The biodegradation of equimolar phenol and sodium salicylate (SA) by Pseudomonas putida CCRC 14365 at 30 degrees C and pH 7 was examined in a microporous polypropylene (PP) hollow fiber module. The fibers were pre-wetted by contacting with ethanol to make them more hydrophilic. The initial cell concentration was fixed at 0.025 g/L (0.064 OD). Even though the total substrate level was sufficiently high (up to 63.8 mm), both substrates could be completely biodegraded with the help of the membrane-attached biofilm. A simplified model was proposed, which combines the steady mass transfer equations and dynamic growth kinetics of suspended cells but assumes the absence of membrane-attached biofilm, to quantitatively evaluate the role of biofilm. Under the total substrate level ranges studied (5.32-63.8 mM), it was shown that more than 50% of substrate biodegradation was caused by the biofilm. Moreover, the contribution of the biofilm remained more than 78% when the total substrate level was increased up to 37.2 mM and more. (c) 2006 Elsevier B.V. All rights reserved.