Effect of salinity on extracellular polymeric substances of activated sludge from an anoxic–aerobic sequencing batch reactor

•LB-EPS and TB-EPS in a SBR were positively correlated with the salinity.•PN/PS ratios in LB-EPS and TB-EPS decreased with the increase of salinity.•FTIR spectra showed salinity variation had no distinct effect on EPS composition.•Protein- and humic acid-like substance in LB-EPS and TB-EPS were found by 3D-EEM.•SVI showed linear correlations with LB-EPS and TB-EPS under different salinities. The effect of salinity on extracellular polymeric substances (EPS) of activated sludge was investigated in an anoxic–aerobic sequencing batch reactor (SBR). The contents of loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS) were positively correlated with the salinity. The polysaccharide (PS) and protein (PN) contents in both LB-EPS and TB-EPS increased with the increase of salinity. With the increase of salinity from 0.5% to 6%, the PN/PS ratios in LB-EPS and TB-EPS decreased from 4.8 to 0.9 and from 2.9 to 1.4, respectively. The four fluorescence peaks in both LB-EPS and TB-EPS identified by three-dimensional excitation–emission matrix fluorescence spectroscopy are attributed to PN-like substances and humic acid-like substances. The Fourier transform infrared spectra of the LB-EPS and TB-EPS appeared to be very similar, but the differences across the spectra were apparent in terms of the relative intensity of some bands with the increase of salinity. The sludge volume index showed a linear correlation with LB-EPS (R2=0.9479) and TB-EPS (R2=0.9355) at different salinities, respectively.