A nitrifying biofilm airlift suspension (BAS) reactor was operated at an ammonia loading rate (ALR) of 5 kg N-NH4+/m3•d, obtaining ammonia conversions of 99%. Neither denitrification nor nitrous oxide production was detected when a suitable carbon source was not present in the system over a long experimental period. However, in the presence of organic matter coming from biofilm hydrolysis or added in the influent nitrous oxide production was found. The addition of 55 mg/l formaldehyde to the influent produced rises in the nitrous oxide production of up to 2 kg N-N2O/m3•d. Nitrogen balance showed that nitrous oxide was the sole or major product of denitrification in the system.

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