In this paper a analysis of state controllability for the dynamical behavior of a wastewater treatment plant is presented. This plant is a large non-linear system subject to large disturbances. Based on a state controllability analysis a redesign for the plant is proposed. State controllability analysis is done by using set theory. This allows checking the controllability limits for the system. Indeed it allows including disturbance limits and constraints on control inputs. In this way, nominal design parameters determine the system controllability while other parameters would improve this system property. Finally, improvements in controllability issues for the system are presented.