Effect of pH on floc properties and membrane fouling in coagulation–ultrafiltration hybrid process with different Al-based coagulants

Impacts of pH on floc properties and membrane fouling in coagulation–ultrafiltration process with different Al-based coagulants were investigated. Polyaluminum chloride (PAC) was prepared with alkali-titration method and Al speciation distribution was tested with Al-Ferron method. Three kinds of coagulants (AC(AlCl₃), PACb, and PACc) consisted primarily of Ala (monomeric species), Alb (medium polymer species), and Alc (colloidal or solid species), respectively, were used as coagulants under pH conditions of 4.0, 6.0, and 8.0. Floc properties were evaluated using a laser diffraction particle sizing device and ultrafiltration experiments were conducted by a dead-end batch unit. The results indicated that pH condition and aluminum species had significant influence on size and fractal dimension value of flocs. For the three Al species, relatively low pH condition was favorable to form larger flocs, while it would diminish the floc growth rate. And the loosest flocs were formed at pH 4.0. Under the same pH conditions, Ala tended to form the largest flocs and Alc achieved the loosest floc structure. For each coagulant, membrane fouling under different pH conditions was in the following order: pH 4.0 > pH 8.0 > pH 6.0. Alb and Alc showed a wider tolerance for changes of pH conditions than Ala. And under the same pH conditions, Alc achieved the lightest membrane fouling.