During municipal wastewater treatment, the pollutant load is oxidized or ending up in the sludge fraction, while the treated water is discharged. The composition of the solid fraction of the sludge does not evolve a lot through the different sludge treatment steps, except during digestion where organic matter is transformed into biogas. However, the liquid fraction of the sludge flows is more dynamic, and its composition varies greatly in the different units. For example, when the sludge encounters anaerobic conditions (typically during thickening and digestion), fermentation occurs and volatile fatty acid (VFA) are progressively produced . It creates a pH drop that influences the solubility of several inorganic compounds present in the sludge, and thus the composition of the liquid fraction. Digestion is usually the final solid treatment step and provokes a big increase in bicarbonate and ammonium concentration associated with a slight increase in pH. Additionally, WWTPs process designs are numerous, and different succession of units will lead to different soluble phase composition. For example, a digestate contains higher P and K concentrations if produced in a WWTP using Enhanced Biological Phosphorus Removal (EBPR) compared to Chemical Phosphorus Removal (CPR) . Considering that the composition of wastewater and sludge soluble phase can vary a lot, evaluating their typical composition is complicated. Ionic strength can be deducted from the composition of the soluble phase. Ionic strength is an important parameter in wastewater treatment since it impacts for example nitrogen removal or the stability of sludge flocs . Especially all kind of precipitation reactions will strongly depend on the ionic strength of the solution since the activity coefficient are calculated from ionic strength . However, ionic strength is often misused in literature, by considering extremely wide ranges , or unrealistic values (76, 83), for example. Moreover, the liquid composition of the different sludge streams, and thus their ionic strength, is not widely available in literature. Ionic strength should preferably be deducted from thorough analyses of the liquid phase composition, but this is not always possible. Therefore, such a study could provide a way to quickly estimate the ionic strength of a sludge stream without the need of a complete characterization. We reviewed the literature for data on the main compounds influencing the ionic strength of wastewater and sludge, and critically evaluated the data availability. Ionic strength ranges were eventually calculated and used to evaluate the current choices of ionic strength in literature.