The present research studies the evolution of sulphur concentrations during wastewater treatment for various full-scale treatment plants and reveals that a significant amount of S is released as highly soluble sulphates with the sludge water during sludge thickening when sludge is still in aerobic conditions. During digestion, sulphur concentrations in the sludge (expressed as mg S/kg of dry solids) generally increased due to destruction of organic material, and relatively low release of H2S. During sludge dewatering following anaerobic digestion, only a minor fraction of sulphur is found in sludge water, because sulphates have already been reduced into insoluble sulphides. To limit the sulphur concentration in the waste sludge, it is beneficial to extensively dewater the sludge when still under aerobic conditions, that is, before long-term sludge storage. If anaerobic digestion is used, the formation of sulphides is inevitable, as insoluble sulphides, and to a lesser extent H2S.