Exploring the Growth Pattern and Hydraulic Resistance of BioMnO_x Cake Layer under Different Fluxes in Ultrafiltration Processes

Biogenic manganese oxides (BioMnOx) are known to self-catalyze Mn(II) oxidation and are unfavorable in conventional ultrafiltration (UF) processes for Mn(II)-containing water purification because they can accumulate on membrane surfaces and cause severe cake fouling. However, recent studies based on gravity-driven membrane (GDM) have found that the fouling resistance of the BioMnOx cake layer is surprisingly low under low flux conditions (several L m–2 h–1), but the underlying mechanism is not clear. Therefore, in this study, we investigated the formation and hydraulic resistances of the naturally formed BioMnOx cake layers at various fluxes (5, 10, 15, and 20 L m–2 h–1). The results showed that the BioMnOx could form a heterogeneous cake layer with many mounds, and these BioMnOx cake layers could remove nearly 100% of 2 mg L–1 Mn(II) after 34, 49, 55, and 64 days of dead-end filtration at fluxes of 5, 10, 15, and 20 L m–2 h–1, respectively. The hydraulic resistance of the BioMnOx cake layer rapidly increased at high fluxes of 10, 15, and 20 L m–2 h–1, while it remained very low (12 m–1) at a low flux of 5 L m–2 h–1. In-situ optical coherence tomography (OCT) observation revealed that at low flux, the BioMnOx mounds of the cake layer had a higher height–width ratio and lower coverage and thus more porous structures. We attributed this structure to the unique growth pattern of BioMnOx at low fluxes. Specifically, because the advection velocity of Mn(II) was low at low fluxes, new BioMnOx was mainly grown in the upper parts of the cake layer with enough time for Mn(II) diffusion and oxidation. Then, the upper parts depleted Mn(II), and consequently, the lower parts maintained a high porosity with few formations of BioMnOx at their confined pores

Polyamide Thin-Film Composite Janus Membranes Avoiding Direct Contact between Feed Liquid and Hydrophobic Pores for Excellent Wetting Resistance in Membrane Distillation

Hydrophobic membranes are very susceptible to pore wetting when they contact the feed water containing surfactants or low-surface-tension liquids in membrane distillation (MD). Avoiding direct contact between feed water and hydrophobic membrane pores is a potential strategy to control membrane pore wetting. In this study, we successfully fabricated a polyamide thin-film composite (TFC) Janus membrane through interfacial polymerization, with a hydrophobic microporous membrane as the substrate. The fabricated TFC Janus membrane showed a super antiwetting ability when treating the hypersaline water containing surfactants (>0.4 mM sodium dodecyl sulfate) or ethanol (>40% v/v). The optical coherence tomography (OCT) observation revealed that no liquid water was present at the distillate-facing side of the polyamide layer. Therefore, we ascribed the super antiwetting ability to the fact that the polyamide layer could prevent the feed liquid from directly contacting hydrophobic pores. The TFC Janus membrane could also avoid the wetting induced by gypsum scaling because the polyamide layer could act as a barrier to hinder the intrusion of gypsum crystals into hydrophobic pores. In addition to the antiwetting ability, the TFC Janus membrane showed 10–20% increases in vapor flux, despite the existence of a dense polyamide layer. Because interfacial polymerization is the most commonly used method for the fabrication of commercial TFC membranes, this study provides a facile and scalable method to fabricate membranes with robust antiwetting ability

Correlating ultrafiltration membrane fouling with membrane properties, water quality, and permeate flux

<div><p></p><p>The effect of membrane properties, feed water quality, and permeate flux on ultrafiltration (UF) membrane fouling was systematically investigated. Fouling tests were carried out with three types of commercially available UF membrane and a variety of influents. The membrane fouling was assessed by the normalized fouling rate (<i>F</i>₅₀₀). The results showed that the PVDF membrane with smaller contact angle was more resistant to membrane fouling than the PVC membrane. As for feed water parameters, significant correlations were observed between turbidity, total organic carbon, UV₂₅₄, fluorescence intensity, and membrane fouling rates using Pearson Correlation Analysis. This was especially true for hydraulically irreversible fouling rate (<i>F</i>_irr). Moreover, significant correlations between permeate fluxes and membrane fouling rates were observed. With these correlations, the critical flux and critical flux for irreversibility were calculated. It was found that the critical flux is strongly depended on feed water composition rather than membrane properties. Particulate matter, with a size of 0.45–1.2 μm in diameter, was proved to increase the critical flux and critical flux for irreversibility.</p></div