Denitrification Kinetics Research Articles

Anaerobic oxidation of methane coupled to denitrification: fundamentals, challenges, and potential

Anaerobic treatment of dilute wastewater (e.g., municipal sewage) is promising due to energy recovery and lessened operating costs, both of which can significantly improve the sustainability of wastewater management over traditional activated sludge treatment. Discharge of dissolved methane and the lack of nitrogen removal are challenges for widespread adoption of anaerobic wastewater treatment. Anaerobic oxidation of methane coupled to denitrification (AOM-D) can overcome both challenges in a complementary way, thus preserving the benefits of anaerobic treatment of dilute wastewater. This review first presents the principles of AOM-D, focusing on pathways and thermodynamics. Second, energy and economic benefits are assessed for anaerobic treatment of dilute wastewater. Third, the review addresses the technical challenges inherent to implementing AOM-D in suspended-growth systems and outlines the advantages of membrane biofilm reactors (MBfRs) for AOM-D applications. Finally, the review develops a model of AOM-D in an MBfR and uses the model to show the conditions that achieve fast denitrification kinetics simultaneously with a very low concentration of dissolved methane.

Denitrification Kinetics of High‐Strength Nitrate in Granular Sludge Reactors

Biological denitrification of high‐strength nitrate is challenging due to building‐up of toxic nitrite to high levels. In this study, denitrification kinetics of high‐strength nitrate is investigated in a granular sludge reactor operated in sequencing batch mode. The nitrate concentrations are increased from 677, 1355, 2032, 2710, 3387, 4065–4742 mg L−1 NO3N in the reactor. Carbon and an electron source (acetate) are supplied at a fixed acetate‐carbon to nitrate‐nitrogen ratio of 1.5. At steady state, complete denitrification of 4742 mg L−1 NO3N in the feed is observed within the first 20 h of sequencing batch reactor (SBR) cycle. High denitrification rate constants are observed for NO3N concentrations from 677 to 2032 mg L−1. However, a decrease in denitrification rate constants is observed for nitrate‐N concentrations of 2710–4742 mg L−1, due to inhibition caused by transient accumulation of high levels of nitrite. Denitrifying biomass formed in the SBR is predominantly consisted of granular sludge, containing cocci, and rod‐shaped microorganisms. Cultivation of nitrite‐tolerant, denitrifying microbial community in granular sludge will help in the development of high rate reactors needed for process application.

The application of Monod equation to denitrification kinetics description in the moving bed biofilm reactor (MBBR)

In this paper, the kinetic constants Vmax and KCOD occurring in the Monod equation, which describe the denitrification process in the moving bed, are determined. For this purpose, a laboratory moving bed biofilm reactor (MBBR) was used. The filling of the reactor consisted of EvU Perl carriers. The experiment was carried out with an excess of nitrate, and denitrification rate was dependent on the concentration of external organic carbon, which constituted the Brennta Plus preparation. Determination of constants was made by Hofstee–Eadie method, whereby there was obtained: Vmax = 0.78 g NO3−–N/g D.M./day and KCOD = 16.97 g O2/m3. The new Monod equation was verified using MBBR constructed on an industrial scale in wastewater treatment plant in Gronowo Górne (Poland). After joining the MBBR to the technological system and after a period of biomass adaptation, total nitrogen removal efficiency increased from 53.5 to 86.0%. The results of the research have been discussed with several similar researches.

The link between the microbial ecology, gene expression, and biokinetics of denitrifying polyphosphate-accumulating systems under different electron acceptor combinations.

The emission of the greenhouse gas nitrous oxide (N2O) can occur during biological nutrient removal. Denitrifying enhanced biological phosphorus removal (d-EBPR) systems are an efficient means of removing phosphate and nitrogen, performed by denitrifying polyphosphate-accumulating organisms (d-PAOs). The aim of this work was to study the effect of various combinations of electron acceptors, nitrate (NO3-), nitrite (NO2-), and N2O, on the denitrification pathway of a d-EBPR system. Batch tests were performed with different electron acceptor combinations, to explore the denitrification pathway. Reverse transcriptase-qPCR (RT-qPCR) and high-throughput sequencing, combined with chemical analysis, were used to study gene expression, microbial diversity, and denitrification kinetics. The potential for N2O production was greater than the potential for its reduction in most tests. A strong correlation was observed between the N2O reduction rate and the relative gene expression of nitrous oxide reductase per nitrite reductase (nosZ/(nirS + nirK)), suggesting that the expression of denitrifying marker genes is a strong predictor of the N2O reduction rate. The d-EBPR community maintained a core population with low variations throughout the study. Furthermore, phylogenetic analyses of the studied marker genes revealed that the organisms actively involved in denitrification were closely related to Thauera sp., Candidatus Accumulibacter phosphatis, and Candidatus Competibacter denitrificans. Moreover, Competibacter-related OTUs seem to be important contributors to the N2O reduction capacity of the system, likely scavenging the N2O produced by other organisms. Overall, this study contributes to a better understanding of the microbial biochemistry and the genetics involving biological denitrification removal, important to minimize N2O emissions in wastewater treatment plants.

