Role of extracellular polymeric substance in determining the high aggregation ability of anammox sludge.

The high aggregation ability of anammox sludge has been extensively observed, but the cause for their aggregation is challenging. Here the structure and composition of extracellular polymeric substance (EPS) excreted from anammox sludge were systematically investigated to interpret the high aggregation ability. We combine results of contact angle, zeta potential and surface thermodynamics analysis as well as the following DLVO theory to address this issue. The results show that hydrophobic interaction is the main force determining the aggregation of anammox sludge. To go insight into inherent mechanism, Fourier transform infrared (FTIR) and x-ray photoelectron (XPS) spectroscopy were conducted and demonstrated there were comparatively few hydrophilic functional groups in the EPS of anammox sludge, compared to that of activated sludge, nitrifying and denitrifying sludge. Further, amino acid composition and secondary structure analyses of protein indicated that large amounts of hydrophobic amino acids and high level of protein loose structure for exposing inner hydrophobic groups of protein in EPS significantly contributed to the hydrophobic interaction and further to the high aggregation ability of anammox sludge, which is the critical finding of this work. This investigation is useful for understanding anammox bacteria and then for accelerating the application of the anammox process in wastewater treatment.