Article pubs.acs.org/est
Modeling of Simultaneous Anaerobic Methane and Ammonium Oxidation in a Membrane Biofilm Reactor Xueming Chen, Jianhua Guo, Ying Shi, Shihu Hu, Zhiguo Yuan, and Bing-Jie Ni* Advanced Water Management Centre, The University of Queensland, St. Lucia, Brisbane, QLD 4072, Australia S Supporting Information *
ABSTRACT: Nitrogen removal by using the synergy of denitrifying anaerobic methane oxidation (DAMO) and anaerobic ammonium oxidation (Anammox) microorganisms in a membrane biofilm reactor (MBfR) has previously been demonstrated experimentally. In this work, a mathematical model is developed to describe the simultaneous anaerobic methane and ammonium oxidation by DAMO and Anammox microorganisms in an MBfR for the first time. In this model, DAMO archaea convert nitrate, both externally fed and/or produced by Anammox, to nitrite, with methane as the electron donor. Anammox and DAMO bacteria jointly remove the nitrite fed/produced, with ammonium and methane as the electron donor, respectively. The model is successfully calibrated and validated using the long-term (over 400 days) dynamic experimental data from the MBfR, as well as two independent batch tests at different operational stages of the MBfR. The model satisfactorily describes the methane oxidation and nitrogen conversion data from the system. Modeling results show the concentration gradients of methane and nitrogen would cause stratification of the biofilm, where Anammox bacteria mainly grow in the biofilm layer close to the bulk liquid and DAMO organisms attach close to the membrane surface. The low surface methane loadings result in a low fraction of DAMO microorganisms, but the high surface methane loadings would lead to overgrowth of DAMO bacteria, which would compete with Anammox for nitrite and decrease the fraction of Anammox bacteria. The results suggest an optimal methane supply under the given condition should be applied not only to benefit the nitrogen removal but also to avoid potential methane emissions.
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recently, Haroon et al.5 discovered a novel archaeon, Candidatus Methanoperedens nitroreducens, which is able to reduce nitrate to nitrite using electrons derived from methane oxidation via reverse methanogenesis. The anaerobic ammonium oxidation (Anammox) bacteria are capable of autotrophic oxidation of ammonium to N2 with nitrite as the electron acceptor in the absence of molecular oxygen.9 The Anammox process is an energy-efficient and economical nitrogen removal process, as it does not require organic carbon or aeration, and produces much less sludge in comparison to conventional processes. However, this process does not remove nitrate present in the wastewater, and in fact converts part (20%) of the nitrite to nitrate.10,11 Thus, highlevel nitrogen removal may not be achieved although the produced nitrate load would be relatively small compared to the overall nitrogen load to the plant. Heterotrophic denitrification could potentially reduce the nitrate, yet an external carbon source would be required and the plant footprint would increase. The discovery of DAMO microorganisms provides a
INTRODUCTION The discovery of denitrifying anaerobic methane oxidation (DAMO) process, in which methane is oxidized anaerobically to provide electrons for denitrification,1−5 forms an important link between two major global nutrient cycles, i.e., the carbon and nitrogen cycles.2,6 It not only stimulates the appreciation of the ecological significance of DAMO microorganisms, but also opens some avenues to develop more sustainable wastewater treatment processes,7 e.g., achieving high levels of nitrogen removal from wastewater with a minimized carbon footprint through using methane as the electron donor. Methane is an inexpensive, widely available carbon source as compared to other electron donors such as methanol and ethanol. It could be generated onsite at a wastewater treatment plant (WWTP) through anaerobic sludge digestion.8 To date, the reported DAMO microorganisms in literature include a bacterial group affiliated to the candidate division NC10 and an archaeal group distantly related to anaerobic methanotrophic archaea (ANME).2,3,5 Ettwig et al.3 identified a DAMO bacterium, Candidatus Methylomirabilis oxyfera, and confirmed that it can first reduce nitrite to nitric oxide and then convert nitric oxide to nitrogen and oxygen as diatomics. The oxygen generated is then further used for methane oxidation via the canonical aerobic pathway by the same organism. More © 2014 American Chemical Society