Identifying and Quantifying the Intermediate Processes during Nitrate-Dependent Iron(II) Oxidation.

Microbially driven nitrate-dependent iron (Fe) oxidation (NDFO) in subsurface environments has been intensively studied. However, the extent to which Fe(II) oxidation is biologically catalyzed remains unclear because no neutrophilic iron-oxidizing and nitrate reducing autotroph has been isolated to confirm the existence of an enzymatic pathway. While mixotrophic NDFO bacteria have been isolated, understanding the process is complicated by simultaneous abiotic oxidation due to nitrite produced during denitrification. In this study, the relative contributions of biotic and abiotic processes during NDFO were quantified through the compilation and model-based interpretation of previously published experimental data. The kinetics of chemical denitrification by Fe(II) (chemodenitrification) were assessed, and compelling evidence was found for the importance of organic ligands, specifically exopolymeric substances secreted by bacteria, in enhancing abiotic oxidation of Fe(II). However, nitrite alone could not explain the observed magnitude of Fe(II) oxidation, with 60-75% of overall Fe(II) oxidation attributed to an enzymatic pathway for investigated strains: Acidovorax ( A.) strain BoFeN1, 2AN, A. ebreus strain TPSY, Paracoccus denitrificans Pd 1222, and Pseudogulbenkiania sp. strain 2002. By rigorously quantifying the intermediate processes, this study eliminated the potential for abiotic Fe(II) oxidation to be exclusively responsible for NDFO and verified the key contribution from an additional, biological Fe(II) oxidation process catalyzed by NDFO bacteria.

Using natural biomass microorganisms for drinking water denitrification

Among the methods that are studied to eliminate nitrate from drinking water, biological denitrification is an attractive strategy. Although several studies report the use of denitrifying bacteria for nitrate removal, they usually involve the use of sewage sludge as biomass to obtain the microbiota. In the present study, denitrifying bacteria was isolated from bamboo, and variable parameters were controlled focusing on optimal bacterial performance followed by physicochemical analysis of water adequacy. In this way, bamboo was used as a source of denitrifying microorganisms, using either Immobilized Microorganisms (IM) or Suspended Microorganisms (SM) for nitrate removal. Denitrification parameters optimization was carried out by analysis of denitrification at different pH values, temperature, nitrate concentrations, carbon sources as well as different C/N ratios. In addition, operational stability and denitrification kinetics were evaluated. Microorganisms present in the biomass responsible for denitrification were identified as Proteus mirabilis. The denitrified water was submitted to physicochemical treatment such as coagulation and flocculation to adjust to the parameters of color and turbidity to drinking water standards. Denitrification using IM occurred with 73% efficiency in the absence of an external carbon source. The use of SM provided superior denitrification efficiency using ethanol (96.46%), glucose (98.58%) or glycerol (98.5%) as carbon source. The evaluation of the operational stability allowed 12 cycles of biomass reuse using the IM and 9 cycles using the SM. After physical-chemical treatment, only SM denitrified water remained within drinking water standards parameters of color and turbidity.

