This paper presents the results of an investigation on the effects of concentration of cations on activated sludge properties and membrane fouling in submerge membrane bioreactors. The working volume of the experimental setup was two liters. The cellulose acetate membrane was immersed in the bioreactor. The flocculability, settling properties and fouling propensity of activated sludge was measured in various concentrations of cations such as sodium, potassium, magnesium and calcium. Results showed that cations bind with biopolymers and induce flocculation in bioreactors. This phenomenon induced in divalent cations so that flocculability of activated sludge in presence of divalent cations was two-fold of flocculability of activated sludge. On the other hand, cations improve settleability and dewatering properties of activated sludge and especially mitigated membrane fouling, for example, calcium reduced the membrane fouling intensity by 25% of the reference activated sludge.