Domingo-Felez, Carlos

Smets, Barth F.

A pseudo-mechanistic model describing three biological nitric oxide (NO) and nitrous oxide (N2O) production pathways was calibrated for an activated sludge mixed culture biomass treating municipal wastewater with laboratory-scale experiments. The model (NDHA) comprehensively describes N2O producing pathways by both autotrophic ammonium oxidizing and heterotrophic bacteria. Extant respirometric assays and anaerobic batch experiments were designed to calibrate the endogenous, heterotrophic denitrification and autotrophic ammonium/nitrite oxidation processes together with the associated net N2O production. Ten parameters describing heterotrophic processes and seven for autotrophic processes were estimated accurately (variance/mean &lt; 25%). The model predicted the N2O and NO dynamics at varying dissolved oxygen, ammonium and nitrite levels and was validated with a different set of batch experiments with the same biomass. Aerobic ammonium oxidation experiments at two oxygen levels used for model evaluation (2 and 0.5 mg/L) indicated that the nitrifier denitrification (42, 64%) and heterotrophic denitrification (7, 17%) pathways increased and dominated the total N2O production at high nitrite and low oxygen concentrations; while the nitrifier nitrification pathway showed the largest contribution at high dissolved oxygen levels (51, 19%). The uncertainty of the biological parameter estimates was propagated to N2O model outputs via Monte Carlo simulations as 95% confidence intervals. The accuracy of the estimated parameters corresponded to a low uncertainty of the N2O emission factors (4.6 ± 0.6% and 1.2 ± 0.1%)

