An algebraic expression in dimensionless form, relating effluent substrate concentration to retention time for plug-flow reactors is developed. Monod kinetics is used, and longitudinal concentration gradients are considered. An expression for mean cell residence time is also derived. The model is illustrated by an example using the data of a field study. Results of the developed model are compared with those of constant biomass assumption and differences in biomass and effluent substrate concentrations found more meaningful for values of (1 + r)theta(x)/theta < 5. A graphical procedure enabling a simplified methodology for process design is presented. The method can be used in the design of batch or plug-flow reactors.