[Combination of Ecological Ditch and Bioretention Pond to Control Rural Runoff Pollution]

Ecological ditches and bioretention ponds have received widespread attention and application due to their runoff pollution control capabilities and ecological benefits. However, a single ecological ditch or bioretention pond often has problems, such as unstable nitrogen and phosphorus removal and substrate clogging in rural runoff pollution control. Thus, we connected the two facilities in a series to construct a combined system, using the ecological ditch to pretreat, therefore reducing the pollution load of the bioretention pond and mitigating substrate clogging. At the same time, the submerged area was set and an external natural carrier carbon source was added in the bioretention pond to improve the nitrogen removal. The effects of the carrier carbon source, rainfall intensity, and alternating wet and dry conditions on the control of rural runoff pollution by the combined system were explored. The results showed that adding straw and sawdust as carrier carbon sources could increase the TN removal of the bioretention pond by 19.9% and 20.4%, respectively. When the simulated rainfall intensity increased from light rain to heavy rain, the removal efficiencies of COD, NH₄⁺-N, TN, and TP in the combined system with external carbon source decreased by 17.0%, 16.8%, 20.4%, and 17.2% on average, respectively. The contribution of the ecological ditch to the removal of the four pollutants decreased by 16.3%, 13.0%, 24.2%, and 22.1% on average. Alternating dry and wet operation can improve the pollutant removal. Compared with continuous inflow, the average TN removal of the sawdust group increased by 12.3% after three weeks of drought. The results of microbial community analysis showed that the α-diversity of the bioretention pond in the sawdust group and the straw group was higher than that in control group. The abundance of Thiobacillus was significantly higher in the submerged area of bioretention ponds with carbon sources than that in the control group. These research results are expected to provide technical support for the practical application of the combined system.