Denitrification Kinetics Research Articles

Nitrogen removal from poultry slaughterhouse wastewater in anaerobic-anoxic-aerobic combined reactor: Integrated effect of recirculation rate and hydraulic retention time

The aim of this work was to assess the nitrogen removal from slaughterhouse wastewater in an anaerobic-anoxic-aerobic combined reactor, evaluating the integrated effect of recirculation rate and hydraulic retention time. The recirculation of the liquid phase from the aerobic zone to the anoxic zone was applied to promote the denitrification through the use of endogenous electron donors. Three recirculation rates (R: 0.5, 1 and 2) and three hydraulic retention times (14, 11 and 8 h) were applied. The operation of the reactor was divided into 3 steps (I, II, and III) according to the factors evaluated (recirculation rate and HRT), to achieve operational conditions that would allow satisfactory performance in the different compartments of the reactor. During the experiment the reactor was fed with average total nitrogen (TN) and chemical oxygen demand (COD) of 65 mg L−1 and 580 mg L−1, respectively. The denitrification efficiency (theoretical) and kinetics parameters for COD decay were calculated. The highest performance was verified in the Step III (R = 2) and HRT of 11 h with NH4+ and TN removals of 84% and 65%, respectively. The TN removal efficiency (65%) was considered satisfactory, since the theoretical denitrification efficiency expected for this condition (R = 2) is 67%, without addition of an external carbon source. The lowest nitrification efficiency values were obtained in HRT of 8 h in the Step I and II (R = 0.5 and 1, respectively), indicating that the nitrification time (3 h - aerobic phase) may be the limiting factor in this HRT. The COD removal efficiency was high in all assays (>95%). The values of the kinetic degradation constants of organic matter were close for all recirculation rates, and the highest values were recorded for the HRT of 8 h and R = 1 and R = 2 (−0.48 and −0.43, respectively).

Enhancement of denitrification efficiency using municipal and industrial waste fermentation liquids as external carbon sources

The addition of external carbon source for nitrogen removal from wastewater is an essential step in wastewater treatment. In this study, various external carbon sources from the fermentation of primary sludge (PS), thickened waste activated sludge (TWAS), food waste (FW), bakery processing & kitchen waste (BP + KW), fat, oil, & grease (FOG), and whey powder (WP) were successfully employed for wastewater denitrification. Methanol and acetate were also used as controls due to their common use as external carbon sources for wastewater denitrification. The denitrification performance and kinetics such as the specific denitrification rate (SDNR), denitrification potential (PDN), and the biomass yield were studied at a constant TVFA as COD/N ratio of 5 for all substrates. Complete denitrification was achieved with a NO3−-N removal efficiency of 98–99%, and no NO2− accumulation was observed at the end of the experiments for all substrates. The results revealed that the liquid fermentation filtrates exhibited higher SDNRs than methanol and acetate. This indicates the high organic matter utilization efficiency and better denitrification ability of fermentation filtrates over conventional carbon sources. WP exhibited the highest SDNR of 17.6 mg NOx − N/g VSS/h, which is approximately four times that of methanol (4.6 mg NOx − N/g VSS/h). The other carbon sources had SDNRs two to three times higher than that of methanol. However, the fermentation filtrates exhibited higher biomass yields of 0.26–0.37 mg VSS/mg COD compared to methanol of 0.21 mg VSS/mg COD, which could lead to higher sludge handling costs. Moreover, methanol exhibited higher PDN of 0.25 g N/g COD compared to all the fermentation filtrates.

Characteristics of nutrients removal under partial denitrification initiated by different initial nitrate concentration.

