Enhanced biological phosphorus removal (EBPR) failure due to nitrate/nitrite presence in the anaerobic reactor is a common problem in full-scale WWTPs aiming at simultaneous C/N/P removal. This work evaluates the performance of two common EBPR configurations (A(2)/O and JHB) under normal conditions and under two detrimental scenarios: (i) increase of ammonium nitrogen in the influent and (ii) increase of nitrite in the external recycle. EBPR failure due to nitrate/nitrite entering the anaerobic phase can be avoided with a controlled addition of a carbon source. In this work, crude glycerol, a biodiesel by-product, was used as a low-cost external carbon source. A control strategy to dose an optimal amount of crude glycerol was theoretically designed through a modelling-based study. Hence, a model was firstly calibrated and validated with the open-loop experimental data. Then, the optimal control strategy was experimentally evaluated in the pilot plant under both configurations and under the same detrimental scenarios revealing the real benefits of the control action. JHB obtained the best results under open- and closed-loop conditions: P was satisfactorily controlled with 18% less crude glycerol dosage than A(2)/O. Finally, two control alternatives were tested in silica to overcome problems derived from the slow dynamics of P effluent concentration with respect to glycerol addition resulting in a better control performance. (C) 2015 Elsevier B.V. All rights reserved.