Effect of high salinity in wastewater on surface properties of anammox granular sludge

Bacterial surface properties fundamentally affect the stability and aggregation of anammox granular sludge. The variation in the surface properties of the granular sludge at different salinities were investigated to further clarify the effect of salinity on the aggregation of anammox granular sludge in this study. High anammox activity was obtained at a salinity of 30 g/L NaCl, and the average removal efficiency of NH₄⁺N, NO₂^--N and TN reached 91.9% ± 1.4%, 97.3% ± 0.4% and 86.3% ± 0.9%, respectively. The sludge particle size in Reactor 1 (with 0 g/L NaCl as control) and Reactor 2 (with 0, 15 and 30 g/L NaCl) increased from 1.62 ± 0.16 mm and 1.59 ± 0.12 mm to 2.71 ± 0.23 mm and 2.44 ± 0.19 mm, respectively, during total operation. PN gradually decreased from 30.58 ± 2.5 mg/g to 18.11 ± 2.1 mg/g, and PS sharply increased from 1.48 ± 0.09 mg/g to 10.52 ± 0.50 mg/g with the increase in salinity. The PS/PN ratio of extracellular polymeric substances (EPS) rapidly increased from 0.05 to 0.58 with an increase of salinity. Fourier transform infra-red spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) results showed that salinity inhibited the expression of anammox sludge hydrophobicity by changing surface groups. Binding between multivalent metal ions and EPS was significantly hindered by the high Na⁺ concentration. The results of this study provided a better understanding of the effect of salinity on the stability and aggregation of anammox granular sludge in saline wastewater treatment.