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Partial nitration‐anammox is a resource‐efficient pathway for nitrogen removal from wastewater. However, the advantages of this nitrogen removal technology may be counter‐acted by the emission of N2O, a potent greenhouse gas. In this study, mathematical modelling was applied to analyse N2O formation and emission dynamics and to develop N2O mitigation strategies for a one‐stage partial nitritation‐anammox granular sludge reactor. Dynamic model calibration for such a full‐scale reactor was performed, applying a one‐dimensional biofilm model and including several N2O formation pathways. Simultaneous calibration of liquid phase concentrations and N2O emissions leads to improved model fit compared to their consecutive calibration. The model could quantitatively predict the average N2O emissions and qualitatively characterize 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 higher aeration energy consumption. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This study deals with the influence of heterotrophic growth on autotrophic nitrogen removal from wastewater in a granular sludge reactor. A mathematical model was set-up including autotrophic and heterotrophic growth and decay in the granules from a partial nitritation-anammox process. A distinction between heterotrophic bacteria was made based on the electron acceptor (dissolved oxygen, nitrite or nitrate) on which they grow, while the nitrogen gas produced was ‘labelled’ to retrieve its origin, from anammox or heterotrophic bacteria. Taking into account heterotrophic growth resulted in a lower initial nitrogen removal, but in a higher steady state nitrogen removal compared with a model in which heterotrophic growth was neglected. The anammox activity is related with the fact that heterotrophs initially use nitrite as electron acceptor, but when they switch to nitrate the produced nitrite can be used by anammox bacteria. Increased anammox activity in the presence of heterotrophs, therefore, resulted in a marginally increased N2production at steady state. Heterotrophic denitrification of nitrate to nitrite also explains why small amounts of organic substrate present in the influent positively affect the maximum nitrogen removal capacity. However, the process efficiency deteriorates once the amount of organic substrate in the influent exceeds a certain threshold. The bulk oxygen concentration and the granule size have a dual effect on the autotrophic nitrogen removal efficiency. Besides, the maximum nitrogen removal efficiency decreases and the corresponding optimal bulk oxygen concentration increases with increasing granule size. [ABSTRACT FROM AUTHOR]

Abstract: Key biological processes such as nitrification do not result from the work of a single bacterial species but are performed by a wide variety of bacteria. However, this microbial diversity is usually not tracked during reactor operation and mostly neglected in present mathematical models, which distinguish at the most between ammonium oxidizers and nitrite oxidizers, assuming the same properties for all bacteria of each group. Nevertheless, experimental evidence is available that different process conditions may favour the selection of different types of bacteria. This contribution deals with observed differences between two nitrifying inverse turbulent bed reactors (ITBRs) with different solid hold-ups. It also contains additional experimental data, revealing a microbial population shift upon changing the operating conditions in one of these reactors. The reactor behaviour is described with a basic two-step nitrification model, which includes the competition between two different types of ammonium oxidizers, besides nitrite oxidizers. Considering the oxygen concentration as the key variable governing the population shift, the influence of microbial growth parameters on the competition dynamics is assessed. [Copyright &y& Elsevier]

Highlights • A partial nitritation-anammox model with 3 N 2 O formation pathways was set up. • Nitrifier denitrification was the main source of N 2 O in the all studied conditions. • Heterotrophic denitrification was a sink of N 2 O. • Influent organics allow simultaneous high N removal and low N 2 O emission. Abstract One-stage partial nitration-anammox granular sludge technologies are widely applied to treat nitrogen-rich wastewater. However, the strong greenhouse gas nitrous oxide (N 2 O) can be released as a by-product. In this study, a mathematical model of a granular sludge reactor was applied to investigate the factors that influence N 2 O emissions from these systems. Individual changes of operating parameters led to large and often non-monotonic changes in the simulated N 2 O 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 N 2 O. Heterotrophic denitrification acted as a sink in deeper layers, even though this pathway has been neglected in previous modelling studies. The dissolved oxygen (DO) concentration that allows simultaneous low N 2 O 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 optimization easier because it can stimulate consumption of N 2 O via heterotrophic denitrification. [ABSTRACT FROM AUTHOR]

