Factors Affecting the Morphology of Granular Sludge in Phosphorus-Accumulating Organism (PAO) and Denitrifying PAO (DPAO) Sequencing Batch Reactors

This study aimed to investigate the influencing factors and characteristics of granule morphology through approximately 500 d of long-term monitoring of two types of anaerobic–aerobic phosphorus-accumulating organism (PAO) and anaerobic–anoxic denitrifying PAO (DPAO) sequencing batch reactors (SBRs). The results show that granules were present in the DPAO SBR and PAO SBR after 200 d and 250 d of operation, respectively. The average diameters of the granules were 2.2 ± 0.7 mm in the DPAO SBR and 0.4 ± 0.3 mm in the PAO SBR, respectively. The DPAO granular sludge contained rod-shaped microorganisms, whereas the PAO granular sludge contained cocci-type microorganisms. A precipitated core consisting of hydroxyapatite was found in the DPAO granules. A comparative analysis conducted under various operating conditions revealed that the availability and type of the electron acceptors (EAs) may have a significant impact on granulation. This observation suggests that the presence and diversity of EAs are crucial factors for the development of different granule sizes and morphologies.