Synergistic improvement of COD removal and nitrogen recycling of swine wastewater with Myriophyllum aquaticum

For the biological treatment of swine wastewater, accelerating the degradation of COD usually leads to increased microbial nitrification, resulting in a conflict between pollutant removal and nitrogen recycling. In this study, the addition of hematite-biochar mixture and the nitrification inhibitor dicyandiamide (DCD) was proposed and applied to simultaneously enhance COD removal and nitrogen recycling efficiency in a Myriophyllum aquaticum-based swine wastewater treatment process. The results showed that addition of hematite-biochar mixture achieved a 1 times increase on COD removal rate. Meanwhile, the addition of DCD effectively suppressed microbial nitrification but slightly increased nitrogen removal by enhancing nitrogen utilization with Myriophyllum aquaticum. Eventually, the addition of hematite-biochar and DCD simultaneously improved the COD removal and nitrogen recycling rate to 96.9% (vs. 46.6% for control) and 72.8% (vs. 39.9% for control), respectively. Furthermore, microbial community analysis indicated that the developed strategy enhanced the abundance of Firmicutes and the genus Comamonas (strengthening COD removal), while reducing the abundance of nitrifying bacteria (phylum Proteobacteria) (repressing the nitrification process). This work provided a practical approach to accelerate pollutants removal while preserving nitrogen for plant utilization, which would be a promising solution for nitrogen recycling from swine wastewater.