[Effect of NaCl Salinity on Extracellular Polymeric Substances and Bioflocculation of Anoxic Sludge in A²/O Process]

In order to improve the biological removal efficiency of nitrogen and phosphorus and bioflocculation performance of salt-containing wastewater, the effect of NaCl salinity on the efficiency of denitrification and phosphorus removal in the anoxic zone of an A²/O process was investigated. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were used to analyze the composition and structure of extracellular polymeric substances (EPS) in activated sludge of the anoxic zone, to discern the effect of salinity on bioflocculation. Results showed that when NaCl salinity was 0-5 g·L^-1, flocculation ability (FA) in A²/O anoxic zone was about 44% and the sludge particle size was 45.5 μm. EPS content increased from 52.3 mg·L^-1 to 62 mg·L^-1 and protein (PN)/polysaccharide (PS) remained at 2.1. When NaCl salinity increased from 10 g·L^-1 to 40 g·L^-1, bioflocculation of sludge significantly decreased. FA decreased from 40% to 22% and sludge particle size decreased from 43.7 μm to 32.1 μm. EPS content increased from 76.5 mg·L^-1 to 101.0 mg·L^-1 and PN/PS decreased from 1.5 to 1.3. Based on FTIR analysis, with increase in salinity, the main components of EPS were always amino, amideⅠ, and carboxyl. Based on XPS analysis, increasing salinity led to charge transfer of some groups (such as C, O, and N groups) during the interaction between EPS and Na⁺, but its form did not change.