Enhancement of filterability in MBR achieved by improvement of supernatant and floc characteristics via filter aids addition

Reduction of membrane fouling in membrane bioreactors (MBR) by addition of three typical filter aids (aluminum sulfate (Al2(SO4)3), polymeric ferric sulfate (PFS) and Chitosan) was investigated. The effects of filter aids on membrane pore blocking, gel layer and cake layer resistance were analyzed respectively. Significant improvement of the sustainable filtration was demonstrated in the filter aids added MBRs. The membrane fouling rate of the MBRs operated under 20 L/m2 h flux was in the order of Control MBR (no filter aid added) > Al2(SO4)3 added MBR > Chitosan added MBR > PFS added MBR. Membrane inner fouling due to pore blocking was analyzed by means of Fourier-transform infrared microscope (FTIR). Compared to the control MBR, significantly low protein and carbohydrate concentrations were measured in the membranes of the filter aids added MBRs, indicating that filter aids could effectively alleviate membrane pore blocking. Gel Permeation Chromatography (GPC) analysis suggested that both the concentration and molecular weight distribution of the macromolecules in supernatant play an important role in gel layer formation and loss of membrane porosity. The reduction of fouling rate in the filter aids added MBRs could be attributed to lower concentration and reduction in molecular weight of macromolecules in supernatant. The specific cake resistance (αc), mean floc size (dp) and fractal dimension of the flocs (df) in the filter aids added MBRs were also investigated. It was demonstrated that αc decreased with the increase of dp and with the decrease of df, which is in consistent with the model prediction.