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, 3, 4 & 5), whereas, if the residence time is above the critical value then the performance of an n-reactor cascade (n = 2, 3, 4, & 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.