Simultaneous removal of SO2 and NOx from flue gas by wet scrubbing using a urea solution

ABSTRACTNitrogen oxides (NOx) and sulfur dioxide (SO2) are major air pollutants, so simultaneously removing them from gases emitted during fossil fuel combustion in stationary systems is important. Wet denitrification using urea is used for a wide range of systems. Additives have strong effects on wet denitrification using urea, and different mechanisms are involved and different effects found using different additives. In this study, the effects of different additives, initial urea concentrations, reaction temperatures, initial pH values, gas flow rates, and reaction times on the simultaneous desulfurization and denitrification efficiencies achieved using wet denitrification using urea were studied in single factor experiment. The optimum reaction conditions for desulfurization and denitrification were found. Desulfurization and denitrification efficiencies of 97.5% and 96.3%, respectively, were achieved at a KMnO4 concentration 5 mmol/L, a reaction temperature of 70°C, initial urea solution pH 8, a urea concentration of 9%, and a gas flow rate of 40 L/h. The concentrations of the desulfurization and denitrification reaction products in the solution were determined. NOx was mainly transformed into N2, and the and concentrations in the solution became very low. The reactions involved in SO2 and NOx removal using urea were analyzed from the thermodynamic viewpoint. Increasing the temperature was not conducive to the reactions but increased the rate constant, so an optimum temperature was determined. The simultaneous desulfurization and denitrification kinetics were calculated. The urea consumption and , , and generation reactions were all zero order. The generation rate was greater than the generation rate. The simultaneous desulfurization and denitrification process and mechanism were studied. The results provide reference data for performing flue gas desulfurization and denitrification in factories.

DEVELOPMENT OF AUTOTROPHIC DENITRIFICATION FOR NITRATE-CONTAMINATED INDUSTRIAL WASTEWATER

An autotrophic denitrification system was developed for treatment wastewater from steel industry. The aim was to evaluate the kinetics and capability of the system in reducing nitrate content in such industrial wastewater. The experiments were conducted in batch suspension and continuous runs. The denitrification kinetics in suspension runs obeys first order reaction with the rate constant k1 and k2 were determined to be 0.014 and 0.004 g-N/m3.d, respectively. The continuous runs used a column packed with mixture of granular sulfur and limestone. The microbes of Thiobacillus denitrificans were attached on the surface of granular sulfur in the form of biofilm. The biofilm thickness was investigated to be approximately 40 μm. The denitrification kinetics in the packed column obeys half-order reaction with the rate constant k of 0.172 g-N1/2.m1/2/(kg-S.d). The lower denitrification extent of industrial wastewater compared with the synthetic wastewater might be due to the lack of alkalinity. Keywords: Autotrophic denitrification, sulfur, packed column, thiobacillus denitrificans.

Kinetics of denitrification with crude glycerine as a carbon source in landfill leachate treatment

Kinetics of denitrification with crude glycerine as a carbon source in landfill leachate treatment

Effect of the type of the external carbon source on denitrification kinetics of wastewater

Effect of the type of the external carbon source on denitrification kinetics of wastewater

Removal of pharmaceuticals in pre-denitrifying MBBR – Influence of organic substrate availability in single- and three-stage configurations

Due to the limited efficiency of conventional biological treatment, innovative solutions are being explored to improve the removal of trace organic chemicals in wastewater. Controlling biomass exposure to growth substrate represents an appealing option for process optimization, as substrate availability likely impacts microbial activity, hence organic trace chemical removal. This study investigated the elimination of pharmaceuticals in pre-denitrifying moving bed biofilm reactors (MBBRs), where biofilm exposure to different organic substrate loading and composition was controlled by reactor staging. A three-stage MBBR and a single-stage reference MBBR (with the same operating volume and filling ratio) were operated under continuous-flow conditions (18 months). Two sets of batch experiments (day 100 and 471) were performed to quantify and compare pharmaceutical removal and denitrification kinetics in the different MBBRs. Experimental results revealed the possible influence of retransformation (e.g., from conjugated metabolites) and enantioselectivity on the removal of selected pharmaceuticals. In the second set of experiments, specific trends in denitrification and biotransformation kinetics were observed, with highest and lowest rates/rate constants in the first (S1) and the last (S3) staged sub-reactors, respectively. These observations were confirmed by removal efficiency data obtained during continuous-flow operation, with limited removal (<10%) of recalcitrant pharmaceuticals and highest removal in S1 within the three-stage MBBR. Notably, biotransformation rate constants obtained for non-recalcitrant pharmaceuticals correlated with mean specific denitrification rates, maximum specific growth rates and observed growth yield values. Overall, these findings suggest that: (i) the long-term exposure to tiered substrate accessibility in the three-stage configuration shaped the denitrification and biotransformation capacity of biofilms, with significant reduction under substrate limitation; (ii) biotransformation of pharmaceuticals may have occurred as a result of cometabolism by heterotrophic denitrifying bacteria.

