Nitrogen contribution and microbial community of size-fractionated anammox sludge in continuous stirred-tank reactors

•Proper particle size (<1 mm) contributed to 93 % of the nitrogen removal rate.•Ca. Kuenenia’s relative abundance in granules was 2.3 times of that in fine sludge.•Microbial diversity shifts into a simpler model as the granular sizes increase.•Fine sludge might be the potential major source of NO/N2O production.