Nitrous oxide emissions and dissolved oxygen profiling in a full-scale nitrifying activated sludge treatment plant
Graphical abstract
Highlights
► Online monitoring of N2O emissions and DO in full-scale wastewater treatment. ► High diurnal and spatial variability of emissions and DO. ► Strong negative correlation between emissions and DO. ► DO fluctuations trigger conditions that favour biological N2O production. ► Emissions add to 13% of carbon footprint of monitored ASP.
Introduction
Nitrous oxide (N2O) is a greenhouse gas with a global warming potential approximately 310 times higher than carbon dioxide (IPCC, 2006), and therefore unwanted even at small levels. Additionally, N2O reacts with atomic oxygen to form nitric oxide, which leads to the destruction of the stratospheric ozone layer (Tallec et al., 2008). Research shows that N2O emissions can be released during wastewater treatment, particularly during nitrification and denitrification (Colliver and Stephenson, 2000; Kimochi et al., 1998). Denitrification is the multi-stepped, anoxic reduction of nitrate to dinitrogen gas (N2) by heterotrophic microorganisms. Nitrification consists of the aerobic oxidation of ammonia to via nitrite , carried out in a two-stepped reaction by ammonia-oxidising bacteria that oxidise to , and by nitrite-oxidising bacteria that oxidise to .
It is generally agreed that the mechanisms of N2O emission during wastewater treatment are process-specific and therefore, related to operating conditions (Ahn et al., 2010; Rassamee et al., 2011). An operating parameter believed to play a critical role in influencing emissions is dissolved oxygen (DO; Tallec et al., 2008). An insufficient supply of oxygen in a nitrifying process can lead to incomplete nitrification, whereby autotrophic ammonia-oxidising bacteria (AOB) reduce to N2O, instead of oxidation to (Colliver and Stephenson, 2000), in a pathway known as nitrifier denitrification. Another route also linked to the production of nitrous oxide by AOB during nitrification is that of hydroxylamine (NH2OH) oxidation (Kampschreur et al., 2009; Wrage et al., 2001; Wunderlin et al., 2012), either by biological oxidation or by chemodenitrification (Wunderlin et al., 2012). However, the production of N2O via this route is likely to be minimal during biological wastewater treatment, as the concentrations of NH2OH in full-scale are in much lower magnitude than those found under experimental conditions (Wunderlin et al., 2012).
On the other hand, heterotrophic denitrification relies on the absence of oxygen, and the availability of nitrate and organic carbon. The presence of oxygen can inhibit denitrification enzymes, particularly nitrous oxide reductase, which converts nitrous oxide to dinitrogen gas (N2). This results in incomplete denitrification, whereby N2O is generated as the end product instead of N2. Therefore, DO may be key in determining the metabolic mechanisms that trigger N2O production, from either nitrifying or denitrifying microorganisms, depending on whether conditions are aerobic and/or anoxic (Rassamee et al., 2011). This is more relevant in situations where transient DO levels are frequent (Kampschreur et al., 2008a), thereby creating a favourable environment for anoxic and aerobic conditions to co-exist.
Dissolved oxygen, however, is the target of most efficiency-driven interventions on an activated sludge process (ASP). The aeration demand required to keep adequate DO conditions in a nitrifying ASP account for 55% of the total energy consumption of a sewage treatment works (George et al., 2009). Thus, it has a clear impact on the running costs of any given ASP. The reduction of energy use by lowering the DO set point at an ASP also has a positive effect on operational carbon emissions, as there is a direct link between energy use and carbon equivalents in currently available carbon accounting tools (UKWIR, 2008). Strategies to mitigate carbon footprint or improve nitrogen removal must thus consider the impact on process emissions.
The challenging, difficult to control full-scale environment however restricts a real-time, comprehensive approach to monitoring emissions and influencing factors. And although measurements may consider gaseous or dissolved N2O emissions, these often rely on off-line analysis (Foley et al., 2010; Kampschreur et al., 2008b) or spot readings (Ahn et al., 2010; Desloover et al., 2011). As a result, the link between dissolved oxygen and nitrous oxide emissions, although believed to be a critical one, is only partially understood. This paper aims to address this knowledge gap, by reporting the findings from online, continuous monitoring of dissolved and gaseous nitrous oxide, and establishing if there is a direct relationship between dissolved oxygen and emissions in a full-scale nitrifying ASP. The associated carbon impact of the process emissions is calculated and reported in relationship to current carbon accounting practices.
Section snippets
Monitoring site
The study was carried out in a plug-flow, three-pass, full-scale sewage treatment works in the Midlands, UK, with a capacity to treat 210,000 population equivalents. The chosen ASP unit is one of three at the works designed to nitrify, and it consists of two identical aeration lanes, each preceded by a small anoxic zone. This is the most common configuration in Severn Trent Water's 57 activated sludge plants. The ASP is run on DO control set point of 1.5 mgO2/L, employing blowers in
Results
The treatment works achieves over 95% removal rates, with average effluent concentrations of 0.25 mgNH4-N/L for ammonia, 25 mg/L for COD and 12 mg/L for total suspended solids. This is within the expected performance of the works and notably, well below the 5 mgNH4-N/L consent for effluent ammonia. Average ammonia concentrations entering the lane were 15.3 mg/L, with most of nitrification taking place by zone 6 (Fig. 2). In-lane measurements during the pre-assessment period showed the majority
Discussion
Nitrous oxide can be emitted from ASPs under aerobic and anoxic conditions due to stressed nitrification and incomplete denitrification, and the resulting emissions can have a significant impact on the carbon footprint associated with the normal operation of an activated sludge plant. Although links have been established between N2O emissions and oxygenation levels at lab-scale level (Tallec et al., 2008; Rassamee et al., 2011; Wunderlin et al., 2012), evidence of these links at full-scale
Conclusions
The aim of this study was to profile N2O emissions and the relationship with operational DO in a full-scale nitrifying ASP. The main findings include:
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Great variability in emissions and DO conditions within the lane was observed, with greater emissions found at the beginning (zones 1 and 2) and at the end of the lane (zones 6–8).
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Nitrous oxide emissions in the dissolved and gaseous presented a clear diurnal pattern.
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There is a negative, direct correlation between dissolved oxygen and nitrous oxide
Acknowledgements
The authors gratefully thank Severn Trent Water for funding this research through a PhD sponsorship and for the helpful support of the operations team at Coleshill Sewage Treatment Works, Birmingham, UK, in facilitating the field work. Thank you to Patricia Bellamy, for the technical expertise on statistical analysis and helpful suggestions on experimental design.
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