Ultrafiltration membrane fouling control by two-stage coagulant dosing with moderate pH adjustment

Abstract

For the coagulation-ultrafiltration process, it has been recognized that coagulation under acidic condition could greatly alleviate membrane fouling, but the high chemical consumption for pH adjustment and the diminished alkalinity of effluent severely restrict the practical application of this operation. In this study, it was found that two-stage coagulant dosing (separate dosing during rapid mixing and medium speed mixing) with pH adjusted to 6.5 could alleviate membrane fouling as that by one-time coagulant dosing at pH 5.5, which could significantly lower the chemical consumption for pH adjustment. Both the favorable aluminum hydrolysis speciation on the floc surface and the reduced interface interaction between membrane and foulant at pH 6.5 contributed to the effectiveness of this method on membrane fouling control. Compared with the scenarios at pH 5.5 and 7.5, the two-stage dosing at pH 6.5 could promote more aluminum species transformation to amorphous Al(OH)3 on the surface of flocs, resulting in less residual Al in the effluent. The acid-base interaction between foulant and membrane dominated membrane fouling. The degree of membrane fouling by separate dosage ratio of 3:7 at pH 6.5 was as low as that by one-time dosing at pH 5.5 because of the similarly low acid-base interaction energy.