This paper examines the implications of pretreatment in the context of the likely behaviour of flocs in DAF units. Evidence indicates that the maximum floc size (dmax) of alumino-humic is scaled by the Kolmogorov length (η) with dmax/η ˜ 1. A larger ratio exists when polymer is present. Data from the literature suggests that velocity gradient in the inlet chamber may exceed 1000 s−1, suggesting η ˜ 40 μm. This view of the response of floc to shear is supported by shear tests carried out on samples of floc withdrawn from flocculators at full scale water treatment works. Using an underwater CCTV camera, coupled with image analysis, it is shown that floc sizes in conventionally designed flocculators are much bigger than the sizes which are likely to exist in DAF units. Circumstantial evidence gained from water treatment works suggests that DAF units perform satisfactorily whatever the size of the incoming floc. From such evidence, the authors question the conventional design of the flocculators in association with DAF units.

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