Nitrous Oxide Emissions from Wastewater Treatment Plants. A Balancing Act

Acknowledgement of the role of nitrification on the emission of N2O from wastewater treatment plants is generating a renewed interest in the subject. Actual emission factors for N2O in WWTP seem to be higher than previously estimated. The impact of temperature, dissolved oxygen, nitrites, nitrates, C/N, pH, ammonia, H2S is reviewed. It is argued that actual emissions from the plant are result of interactions among nitrifiers, denitrifiers and the stripping effect of aeration. Nitrifiers are mostly a source through the nitrifiers-denitrification pathway. Denitrifying bacteria can act as a source or a sink. Denitrifiers play a significant role in reducing emissions in anoxic tanks. Locations prone to nitrous oxide emission in treatment plants are proposed. The impact of dynamic conditions in the plant is emphasized. Emission measurement protocols need to account for the diurnal variability of N2O emissions in treatment plants due to changing influent and operational conditions. Steady state evaluation in the laboratory can not capture the effects observed in the field. Some trends observed in mixed cultures of bacteria in treatment plants can be explained based on fundamental studies in pure cultures, others are not. Increased nitrous oxide emissions at high dissolved oxygen concentration often seen in mixed cultures from treatment plants are not directly inferred from the behaviour of nitrifiers or denitrifiers in pure culture. The impact of potentially higher nitrous oxide emission factors on green-house-gas emissions from the plant is quantified. If current trends are confirmed in full scale facilities the wastewater sector would have a larger impact than previously estimated. Alternative strategies for nitrous oxide control are reviewed. The need to remove nutrients from the water environment might lead to a different type of pollution in the atmosphere; there is a need to balance and minimize the overall effect.