Influence of colloidal fouling and feed water recovery on salt rejection of RO and NF membranes

摘要

The influence of colloidal fouling and feed water recovery (or concentration factor,CF) on salt rejection of thin-film composite reverse osmosis (RO) and nanofiltration (NF) membranes was investigated.Fouling experiments were carried out using laboratory-scale crossflow test unit with continuous permeate disposal to simualte the CF and recovery as commonly observed in full-scale RO/NF systems.For feed waters containing only salt (NaCl),permeate flux declined linearly as CF was increased and salt rejection was nearly constant for both RO and NF membranes.On the other hand,a sharp decrease in permeate flux and significant decline in salt rejection with icnreasing CF were observed under conditionsw here colloidal fouling takes place.For both RO and NF membranes,the marked permeate flux decline was attributed to the so-called "cake-enhanced osmotic pressure".The declien in salt rejection when colloidal fouling predominated was much more substantial for NF than for RO membranes.In all cases,the decline in salt rejection was higher under conditions of more severe colloidal fouling,namely at higher ionic strength and initial permeate flux.