Abstract During biological wastewater treatment, substrates undergo simultaneous diffusion and reactions inside microbial aggregates, creating microscale spatial substrate gradients and limiting the macroscale reaction rates. For flocculent and anaerobic granular sludge, this rate-limiting effect of diffusion is often lumped in model parameters, like the half-saturation coefficients of Monod kinetics in activated sludge models (ASM). Yet, an explicit description of the reaction-diffusion process with biofilm models is more common for aerobic granular sludge. This work investigates whether apparent half-saturation coefficients could have applications for aerobic granular sludge as well and examines the implications of this simplification. To this end, the macroscopic reaction rates predicted with a one-dimensional biofilm (1D) model were fitted with Monod kinetics. The results showed that the macroscale rates could indeed be described using apparent kinetics, at the very least over a time scale where the microbial population distribution stays fixed. However, the coefficients were sensitive to changes in the microbial population distribution, which can be affected by long-term changes in operating conditions. Also the activity of organisms that compete for the same substrates affect the parameter value. Be that as it may, apparent kinetics also depend on the operating conditions for flocculent and anaerobic granular sludge, but they have still been used successfully for design and optimization. Therefore, the last section of this work illustrates that they may also have applications for aerobic granular sludge. A simple model for ammonium removal using apparent half-saturation coefficients for oxygen and ammonium is applied to a full-scale reactor, taking advantage of the batch-wise operation and on-line monitoring data for regular recalibration. Graphical abstract Image 1 Highlights • Reaction-diffusion in aerobic granules can be lumped in apparent kinetic parameters. • Apparent half-saturation coefficients can change by changing operating conditions. • Changing microbial population distributions and competition create these changes. • Full-scale ammonium removal was simulated with regularly recalibrated parameters. [ABSTRACT FROM AUTHOR]

Gas bubbles are introduced in water to absorb or strip volatile substances in a variety of unit operations, for example during (waste)water treatment. To calculate the transfer rate of substances between the liquid phase and the gas phase, different assumptions have been made in literature regarding the gas phase composition and hydraulic pressure, which both vary along the reactor height. In this study, analytical expressions were derived for the total (macroscopic) liquid-gas transfer rate, using either the complete gradients of the mole fraction and pressure (comprehensive approach) or a uniform value, for one or both of them. Simulations with the comprehensive model were performed to understand the effect of the type of volatile substance and of the reactor design and operating conditions on the total liquid-gas transfer rate. These effects were found to be highly interactive and often non-linear. Next, the simulation results of the comprehensive model were compared with those from models that assume either a uniform mole fraction or a uniform pressure in the complete reactor volume. This illustrated that for soluble substances, the mole fraction gradient strongly affects the total liquid-gas transfer rate, while the pressure gradient became only important under operating conditions that promote stripping (i.e., for a high concentration in the liquid phase and low concentration in the inlet gas). For very poorly soluble substances, the pressure became more important under conditions that promote absorption. These results on the importance of the mole fraction and pressure gradients remained equally valid when explicitly considering a typical variation of the volumetric overall transfer coefficient (K L a) along the reactor height. Finally, a simple and fast procedure was made available through a spreadsheet to select appropriate simplifying assumptions in reactor or plant-wide models. By applying the procedure to oxygen (O 2), carbon dioxide (CO 2), methane (CH 4), nitrous oxide (N 2 O) and nitrogen gas (N 2) in an aerobic biological wastewater treatment reactor, it was demonstrated that some common simplifications can lead to significant errors, for which corrections were proposed. Image 1 • Composition and pressure of bubbles can differ between top and bottom of reactors. • Mole fraction gradient affects liquid-gas transfer of soluble substances especially. • Pressure affects stripping of soluble and absorption of poorly soluble substances. • For wastewater treatment, effects of gradients can be approached via mean values. • An Excel file is provided as a tool for fast and rational model selection. [ABSTRACT FROM AUTHOR]