Received: Revised: Accepted: Published: 9540
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potential solution to the problem. Indeed, nitrogen removal from wastewater using cocultures of Anammox and DAMO microorganisms has been proposed recently.12,13 The combination of DAMO and Anammox processes has been demonstrated to simultaneously remove nitrite, nitrate, and ammonium, with N2 as the sole end product. Because both DAMO14 and Anammox microorganisms15 are known to grow slowly, the development of proper reactor configurations is critical to sustain or support sufficient biomass. Biofilms can retain microorganisms with very slow growth kinetics, and biomass can be naturally accumulated in the biofilm at different depths. Compared to the conventional biofilm reactors, the membrane biofilm reactor (MBfR) is particularly suitable,16−19 due to its counter-diffusion design of gas and liquid substrates.20,21 Recently, Shi et al.13 successfully developed a coculture of DAMO and Anammox microorganisms in a lab-scale MBfR system, which was fed with methane from inside of membranes and with ammonium and nitrate provided in the liquid outside the membranes. By growing microorganisms directly on the membranes, counter diffusing fluxes of methane and nitrogen and other nutrients are generated, resulting in controlled redox stratification of the biofilm. Mathematical models are widely applied to predict nitrogen removal during wastewater treatment.22 They provide a powerful tool for gaining an in-depth understanding of the processes and also strongly support the design and optimization of biological treatment systems. Although the coculture of Anammox and DAMO microorganisms has been successfully established, little effort has been dedicated to modeling this coupling system, particularly the DAMO processes. Raghoebarsing et al.2 and Lopes et al.23 reported the methane affinity constant for DAMO bacteria to be around 0.6 μM, however, they did not model the DAMO archaea and DAMO bacteria processes separately. He et al.24 proposed a kinetic model for DAMO bacteria. However, the full verification of the model with experimental data was still missing and DAMO archaea have not been modeled. The objective of this work was to develop a mathematical model to describe the simultaneous anaerobic methane and ammonium oxidation with the coculture of DAMO and Anammox microorganisms in an MBfR. The established model was calibrated and validated using the experimental data from both the long-term operation (over 400 days) and the batch tests at different operational stages of the MBfR. It is expected that the developed model would provide support for further development of a more efficient nitrogen removal process driven by the coculture of DAMO and Anammox microorganisms.
Figure 1. Concept of a membrane biofilm reactor with the coculture of DAMO archaea, DAMO bacteria, and Anammox bacteria. (A) Biofilm grown on a gas-permeable membrane with counter diffusing fluxes of methane and substrates; and (B) microbial interactions and the biochemical reactions in the biofilm.
electron donor (eq 1). DAMO and Anammox bacteria jointly removed the nitrite produced, with methane and ammonium as the electron donors, respectively (eqs 2 and 3). Hence, these three biochemical reactions (as described in eqs 1, 2, and 3) are considered in the biological model to describe the simultaneous consumption of methane, ammonium, and nitrate in the MBfR system (Figure 1B). Nitrate reduction to nitrite by DAMO archaea:5 NO3− + 2/8CH4 → NO2− + 2/8CO2 + 4/8H 2O
Nitrite reduction to N2 by DAMO bacteria:
(1)
2
NO2− + 3/8CH4 + H+ → 1/2N2 + 3/8CO2 + 10/8H 2O
(2)
Nitrite reduction with ammonium oxidation by Anammox:25 NO2− + 1/1.32NH4 + → 1.02/1.32N2 + 0.26/1.32NO3− (3)
A multispecies and multisubstrate one-dimensional biofilm model was then constructed through employing the software AQUASIM 2.1d26 to simulate the bioconversion processes and microbial community structure for simultaneous anaerobic methane and ammonium oxidation in the MBfR. The biochemical reaction model contains terms describing the consumption and/or production of methane, ammonium, nitrate, and nitrite among DAMO archaea, DAMO bacteria, and Anammox bacteria in the biofilm. The model describes the relationships among five soluble species, i.e., ammonium (SNH4), nitrate (SNO3), nitrite (SNO2), methane (SCH4), and nitrogen (SN2), and four particulate species, i.e., DAMO archaea (XDa), DAMO bacteria (XDb), Anammox bacteria (XAn), and inert biomass (XI). For all the microorganisms, both growth and endogenous respiration