Effects Of Inorganic Cations In Aqueous Solution On The Coagulation-ultrafiltration Process

Effects of inorganic cations (calcium ion, magnesium ion and sodium ion) on the coagulation-ultrafiltration (coagulation-UF) process in different pH conditions were evaluated in this study. Humic acid (HA) water (HA raw water and HA-calcium raw water) and Fulvic acid (FA) water (FA raw water, FA-magnesium raw water and FA-sodium raw water) were employed as water samples in this study. Polyaluminum chloride (PAC) of different basicity values (B,OH/A1molar ratio) were used as coagulants to investigate the effects of inorganic cations on the coagultion process. The optimal pH values were selected under optimal PAC dosages in coagulation process. In the optimal coagulation conditions, the size and fractal dimension of flocs were obtained by on-line monitor of coagulation. In the optimized conditions, different water samples were treated in coagulation-UF process and the impacts of these inorganic cations on the mrmbrane foulings were premilinary investigated, respectively. Main conclusions are as follows:(1) Removal efficiencies of organic materials were higher while the B value of PAC was higher. The organics removal efficiencies were affected by the addition of inorganic cations. The influences of different cations on the organics removal efficiencies were different. The optimal PAC dosages of HA and FA water samples were3mg/L and7mg/L, respectively. pH6.00,7.00and raw water pH were selected as the optimal pH in the coagulation-UF process of HA water sample, while pH6.00,7.00and8.00were the optimal pH of FA water sample.(2) The floc sizes of water samples were greater whlie the B value PAC was higher in the optimal coagulation conditions. The HA floc sizes were greater when pH value was smaller. The FA floc sizes complied with the following order: pH6.00> pH8.00> pH7.00. The floc sizes were enhanced by the addition of calcium and magnesium ions. The impacts of sodium on the floc size were different. The effects of three inorganic cations on the fractal dimension of the floc were different. (3) Flux declines of raw water samples in UF tests were more severe in the presence of three inorganic cations. Without pre-coagulation, the flux decline of HA raw water was more at lower pH values, while the flux decilne of FA raw water was smaller. Membrane fluxes were advanced by the addition of coagulants. In the coagulation-UF process, the membrane fouling of water samples were more severe when the B value of coagulant was greater. At lower pH, the flux declines of HA and HA-calcium water samples were smaller in coagulation-UF process. Membrane flux in coagulation-UF was advanced by the addition of calcium and the benefit of calcium on the membrane fouling for PAC with smaller B value was evident. The flux decline of FA water for PAC (B=1.6) was smaller at lower pH value. For PAC (B=2.0) and PAC (B=2.4), the membrane fouling severity of FA water increased in the following order:pH6.00, pH8.00and pH7.00. The flue of FA water was increased in the presence of magnesium, whlie the flux was reduced by the addition of sodium. The impacts of magnesium and sodium on the flux in coagulation-UF process were distinct under different coagulation conditions.(4) Permeate pH in coagulation-UF process was affected by the addition of three cations. Without pre-coagulation, the permeate pH of HA raw water was higher than the feed pH. And the permeate pH of FA raw were higher than the feed pH at pH6.00and pH7.00, while the permeate pH and the feed pH were comparable at pH8.00. In coagulation-UF process, the permeate pH of water samples were greater when the B value of PAC was higher. The premcate pH were increased by the addition of three cations regardless of pre-coagulation process.The results above indicated that the positive or negative impacts of three inorganic cations on the coagulation-UF process were occurred in different coagulation coaditions (pH value, coagulant and coagulant dosage).In water treatment process, the optimal coagulation conditions were selected according to the practical application to achieve the best water treatment effect.