_Article_  
**Wetting Resistance of Commercial Membrane**  
**Distillation Membranes in Waste Streams**  
**Containing Surfactants and Oil**  
[PERSON] 1,2,*, [PERSON] 1, [PERSON] 1, [PERSON] 1, [PERSON] 3 and [PERSON] 2,4  
Footnote 1: VITO—Flemish Institute for Technological Research, Boeretang 200, 2400 Mol, Belgium; [PERSON]K.D.S. ); [EMAIL_ADDRESS] (C.D. ); [PERSON]] (W.D.S.) feed and therefore the process is able to operate properly up to saturation [3; 4]. Recently, membrane distillation has been applied more and more for challenging water streams with a much higher load of contaminants compared to seawater desalination. Examples include reverse osmosis (RO) brines [5; 6; 7], industrial waste water [8; 9] and produced water [10; 11]. In many of the applications, the contaminants include a combination of less soluble salts, organic foulants or components lowering the surface tension of the process fluids. The occurrence of scaling and fouling of these contaminants can decrease the performance of the membrane [6; 12; 13; 14; 15; 16; 17; 18; 19]. Another less investigated phenomenon is membrane wetting, where despite the hydrophobicity and controlled pore size, liquid is able to penetrate into the membrane. The liquid entry pressure (LEP) indicates the minimum pressure difference over the membrane at which membrane wetting will occur and is given by [20]:  
\[LEP=\ -\frac{2\gamma_{\mathrm{L}}\cos\theta}{r_{\mathrm{max}}} \tag{1}\]  
where \(\gamma_{\mathrm{L}}\) is the surface tension of the liquid, \(\theta\) the contact angle of the liquid with the surface and \(r_{\mathrm{max}}\) the maximum pore radius. Multiple origins of membrane wetting are known. Different MD studies show that the reduced hydrophobicity (\(\theta\)) due to membrane fouling is a possible cause of membrane wetting [21; 22]. Furthermore, defects present in the membrane influence \(r_{\mathrm{max}}\) and therefore strongly affect the liquid entry pressure [23; 24]. The main focus of this article is the presence of organics lowering the surface tension (\(\gamma_{\mathrm{L}}\)) of the feed stream, which can cause wetting as well [20; 25; 26]. The effect of membrane wetting for a series of alcohols, organic acids and solvents was already investigated [20; 25]. Different approaches were used to quantify the wetting resistance of a membrane towards the presence of an organic component:  
The determination of the concentration and surface tension (\(\gamma_{\mathrm{pd}}\)) at which a droplet wets the membrane (penetrating drop method) for each organic component. Determination of the theoretical maximum allowable surface tension in the process (\(\gamma_{\mathrm{PC}}\)) using [25]:  
\[\gamma_{\mathrm{pc}}=\ \gamma_{\mathrm{pd}}+\frac{\Delta\mathrm{P}\cdot r_{ \mathrm{max}}}{2B} \tag{2}\]  
Where \(\Delta\mathrm{P}\) is the pressure drop over the membrane, \(r_{\mathrm{max}}\) the maximum membrane pore size and B a geometric factor between 0 and 1, where 1 indicates a perfectly cylindrical pore. Measurement of the liquid entry pressure occurs as a function of the liquid composition. Whereas the effect of alcohols, organic acids and solvent on the wetting behavior is described in literature [20; 25], the effect of surfactants and oil traces on the membrane distillation performance has been less studied. [PERSON] et al. [26] observed membrane wetting of a 0.45 \(\upmu\)m polytetrafluorethen (PTFE) membrane at a sodium dodecyl sulfate (SDS) concentration of 28 mg/L, while a chemically-modified omniphobic membrane was able to resist wetting up to 115 mg/L SDS. [PERSON] et al. showed that a 1000 ppm crude oil mixture immediately wets a 0.45 \(\upmu\)m polyvinylidene fluoride (PVDF) membrane [27], while 100 ppm mineral oil is shown to substantially wet the membrane after 21 h [28]. This study aims to improve the understanding of membrane wetting in the presence of surfactants and oil. A commonly used surfactant, sodium dodecyl sulfate, was used to investigate the effect on surface tension, contact angle and membrane wetting in membrane distillation.