Fatty acid fouling of forward osmosis membrane: Effects of pH, calcium, membrane orientation, initial permeate flux and foulant composition

Octanoic acid (OA) was selected to represent fatty acids in effluent organic matter (EOM). The effects of feed solution (FS) properties, membrane orientation and initial permeate flux on OA fouling in forward osmosis (FO) were investigated. The undissociated OA formed a cake layer quickly and caused the water flux to decline significantly in the initial 0.5 hr at unadjusted pH 3.56; while the fully dissociated OA behaved as an anionic surfactant and promoted the water permeation at an elevated pH of 9.00. Moreover, except at the initial stage, the sudden decline of water flux (meaning the occurrence of severe membrane fouling) occurred in two conditions: 1. 0.5 mmol/L Ca2 +, active layer facing draw solution (AL-DS) and 1.5 mol/L NaCl (DS); 2. No Ca2 +, active layer-facing FS (AL-FS) and 4 mol/L NaCl (DS). This demonstrated that cake layer compaction or pore blocking occurred only when enough foulants were absorbed into the membrane surface, and the water permeation was high enough to compact the deposit inside the porous substrate. Furthermore, bovine serum albumin (BSA) was selected as a co-foulant. The water flux of both co-foulants was between the fluxes obtained separately for the two foulants at pH 3.56, and larger than the two values at pH 9.00. This manifested that, at pH 3.56, BSA alleviated the effect of the cake layer caused by OA, and OA enhanced BSA fouling simultaneously; while at pH 9.00, the mutual effects of OA and BSA eased the membrane fouling.

In this study, the effects of feed solution (FS) properties (pH and Ca2 +), membrane orientation and initial permeate flux on octanoic acid (OA) fouling in forward osmosis (FO) process were investigated. The result showed that membrane fouling formed only in very special cases: 1. enough foulants attached on the membrane surface; and 2. the high water permeation provided the force to push foulant deposit into the porous support substrate. The high permeate flux could make a transition of fouling layer from a looser structure to a much more compact cake layer, making membrane fouling more severer.Figure optionsDownload as PowerPoint slide