Substrate controlled development of anaerobic acidifying aggregates at different shear rates in a gas lift reactor

The influence of liquid shear rates on the development of acidifying mixed-culture aggregates was studied in a gas-lift reactor. The glucose concentration was kept at a constant and relatively high level by operating the reactor in pH-auxostat mode. Size, strength, and wet density of aggregates cultivated at different superficial gas velocities (Ug) were investigated. Image analysis showed that the Sauter mean diameter (Ds) decreased with increasing Ug. A stirred tank was used to characterize the surface detachment rate (Rd) under non-growth conditions. An exponential decrease was observed in Rd with the applied Ug during cultivation, i.e., aggregates became stronger. The increased strength coincided with an increase in aggregate wet density. Size classified aggregates showed an increase in Rd with the square of the aggregate diameter (Dp), however, this contribution was much smaller than the effect of adaptation. Experiments in a similar gas-lift reactor under dynamic conditions without adaptation, showed that Rd increased exponentially with increasing Ug. So, two important contributions to Rd can be distinguished: adaptation, which induces stronger aggregates, and aggregate size, which makes them less susceptible to hydrodynamic shear. A general expression for Rd was derived, which depends on Dp and Ug. Combining this equation with the surface biomass growth rate (Rg) allowed for the estimation of the maximal diameter (Dmax) aggregates can reach at any Ug, and it was found that the estimated and measured Dmax were in good agreement.