Journ~ of Membrane Science 129 (1997) 221-235
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
Two a-aluminaceramic 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 2 h of filtration) for membranes at 250ppmoil 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 10 ppm in water can plug injection wells, foul equipment, or form heat-insulating films. Moreover,regulations require that maximum total oil and grease concentrations in discharge waters be 5-40 ppm, with a typical requirement of 10-15 ppm . Thus, substantial removal of small oil droplets and particles from produced waters is required prior to reuse or discharge. Since conventional-treatment technologies such as gravity separators and coalescer plates cannot meet the high purityrequirements for discharge or reinjection, new or improved technologies have been investigated. Among them, membrane microfiltration (MF) has been used in a growing number of cases for successful treatment of oily wastewaters [5-7]. Systems have already been pilot-tested on offshore oil platforms and onshore facilities [3,8-10]. These tests have shown that membrane systems can be successful in the field fortreating produced water. Distinct advantages of membrane technology for treatment of produced water include reduced sludge, high quality of permeate, and the possibility of total recycle water systems. When considering these advantages along with the small space requirements, moderate capital costs, and ease of operation, membrane technology provides a very competitive alternative to conventionaltechnologies. Although MF membranes can successfully treat produced waters, they experience a decline in permeate throughput or flux as a result of fouling. This flux decline is due to the adsorption and accumulation of rejected oil, suspended solids, and other components of produced water on the membrane surface (external fouling) or in the membrane pores (internal fouling). This fouling can beirreversible or resistant to cleaning, making the original flux unrecoverable. Fouling can be reduced through the use of different or surfacemodified membrane materials, various operating stra-
tegies and pretreatments, and hydrodynamic techniques . However, specific fouling mechanisms and reduction strategies during microfiltration of produced water are not well understood. Although thereis relatively little understanding of fouling mechanisms for oily wastewaters in microfiltration processes, some results have been reported for ultrafiltration (UF) membranes used for the removal of lubricating and cutting oils used in the metal industry. Bhattacharyya et al.  observed internal and external fouling during UF of a lubricating oil-nonionic detergent-water solution throughnoncellulosic, tubular membranes. They noted that membrane fouling and cleaning requirements depend on the type of oilywater systems and membranes. Lee et al.  studied concentration polarization and fouling during UF of a soluble oil-surfactant-water emulsion through a polymeric membrane in a stirred filtration cell. They found that fouling was due to adsorption of oil on the membrane structure....