The partial denitrification (PD) is a very promising process developed in the last decade, to study the comprehensive influence of influent carbon to nitrogen (C/N) on the activated sludge system under PD, six sequencing batch reactors (SBRs) were operated in parallel at C/N of 2.75, 3.30, 4.13, 5.50, 8.25 and 16.50, the nitrogen removal, phosphorus removal and sludge settleability of PD were investigated. The results showed that PD was observed treating synthetic wastewater in all the six SBRs, and the nitrite accumulation rate (NAR) was highest at C/N of 5.50 (NAR of 82.30%). However, due to the alternate inhibition of NO2--N and free nitrous acid (FNA) produced by a limited carbon source, both the sludge settleability and phosphorus removal deteriorated. The average SVI at C/N of 8.25 was 130% lower than C/N of 3.30, and the average amount of PO43--P released at C/N of 16.5 was 189% higher than C/N of 2.75. Kinetic analysis showed that the denitrification kinetics of PD and complete denitrification were similar, and the nitrite accumulation was caused by the difference between nitrate reduction rate and nitrite reduction rate. Variations of on-line parameters (pH and ORP) revealed that nitrite accumulation could be indicated by judging the nitrate turning point and nitrite turning point on pH and ORP curves, which provided guidance for the setup of PD.

Microbial responses underlying the denitrification kinetic shifting exposed to ng/L- and μg/L-level lomefloxacin in groundwater

The emerging co-contaminant of antibiotics and nitrate has acquired great concerns worldwide, which poses a potential impact on denitrification in the ecological environment, but little is known about the groundwater system at lower antibiotic concentration, especially ng/L-level. Herein the frequently detected Lomefloxacin (LOM) in groundwater was selected to explore its influences on denitrification kinetics and microbial dynamic responses. The NO3−-N removals in ng/L–μg/L LOM-amended reactors (8.7–44.9%) performed far lower than that in control (76.1%). LOM can inhibit denitrification even at ng/L-level. The kinetic characteristic shifted from zero- to first-order once inhibition occurred. This observation is the synergistic effects of microbial community, enzyme activity, and antibiotic resistance genes (ARGs). The enzyme activities were inhibited immediately, whereas microbial community and ARGs exhibited hysteresis responses at ng/L-level. The enrichment of non-corresponding ARG types suggested LOM's co-selection effects. Brevundimonas were potential antibiotic resistant bacteria. Exposed to μg/L-level LOM, denitrification underwent a 6-d lag phase. The more sensitive enzyme activities and microbial community and the enrichment of ARGs with less abundance were investigated. These findings clarify the microbial response mechanism underlying the denitrification kinetic shifting exposed to low-concentrations of LOM, which is the potential process for heightening nitrate accumulation in groundwater.

Denitrification Biokinetics: Towards Optimization for Industrial Applications.

Denitrification is a microbial process that converts nitrate (NO3–) to N2 and can play an important role in industrial applications such as souring control and microbially enhanced oil recovery (MEOR). The effectiveness of using NO3– in souring control depends on the partial reduction of NO3– to nitrite (NO2–) and/or N2O while in MEOR complete reduction of NO3– to N2 is desired. Thauera has been reported as a dominant taxon in such applications, but the impact of NO3– and NO2– concentrations, and pH on the kinetics of denitrification by this bacterium is not known. With the goal of better understanding the effects of such parameters on applications such as souring and MEOR, three strains of Thauera (K172, NS1 and TK001) were used to study denitrification kinetics when using acetate as an electron donor. At low initial NO3– concentrations (∼1 mmol L–1) and at pH 7.5, complete NO3– reduction by all strains was indicated by non-detectable NO3– concentrations and near-complete recovery (> 97%) of the initial NO3-N as N2 after 14 days of incubation. The relative rate of denitrification by NS1 was low, 0.071 mmol L–1 d–1, compared to that of K172 (0.431 mmol L–1 d–1) and TK001 (0.429 mmol L–1 d–1). Transient accumulation of up to 0.74 mmol L–1 NO2– was observed in cultures of NS1 only. Increased initial NO3– concentrations resulted in the accumulation of elevated concentrations of NO2– and N2O, particularly in incubations with K172 and NS1. Strain TK001 had the most extensive NO3– reduction under high initial NO3– concentrations, but still had only ∼78% of the initial NO3-N recovered as N2 after 90 days of incubation. As denitrification proceeded, increased pH substantially reduced denitrification rates when values exceeded ∼ 9. The rate and extent of NO3– reduction were also affected by NO2– accumulation, particularly in incubations with K172, where up to more than a 2-fold rate decrease was observed. The decrease in rate was associated with decreased transcript abundances of denitrification genes (nirS and nosZ) required to produce enzymes for reduction of NO2– and N2O. Conversely, high pH also contributed to the delayed expression of these gene transcripts rather than their abundances in strains NS1 and TK001. Increased NO2– concentrations, N2O levels and high pH appeared to cause higher stress on NS1 than on K172 and TK001 for N2 production. Collectively, these results indicate that increased pH can alter the kinetics of denitrification by Thauera strains used in this study, suggesting that liming could be a way to achieve partial denitrification to promote NO2– and N2O production (e.g., for souring control) while pH buffering would be desirable for achieving complete denitrification to N2 (e.g., for gas-mediated MEOR).

