Bioflocculation as a microbial response to substrate limitations.

Previous theories of nutrient supply to microbial floes assumed that transport within the flocs was by molecular diffusion, and they predict that overall nutrient uptake is reduced in floes compared to dispersed cells. Calculations, supported by recent advances in understanding fluid flow through suspended aggregates, however, have shown that substantial fluid flow may occur through highly permeable bacterial floes. Since bioflocculation of microorganisms in bioreactors is known to occur under conditions of low substrate availability, the rate of substrate uptake is assumed to be mass transfer limited. The hydrodynamic environment of a cell then determines cellular uptake rates. Through development of a relative uptake factor, the overall uptake by cells in flocs in sheared fluids and floes attached to bubbles are compared with the uptake by an identical quantity of dispersed cells. Bioflocculation is found to increase the rate of substrate transport to cells in permeable floes compared to dispersed cells, particularly for large-molecular-weight substrates and when bubbles are present.