The sharp increase in raw-water turbidity following heavy rain presents considerable treatment problems due to low sedimentation rates. The objective of this study was to evaluate the removal of negatively charged suspended solids (SS) in surface water treatment using magnetic particles (nFeO; FeO · Fe2O3) synthesised in our laboratory. The performance of nFeO in removing turbidity, dissolved organic matter (DOM) and trihalomethanes (THMs) was compared to the performance of coagulation with polyaluminium chloride (PACl) and to that of a combined nFeO–PACl system. High-turbidity water was prepared by re-dispersing SS with abundant SiO2 and Al2O3 into tap and river waters. The SS were collected from the Ara River in Japan after heavy rain. By adding nFeO to the turbid water, magnetic flocs of several microns in diameter were formed. The nFeO dosage, the solution pH and DOM influenced the turbidity removal by the nFeO. Introducing a low nFeO dose of 6.3 mg/L followed by 20 mg/L PACl reduced turbidity from 51 to 1.6 NTU, an effect that was associated with a total THM removal of 54%. Surface charge, aggregation kinetics and scanning electron microscopy analysis revealed that both physical and surface charge interactions contributed to flocculation of SS by magnetic seeding.

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