Phosphorus removal of retrofit bioretention systems on urban surface runoff

摘要

Bioretention systems rely on mixtures of soil, sand, and organic materials to manage stormwater runoff. However, bioretention systems can also leach nutrients, and the media has unstable effect in infiltration rates and downstream pollution loads. In this study, 10 bioretention basins were constructed by setting different configurations with modified media which a mixture of water treatment residual (WTR), green zeolite, fly ash, and coconut bran and traditional bioretention soil media (BSM, 65% sand+30% soil+5% sawdust, by mass), respectively. The steady infiltration rates of the modified packing bioretention systems were 3.25-62.78 times that of undisturbed soil, which was 0.24-3.3 times that of traditional bioretention soil. Results showed that the removal rate of total phosphorus in bioretention basin with coconut bran as modifier was lower than others (79.14%), and the removal rates of total phosphorus were 86.22%-96.87% in other systems. The effluent concentrations of total phosphorus in 10 bioretention systems were basically better than Class. limitation (0.3 mg/L) of Environmental Quality Standards for Surface Water in China. The probabilities of BSM and fly ash mixed (#7), BSM and fly ash layered (#8), BSM and zeolite mixed (#9), and BSM and coconut bran mixed (#10) bioretention basins outflow concentration inferior to Class II limitation (0.1mg/L) were relatively high, which were 93%, 94%, 40%, and 91%, respectively. The total phosphorus load reduction rate decreased by approximately 20% for #1, and 15% for #7 bioretention basin, when the design recurrence interval increased from 0.5 to 3 years, or the contribution area ratio increased from 10 to 20. In 10 simulated rainfall experiments, the total phosphorus load reached 4.8 kg in each bioretention basin, and the total phosphorus load reduction rate reached 82.39% (#1)-98.32% (#3).