Rethinking the effect of salinity on nitrogen removal capacity of aerobic granular sludge from the perspective of size distribution and granular morphology

Granular size induces the operation performance variation of aerobic granular sludge reactor, but the profound reasons are unrevealed. This study investigated the influence of granular size distribution on the reactor operation under salt stress. The effective nitrogen removal was achieved at ≤4% salinity, but declined at 6% salinity. The phenomenon was determined by the granular size fraction. The small granules (d = 200–600 μm) fraction was 77%–81% at ≤4% salinity, while only 57.32% at 6% salinity. That was positively correlated with nitrite reductase (NIR) activity significantly (p < 0.01). Moreover, small granules exhibited smooth surface at ≤4% salinity. The efficient mass transfer area of granules was enlarged by the smooth surface, accelerating substrates mass transfer. Consequently, ammonia monooxygenase (AMO) activity was enhanced significantly (p < 0.05). Ammonia-oxidation bacteria, Nitrosomonas and nitrite-reduction bacteria, Paracoccus were dominated in small granules at 4% salinity, while loss at 6% salinity. Overall, small granules with smooth surface favored the enrichment of nitrogen removal microbes via substrate transfer enhancement, and improved the activity of AMO and NIR. Thus, the favorable nitrogen removal performance of aerobic granular sludge reactor was achieved at ≤4% salinity.