effluent. For this control the principle of a chemostat system is used: at a constant dilutionrate the effluent substrate concentration will be constant. It has been shown that NH3ratherthen NH4+ is the active substrate (Hellinga et al., 1999). If the pH is increased, a constantammonia level means a lower ammonium level. i.e. by increasing the pH the amount ofammonium in the effluent decreases rapidly. It was shown in period 3 and 5 that indeed aslight change in pH already leads to a large change in the effluent ammonium/nitrite ratio.Without control already a total conversion of >90% can be obtained, it is therefore ques-tionable whether the extra removal by a pH control is economically worthwhile