Ammonium removal is a key step in wastewater treatment which can be accomplished biologically. An interesting process option for this purpose is coupling partial nitritation with the Anammox process. The goal of the partial nitritation process is to convert half of the ammonium in the influent stream into nitrite, so both can be later converted into dinitrogen gas by the Anammox reaction. To obtain a stable partial nitritation, ammonium oxidizing bacteria (AOB) have to prevail over nitrite oxidizing bacteria (NOB) so as to avoid further conversion of nitrite into nitrate. The dissolved oxygen concentration is a key variable for the functional group selection. In this study, a constrained combination of cascade and feedforward control is proposed for reactors with biomass retention, aimed at suppressing unwanted NOB while keeping a nitrite:ammonium ratio suitable for coupling with Anammox. The master controller, aimed to regulate this effluent ratio, generates the set-point for the dissolved oxygen concentration slave controller. In addition to the cascade controller feedback loop, a feed-forward controller calculates the optimal dissolved oxygen concentration based on the current influent stream flow rate and concentrations. The resulting dissolved oxygen concentration set-point is compared to constraints that guarantee the suppression of NOB and survival of AOB. The proposed control strategy is simple to apply in common wastewater treatment plants with biomass retention. A sensitivity analysis is performed to assess the effect of model parameters uncertainty on the controller constraints and to determine which parameters need to be identified with more precision to avoid instability or poor results. The performance and the effect of the uncertainty of the most sensitive parameters on the proposed control algorithm are assessed through simulation using realistic streams as inputs of the process. • A combination of cascade control, feed-forward and constraints is proposed. • Objectives are suppressing unwanted bacteria and setting the nitrite:ammonium ratio. • Sensitivity analysis is performed on the feed-forward and constraints. • The analysis shows the significance of the maximum growth rates. • Simulation results show that the controller can achieve both objectives. • When the effluent ratio cannot be set, the unwanted bacteria are still suppressed. [ABSTRACT FROM AUTHOR]

[Display omitted] • The alpha dynamics are an essential part of aerobic granular sludge (AGS) models. • Alpha dynamics influence energy use and simultaneous nitrification denitrification. • Process optimisation via volume exchange ratio, aeration capacity and granule size. • Soluble slowly biodegradable organics need to be considered as a state variable. • The dynamic nature of alpha affects the model calibration. The aerobic granular sludge (AGS) process treats wastewater with a significantly lower footprint and energy consumption compared to conventional activated sludge systems. Nevertheless, there is still potential for optimizing its performance, and mathematical models are most valuable tools to this end. Aeration energy consumption deserves particular attention, as it is the largest remaining operating cost for AGS systems. Batch-wisely operated reactors show an increasing oxygen transfer efficiency during aeration, which translates into a dynamic alpha factor. However, the dynamic nature of alpha is neglected in current models. The impact of this simplification on the operating performance was addressed for the first time in this study. Through the development of a novel 1-D biofilm reactor model, calibrated to a full-scale AGS plant, it was shown that the alpha dynamics affect both model structure and calibration, as well as the process performance. The description of the dynamic nature of alpha through the empirical relationship with the soluble biodegradable organic carbon required the addition of the state variable representing soluble slowly biodegradable organic carbon (S CB) to the biokinetic ASM2d model. Simulation results showed that alpha dynamics significantly influences simultaneous nitrification and denitrification and therefore need to be included in mathematical models to optimize AGS process performance. Different process variables such as volume exchange ratio, aeration capacity and granule size can be manipulated to improve reactor design and performance. The practical application of these new insights were discussed regarding the optimization of AGS systems, as well as other batch-wisely operated aerobic wastewater treatment systems. [ABSTRACT FROM AUTHOR]

