Influence of NaCl salinity on the aggregation performance of anammox granules

Although the influence of salinity on the aggregation of anaerobic ammonia oxidation (anammox) granules has been investigated, the mechanism behind its effect remains poorly understood. Herein, the changes in the surface characteristics, extracellular polymeric substance (EPS) components, and EPS surface groups of anammox granules were studied at 0, 15, and 30 g/L NaCl salinity to elucidate this mechanism. The results showed that as the salt concentration in wastewater increased (0, 15, and 30 g/L NaCl), the size of anammox granules decreased (2.85 +/- 0.20 mm, 2.17 +/- 0.13 mm and 1.75 +/- 0.17 mm, respectively), the aggregation index (AI) of sludge decreased (21.3 % +/- 1.5 %, 14.6 % +/- 0.8 % and 7.1 % +/- 0.9 %, respectively), and the ratio of extracellular protein (PN) to extracellular polysaccharide (PS) decreased (8.8, 6.2 and 4.2, respectively). Analysis of the surface free energy of the anammox granules based on extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) curves showed that increasing salinity decreased the zeta potential and hydrophobicity of the sludge but increased the repulsive barrier in the sludge. Fourier transform infrared (FTIR) spectroscopy and three-dimensional excitation emission matrix (3D-EEM) fluorescence spectroscopy showed that increasing salinity decreased the anammox granule hydrophobicity by changing the EPS surface groups, and the reduction in hydrophobicity was not conducing to the aggregation of anammox granules. These results provide further useful guidance for practical applications of the anammox process to saline wastewater treatment.