Synergistic improvement of COD removal and nitrogen recycling of swine wastewater with <i>Myriophyllum aquaticum</i>

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 <i>Myriophyllum aquaticum</i>-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 <i>Myriophyllum aquaticum</i>. Eventually, the addition of hematite-biochar and DCD simultaneously improved the COD removal and nitrogen recycling rate to 96.9% (<i>vs.</i> 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 <i>Firmicutes</i> and the genus <i>Comamonas</i> (strengthening COD removal), while reducing the abundance of nitrifying bacteria (phylum <i>Proteobacteria</i>) (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.