The long-term effect of nitrite on the granule-based enhanced biological phosphorus removal system and the reversibility

► Nitrite deteriorated the settleability, stability and integrity of the granules. ► Nitrite could inhibit the excretion of polysaccharides. ► Granules with lower proteins/polysaccharides had better structure and function. ► The community composition was not reversible by recovery. ► GAOs had stronger resistibility and higher recovery rate than PAOs. This study investigated the long-term effect of nitrite on the granule-based enhanced biological phosphorus removal (EBPR) system and the reversibility from macro- to micro-scale. Nitrite was found to seriously deteriorate the EBPR performance and result in severe sludge bulking. The inhibited polysaccharides excretion could lead to breaking the stability and integrity of the granules. Therefore, the reduced particle size and granule disintegration were observed. In this study, granules with lower ratio of proteins to polysaccharides (1.76) had better structure and function than the higher (3.84). Experimental results demonstrated that the microbial community structure was largely changed due to the presence of nitrite. In comparison, glycogen accumulating organisms (GAOs) had stronger resistibility and higher recovery rate than poly-phosphate accumulating organisms (PAOs). Interestingly, the community composition was unable to recover (Dice coefficients, 33.0%), although good EBPR performance was achieved only by propagating other types of PAOs.