Digital macro photography and settling tests were carried out to investigate the morphological and settling characteristics of ANAMMOX granules in a high-rate reactor. The ANAMMOX granules could be divided into settling and floating granules. The settling granules with an average diameter of 2.96 +/- 0.99 mm were smaller than the floating granules with an average diameter of 4.58 +/- 1.22 mm. A settling model was established and validated to correlate the settling velocity with the density (rho(G)), mass shape factor (psi(mass)), shape-correction factor (characterized by sphericity (Phi') or roundness (xi) and projected area equivalent sphere diameter (d(P)) of ANAMMOX granules. The sphericity was more suitable than the roundness for describing the settling behavior. The sensitivity of four parameters was in the order of p(G), psi(mass), d(P), and Phi'. Based on the settling model, ANAMMOX granules with diameter of 1.75-4.00 mm were supposed to be optimal for the ANAMMOX process. (c) 2013 Elsevier Ltd. All rights reserved.