Phosphorus recovery as struvite and hydroxyapatite from the liquid fraction of municipal sewage sludge with limited magnesium addition

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.