Application of stabilized sludge to extensive green roofs in Shanghai: Feasibility and nitrogen leaching control

Extensive green roofs, in which commercial compost is usually used as organic component, have great potential to mitigate some environmental problems caused by urbanization, but carry risks of nutrients leaching into downstream aquatic. Stabilized sludge (SS) from wastewater treatment plants could be potentially used as nutrient component for green roof, but the effects on effluent quality are uncertain. To investigate the problem, a pilot experiment was conducted under field conditions, the effluent quality of green roof using SS was compared with green roofs using peat soil and controlled release fertilizer. In the field experiment, the nutrient concentrations in effluent of the green roof using SS (TN, NO₃^--N, NH₄⁺-N and TP were 3.27 mg/L, 1.75 mg/L, 1.14 mg/L and 0.34 mg/L, respectively) were not significantly different from the green roofs using peat soil and controlled release fertilizer, and the chemical oxygen demand level (92 mg/L) was lower than the roofs using compost or commercial substrate. To reduce the environmental risks caused by the application of SS to green roofs, a laboratory test was carried out to analyze the effects of biochar and dual-substrate structure on nitrogen leaching. The results showed that both biochar and dual-substrate reduced nitrogen leaching, and nitrogen leaching from green roofs using SS was a combined effect of organic nitrogen mineralization during dry period and biological processes during wet period. A high temperature and low humidity environment which is common in green roofs reduced nitrate accumulation during dry period, and nitrate was transformed to other substances in gaseous form by denitrification, which tended to occur in long duration, low intensity rainfall events. The results suggest that the application of stabilized sludge to green roofs is feasible in area where average rain intensity is not high, preferably combined with amendment of biochar and a dual-substrate structure.