In comparison to a single-stage deammonification system, large differences of N2O emissions in double stage treatment with nitrification have been documented. Experiences are presented from pilot-scale nitrification plants employing continuous feeding and clarification as well as sequencing-batch reactor systems. During treatment of digestion centrate with high NH4-N concentrations, nitrous oxide gas was identified in reactors and exhaust gases. With similar NH4 reduction, the results revealed an order of a magnitude lower N2O emissions during wastewater treatment in a single-stage deammonification system.