Deammonification represents a short-cut in the N-metabolism pathway and comprises 2 steps: about half the amount of ammonia is oxidised to nitrite and then residual ammonia and nitrite is anaerobically transformed to elementary nitrogen. Implementation of the pH-controlled DEMON process for deammonification of reject water in a single-sludge SBR system at the WWTP Strass (Austria) contributed essentially to energy self-sufficiency of the plant. The specific energy demand of the side-stream process equals 1.16 kWh per kg ammonia nitrogen removed comparing to about 6.5 kWh of mainstream treatment. Has this resource saving technology already approached state of the art? Deammonification has been operated in full-scale now for almost 3 years without interruption reaching annual ammonia removal rates beyond 90%. Biomass enrichment and DEMON-start-up in Strass took a period of 2.5 years whereas start-up period at the WWTP Glarnerland (Switzerland) was reduced to 50 days due to transfer of substantial amounts of seed sludge.