AbstractA well‐mixed biological reactor with continuous addition of biological solids was subjected to sinusoidal variations in inlet substrate concentration. The phase lag between inlet and outlet concentrations increased with increasing frequency. The deviation of measured substrate concentrations from initial steady‐state concentrations was observed to have a maximum near a frequency of 1 cycle per residence time. Measured concentrations of substrate and biomass in the reactor differed significantly from concentrations predicted by the Monod kinetic mo