The pre-treatment of submerged ultrafiltration membrane by coagulation—Effect of polyacrylamide as a coagulant aid

The application of polyacrylamide (PAM) as a coagulant aid before membrane ultrafiltration was investigated to examine its potential benefit and impact on membrane fouling. The benefit of PAM with alum coagulation was found to depend on the dose and the time of addition during the flocculation process. The optimal conditions, corresponding to the lowest rate of TMP increase, were at a PAM dosage of 0.2 mg/L, applied 5 min after the start of flocculation; a higher PAM dosage of 1 mg/L led to a significant increase in the rate of TMP increase. Since NOM removal was nearly the same for all the pre-treatments, the character of the flocs was the main reason for the variation in fouling. Dynamic light scattering indicated that nano-scale particles (<5 nm) from different pretreatments did not contribute to membrane fouling. Scanning electron microscopy (SEM) images showed that the optimal, low dosage of PAM formed relatively larger floc clusters and thereby increased the porosity/permeability of the cake layer. In contrast, a higher dosage of PAM caused higher residual PAM on the surface of the cake layer and membrane. Crystallization of the alum cake layer may be an important cause of increased membrane fouling with time, and there is evidence that the addition of a low dosage of PAM beneficially inhibited this phenomenon.