Chloride and sodium concentrations should not change during digestion since they are not converted during the process (4, 21, 22, 23) and not present in large amounts in the waste sludge. Concentration of 3500-5000 mg/L for sodium and 6000 mg/L for chloride can inhibit the digestion and should be avoided . High concentration of these two ions can be found in case of industrial wastewater treatment, intrusion (or use) of seawater, or control of H2S production by iron chloride salts addition . Since data on chloride concentration in digesters are rarely reported in literature , information was derived from the composition of dewatered sludge from Slibverwerking Noord-Brabant (SNB), which incinerates roughly 25% of all sewage sludge produced in the Netherlands. . Such observations could be due to error in the analyses.