Effect of pH on nitrous oxide production and emissions from a partial nitritation reactor under oxygen-limited conditions

• In a PN reactor, N2O emissions decreased with the increase in initial pH.
• N2O production inside sludge aggregates were quantified by using microelectrodes.
• A decrease in pH led to increased N2O emissions from denitrification.

The effect of pH on nitrous oxide (N2O) emissions from a laboratory-scale partial nitritation sequencing batch reactor under oxygen-limited conditions was investigated from both macro- and microscopic viewpoints. During the aeration period of a single cycle, N2O emissions decreased when the initial pH increased from 7.5 to 8.5. By application of microelectrodes, N2O production was observed inside entire sludge aggregates, and it increased with decreasing pH from 8.5 to 7.0. At pH 8.0 and 8.5, N2O was mainly produced in the outer layer (<1000 μm) of sludge aggregates, where nitritation mainly occurred. At pH 7.0 and 7.5, N2O production was mainly observed in the inner layer (>1000 μm), where the dissolved oxygen was almost depleted, revealing that the dominant pathway here was denitrification. Under oxygen-limited conditions, a decrease in pH led to increased N2O emissions from denitrification pathway.

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