Effect of humic-acid fouling on membrane distillation

•Humic-acid fouling studied for membrane distillation using PTFE and PVDF membranes.•Fouling layer shown to not offer any significant heat- or mass-transfer resistance.•Experiments isolate effect of vapor-pressure reduction owing to fouling layer.•Kelvin effect caused 12.4–16.6% decrease in feed-side vapor-pressure.•Pore diameter in fouling layer of 13.2±5.5 nm determined from vapor-pressure depression.

Membrane distillation (MD) can concentrate non-volatile solutes or remove volatiles and dissolved gases from an aqueous feed. A microporous hydrophobic membrane provides a barrier between the hot feed and cold distillate. Although MD can operate at ambient pressure and moderate temperatures, use waste heat, and treat wastewater via an MD-bioreactor, it has problems such as temperature polarization, liquid weeping to the distillate side, and membrane fouling. Prior studies speculated that fouling can add a heat- or mass-transfer resistance, or cause a vapor-pressure reduction owing to the Kelvin effect, but did not isolate these effects. This study confirms that the vapor-pressure depression owing to the concave interface in the small pores of the fouling layer is a dominant cause of the 25–63% flux reduction observed for humic-acid fouling on PTFE and PVDF membranes. This study underscores the importance of selecting MD membranes based on their pore-size distribution rather than just their nominal diameter in order to maximize the contribution of Knudsen diffusion. It suggests the development of dual-layer membranes having a thin hydrophilic layer with relatively large pores overlying a hydrophobic layer with a typical MD membrane structure in order to mitigate the vapor-pressure reduction owing to membrane fouling.

Graphical abstractMembrane-distillation flux N   as a function of the adjusted vapor-pressure driving force ph∘e−0.132−pc∘, where ph∘ and pc∘ are the normal vapor pressures on the hot and cold sides of the membrane, for humic-acid fouling of a PVDF membrane. Three replicate runs are shown by different markers. The predicted flux, shown by the dashed line, fits the data with an R2=0.982.R2=0.982. The exponent in the vapor-pressure correction to ph∘ is a measure of the Kelvin effect whereby small pores in the fouling layer cause a depression in the vapor pressure. It implies an average pore size in the fouling layer of 13.9±5.8 nm that caused a reduction of 12.4% in the vapor pressure on the feed side. These data and those for a PTFE membrane provide convincing evidence that a humic-acid fouling layer can cause a significant vapor-pressure depression owing to the Kelvin effect that decreases the permeation flux markedly.Figure optionsDownload full-size imageDownload high-quality image (107 K)Download as PowerPoint slide