Abstract Wastewater treatment processes with granular sludge are compact and are becoming increasingly popular. Interest has been accompanied by the development of mathematical models. This contribution simultaneously reviews available models in the scientific literature for anaerobic, aerobic and partial nitritation-anammox granular sludge reactors because they comprise common phenomena (e.g. liquid, gas and granule transport) and thus pose similar challenges. Many of the publications were found to have no clearly defined goal. The importance of a goal is stressed because it determines the appropriate model complexity and helps other potential users to find a suitable model in the vast amount of literature. Secondly, a wide variety was found in the model features. This review explains the chosen modelling assumptions based on the different reactor types and goals wherever possible, but some assumptions appeared to be habitual within fields of research, without clear reason. We therefore suggest further research to more clearly define the range of operational conditions and goals for which certain simplifying assumptions can be made, e.g. when intragranule solute transport can be lumped in apparent kinetics and when biofilm models are needed, which explicitly calculate substrate concentration gradients inside granules. Furthermore, research is needed to better mechanistically understand detachment, removal of influent particulate matter and changes in the mixing behaviour inside anaerobic systems, before these phenomena can be adequately incorporated in models. Finally, it is suggested to perform full-scale model validation studies for aerobic and anammox reactors. A spreadsheet in the supplementary information provides an overview of the features in the 167 reviewed models. Graphical abstract Image 1 Highlights • Granular sludge reactor models for various processes present common features. • Differences are explained by different reactor types and modelling goals. • Some assumptions seem habitual for certain reactor types. • Knowledge gaps were identified. • A spreadsheet with assumptions of the 167 models reviewed is provided. [ABSTRACT FROM AUTHOR]

During anaerobic treatment of sulfate-rich wastewater, biogas with a high concentration of hydrogen sulfide (H 2 S) is produced. Since H 2 S is toxic to humans and can cause corrosion of concrete and steel, it needs to be removed before using the biogas for energy and heat production. Biogas desulfurization can be achieved by blowing small amount of air into the anaerobic reactor, a process which is termed “microaeration”. In this study, the generally accepted Anaerobic Digestion Model No. 1 (ADM1) was extended with sulfate reduction and sulfide oxidation to optimize the microaeration process during the anaerobic treatment of sulfate-rich wastewater. The resulting model, termed ADM1‐S/O, was validated against experimental data from two reactors operated under anaerobic and microaerobic conditions, showing a good description of H 2 S concentrations in the biogas. The biomass composition in both reactors was not significantly affected by microaeration. Additionally, scenario analyses were carried out to assess the effect of the influent S:COD ratio and the aeration intensity (O 2 :S ratio) on the steady state reactor behavior. [ABSTRACT FROM AUTHOR]

Anammox bacteria can perform dissimilatory nitrate reduction to ammonium (DNRA) with nitrite as intermediate coupled to the oxidation of volatile fatty acids (VFA). Batch tests with enriched anammox and a co-culture of anammox and heterotrophic bacteria showed the capacity of Candidatus ‘Brocadia fulgida’ to perform the DNRA coupled to the anammox reaction (DNRA-anammox) at a high rate although the culture was not previously adapted to VFA. From thermodynamic calculations it could be stated that low COD/N influent ratios favour the DNRA-anammox transformation over heterotrophic conversions since more free energy is gained. A process scheme is proposed for an innovative nitrogen removal system in which the nitrate produced by nitrite oxidizing bacteria and/or anammox bacteria is converted during DNRA-anammox pathway, resulting in a sustainable nitrogen removal from municipal wastewater while circumventing the troublesome out-selection of nitrite oxidizing bacteria encountered in mainstream applications. [ABSTRACT FROM AUTHOR]

