A method to measure the fraction of biofilm bacteria active in respiration was developed and applied to a three-phase fluidized-bed biofilm reactor. The substrate flux (J) and the biofilm detachment coefficient (bs) were controlled independently. The experimental results showed that the amount of biofilm per unit of carrier surface area (XfLf) was controlled by J and bs. However, the biofilm's active fraction was proportional to J, but independent of bs for these experiments with relatively large bs values. When J was increased, the biofilm had less inactive biomass, because the substrate concentration was higher and allowed the cells to maintain a higher growth rate. Accompanying the increased activity were an increase in the observed biomass yield and a decrease in the oxygen consumption. These results provide a tool to control bioparticle fluidization, oxygen consumption, and sludge production.

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