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Downloaded from orbit.dtu.dk on: Apr 10, 2018Application of the NDHA model to describe N2O dynamics in activated sludge mixedculture biomassDomingo-Felez, Carlos; Smets, Barth F.Publication date:2018Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Domingo-Felez, C., & Smets, B. F. (2018). Application of the NDHA model to describe N2O dynamics inactivated sludge mixed culture biomass. Abstract from 6th IWA/WEF Water Resource Recovery ModellingSeminar (WRRmod 2018), Quebec, Canada. Domingo-Félez and Smets 1 Application of the NDHA model to describe N2O dynamics in activated sludge mixed culture biomass.   Carlos Domingo-Félez1 and Barth F. Smets1      1Department of Environmental Engineering, Technical University of Denmark, Miljøvej 115, 2800 Kgs. Lyngby, Denmark (Email: cadf@env.dtu.dk; bfsm@env.dtu.dk)  Abstract A pseudo-mechanistic model describing three biological nitric oxide (NO) and nitrous oxide (N2O)  production pathways was calibrated for an activated sludge mixed culture biomass treating municipal wastewater with laboratory-scale experiments.  The model (NDHA) comprehensively describes N2O producing pathways by both autotrophic ammonium oxidizing and heterotrophic bacteria. Extant respirometric assays and anaerobic batch experiments were designed to calibrate the endogenous, heterotrophic denitrification and autotrophic ammonium/nitrite oxidation processes together with the associated net N2O  production. Ten parameters describing heterotrophic processes and seven for autotrophic processes were estimated accurately (variance/mean < 25%). The model predicted the N2O and NO dynamics at varying dissolved oxygen, ammonium and nitrite levels and was validated with a different set of batch experiments with the same biomass. Aerobic ammonium oxidation experiments at two oxygen levels used for model evaluation (2 and 0.5 mg/L) indicated that the nitrifier denitrification (42, 64%) and heterotrophic denitrification (7, 17%) pathways increased and dominated the total N2O production at high nitrite and low oxygen concentrations; while the nitrifier nitrification pathway showed the largest contribution at high dissolved oxygen levels (51, 19%). The uncertainty of the biological parameter estimates was propagated to N2O model outputs via Monte Carlo simulations as 95% confidence intervals. The accuracy of the estimated parameters corresponded to a low uncertainty of the N2O emission factors (4.6 ± 0.6% and 1.2 ± 0.1%). Keywords Modelling; ASM; Nitrous oxide, Uncertainty, Activated sludge   INTRODUCTION AND OBJECTIVES N2O is a greenhouse gas emitted in wastewater treatment plants. In this study we aim to: (a) quantify N2O dynamics from mixed liquor biomass via extant respirometric assays, (b) calibrate the NDHA model to describe N-removing processes and N2O production and assess the accuracy of estimated parameters, (c) evaluate the predictive ability of the calibrated model against a different mixed liquor biomass and (d) quantify the uncertainty of N2O emissions during aerobic NH4+ removal.  MATERIALS AND METHODS Model structure The NDHA was proposed as a consilient model to describe NO/N2O dynamics under a variety of conditions for biomass containing autotrophic and heterotrophic fractions (Domingo-Félez and Smets, 2016). Three biological pathways are considered: nitrifier nitrification (NN), nitrifier denitrification (ND) and heterotrophic denitrification (HD). Experimental design Respirometric approaches were taken (on-line, high-rate O2 and N2O measurements) to obtain informative data on N2O dynamics from mixed liquor biomass (Table 1). Separate batch Session xxx WRRmod2018 2 nitrification experiments were executed in a 3-L lab-scale reactor with mixed liquor biomass from the same WWTP (Domingo-Félez et al., 2017b). Table 1 – Experimental design for lab-scale respirometric assays (A-C) and for model validation (D-E). Scenario      Oxygen level      Pulses      Monitoring      Targeted ProcessesAnoxic NO3-, NO2-, N2O, COD NO3-, NO2-, N2O, NH4+ Heterotrophic denitrification, hydrolysisFrom excess DO (air-sat) into anoxia COD DO Biomass decay, hydrolysis(B) Anoxic NO3-, NO2- N2O, NO HB-driven NO/N2O dynamics(C) From excess DO (O2-sat) into anoxia NH4+, NH2OH, NO2- DO, N2O, NONH4+, NO2- removalAOB/HB-driven NO/N2O dynamics(D) Constant aeration (high and low DO) NH4+   DO, N2O, NH4+, NO2- NH4+, NO2- removal, N2O dynamics(E) Constant aeration (high and low DO) NH4+, NO2-, NO3-DO, N2O, NH4+, NO2- NH4+, NO2- removal, N2O dynamics(A) Sensitivity analysis, Parameter estimation procedure and Uncertainty analysis A global sensitivity analysis (GSA) was performed to identify the most determinant parameters on model outputs via Monte Carlo simulations using the SRC method. To test the validity of the model response the interdependency of residuals (ysim,i - yobs,i) was analysed by autocorrelation for different lag times. The uncertainty of newly estimated parameters was compared to a reference case from literature and evaluated via Monte Carlo simulations (n = 500). More information can be found elsewhere (Domingo-Félez et al., 2017a).  RESULTS  Sensitivity analysis on a nitrification/denitrification case study  Results from the GSA highlight the importance of AOB on N2O production from a mixed culture biomass during aerobic NH4+ oxidation. Up to four of the ten most sensitive parameters for N2O and NO liquid concentrations corresponded to AOB processes. Interestingly, NOB and HB were also sensitive, highlighting the importance of microbial interactions in complex communities. Biomass activity tests: example heterotrophic activity The specific denitrification rates and oxygen uptake rate were significantly higher in the presence of excess electron donor (mix of C-sources) compared to endogenous conditions: Denitrification (1.5, 2.5 and 4.7 vs. 7, 6.2 and 12 mgN/gVSS.h), C-removal (4.5-7 and 35 mgCOD/gVSS.h). The N2O reduction rate varied 3-fold in the pH range 6.5 - 9, with a maximum at around pH = 8 and lower rates towards higher and lower pH values (Figure 1). Calibration results: Heterotrophic and autotrophic N-removal Based on the overall good fit of model predictions and experimental data the NDHA model described the dynamics of the measured DO, NH4+, NO2-, NO3-, N2O and NO (R2 ≥ 0.94, F-test = 1 for 10/11 datasets). A total of 17 parameters were estimated with bounded approximate confidence regions indicating good identifiability (CV < 25%) (Figure 1, Table 2).        