Theory is developed to represent the macroscopic features of a floc blanket held in steady state. Flocs are treated as non-porous entities possessing a fractal structure. A particular feature of the analysis is the recognition of floc strength, floc sizes adjusting to the prevailing rate of energy dissipation within the blanket. Account is taken of the floc size distribution, with expressions derived to describe the blanket settling velocity and mass flux. Analysis of experimental data yielded detail of the blanket floc size scale (dvo), water content and strength. The mass flux was maximised when the floc volume fraction was close to 0.17. It was shown that the upflow velocity (u) complied with the dependence uSo/(Md max 5−D) in which dmax is a maximum floc size, M the blanket solids concentration, D the fractal dimension and So a strength scaling factor. Consideration was given to blanket characteristics in non-stationary conditions. The final section of the paper focuses on the attenuation capacity. It was argued that dvo should be as low as practicable in order to promote the capture of feed flocs by interception, to reduce the water content of the solids waste stream when using hydrolysing coagulants, and may also enhance the mass flux.

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