BACTERIAL AGGREGATES OF VARIOUS AND VARYING SIZE AND DENSITY - A STRUCTURED MODEL FOR BIOMASS RETENTION

Models for completely-mixed continuous-flow systems that retain bacterial aggregates have until now only considered reactors containing a single type of aggregate. A novel model is presented, which accounts for the dynamic behaviour of a variety of aggregates in such a reactor during steady state operation. The model predicts reactor performance (in particular biomass concentrations) as resulting from individual aggregate characteristics, which are size dependent. As the main feature a life cycle for an individual aggregate is incorporated, consisting of the growth of small aggregates and the fragmentation of large ones. Wash-out of individual aggregates or, conversely, their retention, is formulated as a function of their settling velocity. The overall biomass hold-up is therefore a complex function, resulting from the interaction of various processes. It is shown to be variable and to decrease with dilution rate (D). The reactor biomass concentration depends in a complicated way on D and on the mechanism of fragmentation. The influent substrate concentration (sin) positively affects the reactor and the effluent biomass concentrations, but does not influence retention. The distribution of aggregate dimensions is independent of sin, but is strongly determined by D. Increasing D provokes a general increase in aggregate diameter. In nearly all cases, a bimodal distribution of substrate conversion over diameter categories is observed; at elevated D values especially the contribution of intermediate aggregate sizes to total conversion is negligible. © 1990.