Struvite Production From Wastewater: Effect of Magnesium‐to‐Ammonia Ratio and Stirring Velocity on Precipitation Efficiency and Settling Behavior

ABSTRACT

The recovery of ammoniacal nitrogen from effluents through struvite precipitation is an effective, sustainable method for nutrient removal, producing a valuable fertilizer for macroalgae or ornamental plant cultivation, among other applications. However, controlling crystal size and improving solid–liquid separation remain key challenges. This study investigated the effects of a slight stoichiometric excess of Mg ²⁺ and stirring velocity on struvite precipitation yield, granulometry, and settling behavior. Both synthetic and an industrial effluent (IE) were investigated. A two‐factor factorial experimental design (2 ²  + 2 central points) was applied using jar‐test experiments to determine the influence of the Mg ²⁺ :NH ₄ ⁺ molar ratio and stirring velocity on N and P removal efficiencies, precipitation yield, and settling properties. The results show that optimized synthesis conditions can achieve high nutrient recovery and rapid solid–liquid separation without the use of flocculants. All tested conditions produced precipitates with excellent thickening properties (<19 mL/g) and high settling velocities (5.5–10.1 cm/min). Notably, the Mg ²⁺ :NH ₄ ⁺ molar ratio was identified as the dominant factor controlling the process. A slow and continuous addition of Mg ²⁺ over 10 min, reaching a final stoichiometric excess of 10%, maintained slight supersaturation throughout the reaction. This strategy significantly enhanced PO ₄ ³⁻ removal and promoted the formation of larger particles (>0.84 mm), whereas stirring velocity mainly affected particle settling behavior. Furthermore, the optimized conditions were successfully applied to a complex IE, confirming its viability. However, competitive ion interference, particularly K ⁺ , must be considered to ensure reliable performance under real conditions.