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. Resul ts for a variety of colloidal systems confirm that aggregates can be consid ered to exhibit mass fractal properties with an estimate of fractal dimensi on 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 ra nge are typical of reaction limited aggregation (RLA). The fractal structure of aluminium oxyhydroxide aggregates exhibiting the p resence of highly repulsive short range forces are reviewed. Fractal dimens ions typical of reaction limited conditions are obtained under conditions w here strong repulsive forces are present, even when rapid aggregation is in duced at very high salt concentrations. In the presence of the apparently " structure breaking" sulfate anion, loose structures more typical of DLA con ditions are obtained. Small angle light scattering provides useful structural information on rela tively large aggregates of amorphous iron oxide with more compact aggregate s being formed under low salt conditions, The:he structures so formed (unde r both RLA and DLA conditions) are highly susceptible to restructuring. Ext reme caution must be exercised in interpreting the scattering exponent as a measure of structure where heterogeneity in the nature of the primary scat terers is present. Surprisingly consistent land interpretable results appea r to be produced on applying small angle light scattering to bacterial asse mblages, in part perhaps because of the similarity in refractive index of s uch particles to water. Addition of polymer to bacterial suspensions clearl y modifies the structure of the resultant particulate system but further wo rk is required before static light scattering can be readily applied as a t ool for structure analysis in such systems. The structure of particle aggregates has clear implications to many process es of importance in water and wastewater treatment. Preliminary results of investigations into the impact of aggregate structure on pressure filter de watering efficiency and ultrafiltration membrane fouling are reviewed. In b oth cases, looser aggregates appear to result in improved process performan ce though the increased tendency of the resulting porous cakes to restructu re (with subsequent lowering in process performance) is noted. (C) 1999 Els evier Science B.V. All rights reserved.