Green Roofs - Living Architecture Monitor - Summer 2017 - 10

Additionally, the UCF studies examined the addition of a
pollution control media (PCM) layer, or soil amendment layer, to
improve water quality. Two different approaches were examined,
namely one where the PCM was used as a growth media and one
where the PCM was used as a layer under the growth media.
The PCM examined consisted mostly of tire crumb and expanded
clay. This material can absorb phosphorus, removing it from
green roof filtrate as it flows through the media (Wanielista,
et al., 2007; Wanielista, et al., 2008; Wanielista & Chang, 2008).
The UCF studies showed that green roof test chambers that
incorporated this media into their design, in general, had lower
total phosphorus (TP) concentrations in the green roof filtrate
than green roofs that used just a growth media with more than
half of the test chambers being significantly lower (a=0.05). The
addition of a PCM layer did not appear to affect total nitrogen
(TN) concentrations. It is noted that the vegetation did not grow
as well in the test chambers that utilized just the PCM as growth
media. It was speculated that this was due to the dark color of the
PCM, which resulted in burning the plants. The test chambers
that utilized the PCM as a layer, under the growth media,
exhibited healthy plant growth. All green roof test chambers in
the study that examined using the PCM as a growth media were
shown to have significantly higher concentrations of TP compared to the control chambers (a=0.05). The study that examined
the use of PCM as a layer under the growth media showed that
about half of the test chambers had significantly lower concentrations than the control test chambers with the others not being
significantly different (a=0.05). TN concentrations, in general,
were shown not to be significantly different (a=0.05) from the
control chambers.
The UCF studies also showed that vegetation plays an important role in reduction of TN and TP from green roof filtrate.
The UCF study showed that test chambers with vegetation had,
in general, significantly reduced TN and TP concentrations compared to test chambers with only media (a=0.05). This is expected
since nitrogen and phosphorus are macro nutrients required for
plant growth. Comparing the vegetated test chambers to the
control test chambers, TN concentrations were found to not be
significantly different (a=0.05) while TP concentrations were
found to be significantly higher (a=0.05) for the test chambers
without the PCM.
Finally, the UCF studies showed that the use of a cistern can
also help in reducing TP concentrations in runoff discharged
from a green roof. This is likely due to settling that occurs in the
cistern. It was shown that most of the test chamber cisterns had
lower TP concentrations than the filtrate (a=0.1). TN concentrations, in general, were shown to not be significantly different
between the green roof filtrate and cisterns.
As noted previously, the volume reduction achieved by green
roofs can result in a decrease in mass of TN and TP relative to
conventional roofs. Figure 2 shows the mass of TN and TP
LIVING ARCHITECTURE MONITOR / SUMMER 2017 / 10

"BASED ON THE RESULTS FROM
THESE STUDIES, GREEN ROOFS
CAN BE USED TO REDUCE THE
NUTRIENT LOADING DUE TO
STORMWATER RUNOFF FROM
ROOF TOPS."

- MIKE HARDIN

discharged from the green roof and control roof test chambers.
It can be seen that in green roofs that utilize a pollution control
media, vegetation, and a cistern (sized to 5 inches over the green
roof area) to capture and reuse green roof filtrate, a significant
mass reduction can be achieved.
Based on the results from these studies, green roofs can be
used to reduce the nutrient loading due to stormwater runoff
from roof tops. Further, the use of a PCM layer was shown to
help reduce the TP loading from green roofs. The addition of a
cistern to capture and reuse the green roof filtrate further helps
to reduce the TP and TN. Finally, the establishment of healthy
vegetation contributes to the reduction of nutrients from a green
roof. Incorporating all these elements in a green roof design can
result in a green roof that discharges less nutrient mass than a
conventional roof.

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Mike Hardin is a Water Resources Engineer with Geosyntec Consultants, Inc., Orlando, Florida. For more information http://geosyntec.co/
geosyntec/pdf/The-Control-of-Nutrients-in-Green-Roof-Runoff.pdf.
References: Wanielista, M., Kelly, M., & Hardin, M., "A Comparative
Analysis of Greenroof Designs Including Depth of Media, Drainage
Layer Materials, and Pollution Control Media", FDEP Final Report
WM 864, September 2008.
Wanielista, M., Hardin, M., & Kelly, M., "The Effectiveness of Green
Roof Stormwater Treatment Systems Irrigated with Recycled Green
Roof Filtrate to Achieve Pollutant Removal with Peak and Volume
Reduction in Florida", FDEP Final Report WM 864, May 2007.
Wanielista, M., & Chang, N., "Alternative Stormwater Sorption Media
for the Control of Nutrients", Southwest Florida Water Management
District Final Report B236, September 2008.
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ttp://geosyntec.co/geosyntec/pdf/The-Control-of-Nutrients-in-Green-Roof-Runoff.pdf. http://www.LIVINGARCHITECTUREMONITOR.COM

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