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Abstract Water resource recovery facilities (WRRFs) contribute to climate change and air pollution, as they are anthropogenic potential sources of direct and indirect emission of greenhouse gases (GHGs). Studies concerning the monitoring and accounting for GHG emissions from WRRFs are of increasing interest. In this study, the floating hood technique for gas collection was coupled with the off-gas method to monitor and apportion nitrous oxide (N 2 O) and carbon dioxide (CO 2) emissions from both aerated and non-aerated tanks in a municipal water resource recovery facility, in order to investigate its carbon footprint (CFP). To our knowledge, this is the first time that the chamber technique was applied to evaluate gas fluxes from the settler, where an emission factor (EF) of 4.71 ∗ 10−5 kg CO2,eq kg bCOD −1 was found. Interesting results were found in the disinfection unit, which was the major contributor to direct N 2 O emissions (with a specific emission factor of 0.008 kg CO2,eq kg bCOD −1), due to the chemical interaction between hydroxylamine and the disinfectant agent (hypochlorite). The specific emission factor of the biological aerated tank was 0.00112 kg CO2,eq kg bCOD −1. The average direct CO 2 emission was equal to 0.068 kg CO2 kg bCOD −1 from the activated sludge tank and to 0.00017 kg CO2 kg bCOD −1 from the secondary clarifier. Therefore, taking into account the contribution of both direct N 2 O and CO 2 emissions, values of 0.069 kg CO2,eq kg bCOD −1, 0.008 kg CO2,eq kg bCOD −1 and 0.00022 kg CO2,eq kg bCOD −1 , were found for the net CFP of the aerated compartment, the disinfection unit and the clarifier, respectively. The plant energy Footprint (eFP) was also evaluated, confirming that the aeration system is the major contributor to energy consumption, as well as to indirect CO 2 emission, with a specific eFP of 1.49 kWh kg bCOD −1. Graphical abstract Unlabelled Image Highlights • CO 2 and N 2 O emitted during wastewater treatment in water resource recovery facilities • Floating hood technique utilised for gas collection • CO 2 and N 2 O emissions from aerated and non-aerated tanks • Comparison of two methods for the measurement of gas fluxes from non-aerated tanks • Contribution of disinfection process to N 2 O emissions from WRRFs [ABSTRACT FROM AUTHOR]

Nitrous oxide, carbon dioxide and methane are greenhouse gases (GHG) emitted from wastewater treatment that contribute to its carbon footprint. As a result of the increasing awareness of GHG emissions from wastewater treatment plants (WWTPs), new modelling, design, and operational tools have been developed to address and reduce GHG emissions at the plant-wide scale and beyond. This paper reviews the state-of-the-art and the recently developed tools used to understand and manage GHG emissions from WWTPs, and discusses open problems and research gaps. The literature review reveals that knowledge on the processes related to N 2 O formation, especially due to autotrophic biomass, is still incomplete. The literature review shows also that a plant-wide modelling approach that includes GHG is the best option for the understanding how to reduce the carbon footprint of WWTPs. Indeed, several studies have confirmed that a wide vision of the WWPTs has to be considered in order to make them more sustainable as possible. Mechanistic dynamic models were demonstrated as the most comprehensive and reliable tools for GHG assessment. Very few plant-wide GHG modelling studies have been applied to real WWTPs due to the huge difficulties related to data availability and the model complexity. For further improvement in GHG plant-wide modelling and to favour its use at large real scale, knowledge of the mechanisms involved in GHG formation and release, and data acquisition must be enhanced. [ABSTRACT FROM AUTHOR]

The Skjønhaug constructed wetland (CW) is a free surface water (FSW) wetland polishing chemically treated municipal wastewater in southeastern Norway and consists of three ponds as well as trickling, unsaturated filters with light weight aggregates (LWA). Fluxes of nitrous oxide (N2O) and methane (CH4) have been measured during the autumn, winter and summer from all three ponds as well as from the unsaturated filters. Physicochemical parameters of the water have been measured at the same localities. The large temporal and spatial variation of N2O fluxes was found to cover a range of −0.49 to 110 mg N2O–N m−2 day−1, while the fluxes of CH4 was found to cover a range of −1.2 to 1900 mg m−2 day−1. Thus, both emission and consumption occurred. Regarding fluxes of N2O there was a significant difference between the summer, winter and autumn, with the highest emissions occurring during the autumn. The fluxes of CH4 were, on the other hand, not significantly different with regard to seasons. Both the emissions of N2O and CH4 were positively influenced by the amount of total organic carbon (TOC). The measured fluxes of N2O and CH4 are in the same range as those reported from other CWs treating wastewater. There was an approximately equal contribution to the global warming potential from N2O and CH4. [Copyright &y& Elsevier]