• Sludge obtained from primary clarifier, A-stage, and CEPT were characterized. • The characteristics of all sludges differed significantly from each other. • Primary sludge had the highest methane yield, while CEPT sludge had the lowest. • Energy consumption was the highest using A-stage due to aeration energy demand. • CEPT had the highest operational costs due to chemical use. Biogas production from anaerobic sludge digestion plays a central role for wastewater treatment plants to become more energy-efficient or even energy-neutral. Dedicated configurations have been developed to maximize the diversion of soluble and suspended organic matter to sludge streams for energy production through anaerobic digestion, such as A-stage treatment or chemically enhanced primary treatment (CEPT) instead of primary clarifiers. Still, it remains to be investigated to what extent these different treatment steps affect the sludge characteristics and digestibility, which may also impact the economic feasibility of the integrated systems. In this study, a detailed characterization has been performed for sludge obtained from primary clarification (primary sludge), A-stage treatment (A-sludge) and CEPT. The characteristics of all sludges differed significantly from each other. The organic compounds in primary sludge consisted mainly of 40% of carbohydrates, 23% of lipids, and 21% of proteins. A-sludge was characterized by a high amount of proteins (40%) and a moderate amount of carbohydrates (23%), and lipids (16%), while in CEPT sludge, organic compounds were mainly 26% of proteins, 18% of carbohydrates, 18% of lignin, and 12% of lipids. The highest methane yield was obtained from anaerobic digestion of primary sludge (347 ± 16 mL CH 4 /g VS) and A-sludge (333 ± 6 mL CH 4 /g VS), while it was lower for CEPT sludge (245 ± 5 mL CH 4 /g VS). Furthermore, an economic evaluation has been carried out for the three systems, considering energy consumption and recovery, as well as effluent quality and chemical costs. Energy consumption of A-stage was the highest among the three configurations due to aeration energy demand, while CEPT had the highest operational costs due to chemical use. Energy surplus was the highest by the use of CEPT, resulting from the highest fraction of recovered organic matter. By considering the effluent quality of the three systems, CEPT had the highest benefits, followed by A-stage. Integration of CEPT or A-stage, instead of primary clarification in existing wastewater treatment plants, would potentially improve the effluent quality and energy recovery. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • The alpha factor increases over the aeration phase length of a batch reactor. • Alpha dynamics follow a first-order relation. • Alpha was related to the removal of soluble biodegradable organic carbon (bCODs) • Adsorption of bCODs was described by Langmuir isotherm. • Exchange ratio and temperature influences alpha as well. In the pursuit of reducing carbon footprint and in view of increasing energy prices, energy efficiency is more important than ever before. Batch-wise operated aerobic granular sludge reactors consume up to 50% less energy compared to conventional activated sludge systems because pumping energy is reduced and mixing equipment is not needed. Further energy reduction efforts should therefore target aeration energy requirements. The alpha factor is an important factor influencing the oxygen transfer efficiency, however the dynamic behaviour of alpha has hardly been investigated in general and never for an aerobic granular sludge reactor. This study showed that alpha increases during the aeration phase of a cycle due to the influence of different process parameters. Through a data analysis study of 175 batch cycles of the Prototype Nereda® installation in Utrecht over the summer and winter period of 2020–2021, the exchange ratio and temperature were identified as the main influencing factors on the rate of increase of alpha in a batch cycle. A higher exchange ratio was related to a slower increase in alpha over the aeration phase, while a higher temperature was related to a faster increase in alpha. Moreover, alpha was characterized by a same minimal value at the beginning of every aeration phase, which could be explained by the adsorption of soluble biodegradable organic carbon described by a Langmuir adsorption model. Two mathematical models, a decreasing exponential and a first order model, were set up to unravel the dynamic behaviour of alpha. Both models were discussed in view of their practical implications for the design and performance optimization of aerobic granular sludge reactors and other batch-wise operated aerobic wastewater treatment systems. [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]

Plant-wide models of wastewater treatment (such as the Benchmark Simulation Model No. 2 or BSM2) are gaining popularity for use in holistic virtual studies of treatment plant control and operations. The objective of this study is to show the influence of ionic strength (as activity corrections) and ion pairing on modelling of anaerobic digestion processes in such plant-wide models of wastewater treatment. Using the BSM2 as a case study with a number of model variants and cationic load scenarios, this paper presents the effects of an improved physico-chemical description on model predictions and overall plant performance indicators, namely effluent quality index ( EQI ) and operational cost index ( OCI ). The acid-base equilibria implemented in the Anaerobic Digestion Model No. 1 (ADM1) are modified to account for non-ideal aqueous-phase chemistry. The model corrects for ionic strength via the Davies approach to consider chemical activities instead of molar concentrations. A speciation sub-routine based on a multi-dimensional Newton–Raphson (NR) iteration method is developed to address algebraic interdependencies. The model also includes ion pairs that play an important role in wastewater treatment. The paper describes: 1) how the anaerobic digester performance is affected by physico-chemical corrections; 2) the effect on pH and the anaerobic digestion products (CO 2 , CH 4 and H 2 ); and, 3) how these variations are propagated from the sludge treatment to the water line. Results at high ionic strength demonstrate that corrections to account for non-ideal conditions lead to significant differences in predicted process performance (up to 18% for effluent quality and 7% for operational cost) but that for pH prediction, activity corrections are more important than ion pairing effects. Both are likely to be required when precipitation is to be modelled. [ABSTRACT FROM AUTHOR]

