(117ak) Combining Ion Exchange and Deammonification for Sustainable Ammonium Removal from Mainstream Wastewater

Excess reactive nitrogen discharges severely impact surface and groundwater, resulting in environmental, economic, and public health problems. Domestic wastewater is considered a major point source of nitrogen pollution. Conventional biological nitrogen removal (BNR) processes are widely used to remove inorganic nitrogen from domestic wastewater through the processes of nitrification and denitrification. Conventional BNR processes typically require a high energy demands to provide oxygen for nitrification and high chemical demands to provide organic carbon for denitrification. Biological deammonification is a novel BNR process that combines partial nitritation (NH₄⁺ => NO₂^-) with anaerobic ammonium oxidation (anammox; NH₄⁺ + NO₂^- => N₂) (Ergas and Aponte-Morales, 2014). The combined partial nitritation/anammox (PN/A) process considerably reduces the oxygen and organic carbon requirements at full-scale wastewater treatment plants (WWTPs). Therefore, deammonification is promising as a more sustainable BNR alternative with lower energy and chemical requirements than conventional BNR (Cao et al., 2017).

Implementation of deammonification has been successful for treatment of high-ammonium strength wastewaters, such as sidestreams from anaerobic digestion process. Full-scale sidestream PN/A plants are in operation in the US, Europe, and Australia (Bailey et al., 2018). Elevated ammonium concentrations result in high free ammonia concentrations, which are suitable to inhibit competitor microbes that otherwise interfere with the PN/A process. However, typical domestic (mainstream) wastewater has a much lower ammonium (and free ammonia) content, making the PN/A process challenging to implement and control (Anthonisen et al., 1976). To date, there are no reported full-scale mainstream PN/A plants operating worldwide (Izadi et al., 2021).

Our research project has implemented a strategy of concentrating ammonium on chabazite, a natural zeolite mineral with a high ion exchange (IX) capacity and selectivity for ammonium ions. The fundamental basis of this process is to pass large volumes of mainstream wastewater through the chabazite media bed until saturation is reached. Subsequently, the mineral is bioregenerated via PN/A by microbial biofilms attached to the chabazite surface that consume ammonium. The bioregeneration process allows chabazite to be reused for many treatment cycles without having to add fresh zeolite or regenerant brines (Aponte-Morales et al., 2016, 2018).

The overall goal of the proposed project is to implement IX-PN/A process in a single-stage for ammonium removal from mainstream wastewater. Specific objectives are to: a) evaluate whether chabazite is effective in concentrating ammonium and providing the environmental conditions needed for PN/A, b) demonstrate that the microbes involved in deammonification can bioregenerate chabazite for reuse over many IX-PN/A cycles, and c) identify the diversity among the microbial community governing the IX-PN/A process.

Methodology

A 2-L sequencing batch biofilm reactor (SBBR) filled with an adsorptive medium containing chabazite was used to conduct IX-PN/A as a single process. Once the IX process reached a complete breakthrough curve (C_effluent/C_influent ≈ 100%) (Figure 1, left), the reactor was inoculated with biofilms from PN/A enriched cultures, and bioregeneration was initiated by applying recirculation to provide aeration and aid mass transfer. The steps in a typical IX-PN/A treatment cycle consisted of IX, sidestream addition, and bioregeneration (by PN/A). During IX, the IX-PN/A bioreactor was fed with mainstream wastewater (~40 mg/L ammonium as nitrogen) using variable feeding rates (22-34 mL/min) to reach a target breakpoint. Sodium and ammonium concentrations were measured using Standard Method 3500 (APHA) by Metrohm 881 Compact IC Pro Ion Chromatography. Subsequently, synthetic sidestream (~500 mg/L ammonium as nitrogen) was added to ensure a high free ammonia content in the liquid phase to favor conditions for PN/A. During bioregeneration, samples were taken periodically to measure ammonium, nitrite, and nitrate concentrations over time and determine the nitrogen removal and zeolite bioregeneration. Measurements of nitrogen species (NH₄⁺, NO₂^-, and NO₃^-) and other ion species (Na⁺, SO₄^2-, and PO₄^3-) concentrations were done by Chromatography. The pH, temperature, turbidity, and concentrations of dissolved oxygen, sulfide, and chemical oxygen demand (COD) were measured by the end of each IX-PN/A cycle following APHA Standard Methods.

