So far, extremely efficient metabolic pathways for nitrogen removal exclusively by autotrophic organisms are well established in scientific literature but not in practice. This paper presents results from the successful implementation of rejection water deammonification in a full-scale single sludge system at the WWTP Strass, Austria. Anaerobic ammonia oxidising biomass has been accumulated during a 2.5 year start-up period when the reactor size was gradually scaled up in the steps. The pH-controlled deammonification system (DEMON) has reached a design capacity of eliminating approximately 300 kg of nitrogen per day. Energy savings outperform expectations, decreasing the mean specific demand for compressed air from 109 m3(kg N)(-1) to 29 m3(kg N)(-1). Dominance of autotrophic metabolism is confirmed by organic effluent loads topping influent loads.