Methane is a potent greenhouse gas and its emission from municipal wastewater treatment plants (WWTPs) should be prevented. One way to do this is to promote the biological conversion of dissolved methane over stripping in aeration tanks. In this study, the well-established Activated Sludge Model n°1 (ASM1) and Benchmark Simulation Model n°1 (BSM1) were extended to study the influence of process design and operating parameters on biological methane oxidation. The aeration function used in BSM 1 was upgraded to more accurately describe gas–liquid transfer of oxygen and methane in aeration tanks equipped with subsurface aeration. Dissolved methane could be effectively removed in an aeration tank at an aeration rate that is in agreement with optimal effluent quality. Subsurface bubble aeration proved to be better than surface aeration, while a CSTR configuration was superior to plug flow conditions in avoiding methane emissions. The conversion of methane in the activated sludge tank benefits from higher methane concentrations in the WWTP's influent. Finally, if an activated sludge tank is aerated with methane containing off-gas, a limited amount of methane is absorbed and converted in the mixed liquor. This knowledge helps to stimulate the methane oxidizing capacity of activated sludge in order to abate methane emissions from wastewater treatment to the atmosphere. [ABSTRACT FROM AUTHOR]

Abstract: In the last few years, the emission of nitrous oxide from wastewater treatment plants has become a topic of increased interest, given its considerable impact on the overall climate footprint of wastewater treatment plants. Various sampling strategies to estimate nitrous oxide emission from wastewater treatment plants have been applied in different studies. The present study addresses the influence of sampling strategies on the estimated emission by analysing the variability of an extensive dataset of nitrous oxide emissions resulting from a long-term online monitoring campaign at a full-scale municipal wastewater treatment plant. It is shown that short-term sampling is inadequate to accurately estimate the average nitrous oxide emissions from a particular wastewater treatment plant, while online monitoring is indispensable to capture the short-term variability (diurnal dynamics). [Copyright &y& Elsevier]

Abstract: Municipal wastewater treatment plants emit methane. Since methane is a potent greenhouse gas that contributes to climate change, the abatement of the emission is necessary to achieve a more sustainable urban water management. This requires thorough knowledge of the amount of methane that is emitted from a plant, but also of the possible sources and sinks of methane on the plant. In this study, the methane emission from a full-scale municipal wastewater facility with sludge digestion was evaluated during one year. At this plant the contribution of methane emissions to the greenhouse gas footprint were slightly higher than the CO2 emissions related to direct and indirect fossil fuel consumption for energy requirements. By setting up mass balances over the different unit processes, it could be established that three quarters of the total methane emission originated from the anaerobic digestion of primary and secondary sludge. This amount exceeded the carbon dioxide emission that was avoided by utilizing the biogas. About 80% of the methane entering the activated sludge reactor was biologically oxidized. This knowledge led to the identification of possible measures for the abatement of the methane emission. [Copyright &y& Elsevier]