Determination of nitrate removal kinetics model parameters in woodchip bioreactors

Woodchip bioreactors have emerged as a viable water management tool to reduce nitrate contamination from agricultural subsurface drainage, wastewater, and stormwater. Understanding of denitrification kinetics is critical to the design and application of field woodchip bioreactors. The denitrification process in woodchip bioreactors generally obeys a model of Michaelis-Menten type enzyme kinetics. The objective of this study was to determine Michaelis-Menten model parameters for nitrate removal in laboratory bioreactors using the fresh, composted and aged woodchips. The results showed that the maximum nitrate removal rates (Vmax) were 2.09, 0.88 and 0.30 mg N/L/h, and the half saturation constants (Km) were 2.60, 2.16 and 2.01 mg N/L for the composted, fresh and aged woodchip bioreactors at 22 °C. The Vmax values decreased to 0.26 and 0.05 mg N/L/h, and the Km values decreased to 1.74 and 1.19 mg N/L when the composted and fresh woodchip bioreactors were operated at 5 °C. Denitrification in woodchip bioreactors can be operationally defined as a zero-order reaction when treating contaminated water with nitrate much higher than the Km values. The nitrate removal efficiency of the bioreactors followed the order of composted woodchips > fresh woodchips > aged woodchips. The average nitrate load reduction rates were 8.81-21.0, 7.36-9.78, and 2.46-3.54 g N/m3/d for the composted, fresh, and aged woodchip bioreactors at influent nitrate concentrations of 10-50 mg N/L and 22 °C. Woodchip composting before bioreactor installation can be used as a practical strategy to enhance denitrification performance of bioreactors.

Nitrite accumulation and microbial behavior by seeding denitrifying phosphorus removal sludge for partial denitrification (PD): The effect of COD/NO3− ratio

Effect of influent COD/NO3− ratios (2.0–5.0) on partial denitrification (PD, NO3− to NO2−) was investigated by seeding denitrifying phosphorus removal (DPR) sludge at ambient temperature (16 ± 2℃). At COD/NO3− ratio of 2.5, the optimal NO2− effluent and nitrate-to-nitrite transformation ratio (NTR) reached up to 27.18 mg/L and 82.18%, respectively, and nitrate removal efficiency (NRE) (59.34 → 97.98%) showed positive relationship with increasing COD/NO3− ratios. The variations were further illustrated by denitrification kinetics, where faster COD degradation (1.55 g COD/(gVSS·d)), more NO3− consumption (0.55 gN/(gVSS·d)) and higher NO2− production (0.52 gN/(gVSS·d)) were observed although the NO2− peaks happened at anoxic 30 min. Microbial analysis showed lower community diversity and more concentrated composition with dominated genera Thauera (14.10%), Terrimonas (9.40%), Saprospiraceae (13.50%) and Flavobacterium (28.23%) at COD/NO3− ratio of 2.5. Based on the achievement of PD, the application feasibility of integrated PD-DPR-Anammox in a two-sludge DPR system for deep-level nutrient removal was discussed.

