Heterogeneous microstructure induces floatation in high-rate anammox granules

The floatation of anammox granules can be a serious challenge in practical wastewater treatment, as it can deteriorate reactor performance and cause bacterial loss. To deepen the understanding of floatation mechanism, in this study, both the floating (F-AnGS) and settling anammox granules (S-AnGS) from a high-rate anammox reactor were comparatively investigated. F-AnGS demonstrated 1.6 times higher specific anammox activity compared to S-AnGS, but only 65 % of produced gas could be successfully released, as quantified by anaerobic respirometry. In addition to the overall EPS accumulation, F-AnGS exhibited a heterogeneous microstructure distinct from that of S-AnGS, as revealed by 3D X-ray microscopic imaging at the single granule level. The heterogeneous distribution of EPS, which can form a dense surface layer, was the main cause for granule floatation. The heterogeneous microstructure of F-AnGS can reduce the distance between microorganisms and enhance the metabolic interaction between anammox bacteria and heterotrophs. The abundance of community members did not have a significant variation, but the functional genes related to anammox and partial denitrification pathway were significantly increased, indicating the enhanced nitrite loop in F-AnGS. This study proposed new structural insights into mechanism of anammox granule floatation, suggesting the appropriate activity control of granule-based anammox process.