2019 Curtin University and John Wiley & Sons, Ltd. We analyse the steady-state operation of two process configurations that employ a reactor cascade with recycle. The first process configuration is a two-reactor cascade in which the feed stream enters into the first reactor. The second process configuration is a two-reactor step-feed cascade in which the feed stream enters the second reactor. In each process configuration, part of the effluent stream is recycled back into the first reactor. The reactors in the cascade need not be of equal volume. The reaction is assumed to be a biological process governed by Monod growth kinetics with a decay coefficient for the microorganisms. The stability of the washout solution is determined analytically for both process configurations. It follows from the stability analysis that that there is a range of residence times over which the effluent concentration leaving a step-reactor cascade is lower than that leaving a standard reactor cascade. Steady-state diagrams are presented showing the effluent concentration as a function of the residence time. We find that if the desired effluent concentration is not too low, then the process configuration that minimises the residence time is the step-feed reactor cascade. However, for much cleaner waste streams, the process configuration that minimises the residence time is the standard reactor cascade.
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