Contingent Effects of Liming on N2O-Emissions Driven by Autotrophic Nitrification

Liming acidic soils is often found to reduce their N2O emission due to lowered N2O/(N2O + N2) product ratio of denitrification. Some field experiments have shown the opposite effect, however, and the reason for this could be that liming stimulates nitrification-driven N2O production by enhancing nitrification rates, and by favoring ammonia oxidizing bacteria (AOB) over ammonia oxidizing archaea (AOA). AOB produce more N2O than AOA, and high nitrification rates induce transient/local hypoxia, thereby stimulating heterotrophic denitrification. To study these phenomena, we investigated nitrification and denitrification kinetics and the abundance of AOB and AOA in soils sampled from a field experiment 2–3 years after liming. The field trial compared traditional liming (carbonates) with powdered siliceous rocks. As expected, the N2O/(N2O + N2) product ratio of heterotrophic denitrification declined with increasing pH, and the potential nitrification rate and its N2O yield (YN2O: N2O-N/NO3–-N), as measured in fully oxic soil slurries, increased with pH, and both correlated strongly with the AOB/AOA gene abundance ratio. Soil microcosm experiments were monitored for nitrification, its O2-consumption and N2O emissions, as induced by ammonium fertilization. Here we observed a conspicuous dependency on water filled pore space (WFPS): at 60 and 70% WFPS, YN2O was 0.03-0.06% and 0.06–0.15%, respectively, increasing with increasing pH, as in the aerobic soil slurries. At 85% WFPS, however, YN2O was more than two orders of magnitude higher, and decreased with increasing pH. A plausible interpretation is that O2 consumption by fertilizer-induced nitrification cause hypoxia in wet soils, hence induce heterotrophic nitrification, whose YN2O decline with increasing pH. We conclude that while low emissions from nitrification in well-drained soils may be enhanced by liming, the spikes of high N2O emission induced by ammonium fertilization at high soil moisture may be reduced by liming, because the heterotrophic N2O reduction is enhanced by high pH.

Denitrification kinetics indicates nitrous oxide uptake is unaffected by electron competition in Accumulibacter

Denitrifying phosphorus removal is a cost and energy efficient treatment technology that relies on polyphosphate accumulating organisms (DPAOs) utilizing nitrate or nitrite as terminal electron acceptor. Denitrification is a multistep process, but many organisms do not possess the complete pathway, leading to the accumulation of intermediates such as nitrous oxide (N2O), a potent greenhouse gas and ozone depleting substance. Candidatus Accumulibacter organisms are prevalent in denitrifying phosphorus removal processes and, according to genomic analyses, appear to vary in their denitrification abilities based on their lineage. Denitrification kinetics and nitrous oxide accumulation in the absence of inhibition from free nitrous acid is a strong indicator of denitrification capabilities of Accumulibacter exposed long-term to nitrate or nitrite as electron acceptor. Thus, we investigated the preferential use of the nitrogen oxides involved in denitrification and nitrous oxide accumulation in two enrichments of Accumulibacter and a competitor – the glycogen accumulating organism Candidatus Competibacter. We modified a metabolic model to predict phosphorus removal and denitrification rates when nitrate, nitrite or N2O were added as electron acceptors in different combinations. Unlike previous studies, no N2O accumulation was observed for Accumulibacter in the presence of multiple electron acceptors. Electron competition did not limit denitrification kinetics or lead to N2O accumulation in Accumulibacter or Competibacter. Despite the presence of sufficient internal storage polymers (polyhydroxyalkanoates, or PHA) as energy source for each denitrification step, the extent of denitrification observed was dependent on the dominant organism in the enrichment. Accumulibacter showed complete denitrification, whereas Competibacter denitrification was limited to reduction of nitrate to nitrite. These findings indicate that DPAOs can contribute to lowering N2O emissions in the presence of multiple electron acceptors under partial nitritation conditions.

