The paper examines the links between floc structure and the operational problems which surround the use of alum and similar hydrolysing salts in the sweep floc domain. Structure is described on the basis of primary particle bonding, and arising from the rheological properties of the bulk precipitate within the floc water. Evidence suggests that primary particle bonding is controlled by electrostatic forces and that floc strength deteriorates when this basic mechanism is impaired. It is argued that effective rapid mixing is a vital feature of design when employing sweep coagulation. Difficulties in dewatering and the adsorption of a nonionic polymer added for strengthening and sludge conditioning are attributed to the behaviour of the bulk precipitate.

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