Characterization of ion transfer and modeling of fouling in nanofiltration and reverse osmosis membranes

The first goal of this study is to provide a better understanding of the transfer mechanisms involved by nitrate, fluoride, chloride, and sulfate ions contained in nitrated brackish water. This part of the study is done by Spiegler-Kedem-Katchalsky (SKK) model that relates permeate flux to the rejection of the four anions by using a polyamide NF90 and BW30 membranes and leads to distinguish diffusion and convection phenomena. Secondly, the transport parameters determined using the SKK model (reflection coefficients and solute permeability P-s) are used in the steric hindrance pore (SHP) model to calculate the structural characteristics of the NF90 membrane. The results show that the permeabilities of ions in nanofiltration (NF) membrane are greater than that of reverse osmosis (RO) and, reflection coefficients (sigma) are almost similar for both membranes. For the transfer process, the mechanism is convective and diffusional for the NF membrane, whereas for the RO membrane it is diffusional. The estimated average of r(p) and A(k)/Delta x for NF membrane are r(p) = 0.16 nm and Delta x/A(k) = 25.50 x 10(-7) m. Thirdly, this study is completed by a fouling investigation which is based on two experiments carried out separately. The first one is performed by studying the permeate flux decline vs. time. Based on the insight gained from these experiments, Hermia model is applied to determine exactly the type of fouling phenomenon. The second experiment for fouling investigation is based on the determination of different pure water permeability. The experimental data is applied to the resistance in series model to calculate the total resistance (R-t) related to both membranes, NF90 and BW30. The results of both fouling models obtained complemented each other and reveal the problems caused by the accumulation of deposits on the membrane surface. The results indicate that RO membrane is severely fouled than the NF membrane and, the fouling mode responsible for permeate flux decline for both membranes is the cake fouling.