A system for recovering phosphorus from membrane-filtrate from a sludge reduction process containing high phosphorus concentrations was developed. In this system, referred to as the completely mixed phosphorus crystallization reactor, powdered converter slag was used as a seed material. In a preliminary experiment, the optimal pH range for metastable crystallization of phosphorus from membrane-filtrate containing about 100 mg/L PO4-P was found to be 6.6-7.0. The laboratory scale completely mixed phosphorus crystallization reactor, actually operated in pH range of 6.8-7.6 for influent 72.9 mg/L PO4-P, achieved an average efficiency of phosphorus removal from the membrane-filtrate of 52.4% during a 30-day experiment. Mixed-liquor suspended solids (MLSS) measurements revealed that, out of 0.24 kg PO4-P in the original membrane-filtrate fed into the reactor, 0.12 kg PO4-P was recovered on the seed particles after 30 days. X-ray diffraction (XRD) pattern and Fourier transform infrared (FT-IR) spectra of the crystalline material deposited on the seed particles showed peaks consistent with hydroxyapatite. Scanning electron micrograph (SEM) images exhibited that finely distributed crystalline material was formed on the surfaces of seed particles. Energy dispersive X-ray spectroscopy (EDS) mapping analysis revealed that the molar composition ratio of Ca/P of the crystalline material was 1.84. The Ca/P molar ratio > 1.67 for crystalline substance might result from the presence of CaCO3 on the crystalline surfaces. A particle size distribution analysis showed that the average particle size increased from 22 mu m for the original converter slag seed particles, to 94 mu m after 30 days of phosphorus crystallization. Collectively, the present results suggest that the proposed phosphorus crystallization recovery system is an effective tool for recycling phosphorus from phosphate solution. (c) 2006 Elsevier B.V. All rights reserved.