Marlies J. Kampschreur

Nitrous oxide emission during wastewater treatment

Nitrous oxide (N(2)O), a potent greenhouse gas, can be emitted during wastewater treatment, significantly contributing to the greenhouse gas footprint. Measurements at lab-scale and full-scale wastewater treatment plants (WWTPs) have demonstrated that N(2)O can be emitted in substantial amounts during nitrogen removal in WWTPs, however, a large variation in reported emission values exists. Analysis of literature data enabled the identification of the most important operational parameters leading to N(2)O emission in WWTPs: (i) low dissolved oxygen concentration in the nitrification and denitrification stages, (ii) increased nitrite concentrations in both nitrification and denitrification stages, and (iii) low COD/N ratio in the denitrification stage. From the literature it remains unclear whether nitrifying or denitrifying microorganisms are the main source of N(2)O emissions. Operational strategies to prevent N(2)O emission from WWTPs are discussed and areas in which further research is urgently required are identified.

Mechanisms and specific directionality of autotrophic nitrous oxide and nitric oxide generation during transient anoxia.

The overall goal of this study was to determine the molecular and metabolic responses of chemostat cultures of model nitrifying bacteria to imposition of and recovery from transient anoxic conditions. Based on the study, a specific directionality in nitrous oxide (N(2)O) and nitric oxide (NO) production was demonstrated. N(2)O production was only observed during recovery to aerobic conditions after a period of anoxia and correlated positively with the degree of ammonia accumulation during anoxia. NO, on the other hand, was emitted mainly under anoxia. The production of NO was linked to a major imbalance in the expression of the nitrite reductase gene, which was overexpressed during transient anoxia. In contrast, genes coding for ammonia and hydroxylamine oxidation and nitric oxide reduction were generally under-expressed during transient anoxia. These results are different from the observed parallel expression and activity of nitrite and nitric oxide reductase in heterotrophic bacteria subjected to transient oxygen cycling. Unlike NO, the production of N(2)O could not be solely correlated to gene expression patterns and likely involved responses at the enzyme activity or metabolic levels. Based on experimental data, the propensity of the nitrifying cultures for N(2)O production is related to a shift in their metabolism from a low specific activity (q < q(max)) toward the maximum specific activity (q(max)).

Emission of nitrous oxide and nitric oxide from a full-scale single-stage nitritation-anammox reactor

At a full-scale single-stage nitritation-anammox reactor, off-gas measurement for nitric oxide (NO) and nitrous oxide (N(2)O) was performed. NO and N(2)O are environmental hazards, imposing the risk of improving water quality at the cost of deteriorating air quality. The emission of NO during normal operation of a single-stage nitritation-anammox process was 0.005% of the nitrogen load while the N(2)O emission was 1.2% of the nitrogen load to the reactor, which is in the same range as reported emission from other full-scale wastewater treatment plants. The emission of both compounds was strongly coupled. The concentration of NO and N(2)O in the off-gas of the single-stage nitritation-anammox reactor was rather dynamic and clearly responded to operational variations. This exemplifies the need for time-dependent measurement of NO and N(2)O emission from bioreactors for reliable emission estimates. Nitrite accumulation clearly resulted in increased NO and N(2)O concentrations in the off-gas, yielding higher emission levels. Oxygen limitation resulted in a decrease in NO and N(2)O emission, which was unexpected as oxygen limitation is generally assumed to cause increased emissions in nitrogen converting systems. Higher aeration flow dramatically increased the NO emission load and also seemed to increase the N(2)O emission, which stresses the importance of efficient aeration control to limit NO and N(2)O emissions.

Modelling nitrite in wastewater treatment systems: a discussion of different modelling concepts

Originally presented at the 1st IWA/WEF Wastewater Treatment Modelling Seminar (WWTmod 2008), this contribution has been updated to also include the valuable feedback that was received during the Modelling Seminar. This paper addresses a number of basic issues concerning the modelling of nitrite in key processes involved in biological wastewater water treatment. To this end, we review different model concepts (together with model structures and corresponding parameter sets) proposed for processes such as two-step nitrification/denitrification, anaerobic ammonium oxidation and phosphorus uptake processes. After critically discussing these models with respect to their assumptions and parameter sets, common points of agreement as well as disagreement were elucidated. From this discussion a general picture of the state-of-the-art in the modelling of nitrite is provided. Taking this into account, a number of recommendations are provided to focus further research and development on nitrite modelling in biological wastewater treatment.

Reduced iron induced nitric oxide and nitrous oxide emission

Formation of the greenhouse gas nitrous oxide in water treatment systems is predominantly studied as a biological phenomenon. There are indications that also chemical processes contribute to these emissions. Here we studied the formation of nitric oxide (NO) and nitrous oxide (N(2)O) due to chemical nitrite reduction by ferrous iron (Fe(II)). Reduction of nitrite and NO coupled to Fe(II) oxidation was studied in laboratory-scale chemical experiments at different pH, nitrite and iron concentrations. The continuous measurement of both NO and N(2)O emission showed that nitrite reduction and NO reduction have different kinetics. Nitrite reduction shows a linear dependency on the nitrite concentration, implying first order kinetics in nitrite. The nitrite reduction seems to be an equilibrium based reaction, leading to a constant NO concentration in the liquid. The NO reduction rate is suggested to be most dependent on reactive surface availability and the sorption of Fe(II) to the reactive surface. The importance of emission of NO and N(2)O coupled to iron oxidation is exemplified by iron reduction experiments and several examples of environments where this pathway can play a role.