processes are included in the model. Kinetic control of all the enzymatic reaction rates is described by the Michaelis−Menten equation. The rate of each reaction is modeled by an explicit function of the concentrations of all substrates involved in the reaction. The effects of pH on biological reactions are not considered currently in the model since the pH was controlled at a constant level (around 7.0) in the experimental MBfR of this work. The inhibition of free nitrous acid (FNA) and free ammonia (FA) on microorganisms is also not included due to the fact that nitrite accumulation was not observed during the whole MBfR operation and that the FA level (0.25−0.90 mg N L−1) in
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MATERIALS AND METHODS Model Development. In the MBfR enriched with coculture of DAMO and Anammox microorganisms, methane was supplied through gas-permeable membranes, which also served as biofilm support, to the base of the biofilm, while other substrates, namely ammonium and nitrate, were supplied from the bulk liquid phase, as shown in Figure 1A. Figure 1B illustrates the main biochemical reactions and the potential interactions among microorganisms in the biofilm. Based on the known metabolisms of DAMO and Anammox microorganisms, mass balance, and isotope analysis, Shi et al.13 revealed that DAMO archaea converted nitrate, both externally fed and produced by Anammox, to nitrite, with methane as the 9541
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then operated as an SBR. The cycle length was set to 1 day, and 150 mL of fresh medium was fed within 5 min at the beginning of each cycle, thus resulting in a hydraulic retention time (HRT) of 3 days. During the whole SBR phase, the inner hollow fiber membranes were connected to the gas cylinder at all times in order to maintain the pressure at 1.3 atm. With the improvement of the MBfR performance, the feeding ammonium and nitrate concentrations were stepwise increased from the initial level of 200 to 300 mg N L−1 and 600 mg N L−1, respectively, during the over 400-day SBR operation. The change of biomass activity during the SBR operation was regularly determined using SBR cycle profiles. Fluorescent in situ hybridization (FISH) results demonstrated the coexistence and joint dominance of DAMO archaea, DAMO bacteria, and Anammox bacteria in the biofilm during the SBR operation.13 At the end of each SBR cycle, nitrate, nitrite, and ammonium concentrations in the effluent were measured using a Lachat QuikChem8000 Flow Injection Analyzer (Lachat Instrument, Milwaukee, WI). Two Batch Tests, A and B, were conducted at different operational stages during the SBR operation of the MBfR (on Days 130 and 440 of the SBR operation), as indicated in the two shadow regions in Figure 2. To measure the consumption of methane, the MBfR was disconnected from the gas cylinder to stop methane supply. Then both the interior of the hollow fiber membranes and the liquid phase of the MBfR were connected to the overflow bottle and filled with methanesaturated liquid medium. The auto-overflow point was locked
the reactor was much lower than the inhibitory level previously reported in literature.27 Continuity checking was performed according to Hauduc et al., to ensure correct mass balance of all components.28 The stoichiometrics and kinetics of the developed model are summarized in Table S1 in the Supporting Information (SI). Table S2 in the SI lists the definitions, values, units, and sources of all parameters used in the developed model. The MBfR is modeled to consist of two different compartments, i.e., a completely mixed gas compartment, representing the membrane lumen operated as flowthrough, and a biofilm compartment, containing the biofilm and bulk liquid. The specifications and the influent conditions in the model are set according to the experimental conditions. The gas compartment is connected to the base of the biofilm through a diffusive link. The biofilm compartment is modeled according to Ni et al.29 More details of the biofilm model can be found in the SI. The gaseous concentration of methane in the gas compartment is determined by the gas flow rates together with the applied gas pressure. The flux of methane (LCH4) from the gas to the biofilm matrix compartment through the membrane is modeled using the following equation according to Ni et al.30 ⎞ ⎛ SCH ,g 4 LCH4 = k CH4⎜⎜ − SCH4⎟⎟ ⎠ ⎝ HCH4
(4)
