Membrane Water Treatment, Vol. 6, No. 3 (2015) 237-249 DOI: http://dx.doi.org/10.12989/mwt.2015.6.3.237 237 Copyright © 2015 Techno-Press, Ltd. http://www.techno-press.org/?journal=mwt&subpage=7 ISSN: 2005-8624 (Print), 2092-7037 (Online) Experimental determination of liquid entry pressure (LEP) in vacuum membrane distillation for oily wastewaters Gábor Rácz 1 , Steffen Kerker 2 , Oliver Schmitz 2 , Benjamin Schnabel 2 , Zoltán Kovács 1,2 , Gyula Vatai 1 , Mehrdad Ebrahimi 2 and Peter Czermak 2,3 1 Department of Food Engineering, Corvinus University of Budapest, 44 Ménesi street, Budapest H-1118, Hungary 2 Institute of Bioprocess Engineering and Pharmaceutical Technology, University of Applied Sciences Mittelhessen, Giessen, Germany 2 Department of Chemical Engineering, Kansas State University, Manhattan, USA (Received December 29, 2013, Revised February 02, 2015, Accepted February 23, 2015) Abstract. In this study we investigate a laboratory scale vacuum membrane distillation system to produce pure water from model oil in water emulsion. Experimental determination of liquid entry pressure (LEP) of a commercial Durapore TM GVPH flat sheet membrane using model emulsions in various oil concentrations has been carried out. Two different methods of liquid entry pressure determination – a frequently used, so-called static and a novel dynamic method – have been investigated. In case of static method, LEP value was found to be 2.3 bar. No significant effect of oil content on LEP was detected up to 3200 ppm. In contrast, LEP values determined with dynamic method showed strong dependence on the oil concentration of the feed and decreased from 2.0 bar to a spontaneous wetting at 0.2 bar in the range 0-250 ppm, respectively. Vacuum membrane distillation tests were also performed. The separation performance is evaluated in terms of flux behavior, total organic carbon removal and droplet size distribution of the feed and final retentate. No significant effect of oil content on the flux was found (5.05 ± 0.31 kgm -2 h -1 ) up to 250 ppm, where a spontaneous wetting occurred. High separation performance was achieved along with the increasing oil concentration between 93.4-97.0%. Keywords: liquid entry pressure; wetting phenomenon; wetted membranes; vacuum membrane distillation; oil in water emulsion; produced water purification 1. Introduction One of the recently developed membrane processes proposed for the production of pure water from contaminated effluents is membrane distillation (MD). MD is a thermally-driven transport of vapor (mainly water vapor) through non-wetted porous hydrophobic membranes, the driving force being the vapor pressure difference across the membrane. The liquid feed stream to be treated by MD must be in direct contact with one side of the membrane. If the transmembrane pressure higher than the membrane liquid entry pressure (LEP), MD cannot be maintained. Different MD configurations have been developed such as direct contact membrane distillation Corresponding author, Ph.D. Candidate, E-mail: racz.g@sooswrc.hu