ELSEVIER Journ~ of Membrane Science 129 (1997) 221-235 iournalof MEMBRANE SCIENCE Crossflow microfiltration of oily water Jeffrey Mueller 1, Yanwei Cen 2, Robert H. Davis* Department of Chemical Engineering, University of Colorado at Boulder, Boulder, CO 80309-0424, USA Received 19 July 1996; received in revised form 12 November 1996; accepted 14 November 1996 Abstract Two a-alumina ceramic membranes (0.2 and 0.8 jam pore sizes) and a surface-modified polyacrylonitrile membrane (0.1 jam pore size) were tested with an oily water, containing various concentrations (250-1000 ppm) of heavy crude oil droplets of 1-10 ~tm diameter. Significant fouling and flux decline were observed. Typical final flux values (at the end of experiments with 2h of filtration) for membranes at 250ppm oil in the feed are ~30--40kg m -2 h -1. Increased oil concentrations in the feed decreased the final flux, whereas the crossflow rate, transmembrane pressure, and temperature appeared to have relatively little effect on the final flux. In all cases, the permeate was of very high quality, containing <6 ppm total hydrocarbons. The addition of suspended solids increased the final membrane flux by one order of magnitude. It is thought that the suspended solids adsorb the oil, break up the oil layer, and act as a dynamic or secondary membrane which reduces fouling of the underlying primary membrane. Resistance models were used to characterize the type of fouling that occurs. Both the 0.2 jam and the 0.8 jam ceramic membranes appeared to exhibit internal fouling followed by external fouling, whereas external fouling characterized the behavior of the 0.1 jam polymer membrane from the beginning of filtration. Examination of the external fouling layer showed a very thin hydrophobic oil layer adsorbed to the membrane surface. This oil layer made the membrane surface hydrophobic, as demonstrated by increased water-contact angles. The oil layer proved resistant to removal by hydrodynamic (shear) methods. By extracting the oil layer with tetrachloroethylene, followed by IR analysis, its average thickness at the end of a 2 h experiment under typical conditions was determined to be 60 ~tm for the 0.2 jam ceramic membrane and 30 jam for the 0.1 jam polymer membrane. These measured amounts of oil associated with the membrane at the end of the experiments are in good agreement with those determined from a simple mass balance, in which it is assumed that all of the oil associated with the permeate collected is retained on or in the membrane, indicating that the tangential flow did not sweep the rejected oil layer to the filter exit. Keywords: Fouling; Water treatment; Produced water; Microfiltration *Corresponding author. Tel.: (303) 492-7314; fax: (303) 492- 4341, e-mail: davisr@spot.colorado.edu. tPresent address: Harrison-Western Environmental, 1208 Quail Street, Lakewood, CO 80215, USA. 2present address: Chusei, Inc., 12500 Bay Area Boulevard, Pasadena, TX 77507, USA. 0376-7388/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0376-7388(96)00344-4 1. Introduction Wastewaters containing dispersed oils and sus- pended particles are produced from diverse industrial sources, such as general metal-working, food proces- sing, transportation, and gas and oil production. Waste streams from onshore and offshore oil and gas opera-