Monitoring and modelling of N2O emissions from innovative nitrogen removal processes

The emissions of greenhouse gases (such as N2O) from wastewater treatment is a matter of growing concern. The current atmospheric concentration of N2O, a potent greenhouse gas, is the highest in history. Conventional biological nitrogen removal is based on nitrification, i.e. conversion of ammonium to nitrate, followed by denitrification, i.e. conversion of nitrate to N2. Over the last 20 years, innovative nitrogen removal processes have been developed as an alternative, such as those based on the combined partial nitritation-anammox conversions which result in savings in aeration energy, no external carbon source, less CO2 emissions and sludge production. The overall goal of this PhD thesis was to elucidate the formation mechanisms of N2O from innovative nitrogen removal processes. To reach this goal, models were developed and applied in simulation studies. One of the first mechanistic models describing N2O formation by ammonia oxidizing bacteria was developed and formed the basis for later adaptations and extensions reflecting additional insights gathered. Monitoring campaigns were conducted on full-scale reactors for innovative nitrogen removal, including the development and application of a novel monitoring method and rigorous assessment of the gathered experimental data. The carbon footprint of the monitored full-scale partial nitritation reactor consisted almost entirely (92%) of N2O emissions. A novel method to measure dissolved N2O concentration on a minutely time scale was theoretically developed and applied on the full-scale reactor. The reactor off-gas N2O profile showed large variations during an operating cycle. This transient behaviour was exploited, enabling monitoring of the interphase transfer rate kLa and average N2O formation rates under different conditions, the latter was validated with the dissolved N2O measurements. By combining simulation and experimental results, it was found that the majority of N2O emissions was related to AOB, both under aerobic and anoxic conditions

Dynamic simulation of N2O emissions from a full-scale partial nitritation reactor

This study deals with the potential and the limitations of dynamic models for describing and predicting nitrous oxide (N2O) emissions associated with biological nitrogen removal from wastewater. The results of a three-week monitoring campaign on a full-scale partial nitritation reactor were reproduced through a state-of-the-art model including different biological N2O formation pathways. The partial nitritation reactor under study was a SHARON reactor treating the effluent from a municipal wastewater treatment plant sludge digester. A qualitative and quantitative comparison between experimental data and simulation results was performed to identify N2O formation pathways as well as for model identification. Heterotrophic denitrifying bacteria and ammonium oxidizing bacteria (AOB) were responsible for N2O formation under anoxic conditions, whereas under aerated conditions the AOB were the most important N2O producers. Relative to previously proposed models, hydroxylamine (NH2OH) had to be included as a state variable in the AOB conversions in order to describe potential N2O formation by AOB under anoxic conditions. An oxygen inhibition term in the corresponding reaction kinetics was required to fairly represent the relative contribution of the different AOB pathways for N2O production. Nevertheless, quantitative prediction of N2O emissions with models remains a challenge, which is discussed

Modelling nitrous and nitric oxide emissions by autotrophic ammonia-oxidizing bacteria

The emission of greenhouse gases, such as N2O, from wastewater treatment plants is a matter of growing concern. Denitrification by ammonia-oxidizing bacteria (AOB) has been identified as the main N2O producing pathway. To estimate N2O emissions during biological nitrogen removal, reliable mathematical models are essential. In this work, a mathematical model for NO (a precursor for N2O formation) and N2O formation by AOB is presented. Based on mechanistic grounds, two possible reaction mechanisms for NO and N2O formation are distinguished, which differ in the origin of the reducing equivalents needed for denitrification by AOB. These two scenarios have been compared in a simulation study, assessing the influence of the aeration/stripping rate and the resulting dissolved oxygen (DO) concentration on the NO and N2O emission from a SHARON partial nitritation reactor. The study of the simulated model behavior and its comparison with previously published experimental data serves in elucidating the true NO and N2O formation mechanism

Novel method for online monitoring of dissolved N2O concentrations through a gas stripping device

Nitrous oxide emissions from wastewater treatment plants are currently measured by online gas phase analysis or grab sampling from the liquid phase. In this study, a novel method is presented to monitor the liquid phase N2O concentration for aerated as well as non-aerated conditions/reactors, following variations both in time and in space. The monitoring method consists of a gas stripping device, of which the measurement principle is based on a continuous flow of reactor liquid through a stripping flask and subsequent analysis of the N2O concentration in the stripped gas phase. The method was theoretically and experimentally evaluated for its fit for use in the wastewater treatment context. Besides, the influence of design and operating variables on the performance of the gas stripping device was addressed. This method can easily be integrated with online off-gas measurements and allows to better investigate the origin of the gas emissions from the treatment plant. Liquid phase measurements of N2O are of use in mitigation of these emissions. The method can also be applied to measure other dissolved gasses, such as methane, being another important greenhouse gas

N2O and NO emissions during autotrophic nitrogen removal in a granular sludge reactor: a simulation study

This contribution deals with NO and N2O emissions during autotrophic nitrogen removal in a granular sludge reactor. Two possible model scenarios describing this emission by ammonium- oxidizing biomass have been compared in a simulation study of a granular sludge reactor for one-stage partial nitritation–Anammox. No significant difference between these two scenarios was noticed. The influence of the bulk oxygen concentration, granule size, reactor temperature and ammonium load on the NO and N2O emissions has been assessed. The simulation results indicate that emission maxima of NO and N2O coincide with the region for optimal Anammox conversion. Also, most of the NO and N2O are present in the off-gas, owing to the limited solubility of both gases. The size of granules needs to be large enough not to limit optimal Anammox activity, but not too large as this implies an elevated production of N2O. Temperature has a significant influence on N2O emission, as a higher temperature results in a better N-removal efficiency and a lowered N2O production. Statistical analysis of the results showed that there is a strong correlation between nitrite accumulation and N2O production. Further, three regions of operation can be distinguished: a region with high N2O, NO and nitrite concentration; a region with high N2 concentrations and, as such, high removal percentages; and a region with high oxygen and nitrate concentrations. There is some overlap between the first two regions, which is in line with the fact that maximum emission of NO and N2O coincides with the region for optimal Anammox conversion

Modelling nitrous and nitric oxide emissions by autotrophic ammonium oxidizing bacteria

In this work, a mathematical model has been set up to describe NO and N2O formation by autotrophic ammonium oxidizing bacteria in a partial nitritation (SHARON) reactor. Based on mechanistic grounds, two possible reaction mechanisms for NO and N2O formation were proposed. These two scenarios have subsequently been compared in a simulation study. The influence of intermittent versus continuous aeration on NO and N2O formation for the same aerated retention time has been addressed as well. Applying continuous aeration a maximal N2O formation was found at intermediate dissolved oxygen (DO) concentrations of about 2 mg O2.l-1