where SCH4,g and SCH4 are the concentrations of methane in the gas and biofilm matrix compartments (g COD m−3), respectively, kCH4 is the overall mass transfer coefficient of methane (m h−1), which was determined according to PellicerNacher et al.,31 and HCH4 is the Henry coefficient for methane (mol CH4 m−3 gas/mol CH4 m−3 liquid). Experimental Data for Model Evaluation. Experimental data from the MBfR coupling Anammox and DAMO processes previously reported in Shi et al.13 are used to calibrate and validate the developed model. The MBfR had a total volume of 1150 mL including 400 mL of hollow fiber membranes, 300 mL of interior space for gas supply, and 450 mL of external space outside the membranes for completely mixed liquid (see Figure S1 in the SI). The total surface area of the membrane was 1 m2. The interior of the hollow fiber membranes was connected to a feeding gas cylinder. The liquid was continuously recirculated through an overflow bottle (150 mL liquid volume and 180 mL as headspace), which also had a liquid sampling point on the bottom and a gas sampling point on the top. More information on the reactor setup can be found in the SI. The temperature and pH were monitored and controlled at 22 ± 2 °C and 7.0 ± 0.2, respectively. The MBfR was fed with a synthetic wastewater (the detailed wastewater composition was described in Shi et al.13) as substrates containing nitrate and ammonium at 200− 600 mg N L−1 and 200−300 mg N L−1, respectively. The methane flux into the MBfR was set at around 1 × 10−5 m3 h−1 through adjusting a gas regulator to control the gas pressure. During the startup phase, the MBfR was fed with concentrated nitrate (80 g N L−1) and ammonium (48 g N L−1) solutions weekly to keep both nitrate and ammonium concentrations at 200 mg N L−1 after feeding. This phase was to cultivate biofilm and enhance the activities of microorganisms. To ensure enough supply of methane, the inner membranes were manually repressurized to 1.5 atm when the pressure decreased to 1.2 atm. After 290 days, the MBfR was
Figure 2. Model calibration results based on the data from the longterm SBR operation of the MBfR (influent data, dashed line; measured effluent data, symbols; model effluent predictions, straight line). (A) NH4+ profiles; (B) NO3− profiles; and (C) dynamic removal rates of NH4+ and NO3−. Shadow region on the left indicates Batch Test A period, and shadow region on the right indicates Batch Test B period. 9542
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ammonium and nitrate concentrations gradually decreased over time, reaching the total nitrogen (TN) removal efficiency of over 85%, which was calculated as the percentage of TN (including nitrite, nitrate, and ammonium) removed from the system. In response to the enhanced TN removal activity, the influent nitrate and ammonium concentrations were stepwise increased. At each elevation of ammonium and nitrate concentrations in the influent, both effluent concentrations increased first and then decreased quickly due to the activity improvement (Figure 2A and B). The model captures all these trends well. The good agreement between model simulations and measured data in Figure 2A and B supports the capability of the developed model in describing the nitrogen conversion profiles between DAMO and Anammox microorganisms. Nitrite as the intermediate was measured to be very low (below 0.05 mg N L−1) during the operation period, which was also well predicted by the model (data not shown). The measured removal rates of ammonium and nitrate by the MBfR during the entire course of the SBR operation are shown in Figure 2C, together with the model predictions. With the increasing microbial activity and system performance, the removal rates of ammonium and nitrate gradually increased from the initial 15 and 29 mg N L−1 d−1 to the final 60 and 190 mg N L−1 d−1, respectively. These experimentally obtained TN removal rates in this proof-of-concept work are still lower than the literature-reported TN removal rates of well-established Anammox systems.7,34,35 Nevertheless, the feasibility of coupling DAMO and Anammox microorganisms in the MBfR for nitrogen removal has been demonstrated successfully. The reaction rate could be improved through future operational optimization. The model predictions match the experimental results well (Figure 2C), again supporting the validity of the developed model. In addition, the model-predicted active biomass fractions of DAMO archaea, DAMO bacteria, and Anammox bacteria in the biofilm at the end of the SBR operation of the MBfR were about 40%, 25%, and 35%, which were comparable with the FISH results by Shi. et al.13 Quantitative FISH revealed that each of these groups represented about 20−30% of