Background: Membrane bioreactor systems are extensively used in biological treatment of the municipal and industrial wastewater. Despite the efficiency of these bioreactors in biological wastewater treatment, several problems, including bacterial biofilm formation, have limited their use. This study aimed to identify the dominant bacteria in the biofilm of such systems as well as to investigate the lethal effect of the silver nanoparticles in small size (4 nm) and higher dilutions in wastewater.

Materials and Methods: This laboratory-scale experimental study was performed on 140 samples the dominant isolates were identified using the differential biochemical tests. Using Broth micro dilution, the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were calculated. Standard discs were prepared by different concentrations of nanosilver particles (0.5, 1, 2, 4, 8 ppm) and evaluated through the disc agar diffusion method on Muller-Hinton agar plates. Finally, the inhibition zone diameter was measured.

Results: The frequencies of isolates were Escherichia coli, Pseudomonas aeruginosa, Proteus, Bacillus subtilis, Bacillus cereus, Citrobacter and Bacillus anthracis, respectively. Moreover, The MIC of sliver nanoparticles (4nm) against the isolates were as follows: Escherichia coli, 2ppm Citobacter, 1ppm Bacillus cereus, 1ppm Bacillus anthracis, 0.5 ppm Bacillus subtilis, 1ppm Pseudomonas aeruginosa, 4ppm and Proteus, 2ppm.

Conclusion: It can be concluded that silver nanoparticles, with a particle size of <10 nm, have a positive impact on the removal of dominant resistant isolates, especially gram-positive bacteria.