A fundamental analysis of continuous flow bioreactor models governed by Contois kinetics. IV. Recycle around the whole reactor cascade

Prior to discharge into rivers municipal and industrial waste waters may be treated in a reactor cascade that employs a settling unit to recycle biomass from the final cascade reactor to the first. In this paper we use steady-state analyse to examine the process efficiency of such a reactor configuration. The Contois specific growth rate model is used to describe biomass growth.It is found that there is a critical value of the total residence time which identifies a turning point in the performance of the reactor cascade. In particular, if the total residence time is below the critical value then the settling unit improves the performance of an n-reactor cascade (n = 2, … , 5), whereas, if the residence time is above the critical value then the performance of an n-reactor cascade (n = 2, … , 5) with the settling unit is inferior to that of a cascade without one. It is shown that the critical values of residence time depends upon the values of the recycle ratio R and the concentration factor C.We compare the performance of a reactor configuration employing recycle around the whole cascade with that of a cascade in which the settling unit recycles the effluent stream leaving the ith reactor into the feed stream for the ith reactor.

► Examined reactor cascade where recycle from the last reactor to the first reactor. ► Showed that it can improve performance at low residence times. ► At high residence time: a cascade performance with recycle is worse than no recycle. ► For a two-reactor cascade is it better to add a settling unit or a third reactor?