Investigating the nitrification and denitrification kinetics under aerobic and anaerobic conditions by Paracoccus denitrificans ISTOD1

Municipal wastewater contains multiple nitrogen contaminants such as ammonia, nitrate and nitrite. Two heterotrophic nitrifier and aerobic denitrifiers, bacterial isolates ISTOD1 and ISTVD1 were isolated from domestic wastewater. On the basis of removal efficiency of ammonia, nitrate and nitrite under both aerobic and anaerobic conditions, ISTOD1 was selected and identified as Paracoccus denitrificans. Aerobically, NH4+-N had maximum specific nitrogen removal rate (Rxi) of 7.6g/gDCW/h and anaerobically, NO3−N showed Rxi of 2.5*10−1g/g DCW/h. Monod equation described the bioprocess kinetic coefficients, µmax and Ks, obtained by regression. Error functions were calculated to validate the Monod equation experimental data. Aerobic NO3−N showed the highest YW of 0.372mg DCW/mg NO3−N among the five conditions. ISTOD1 serves as a potential candidate for treating nitrogen rich wastewater using simultaneous nitrification and aerobic denitrification. It can be used in bioaugmentation studies under varied condition.

Differential mode of denitrification by Pseudomonas sp. KY1 using molasses as a carbon source

A new kinetic model that combines Monod kinetics and a constant denitrification rate is proposed to predict bacterial nitrate removal in groundwater. The model, which was developed with an indigenous bacterium Pseudomonas sp. KY1 isolated from a nitrate-contaminated site using molasses as a carbon source, takes into account the dual-mode substrate utilization pattern depending on the degradability of the compounds that constitute molasses. At the early stage of a batch reactor study with various C/N ratios, the nitrate reduction and molasses degradation was likely to be associated with microbial growth, while at the later stage of the total 48 hr of the study, a significant nitrate reduction occurred without substantial cell growth and molasses degradation. The new model was able to simulate the differential substrate utilization pattern, which could not be explained by either Monod kinetics or a constant denitrification rate model. Although further validation using other types of substrates and inoculums should follow, the new model shows the potential to accurately predict the denitrification kinetics in a heterogeneous carbon substrate system with minimum input parameter determination requirements.

Quantifying an aquifer nitrate budget and future nitrate discharge using field data from streambeds and well nests

AbstractNovel groundwater sampling (age, flux, and nitrate) carried out beneath a streambed and in wells was used to estimate (1) the current rate of change of nitrate storage, /dt, in a contaminated unconfined aquifer, and (2) future [ ]FWM (the flow‐weighted mean nitrate concentration in groundwater discharge) and (the nitrate flux from aquifer to stream). Estimates of /dt suggested that at the time of sampling (2013) the nitrate storage in the aquifer was decreasing at an annual rate (mean = −9 mmol/m2yr) equal to about one‐tenth the rate of nitrate input by recharge. This is consistent with data showing a slow decrease in the [ ] of groundwater recharge in recent years. Regarding future [ ]FWM and , predictions based on well data show an immediate decrease that becomes more rapid after ∼5 years before leveling out in the early 2040s. Predictions based on streambed data generally show an increase in future [ ]FWM and until the late 2020s, followed by a decrease before leveling out in the 2040s. Differences show the potential value of using information directly from the groundwater—surface water interface to quantify the future impact of groundwater nitrate on surface water quality. The choice of denitrification kinetics was similarly important; compared to zero‐order kinetics, a first‐order rate law levels out estimates of future [ ]FWM and (lower peak, higher minimum) as legacy nitrate is flushed from the aquifer. Major fundamental questions about nonpoint‐source aquifer contamination can be answered without a complex numerical model or long‐term monitoring program.

Distinctive denitrifying capabilities lead to differences in N2O production by denitrifying polyphosphate accumulating organisms and denitrifying glycogen accumulating organisms

This study aims at investigating the denitrification kinetics in two separate enriched cultures of denitrifying polyphosphate accumulating organisms (dPAO) and denitrifying glycogen accumulating organisms (dGAO) and compare their N2O accumulation potential under different conditions. Two sequencing batch reactors were inoculated to develop dPAO and dGAO enriched microbial communities separately. Seven batch tests with different combinations of electron acceptors (nitrate, nitrite and/or nitrous oxide) were carried out with the enriched biomass from both reactors. Results indicate that in almost all batch tests, N2O accumulated for both cultures, however dPAOs showed a higher denitrification capacity compared to dGAOs due to their higher nitrogen oxides reduction rates. Additionally, the effect of the simultaneous presence of several electron acceptors in the reduction rates of the different nitrogen oxides was also assessed in dPAOs and dGAOs.

