The origin of foulants that causes physically irreversible fouling in pilot-scale membrane bioreactors (MBRs) treating real municipal wastewater was investigated. Utilization-associated products (UAP), biomass-associated products (BAP), and organic matter contained in the raw wastewater could be considered as potential sources of foulants in MBRs. Organic matter contained in the raw wastewater and BAP were characterized to assess their contributions to the evolution of physically irreversible fouling. Fourier transform infrared (FTIR) analysis, fluorescence excitation-emission matrices (EEM), and monosaccharide/amino acid composition analysis were used to characterize the organic matter. The results revealed that the organic matter contained in the raw wastewater was minor among the foulants regardless of the SRT. The characteristics of the foulant of the MBR operated with a long SRT were relatively similar to those of BAP, while no obvious similarity was observed between the characteristics of BAP and that of the foulant in the MBR operated with a short SRT. It was suggested that the contribution of UAP to the physically irreversible fouling increases as the SRT is shortened. The foulant of the MBR operated with a short SRT exhibited a higher specific filtration resistance than that of the MBR operated with long SRT. UAP causes severe physically irreversible fouling and it is important to minimize the accumulation of UAP in the mixed liquor of MBRs.
