Recognition of membrane fouling: testing of theoretical approaches with data on NF of salt solutions containing a low molecular weight surfactant as a foulant

Membrane fouling may be dissected into four types: complete pore blocking, incomplete pore blocking, standard pore blocking and external surface fouling (cake/gel deposition). Using our experimental data on nanofiltration (NF) of NaCl solutions containing biosodiummethylene-bis-naphthalene sulphonate (BSMNS) as a foulant, we have tested some theoretical approaches with these data to recognize the type of membrane fouling. For fouling identification the following tools were used: (1) the diagrammatic representation of the fouling kinetics in the characteristic coordinates for constant pressure dead-end filtration; (2) the fitting of flux-time theoretical relationships recently modified for crossflow filtration to the experimental data; (3) the flux-pressure diagrams. The applicability of both the dead-end and crossflow filtration relationships to recognize the types of fouling at the initial stage of NF of the solutions studied has been shown. For a dilute BSMNS solution with a low salt concentration, pore blocking has been revealed as the contributory form of fouling. For the solution with a high salt concentration, the cake/gel build-up is the dominant mechanism of fouling. The discovered forms of fouling have been confirmed by the profile of the flux-pressure diagrams.

1. In NF of the BSMNS dilute solution, the contributory form of fouling is clearly recognized at the initial stage of fouling using both the deadend and cross-flow filtration theoretical approaches. The flux-pressure diagram is helpful to recognize pore blocking fouling with no cake and the presence of cake/gel and/or concentration polarization layers. 2. The fouling mechanisms are different for feed solutions with the low and high NaCl concentrations. That can be associated with the different state of the foulant in the feed solutions, namely, preferably a monomolecular form of BSMNS in the 0.15% NaCl and a micellar form of BSMNS in the 5% NaCl.