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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]

Bio-trenches are a sustainable option for treating nitrate contamination in groundwater. However, a possible side effect of this technology is the production of nitrous oxide, a greenhouse gas that can be found both dissolved in the liquid effluent as well as emitted as off gas. The aim of this study was to analyze NO 3 − removal and N 2 O production in lab-scale column trials. The column contained olive nut as organic carbon media. The experimental study was divided into three phases (I, II and III) each characterized by different inlet NO 3 − concentrations (30, 50, 75 mg NO 3 -N L −1 respectively). Sampling ports deployed along the length of the column allowed to observe the denitrification process as well as the formation and consumption of intermediate products, such as nitrite (NO 2 − ) and nitrous oxide (N 2 O). In particular, it was observed that N 2 O production represent only a small fraction of removed NO 3 − during Phase I and II, both for dissolved (0.007%) and emitted (0.003%) phase, and it was recorded a high denitrification efficiency, over 99%. Nevertheless, significantly higher values were recorded for Phase 3 concerning emitted phase (0.018%). This fact is due to increased inlet concentration which resulted in a carbon limitation and in a consequent decrease in denitrification efficiency (76%). [ABSTRACT FROM AUTHOR]

Nitrogen pollution is a worldwide problem and has been extensively treated by canonical denitrification (CDN) process. However, the CDN process generates several issues such as intensive greenhouse gas (GHG) emissions. In the past years, a novel biological nitrogen removal (BNR) process of oxygenic denitrification (O2DN) has been proposed as a promising alternative to the CDN process. The classic denitrification four steps are simplified to three steps by O2DN bacteria without producing and releasing the intermediate nitrous oxide (N 2 O), a potent GHG. In this article, we summarized the findings in previous literatures as well as our results, including involved microorganisms and metabolic mechanisms, functional genes and microbial detection, kinetics and influencing factors and their potential applications in wastewater treatment. Based on our knowledge and experience, the benefits and limitations of the current O2DN process were analyzed. Since O2DN is a new field in wastewater treatment, more research and application is required, especially the development of integrated processes and the quantitative assessment of the contribution of O2DN process in natural habitats and engineered systems. [ABSTRACT FROM AUTHOR]

In this study, nitrous oxide (N 2 O) emissions from a full-scale denitrifying biofilter plant were continuously monitored over two periods (summer campaign in September 2014 and winter campaign in February 2015). Results of the summer campaign showed that the major part (> 99%) of N 2 O flux was found in the liquid phase and was discharged with the effluent. N 2 O emissions were highly variable and represented in average 1.28 ± 1.99% and 0.22 ± 0.31% of the nitrate uptake rate during summer and winter campaigns, respectively. Denitrification was able to consume a large amount of dissolved N 2 O coming from the upstream nitrification stage. In the absence of methanol injection failure and with an influent BOD/NO 3 -N ratio higher than 3, average reduction of N 2 O was estimated to be of 93%. The control of exogenous carbon dosage is essential to minimize N 2 O production from denitrifying biofilters, in correlation to NO 2 -N concentrations in the filter. [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]

During nitrogen removal in conventional activated sludge processes, nitrous oxide can be emitted. With a global warming potential of 298 CO 2 -equivalents it is an important greenhouse gas that affects the sustainability of wastewater treatment. The present study reports nitrous oxide emission data from a 16 month monitoring campaign on a full-scale municipal wastewater treatment. The emission demonstrated a pronounced diurnal and seasonal variability. This variability was compared with the variability of a number of process variables that are commonly available on a municipal wastewater treatment plant. On a seasonal timescale, the occurrence of peaks in the nitrite concentration correlated strongly with the emission. The diurnal trend of the emission coincided with the diurnal trend of the nitrite and nitrate concentrations in the tank, suggesting that suboptimal oxygen concentrations may induce the production of nitrous oxide during both nitrification and denitrification. This study documents an unprecedented dataset that could serve as a reference for further research. [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]

Abstract: Nitrous oxide (N2O) is a primary ozone-depleting substance and powerful greenhouse gas. N2O emissions from secondary-level wastewater treatment processes are relatively well understood as a result of intensive international research effort in recent times, yet little information exists to date on the role of sewers in wastewater management chain N2O dynamics. Here we provide the first detailed assessment of N2O levels in the untreated influent (i.e. sewer network effluent) of three large Australian metropolitan wastewater treatment plants. Contrary to current international (IPCC) guidance, results show gravity sewers to be a likely source of N2O. Results from the monitoring program revealed hydraulic flow rate as a strong driver for N2O generation in gravity sewers, with microbial processes (nitrification and possibly denitrification) implicated as the main processes responsible for its production. Results were also used to develop a presumptive emission factor for N2O in the context of municipal gravity sewers. Considering the discrepancy with current IPCC Guidelines, further work is warranted to assess the scale and dynamics of N2O production in sewers elsewhere. [Copyright &y& Elsevier]

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]

