The effect of the nitrification on dissolved phosphorus during the treatment of piggery wastewater by a modified sequencing batch reactor has been observed in a previous study. The high solid mineral phosphorus content in the piggery wastewater and the chemical mechanism induced by the fall in pH during the nitrification were proposed to explain this effect. In this work, trials using modified sequencing batch reactors were performed to study the influence of the amount of nitrified nitrogen on pH (and thus solubility of the phosphorus) and determine whether the pH was the main cause of the increased phosphorus dissolving. The dissolved phosphorus was found to increase (up to 3 fold) with the increase in nitrified nitrogen. However, even if the pH was regulated this dissolving still occurred. This could be due to local variations of the pH near the bacterial floc. The modified sequencing batch reactor process management (hydraulic retention time and anaerobic/aerobic step duration) allowed the control of the amount of nitrogen nitrified in one cycle and hence the amount of the phosphorus dissolved.