This article is aimed to propose an alternative method to characterize an air/water flow and new designs of rotating air distributors for membrane fouling control in a side-stream membrane module for a membrane bioreactor. A new optical sensor technique was experimentally demonstrated for the characterization of slug flows in a tubular ultrafiltration membrane module. The experimental results showed that the optical sensor system had high sensitivities for measurements of slug velocities, lengths, and frequencies in each membrane tube. The phenomena of slug coalescence/spilt were discussed. The rotating air distributor significantly improved the air slug distributions and had the potential to reduce the membrane fouling.