@phdthesis{8700160,
  abstract     = {{As a result of population growth and increasing living standards, the amount of wastewater produced and discharged keeps increasing worldwide. This evolution puts an increasing demand on wastewater treatment facilities in terms of capacity, consumption of energy and other resources, as well as carbon footprint. Over the last decades, several innovative wastewater treatment technologies have been put forward to reach sustainable wastewater management. For example, technologies for biological nitrogen removal from wastewater based on the partial nitritation-anammox pathway require about 60% less aeration energy, no external organic carbon addition, emit less CO2, and produce 70-80% less sludge compared to conventional nitrification-denitrification processes (Siegrist et al., 2008). 
Wastewater treatment entails greenhouse gases emissions. During biological nitrogen removal from wastewater, nitrous oxide (N2O) may be formed and emitted. N2O is a potent greenhouse gas that is 298 times stronger than CO2 over a 100-year time frame (IPCC, 2013). Even small amounts of N2O emission can thus contribute significantly to the carbon footprint of a wastewater treatment plant (Daelman et al., 2013).  
The main goal of this thesis is to minimise greenhouse gas emissions during biological nitrogen removal in partial nitritation-anammox reactors. The focus lies on one-stage granular sludge reactor configurations treating side-stream wastewater. The term ‘one-stage’ refers to the partial nitritation and anammox reactions taking place simultaneously, in a single reactor. ‘Granular sludge’ refers to the fact that the bacteria carrying out these conversions grow in dense, fast-settling granules, resulting in a compact process. ‘Side-stream wastewater’ refers to the ammonium-rich water flow resulting from the sludge treatment. In this work, the N2O formation and emissions mechanisms and the potential N2O mitigation strategies are explored via mathematical modelling and simulation (Chapter 2 and Chapter 3), lab-scale experiments (Chapter 4), and full-scale data analysis (Chapter 5). 
Chapter 1 gives a general introduction, outlining conventional biological nitrogen removal process and the partial nitritation-anammox process. An overview of the main N2O formation pathways is given and related to various N2O formation models proposed in literature. Moreover, a few challenges in the operation and control of a one-stage partial nitritation-anammox are detailed and related to the objectives of this work. The N2O emissions reported in literature show significant variations, and some apparent contradictions were found regarding the effect of operating conditions such as dissolved oxygen (DO) concentration, granule size, nitrogen load, temperature and organic load. It was hypothesised that this may be due to the different (combinations of) values applied to the operating conditions in different studies. This hypothesis was verified in Chapter 2 by applying a mathematical model of a granular sludge partial nitritation-anammox reactor to investigate the effect of multiple factors influencing N2O emissions from these systems. Individual changes of operating parameters led to large and often non-monotonic changes in the simulated N2O emissions, which could explain the large variety of emissions factors and some apparent contradictions found in literature. Nitrifier denitrification in the outer layer of granules was found to be the main source of N2O. Heterotrophic denitrification acted as a sink in deeper layers, even though this pathway has been neglected in previous modelling studies. The DO concentration that allows simultaneous low N2O emissions and high nitrogen removal appears to be very site-specific, depending on the wastewater composition and granule size. The presence of organic substrates makes process optimisation easier because it can stimulate N2O consumption via heterotrophic denitrification.
The aeration flow rate in a partial nitritation-anammox reactor is typically adjusted to keep a the bulk DO concentration at a fixed setpoint (Lackner et al., 2014). However, the fluctuations in the amounts of flocs, which are deliberately or unavoidable present besides granules, can alter the optimal DO setpoint for maximal nitrogen removal (Corbalá-Robles et al., 2016; Hubaux et al., 2015), which may also affect the N2O formation. Chapter 3 deals with the effect of aeration control strategies on nitrogen removal efficiency and nitrous oxide (N2O) emissions in a partial nitritation–anammox reactor with granular sludge. More specifically, DO control, constant airflow and effluent ammonium control strategies were compared through simulation. Particular attention was paid to the effect of flocs in this type of reactor. When applying DO control, DO setpoints had to be adjusted to the amount of flocs present in the reactor to maintain high nitrogen removal and reduce N2O emissions, which is difficult to realise in practice because of variable floc fractions. Constant airflow rate control could maintain a good nitrogen removal efficiency independent of the floc fraction in the reactor but failed in N2O mitigation. Controlling the effluent ammonium concentration resulted in both a high nitrogen removal efficiency and relatively low N2O emissions, also in the presence of flocs. Fluctuations in floc fractions caused significant upsets in nitrogen removal and N2O emissions under DO control but had less effect at constant airflow and effluent ammonium control. Still, rapid and sharp drops in flocs led to a peak in N2O emissions at constant airflow and effluent ammonium control. Overall, effluent ammonium control reached the highest average nitrogen removal efficiency and lowest N2O emissions and consumed the lowest aeration energy under fluctuating floc concentrations. 
