Nutrients recovery from wastewater centrate via struvite precipitation with brine-derived Magnesium Hydroxide

Struvite (MgNH₄PO₄·6H₂O) crystallization from wastewater streams is an increasing adopted strategy to recover nitrogen and phosphorus to valorize nutrient-rich streams into a slow-release fertilizer product. So far, struvite crystallization has been mainly based on the use of magnesium chloride or magnesium sulphate sources, requiring external alkaline reagents (e.g., NaOH) to reach the reaction pH [1]. On the other hand, today much effort is focused on the recovery of magnesium hydroxide, Mg(OH)2, solids from Mg2+ rich waste brines from desalination plants and saltworks industries. In this context, this work explores an integrated approach that couples the recovery of nutrients from a real centrate (clarified stream from centrifugation of digested sludge) from municipal wastewater treatment (Acqua dei Corsari, Palermo), rich in NH₄⁺ and PO₄³⁻, with struvite precipitation via direct addition of magnesium hydroxide, Mg(OH)₂, extracted from Mg-rich saltworks brines [2]. In particular, Mg(OH)₂ suspensions were produced via reactive crystallisation within three different reaction configurations, leading to very different particle size distribution and morphology. Struvite batch crystallization experiments were conducted at ambient temperature with continuous monitoring of pH and ion concentrations over time. Particular emphasis was given to the interplay between Mg(OH)₂ characteristics (particle size distribution, specific surface area, morphology) and process variables such as aging time and feeding flow rate. SEM analysis of struvite samples revealed a shift in crystal habit and aggregation based on Mg(OH)₂ properties, indicating that struvite features can be tuned through the magnesium precursor source. Thermogravimetric analysis (TGA) consistently showed struvite purities close to 100%, even when using a real concentrated wastewater centrate feed. Results marked that struvite crystallization using Mg(OH)₂ as co-reactant (i) is an effective alternative to conventional magnesium salts, (ii) can lead to high product purity, and (iii) allows morphology and crystal size of the struvite product to be tuned based on the initial characteristics of Mg(OH)2 solids. Keywords: Wastewater valorisation; struvite; mineral recovery; crystallization; brine valorisation. Acknowledgments This work has received funding from the European Union’s LIFE 2021-2027 programme under Grant Agreement 101147407 - LIFE23-ENV-IT-MAREMAG LIFE. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or CINEA. Neither the European Union nor CINEA can be held responsible for them. The authors acknowledge the financial support provided by the Erasmus+ Traineeship Programme, as well as the Universitat Politècnica de València for granting the FPI-UPV scholarship to Samuel Navajas Valiente. References [1] A. Siciliano, C. Limonti, G.M. Curcio, R. Molinari, Advances in struvite precipitation technologies for nutrients removal and recovery from aqueous waste and wastewater, Sustainability (Switzerland) 12 (2020). https://doi.org/10.3390/su12187538. [2] F. Miciletta, G. Battaglia, G. Scelfo, F. Vassallo, F. Vicari, A. Tamburini, A. Cipollina, G. Micale, A Novel Pilot-scale Crystallizer for the Production of Mg(OH)2 Powders from Saltworks Bitterns, Chemical Engineering Transactions 119 (2025) 7–12. https://doi.org/10.3303/CET25119002.