Following the information found in literature and discussed above, NH4 + and NO3 -were considered to be the only soluble nitrogen compounds in the influent/primary sludge and in the WAS, respectively. Similarly, SO4 2-was taken as the only sulphur compound in the influent while H2S alone was considered in the digester. Lastly, VFA were considered to be acetate and P to be HPO4 2-(according to the pH). Since ionic strength in sludge streams is rarely determined in literature, it is complicated to verify the ranges proposed in this study. In , the ionic strength of five digestates was calculated and ranged from 0.018 to 0.094M with an average of 0.054M. Overall, their results are consistent with the range proposed in this study. An ionic strength of 0.1M, consistent with our range was given for a EBPR digestate in , but not calculation details were given. The major conclusion that can be drawn from Figure is that the main contributors to the ionic strength vary depending on the sludge stream studied. Salts whose concentration won't be too influenced by the treatment process (Na + , Cl -, Mg 2+ , Ca 2+ ) represent up to 50-70% of the ionic strength for the streams before digestion. Their contribution progressively decreases with the increase of the NH4 + and HCO3 -concentrations, which will eventually account for around 60-80% of the ionic strength in digestate.