  Given hydric capacity and nutrient flow of a chemostat-like system, we
analyse the influence of a spatial structure on the output concentrations at
steady-state. Three configurations are compared: perfectly-mixed, serial and
parallel with diffusion rate. We show the existence of a threshold on the input
concentration of nutrient for which the benefits of the serial and parallel
configurations over the perfectly-mixed one are reversed. In addition, we show
that the dependency of the output concentrations on the diffusion rate can be
non-monotonic, and give precise conditions for the diffusion effect to be
advantageous. The study encompasses dead-zone models
