We consider classes of nonlinear block triangular systems, for which we design interval observers. They possess the well known robustness property of Input to State Stability with respect to the bounds of the time varying disturbances. The systems under study are in general not cooperative and not globally Lipschitz. We illustrate the constructions by two bioreactors models, the first one dealing with competition of two bacterial species and the second one representing a cascade of anaerobic bioreactions.