Recovery of high-quality struvite fertilizer product from swine wastewater using fluidized bed homogeneous crystallization

Abstract

Recovering phosphorus (PO₄³⁻) and nitrogen (NH₄⁺) from swine wastewater is critical because their excess release can trigger eutrophication, severely harming aquatic ecosystems. This study introduces a pioneering fluidized-bed homogeneous crystallization (FBHC) process for single-step co-recovery of PO₄³⁻ and NH₄⁺. The method ensures compliance with discharge standards and produces a reusable nutrient product. Preliminary jar-test experiments were performed to identify the feasibility of struvite (MgNH₄PO₄·6H₂O) precipitation and to define the appropriate range for key operational parameters. Subsequently, operational parameters including pH, reaction time, up-flow velocity, cross-sectional loading, and bed height were systematically optimized. Under optimal conditions with a working medium at pH 9, reaction time of 24 min, up-flow velocity of 36 m h⁻¹, cross-sectional loading of 0.27 kg m⁻² h⁻¹, and bed height of 10 cm, the total removal efficiencies reached 97.42% for PO₄³⁻ and 86.55% for NH₄⁺. The corresponding crystallization ratios were 95.55% and 83.18%, respectively. The FBHC process exhibited high crystallization efficiency (>95%), contributing to reduced impurity levels and improved energy efficiency compared to conventional fluidized bed and chemical precipitation methods. The recovered product was identified as high-purity struvite (94.3%), predominantly comprising particles smaller than 0.75 mm. These results demonstrate the effectiveness of the FBHC strategy for simultaneous nutrient removal and resource recovery through the production of a value-added fertilizer.