Identifying potential uses for green roof discharge based on its physical–chemical-microbiological quality

Abstract

Green roofs are promising tools in sustainable urban planning, offering benefits such as stormwater management, energy savings, aesthetic appeal, and recreational spaces. They play a crucial role in creating sustainable and resilient cities, providing both environmental and economic advantages. Despite these benefits, concerns persist about their impact on water quality, especially for non-potable use, as conflicting results are found in the literature. This study presents a comparative analysis of the quantity and quality of water drained from an extensive green roof against an adjacent conventional rooftop made of fiber–cement tiles in subtropical Brazil. Over a 14-month period, the water drained from both roofs was evaluated based on physical (turbidity, apparent color, true color, electrical conductivity, total solids, total dissolved solids, suspended solids), chemical (pH, phosphate, total nitrogen, nitrate, nitrite, chlorides, sulfates, and BOD), microbiological (total coliforms and E. coli), and metal (copper, iron, zinc, lead, and chrome) concentration parameters. The discharge from the green roof was 40% lower than its counterpart measured at the control roof, while the water quality from both roofs was quite similar. However, the green roof acted as source of chlorides, electrical conductivity, color, BOD, total hardness, E. coli, phosphate, sulfate, and turbidity. On the other side, the green roof neutralized the slightly acidic character of rainwater, showcasing its potential to mitigate the effects of acid rain. The study’s results underscored that the water discharged from the green roof generally aligned with non-potable standards mandated by both Brazilian and international regulations. However, the findings emphasized the imperative need for pre-treatment of the green roof discharge before its utilization, specifically adjusting parameters like turbidity, BOD, total coliforms, and E. coli, which were identified as crucial to ensure water safety and compliance with non-potable use standards.

Appendix. Summary of guideline values for each parameter according to Brazilian and international organizations

Brazilian and international standards
	Parameter	NBR 15527	NBR 16783	NBR 13969				NRMMCb	2012 GWRc	Regulation 2020/741d
				CL1a	CL2a	CL3a	CL4a
Physical	Turbidity (NTU)	< 5.0	< 5.0	< 5.0	< 5.0	< 10.0		–	≤ 2	≤ 5
	SS (mg·L−1)	–	–	–	–	–	–	< 30	–	≤ 10
	TS (mg·L−1)	–	–	–	–	–	–	–	–	–
	TDS (mg·L−1)	–	< 2000	< 200	–	–	–	–	–	–
	Apparent color (mC)	–	–	–	–	–	–	–	–	–
	True color (mC)	–	–	–	–	–	–	–	–	–
	Electric cond. (µS·cm−1)	–	< 3200	–	–	–	–	–	–	–
Chemical	Phosphate (mg·L−1)	–	–	–	–	–	–	–	–	–
	TKN (mg·L−1)	–	–	–	–	–	–	–	–	–
	Nitrite (mg·L−1)	–	–	–	–	–	–	–	–	–
	Nitrate (mg·L−1)	–	–	–	–	–	–	–	–	–
	BOD (mg·L−1)	–	< 20	–	–	–	–	< 20	≤ 10	≤ 10
	Chlorides (mg·L−1)	–	–	–	–	–	–	–	–	–
	Sulfate (mg·L−1)	–	–	–	–	–	–	–	–	–
	Iron (mg·L−1)	–	–	–	–	–	–	–	–	–
	Zinc (mg·L−1)	–	–	–	–	–	–	–	–	–
	Copper (mg·L−1)	–	–	–	–	–	–	–	–	–
	Lead (mg·L−1)	–	–	–	–	–	–	–	–	–
	Chromium (mg·L−1)	–	–	–	–	–	–	–	–	–
	pH (–)	6.0 to 9.0	6.0 to 9.0	6.0 to 8.0	–	–	–	–	6.0 to 9.0	–
Microbiological	E. coli (NMP 100 mL−1)	< 200	< 200	–	–	–	–	< 100	–	≤ 10
	Total coliforms (NMP 100 mL−1)	–	–	< 200	< 500	< 500	< 5000	–	–	–

1. ^aCL—class for uses: 1—car washing and other uses that require direct user contact with water, with possible aspiration of aerosols by the operator; 2—washing of floors, sidewalks, and irrigation of gardens, maintenance of lakes and channels for landscaping purposes; 3—reuse in toilet flushing; 4—reuse in orchards, cereals, forages, pastures for cattle and other crops through surface runoff or irrigation system
2. ^bMunicipal use, with restricted access and application
3. ^cUrban reuse—unrestricted
4. ^dClass A