B. M. Wagner 1, G. T. Daigger 1*, and N. G. Love 1
1 Department of Civil and Environmental Engineering, University of Michigan, 1351 Beal Avenue, Ann Arbor, MI 48109, USA
* Corresponding author: [gdaigger@umich.edu]
Summary: Partial nitritation anammox (PNA) membrane aerated biofilm reactors (MABR) have the potential to be employed in mainstream wastewater treatment and can drastically decrease the energy and carbon requirements for nitrogen removal. Previous PNA MABR studies have looked at 1-stage systems, but no study has holistically compared the performance of different MABR configurations. In this study, a PNA MABR was mechanistically modelled to determine the impact of the reactor configuration (1-stage, hybrid, or 2-stage system) on the location of the preferred niche for anammox bacteria and the overall nitrogen removal performance. Results from this study show that the 2-stage configuration which used an MABR with a thin biofilm for nitritation and a moving bed biofilm reactor for anammox had a 20% larger nitrogen removal rate than the 1-stage or hybrid configurations. This suggests that an MABR should focus on maximizing nitrite production with anammox implemented in a second-stage biofilm reactor to achieve the most cost-effective nitrogen removal.
Keywords: Membrane Aerated Biofilm Reactor; Partial Nitritation Anammox; Mainstream Nitrogen Removal Introduction
Partial nitritation anammox (PNA) membrane aerated biofilm reactors (MABR) have the potential to be employed in mainstream wastewater treatment and can drastically decrease the energy and carbon requirements for nitrogen removal.1 PNA MABR research has thus far primarily focused on 1-stage (MABR only) configurations rather than hybrid (MABR + suspended growth) or 2-stage (MABR + traditional biofilm reactor) configurations, shown in Figure 1.1–3 The main benefit of 1-stage reactors is that partial nitritation and anammox can be performed within one biofilm. However, 1-stage PNA MABRs have several disadvantages. They require a thicker biofilm and a low oxygen loading to create separate redox zones for ammonia oxidizing organisms (AOO) and anammox bacteria (AMX) which would increase diffusion limitations in mainstream wastewaters.4 In addition, oxygen sensitive AMX may become inhibited, as the membrane air loadings are adjusted to dynamic wastewater flows. Given these drawbacks, it is advantageous to explore other alternative reactor configurations that are less susceptible to diffusion resistance and AMX inhibition.
Alternatively, the PNA performance may increase in a hybrid or 2-stage configuration by decreasing the biofilm thickness in the MABR and by separating the aerobic microorganisms from AMX. A thinner, nitrite producing biofilm populated with AOO would be less susceptible to ammonia diffusion limitations in mainstream conditions than a thicker PNA biofilm since the low ammonia concentration commonly found in mainstream wastewaters would have to diffuse a shorter distance to reach AOOs located at the membrane-biofilm interface. Oxygen sensitive AMX would also be located further away from the oxygen source at the base of the membrane and would be less susceptible to inhibition. In the hybrid and 2-stage configurations, it may be possible to increase the membrane air loading in the MABR which could produce more nitrite and reduce the required MABR surface area without inhibiting AMX. In addition since the biofilm is less susceptible to ammonia diffusion limitations, the ammonia concentration is larger at the membrane-biofilm interface, and the biofilm may be able to continue suppressing NOB at this increased membrane
124 oxygen loading. Thus, it is important to compare the performance of 1-stage, hybrid, and 2-stage reactors.
The purpose of this study is to use mechanistic modeling to identify the preferred niche for AMX and to quantify the performance tradeoffs between 1-stage, hybrid, and 2-stage PNA MABRs under different influent and operating conditions. The impact of increasing the membrane oxygen loading on NOB suppression and AMX inhibition in the hybrid and 2-stage configurations are presented.
The most favorable configurations that could be considered for experimental evaluation are identified, and the ability to reach low effluent total nitrogen concentrations is analyzed.
Materials and methods
Using SUMO wastewater modeling software,5 an MABR model was created to treat mainstream wastewater with an ammonia concentration of 50 mg N/L and an influent nitrogen flux of 4.1 g N/m2-day. The biofilm was modeled as five layers with a variable thickness and specific mass, and the boundary layer thickness was 100µm. All other kinetic and state variables were SUMO default values. The configurations that were tested are shown in the diagram in Figure 1 and included (1) a 1-stage MABR with AMX and AOO located in a thicker 700µm biofilm, (2) a 1-stage Hybrid system with AOO located in a thinner 300µm biofilm in the MABR and AMX located in the suspended biomass at a suspended solids retention time (SRT) of 15 days, and (3) a 2-stage system with AOO located in a thinner 300µm biofilm in the MABR and AMX located in a second stage moving bed bioreactor (MBBR). The configurations were simulated with different SRTs, carbon to nitrogen ratios, membrane air loadings, membrane surface areas, and MBBR biofilm surface areas (for the 2-stage configuration only) to determine their performance under a wide range of conditions.
Figure 1: Overview of PNA MABR configurations that highlight the differences in the location of AMX. The color of the metabolisms under each diagram corresponds to the location where each metabolism occurs with red corresponding to the biofilm in the MABR, blue corresponding to the bulk liquid, and purple corresponding to a biofilm in a 2nd stage moving bed biofilm reactor (MBBR).