Enhancement of the denitrification performance of an activated sludge using an electromagnetic field in batch mode

The influence of electromagnetic fields on bacterial denitrification has been tested on synthetic media with sludges from wastewater treatment stations, in batch mode. The effects of the intensity of the magnetic induction ratio B (mT), reaction volume and initial biomass concentration on the kinetics of the denitrification process were studied. Magnetic field had both an optimal stimulating effect on the activity of the denitrifying flora for B (mT)/mgx values of the order of 0.212, and an inhibitory effect for the values beyond the latter.Sludges underwent multiple exposure cycles to magnetic fields. It was shown that, after three exposure cycles, denitrification kinetics went from 6.5 to 12.7 mg N–NO-3.L−1.h−1 which corresponds to a 2.7 fold improvement. The improved performance persists even after the cessation of the magnetic field. Observation of the sludge by the environmentalelectron microscope shows that the microbial population forming the starting sludge; changed following exposure to the magnetic field. The action of the; electromagnetic field on the microbial populations in denitrification resulted in the modification of the diversity of the flora that is initially present, favoring the development of Proteo bacteria, particularly the Betaproteo bacteria subclass, which results in improved denitrification.

Denitrification performance evaluation and kinetics analysis with mariculture solid wastes (MSW) derived carbon source in marine recirculating aquaculture systems (RAS)

Biological denitrification using mariculture solid wastes (MSW) carbon source is a promising solution for removing nitrate (NO3−-N) and disposing MSW in marine recirculating aquaculture systems (RAS). To enhance denitrification performance, heating (HT), rhamnolipid (RL), alkali (AL), thermophilic bacteria (TB) pre-treated MSW acidogenic fermentation effluents were prepared as carbon sources. Profiles of soluble organics in four types of fermentation effluents were first evaluated. The highest volatile fatty acids (VFAs) yield (52.1%) was obtained from TB treated MSW after acidification. RL and TB treated MSW acidogenic fermentation effluents showed high NO3−-N removal efficiency (NRE) (around 97%). Acidogenic fermentation effluent from TB treated MSW presented a high biodegradability, with the minimum effluent chemical oxygen demand (COD) amount (35 mg/L). Denitrification kinetics parameters were also analyzed; high fraction (74.5%) of the most readily biodegradable organics (SS) demonstrated that TB treated MSW acidogenic fermentation effluent is a high-quality carbon source for enhancing denitrification.

Denitrification Kinetics of Nitrate by a Heterotrophic Culture in Batch and Fixed-Biofilm Reactors

Herein, the progress of nitrate removal by a heterotrophic culture in a batch reactor and continuous-flow fixed-biofilm reactor was examined. Two batch experiments for nitrate reduction with acetate degradation using 250 mL batch reactors with acclimated denitrifying biomass were conducted. The experimental results indicated that the nitrate was completely reduced; however, the acetate remained at a concentration of 280 mg/L from initial nitrate concentration of 100 mg/L. However, the acetate was fully biodegraded by the denitrifying biomass at an initial nitrate concentration of 300 mg/L. To evaluate the biokinetic parameters, the concentration data of nitrate, nitrite, acetate, and denitrifying biomass from the batch kinetic experiments were compared with those of the batch kinetic model system. A continuous-flow fixed-biofilm reactor was used to verify the kinetic biofilm model. The removal efficiency of nitrate in the fixed-biofilm reactor at the steady state was 98.4% accompanied with 90.5% acetate consumption. The experimental results agreed satisfactorily with the model predictions. The modeling and experimental approaches used in this study could be applied in the design of a pilot-scale, or full-scale, fixed-biofilm reactor for nitrate removal in water and wastewater treatment plants.

