Biological wastewater treatment reactor are designed to reduce the pollutant content of a wastewater to an acceptable level often fixed by wastewater discharge regulations. The reactor design is often based on average wastewater flow and composition patterns. However, industrial wastewater treatment reactors are often subject to unexpected perturbations (variations in wastewater flow, composition or shift in the microbial communities). Hence the capacity of the reactor to recover from these perturbations in reasonable times should also be considered as an important design criteria. In this work we illustrate how this property can be quantified by calculating reactor resilience using return time concept. We show that a reactor design that maximizes the productivity (in term of mass of pollutant removed per unit of time and unit of reactor volume) may not be optimal in term of resilience. Hence, a compromise between productivity and resilience should be targeted when designing biological wastewater treatment reactors.