Bulking and floatation of the anammox-HAP granule caused by low phosphate concentration in the anammox reactor of expanded granular sludge bed (EGSB)

•The N and P removal were achieved in P concentration high than 11.4 mg L−1.•The sludge bulking was an omen for floatation.•Increased EPS resulted in the deterioration of granules.•The main reason of floatation was unreleasable gas bubbles attached granules.•A sufficient supply of P was a controlling strategy for stable operation. The effect of phosphate concentration on the anammox-HAP process was investigated in this work. A high total nitrogen removal efficiency (>82.6%) and a stable total phosphate removal efficiency (>56.2%) was achieved in reactor with the phosphate concentration over 11.4 mg L−1. However, a phosphate concentration below to 5.7 mg L−1, a floatation of sludge occurred caused the deterioration of process. A new understanding for the floatation was divided into three stages: the stable stage, bulking stage and floating stage. First, anammox biofilm coupled with HAP for granulation in the stable stage. Second, the aggregation of bulking sludge resulted in changes in viscoelastic properties of the sludge. Third, the floatation resulted from unreleasable gas bubbles in the granules wrapped a high concentrations of slime layer proteins. Overall, this paper suggests that a control strategy was a sufficient supply of phosphate for the stable operation.