[Display omitted] • Settling and diffusion properties of granules (AGS) oppositely affect conversion rates. • The treatment capacity of a continuous plant increases with better settling AGS. • Diffusion limitations negatively affect treatment capacity and energy consumption. • Full benefits of continuous AGS plants are obtained by optimized process operation. The application of aerobic granular sludge in continuous wastewater treatment plants is receiving increased attention. The introduction of better settling sludge in existing installations is expected to increase the plant's treatment capacity by allowing a higher biomass concentration in the reactor. A lot of recent research has therefore focused on how to get stable granules in a continuous flow plant, which make up the majority of current infrastructure. Still, the effect of aerobic granular sludge on the treatment capacity and energy requirements has not thoroughly been studied so far and is not that straightforward. While it is clear that the introduction of aerobic granular sludge will result in better settling, it will also bring about a higher diffusion limitation – which is often overlooked. This study scrutinized the effect of better settling versus diffusion limitation in a typical continuous activated sludge plant (predenitrification type) through a simulation study. When only considering the improved settling velocity, the treatment capacity increased by 40% compared to conventional activated sludge. However, diffusion limitation could almost totally counteract the positive effect of better settling for a non-improved continuous system. The continuous system could be improved for aerobic granular sludge by increasing the aerobic reactor volume fraction, while lowering the oxygen set-point to benefit from simultaneous nitrification–denitrification. The optimization led to a 20% improvement in treatment capacity and a 10% reduced energy consumption compared to a conventional activated sludge system. Overall, the performance of continuous wastewater treatment plants could indeed benefit from the excellent settling properties of aerobic granular sludge, as long as process operation was improved in order to take into account diffusion limitation. [ABSTRACT FROM AUTHOR]

• Fate of dissolved methane in aerobic granule sludge reactors was modelled. • Separating feeding and aeration in time strongly decreased methane oxidation. • Intragranule transport, retention time, reactor height and influent had less effect. • Mitigation strategies for methane emissions are proposed based on these findings. In municipal wastewater treatment plants, some dissolved methane can enter the aerobic bioreactors. This greenhouse gas originates from sewers and return flows from anaerobic sludge treatment. In well-mixed conventional activated sludge reactors, methane emissions are largely avoided because methane oxidizing bacteria consume a large fraction, even without optimizing for this purpose. In this work, the fate of dissolved methane is studied in aerobic granular sludge reactors, as they become increasingly popular. The influence of the characteristic design and operating conditions of these reactors are studied with a mathematical model with apparent conversion kinetics and stripping: the separation of feeding and aeration in time, a higher substrate transport resistance, a high retention time of granular biomass and a taller water column. Even for a best-case scenario combining an unrealistically low intragranule substrate transport resistance, a high retention time, a tall reactor, an extremely high influent methane concentration and no oxygen limitation, the methane conversion efficiency was only 12% when feeding and aeration were separated in time, which is lower than for continuous activated sludge reactors under typical conditions. A more rigorous model was used to confirm the limited conversion, considering the multi-species and multi-substrate biofilm kinetics, anoxic methane consumers and the high substrate concentration at the bottom during upward plug flow feeding. The observed limited methane conversion is mainly due to the high concentration that accumulates during unaerated feeding phases, which favours stripping more than conversion in the subsequent aeration phase. Based on these findings, strategies were proposed to mitigate methane emissions from wastewater treatment plants with sequentially operated reactors. [Display omitted] [ABSTRACT FROM AUTHOR]

• First-time simulation of experimentally measured N 2 O emissions from SHARON reactor. • Dynamic reactor behaviour, including off-gas N 2 O and NO, was described. • Modelling and simulation are powerful tools to identify N 2 O formation pathways. • Comparison with experimental data allowed model structure identification. This study deals with the potential and the limitations of dynamic models for describing and predicting nitrous oxide (N 2 O) 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 N 2 O 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 N 2 O formation pathways as well as for model identification. Heterotrophic denitrifying bacteria and ammonium oxidizing bacteria (AOB) were responsible for N 2 O formation under anoxic conditions, whereas under aerated conditions the AOB were the most important N 2 O producers. Relative to previously proposed models, hydroxylamine (NH 2 OH) had to be included as a state variable in the AOB conversions in order to describe potential N 2 O 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 N 2 O production. Nevertheless, quantitative prediction of N 2 O emissions with models remains a challenge, which is discussed. [ABSTRACT FROM AUTHOR]