Improving mechanical stability of anammox granules with organic stress by limited filamentous bulking.

Under organic stress, the limited filamentous bulking (FB) was demonstrated to improve anammox capability by inhibiting granule disintegration and washout. The accumulation of internal stress played a more important role than the adverse physicochemical properties (low viscoelasticity and hydrophobicity) of granules in limiting granular strength by consuming the granular elastic energy. Different from the floc-forming heterotrophic bacteria (HB) that stored its growth stress as internal stress by pushing the surrounded anammox micro-colonies outwards under the spatial constraint of elastic anammox "shell", the filamentous HB grew into a uniform network structure within granules, endowed granules low internal stress and acted as the granular skeleton due to its rich amyloid substance, which was benefited from the elimination of inhomogeneous growth and the consequent expansion competition for living space. Combined with the mechanical instability and sticking-spring models, controlling FB at limited level was effective for improving granular strength without affecting sludge-water separation.