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- Environmental Advances 4 (2021) 100059
Contents lists available at ScienceDirect
Environmental Advances
journal homepage: www.elsevier.com/locate/envadv
Impact of green roof plant species on domestic wastewater treatment
Lijiao Liu, Junjun Cao, Mehran Ali, Jiaxin Zhang, Zhaolong Wang∗
School of Agriculture and Biology, Shanghai Jiaotong University, Shanghai, 200240, P. R. China
a r t i c l e i n f o a b s t r a c t
Keywords: Green roof is one of the nature-based solutions to provide environmental and social benefits for sustainable urban
Green roof development. Green roofs irrigated with domestic wastewater solves not only their irrigation water resource but
Domestic wastewater treatment also the urban wastewater treatment. However, it is unknown whether the capacity of green roofs in pollutant
C4 plants
removal meets the requirement of domestic wastewater treatment. This study was to investigate the capacity of
C3 plants
pollutant removal by the green roofs with C4, C3, and CAM plant species when irrigated with domestic wastewa-
CAM plants
Irrigation ter. Results showed that green roofs removed 79.27~97.38% of total suspended solids, 79.94~98.92% of chemi-
Water use strategies cal oxygen demand, 65.26~90.52% of total nitrogen, 83.32~96.31% of ammonium nitrogen, 77.83~93.97% of
nitrate nitrogen, and 93.77~98.94% of total phosphorus, respectively. C4 and C3 plants contributed significantly
higher runoff reduction, removal of total nitrogen, chemical oxygen demand, total nitrogen, ammonium nitrogen,
nitrate nitrogen, phosphates, and total phosphorus than CAM plants. Water quality indicated by chemical oxygen
demand, total nitrogen, ammonium nitrogen, and total phosphorus from the effluent complied with the widely
accepted limits on domestic wastewater discharge. Our results demonstrate that green roofs can be designed for
the ecological treatment of domestic wastewater on the household scale.
1. Introduction 2019), which lead to the contradiction between water consumption for
green roof plants and water consumption for urban residents, especially
The current urban population accounts for 55% of the world’s pop- in the dry season (Darbandsari et al., 2020). On the other hand, a large
ulation, which will increase to 68% by 2050 (United Nations, 2018). amount of wastewater is produced by urban residents, which needs to be
The densification of the urban population has caused serious urban transported to the wastewater treatment plant and be discharged after
environmental degradation and a great threat to sustainable cities the treatment and meeting the discharge standard (Diaz-Elsayed et al.,
(Nitoslawski et al., 2019). Green infrastructure is a natural ecosystem 2019). Most pollutants in the domestic wastewater are nitrogen, phos-
to mitigate urban pollution, increase climate resilience, and address in- phorus, and organic matter which could be used as nutrients for plant
clusive urban regeneration (Schaubroeck, 2017; Yang & Bou-Zeid, 2019; growth (Jennett & Zheng, 2018.). The recycling of domestic wastewater
Tan et al., 2020). for green roof irrigation provides not only simultaneous in situ wastew-
Green roofs have been widely accepted as a nature-based solu- ater treatment to reduce the load on municipal wastewater plants but
tion in highly-populated urban centers because land for green in- also a cost-effective and sustainable alternative to the irrigation water
frastructure construction is very limited (Jim, 2017; Thuring & Dun- source. It has been adopted in the future community design, but there
nett, 2019). Green roofs present numerous ecological and social ben- are still many technical problems to be solved.
efits to the built environment such as the reduction of building en- Sedum and turfgrass are the most widely used plant species in ex-
ergy consumption (Cai et al., 2019; Susca, 2019; Tabatabaee et al., tensive or semi-extensive green roofs. Our previous studies demon-
2019; Simoes et al., 2020), stormwater management (Ebrahimian et al., strated that plant species with different photosynthetic strategies (CAM-
2019; Nguyen et al., 2019; Sims et al., 2019), mitigation of urban crassulacean acid metabolism, C3, and C4) performed a significant dif-
heat island effect (Bevilacqua et al., 2017; Sanchez & Reames, 2019; ference in their water use efficiency (Cao et al., 2019) and water re-
Dong et al., 2020), improvement of air pollution (Gourdji, 2018; tention capacity of green roofs (Li et al., 2018). However, no study has
Ramasubramanian et al., 2019), and increased green aesthetics of build- been conducted to elucidate the different effects of CAM, C3, and C4
ings (Southon et al., 2017; Besir & Cuce, 2018). plant species on the removal of pollutants when exposed to wastewater
Green roofs need a lot of water to maintain the plant growth because irrigation. Therefore, the objectives of this study were to investigate: 1)
of their shallow depths and low water availability of substrates (Du et al., the removal capacity of pollutants by the extensive green roofs; 2) the
wastewater treatment volumes by CAM, C3, and C4 plant species; 3)
∗
Corresponding author.
