Enhanced Phosphorus Removal from Municipal Wastewater by Coagulation with Alum and Iron Salts

Phosphorus is often a limiting factor for the eutrophication of surface waters. An efficient removal of this constituent from the effluent of municipal wastewater treatment plants has been of growing interest. Alum salts (AlCl₃, Al₂(SO₄)₃) and iron salts (FeCl₃, FeSO₄) are the commonly used chemical coagulants for the removal of phosphorus. The aim of the present work was to evaluate the coagulation performances of the four coagulants by treating the effluent of sewage treatment plants. The effects of coagulation dosage, pH and coagulation time on the coagulation ability were systematic investigated. The results indicated that the coagulant dosage and pH were the major factors influencing the removal of the total phosphorus (TP). At the optimum coagulation time of 15min, the optimum dosages for AlCl₃, Al₂(SO₄)₃, FeCl₃ and FeSO₄ were 3.5, 4.0, 4.0 and 4.0 mg/L with the phosphorus removal efficiencies of 69.70%, 70.34%, 68.48% and 68.04%, respectively. At slightly acidic and neutral pH, AlCl₃ was more effective for the TP removal, while Al₂(SO₄)₃, FeCl₃ and FeSO₄ were more effective at slightly alkaline pH. The residual phosphorus concentrations for the four coagulants were all below the effluent discharge limitation of 0.5 mg/L. The coagulation with alum and iron salts can be used as an option for the tertiary removal of phosphorus from the effluent of wastewater treatment plants.