Struvite precipitation for nutrient recovery from clarified shrimp pond sludge supernatant under brackish conditions

Abstract

Coastal intensive shrimp ponds generate sludge-derived supernatant containing reactive nitrogen and phosphorus that may contribute to eutrophication if discharged without appropriate treatment. This study evaluated struvite precipitation for nutrient recovery from clarified supernatant of white-leg shrimp pond sludge collected in Thuan An Ward, Hue City, Vietnam, under post-flood brackish conditions. The collected supernatant retained a relatively high native Mg²⁺ concentration (100.7 ± 9.2 mg/L), whereas NH₄⁺-N and PO₄³⁻-P were substantially diluted after flooding; therefore, NH₄⁺ and PO₄³⁻ were supplemented to restore the stoichiometric window required for crystallisation. Batch jar tests evaluated the effects of pH (8.0–9.5), Mg/N/P molar ratio (1:1:1–1.5:1:1), and reaction time (30–120 min). The optimum conditions were pH 9.0, Mg/N/P = 1:1:1, and 30 min, achieving recovery efficiencies of 61.6 ± 1.0% for NH₄⁺-N and 82.1 ± 0.3% for PO₄³⁻-P. XRD identified struvite as the predominant crystalline phase, while SEM and optical microscopy showed prismatic crystals consistent with struvite morphology. These findings demonstrate that, even under post-flood brackish conditions, clarified shrimp sludge supernatant can serve as a magnesium-rich matrix for struvite-based nutrient recovery, highlighting a promising route for resource recovery in coastal aquaculture.