Selective Organic Carbon Enrichment Influences Nitrous Oxide Reduction by Denitrifiers: Electron Competition Insights

Abstract

A promising approach to reduce nitrous oxide (N 2 O) emission from wastewater treatment plants (WWTPs) would be to enrich highly efficient heterotrophic denitrifiers that are capable of regulating N 2 O reduction. The specific denitrifying community structure could be influenced by the types of organic carbon fed, while their corresponding N 2 O reduction capacity and electron competition mechanism remain unclear. In this study, the N 2 O reduction capacity by methanol-, ethanol-, and sodium acetate-enriched denitrifiers was systematically explored at varying carbon-loading rates. The effect of electron competition on N 2 O reduction was investigated by the combination of the microbial community and biochemical experiments. The results indicated that sodium acetate-enriched denitrifiers harboring abundant N 2 O reductase (Nos) genes invariably maintained the highest N 2 O reduction rate (r N 2 O ). However, the concurrent presence of multiple electron acceptors (i.e., NO 3 − and/or NO 2 − ) resulted in the higher susceptibility of r N 2 O for sodium acetate-enriched denitrifiers, which could explain the decrease in the N 2 O reduction rate under sufficient carbon supply. However, under insufficient organic carbon supply, Nos was confronted with fiercer electron competition from the upstream electron pool, causing more N 2 O accumulation. Moreover, a quantitative investigation of the electron distribution corroborated the aforementioned N 2 O reduction data. Further studies are warranted to optimize organic carbon-feeding strategies to utilize denitrifiers as an effective N 2 O sink at WWTPs.

Biodegradable Microplastics Increase N2O Emission from Denitrifying Sludge More Than Conventional Microplastics

“…A smaller difference between the potential electron consumption rate (PEC) and maximum electron supply rate (MES) generally means stronger electron competition intensity. 20,36 As expected, electron competition among reductases was more intense at lower COD loading rates (Table S7). Meanwhile, the effect of COD loading rate on denitrification activity and electron competition was concluded to be most remarkable in the reactor with BMPs.…”

“…Although the electron consumption rates were greatly reduced in both experimental reactors, nitrate reductase (Nar) possessed the highest and approximately constant electron consumption rates during a set of batch tests (i.e., a–g) for each reactor (Figure B), suggesting that electrons were prioritized to Nar regardless of microplastic exposure. A smaller difference between the potential electron consumption rate (PEC) and maximum electron supply rate (MES) generally means stronger electron competition intensity. , As expected, electron competition among reductases was more intense at lower COD loading rates (Table S7). Meanwhile, the effect of COD loading rate on denitrification activity and electron competition was concluded to be most remarkable in the reactor with BMPs.…”

“…The results demonstrated that Nos was favored to gain electrons above Nir in a more fiercer electron competition, while Nar showed a reduced electron distribution fraction. Generally, denitrifying sludge supported by different carbon sources may have different inherent electron distribution strategies. − Furthermore, larger COD-induced changes in the electron distribution were observed in experimental reactors, suggesting that microplastics would increase the sensitivity to the COD concentration.…”

Smaller Aerobic Granules Significantly Reduce N2O Production by Ammonia-Oxidizing Bacteria: Evidences from Biochemical and Isotopic Analyses

“…Regardless of the NO 2 – level, selecting smaller granules would achieve significant N 2 O reduction. When organic carbon sources in the influent were extremely insufficient, the remaining carbon sources would first be utilized by the self-growth process of heterotrophic denitrifying bacteria, resulting in the stagnation of denitrification due to lack of electron donors, consequently obtaining higher NO 2 – levels. Moreover, shortcut nitrification will become the main process in the future due to its advantages in cost saving.…”