Closed-loop control of ammonium concentration in nitritation: Convenient for reactor operation but also for modeling

- Nitritation with granular reactor under ratio control was described mathematically.
- Closed-loop control of ammonium concentration enhance stability of full nitritation.
- Granules size larger than 1 mm enhances full nitritation.
- Full nitritation with granular reactors is feasible at low temperatures.