Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

Martha Dagnew

Abstract

Second-generation total nitrogen (TN) removal processes, such as shortcut pathways and nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO), play a key role in establishing organic carbon neutral footprint treatment processes. Lower organic carbon demand by second-generation processes enables the paradigm shift in wastewater treatment plants to capture carbon upstream of secondary treatment processes and divert it for energy recovery. In addition to processes, technologies need to be developed to realize sustainable wastewater treatment goals related to energy, footprint and by-product generation. A membrane biofilm reactor (MBfR) is an innovative technology that selectively diffuses gas substrates such as oxygen to a biofilm that naturally forms on the membrane surface. The technology is energy efficient and requires a smaller footprint making it attractive to implement second-generation TN removal processes from municipal wastewater. This research aimed at developing three different second-generation TN removal processes, simultaneous nitrification and denitrification (SND), nitrite shunt, and n-DAMO, in MBfR using carbon-captured municipal wastewater.

Using low soluble COD:N ratio of 1.9±1 and short 2.5 h hydraulic retention time (HRT), one-stage biofilm-based SND process was established in MBfR with nitrification rate of 1.1±0.58 g NH4-N/m2.d and denitrification rate of 0.7±0.3 g NO3-N/m2.d using intermittent aeration mode. It was also observed that reducing carbon in the liquid train positively impacted nitrification by achieving 87 ±12% (1.4±0.4 g/m2.d) ammonia removal with an effluent 2.5±2.8 mg/L ammonia concentration using continuous aeration at HRT of 4 h. Different aeration schemes also significantly impacted microbial dynamics in MBfR, where respective relative abundance for denitrifier bacteria was found as 20±2.8% and 40.5±3.1% under continuous and intermittent aeration that aligned well with denitrification efficiency under the two different aeration modes. To establish a mainstream nitrite shunt, despite the common strategy of “fully suppressing nitrite-oxidizing bacteria (NOB)” reported in the literature, this study followed a sustainable coexistence approach. In this approach, all the involved microbial groups exist in the bioreactor and their activity/growth was controlled by adjusting operating conditions such as the amount of process air, COD:N ratio, and scouring intensity as a biofilm thickness control strategy. Having used the coexistence approach, a successful nitrite shunt process was established, where normally-scoured MBfRs achieved the highest observed nitrification rate in this experiment by 1.58±0.6 g NH4-N/m2.d and non-scoured MBfRs showed the highest TN removal capability by achieving 0.86±0.5 g NO3-N/m2.d. In the last phase of this work, successful n-DAMO enrichment and experimentally determined temperature and half-saturation coefficients in MBfR were reported for the first time using typical wastewater treating sludge as inoculum. Also, comparing n-DAMO performance in two different bioreactor settings at varying operating temperatures showed significantly higher TN removal rates of 19.9±0.6 mg/L.d (0.28±0.009 g/m2.d) and 20.7±0.6 mg/L.d (0.3±0.012 g/m2.d) in MBfR vs. 10.9±1 and 11.8±1 mg/L.d in suspended growth system at 22 ℃ and 35 ℃, respectively.

In summary, MBfR showed significant potential to address the challenges and opportunities associated with implementing second-generation TN removal processes for carbon-captured municipal wastewater. Unique characteristics of MBfR, such as the bubble-less gas diffusion and counter-diffusional substrate supply to the biofilm, led to the successful establishment of robust and resilient TN removal through SND, nitrite shunt, and n-DAMO processes. The developed mainstream second-generation TN removal processes in this study can be used in real-world applications as secondary (SND and nitrite shunt) or tertiary/polishing (n-DAMO) processes depending on the treatment goals.

Summary for Lay Audience

As a response to global energy and carbon crisis, wastewater treatment processes have been increasingly considered to shift in sustainable paradigms with low or even neutral energy input. Energy and carbon footprint are the two main concerns in treating the massive and increasing load of receiving wastewater as a result of rapid urbanization and population growth. One of the practices towards low energy and carbon demand treatments is capturing carbon upstream of wastewater treatment terrain and redirect it to energy recovery units. However, conventional first-generation nitrification and denitrification that has been used for decades, is not compatible with such sustainable practice since it has quite high oxygen (as one of the indicators for energy consumption) and carbon demand. Therefore, second-generation total nitrogen (TN) removal processes that requires significantly lower oxygen and organic carbon demand has recently attracted considerable interest. However, there has been several challenges associated with adopting second-generation TN removal processes for mainstream municipal wastewater. Membrane biofilm reactor (MBfR) is an emerging technology with high potential of reducing energy and carbon footprints of processes. MBfR uses pressurized hollow fiber membrane to diffuse the gaseous substrate (such as air, pure oxygen, etc.) into the microbial community that is formed on the membrane surface. Unique characteristics of MBfR, such as the bubble-less gas diffusion, has made it an appropriate alternative to implement second-generation TN removal processes for municipal wastewater. This study aimed at evaluating the implementation of three different types of second-generation TN removal processes, namely simultaneous nitrification and denitrification (SND), nitrite shunt, and nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO) in MBfR.

Having studied different operating conditions, successful implementation of mainstream SND and nitrite shunt were obtained in MBfR. Also, successful n-DAMO enrichment in MBfR using typical wastewater treating sludge as inoculum was achieved in this study for the first time. The findings of this study can be a roadmap to adopt second-generation TN removal processes in real-world applications as secondary or tertiary/polishing processes depending on the treatment goals.

Available for download on Monday, October 30, 2023

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