Nitrous Oxide Emissions from Wastewater Treatment Plants. A Balancing Act Eugenio Giraldo Innovation and Environmental Stewardship American Water, 1025 Laurel Oak Rd, Voorhees, NJ Eugenio.giraldo@amwater.com ABSTRACT Acknowledgement of the role of nitrification on the emission of N 2 O from wastewater treatment plants is generating a renewed interest in the subject. Actual emission factors for N 2 O in WWTP seem to be higher than previously estimated. The impact of temperature, dissolved oxygen, nitrites, nitrates, C/N, pH, ammonia, H 2 S 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 N 2 O 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. Keywords nitrous oxide, wastewater, inventory, green house gas, nitrification, denitrification, review Introduction Nitrous oxide (N 2 O) is a stable gas with an atmospheric life time of approximately 120 years that can trap heat acting as a greenhouse gas. Due to these characteristics it is calculated that its heat retention effect in the atmosphere is 310 times more potent than carbon dioxide on a per molecule basis. Concentrations of nitrous oxide in the atmosphere continue to increase at a rate of 0.25% per year (EPA, 2009). Nitrous oxide has both natural and human related sources. A recent report from EPA establishes that nitrous oxide contributes to 5.2% of the total anthropogenic emissions of greenhouse gases for the United States in 2006 (EPA, 2008). Emissions of nitrous oxide within the wastewater treatment sector account for approximately one third of the total direct emissions. The emissions of nitrous oxide attributed to wastewater treatment are due to direct emissions at the treatment plant, and emissions due to discharge of nitrogenous compounds to the environment, e.g. a river. Nitrous oxide is produced during nitrification and denitrification processes that occur in treatment plants and water bodies receiving treatment plant effluents. Early studies used to obtain emission factor of nitrous oxide in wastewater treatment plants were focused on denitrification as the principal source, however, it is now recognized that nitrification also plays a significant role in the emissions at the plant. As a result there are very few studies measuring emissions in treatment plants that account for the contribution of nitrifiers to the overall inventory of the plant. Recent published information on the emission factors from nitrification processes suggest that actual emissions from treatment plants are higher than originally estimated. In contrast with this, recent 357 Nutrient Removal 2009 Copyright ©2009 Water Environment Federation. All Rights Reserved.