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 behaviour and its comparison with previously published experimental data serves in elucidating the true NO and N2O formation mechanism.

Effect of Nitric Oxide on Anammox Bacteria

ABSTRACT The effects of nitrogen oxides on anammox bacteria are not well known. Therefore, anammox bacteria were exposed to 3,500 ppm nitric oxide (NO) in the gas phase. The anammox bacteria were not inhibited by the high NO concentration but rather used it to oxidize additional ammonium to dinitrogen gas under conditions relevant to wastewater treatment.

Physiological and phylogenetic study of an ammonium-oxidizing culture at high nitrite concentrations

Oxidation of high-strength ammonium wastewater can lead to exceptionally high nitrite concentrations; therefore, the effect of high nitrite concentration (> 400 mM) was studied using an ammonium-oxidizing enrichment culture in a batch reactor. Ammonium was fed to the reactor in portions of 40-150 mM until ammonium oxidation rates decreased and finally stopped. Activity was restored by replacing half of the medium, while biomass was retained by a membrane. The ammonium-oxidizing population obtained was able to oxidize ammonium at nitrite concentrations of up to 500 mM. The maximum specific oxidation activity of the culture in batch test was about 0.040 mmol O(2)g(-1)proteinmin(-1) and the K(s) value was 1.5 mM ammonium. In these tests, half of the maximum oxidation activity was still present at a concentration of 600 mM nitrite and approximately 10% residual activity could still be measured at 1200 mM nitrite (pH 7.4), or as a free nitrous acid (FNA) concentration of 6.6 mg l(-1). Additional experiments showed that the inhibition was caused by nitrite and not by the high sodium chloride concentration of the medium. The added ammonium was mainly converted into nitrite and no nitrite oxidation was observed. In addition, gaseous nitrogen compounds were detected and mass balance calculations revealed a nitrogen loss of approximately 20% using this system. Phylogenetic analyses of 16S rRNA and ammonium monooxygenase (amoA) genes of the obtained enrichment culture showed that ammonium-oxidizing bacteria of the Nitrosomonas europaea/Nitrosococcus mobilis cluster dominated the two clone libraries. Approximately 25% of the 16S rRNA clones showed a similarity of 92% to Deinococcus-like organisms. Specific fluorescence in situ hybridization (FISH) probes confirmed that these microbes comprised 10-20% of the microbial community in the enrichment. The Deinococcus-like organisms were located around the Nitrosomonas clusters, but their role in the community is currently unresolved.

Role of nitrogen oxides in the metabolism of ammonia-oxidizing bacteria

Ammonia-oxidizing bacteria (AOB) can use oxygen and nitrite as electron acceptors. Nitrite reduction by Nitrosomonas is observed under three conditions: (i) hydrogen-dependent denitrification, (ii) anoxic ammonia oxidation with nitrogen dioxide (NO(2)) and (iii) NO(x)-induced aerobic ammonia oxidation. NO(x) molecules play an important role in the conversion of ammonia and nitrite by AOB. Absence of nitric oxide (NO), which is generally detectable during ammonia oxidation, severely impairs ammonia oxidation by AOB. The lag phase of recovery of aerobic ammonia oxidation was significantly reduced by NO(2) addition. Acetylene inhibition tests showed that NO(2)-dependent and oxygen-dependent ammonia oxidation can be distinguished. Addition of NO(x) increased specific activity of ammonia oxidation, growth rate and denitrification capacity. Together, these findings resulted in a hypothetical model on the role of NO(x) in ammonia oxidation: the NO(x) cycle.

Metabolic modeling of denitrification in Agrobacterium tumefaciens: a tool to study inhibiting and activating compounds for the denitrification pathway

A metabolic network model for facultative denitrification was developed based on experimental data obtained with Agrobacterium tumefaciens. The model includes kinetic regulation at the enzyme level and transcription regulation at the enzyme synthesis level. The objective of this work was to study the key factors regulating the metabolic response of the denitrification pathway to transition from oxic to anoxic respiration and to find parameter values for the biological processes that were modeled. The metabolic model was used to test hypotheses that were formulated based on the experimental results and offers a structured look on the processes that occur in the cell during transition in respiration. The main phenomena that were modeled are the inhibition of the cytochrome c oxidase by nitric oxide (NO) and the (indirect) inhibition of oxygen on the denitrification enzymes. The activation of transcription of nitrite reductase and NO reductase by their respective substrates were hypothesized. The general assumption that nitrite and NO reduction are controlled interdependently to prevent NO accumulation does not hold for A. tumefaciens. The metabolic network model was demonstrated to be a useful tool for unraveling the different factors involved in the complex response of A. tumefaciens to highly dynamic environmental conditions.