Simultaneous Phosphate Precipitation and Particle Destabilization Using Aluminium Coagulants of Different Basicity

Abstract

Chemical treatment of wastewater was investigated by conducting jar tests with a synthetic testwater. The influence of different variables (pH, ortho-P concentration, particle concentration, hardness, humus concentration) on the competitive reactions of aluminium coagulants with ortho-P and hydroxyl ions, respectively, was studied under controlled conditions. The data were evaluated by assuming that basic aluminium phosphate A1_1.4PO₄(OH)_1.2, referred to as Alp and A1(0H)₃ are precipitated simultaneously. The ratio A1P/A1_T was found to depend on initial pH, initial ortho-P concentration, initial particle concentration and the coagulant’s basicity. In a soft water matrix, the amount of coagulant not consumed by P-precipitation was about as efficient for particle destabilization as an equal amount in the same, but ortho-P free, raw water. In hard water matrices, however, P-precipitation was found to enhance particle destabilization. Co-precipitation of calcium even at pH = 6 was identified as the main reason for this effect. Humus concentrations on the order of 5–10 mg/l can considerably increase the coagulant demand for particle destabilization. A procedure for advanced characterization and evaluation of wastewater with respect to chemical treatment is suggested.