Anammox granular sludge in low-ammonium sewage treatment: Not bigger size driving better performance.

An integrated investigation to document high anammox abundance, activity and diversity in upflow anaerobic sludge blanket (UASB) reactor treating low-strength ammonium loading sewage was performed and showed that the optimal anammox granular sludge sizes could mitigate undesirable N2O emission. The enhanced anammox bacterial abundance, activity and specific anammox rate were achieved with optimal granules sludge sizes of 0.5-0.9 mm with multiple "Jettenia", "Brocadia", and "Anammoxoglobus" species. The tightly-bound extracellular polymeric substance (TB-EPS) was the main EPS layer found in anammox granular sludge, in which polysaccharides play an important structural role. Over this granular sludge sizes, the anammox bacterial abundance and activity did not significantly decrease, but N2O emission significantly increased. High throughput sequencing and ecological networks demonstrated the patterns of anammox and their co-occurring bacteria, with availability N2O-producer and N2O-reducer functional genes. Incomplete denitrification and insufficient carbon source mainly contributed to N2O production in granular sludge, as supported by results of stratification analysis.