The effect of sulfate on nitrite-denitrifying anaerobic methane oxidation (nitrite-DAMO) process

Sulfate is generally found in natural water and wastewater. Nitrite-DAMO bacteria live in natural water or wastewater containing different sulfates. To determine the effect of sulfate on the nitrite-DAMO process, we conducted batch tests and continuous tests to investigate the performance and microbial structure of the nitrite-DAMO system at different sulfate concentrations. The results indicated that the nitrogen removal performance of the nitrite-DAMO system was initially promoted and then inhibited at 0–200 mg SO42−/L, and the denitrification rate was highest at 80 mg SO42−/L which was 1.26 mgN/(L·d). When stimulated by sulfate, protein stabilized nitrite-DAMO bacteria. The denitrification kinetics conformed to the Edward equation, and the initial inhibitory concentration of the nitrite-DAMO system was 189.70 mg SO42−/L. Changes in the proportion of unclassfied_c_ABY1 of the phylum Patescibacteria and norank_f_LD-RB-34 of the phylum Bacteroidetes were the main factors influencing how the nitrogen removal rate of the nitrite-DAMO system responded to sulfate.

Macroscopic Kinetics of Simultaneous Desulfurization and Denitrification by MgO/NaClO2 Solution

Based on the early research results of reaction characteristics of the simultaneous desulfurization and denitrification by MgO/NaClO2 solution, the macroscopic reaction kinetics was investigated in a small-scale bubbling reactor. The effect of various factors on the reaction rate was investigated in the experiments, i.e. initial NaClO2 concentration, solution pH and reaction temperature. The reaction rate equations and the reaction order of NO, SO2 and NaClO2 were also obtained by experiments and analysis. The results showed that reaction orders of NO, SO2 and NaClO2 during simultaneous desulfurization and denitrification were all pseudo-first-order. And the reaction equations of NO, SO2 and NaClO2 were r=0.079C0.984 NO , r=0.066C0.989 SO 2 and r=0.066C0.998 SO 2 , respectively. The reaction rate of NO, SO2 and NaClO2 were proportional to the NaClO2 concentration and inversely proportional to the solution pH; the reaction rate of SO2 was less affected by the concentration and pH of NaClO2. When the reaction temperature was 50 °C, the reaction rate of NO and SO2 was the largest, and the reaction temperature had little effect on the reaction rate of NaClO2.

A semi-empirical method to quantify the impact of mass transfer on lab-scale SCR deNOx catalyst activity testing

Industrial denitrification catalyst monoliths are regularly tested in controlled lab-scale conditions to quantify their activity. As mass transfer phenomena occurring inside the channels of the monoliths have an important impact on the global denitrification kinetics, activity measurements must be conducted in well-known conditions in terms of sample geometry and gaseous flow regimes, among others. The lack of accurate mass transfer correlations for the complex flows at stake however prevents any accurate generalisation of the test results. In this paper, we propose a semi-empirical method for the quantification of contribution of mass transfer to the global denitrification kinetics for any given test bench. It is based on the experimental adjustment of mass transfer correlations presenting a suitable form. Mass transfer can therefore be decoupled from the intrinsic kinetics and large ranges of conditions can be covered by conducting a limited number of measurements.

Denitrification performance and kinetics of an attapulgite lightweight ceramsite biofilter

Denitrification performance and kinetics of an attapulgite lightweight ceramsite biofilter

Comparative study on denitrification kinetics of nitrified effluent with and without organic carbon by activated sludge process

Comparative study on denitrification kinetics of nitrified effluent with and without organic carbon by activated sludge process

Characterisation of Pseudomonas stutzeri T13 for aerobic denitrification: Stoichiometry and reaction kinetics

