Based on coagulation experiments, electrophoretic mobility measurements, and literature studies, mechanisms for particle and phosphate removal during coagulation with coagulants of different basicities are discussed. In wastewater treatment, particles are expected to be removed by both the adsorption -charge neutralization and sweep floc mechanisms. However, when coagulating with low basicity coagulants, the dominant mechanism for particle removal is suggested to be the sweep floc mechanism, while the dominant mechanism for phosphate removal is adsorption-precipitation by initial formation of aluminium - hydroxo -complexes. When coagulants with high basicities are used, the particles are removed dominantly by the adsorption - charge neutralization mechanism, while phosphate removal will be favoured by adsorption of phosphate ions on to the hydrolysis products.

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