An innovative two-step methodology for struvite recovery from wastewater with high purity and agronomic potential
Andreia F. Santos, Raquel L. Abreu, Paula Alvarenga, Licínio M. Gando‐Ferreira, Margarida J. Quina
Abstract
Recovering phosphorus (P) from wastewater is a central strategy for environmental protection and sustainable development, with a contribution to resource scarcity issues. This research aims to fill a gap in the understanding of selective calcium removal prior to struvite precipitation to increase its purity. A previous assessment showed that the stream before anaerobic digestion is the best hot spot for precipitate struvite, as it gathers the following conditions: soluble reactive P (orthophosphates) > 30 mg P/L, Mg/P molar ratio > 1, highest Mg/Ca molar ratio, and lowest Ca/P molar ratio. As expected, calcium had a major influence in the process, with 90 % of P recovered within the first 2.5 min of reaction in its presence. However, the magnesium removal efficiency was reduced to about 30 %, which affected the purity of the product (confirmed by microscopy). Increasing the Mg/Ca molar ratio to minimize the effect of Ca is not beneficial. The removal efficiency of Ca from the solution was not significantly affected by the increase of the molar ratio, despite more Mg removed associated with P. The presence of carbonate ions in the wastewater can reduce the purity of the precipitated solid, as it reacts with Ca. A two-step precipitation alternative was developed relying on pH changes to selectively precipitate Ca in the first step (pH 7), and precipitate Mg with P in the second step (pH 8). The solid obtained after the second step was mostly struvite, with a negligible contribution of Ca, which revealed good availability of P after soil application, measuring its extractability in incubation experiments. • The stream before anaerobic digestion is the best hot spot to precipitate struvite. • Ca presence led to 90 % of P recovery within the first 2.5 min of reaction. • Increasing the Mg/Ca molar ratio to minimize the effect of Ca is not beneficial. • The solid after two-step precipitation is mostly struvite, with low Ca contribution. • The produced struvite revealed good availability of P after soil application.