The ionic composition of the liquid in the different sludge streams of a WWTP largely depends on the influent wastewater and on the process scheme of the WWTP. From an extensive literature review, three ranges of concentration were proposed for the main constituents of influent/effluent, undigested sludge, CPR and EBPR digestate. From these data, the ionic strength of the different sludge streams was calculated. This study allows the reader to quickly estimate the ionic strength based on the concentration of the compounds influencing it the most. Reviewing numerous studies showed that ionic strength is a very important parameter since it impacts important wastewater treatment processes. Nevertheless, the choice of the studied range is rarely motivated and not always adequate in literature, which can weaken the conclusion.