125 Results and discussion
Simulations comparing the nitrogen removal performance across the different configurations have led to several findings regarding the optimal configuration of PNA MABRs and the ideal niche for AMX. First, AMX was most abundant in the outer portion of the thick biofilm in the MABR, in the suspended growth at a suspended SRT of 15 days, and in the biofilm in the MBBR for the 1-stage, hybrid, and 2-stage configurations, respectively. Because AMX was located outside the MABR’s biofilm in the hybrid and 2-stage configurations, the air flux to the membrane of the MABR was able to be increased from 4.1 to 11.5 g O2/m2-d when compared to the 1-stage configuration without increasing nitrate production and without inhibiting AMX. This increase in the membrane air loading rate resulted in a 180% increase in the nitrite production rate and an increase in the mass of AOOs by a factor of 2.6 from the 1-stage to the hybrid and 2-stage configurations, shown in Figure 2. The 1-stage configuration did have almost 1.8 times more AMX mass than the hybrid and 2-stage configurations, but it was not able to capitalize on this since it was nitritation (AOO and oxygen) limited.
Figure 2: Biomass comparison for the different MABR configurations. Arrows highlight the key differences in the AOO mass between the configurations.
Second, the 2-stage configuration achieved up to a 20% increase in performance when compared to the 1-stage and hybrid configurations, seen in Figure 3. This increase in performance was enabled by the increased nitritation rate from the thinner MABR biofilm that was mentioned previously.
Since the AMX was not located in the biofilm in the MABR, the increase in oxygen flux did not induce oxygen inhibition for AMX, and the biofilm was less affected by diffusion resistance.
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Figure 3: Comparison of TIN removal rates for the different MABR configurations for variable C:N ratios. The removal rates are for all biomass present in the system, not just the MABR.
Finally by increasing the biofilm surface area in the MBBR, membrane air loading, and membrane surface area, the 2-stage configuration was able to achieve an effluent TIN concentration that was approximately 56% lower than the minimum TIN concentration achieved with the 1-stage configuration (data not shown). The 2-stage configuration also had a wider operational range which could result in easier transitions to variable loadings and improved performance (data not shown).
Conclusions:
All PNA MABR configurations can be implemented in mainstream nitrogen removal, but the 1-stage configuration required a lower air loading to prevent oxygen inhibition of AMX, produced less nitrite, and was less efficient than the MABRs found in the hybrid and 2-stage configurations. AMX were most abundant in the bulk liquid and in the MBBR for the hybrid and 2-stage configurations respectively, which decreased AMX’s potential to be inhibited by oxygen. The oxygen flux in the MABR could therefore be increased, resulting in better TIN removal rates. As a result of the increased oxygen flux, fewer MABR modules would be needed in the hybrid and 2-stage configurations to produce the same quantity of nitrite as the MABR modules in the 1-stage. In full-scale treatment, it will likely be more cost-effective to use less expensive MBBR media for anaerobic metabolisms (anammox and heterotrophic denitrification) and to focus on maximizing oxidation rates with the more costly MABRs. Experimental validation will be important to confirm these modeling results and will help increase the likelihood of PNA MABR adoption in mainstream nitrogen removal.
References
1. Bunse, P., Orschler, L., Agrawal, S. & Lackner, S. Membrane aerated biofilm reactors for mainstream partial nitritation/anammox: Experiences using real municipal wastewater. Water Res. X 9, 100066 (2020).
2. Gilmore, K. R., Terada, A., Smets, B. F., Love, N. G. & Garland, J. L. Autotrophic Nitrogen Removal in a Membrane-Aerated Biofilm Reactor Under Continuous Aeration: A Demonstration. Environ. Eng. Sci.30, 38–45 (2013).
3. Pellicer-Nàcher, C. et al. Sequential aeration of membrane-aerated biofilm reactors for high-rate autotrophic nitrogen removal: Experimental demonstration. Environ. Sci. Technol. 44, 7628–7634 (2010).
4. Terada, A., Lackner, S., Tsuneda, S. & Smets, B. F. Redox-stratification controlled biofilm (ReSCoBi) for completely autotrophic nitrogen removal: The effect of co- versus counter-diffusion on reactor performance.
Biotechnol. Bioeng. 97, 40–51 (2007).
5. Dynamita. SUMO 19.2. Sigale, France http://www.dynamita.com/ (2019).
Presenting Author
Mr. Brett Wagner
Department of Civil and Environmental Engineering, University of Michigan, 1351 Beal Avenue, Ann Arbor, MI 48109, USA
Is the presenting author an IWA Young Water Professional? Yes
Bio: Brett Wagner is a PhD student studying environmental engineering at the University of Michigan who is co-advised by Dr. Glen Daigger and Dr. Nancy Love. He received his Master's degree in environmental engineering at the University of Michigan in 2017 and his Bachelor's degree in civil engineering with an environmental emphasis from the University of Kansas in 2016. His research is focused on membrane aerated biofilm reactors (MABR) and partial-nitritation anammox biofilms.
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