The use of dimensionless groups in the design of activated sludge reactors

Dimensional analysis techniques were applied to the differential equations for a well mixed activated sludge reactor to obtain generalized information on the effect of process variables on conversion of substrate. The Monod model was used to represent the reaction kinetics since it also contains first and second order kinetics as special cases. The performance of a steady state reactor could be described in terms of five dimensionless groups involving the important combinations of process variables. A simple criterion was developed to predict washout conditions for reactors without recycle of biological solids. Under certain operating conditions, activated sludge reactors were shown to possess an inherent degree of self control to compensate for changes in feed concentration.