Deciphering the floatation reversibility of anammox sludge: A balance between sludge rheological intensity and external hydraulic shearing.

Sludge floatation is inevitable in anammox granular reactors, which reduces the effective granules participating in anammox reaction and weakens the robust operation of anammox reactors. However, so far, the involved floatation mechanisms as well as the floatation mitigation measurements have not been well proposed. In this study, floating sludge (including irreversibly floating sludge (FS) and reversibly floating sludge (FS)) and settled granule sludge (SGS) were collected from an anammox expanded granular sludge bed (EGSB) reactor and compared in terms of morphological, physical, chemical and microbial properties. The particle size ranked FS > SGS > FS, and cavities were distinctly observed in FS due to the gas pockets and cell lysis. Rheological measurements revealed that the storage modulus (represent intensity of sludge) of FS and SGS were comparable, both of which were approximately1.4 times greater than that of FS. High storage modulus indicated that the hydraulic shear force in the EGSB was not strong enough to destroy FS and release the gases trapped in gas pocket, resulting in the irreversible floatation of FS. Whereas, the dinitrogen gases adhered onto FS were readily stripped from FS under hydraulic shearing, which contributed to their reversible floatation property. It is concluded that sludge floatation is resulted from the gas accumulation or gas adhesion onto the sludges, while the sludge floatation reversibility depends on the sludge intensity and hydraulic shear force. Our findings elucidate the floatation properties of anammox sludge via rheological analysis, which will contribute to the proper sludge floatation control and facilitate the optimization of anammox granule fluidization in EGSB reactor.