E-mail address: turf@sjtu.edu.cn (Z. Wang).
https://doi.org/10.1016/j.envadv.2021.100059
Received 28 August 2020; Received in revised form 7 April 2021; Accepted 21 April 2021
2666-7657/© 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/)
- L. Liu, J. Cao, M. Ali et al. Environmental Advances 4 (2021) 100059
whether green roofs can be the ecological treatment of domestic wastew- (COD) was analyzed using the rapid digestion and spectrophotomet-
ater on the household scale. ric method (HJ/T 399-2007); TN was measured using the alkaline
potassium persulfate digestion and UV spectrophotometric method
2. Materials and Methods (GB11894-89); NH4 + -N was measured using Nessler’s reagent spec-
trophotometric method (HJ535-2009); NO3 − -N was measured using the
2.1. Setup of green roofs phenol disulfonic acid spectrophotometric method (HJ/T 346-2007);
Phosphate was measured using the phosphomolybdenum blue spec-
Experimental green roof plots were set up as described in trophotometric method (GB/T 6913-008); Total phosphorus (TP) was
Li et al. (2018). A total of 28 green roof plots with internal dimen- measured using the persulfate digestion spectrophotometric method (GB
sions of 76 cm long × 36.5 cm wide × 25 cm height were set up on a 11893-89). The quality assurance and quality control (QA/QC) for wa-
fully exposed rooftop of the University Experimental Station (31°12′lat., ter sampling and analysis strictly followed the guidelines of National
121°38′long.). Each green roof plot was in a rectangular high-density standards.
polyethylene plastic lysimeters with an outflow opening (1 cm in di- The removal efficiency of the pollutants was calculated as:
ameter), which was constructed in the lowest part of the lysimeters. A Equation (1,2,3)
semi-extensive green roof was simulated within each Lysimeter, starting 𝑅𝐸 (%) = (𝑖𝑛𝑝𝑢𝑡 𝑎𝑚𝑜𝑢𝑛𝑡 − 𝑜𝑢𝑡𝑝𝑢𝑡 𝑎𝑚𝑜𝑢𝑛𝑡)∕𝑖𝑛𝑝𝑢𝑡 𝑎𝑚𝑜𝑢𝑛𝑡 × 100% (1)
with a non-rotting synthetic geotextile layer at the bottom to protect soil
escape from the outflow opening. The sand substrate of 100 kg was used The plant contribution to the pollutant removal by green roofs was
for plant growth and placed on top of the geotextile layer with 22.5 cm calculated as:
in height. 𝑃 𝐶 (%) = (𝑝𝑜𝑙𝑙𝑢𝑡𝑎𝑛𝑡 𝑟𝑒𝑚𝑜𝑣𝑎𝑙 𝑏𝑦 𝑡ℎ𝑒 𝑔𝑟𝑒𝑒𝑛 𝑟𝑜𝑜𝑓 − 𝑝𝑜𝑙𝑙𝑢𝑡𝑎𝑛𝑡 𝑟𝑒𝑚𝑜𝑣𝑎𝑙𝑏𝑦 𝑡ℎ𝑒 𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒)∕
𝑝𝑜𝑙𝑙𝑢𝑡𝑎𝑛𝑡 𝑟𝑒𝑚𝑜𝑣𝑎𝑙 𝑏𝑦 𝑡ℎ𝑒 𝑔𝑟𝑒𝑒𝑛 𝑟𝑜𝑜𝑓 × 100% (2)
2.2. Experiment design
Plant contribution to runoff reduction was calculated as:
The experiment was arranged in a randomized complete block de-
𝑃 𝐶(%)𝑜𝑓 𝑟𝑢𝑛𝑜𝑓 𝑓 𝑟𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛 = (𝑑𝑟𝑎𝑖𝑛𝑎𝑔𝑒 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒 𝑐𝑜𝑛𝑡𝑟𝑜𝑙 − 𝑑𝑟𝑎𝑖𝑛𝑎𝑔𝑒
sign with four replicates for each treatment. Plant species treatments in-
𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑔𝑟𝑒𝑒𝑛 𝑟𝑜𝑜𝑓 )∕𝑑𝑟𝑎𝑖𝑛𝑎𝑔𝑒 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒 𝑐𝑜𝑛𝑡𝑟𝑜𝑙 × 100% (3)
cluded two C4 turfgrass species (Eremochloa ophiuroides (Munro) Hack
cv. ’Civil’ and Cynodon dactylon (L.) Pers cv. ’Tifdwarf’), two C3 turfgrass All data are presented as means of four replicated measurements.
species (Poa pratensis L. cv. ’Midnight’ and Festuca arundinacea Schreb. Statistical analysis was performed with the software SAS (version 9.1,
cv. ’Jaguar 4G’), and two CAM plant species (Sedum lineare Thunb. and SAS Institute Inc., Cary, NC) using the general linear model (GLM) pro-
Callisia repens L.), which were most commonly used for extensive green cedure, and the least significant difference (LSD) at a 0.05 probability
roofs, and non-vegetated substrate control. The experimental green roof level was used to detect the differences between treatment means.
