Advanced removal of phosphorus from urban sewage using chemical precipitation by Fe-Al composite coagulants

Abstract

Phosphorus (P) removal is a significant issue in wastewater treatment. This study applies Fe-Al composite coagulant to the advanced treatment of different P forms in biological effluent. For 90% total P removal, the dosage of FeCl₃-AlCl₃ composite coagulant reduces by 27.19% and 43.28% than FeCl₃ and AlCl₃ only, respectively. Changes in effluent P forms could explain the phenomenon of composite coagulant dosage reduction. The suspended P in the effluent of composite coagulant is easier removed by precipitation than single coagulant. In this study, the hydrolysis speciations of Fe³⁺, Fe²⁺, and Al³⁺ at a pH range are calculated by Visual MINTEQ. Changes in the morphology of metal hydroxides correlate with P removal at pH 4–9. Besides, analyses of scanning electron microscope (SEM), Fourier transformed infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) are performed on the coagulation precipitations. Fe²⁺ reacts directly with P to form flocs of Fe₃(PO₄)₂, and Al₂(SO₄)₃ assists in the sedimentation of the small-volume flocs. Al₁₃ is a significant hydrolysis product of Al³⁺, and Fe and P would substitute for the peripheral Al^VI of the Al₁₃ structure to form stable Fe–O–Al covalent bonds.