Effect of calcium on floc properties and membrane foulings in coagulation–ultrafiltration process by polyaluminum chloride (PACl) of different OH/Al3+ values

The effects of calcium on flocs formed by PACl of different basicity values (B, OH/Al molar ratio) and the subsequent impacts on membrane fouling in humic acid (HA) treatment by coagulation–ultrafiltration (UF) process were evaluated in this study. Two types of HA water containing different quantities of calcium were employed, which were high hardness HA raw water and HA raw water. A series of jar tests were initially conducted to investigate the impacts of calcium on the floc breakage and re-aggregation potential. Flux declines in the UF experiments were used to indicate the membrane foulings in the process. The results indicated that calcium could enhance the sizes and strength of flocs formed by PACl. The beneficial effect of calcium on HA–PACl flocs was more obvious for PACl with smaller B value. The results of UF tests showed that the membrane fouling was significantly dependent on the floc size and smaller flocs resulted in more severe flux decline regardless the presence of calcium. The fluxes of both PACl were advanced in the presence of calcium. The benefit of calcium on membrane fouling for PACl with smaller B value was evident.

► Calcium makes flocs larger and stronger.
► Severity of UF flux decline is decreased by calcium regardless of coagulants.
► Calcium affects more on PACl (B = 1.6) in coagulation–UF process.