the whole microbial community at the end of the SBR operation in the MBfR. Db Parameter values (YDa, YDb, μDa, μDb, KDa NO3, and KNO2) giving the optimal model fits with the experimental data are listed in Table S2 in the SI. YDa (0.071 g COD g−1 COD) and YDb (0.055 g COD g−1 COD) are of the same order of magnitude, indicating a very low biomass yield for the growth of DAMO microorganisms. The value of μDa (0.00151 h−1, resulting in the doubling time of 19 d) is also commensurate with μDb (0.0018 h−1, resulting in the doubling time of 16 d), while both are much smaller than μAn (0.003 h−1, resulting in the doubling time of 10 d), suggesting the even slower growth rates and longer doubling time for DAMO archaea and DAMO bacteria compared with those for Anammox bacteria. In particular, the parameter value of μDb in this study is consistent with that reported by He et al.24 These results suggest that the slowly growing DAMO microorganisms should grow in granules or biofilms in real applications. Granules or biofilms consist of aggregation of cells and abiotic particulates within organic polymeric matrices of microbial origin, which would achieve higher cell density in the system. The estimated values of KDa NO3 and KDb NO2 represent the affinity of DAMO archaea and DAMO bacteria to nitrate and nitrite, respectively, with lower values indicating higher affinity. The value of KDa NO3 was calibrated to −3 be 0.11 g N m−3. The difference between KDb NO2 (0.01 g N m )
so that the headspace in the overflow bottle became the only gas phase of the whole system and the consumption of methane could be measured. Batch Test A was operated in the batch mode for 30 h after a 12-h equilibrating period, while Batch Test B was operated for 3 h due to the increasing activities of microorganisms. In addition to ammonium, nitrate, and nitrite concentrations, nitrogen and methane gas partial pressures were also measured on a gas chromatograph (Shimazu, Japan). The methane and nitrogen concentrations in liquid phase were determined from the gas phase data using the Henry’s law. More details of the MBfR operation, batch tests, and analysis methods can be found in Shi et al.13 Model Calibration, Uncertainty Analysis, and Model Validation. The model includes 6 species-specific biochemical processes and 20 stoichiometric and kinetic parameters, as summarized in Tables S1 and S2 (SI). Some of these parameters are well established in previous studies (e.g., Anammox related kinetics). Thus, literature values were directly adopted for these parameters (SI Table S2). However, limited information is available in literature for the parameters related to DAMO microorganisms,23,24 e.g., YDa, YDb, μDa, μDb, KDa NO3, Db Db KDa , K , and K . Preliminary data analysis reveals that CH4 NO2 CH4 Da Db some of the parameters (e.g., KCH4 and KCH4 ) are not identifiable from the data set available. For these parameters, we adopted their values available in literature. Parameter estimation based on experimental measurements was then only carried out for six parameters, namely YDa, YDb, μDa, μDb, KDa NO3, and KDb NO2 (see SI Table S2). These six parameter values were estimated through minimizing the sum of squares of the deviations between the experimental measurements and the model predictions for the long-term dynamic operational data of effluent ammonium, nitrite, and nitrate concentrations, as well as ammonium and nitrate removal rates (see SI Table S3). Parameter estimation and parameter uncertainty evaluation were conducted according to Batstone et al.,32 with a 95% confidence level for significance testing and parameter uncertainty analysis. The standard errors and 95% confidence intervals of individual parameter estimates were calculated from the mean square fitting errors and the sensitivity of the model to the parameters. The determined F-values were used for parameter combinations and degrees of freedom in all cases. A modified version of AQUASIM 2.1d was used to determine the parameter surfaces.33 Model validation was then conducted with the calibrated model parameters using the independent experimental data sets from the two Batch Tests A and B under different substrate and operational conditions (see SI Table S3). Batch Test A was conducted on day 130 of the SBR operation of the MBfR with the initial methane, ammonium, and nitrate set at 7.59, 4.81, and 6.59 mmol, respectively. However, Batch Test B was conducted at the end of the SBR operation of the MBfR on day 440 with the initial methane, ammonium, and nitrate set at 5.16, 6.66, and 6.82 mmol, respectively.
