Split injection of coagulant to enhance the water permeability of a cake layer in a coagulation–microfiltration process

A split injection of an aluminum coagulant at the initial stage of rapid mixing was investigated to improve the water permeability of a cake layer composed of chemical flocs on a microfiltration membrane surface. The split ratio of the first to second injections was changed to find an optimized injection condition. With an increase in the split ratio, the cake compressibility index decreased because compact chemical flocs were produced with reduced dosage of the second injection, but the specific cake resistance increased due to small size of chemical flocs with the insufficient amount of coagulant at the second injection. At a small split ratio, the particle size distribution of the split injection was similar to that of normal single injection, and the compressibility index was small, leading to better water permeability of the cake layer at higher pressure.