Significant NH ₄-N balance deficits were found during the measurement campaigns for the data collection for dynamic simulation studies at five full-scale sequencing batch reactor (SBR) waste water treatment plants (WWTPs), as well as during subsequent calibrations at the investigated plants. Subsequent lab scale investigations showed high evidence for dynamic, cycle- specific NH ⁺ ₄ adVdesorption to the activated flocs as one reason for this balance deficit. This specific dynamic was investigated at five full-scale SBR plants for the search of the general causing mechanisms. The general mechanism found was a NH ⁺ ₄ desorption from the activated flocs at the end of the nitrification phase with subsequent nitrification and a chemical NH ⁺ ₄ adsorption at the flocs in the course of the filling phases. This NH ⁺ ₄ adVdesorption corresponds to an antiparallel K ⁺ ad/-desorption. One reasonable full-scale application was investigated at three SBR plants, a controlled filling phase at the beginning of the sedimentation phase. The results indicate that this kind of filling event must be specifically hydraulic controlled and optimised in order to prevent too high waste water break through into the clear water phase, which will subsequently be discarded.