In Chapter 4, a long-term experimental study with a granular sludge one-stage partial nitritation-anammox reactor was carried out to validate the simulation results obtained in Chapter 2 and Chapter 3. The effect of aeration control strategies, influent organics, and floc removal on both nitrogen removal and N2O emissions were investigated, and the interpretation was complimented with mathematical modelling. The constant DO control failed to maintain low effluent nitrite and nitrate concentrations and led to significant fluctuations in N2O emissions. In comparison, applying a constant airflow rate led to a more stable effluent quality and less N2O emissions, which confirmed the simulation results of Chapter 3. The presence of influent organic carbon (COD/N ratio of 1) helped to suppress nitrite oxidation bacteria (NOB) and reduced the overall N2O emissions without sacrificing nitrogen removal efficiency, which is in line with Chapter 2. Floc removal had contrasting effects on N2O emissions at different aeration control strategies. At constant DO, removing flocs resulted in a relatively high reduction of ammonia-oxidizing bacteria (AOB), which was abundant in flocs, leading to lower ammonium oxidation rate and thus less N2O emissions. However, if constant airflow was applied, floc removal led to higher DO, which compensated the influence of AOB reduction on ammonium oxidation and inhibited the nitrite consumption by anammox, leading to slightly higher N2O emissions. These contrasting effect of floc removal on N2O emissions was also confirmed by simulation studies. Occasional pH drops in the reactor led to a drop in N2O emissions at constant DO. However, at constant airflow rate, an N2O peak occurred at the start and recovering of the pH drop, which is more pronounced at the presence of influent organics. Anammox can significantly reduce the N2O production during heterotrophic denitrification, probably by competing for nitric oxide with denitrifiers.
In Chapter 5, experimental data from a full-scale one-stage partial nitritation-anammox granular sludge reactor were analysed through mathematical modelling and simulation, in order to gain an understanding of N2O formation and emission dynamics and to develop N2O mitigation strategies.  Dynamic model calibration for such full-scale reactor was performed for the first time, applying a 1-dimensional biofilm model and including several N2O formation pathways. Simultaneous calibration of liquid-phase concentrations and N2O emissions led to improved model fit compared to their consecutive calibration. The calibrated model could quantitatively predict the average N2O emissions and qualitatively characterise the N2O dynamics, adjusting only seven parameter values. The model was validated with N2O data from an independent data set at different aeration conditions. Nitrifier nitrification was identified as the dominating N2O formation pathway.  Off-gas recirculation as a potential N2O emission reduction strategy was tested by simulation and showed indeed some improvement, be it at the cost of a higher aeration energy consumption.
The thesis is concluded with Chapter 6, which summarises the main findings of this work and gives perspectives for further research and practical applications to reduce N2O emissions from one-stage partial nitritation-anammox reactors.}},
  author       = {{Wan, Xinyu}},
  isbn         = {{9789463573986}},
  keywords     = {{wastewater treatment,N removal,Greenhouse gas Emissions}},
  language     = {{eng}},
  pages        = {{XIV, 146}},
  publisher    = {{Universiteit Gent. Faculteit Bio-ingenieurswetenschappen}},
  school       = {{Ghent University}},
  title        = {{Influencing factors and strategies to mitigate N2O emissions from one-stage partial nitritation-anammox reactors}},
  year         = {{2021}},
}