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RESULTS Model Calibration. Figure 2A and B illustrate the dynamic profiles of ammonium and nitrate in the influent and effluent fluxes during the over 400-day long-term SBR operation of the MBfR, which were used for model calibration. The six key Db parameters, i.e., YDa, YDb, μDa, μDb, KDa NO3, and KNO2, which regulate the reactor performance and microbial composition, were estimated by fitting simulation results to the monitored data. At the beginning of the SBR operation, the effluent 9543
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−3 and KAn NO2 (0.05 g N m ) reflects a relatively higher affinity of DAMO bacteria to compete against Anammox bacteria over nitrite to achieve the ultimate N2 production under the condition of low nitrite accumulation. Parameter Identifiability. Parameter uncertainty analysis of a model structure is important as it indicates which parameter combinations can be estimated under given measurement accuracy and quantity. The obtained parameter correlation matrix during model calibration indicated most of the parameter combinations have low correlation coefficients (< 0.8), except for four of them with correlation coefficients greater than 0.8. Thus, these four parameter combinations were further analyzed to evaluate the uncertainty associated with their estimates. In the uncertainty analysis, 95% confidence regions for the different parameter combinations were investigated to evaluate their identifiability. SI Figure S2 shows all the four joint 95% confidence regions for different parameter combinations, together with the confidence intervals for all the parameters. Overall, the 95% confidence regions for all the four pairs are small, with mean values lying at the center. The 95% confidence intervals for all the individual parameters are also small, which are generally within 10% of the estimated values (SI Figure S2). These indicate that these parameters have a good level of identifiability and the estimated values are reliable. Model Validation. Model and parameters validation was based on the comparison between the model predictions using the calibrated parameter values and independent experimental data collected from both Batch Tests A and B under different initial conditions (not used for model calibration). The model and its parameters were first evaluated with the 30 h data from Batch Test A. Different from the data for calibration, experimental data for validation were obtained in the enclosed MBfR with methane and nitrogen gas measured likewise. Having experimental measurements of methane and nitrogen gas profiles in this batch experiment allows us to directly test the model’s ability to represent methane consumption and nitrogen conversion by DAMO microorganisms. The model predictions and the experimental results for Batch Test A are shown in Figure 3A. The validation results show that the model predictions match the measured data in terms of methane consumption, ammonium utilization, nitrate reduction, nitrite profile, and nitrogen gas production, which supports the validity of the developed model. The experimental data from Batch Test B were also used to further validate the developed model. The initial conditions for this experiment were substantially different from those in Batch Test A. Also, the initial microbial activities of Batch Test B were much higher than those in Batch Test A. The experimental and simulated results of the methane, ammonium, nitrate, nitrite, and nitrogen gas profiles are shown in Figure 3B. The good agreement between simulations and the measured results again supports the validity of the proposed model to describe the methane consumption and nitrogen conversion in the MBfR with the coexistence of DAMO archaea, DAMO bacteria, and Anammox bacteria.
Figure 3. Model validation results using experimental data (methane, ammonium, nitrate, nitrite, and nitrogen gas) from the two independent batch tests (measured data, symbols; model predictions, straight line). (A) Batch Test A; and (B) Batch Test B.
biofilm system with the coculture of DAMO and Anammox microorganisms. In this work, an integrated biofilm model considering the coculture of DAMO and Anammox microorganisms in an MBfR was constructed for the first time based on the known metabolisms of DAMO and Anammox microorganisms. In this model, DAMO archaea converted nitrate, both externally fed and produced by Anammox, to nitrite, with methane as the electron donor. Anammox and DAMO bacteria jointly removed the nitrite produced, with ammonium and methane as the electron donor, respectively. Both anabolic and catabolic processes of DAMO archaea, DAMO bacteria, and Anammox bacteria were incorporated in the model with the enzymatic reaction rates described by the Michaelis−Menten equation. Two stoichiometric parameters (YDa and YDb) and four kinetic parameters (μDa, μDb, KDa NO3, and KDb NO2) describing DAMO archaea and bacteria were estimated from long-term (over 400 days) experimental data. The set of best-fit parameter values is shown in SI Table S2. The parameter values obtained were generally robust in their ability to predict ammonium and nitrate dynamics in the long term operation of the MBfR and the retrieved parameters appear realistic. The uncertainty analysis confirmed that these parameters could be reliably estimated. However, the kinetic parameter values might be environment-specific,22 and their applicability to other systems remains to be further verified. The validity of this developed model was confirmed by independent batch experimental data with the biofilm coculture of DAMO and Anammox microorganisms. The successful application of the model to the MBfR system in this work indicates that the model is applicable to the coupling DAMO and Anammox biofilm system. In addition, the model can be integrated with other wastewater treatment models such as the well-accepted activated sludge models (e.g., ASM1) and the anaerobic digestion models (e.g., ADM1).37 The integrated model can be used to describe plant-wide performance rather than parts of the plant. In fact, the model is developed by employing kinetic approaches and nomenclature similar to the
