Advance in the sulfur-based electron donor autotrophic denitrification for nitrate nitrogen removal from wastewater

Enhanced electron transfer for the improvement of nitrogen removal efficiency and N2O reduction at low temperatures

Low temperature generally restricts biological activity, slowing down electron transfer in biogeochemical cycles and causing a series of environmental problems such as nitrogen pollution. We present a strategy to boost electron transfer in microbial cell at low temperatures via stimulation with low current. It is demonstrated by establishing a constructed wetland system coupled with solar powered microbial electrolysis cell, which enhances microbial activity through external micro currents (18.9 ± 5.5 μA) for removing nitrogen pollution in winter (average temperature from -6.6 to 4.5 °C). We investigated the efficiency of pollutants removal, microbial activity, N2O production and its mechanisms using complexes activity detection, RT-qPCR, incubation, and 15N-18O dual-isotope labeling techniques. The activity of complexes I, II, III, and IV collectively represent the microbial electron transfer rate. Results indicated that the microcurrents increased the activity of complexes II, III and IV by 96 %, 172 %, and 313 %, respectively. The transcription abundance of amoA genes in ammonia oxidation and nirS/K genes in denitrification by 263 % and 51 %, respectively. Consequently, NH4+-N removal efficiency improved from 23 % to 35 %, and NO3--N removal efficiency from 21 % to 31 %. Moreover, microcurrents reduced N2O emission by 44 %. However, external microcurrent stimulation did not alter the microbial production pathway of N2O as determined by the 15N-18O dual isotope labeling technique. The relative abundance of the nitrifying bacteria Nitrosomonas, Nitrosospira, and Nitrospira, as well as the denitrifying bacteria Methylotenera, significantly increased due to microcurrent stimulation. Specifically, Nitrospira exhibited the highest increase of 156 %. Our findings provide a novel way to enhance N removal efficiency and simultaneously reduce N2O emission of biological system like constructed wetlands in winter conditions.

N2O recovery from wastewater and flue gas via microbial denitrification: Processes and mechanisms

Nitrous oxide (N2O) is increasingly regarded as a significant greenhouse gas implicated in global warming and the depletion of the ozone layer, yet it is also recognized as a valuable resource. This paper comprehensively reviews innovative microbial denitrification techniques for recovering N2O from nitrogenous wastewater and flue gas. Critical analysis is carried out on cutting-edge processes such as the coupled aerobic-anoxic nitrous decomposition operation (CANDO) process, semi-artificial photosynthesis, and the selective utilization of microbial strains, as well as flue gas absorption coupled with heterotrophic/autotrophic denitrification. These processes are highlighted for their potential to facilitate denitrification and enhance the recovery rate of N2O. The review integrates feasible methods for process control and optimization, and presents the underlying mechanisms for N2O recovery through denitrification, primarily achieved by suppressing nitrous oxide reductase (Nos) activity and intensifying competition for electron donors. The paper concludes by recognizing the shortcomings in existing technologies and proposing future research directions, with an emphasis on prioritizing the collection and utilization of N2O while considering environmental sustainability and economic feasibility. Through this review, we aim to inspire interest in the recovery and utilization of N2O, as well as the development and application of related technologies.

Revisit the role of hydroxylamine in sulfur-driven autotrophic denitrification

Through dedicated batch tests using the enriched sludge dominated by sulfur-oxidizing bacteria (SOB), the potential transformation of hydroxylamine (NH2OH) by SOB and the effects of NH2OH on the rate-limiting sequential reduction processes of sulfur-driven autotrophic denitrification (SDAD) were systematically explored in this study. The results indicated that NH2OH might be first converted to NO by SOB and then participate in the SDAD process, thus accelerating the utilization of S2- and contributing to the formation of N2O. Up to 3.5 mg-N/L NH2OH didn't affect the NO3- or NO2- reduction of SDAD, during which no significant changes were observed for the NH2OH concentration. Comparatively, even though NH2OH had no direct impact on the N2O reduction of SDAD, it could be consumed and therefore affect the depletion of N2O indirectly by regulating the toxic effect and electron supply of S2-. These findings provide novel implications for applying NH2OH to SDAD-based integrated processes for biological nitrogen removal.

Comparison of nitrification performance in SBR and SBBR with response to NaCl salinity shock: Microbial structure and functional genes

Ammonia removal by nitrifiers at the extremely high salinity poses a great challenge for saline wastewater treatment. Sequencing batch reactor (SBR) was conducted with a stepwise increase of salinity from 10 to 40 g-NaCl·L-1, while sequencing batch biofilm reactor (SBBR) with one-step salinity enhancement, their nitrification performance, microbial structure and interaction were evaluated. Both SBR and SBBR can achieve high-efficiency nitrification (98% ammonia removal) at 40 g-NaCl·L-1. However, SBBR showed more stable nitrification performance than SBR at 40 g-NaCl·L-1 after a shorter adaptation period of 4-15 d compared to previous studies. High-throughput sequencing and metagenomic analysis demonstrated that the abundance and capability of conventional ammonia-oxidizing bacteria (Nitrosomonas) were suppressed in SBBR relative to SBR. Gelidibacter, Anaerolineales were the predominant genus in SBBR, which were not found in SBR. NorB and nosZ responsible for reducing NO to N2O and reducing N2O to N2 respectively had s strong synergistic effect in SBBR. This study will provide a valuable reference for the startup of nitrification process within a short period of time under the extremely high NaCl salinity.