plots were established in 2016 and maintained consistently under 100%
plant coverage (no visible growth substrate) in 2017 and 2018. Domes- 3. Results and Discussion
tic wastewater was from the local household’s wastewater tank by col-
lecting the kitchen use and rainwater runoff. The total suspended solids 3.1. Drainage effluent volume
(TSS), chemical oxygen demand (COD), total nitrogen (TN), ammonium
nitrogen (NH4 + -N), nitrate-nitrogen (NO3 − -N), phosphates, and total Green roof is one of the most important green infrastructures for
phosphorus (TP) of the experimental wastewater were 270.0, 483.5, runoff reduction in dense urban areas (Hellies et al., 2018; Liu et al.,
14.47, 7.32, 5.37, 3.24, and 4.59 mg L−1 , respectively, which exceed 2019). Numerous studies focused on the water retention capacity of
the standard limitations of TSS (50 mg L−1 ), COD (120 mg L−1 ), and TP green roof substrates (Bouzouidja et al., 2018; Gong et al., 2019;
(3 mg L−1 ) for domestic wastewater discharge. The wastewater was irri- Bollman et al., 2019). However, the water retention capacity of sub-
gated onto the plant canopy of green roofs regularly in a 2-days interval strates only determines the initial water volume retained in the green
to replenish the evapotranspiration water loss. Five different wastewater roof (Krebs et al., 2016; Viola et al., 2017). The water consumption by
volumes (5, 10, 25, 50, 100 L m-2) were irrigated to the experimental plants is more important to the long-term water retention capacity of the
green roof plots on September 20, 22, 24, and 26. The moisture of sand green roof because it creates a room where the green roof can absorb wa-
substrate in all experimental green roofs was at 22.2~22.9% when the ter in the sequent irrigation event (Poe et al., 2015; Zhang et al., 2018;
volume treatment started. Irrigation was applied by an intensity of 5 L Cascone et al., 2019). CAM, C3, and C4 plants have different water-use
m−2 for a duration of 15 min on the green roofs from an artificial rain strategies, which resulted in significant differences in water evapotran-
generator (No. 25 nozzles with 15 psi pressure regulator) according to spiration (Cao et al., 2019).
Li et al. (2018). Five wastewater treatment volumes (5, 10, 25, 50, 100 L Under the wastewater volumes at 5 and 10 L m−2 , no drainage efflu-
m−2 ) were achieved by 1, 2, 5, 10, 20 times of each irrigation in 15, 30, ent occurred because the green roof retained all water in the substrate
75, 150, 300 min, respectively. The drainage effluent from the tested layers (Fig. 1). The drainage effluent occurred when wastewater vol-
green roof was collected and recorded at 5 h after the irrigation stopped umes reached 25 L m−2 . The effluent volumes were increased with the
when no more water was discharged out of each lysimeter. Water sam- increases of the wastewater treatment volumes in all green roofs and
ples were collected and sent to the laboratory for the measurements of substrate control. Green roofs significantly reduced effluent volumes in
TSS, COD, TN, NH4 + -N, NO3 − -N, phosphates, and TP. all tested wastewater volumes, when compared to the substrate control.
Green roofs with C4 (Eremochloa ophiuroides and Cynodon dactylon) and
2.3. Measurements C3 (Poa pratensis and Festuca arundinacea) plant species showed signif-
icantly lower effluent volumes than that of CAM plant species (Sedum
Daily evapotranspiration was measured by the weighting method ac- lineare and Callisia repens) under 25, 50, and 100 L m−2 wastewater
cording to Li et al. (2018). Each green roof lysimeter was weighted and treatments. The lower effluent volume in the C4 and C3 green roofs can
recorded daily. The change of the lysimeter weight represents the water be interpreted by their higher evapotranspiration, as shown in Fig. 2.
lost by evapotranspiration in the green roof. During the experimental period, green roofs with C4 and C3 plants per-
Water quality was analyzed according to the national standards. formed the highest daily evapotranspiration with 5.40 mm d−1 in Cyn-
Total suspended solids (TSS) were measured using the filtering, dry- odon dactylon, 5.09 mm d−1 in Eremochloa ophiuroides, 4.64 mm d−1
ing, and weighing method (GB11901-89); Chemical oxygen demand in Poa pratensis, and 4.59 mm d−1 in Festuca arundinacea. The higher
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- L. Liu, J. Cao, M. Ali et al. Environmental Advances 4 (2021) 100059
Fig. 1. Drainage effluent volume by green roofs under
different wastewater treatments
Fig. 2. Daily green roof evapotranspiration with dif-
ferent plant species during the experiment.