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DISCUSSION Recent and increasing interests in DAMO processes have focused on the characterization of microbial community,2,3,5 the enrichment of specific microorganisms,1,14,36 and the potential application extension.12,13 However, so far limited efforts have been dedicated to modeling DAMO processes,24 especially the 9544
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could guide the operation and control of the partial nitritation process (e.g., through control of aeration). In this work, the MBfR performance and microbial community with different ratios of nitrite to ammonium produced by partial nitritation were then investigated using the developed model. The MBfR was set to have continuous feeding with ammonium kept at 300 mg N L−1 while the nitrite concentration was adjusted from 100 to 600 mg N L−1. This would form different influent NO2−/ NH4+ ratios mimicking the different wastewater compositions produced from partial nitritation and the typical nitrogen concentration of anaerobic digestion liquor. The system was run to reach steady state. Figure 4A shows the effects of NO2−/
ASM-type models. This makes the integration a relatively straightforward process. It should be noted that the potential existence of heterotrophic bacteria was not considered in the current model. This is acceptable due to the fact that the MBfR was operated under the condition without external organic carbon supply, which largely limited the growth of heterotrophs. Heterotrophs may also grow using biomass decay products or soluble microbial products. The FISH results reported in Shi et al.13 for the MBfR confirmed that heterotrophic growth on these sources was small (∼ 10%) compared to the dominating DAMO and Anammox microorganisms (∼ 90%). However, the heterotrophic processes could be easily incorporated into the model, if heterotrophic activity would be included in the system. Also, the inhibition of nitrite or FA on microorganisms was not included in the developed model due to the extremely low nitrite accumulation (lower than 0.05 mg N L−1) and very low FA level (0.25−0.90 mg N L−1) observed during the MBfR operation. Lotti et al.38 reported nitrite inhibition on Anammox process at a high nitrite level, which confirmed the negligible effect of nitrite inhibition in this work. However, the developed model can be modified to include these inhibitory effects if necessary in future applications. For example, FA and nitrite have been reported to be the two main inhibitors for Anammox,39 which could be included in the kinetic rate expression of the Anammox process (Process 5 in SI Table S1) by incorporating the corresponding inhibitory terms for FA and/or nitrite. Additionally, the methane affinity constants for Db DAMO microorganisms (KDa CH4 and KCH4) were not calibrated in this work due to the excessive methane supply during the MBfR operation, although the adopted values from literature were able to describe the experimental data well. These values may warrant further evaluation. Shi et al.13 have demonstrated experimentally that it is feasible to grow a coculture of DAMO and Anammox microorganisms in an MBfR to achieve simultaneous ammonium and methane oxidation, which has a significant potential for wastewater treatment. For instance, the Anammox process is being widely used for full-scale nitrogen removal from anaerobic digestion liquor,35,40 which has a high ammonium content (500−1500 mg N L−1) and an unfavorable carbon to nitrogen ratio for conventional nitrification and denitrification treatment. One alternative treatment option could be the partial nitritation (ammonium oxidation to nitrite40,41) followed by the coculture system of Anammox and DAMO (i.e., the MBfR) with complete nitrogen removal. The partial nitritation would produce a mixture of ammonium and nitrite as the feed for the MBfR. Methane can be produced through anaerobic digestion on the treatment plant, and a small fraction could be fed to the Anammox reactor using hollow fiber membrane to support the growth of DAMO microorganisms, thus achieving the removal of nitrate produced by Anammox. This represents a significant advantage over Anammox-only process. Mathematical modeling of this system is an important step toward the development of this process into a practical technology. The model proposed in this work is expected to enhance our ability to predict DAMO processes and can serve as a tool to explore the effects of operational conditions on DAMO systems and optimize this innovative technology for wastewater treatment. For a nitrogen removal system with partial nitritation followed by the coculture system of Anammox and DAMO, the optimal NO2−/NH4+ ratio required for the MBfR feed
Figure 4. Effects of NO2−/NH4+ ratio in the influent on (A) TN removal efficiency and (B) active biomass fraction in the biofilm; and effects of CH4 loading on (C) TN removal efficiency and (D) active biomass fraction in the biofilm.
