Aerobic granular sludge: Impact of size distribution on nitrification capacity

•AGS fed by VFA in the well-mixed anaerobic phase was used to study nitrification•Aerobic zone volume is proportional to amoA gene copy number and nitrification rates•Average granular size and surface area do not correlate well with nitrification rate•Aerobic zone volume density is proposed for monitoring nitrification rate•Nitrification capacity can be enhanced by crushing big granules or increasing MLSS