This research study focuses on the performance of a bench-scale membrane bioreactor (MBR) in a pilot plant at a wastewater treatment plant. We analyzed particle size distribution in the permeate stream under different mixed liquor suspended solids (MLSS) concentrations in order to see how membrane fouling affects membrane bioreactor (MBR) systems. These analyses were performed over a period of 43 weeks. The results of our study show a significant increase in the number of particles in the permeate (particle size between 2 and 5m), which was probably due to the formation of biofilm on the clean side of the membrane. This increase was the result of constant backwashes without chemical products. After chemical cleaning, the number of particles in the permeate stream at first increased dramatically because of the particles detached from the clean side of the membrane. However, when the chemical treatment completely removed the biofilm, the number of particles in the permeate stream decreased. Thus, under actual operating conditions (six experiments were done) the minimum particle numbers per mL detected were in the range between 8 and 254, while after chemical cleaning they were in the range between 51 and 16,720.