A comprehensive quantitative analysis on the non-linear stationary properties for a cascade of stirred tank bioreactors with partial recirculation and thickening of biomass has been carried out. The phenomenon of secondary multiplicities of the steady states discovered in such bioreactors for processes with two trophic levels was discussed in detail. The process analysed occurs according to the predator-prey scheme at the presence of so-called hiding places for prey. An influence of a degree of inaccessibility of the prey, thickening of biomass and distribution of operative volumes in the object on the structure and linear stability of the steady states has been evaluated. An attempt of explanation of the role of protozoa and hiding places for the prey on stationary properties of the bioreactors was undertaken.