Detailed characterisations of membrane-foulant interactions in low-pressure membrane filtration systems

Until now, direct correlation between fouling propensity and organic concentration has not been demonstrated in low-pressure membrane systems. This is due to (1) the expected complex nature of the fouling layer (2) the characterisation of the organics present in feed water rather than those deposited on the membrane surface, (3) the lack of standard methods for organic characterisation, and (4) the limitations of the methods used so far. The proposed research aims to characterise the soluble organics participating membrane fouling through critical assessment of the analytical techniques and comparison with filtration performance. This will allow better evaluation of fouling mechanisms based on the fundamental understanding of the physicochemical interactions occurring during organic deposition on porous membrane used in water and wastewater treatment.In this study, liquid chromatography with organic carbon detector (LC-OCD) demonstrated that higher bulk organic concentrations in the feed does not necessarily result in greater extent of membrane fouling, due to possible foulant-foulant interactions within the feed mixture. It has also been found that, although LC-OCD technique can be considered as a promising advanced analytical tool, it also features relative limitations in precisely characterising the nature of complex organic aggregations formed by carbohydrate compounds. Detailed characterisation of the fouling layers demonstrated that the relative amount of protein and low molecular weight (LMW) compounds presented a strong correlation with the hydraulic resistance associated with this reversible fouling fraction. Fluorescence excitation emission matrices (FEEM) analysis further confirmed these trends, suggesting that fluorescence has also the potential to be used as foulant indicator for domestic wastewater systems. Rigorous foulant extraction protocol was applied to two pilot scale membrane bioreactors (MBRs) operated under different solid retention times (SRT). Surprisingly, the long-term hydraulic resistances for the two MBRs were similar. Given the complex nature of the fouling layers obtained in MBRs, the organic characterisations only could not help to explain the resistances exerted by the different fouling layers. Because of the expected participation of particulates and colloids within the formation of the fouling layer, even advanced characterisation of organic compounds will not always be easily correlated with fouling mechanisms.The initial and the subsequent interactions between membrane and foulant during filtration was also estimated, by considering the novel approach of calculating the free energy of interactions from the extended DerjaguinLandauVerweyOverbeek (xDLVO) theory. It was demonstrated that the surface tension components calculated for the range of tested wastewaters successfully predicted the organic deposition on the membrane surface. The findings in this study imply the importance of considering physicochemical interactions for assessing fouling mechanisms in porous membranes