Results and Discussion

Results show that, during the IX stage, chabazite successfully removed large ammonium loads from low ammonia-strength wastewater. During a typical IX cycle, ammonium was removed from mainstream wastewater until a breakpoint of C_effluent/C_influent ≈ 30-40% was achieved (Figure 1, right). During the bioregeneration stage, the ammonium concentration in the effluent typically reached <1 mg/L (as N) due to the activity of PN/A microorganisms. At the end of the bioregeneration stage, the chabazite was ready to start the next IX-PN/A treatment cycle (Figure 2). The single-stage IX-PN/A was operated for twelve treatment cycles (to March 2023). Sodium and ammonium profiles over a typical cycle are shown in Figure 3. During IX, ammonium capture was accompanied by sodium release, which slowed as the chabazite reached saturation. During bioregeneration, microbial ammonium uptake from the chabazite surface was accompanied by sodium adsorption from the liquid phase onto the surface, providing exchangeable sodium ions for the following IX-PN/A cycle.

The microbial basis on which the IX-PN/A process relied was that ammonium concentration progressively decreased due to the activity of ammonia-oxidizing microorganisms (AOM) and anammox bacteria, with suppression of competitor nitrite-oxidizing bacteria (NOB). Some NOB activity and nitrate accumulation was observed in some treatment cycles (Figure 2) with high recirculation rates provided for microbial detachment and sampling. Microbial analysis by polymerase chain reaction (PCR) also confirmed the existence of AOM and anammox bacteria through the IX-PN/A column.

Conclusions

Deammonification, or partial nitritation/anammox (PN/A), is a novel BNR technology with lower energy and chemical requirements than conventional BNR. Bioregeneration of chabazite with PN/A eliminates the need for chemical regenerant inputs, replacement with fresh media, or brine waste production. As the natural chabazite is reused for many treatment cycles, this process offers a sustainable strategy for a circular economy. Finally, coupling IX with PN/A is an innovative, effective BNR for mainstream wastewater, with lower costs, environmental impacts, and carbon footprint. Future work will focus on a) identifying the effect of operating conditions on PN/A performance through experimental and modeling studies and b) compare system performance with different strategies for biofilm formation: naturally formed PN/A biofilm, chabazite-coated biocarriers for attached biofilm growth, or biocoatings encapsulating PN/A microbes.

References

Anthonisen, A. C., Loehr, R. C., Prakasam, T. B. S., and Srinath, E. G. (1976). Inhibition of nitrification by ammonia and nitrous acid. Journal (Water Pollution Control Federation), 835-852.

Aponte-Morales, V. E., Payne, K. A., Cunningham, J. A., & Ergas, S. J. (2018). Bioregeneration of chabazite during nitrification of centrate from anaerobically digested livestock waste: Experimental and modeling studies. Environmental science & technology, 52(7), 4090-4098.

Aponte-Morales, V. E., Tong, S., & Ergas, S. J. (2016). Nitrogen removal from anaerobically digested swine waste centrate using a laboratory-scale chabazite-sequencing batch reactor. Environmental Engineering Science, 33(5), 324-332.

Bailey, E. L., Bilyk, K., Hanna, A., & Wankmuller, D. (2018). Evaluation of Sidestream Deammonification Process Enhancements for Treating High Strength Filtrate at the City of Raleigh’s Neuse River Resource Recovery Facility. Proc. Water Environ. Fed., 2018, 5286-5307.

Cao, Y., van Loosdrecht, M., and Daigger, G. T. (2017). Mainstream partial nitritation–anammox in municipal wastewater treatment: status, bottlenecks, and further studies. Applied microbiology and biotechnology, 101(4), 1365-1383.

Ergas, S. J., and Aponte-Morales, V. (2014). Biological Nitrogen Removal. Comprehensive Water Quality and Purification: Vol. 3 Remediation of Polluted Waters, S. Sengupta Ed., Elsevier.

Izadi, P., Izadi, P., & Eldyasti, A. (2021). Towards mainstream deammonification: Comprehensive review on potential mainstream applications and developed sidestream technologies. Journal of Environmental Management, 279, 111615.