Exploring the effect of water chemistry regulation on the ultrafiltration performance of a new membrane with multi-walled carbon nanotube modification: based on real source water tests†

Abstract

Membrane fouling hinders ultrafiltration applications of polyvinylidene fluoride (PVDF). Besides membrane modification, water regulation is another promising strategy. However, limited information is available on regulating water quality from actual field water with specific organic compositions, especially when using a modified membrane. We explored the influence of pH values, ionic strength (Na⁺), and water hardness (Ca²⁺) on filtration performance, including the removal of dissolved organic matter and anti-fouling ability. Alkaline conditions hindered hydrophilic removal of organic matter and led to severe fouling in the nascent membrane. In contrast, the modified membrane demonstrated effective hydrophilic organic removal and improved fouling mitigation under the same conditions. The nascent membrane exhibited reduced organic removal and experienced severe fouling as ionic strength increased, particularly at 3 and 100 mM; the negative influence of increased ionic strength (3 mM) can be partially counteracted for the modified membrane. An increase in water hardness promoted organic removal at 1–10 mM, while aggregated macromolecules induced by Ca²⁺ ions caused severe fouling in the nascent membrane; such fouling was alleviated by the modified membrane, with the highest residual flux at 3 mM. According to the fluorescence results, pH values barely affected organic properties in the feed water, but organic properties mattered a lot for Na⁺ and Ca²⁺ ions. This study provides a comprehensive basis for improving filtration performance by regulating water chemistry conditions with a modified membrane as an efficient, low-energy method.