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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

Dagnew, Martha

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.

Summary for Lay Audience

Nitrogen removal from wastewater is crucial because high nitrogen levels can harm aquatic ecosystems, leading to problems like algae blooms, which deplete oxygen in water and threaten fish and other wildlife. Excess nitrogen can also contaminate drinking water, posing health risks to humans. To tackle this, biological nitrogen removal processes are employed, where bacteria break down harmful nitrogen compounds naturally. However, conventional nitrification and denitrification that has been used for decades require high expenses in terms of energy for aeration and external carbon requirements.

An advanced method for biological nitrogen removal is partial nitritation anammox (PNA). In this process, two types of bacteria work together: ammonium oxidizing bacteria (AOB) converts ammonia into nitrite (partial nitritation, which save aeration energy), and the other transforms both nitrite and ammonia directly into harmless nitrogen gas (anammox, without requiring external carbon requirement). PNA is more energy-efficient and cost-effective than traditional methods because it reduces the need for oxygen and no external carbon requirement.

A biofilm is a thin layer of microorganisms that attaches to surfaces in the treatment system, providing a protective environment for bacteria to thrive. The use of biofilms allows for the decoupling of two important concepts in wastewater treatment: Hydraulic Retention Time (HRT) and Solid Retention Time (SRT). HRT refers to the time wastewater spends in the reactor, while SRT is the time that solids (like bacteria) remain in the system. By using biofilms, longer SRTs can be maintained without increasing HRT, which means bacteria can continue to grow and effectively remove nitrogen without needing to hold the water for extended periods.

A Membrane Aerated Biofilm Reactor (MABR) is a biofilm system that has the potential to enhance the PNA process by improving the oxygen transfer efficiency through a membrane fiber and further save energy by reducing aeration. This allows bacteria to grow in layers, encouraging both aerobic and anoxic bacteria to coexist in one system, making nitrogen removal more effective. MABRs optimize the benefits of biofilms, improving energy efficiency and boosting nitrogen removal rates, thus offering a more sustainable and effective solution for wastewater treatment. Anammox is a slow growing microorganism and enriching anammox in a PNA process requires effective inhibiting strategy for nitrite oxidizing bacteria while maintaining the environment for AOB growth.

This study established a comprehensive start-up to achieve PNA in MABR by controlling off-gas oxygen concentrations and explored the effect of different mixing regimes to optimize and reduce start-up time. This study also investigated the capacity of MABR in nitrogen removal using various operating strategies like optimizing aeration strategies, controlling the growth of bacteria on the fiber and different concentration of external alkalinity for AOB growth. Treatment plants are exposed to temperature fluctuations at high altitude during colder months encounter challenges to meet their capacity in nitrogen removal. This study also investigated controlling strategies to achieve PNA in MABR at low temperature using two approaches, temporary heat shock to inhibit NOB growth and chemical additions to enhance AOB and anammox growth.

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