Establishing second-generation total nitrogen removal for carbon-captured authentic wastewater using membrane biofilm reactor
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.