A combined pore blockage, osmotic pressure, and cake filtration model for crossflow nanofiltration of natural organic matter and inorganic salts

The performance of nanofiltration (NF) process for water treatment is affected by flux decline due to membrane fouling. Many models have been applied to explain fouling mechanisms. In this work, a combined pore blockage, osmotic pressure, and cake filtration model was developed and successfully used to determine NF performance and model parameters for crossflow NF. NOM solutions containing sparingly soluble inorganic salts (i.e. CaCO3, CaSO4, and Ca3(PO4)2), showed higher normalized flux decline than those containing soluble inorganic salts (i.e. NaCl and CaCl2). The αblocked and Rm,s parameters for sparingly soluble inorganic salts exhibited higher values than those for soluble inorganic salts, while the Rm,s and αcake parameters were found to be significant for soluble inorganic salts due to increased salt concentration and NOM cake accumulation at the membrane surface. Increased ionic strengths from 0.01 M to 0.11 M resulted in more pronounced flux decline, thus increased model parameters (i.e. αblocked and Rm,s). The membrane surface characteristics examined by the scanning electron microscopy (SEM) images evidently supported the precipitation of sparingly soluble inorganic salts. The flux decline was the most pronounced for phosphate species, corresponding to the lowest water flux recovery, thus increased non-recoverable resistance (Rnon-rec) due to pore plugging from phosphate salt precipitation.

Research Highlights
► A combined pore blockage, osmotic pressure, and cake filtration model was developed.
► It was used to determine NF performance.
► The experiments were performed using feeds containing NOM and inorganic salts.
► The model successfully predicts flux decline and important parameters.
► Flux decline and fouling of NOM with different inorganic salts were compared.