH = 9 by adding NaOH and a MgCl2 dose of 656 mg/L (the higher tested). According to this data, it was estimated that 92.0 tons/year of struvite and 33.2 tons/year of hydroxyapatite could be recovered from the WWTP centrate with a cost for reagent consumption being almost 50% of the mean P market value. An increase of P precipitation was observed when comparing experiments with same pH values but higher Mg2+ dose. Ca2+ addition led to extensive P precipitation but mainly as amorphous phases that interfere with struvite formation. This article is protected by copyright. All rights reserved.