The mechanism of total nitrogen (TN) removal at aerobic condition in wastewater treatment plants (WWTPs) has been one of the most popular research fields. However, the role of aerobic denitrification in TN removal was unclear because of the lack of stoichiometric coefficients and kinetic constants of aerobic denitrification bacterium. Thus, this study aimed to investigate the stoichiometry and kinetics of aerobic denitrification by using Pseudomonas stutzeri T13 as a model aerobic denitrification bacterium. Results indicated that strain T13 obtained the maximum yield coefficient (0.1098 mol biomass-N/mol COD) when using NH4+-N as the sole nitrogen source. This value decreased slightly (0.1077 mol biomass-N/mol COD) during aerobic denitrification, but was still higher than that of conventional denitrification. The half-saturation constants for ammonium, nitrate and nitrite (KNH4+, KNO3- and KNO2-) of strain T13 were fitted based on the experimental data and were 2.72, 18.33 and 209.07 mg/L, respectively. The validity of the stoichiometric coefficients and kinetic constants was tested at two extra conditions and perfect fitting results were obtained. To our knowledge, this is the first time to report the stoichiometric coefficients and kinetic constants of aerobic denitrification. These parameters will be useful in modelling nitrogen removal performance in systems inoculated with aerobic denitrification bacterium. Moreover, this study could provide an experimental basis for further clarifying the mechanism of aerobic denitrification from a quantitative perspective.

Potential of interactive multiobjective optimization in supporting the design of a groundwater biodenitrification process

The design of water treatment plants requires simultaneous analysis of technical, economic and environmental aspects, identified by multiple conflicting objectives. We demonstrated the advantages of an interactive multiobjective optimization (MOO) method over a posteriori methods in an unexplored field, namely the design of a biological treatment plant for drinking water production, that tackles the process drawbacks, contrarily to what happens in a traditional volumetric-load-driven design procedure. Specifically, we consider a groundwater denitrification biofilter, simulated by the Activated Sludge Model modified with two-stage denitrification kinetics. Three objectives were defined (nitrate removal efficiency, drawbacks on produced water, investment and management costs) and the interactive method NIMBUS applied to identify the best-suited design without any a priori evaluation, as for volumetric-load-driven design procedures. When compared to an evolutionary MOO algorithm, the interactive solution process was faster, more understandable and user-friendly and supported the decision maker well in identifying the most preferred solution (main design/operating parameters) to be implemented. Approach strength has been proved through both sensitivity analysis and positive experimental validation through a pilot scale biofilter operated for three months. In synthesis, without any “a priori” evaluation based on practical experience, the MOO design approach allowed obtaining a preferred Pareto optimal design, characterized by volumetric loading in the range 0.85–2.54 kgN m−3 d−1 (EBCTs: 5–15 min), a carbon dosage of 0.5–0.8 gC,dos/gC,stoich, with SRTs in the range 4–27 d.

Methane-based denitrification kinetics and syntrophy in a membrane biofilm reactor at low methane pressure

A methane-based membrane biofilm reactor (MBfR) was assessed for a tertiary nitrogen removal process in domestic wastewater treatment. To mitigate effluent dissolved methane concentrations, the MBfR was operated with a 20% methane mixing ratio and a low pressure of 0.003 atm. The nitrate concentration was reduced from 20 to 4 mg/L with a low methane concentration of 3.3 mg/L in the effluent at 4 h hydraulic retention time (HRT). An in situ dissolved oxygen sensor showed a concentration of 0.045 mg/L in the MBfR, demonstrating methane oxidation under hypoxic conditions. Both 16S rRNA gene sequencing and metagenomic analysis identified bacteria capable of oxidation of methane coupled to denitrification (Methylocystis), whereas other bacteria were implicated in either methane oxidation (Methylococcus) or nitrate reduction (Escherichia). Reduced genetic potential for nitrate reduction to nitrite at a shorter HRT coincided with a decreased efficiency of denitrification, suggesting rate limitation by this initial step of denitrification. Genes encoding nitrite reduction to dinitrogen were at similar relative abundance under both HRT conditions. Our results provide mechanistic evidence for microbial syntrophy between aerobic methanotrophs and denitrifiers in methane-fed MBfRs operated under varying HRTs, with important implications for novel biological nitrogen removal to dilute wastewater.