Measurement and implications of floc structure in water and wastewater treatment

The use of static light scattering in gaining insight into the structure of colloidal and "post-colloidal" aggregates is examined in this paper. Results for a variety of colloidal systems confirm that aggregates can be considered to exhibit mass fractal properties with an estimate of fractal dimension available in many instances from relatively straightforward analysis of their power law light scattering behaviour. Fractal dimensions in the range of 1.7 to 1.8 are widely reported for flocs formed under diffusion limited aggregation (DLA) conditions while fractal dimensions in the 2.2 to 2.3 range are typical of reaction limited aggregation (RLA). The fractal structure of aluminium oxyhydroxide aggregates exhibiting the presence of highly repulsive short range forces are reviewed. Fractal dimensions typical of reaction limited conditions are obtained under conditions where strong repulsive forces are present, even when rapid aggregation is induced at very high salt concentrations. In the presence of the apparently "structure breaking" sulfate anion, loose structures more typical of DLA conditions are obtained. Small angle light scattering provides useful structural information on relatively large aggregates of amorphous iron oxide with more compact aggregates being formed under low salt conditions, The:he structures so formed (under both RLA and DLA conditions) are highly susceptible to restructuring. Extreme caution must be exercised in interpreting the scattering exponent as a measure of structure where heterogeneity in the nature of the primary scatterers is present. Surprisingly consistent land interpretable results appear to be produced on applying small angle light scattering to bacterial assemblages, in part perhaps because of the similarity in refractive index of such particles to water. Addition of polymer to bacterial suspensions clearly modifies the structure of the resultant particulate system but further work is required before static light scattering can be readily applied as a tool for structure analysis in such systems. The structure of particle aggregates has clear implications to many processes of importance in water and wastewater treatment. Preliminary results of investigations into the impact of aggregate structure on pressure filter dewatering efficiency and ultrafiltration membrane fouling are reviewed. In both cases, looser aggregates appear to result in improved process performance though the increased tendency of the resulting porous cakes to restructure (with subsequent lowering in process performance) is noted. (C) 1999 Elsevier Science B.V. All rights reserved.