Characteristics and formation mechanism of hollow Anammox granular sludge in low-strength ammonia sewage treatment

Hollow anaerobic ammonium oxidation (Anammox) granular sludge formed and became the main part in the treatment of low-strength ammonia wastewater. The characteristics and formation mechanism of hollow Anammox granular sludge were investigated in a continuous stirred tank reactor (CSTR) at room temperature. Granular sludge was found to be cavitated, and Candidatus Kuenenia was the dominant bacteria that lived and coexisted with other bacteria in walls under an extracellular polymeric substance (EPS) shield. The EEM-PARAFAC analysis was applied to characterize the EPS, while EPS secretion was the expression of specific Anammox activity. Tryptophan or protein-like was the active component of proteins (PN) as the main part of EPS in granules. These results confirm that hollow Anammox granular sludge is suitable for low-strength ammonia sewage. Finally, a conceptual model of hollow Anammox granules growth and breakage was developed, and cavitation was a key step of regranulation.