Annual variation patterns of the effluent water quality from a green roof and the overall impacts of its structure.

To explore the optimal combination of vegetation type, substrate type, and substrate thickness in a green roof and the interannual variation patterns of the runoff quality, eight green roof units were constructed in Shenzhen, China. Runoff quality of the eight units was monitored for 3 years (24 rainfall events). The rainfall event mean concentrations (EMC) were used to evaluate runoff quality as well as annual pollutant load. An orthogonal L8(24) experiment was designed to verify the significance of different factors. An optimal level of significant factors was selected to determine the optimal design of green roof. The optimal vegetation was Ophiopogon japonicus. The optimal substrate was modified perlite, while optimal substrate thickness was 200 mm. A three-year interannual variation analysis was performed on the optimal green roof. It was found that the interannual variation of each runoff quality index is different. The concentrations of SS, COD, and NH4+-N in the runoff decreased with years. The concentration of NO3--N increased over time, while TP remained stable. The concentration of TN had certain volatility with no significant interannual variation. Overall, the runoff quality of the green roof improves over time. The optimal green roof's runoff quality in the third year including 11 rainfall events was monitored. Results showed that the effluent quality from the green roof was lower than that of precipitation. The average concentrations of SS, COD, NH4+-N, TN, and TP decreased respectively by 37.85%, 28.89%, 30.25%, 14.52%, and 12.93%, but NO3--N increased by 69.91% comparing to the traditional roof.