Discussion of “Mechanistic Model for CaSO4 Fouling on Nanofiltration Membrane” by Che-Jen Lin, Saqib Shirazi, and Pritesh Rao

Abstract

The mass transfer phenomenon that leads to concentration polarization and crystallization of salt on membrane surfaces during filtration of CaSO4 solutions is a dynamic process as presented in Fig. 5 of the original paper . The flux decline observed is due to the increases in both the density and thickness of the concentrated layers of CaSO4 i.e., concentration polarization and salt cake on the membrane surface Li et al. 2005 . Eqs. 1 and 2 in the paper are valid for steady state conditions, as the authors have noted. However, the authors have used these equations to estimate a dynamic parameter i.e., concentrations of CaSO4 on membrane surface at specific times . The estimated concentrations of CaSO4 at the membrane surface Table 1 of the original paper show a decrease in CaSO4 concentration over time as the membrane fouling progresses from point A to point D. With the progression of fouling over time, one would expect the CaSO4 concentration at the membrane surface to increase, as illustrated in Fig. 6 of the original paper , since this is the primary cause of fouling, which causes the flux decline presented in Figs. 3 and 7 of the original paper . Other researchers have used the steady state solution of the mass transfer equation for estimation of salt concentration on membrane surfaces at the time of nucleation or to determine if the precipitation is expected to occur in more complex systems van de Lisdonk et al. 2001; Le Gouellec and Elimelech 2002 . These limited applications of the steady state solution may be appropriate in such cases as the flux appears to be stable during the time of nucleation. For simplification purposes, assuming that the flux decline is entirely due to increase in concentration of CaSO4 at the membrane surface, then, the decrease in flux can be related to the increase in concentration of CaSO4 at the membrane