Total alkalinity increases together with sludge fermentation, and some experimental data are available for this parameter, which is not the case for bicarbonate alkalinity. Since VFA and bicarbonate should be the two main basic compounds contributing to the total alkalinity, bicarbonate concentration was deducted from VFA concentration and total alkalinity. While a lower pH can provoke a dissolution of some precipitates, the biological activity is the main mechanism for the release of PO4 *Secondary sludge/primary sludge No clear differences were noticed in the concentration of PO4 3-, Mg 2+ , Ca 2+ and K + between primary sludge and Waste Activated Sludge (WAS). We believe that those concentrations (except Ca 2+ ) will depend on the amount of phosphorus stored by PAO's, and therefore, on the design of the WWTP. On the other hand, the pH seems to be lower in primary sludge than in WAS , which is in line with the fact that primary sludge starts to ferment immediately into VFA, while VFA are produced slower in WAS and are then directly converted to CH4. A clearer difference is observed for nitrogen since 5-15 times more soluble Nitrogen was measured in primary sludge than in WAS . It seems logical considering that ammonia is removed during secondary treatment, producing a sludge poorer in soluble nitrogen. This observation is backed up by the study of where the NH4 + concentration in 7 WAS ranged from 0 to 50, while it reached up to 480 mg/L in thickened primary sludge in some cases .