Constructed wetlands (CWs) are an environmentally friendly and cost-effective method for treating wastewater, but their effectiveness is often limited by low carbon/nitrogen (C/N) ratios in the wastewater effluent. A low C/N ratio can inhibit the denitrification process, which is critical for removing nitrogen and preventing pollutants like nitrate from entering the environment. In a recent study published in the journal Engineering, Hong-Tao Shi et al. propose a new strategy to address this issue by using a novel substrate: β-cyclodextrin-functionalized biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More (BC@β-CD). This material is a modified form of biochar, a carbon-rich substance made from reed straw, which enhances denitrification by improving the reallocation of carbon metabolism.

The researchers compared the performance of three CW systems: a traditional gravel-based system (control), a biochar-amended system (BC), and a system with β-cyclodextrin-functionalized biochar (BC@β-CD). The study found that the BC@β-CD system achieved the highest nitrogen removal rates. As the C/N ratio in the influent decreased from 4 to 2, the total nitrogen removal rate of the BC@β-CD group was 58.17%, which was significantly higher than the 15.37% in the control and 39.51% in the BC group.

The BC@β-CD substrate significantly improved nitrate removal and mitigated the release of nitrous oxide, a potent greenhouse gas. When the C/N ratio dropped from 4 to 2, nitrous oxide emissions in the BC@β-CD system decreased by 70.57% and 85.45%, respectively, compared to the control. This is a crucial finding, as traditional CWs can see a more than two-fold increase in nitrous oxide emissions when the C/N ratio drops, potentially exacerbating global warming.

The study goes on to explain the mechanisms behind this enhanced performance. Metagenomic and enzymatic analyses showed that BC@β-CD works not by altering the microbial community composition or species diversity but by promoting the activity of individual functional microorganisms. The material’s unique properties, including its high hydrophilicity and specific functional groups, likely contribute to its better biocompatibility and mass transfer properties, making it an ideal habitat for microorganisms.

The researchers discovered that BC@β-CD strategically reallocates carbon metabolic flow to support denitrification. It achieves this by increasing the activity of key enzymes involved in electron generation and transfer. Specifically, the BC@β-CD system showed a significant increase in the activity of NADH dehydrogenase, an enzyme that generates electrons for denitrification. At a C/N ratio of 2, the NADH dehydrogenase activity in the BC@β-CD group was 61.60% higher than in the control. The overall electron transfer system activity (ETSA) was also highest in the BC@β-CD group, with a 22.06% increase compared to the control at a C/N ratio of 2.

By promoting electron generation and transfer, BC@β-CD makes the denitrification process more competitive for the limited carbon sources available in low C/N conditions. This is supported by the finding that the ratio of theoretical COD consumed by denitrification to the actual COD consumed was significantly higher in the BC@β-CD group (55.57%) compared to the control (4.94%) when the C/N ratio was 2. This confirms that the functionalized biochar is highly effective at allocating carbon metabolism to maximize denitrification, providing a novel solution for low-carbon wastewater treatment.

Source: Shi, H.-T., Feng, X.-C., Xiao, Z.-J., Jiang, C.-Y., Wang, W.-Q., Zeng, Q.-Y., Yang, B.-W., Si, Q.-S., Wu, Q.-L., & Ren, N.-Q. (2024). Enhanced Denitrification in Constructed Wetlands with Low Carbon/Nitrogen Ratios: Insights into Reallocation of Carbon Metabolism Based on Electron Utilization. Engineering, 45, 222-233.