Analysis of purification efficiency of pollutants in farmland runoff by the ecological ditch in winter and spring

To improve the purification efficiency of ditches in winter and spring, the ecological ditch was constructed by adding floating blankets with the cold-tolerant plant (Lolium perenne L.) and permeable dams with adsorption filler in the original soil ditch. The removal rates of main pollutants in farmland runoff by the ecological ditch were analyzed, the purification efficiencies of pollutants in different treatment segments were compared and the contributions of plant absorption to the total pollutant flux reduction were estimated. The results showed that the average values of the total removal rates of total nitrogen (TN), nitrate nitrogen (\mathrmNO_ 3^-\text-\mathrmN), total phosphorus (TP), chemical oxygen demand (COD) and suspended solids (SS) were 29.06%, 54.93%, 20.76%, 54.08% and 49.85%, respectively. The removal rates of TN and \mathrmNO_ 3^-\text-\mathrmN were highest in the permeable dam section of the ecological ditch; the removal rates of TP and COD were highest in the permeable dam section combined with floating blankets; the removal rate of SS was highest in the floating blankets section, and the purification efficiencies of \mathrmNO_ 3^-\text-\mathrmN, TP and COD were also better in the floating blankets section. In the nine days with more than 10 mm of rainfall during the experiment, the total reduction amounts of pollutant flux of TN and TP were 11.43 and 0.27 kg, respectively. The nitrogen and phosphorus amounts absorbed by Lolium perenne L. accounted for 20.6% and 55.6% of the total reduction amount of pollution flux of TN and TP, respectively. When the temperature dropped below 0 ℃ after the melting of heavy snow, the concentrations of nitrogen and phosphorus in snow-water runoff increased significantly, which might be due to soil freezing and thawing. Meanwhile, as the purification efficiencies of nitrogen and phosphorus in snow runoff by ecological ditch were not significant, nearby ponds and rivers could be utilized as temporary storage of the runoff, and further enhanced treatment of pollutants could be implemented in combination with ecological measures.