Impact of green roof plant species on domestic wastewater treatment

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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.

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Nội dung Text: Impact of green roof plant species on domestic wastewater treatment

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 2
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., 3
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 4
L. Liu, J. Cao, M. Ali et al. Environmental Advances 4 (2021) 100059 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. 5
L. Liu, J. Cao, M. Ali et al. Environmental Advances 4 (2021) 100059 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. 6
L. Liu, J. Cao, M. Ali et al. Environmental Advances 4 (2021) 100059 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
L. Liu, J. Cao, M. Ali et al. Environmental Advances 4 (2021) 100059 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
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