Figure 1 – Experimental and modelling calibration results for heterotrophic processes (A), nitrous oxide reduction dependency on pH (B), NO2- oxidation (C), NH4+ oxidation (D). Red lines: 95% confidence intervals. Model evaluation  Domingo-Félez and Smets 3 The predictive ability of the calibrated NDHA model was evaluated  on a set  of batch experiments where mixed liquor biomass from the same WWTP was subject to varying N pulses at constant aeration. The model captured the trends of DO, main N-substrates and liquid N2O without any Table 2 – Best-fit values for the parameters estimated (25 °C).  Parameter Units Value Scen. Parameter Unit Value Scen. Parameter Unit Value Scen.pHopt.nosZ ( - ) 7.9 ± 0.1 (A) KHB.S.NIR mgCOD/L 4.3 ± 0.69 (B) μNOB d-11.51 ± 0.07 (C)wnosZ ( - ) 2.2 ± 0.2 (A) KHB.S.NOR mgCOD/L 5.3 ± 0.83 (B) μHB d-17.23 ± 0.16 (A)KHB.N2O mgN/L 0.078 ± 0.020 (A) KHB.S.NOS mgCOD/L 4.1 ± 0.40 (B) εAOB ( - ) 0.0031 ± 0.0001(C)μHB.NAR d-11.71 ± 0.11 (A) KAOB.NH3 μgN/L 7.00 ± 1.17 (C) ηNIR ( - ) 0.22 ± 0.01 (C)μHB.NIR d-11.11 ± 0.07 (A) KNOB.HNO2 μgN/L 0.027 ± 0.006 (C) ηNOR ( - ) 0.36 ± 0.02 (C)μHB.NOS d-11.17 ± 0.02 (A) μAOB.AMO d-10.86 ± 0.02 (C)  parameter modification (R2avg for DO = 0.98; NH4+ = 0.99; NO2- = 0.84; N2O = 0.80). Only the N2O residuals (ysim,i - yobs,i) did not pass the F-distribution test (FN2O = 0). Higher NH4+ pulses yielded a higher N2O fraction (0.6 - 1.7 - 2.5 – 3.2% N2O/NH4+rem) as more NH4+  oxidation occurred at low DO, thus promoting the contribution of denitrification pathways (Figure 2).   Figure 2 –Model evaluation results. Effect of NO3- pulse (A). Main substrates: DO, NH4+, NO2-(left), N2O (middle) and N2O pathway contributions (right). Experimental results (markers), best-fit simulations (black lines), 95% confidence intervals (red lines) (left, middle). NN (cyan), ND (blue), and HD (black) pathway contributions. To gain more insights on the N2O emissions from mixed liquor biomass during aerobic NH4+ oxidation simulations with best-fit estimate parameters were run for a wider range of DO (0.2 – 4 mg/L) and NO2- (0 – 1.4 mgN/L) at excess NH4+. The N2O emission factor and individual pathway contributions to the total N2O pool at pseudo-steady state are shown in Figure 3.  Figure 3 – Model evaluation at varying NO2- and DO concentrations during excess NH4+ removal (pH = 7.2). From left to right: Pathway contributions to total N2O pool NN, ND, HD; N2O emission factor. Uncertainty of N2O emissions from activated sludge biomass The N2O emission factors of simulated NH4+ removal at constant DO (0.5 and 2.0 mg/L) are comparable to Wunderlin et al. (2012) at the same DO levels (3.8 and 2%, Table 3). The uncertainty of N2O model predictions was evaluated and could be used in future studies to discriminate between calibration procedures. By comparing the uncertainty of two cases (from a reference value from literature and the one obtained in this study) the uncertainty estimated with Session xxx WRRmod2018 4 this calibration procedure was only 28% of that simulated with the reference (Table 3). Here we show the impact of the uncertainty of biological parameter estimates in N2O emissions, which will significantly impact the carbon footprint of the process. Unfortunately, no other studies exist on uncertainty of N2O emissions. We believe that future comparison of best-fit simulations together with their uncertainty (e.g. 95% CI) will improve calibration procedures for N2O models. Table 3 – Nitrogen removal, N2O emission factor and N2O pathway contribution for the nitrification/denitrification case study after model calibration. The standard deviations (std) correspond to the uncertainty from estimated parameters in this study (stdt.s.), and a reference value from literature (stdinit) for 500 Monte Carlo simulations. DO ΔTN (mgN/L) N2Oemitted/removed NN stdinit stdt.s. ND stdinit stdt.s. HD stdinit stdt.s.0.5 mg/L 16.8 ± 0.1 4.6 ± 0.6% 19% 11% 2% 64% 11% 2% 17% 7% 2%2.0 mg/L 27.1 ± 0.3 1.2 ± 0.1% 51% 15% 3% 42% 14% 3% 7% 4% 2%N2Opathway_contrib   ACKNOWLEDGEMENT The research work was financed by the LaGas project (Danish Council for Strategic Research). . Dr. Ulf Jeppsson (Lund University) is acknowledged for having provided the codes of the Benchmark Simulation Model no 2 from which this work was developed.   REFERENCES Domingo-Félez, C., Calderó-Pascual, M., Sin, G., Plósz, B.G., Smets, B.F., 2017a. Calibration of the comprehensive NDHA-N 2 O dynamics model for nitrifier-enriched biomass using targeted respirometric assays. Water Res. 126, 29–39. doi:10.1016/j.watres.2017.09.013 Domingo-Félez, C., Pellicer-Nàcher, C., Petersen, M.S., Jensen, M.M., Plósz, B.G., Smets, B.F., 2017b. Heterotrophs are key contributors to nitrous oxide production in activated sludge under low C-to-N ratios during nitrification-Batch experiments and modeling. Biotechnol. Bioeng. 114, 132–140. doi:10.1002/bit.26062 Domingo-Félez, C., Smets, B.F., 2016. A consilience model to describe N 2 O production during biological N removal. Environ. Sci. Water Res. Technol. 2, 923–930. doi:10.1039/C6EW00179C Wunderlin, P., Mohn, J., Joss, A., Emmenegger, L., Siegrist, H., 2012. Mechanisms of N2O production in biological wastewater treatment under nitrifying and denitrifying conditions. Water Res. 46, 1027–37. doi:10.1016/j.watres.2011.11.080                      

Application of the NDHA model to describe N<sub>2</sub>O dynamics in activated sludge mixed culture biomass

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Application of the NDHA model to describe N<sub>2</sub>O dynamics in activated sludge mixed culture biomass

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