Innovative nitrogen removal based on mesophilic anoxic ammonium oxidation (anammox) is among the most distinct and successful innovations in wastewater treatment over the last decades. This technology demonstrates that better effluent quality and sustainability can go hand in hand, and even results in reduced investment and operating costs. In the CalidAmmox proposal, the application span for partial nitritation/anammox (PN/A) will for the first time be extended to thermophilic treatment (40-70oC), with recently discovered thermophilic anammox bacteria. Thermophilic biotechnology is often faster, hygienizes better and can be more cost effective than its mesophilic (<40oC) counterpart. Bioreactors will be used to study microbial stoichiometries, kinetics and competitions in depth, first in separate systems for nitritation and anammox, later for the combined PN/A system. State-of-the-art molecular tools will enable a deeper understanding of the nitrogen conversions and microbial interactions in these systems. Mathematical models will be developed to simulate, predict and optimize reactor performance, based on underlying microbial population dynamics. Emission mitigation of the greenhouse gas N2O will be considered as an additional sustainability aspect. CalidAmmox is highly innovative and can be the gateway to a new domain of sustainable nitrogen cycling biotechnology and ecology.