NH4+ ratio on the TN removal efficiency at steady state. With the increase of NO2−/NH4+ ratio, the TN removal efficiency increased from about 40% to the maximum of 95% at the NO2−/NH4+ ratio of 1 and then decreased to 70%. The relationship between the microbial community fractions and the NO2−/NH4+ ratio is shown in Figure 4B. Anammox bacteria increased with the increasing NO2−/NH4+ ratio. At the NO2−/NH4+ ratio of 1 with the highest TN removal, DAMO archaea and DAMO bacteria would coexist with Anammox bacteria dominating the microbial community in the biofilm, where the active biomass fractions would be 12%, 23%, and 65% for DAMO bacteria, DAMO archaea, and Anammox bacteria, respectively. Further increase of the NO2−/NH4+ ratio would remove the DAMO archaea due to the competition between DAMO archaea and DAMO bacteria over methane, resulting in the decreasing TN removal. Figure 5A shows the simulated microbial population distribution within the biofilm depth at the NO2−/NH4+ ratio of 1. Anammox organism abundance was 85% at the biofilm surface and decreased gradually to 40% at the base of biofilm. The abundance of DAMO organisms was lower than 15% at the biofilm surface and increased to 60% at the base of biofilm. The concentration gradients of methane, ammonium, nitrite, and nitrate would cause stratification of the biofilm in the MBfR, where Anammox bacteria mainly grow in the biofilm layer close to the bulk liquid phase where ammonium and nitrite are available, while DAMO archaea and DAMO bacteria attach close to the membrane surface (biofilm base), where methane, nitrite, and nitrate are 9545
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research should be dedicated to fully analyzing the economic and environmental benefits of the technology.
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ASSOCIATED CONTENT
S Supporting Information *
Additional methods, tables, and figures as mentioned in the text. This material is available free of charge via the Internet at http://pubs.acs.org/
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AUTHOR INFORMATION
Corresponding Author
Figure 5. Microbial community structure in the biofilm of the MBfR with the coculture of DAMO and Anammox microorganisms. (A) Modeling results of population distribution in the biofilm at the NO2−/NH4+ ratio of 1 (point 0 on the x-axis indicates the membrane surface); and (B) schematic representation of the stratification of community structure and interactions as well as the biochemical reactions in the biofilm.
*Phone: +61 7 3346 3230; fax: +61 7 3365 4726; e-mail: b.ni@ uq.edu.au. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by the Australian Research Council (ARC) through Projects DP130103147 and DP120100163. X.C. acknowledges the scholarship support from China Scholarship Council (CSC). B.-J.N. acknowledges the support of ARC Discovery Early Career Researcher Award (DE130100451). J.G. acknowledges the support of ARC Discovery Early Career Researcher Award (DE130101401) and NSFC (51208009).
available with the last produced by Anammox (Figure 5B). These results indicate the significant role of the coexistence of DAMO microorganisms and Anammox bacteria on the system performance in the MBfR. Thus, a proper NO2−/NH4+ ratio from the prepartial nitritation should be controlled (e.g., a NO2−/NH4+ ratio of 1) to optimize the nitrogen removal through achieving suitable relative abundance of DAMO archaea, DAMO bacteria, and Anammox bacteria in the MBfR. Methane as the electron donor for both DAMO archaea and bacteria plays an important role in regulating microbial community structure in the biofilm, thus affecting the operational performance. Furthermore, the high continuous methane supply and its slow consumption by the DAMO microorganisms resulted in the accumulation of methane in the bulk liquid, and its loss in the effluent. The potentially excessive methane supply not only wastes valuable energy but also increases the safety risks and greenhouse gas emission. The methane supply can be controlled through adjusting gas pressure. The gas pressure can be measured using a gas regulator. The optimization of methane supply to the MBfR through pressure (surface methane loading) can also be achieved with the developed model. The dependency of TN removal efficiency and microbial community on the surface methane loading at the NO2−/NH4+ ratio of 1 (the influent feedings of nitrite and ammonium were both set at 300 mg N L−1) are illustrated in Figure 4C and D. The TN removal efficiency would first increase with the increasing surface methane loadings and then decrease slightly, with the maximum removal efficiency over 99%. The low surface methane loadings resulted in a lower availability of methane and the lower fractions of DAMO microorganisms (both archaea and bacteria), but the high surface methane loadings would lead to overgrowth of DAMO microorganisms with high fractions of DAMO bacteria, which would compete with Anammox for nitrite and decrease the fractions of Anammox bacteria (Figure 4D). These modeling results indicate that an optimal methane supply under the given condition should be applied not only to benefit the TN removal but also to avoid the potential methane emissions. Methane-driven denitrification would save the external carbon source; however, methane consumption would decrease its availability for power production. Therefore, a proper methane supply is of great importance to achieving complete nitrogen removal with the lowest methane consumption, as partially demonstrated in Figure 4. Further
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