Sewage treatment with anammox could be implemented through a two-step reactor system, where the first reactor would be devoted to partial nitritation. A process design was sketched including control loops. The control strategy regulates the flow-rate of the rich ammonium sidestream produced after dewatering the digested sludge, to keep the ammonium concentration at a set point in the partial nitritation reactor by DOsing the SIde Stream (DOSIS). A second control loop manages the ammonium concentration set point based on the measurement of the total nitrogen in the partial nitritation reactor. A mathematical model was developed to assess the amount of sidestream required. Even in the case of a strong diurnal variability, simulations show how the control strategy is correctly performing, demonstrating the potential of the proposed technology.