evapotranspiration in two C4 species was due to the higher biomass 3.2. Pollutant removal
production during the summer months when compared to C3 species
(Cao et al., 2019). Green roofs with CAM plants performed significantly 3.2.1. Removal of TSS and COD
lower daily evapotranspiration with only 2.29 mm d−1 in Callisia repens Numerous studies found that plants could absorb and remove pol-
and 2.35 mm d−1 in Sedum lineare. Our previous study also found that lutants from the wastewater in the wetland systems (Vo et al., 2018;
the lower evapotranspiration of CAM plant species led to less runoff re- Pradhan et al., 2019; Boano et al., 2020). The green roof is a xerophytic
duction when compared to C3 grass species (Li et al., 2018). Studies in structure, which is different from the wetland system. Few studies have
hedge species also confirmed that green roof plants with higher evapo- been conducted for pollutant removal in the xerophytic systems. Domes-
transpiration rates contributed the higher runoff reductions (Blanusa & tic wastewater cannot be directly discharged into the environment be-
Hadley, 2019). cause of the high TSS and COD contents (Hench et al., 2003; Yuan et al.,
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- L. Liu, J. Cao, M. Ali et al. Environmental Advances 4 (2021) 100059
Table 1
Removal efficiency of total suspended solids (TSS) by green roofs with different plant species (%). Removal efficiency was calculated according to Formula 1. The
data were presented by means ± standard errors of four replications. Different letters represent the significant differences between the treatments at LSD 0.05.
Wastewater Cynodon Eremochloa Festuca Substrate
volume (L m−2 ) dactylon ophiuroides Poa pratensis arundinacea Sedum lineare Callisia repens control
25 96.95 ± 0.16a 97.38 ± 0.23a 96.96 ± 0.23a 96.99 ± 0.15a 94.86 ± 0.26b 94.71 ± 0.59b 88.19 ± 0.87c
50 91.29 ± 0.23ab 92.18 ± 0.32a 89.58 ± 0.83b 91.45 ± 0.47ab 87.43 ± 0.17c 86.62 ± 0.36c 79.82 ± 2.01d
100 84.39 ± 0.75a 85.08 ± 0.71a 83.67 ± 0.75a 84.79 ± 0.72a 79.45 ± 0.75b 79.27 ± 1.10b 70.79 ± 1.36c
Table 2
Removal efficiency of COD by green roofs with different plant species (%). Removal efficiency was calculated according to Formula 1. The data were presented by
means ± standard errors of four replications. Different letters represent the significant differences between the treatments at LSD 0.05.
Wastewater volume (L m−2 ) Cynodon dactylon Eremochloa ophiuroides Poa pratensis Festuca arundinacea Sedum lineare Callisia repens Substrate control
25 98.92 ± 0.37a 98.91 ± 0.28a 98.83 ± 0.28a 98.79 ± 0.25a 97.01 ± 0.62b 97.48 ± 0.65b 92.08 ± 0.77c
50 94.68 ± 0.29a 94.58 ± 0.30a 94.99 ± 0.48a 94.75 ± 0.56a 91.79 ± 0.69b 93.34 ± 1.14b 81.32 ± 0.30c
100 93.37 ± 0.11a 94.73 ± 0.42a 93.90 ± 0.58a 93.80 ± 0.92a 91.08 ± 0.44b 92.52 ± 0.54b 79.94 ± 0.72c
2017). Green roof irrigated by domestic wastewater provides the possi- 3.2.2. Removal of N and P
bility of simultaneous wastewater treatment and urban water resource Nitrogen and phosphorus are the main pollutants in domestic
regeneration if the green roof can reduce the pollutants to within their wastewater (Batstone et al., 2015), but they are essential nutrients for
limit values (Knapp et al., 2019; Liu et al., 2018). plants (Su et al., 2019). Wastewater irrigation provides not only wastew-
TSS was significantly removed by all green roofs when compared ater treatment to remove nitrogen and phosphorus, but also supplies the
to the input wastewater (270 mg L−1 ). Green roofs with different plant nutrients for plant growth (Libutti et al., 2018).
