Diauxic lag from changing electron acceptors in activated sludge treatment

We have shown experimentally that a diauxic lag can be exhibited when activated sludge bacteria switch from oxygen to nitrate as electron acceptors. This behavior was successfully modeled using a cybernetic modification of the “industry standard” IAWQ Activated Sludge Model 1. The new model accurately predicts oxygen and nitrate uptake, and portrays the growth pattern and diauxie. The model shows that diauxie occurs when the specific level of denitrifying enzyme is low at the outset of the anoxic phase, but does not occur if the specific level is initially high. The present work suggests that caution is needed in the application of conventional models for optimizing periodic processes for nitrogen removal, especially when the phase lengths considered are either much shorter or much longer than those tested under experimental conditions.