| Low operating pressure, relatively high permeate flux and selective rejection of multivalent ions make nanofiltration (NF) an attractive alternative in water treatment. However, one of the major limitations in the application of NF in drinking water treatment is inorganic scaling caused by the accumulation of inorganic salts on the membrane surface, which decreases permeate flux, increases feed pressure, decreases water quality and finally, shortens the membrane life.; This study was performed to investigate inorganic scaling on NF membrane. Two salts (CaSO4 and CaHPO4) were selected in a series of nanofiltration experiments to observe inorganic scaling behavior on NF during the filtration of the salts. Based on the flux decline behavior during CaSO4 fouling on NF, a mechanistic model has been proposed in this study to describe the permeate flux decline during CaSO4 scaling on NF. The proposed model has four stages, concentration polarization, nucleation, cake formation and steady state stage. It was found that increasing operating pressure, decreasing cross-flow velocity and increasing feed concentration promote CaSO4 nucleation and crystallization, thus enhanced membrane fouling. During CaSO4 fouling on NF, flux decline and its dynamics due to the change of operating parameters were assessed quantitatively in this study. Applied transmembrane pressure was found to have the most dominant impact on the inorganic fouling on NF.; It was further established in this study that the resistance caused by CaSO4 fouling on NF was greater than that by CaHPO4 fouling. Ninety-seven percent of flux was reduced due to CaSO4 fouling, while seventy percent of flux was reduced due to CaHPO4 fouling on NF at 345 KPa and 3.2 x 10-3 m/s cross-flow velocity. Moreover, during CaSO4 fouling, resistance was attributed mainly by cake growth, while during CaHPO4 fouling, resistance was attributed both by pore and/or surface adsorption and cake growth. In this study, resistance due to pore/surface adsorption and cake growth during CaHPO4 fouling was assessed quantitatively and it was observed that cake resistance was much higher than irreversible resistance.; This study also demonstrates that mechanism of cake formation was dependent on operating parameters during CaSO4 filtration. Cake formation by CaSO4 was limited by both bulk and surface crystallization at 345 KPa, while the primary mechanism of fouling was surface crystallization at higher operating pressure (e.g., 966 KPa). However, during CaHPO4 filtration, the main mechanism of fouling was limited mainly by surface crystallization, regardless of the operating conditions. These findings indicate that the speciation of salts in feed water as well as operating parameters of the system during filtration needs to be considered when evaluating inorganic fouling on nanofilter membranes. |
