Analysis of Configurational Effects on Hindered Convection of Nonspherical Bacteria and Viruses across Microfiltration Membranes Ruth E. Baltus,* ,† Appala Raju Badireddy, ‡ Wendong Xu, ‡ and Shankararaman Chellam ‡,§ Department of Chemical and Biomolecular Engineering, Clarkson UniVersity, Potsdam, New York 13699-5705, Department of CiVil and EnVironmental Engineering, UniVersity of Houston, Houston, Texas 77204-4003, and Department of Chemical and Biomolecular Engineering, UniVersity of Houston, Houston, Texas 77204-4004 Short-term microfiltration experiments were performed to measure the removal of two gram-negative bacteria (BreVundimonas diminuta and Serratia marcescens), two bacterial viruses (PRD1 and T4), and several spherical silica particles in a stirred cell under convection-dominated conditions before the onset of fouling. These (bio)colloids ranged from being spherical- to rod-shaped, with an aspect ratio in the range of 1-9. Experimental measurements of the removal of spherical silica and spherical PRD1 bacteriophages were in good agreement with theoretical predictions. A comparison of experimental results for the removal of rod-shaped microorganisms to predictions from a previously developed hindered transport model for capsule-shaped particles indicates shear alignment of the rod-shaped particles within the membrane pore, with microbial passage biased toward an “end on” configuration. Results show that, under the conditions of our measurements, microorganism removal by membranes can be conservatively estimated using the rod diameter as the size parameter in hindered convection predictions. 1. Introduction Microfiltration (MF) and ultrafiltration (UF) membranes are increasingly employed in the food processing, biotechnology, and pharmaceutical industries, for water and wastewater treat- ment, etc. 1-3 Preventing microbial contamination of the filtered water (physical disinfection and sterile filtration) is integral to these applications. 4,5 To date, several studies have empirically demonstrated the incomplete rejection of bacteria and viruses by microfilters. 6-10 However, quantitative analyses of colloid passage across MF membranes have predominantly focused on spherical colloids, e.g., see refs 11-14, primarily because they are easily available commercially. In contrast, well-defined nonspherical shapes are more difficult to synthesize reproducibly in the laboratory or purchase commercially. Consequently, a fundamental quantitative analysis of the removal of nonspherical colloids by membranes has not yet been performed. In addition, theoretical analyses of colloidal transport have focused primarily on spherical particles. Because of the complexity of the problem, analyses of the rejection of nonspherical particles have been limited to consideration of configurational effects alone. The problem is an important one because rod-shaped bacteria and nonspherical viruses are frequently encountered in membrane feed waters. Their removal from drinking water supplies is essential to providing biologically stable tap water, from wastewater before discharging to receiving water bodies, and to produce water free of viruses, cells, and cell debris during downstream processing. Studies of colloid and bacterial transport in packed beds have indicated that particle or cell shape is an important parameter in retention. Salerno et al. 15 demonstrated a considerable increase in particle retention within a packed column with increasing particle aspect ratio ()length/diameter) using rodlike polystyrene latex particles with aspect ratios of 2 and 3. In another study of bacterial transport through a packed sand column, Weiss et al. 16 report a chromatographic effect, with changes in the aspect ratio of some bacterial strains as they pass from the inlet to the effluent. Because of the complex geometry in these packed beds, it is difficult to interpret results from these studies using fundamental transport theories that incorporate particle shape effects. Anderson 17 theoretically examined the transport of rigid nonspherical solutes across porous membranes by considering configurational effects for rodlike particles in cylindrical pores. The reflection coefficient for a rod-shaped particle was predicted to be larger (higher removal) than for a spherical particle with equivalent volume. Experimental results with rodlike tobacco mosaic virus generally agreed with predictions from this theory, when corrections were made to consider polydispersity of virus lengths. 18 Prud’homme and co-workers 19 compared the elution char- acteristics of rodlike xanthan polysaccharide and latex spheres in hydrodynamic chromatography measurements performed in a column packed with nonporous spheres. Results indicated that the rodlike xanthan is oriented by the flow field in the chromatography column, eluting from the column with an equivalent volume lying between the volume of a freely tumbling rod and that of a rod which is completely aligned along a streamline. Order of magnitude calculations of the orientational distribution of a rigid dumbbell in unbounded shear flow and in unbounded elongational flow were generally consistent with experimental observations. 20 While the importance of particle shape on membrane or packed bed transport is generally recognized, detailed informa- tion on the removal of nonspherical colloids is still lacking. The principal objective of this research was to quantitatively investigate the role of microorganism aspect ratio on their removal. The passage of rod-shaped bacteria, BreVundimonas (formerly Pseudomonas) diminuta (B. diminuta) and Serratia marcescens (S. marcescens), two bacterial viruses (PRD1 and T4), and spherical silica colloids across clean track-etched membranes is quantitatively interpreted using a previously * To whom correspondence should be addressed. Tel.: 315-268-2368. Fax: 315-268-6654. E-mail: baltus@clarkson.edu. † Clarkson University. ‡ Department of Civil and Environmental Engineering, University of Houston. § Department of Chemical and Biomolecular Engineering, University of Houston. Ind. Eng. Chem. Res. 2009, 48, 2404–2413 2404 10.1021/ie800579e CCC: $40.75  2009 American Chemical Society Published on Web 10/24/2008