Understanding membrane fouling via in-situ monitoring

Abstract

As human life is threatened by diminishing water resources, new strategies for water and wastewater recovery and reuse must be developed. Among them, membrane technology shows remarkable strengths, such as high efficiency, low costs, easy scalability, and minimum environmental impact, which draws worldwide attention. Nevertheless, membrane fouling is the ubiquitous and negligent problem for membrane processes. It causes severe flux decline, reduces separation efficiency, and impairs product quality, eventually increasing the operation costs and leading to shorter membrane life. Therefore, having a better understanding of membrane fouling is essentially necessary. The study investigates the membrane fouling in ultrafiltration via in-situ monitoring the fouling extent and evolution. Firstly, the effects of feed characteristics (feed concentrations and solution environments) and operational conditions (cross-flow velocity (CFV)) on membrane fouling is explored by monitoring the most straightforward flux decline during the ultrafiltration process. Meanwhile, the relationship between membrane fouling and chiral separation is firstly discussed. To further explore the fouling mechanism, the evolution of membrane fouling is studied with electrical impedance spectroscopy (EIS), which is capable of distinguishing different membrane layers and illustrating the foulant accumulation and fouling layer growth. Charged foulants (oil emulsions stabilized by surfactants) are employed for EIS detection and their types and mixtures are studied. Pure anionic surfactants (sodium dodecyl sulphate, SDS) are recommended for preparation of oil emulsions in terms of reduced fouling due to the electrostatic repulsive interactions between foulants-membrane and foulants-foulants. Nevertheless, under low surfactant loads and transmembrane pressure (TMP), cationic surfactants (cetyltrimethylammonium bromide, CTAB) – oil emulsions display an interesting negligible flux decline for short-term ultrafiltration. Revealed by EIS, the reason is the extensive adsorption onto the oppositely charged membrane that deterred oil deposition. Different fouling behaviors of mixed surfactants of SDS and CTAB are also observed through conductances and capacitances of membrane skin layer.