Hydrodynamics of biological aggregates of different sludge ages: An insight into the mass transport mechanisms of bioaggregates

Internal permeation and its role in mass transport to biological aggregates are investigated through detailed settling experiments. Three groups of bacterial aggregates (0.8-3.2 mm), which were different in fractal dimension and porosity, were generated in batch activated sludge reactors with biomass residence times (BRTs) of 5, 10, and 20 days. An apparatus of vertically connected double settling columns, which were filled respectively with water and an EDTA solution of a higher density, was used to characterize the settling and permeability features of individual aggregates. The settling velocities observed in water were in good agreement with those predicted by Stokes' law for porous but impermeable aggregates. The BRT10 and BRT20 aggregates, with porosities that were generally less than 0.92, had fluid collection efficiencies ranging from 0 to 0.1, while the BRT5 aggregates with porosities that were generally higher than 0.97 were almost completely impermeable. It is suggested that bioaggregates with either a tightly or a loosely packed structure cannot be as highly permeable as characterized for nonbiological fractal aggregates. While failing from water into the denser EDTA liquid, many BRT10 and BRT20 aggregates stopped and stayed below the interface of the two liquids for a period that was more than an order of magnitude shorter than the prediction based on the assumption of molecular diffusion as the predominant mass transport mechanism to the aggregates. The results indicate that limited intra-aggregate convection, which may not be important to the hydrodynamic properties of bioaggregates, could significantly enhance mass transport to suspended aggregates in biological wastewater treatment reactors.