Calcium and ash accumulation in aerobic granules treating organic wastewater was investigated systematically in this study. It was found that ash content of granular sludge reached 43% at stable state of reactor operation with calcium carbonate precipitation as the main cause. Since granular sludge in the reactor at the steady state was the mixture of granules with different sizes, granules in the reactor on operation days 90, 120 and 150, respectively, were sorted into 10 categories by size to study the inorganic substances. It was found that calcium was selectively enriched in granules and the precipitation of calcium carbonate was just dependent on granule size instead of operation duration at the steady state. More calcium carbonate was precipitated in bigger aerobic granules, but both calcium and ash contents reached stable values when granule size was bigger than 700 mu m. Analysis by X-ray diffractometer disclosed that aragonite was the only polymorph of calcium carbonate detected in aerobic granules with size bigger than 300 pm. Scanning electron microscope revealed clearly for the first time that bundles of needle shaped aragonite was embedded in extracellular polymeric substances of aerobic granules. A mechanism on calcium carbonate precipitation was proposed to deepen the understanding of biomineralization. (C) 2015 Elsevier B.V. All rights reserved.