Colloidal fouling of ultrafiltration membranes: Impact of aggregate structure and size

A close coupling between the structure and size of hematite flocs formed in suspension and the permeability of the cake that accumulates on ultrafiltr ation membranes is observed. Specific resistances of cakes formed from floc s generated under diffusion-limited aggregation conditions are at least an order of magnitude lower than those of cakes formed from flocs generated un der reaction-limited aggregation conditions. Similar effects are observed w hether the aggregation regime is controlled by salt concentration, pH, or a dded organic anions. This dramatic difference in cake resistance is conside red to arise from the size and fractal properties of the hematite assemblag es. The ease of fluid flow through these assemblages will be influenced bot h by the fractal dimension of the aggregates and by their size relative to primary particle size (since, for fractal aggregates, porosity increases as the size of the aggregate increases). The size and strength of aggregates are also important determinants of the relative effects of permeation drag, shear-induced diffusion, and inertial lift and result, in the studies repo rted here, in relatively similar rates of particle deposition for both rapi dly and slowly formed aggregates. The results presented here suggest that c ontrol of cake permeability (and mass) via control of aggregate size and st ructure is an area with scope for further development though the nature and extent of compaction effects in modifying the fractal properties of aggreg ates generated in suspension requires attention. (C) 1999 Academic Press.