Effects of salinity on performance, extracellular polymeric substances and microbial community of an aerobic granular sequencing batch reactor

The effects of salinity on the performance, extracellular polymeric substances (EPS) and microbial community of granular sludge were evaluated in an aerobic granular sequencing batch reactor (GSBR). The chemical oxygen demand (COD) and NH4+-N removal efficiencies decreased from 92% and 93% to 25% and 21% at 0-8% salinity, respectively. The specific ammonium oxidation rate (SAOR), specific nitrite oxidation rate (SNOR), and specific nitrate reduction rate (SNRR) decreased from 4.6, 5.6 and 43.0 mg N g−1 MLSS h−1 to 0.5, 0.4 and 9 mg N g−1 MLSS h−1 at 0-8% salinity, respectively. The specific oxygen utilization rate (SOUR) of granular sludge increased from 30 to 42 mg O2 g−1 MLSS h−1 at 0-3% salinity and subsequently decreased to 11 mg O2 g−1 MLSS h−1 at 8% salinity. The protein (PN) and polysaccharide (PS) contents in loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS) increased at 0-8% salinity. The relative hydrophobicity (RH) of granular sludge illustrated good positive linear correlations with the PN/PS ratios in TB-EPS and LB-EPS. The chemical structural changes of LB-EPS and TB-EPS at different salinities were analyzed by three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy. A good linear correlation was found between sludge volume index (SVI) and LB-EPS (or TB-EPS). The denaturing gradient gel electrophoresis (DGGE) profile revealed that Lactococcus sp. YM05004, Desulfopila sp. PR5_F09 and Demequina oxidasica were present at all the salinities, suggesting these bacteria were capable of tolerating 8% salinity. Some bacteria could utilize nitrate as an electron acceptor and was present at certain salinities, such as Arcobacter defluvii, Halomonas hydrothermalis and Paracoccus denitrificans.