species showed significant impacts on TSS removal. Green roofs with C4 All green roofs significantly reduced ammonium nitrogen (NH4 + -N),
plants (Cynodon dactylon and Eremochloa ophiuroides) showed the signif- nitrate-nitrogen (NO3 − -N), and total nitrogen (TN) concentrations in
icant highest TSS removals (84.39~ 97.38%), followed by C3 plants their drainage effluents when compared to the input wastewater (7.32,
(Poa pratensis and Festuca arundinacea) at 83.67~96.99% of TSS re- 5.37, and 14.47 mg L−1 , respectively). The effects of NH4 + -N, NO3 − -N,
moval (Table 1). Green roofs with CAM plants (Sedum lineare and Callisia and TN removals performed the best under the 25 L m−2 of wastewater
repens) showed significantly lower TSS removals than that of C4 and C3 treatment and they were decreased with the increases of the volumes
plants. of wastewater, in all green roofs and substrate control (Table 3). Un-
TSS removal capacity of green roofs was reduced with the increases der 25 L m−2 of wastewater treatment, the substrate control reduced
of wastewater treatment volumes (Fig. 3). The normal limitation of TSS NH4 + -N, NO3 − -N, and TN concentrations to 2.90, 2.58, and 7.52 mg L−1
for direct discharge of domestic wastewater is 50 mg L−1 . To meet the and removed 85.84%, 82.78%, and 71.61% of NH4 + -N, NO3 − -N, and
TSS limitation of 50 mg L−1 in its drainage effluent, non-vegetated sub- TN, respectively. Green roofs with CAM plants showed additional plant
strate control could only treat 17.9 L m−2 of wastewater, according to impact on nitrogen removals. Green roof with Sedum lineare removed
the regression of wastewater treatment volume and TSS content. All 92.10% of NH4 + -N, 89.14% of NO3 − -N, and 83.71% of TN, and Green
plant species showed significant promotion in TSS removal. Green roofs roof with Callisia repens removed 91.91% of NH4 + -N, 88.82% of NO3 − -
with C4 (Eremochloa ophiuroides and Cynodon dactylon) and C3 plants N, and 83.16% of TN, respectively. Green roofs with C3 (Poa praten-
(Poa pratensis and Festuca arundinacea) performed significant higher ca- sis and Festuca arundinacea) and C4 (Cynodon dactylon and Eremochloa
pacity of TSS removal than CAM plants, which could treat 41.4, 44.0, ophiuroides) plant species showed the even better effects of nitrogen re-
35.3, and 43.4 L m−2 of wastewater without exceeding the TSS limi- movals and the NH4 + -N removals reached 95.42% ~ 96.31%, NO3 − -
tation in their drainage effluent, while CAM plants (Sedum lineare and N removals reached 93.61% ~ 93.97%, and the TN removals reached
Callisia repens) at only 29.6 and 29.8 L m−2 , respectively. 89.38% ~ 90.52%, respectively, which were significantly higher than
All green roofs significantly reduced COD concentrations in their that in CAM plants.
drainage effluents when compared to the input wastewater (483.5 mg All green roofs significantly reduced phosphate and total phospho-
L−1 ). Green roofs with different plant species showed significant im- rus (TP) concentrations in their runoffs when compared to the input
pacts on COD removal. Green roofs with C4 plants (Cynodon dactylon wastewater (3.24 and 4.59 mg L−1 ). Under 25 L m−2 of wastewater treat-
and Eremochloa ophiuroides) and C3 plants (Poa pratensis and Festuca ment, the substrate control reduced phosphate and TP concentrations to
arundinacea) showed the significant highest COD removals at 93.37~ 1.15 and 1.45 mg L−1 , and removed 87.34% and 79.99% of phosphate
98.92% (Table 2). Green roofs with CAM plants (Sedum lineare and Cal- and TP, respectively. Green roofs with CAM plants showed additional
lisia repens) showed significantly lower COD removals than that of C4 improvement of phosphate and TP removals. Green roof with Sedum lin-
and C3 plants in all three wastewater treatment volumes. eare removed 98.65% of phosphate and 96.71% of TP. Green roof with
COD removal capacity of green roofs was also reduced with the Callisia repens removed 98.09% of phosphate and 96.56% of TP. Green
increases in wastewater volumes (Fig. 4). The standard limitation of roofs with C3 (Poa pratensis and Festuca arundinacea) and C4 (Cynodon
COD for direct discharge of domestic wastewater is 120 mg L−1 in most dactylon and Eremochloa ophiuroides) plant species showed the even bet-
European and Asian countries. All green roofs with C4, C3, and CAM ter effects of phosphate and TP removals and the phosphate removals
plants performed significant COD removal, and none of them exceeded reached 99.23% ~ 99.43%, and the TP removals reached 98.19% ~
the COD limitation under all wastewater treatment volumes. Only non- 98.37%, respectively, which were significantly higher than that in CAM
vegetated substrate control showed exceeding the COD limitation in plants (Table 4). The phosphate and TP removals by green roofs did not
their effluent runoff when wastewater treatment volume reached 42.3 L show a significant decline with the increases of wastewater treatment
m −2 . volumes, in all green roofs and substrate control (Table 4).
Fig. 5
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Fig. 3. Total suspended solid (TSS) in the
runoff from green roofs. The dash line in-
dicates the TSS standard limit for direct
discharge of domestic wastewater.
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Fig. 4. Chemical oxygen demand (COD) in the runoff from green roofs. The dash line indicates the COD standard limit for direct discharge of domestic wastewater.
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Fig. 5. Percentage of pollutant removal contributed
by plants to green roofs under 25 L m−2 of wastewa-
ter treatment. Plant contribution (%) to the pollutant
removal was calculated according to Formula 2. Plant
contribution (%) to runoff reduction was calculated ac-
cording to Formula 3.
