Ultrasonic time domain reflectometry (UTDR) as a non-invasive and real-time technique was employed to monitor the sub critical flux fouling in a submerged hollow fiber membrane module (SHFMM) oriented horizontally. A single polyethersullone hollow fiber membrane was used to filter 5 g/L yeast suspension in the fouling experiment. The effect of aeration rate, fiber length and operating flux on the sub critical flux fouling profiles was investigated. Results showed that the fouling distribution along fiber in the horizontal SHFMM was more uniform than that in the vertical SHFMM under the same conditions. The deposition of foulants gradually migrated from both suction ends to the middle part of fiber in the horizontal SHFMM. Further, a high aeration rate could effectively restrain the deposition of fouling, but could not completely eliminate membrane fouling. Moreover, shortening the length of horizontal SHFMM could not reduce membrane fouling. A low operating flux could effectively prevent from membrane fouling. (C) 2013 Elsevier EN. All rights reserved