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 2.2 mm. ► A mechanism for the granulation and floatation of anammox granules was proposed.