High-Resolution Denitrification Kinetics in Pasture Soils Link N2O Emissions to pH, and Denitrification to C Mineralization.

Denitrification in pasture soils is mediated by microbial and physicochemical processes leading to nitrogen loss through the emission of N2O and N2. It is known that N2O reduction to N2 is impaired by low soil pH yet controversy remains as inconsistent use of soil pH measurement methods by researchers, and differences in analytical methods between studies, undermine direct comparison of results. In addition, the link between denitrification and N2O emissions in response to carbon (C) mineralization and pH in different pasture soils is still not well described. We hypothesized that potential denitrification rate and aerobic respiration rate would be positively associated with soils. This relationship was predicted to be more robust when a high resolution analysis is performed as opposed to a single time point comparison. We tested this by characterizing 13 different temperate pasture soils from northern and southern hemispheres sites (Ireland and New Zealand) using a fully automated-high-resolution GC detection system that allowed us to detect a wide range of gas emissions simultaneously. We also compared the impact of using different extractants for determining pH on our conclusions. In all pH measurements, soil pH was strongly and negatively associated with both N2O production index (IN2O) and N2O/(N2O+N2) product ratio. Furthermore, emission kinetics across all soils revealed that the denitrification rates under anoxic conditions (NO+N2O+N2 μmol N/h/vial) were significantly associated with C mineralization (CO2 μmol/h/vial) measured both under oxic (r2 = 0.62, p = 0.0015) and anoxic (r2 = 0.89, p<0.0001) conditions.

Transient Accumulation of NO2- and N2O during Denitrification Explained by Assuming Cell Diversification by Stochastic Transcription of Denitrification Genes.

Denitrifying bacteria accumulate , NO, and N2O, the amounts depending on transcriptional regulation of core denitrification genes in response to O2-limiting conditions. The genes include nar, nir, nor and nosZ, encoding -, -, NO- and N2O reductase, respectively. We previously constructed a dynamic model to simulate growth and respiration in batch cultures of Paracoccus denitrificans. The observed denitrification kinetics were adequately simulated by assuming a stochastic initiation of nir-transcription in each cell with an extremely low probability (0.5% h-1), leading to product- and substrate-induced transcription of nir and nor, respectively, via NO. Thus, the model predicted cell diversification: after O2 depletion, only a small fraction was able to grow by reducing . Here we have extended the model to simulate batch cultivation with , i.e., , NO, N2O, and N2 kinetics, measured in a novel experiment including frequent measurements of . Pa. denitrificans reduced practically all to before initiating gas production. The production is adequately simulated by assuming stochastic nar-transcription, as that for nirS, but with a higher probability (0.035 h-1) and initiating at a higher O2 concentration. Our model assumes that all cells express nosZ, thus predicting that a majority of cells have only N2O-reductase (A), while a minority (B) has -, NO- and N2O-reductase. Population B has a higher cell-specific respiration rate than A because the latter can only use N2O produced by B. Thus, the ratio is low immediately after O2 depletion, but increases throughout the anoxic phase because B grows faster than A. As a result, the model predicts initially low but gradually increasing N2O concentration throughout the anoxic phase, as observed. The modelled cell diversification neatly explains the observed denitrification kinetics and transient intermediate accumulations. The result has major implications for understanding the relationship between genotype and phenotype in denitrification research.

Metabolism-Induced CaCO3 Biomineralization During Reactive Transport in a Micromodel: Implications for Porosity Alteration.