Table 3
Nitrogen removal by green roofs with different plant species. Removal efficiency was calculated according to Formula 1. The data were presented by means ±
standard errors of four replications. Different letters represent the significant differences between the treatments at LSD 0.05.
Wastewater Cynodon Eremochloa Festuca Substrate
volume (L m−2 ) N forms dactylon ophiuroides Poa pratensis arundinacea Sedum lineare Callisia repens control
25 NH4 + -N mg L −1 1.93 ± 0.02c 1.94 ± 0.02c 1.70 ± 0.04c 1.90 ± 0.04c 2.22 ± 0.03b 2.19 ± 0.02b 2.90 ± 0.1a
Removal 95.42 ± 0.12b 95.82 ± 0.31ab 96.31 ± 0.14a 95.93 ± 0.18ab 92.10 ± 0.07c 91.91 ± 0.20c 85.84 ± 0.66d
efficiency %
NO3 − -N mg L −1 1.98±0.03e 2.06±0.05de 2.06±0.04de 2.11±0.02cd 2.24±0.02b 2.22±0.01bc 2.58±0.11a
Removal 93.61 ± 0.13a 93.97 ± 0.40a 93.92 ± 0.31a 93.83 ± 0.42a 89.14 ± 0.26b 88.82 ± 0.25b 82.78 ± 1.26c
efficiency %
TN mg L−1 5.79 ± 0.03b 5.72 ± 0.02b 5.71 ± 0.10b 5.72 ± 0.07b 5.93 ± 0.10b 5.91 ± 0.16b 7.52 ± 0.19a
Removal 89.38 ± 0.33a 90.19 ± 0.64a 90.41 ± 0.55a 90.52 ± 0.51a 83.71 ± 0.11b 83.16 ± 0.17b 71.61 ± 0.86c
efficiency %
50 NH4 + -N mg L −1 1.92 ± 0.01cd 1.92 ± 0.01cd 1.88 ± 0.03d 1.94 ± 0.02c 2.15 ± 0.02b 2.14 ± 0.01b 2.79 ± 0.02a
Removal 87.51 ± 0.30ab 88.20 ± 0.17a 88.11 ± 0.23a 87.32 ± 0.24b 84.27 ± 0.36c 83.48 ± 0.11d 77.43 ± 0.47e
efficiency %
NO3 − -N mg L −1 1.95 ± 0.01d 1.89 ± 0.01e 2.04 ± 0.01c 2.09 ± 0.02bc 2.09 ± 0.02b 2.09 ± 0.02b 2.43 ± 0.02a
Removal 82.77 ± 0.36b 84.18 ± 0.32a 82.44 ± 0.16b 81.41 ± 0.34c 79.12 ± 0.67d 78.08 ± 0.23e 73.21 ± 0.32f
efficiency %
TN mg L−1 5.50 ± 0.12c 5.48 ± 0.01c 5.59 ± 0.02c 5.58 ± 0.08c 5.82 ± 0.05b 5.83 ± 0.08b 7.85 ± 0.13a
Removal 72.41 ± 0.72a 72.13 ± 1.18a 71.89 ± 0.67a 71.79 ± 0.46a 67.04 ± 0.63b 65.26 ± 0.77b 50.98 ± 1.37c
efficiency %
100 NH4 + -N mg L −1 1.82 ± 0.02cd 1.81 ± 0.01cd 1.79 ± 0.00d 1.84 ± 0.02c 1.98 ± 0.01b 1.97 ± 0.01b 2.47 ± 0.02a
Removal 85.96 ± 0.12b 86.44 ± 0.13a 86.40 ± 0.04a 85.95 ± 0.23b 83.57 ± 0.09c 83.32 ± 0.16c 78.41 ± 0.08d
efficiency %
NO3 − -N mg L −1 1.91 ± 0.02bc 1.76 ± 0.02d 1.87 ± 0.01c 1.94 ± 0.01b 1.91 ± 0.04bc 1.92 ± 0.01bc 2.18 ± 0.01a
Removal 79.88 ± 0.20c 82.05 ± 0.18a 80.61 ± 0.12b 79.87 ± 0.24c 78.34 ± 0.42d 77.83 ± 0.34d 74.03 ± 0.18e
efficiency %
TN mg L−1 4.38 ± 0.06c 4.46 ± 0.09c 4.33 ± 0.07c 4.46 ± 0.19c 5.00 ± 0.04b 4.99 ± 0.19b 6.95 ± 0.04a
Removal 73.80 ± 0.38a 73.76 ± 0.56a 72.42 ± 1.36a 73.71 ± 0.80a 67.90 ± 0.16b 68.55 ± 1.09b 52.98 ± 0.41c
efficiency %
3.3. Plant contribution to pollutant removal 2017; Karczmarczyk et al., 2018). Most pollutants in domestic wastew-
ater are nutrients for plants and can be continuously absorbed and
The pollutant removal by green roof was contributed from both the used by plants (Kasak et al., 2018). The pollutant removal by plants
substrate retention and plant uptake (Berretta et al., 2014; Li et al., was mainly dependent upon its capacity of uptake and accumulation
2018). Numerous studies focused on the characteristics of substrates (Schwammberger et al., 2019). However, the plant contribution to pol-
and their capacity to absorb the pollutants (Bollman et al., 2019; lutant removal has not been well defined.
