Enhanced nitrogen removal via biochar-mediated nitrification, denitrification, and electron transfer in constructed wetland microcosms

This study investigated the effect of biochar on real domestic wastewater treatment by constructed wetlands (CWs). To evaluate the role of biochar as a substrate and electron transfer medium on nitrogen transformation, three treatments of CW microcosms were established: conventional substrate (T1), biochar substrate (T2), and biochar-mediated electron transfer (T3). Nitrogen removal increased from 74% in T1 to 77.4% in T2 and 82.1% in T3. Nitrate generation increased in T2 (up to 2 mg/L) but decreased in T3 (lower than 0.8 mg/L), and the nitrification genes (amoA, Hao, and nxrA) in T2 and T3 increased by 132-164% and 129-217%, respectively, compared with T1 (1.56 x 10(4)- 2.34 x 10(7) copies/g). The nitrifying Nitrosomonas, denitrifying Dechloromonas, and denitrification genes (narL, nirK, norC, and nosZ) in the anode and cathode of T3 were significantly higher than those of the other treatments (increased by 60-fold, 35-fold, and 19-38%). The genus Geobacter, related to electron transfer, increased in T3 (by 48-fold), and stable voltage (similar to 150 mV) and power density (similar to 9 uW/m(2)) were achieved. These results highlight the biochar-mediated enhancement of nitrogen removal in constructed wetlands via nitrification, denitrification, and electron transfer, and provide a promising approach for enhanced nitrogen removal by constructed wetland technology.

Automated summary

This study investigated the effect of biochar on domestic wastewater treatment by constructed wetlands. The results showed that adding biochar as a substrate and electron transfer medium improved nitrogen removal through nitrification, denitrification, and electron transfer processes. The presence of biochar increased nitrate generation in the substrate but decreased it in the electron transfer treatment. Biochar also significantly increased the abundance of nitrifying and denitrifying genes, as well as the presence of Geobacter, a bacteria related to electron transfer. These findings suggest that biochar could enhance nitrogen removal in constructed wetlands.