The structure, density and settlability of anammox granular sludge in high-rate reactors

► Anammox granules were found to include four structures. ► The volume of gas pockets was closely related to the density of anammox granules. ► The volume of gas pockets was related to the settlability of anammox granules. ► The diameter of anammox granules had better to be controlled at less than 2.2mm. ► A mechanism for the granulation and floatation of anammox granules was proposed. Microscopic observation and settling test were carried out to investigate the structure, density and settlability of anammox granules taken from a high-rate upflow anaerobic sludge blanket (UASB) reactor. The results showed that the anammox granules were irregular in shape and uneven on surface, and their structure included granule, subunit, microbial cell cluster and single cell. The gas pockets were often observed in the anammox granules, and they originated from the obstruction of gas tunnel by extracellular polymer substances (EPSs) and the inflation of produced dinitrogen gas. The volume of gas pockets became larger with the increasing diameter of anammox granules, which led to the decreasing density and the floatation of anammox granules. The diameter of anammox granules should be controlled at less than 2.20mm to avoid the granule floatation. A hypothesized mechanism for the granulation and floatation of anammox biomass was proposed.