Conn et al., 2020; Xue & Farrell, 2020). Substrate absorption could Plant species vary in their capacities in nutrient absorption and use
be saturated, thus, some studies found that green roofs became a efficiency from the wastewater (Van et al., 2015; Vo et al., 2018). In this
source of pollution because of the leaching of pollutants (Wang et al., study, plant contribution to the pollutant removal was calculated by the
7
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Table 4
Phosphate and Total Phosphorus removal by green roofs with different plant species. Removal efficiency was calculated according to Formula 1. The data were
presented by means ± standard errors of four replications. Different letters represent the significant differences between the treatments at LSD 0.05.
Waste water Cynodon Eremochloa Festuca Substrate
volume (L m−2 ) P forms dactylon ophiuroides Poa pratensis arundinacea Sedum lineare Callisia repens control
25 Phosphate mg 0.14 ± 0.01c 0.13 ± 0.01c 0.15 ± 0.01c 0.11 ± 0.01c 0.17 ± 0.01bc 0.23 ± 0.01b 1.15 ± 0.07a
L −1
Removal 99.23 ± 0.03a 99.37 ± 0.04a 99.28 ± 0.02a 99.43 ± 0.09a 98.65 ± 0.16b 98.09 ± 0.10c 87.34 ± 0.29d
efficiency %
TP mg L−1 0.27 ± 0.01c 0.28 ± 0.01c 0.27 ± 0.01c 0.27 ± 0.01c 0.33 ± 0.02b 0.33 ± 0.05b 1.45 ± 0.04a
Removal 98.19 ± 0.06a 98.31 ± 0.18a 98.37 ± 0.04a 98.36 ± 0.12a 96.71 ± 0.19b 96.56 ± 0.41b 79.99 ± 0.95c
efficiency %
50 Phosphate mg 0.15 ± 0.01cd 0.11 ± 0.00d 0.11 ± 0.01d 0.11 ± 0.01d 0.23 ± 0.02b 0.21 ± 0.01bc 1.20 ± 0.09a
L −1
Removal 97.82 ± 0.05a 98.52 ± 0.08a 98.38 ± 0.18a 98.37 ± 0.16a 96.23 ± 0.32b 96.28 ± 0.15b 78.09 ± 1.79c
efficiency %
TP mg L−1 0.20 ± 0.01c 0.17 ± 0.00c 0.27 ± 0.01b 0.26 ± 0.00b 0.25 ± 0.01b 0.27 ± 0.01b 1.69 ± 0.06a
Removal 96.40 ± 0.19ab 97.03 ± 0.10a 95.23 ± 0.15bc 95.21 ± 0.12bc 94.73 ± 0.20c 94.17 ± 0.32c 61.35 ± 1.47d
efficiency %
100 Phosphate mg 0.11 ± 0.00de 0.09 ± 0.00e 0.13 ± 0.01cd 0.12 ± 0.01de 0.19 ± 0.01b 0.18 ± 0.00bc 0.87 ± 0.05a
L −1
Removal 98.00 ± 0.07a 98.53 ± 0.07a 97.70 ± 0.13a 97.98 ± 0.15a 96.36 ± 0.21b 96.62 ± 0.13b 82.75 ± 1.05c
efficiency %
TP mg L−1 0.15 ± 0.00c 0.14 ± 0.01c 0.16 ± 0.00c 0.17 ± 0.00c 0.24 ± 0.01b 0.26 ± 0.01b 1.19 ± 0.02a
Removal 96.81 ± 0.07a 96.94 ± 0.12a 96.61 ± 0.10a 96.32 ± 0.12a 94.32 ± 0.12b 93.77 ± 0.17b 70.68 ± 0.50c
efficiency %
difference between each green roof and the substrate control. C4 (Cyn- with C4 (Eremochloa ophiuroides and Cynodon dactylon), C3 plants (Poa
odon dactylon and Eremochloa ophiuroides) and C3 plants (Poa praten- pratensis and Festuca arundinacea), and CAM plants (Sedum lineare and
sis and Festuca arundinacea) contributed an additional 22.3~23.8% of Callisia repens) were 41.4, 44.0, 35.3, 43.4, 29.6 and 29.8 L m−2 , respec-
runoff reduction, 19.1~20.0% of TSS removal, 16.7~17.6% of COD re- tively (Fig. 2). It was estimated that an average person produced 390 L
moval, 19.9~20.7% of TN removal, 17.8~21.4% of NH4 -N removal, of wastewater per day (Li et al., 2014). To adequately clean the do-
14.4~17.7% of NO3 -N removal, 32.4~33.2% of phosphate removal, mestic wastewater and regenerate the urban water resource, the green
and 27.1~27.3% of TP removal, which were significantly higher than roof area required by each person is 18.8, 17.7, 22.0, and 18.0 m2 for
that of CAM plants (Sedum lineare and Callisia repens) in their green roof C4 (Eremochloa ophiuroides and Cynodon dactylon) and C3 plants (Poa
system (Fig. 4). pratensis and Festuca arundinacea), respectively. Green roofs with CAM
The effect of green roofs on wastewater treatment depends on the plants (Sedum lineare and Callisia repens) required more areas (26.4 and
adsorption capacity of the substrate and plant root system to nutri- 26.2 m2 ) for the wastewater treatment. It is calculated that an aver-
ents (Yang et al., 2001). After the adsorption was saturated, pollu- age 3-person family only needs 54~66 m2 of C3 or C4 green roofs to
tants drained out with the water flow (Syversen & Haarstad, 2005; regenerate the domestic wastewater they produced. Our data demon-
Kumwimba et al., 2020). Most of the pollutants in domestic wastew- strate that the extensive green roofs with C4 and C3 plant species could
ater are nutrients for plant growth (Cheuyglintase et al., 2018). Domes- be recommended for the ecological treatment of domestic wastewater
tic wastewater irrigation not only removes pollutants for wastewater at the household scale.