High levels of nitrogen originating from blasting operations, for example at mining sites or quarries, risk contaminating water bodies through leaching from waste rock dumps. Woodchip bioreactors can be a simple and cost-effective way of reducing nitrate concentrations in the leachate. In this study we investigated how bottle sedge, barley straw, and pine woodchips used as electron donors for denitrification influenced microbial community composition and nitrate removal in lab-scale bioreactors during 270 days. The reactors were operated to ensure that nitrate was never limiting and to achieve similar nitrate removal (%). Distinct bacterial communities developed due to the different substrates, as determined by sequencing of the 16S rRNA gene. Sedge and straw reactors shared more taxa with each other than with woodchips and throughout the experimental period, sedge and straw were more diverse than woodchips. Cellulose degrading bacteria like Fibrobacteres and Verrucomicrobia were detected in the substrates after 100–150 days of operation. Nitrate removal rates were highest in the sedge and straw reactors. After initial fluctuations, these reactors removed 5.1–6.3 g N m−3 water day−1, which was 3.3–4.4 times more than in the woodchip reactors. This corresponded to 48%, 42%, and 44% nitrate removal for the sedge, straw, and woodchip reactors respectively. The functional communities were characterized by quantitative PCR and denitrification was the major nitrate removing process based on genetic potential and water chemistry, although sedge and straw developed a capacity for ammonification. Gene ratios suggested that denitrification was initially incomplete and terminating with nitrous oxide. An increase in abundances of nitrous oxide reducing capacity in all substrate types towards the end increased the potential for less emissions of the greenhouse gas nitrous oxide. Unlabelled Image • Sedge and straw bioreactors removed nitrate at higher rates than woodchip reactors. • Denitrification was the dominant nitrate reduction process, but potential for DNRA. • Distinct bacterial communities developed in the lignocellulosic substrates. • The potential for N 2 O reduction increased with time in all tested substrates. [ABSTRACT FROM AUTHOR]

Wastewater is a source of N 2 O emission that is generated, both directly from advanced treatment plants and indirectly from the discharge of wastewater into the natural environment, due to its remaining nitrogen content. There are a variety of methods based on different parameters used to calculate N 2 O emission in wastewater treatment plants. The methodology proposed by the IPCC is used as an international reference for national inventories. In this work, we use five international methodologies to calculate the N 2 O emission of the WWTPs in two areas with high population density: The Metropolitan Area of Barcelona (MAB) and Mexico City (MXC). The MAB has 100% population served and has advanced treatment plants (five WWTP) and traditional wastewater treatment plants (two WWTP), the MXC served 14% of its population and had advanced treatment plants (six WWTP) and traditional plants (nineteen WWTP) in 2016. The results obtained show that the IPCC and Das methodologies underestimate the emission of N 2 O by considering the per capita consumption of proteins as a constant nitrogen value and also by the suggested emission factors. The methodologies that use the operational data of each plant provide emission results closer to those found in the literature. The value of TN should be the parameter to be considered for a correct estimate of the N 2 O emission in the WWTPs. The emission factors currently used are very low, with a low level of confidence of up to 1.3%. The range currently used should be increased and have a minimum range of 0.03 kg N 2 O-N/kg N. The emission factors reported in the literature are very variable and with very high levels of uncertainty, and therefore underestimate the emission of N 2 O in WWTPs. More research should be done to obtain higher and more reliable emission factors than those currently used. Unlabelled Image • The TN should be the parameter used to know the content of N and the N 2 O emission. • The range of EF proposed by the IPCC presents high levels of uncertainty. • N 2 O emission factors reported in the literature show high levels of uncertainty. [ABSTRACT FROM AUTHOR]

• Long-term N 2 O emission data set of three different activated sludge systems. • Significant spatial and temporal emission variation on all three treatment plants. • Guiding principles for monitoring on wastewater treatment plants with open tanks. • Separate reject water treatment can reduce overall N 2 O emissions. Nitrous oxide (N 2 O) emissions from wastewater treatment contribute significantly to greenhouse gas emissions. They have been shown to exhibit a strong seasonal and daily profile in previously conducted monitoring campaigns. However, only two year-long online monitoring campaigns have been published to date. Based on three monitoring campaigns on three full-scale wastewater treatment plants (WWTPs) with different activated sludge configurations, each of which lasted at least one year, we propose a refined monitoring strategy for long-term emission monitoring with multiple flux chambers on open tanks. Our monitoring campaigns confirm that the N 2 O emissions exhibited a strong seasonal profile and were substantial on all three plants (1–2.4% of the total nitrogen load). These results confirm that N 2 O is the most important greenhouse gas emission from wastewater treatment. The temporal variation was more distinct than the spatial variation within aeration tanks. Nevertheless, multiple monitoring spots along a single lane are crucial to assess representative emission factors in flow-through systems. Sequencing batch reactor systems were shown to exhibit comparable emissions within one reactor but significant variation between parallel reactors. The results indicate that considerable emission differences between lanes are to be expected in cases of inhomogeneous loading and discontinuous feeding. For example, N 2 O emission could be shown to depend on the amount of treated reject water: lanes without emitted <1% of the influent load, while parallel lanes emitted around 3%. In case of inhomogeneous loading, monitoring of multiple lanes is required. Our study enables robust planning of monitoring campaigns on WWTPs with open tanks. Extensive full-scale emission monitoring campaigns are important as a basis for reliable decisions about reducing the climate impact of wastewater treatment. More specifically, such data sets help us to define general emission factors for wastewater treatment plants and to construct and critically evaluate N 2 O emission models. [ABSTRACT FROM AUTHOR]