[Control Measure Effects on the Effluent Quality from Extensive Green Roofs Based on Stabilized Sludge Recycling]

Substrate type is an important factor affecting the quality of water deriving from rainfall onto extensively green roofed areas. Here, stabilized sludge was used as the main nutrient component of the substrate combined with biochar and a dual-substrate structure. Five green roof pilot facilities were constructed, and the effect of control measures on effluent quality was analyzed. The results showed that the stabilized sludge dosage was 3%, and the annual average mass concentrations of total nitrogen (TN) and nitrate nitrogen (NO₃^--N) were 3.27 mg·L^-1 and 1.61 mg·L^-1. The use of stabilized sludge as a nutrient component under real rainfall and temperature conditions in Shanghai did not cause significant leaching of TN and NO₃^--N. In order to further improve the quality of the effluent, biochar was used as an amendment measure. As a result, the concentrations of TN and NO₃^--N in the effluent were decreased to 2.16 mg·L^-1 and 1.38 mg·L^-1, respectively. Using an adsorption layer of pumice can alleviate the leaching of total phosphorus (TP) and chemical oxygen demand (COD). For the total nitrogen budget of each pilot facility, the retained TN was about 55% of the original TN after one year of operation. Thus, stabilized sludge could be used as a nutrient substrate to meet the long-term requirements of plants. In the substrate with biochar, the retained TN in the substrate and the NO₃^--N concentration in the effluent was decreased, which was related to the mineralization of organics during dry periods and the enhancement of denitrification during rainfall periods. Stabilized sludge was not a polluting source of N but was a source of P. Using biochar and a dual-substrate structure can effectively reduce the TN and COD load of the tested green roof facilities.