treatment but also saves the fertilizer input for vegetation maintenance
(Magwaza et al., 2020). Previous studies showed that nutrients adsorp-
tion, absorption, and utilization are varied with different plant species 4. Conclusions
(Mei, et al., 2014; Kumwimba et al., 2017). Here we find that C3 and C4
plants perform significantly higher capacity in pollutant removal than Green roofs in this experiment removed 79.27~97.38% of TSS,
CAM plants when exposed to domestic wastewater irrigation. There- 79.94~98.92% of COD, 65.26~90.52% of TN, 83.32~96.31% of NH4 + -
fore, it should be considered not only the substrate adsorption but also N, 77.83~93.97% of NO3 − -N, and 93.77~98.94% of TP, respectively.
the capacity variations of plant species in nutrient absorption and uti- Green roof with C4 and C3 plants contributed significantly higher runoff
lization when designing domestic wastewater irrigation for green roofs. reduction, removal of TSS, COD, TN, NH4 + -N, NO3 − -N, phosphates,
However, the effect of the green roof on pollutant removal under do- and TP than that of CAM plants. Water quality indicated by COD, TN,
mestic wastewater irrigation involves a dynamic balance of pollutant NH4 + -N, and TP from the effluent runoff complied with the widely ac-
input and plant uptake, which needs to be further verified by long-term cepted limits on domestic wastewater discharge. Based on the acquired
experiments. data, some concluding remarks can be reached: ○Green
1 roof could be a
nature-based solution for the complete treatment of domestic wastewa-
3.4. Practical recommendations ter and regenerate the urban water resource; ○Green
2 roofs with C4 and
C3 plants bring better effects on wastewater retention and pollutant re-
TSS, COD, and TP contents in the domestic wastewater in this ex- moval than CAM plants; ○C43 and C3 plants contributed an additional
periment exceeded the normal limits on direct discharge (TSS ≤ 50 mg 16.7~33.2% of pollutant removal besides the substrate.
L−1 , COD ≤ 120 mg L−1 , TP ≤ 3 mg L−1 ) according to the National Stan-
dard (GB 18918-2002, Discharge standard of pollutants for municipal
wastewater treatment plant, China) by 5.4, 4.0, and 1.5 times, respec- Main finding
tively. All green roofs successfully reduced the COD and TP contents to
their limits (Fig. 4, Table 4). However, the reduction of TSS was lim- Domestic wastewater could discharge directly after green roof irri-
ited by the wastewater treatment volume. To meet the criterion of TSS gation. C4 and C3 plants perform better pollutant removal than CAM
limit (50 mg L−1 ), the wastewater treatment volumes by green roofs plants.
8
- L. Liu, J. Cao, M. Ali et al. Environmental Advances 4 (2021) 100059
Declaration of Competing Interest Jim, C.Y., 2017. Green roof evolution through exemplars: Germinal prototypes to modern
variants. Sustain. Cities Soc. 35, 69–82. doi:10.1016/j.scs.2017.08.001.
Karczmarczyk, A., Bus, A., Baryla, A., 2018. Phosphate leaching from green
The author declares no conflict of interest exits in the submission of roof substrates-can green roofs pollute urban water bodies? Water 10, 199.
this manuscript. doi:10.3390/w10020199.
Kasak, K., Truu, J., Ostonen, I., Sarjas, J., Oopkaup, K., Paiste, P., Koiv-Vainik, M., Man-
der, U., Truu, M., 2018. Biochar enhances plant growth and nutrient removal in
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This work was financially supported by the National Key Research structed wetlands as progressive eco-technologies in urban areas. Sustainability 11,
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