The ability of Pseudomonas stutzeri strain DCP-Ps1 to drive CaCO3 biomineralization has been investigated in a microfluidic flowcell (i.e., micromodel) that simulates subsurface porous media. Results indicate that CaCO3 precipitation occurs during NO3(-) reduction with a maximum saturation index (SIcalcite) of ∼1.56, but not when NO3(-) was removed, inactive biomass remained, and pH and alkalinity were adjusted to SIcalcite ∼ 1.56. CaCO3 precipitation was promoted by metabolically active cultures of strain DCP-Ps1, which at similar values of SIcalcite, have a more negative surface charge than inactive strain DCP-Ps1. A two-stage NO3(-) reduction (NO3(-) → NO2(-) → N2) pore-scale reactive transport model was used to evaluate denitrification kinetics, which was observed in the micromodel as upper (NO3(-) reduction) and lower (NO2(-) reduction) horizontal zones of biomass growth with CaCO3 precipitation exclusively in the lower zone. Model results are consistent with two biomass growth regions and indicate that precipitation occurred in the lower zone because the largest increase in pH and alkalinity is associated with NO2(-) reduction. CaCO3 precipitates typically occupied the entire vertical depth of pores and impacted porosity, permeability, and flow. This study provides a framework for incorporating microbial activity in biogeochemistry models, which often base biomineralization only on SI (caused by biotic or abiotic reactions) and, thereby, underpredict the extent of this complex process. These results have wide-ranging implications for understanding reactive transport in relevance to groundwater remediation, CO2 sequestration, and enhanced oil recovery.

PH-driven shifts in overall and transcriptionally active denitrifiers control gaseous product stoichiometry in growth experiments with extracted bacteria from soil.

Soil pH is a strong regulator for activity as well as for size and composition of denitrifier communities. Low pH not only lowers overall denitrification rates but also influences denitrification kinetics and gaseous product stoichiometry. N2O reductase is particularly sensitive to low pH which seems to impair its activity post-transcriptionally, leading to higher net N2O production. Little is known about how complex soil denitrifier communities respond to pH change and whether their ability to maintain denitrification over a wider pH range relies on phenotypic redundancy. In the present study, we followed the abundance and composition of an overall and transcriptionally active denitrifier community extracted from a farmed organic soil in Sweden (pHH2O = 7.1) when exposed to pH 5.4 and drifting back to pH 6.6. The soil was previously shown to retain much of its functioning (low N2O/N2 ratios) over a wide pH range, suggesting a high functional versatility of the underlying community. We found that denitrifier community composition, abundance and transcription changed throughout incubation concomitant with pH change in the medium, allowing for complete reduction of nitrate to N2 with little accumulation of intermediates. When exposed to pH 5.4, the denitrifier community was able to grow but reduced N2O to N2 only when near-neutral pH was reestablished by the alkalizing metabolic activity of an acid-tolerant part of the community. The genotypes proliferating under these conditions differed from those dominant in the control experiment run at neutral pH. Denitrifiers of the nirS-type appeared to be severely suppressed by low pH and nirK-type and nosZ-containing denitrifiers showed strongly reduced transcriptional activity and growth, even after restoration of neutral pH. Our study suggests that low pH episodes alter transcriptionally active populations which shape denitrifier communities and determine their gas kinetics.

Low probability of initiating nirS transcription explains observed gas kinetics and growth of bacteria switching from aerobic respiration to denitrification.

In response to impending anoxic conditions, denitrifying bacteria sustain respiratory metabolism by producing enzymes for reducing nitrogen oxyanions/-oxides (NOx) to N2 (denitrification). Since denitrifying bacteria are non-fermentative, the initial production of denitrification proteome depends on energy from aerobic respiration. Thus, if a cell fails to synthesise a minimum of denitrification proteome before O2 is completely exhausted, it will be unable to produce it later due to energy-limitation. Such entrapment in anoxia is recently claimed to be a major phenomenon in batch cultures of the model organism Paracoccus denitrificans on the basis of measured e−-flow rates to O2 and NOx. Here we constructed a dynamic model and explicitly simulated actual kinetics of recruitment of the cells to denitrification to directly and more accurately estimate the recruited fraction (). Transcription of nirS is pivotal for denitrification, for it triggers a cascade of events leading to the synthesis of a full-fledged denitrification proteome. The model is based on the hypothesis that nirS has a low probability (, h−1) of initial transcription, but once initiated, the transcription is greatly enhanced through positive feedback by NO, resulting in the recruitment of the transcribing cell to denitrification. We assume that the recruitment is initiated as [O2] falls below a critical threshold and terminates (assuming energy-limitation) as [O2] exhausts. With = 0.005 h−1, the model robustly simulates observed denitrification kinetics for a range of culture conditions. The resulting (fraction of the cells recruited to denitrification) falls within 0.038–0.161. In contrast, if the recruitment of the entire population is assumed, the simulated denitrification kinetics deviate grossly from those observed. The phenomenon can be understood as a ‘bet-hedging strategy’: switching to denitrification is a gain if anoxic spell lasts long but is a waste of energy if anoxia turns out to be a ‘false alarm’.