Optimizing Advanced Biological Nitrogen Removal and Nitrous Gas Emission Using Membrane Aerated Biofilm Reactor

S Ahmad Shabir Razavi, The University of Western Ontario

Abstract

The one-stage partial nitritation anammox (PNA) system is favored for nitrogen removal due to its energy efficiency and spatial benefits, making it suitable for facilities with limited capacity. However, the slow growth rate of anammox bacteria, presents a key limitation to the widespread adaptation of PNA processes. Additionally, in one-stage PNA systems, nitrite-oxidizing bacteria (NOB) compete with ammonium-oxidizing bacteria (AOB) for oxygen and nitrite (NO₂), impairing process efficiency. Membrane-aerated biofilm reactors (MABR), a recent innovation, have garnered attention due to their counter-diffusion design. This setup with precise oxygen control and high oxygen transfer efficiency allows for the development of stratified biofilm layers that promote the coexistence of aerobic and anoxic bacteria and reduce oxygen demand. This research dived into establishing a novel and comprehensive start-up strategy to start-up PNA and dive into understanding different operating parameters like aeration pattern, alkalinity and scouring on capacity enhancement of PNA in MABR and also studied the effects of low temperature on PNA process in MABR suggesting controlling strategies like temporary heat shock and chemical addition.

The effects of off-gas oxygen control, mixing modes, and biomass attachment on PN/PNA start-up, biofilm properties, and microbial composition were established in four MABRs. Two reactors used hydraulic and gas mixing, while the other two controlled off-gas oxygen levels. Controlling off-gas to 5% significantly reduced the PN/PNA start-up time and suppressed NOB, with PN achieved in 10 days and PNA in 26 days, reaching maximum activity in 70 days without primary anammox seeding. Gas mixing led to better nitrogen removal rates (NRR) than hydraulic mixing, while biofilm images and microbial analysis revealed decreased biofilm thickness and microbial shifts along the fiber length, driven by oxygen gradients.

Further research explored the capacity of nitrogen removal by studying the effects of aeration strategies, scouring frequencies, and alkalinity concentrations on PNA performance and N₂O emissions on established biofilm. Four MABRs were tested with different aeration cycles, scouring intensities, and alkalinity levels. Results showed that intermittent aeration improved NRR and reduced N₂O emissions. Continuous aeration with alternating flow direction achieved stable PNA with the highest ARR. Compared to reactors with increased aeration time, it demonstrates the advantage of oxygen gradient in controlling NRR. Scouring impacts ARR, NRR and N₂O emission, showing less frequent high intensity scouring enhances NRR and reduces N₂O emission. Increasing alkalinity concentrations improved ARR but raised N₂O emissions, particularly near the membrane surface. Microbial analysis indicated that scouring slightly decreased NOB while promoting AOB and anammox growth with enhancing diffusion of substrate.

The effects of low temperature (10°C) combined with heat shock and chemical additives on PNA performance was also investigated in this research. Four MABRs were tested under low-temperature conditions, with two subjected to different heat shock intervals and two supplemented with iron and hydrazine. Results showed a sudden decline in ARR and NRR at 10°C, with heat shock partially inhibiting NOB and stimulating anammox, while biofilm analysis revealed increased thickness and mineralization. The iron supplemented reactor showed significant performance enhancement with a shift in microbial composition, showing highest relative abundances of AOB and anammox bacteria. N₂O emissions increased at low temperatures, especially in reactors with iron, though heat shock partially mitigated these emissions.

In summary, the study demonstrates that MABRs can achieve rapid start-up through a comprehensive off-gas control strategy. Targeted adjustments to operating parameters, such as scouring, aeration, and alkalinity addition, further enhance MABR removal capacity while reducing N₂O emissions. The feasibility of PNA in MABRs at low temperatures was also established, underscoring the importance of control strategies like heat shock and iron supplementation as reliable methods for achieving efficient autotrophic nitrogen removal.