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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]

In this study, methane (CH4) and nitrous oxide (N2O) emission dynamics of a plug-flow bioreactor located in a municipal full-scale wastewater treatment plant were monitored during a period of 10 weeks. In general, CH4 and N2O gas emissions from the bioreactor accounted for 0.016% of the influent chemical oxygen demand (COD) and 0.116% of the influent total Kjeldahl nitrogen (TKN) respectively. In order to identify the emission patterns in the different zones, the bioreactor was divided in six different sampling sites and the gas collection hood was placed for a period of 2-3 days in each of these sites. This sampling strategy also allowed the identification of different process perturbations leading to CH4 or N2O peak emissions. CH4 emissions mainly occurred in the first aerated site, and were mostly related with the influent and reject wastewater flows entering the bioreactor. On the other hand, N2O emissions were given along all the aerated parts of the bioreactor and were strongly dependant on the occurrence of process disturbances such as periods of no aeration or nitrification instability. Dissolved CH4 and N2O concentrations were monitored in the bioreactor and in other parts of the plant, as a contribution for the better understanding of the transport of these greenhouse gases across the different stages of the treatment system. [ABSTRACT FROM AUTHOR]

In 2001 and 2002, fluxes of N2O, CH4, CO2 and N2 were measured in two constructed wetlands (CW) for domestic wastewater treatment in Estonia. The difference between the median values of N2O, CH4, and N2 fluxes in the horizontal subsurface flow (HSSF) CWs was non-significant, being 1.3–1.4 and 1.4–4.1 mg m−2 d−1 for N2O–N and CH4–C, and 0.16–0.17 g N m−2 d−1 for N2–N respectively. The CO2–C flux was significantly lower (0.6 g C m−2 d−1) in one of the HSSF filters of a hybrid CW, whereas the single HSSF and VSSF filters emitted 1.7 and 2.0 g C m−2 d−1. The median value of CH4–C emission in CWs varied from 1.4 to 42.6 g C m−2 d−1, being significantly higher in the VSSF filter beds. We also estimated C and N budgets in one of the HSSF CWs (312.5 m2) for 2001 and 2002. The total C input into this system was similar in 2001 and 2002, 772 and 719 kg C year−1, but was differently distributed between constituent fluxes. In 2001, the main input flux was soil and microbial accumulation (663 kg C year−1 or 85.8% of total C input), followed by plant net primary production (NPP) (10.2%) and wastewater inflow (3.9%). In 2002, 55.7% of annual C input was bound in plant NPP, whereas the increase in soil C formed 28.5% and wastewater inflow 15.7%. The main C output flux was soil respiration, including microbial respiration from soil and litter, and the respiration of roots and rhizomes. It formed 120 (97.5%) and 230 kg C year−1 (98.2%) in 2001 and 2002 respectively. The measured CH4–C flux remained below 0.1% of total C output. The HSSF CW was generally found to be a strong C sink, and its annual C sequestration was 649 and 484 kg C year−1 per wetland in 2001 and 2002 respectively. However, negative soil and microbial accumulation values in recent years indicate decreasing C sequestration. The average annual N removal from the system was 38–59 kg N year−1 (46–48% of the initial total N loading). The most important flux of the N budget was N2–N emission (22.7 kg in 2001 and 15.2 kg in 2002), followed by plant belowground assimilation (2.3 and 11.9 kg N year−1 in 2001 and 2002), and above-ground assimilation (1.9 and 9.2 kg N year−1, respectively). N2O emission was low: 0.37–0.60 kg N year−1 . [Copyright &y& Elsevier]

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]