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Review

Vertical Green Wall Systems for Rainwater and Sewage Treatment

by
Wen Wang
1,2,†,
Xiaolin Zhou
1,2,†,
Suqing Wu
1,2,
Min Zhao
1,2,
Zhan Jin
1,2,
Ke Bei
1,2,
Xiangyong Zheng
1,2,* and
Chunzhen Fan
1,2,*
1
College of Life and Environmental Science, Wenzhou University, Wenzhou 325000, China
2
National and Local Joint Engineering Research Center of Ecological Treatment Technology for Urban Water Pollution, Wenzhou University, Wenzhou 325000, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Sustainability 2024, 16(17), 7593; https://doi.org/10.3390/su16177593
Submission received: 13 August 2024 / Revised: 28 August 2024 / Accepted: 1 September 2024 / Published: 2 September 2024
(This article belongs to the Section Pollution Prevention, Mitigation and Sustainability)
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Abstract

:
Rainwater and sewage are important pollution sources for surface water bodies. Vertical greening systems (VGSs) are extensively employed for these wastewater treatments due to the green and sustainable characteristics, as well as their high-efficiency in pollutant (organic matter, nitrogen, and phosphorus) removal. At present, more and more VGSs are designed with green buildings, serving city ecosystems. This study provides an overview of different kinds of VGSs for rain and sewage treatment, emphasizing their types, design, mechanisms, selection of plants, and growth substrate. Plants play a crucial role in pollutant removal, and different plants usually obtain different efficiencies of water treatment. Climbing plants and ornamental plants with fast growth rates are priority selections for VGSs, including Canna lilies, Jasmine, Grape vine, Boston ivy, Pittosporum tobira, Pelargonium australe, Mentha aquatica, and Lythrum salicaria. The substrate is the most critical part of the VGS, which plays an important role in regulating water flow, supporting plant growth, promoting biofilm growth, filtering pollutants, and adsorbing nutrients. The single substrate either has a blockage problem or has a short holding time. Therefore, a number of studies have mixed the substrates and integrated the advantages of the substrates to form a complementary effect, thereby improving the overall purification efficiency and stability. Novel substrates (sand, spent coffee grounds, date seeds, coffee grinds, reed-based, etc.) are usually mixed with coco coir, light-weight expanded clay, growstone, or perlite at a certain ratio to obtain optimum treatment performance. Moreover, plants in clay show more significant growth advantages and health statuses than in zeolite or soil. Operating parameters are also significant influences on the treatment performance. This review provides theoretical and technical support for designing sustainable, environmentally friendly, and cost-effective VGSs in treating rainwater and sewage.

1. Introduction

The ecological design concept of VGSs has a profound historical background, and its origin can be traced back to the sky garden of Babylon 2500 years ago [1]. In 1937, Stanley Hart White proposed the concept of “vertical greening”, which is different from “plane greening” and includes the placement of plants horizontally and vertically on the façade [2]. The concept of the green wall was created by E.O. Wilson in 1984 and has been popular for a long time [3]. In 1994, Patrick Blanc went beyond pure green wall creation to showcase the first hydroponic system, “Mur Vegetal”, combined with green living walls [4]. The recent research and application of gray water recycling combined with vertical treatment systems originated in 2007 [5]. Kew et al. [6] explored the use of simulated runoff from the initial rainwater stored in reservoirs to irrigate green walls, providing a scientific basis and empirical support for the application of green walls in water treatment. In the following decades, it can be observed that people’s interest in VGSs for rainwater sewage treatment is growing, and it is expected to appear more frequently in architectural design in the next few years to provide a variety of services for urban ecosystems [7]. VGSs have broad application prospects in rainwater wastewater treatment, but the relevant literature is scattered, and the content may involve the system design (system geometry, plant type, matrix), working principle, purification effect, and operating conditions (irrigation method and hydraulic load) and other aspects. In this paper, the experimental data and case analysis of VGSs for rainwater sewage treatment are analyzed in detail, and the technical principle, treatment effect, and application prospect of VGSs for rainwater sewage treatment are summarized.
The existing standard mode of urban sewage treatment is centralized treatment, which covers the comprehensive collection and treatment of gray water and black water. The process of transporting gray water and black water through the sewage pipe network to the sewage treatment plant is efficient and convenient, but the infrastructure cost is high and the water resource utilization rate is low [8]. Large-scale and long-distance sewage pipe networks are prone to congestion and leakage risks, while the upgrading of existing underground facilities is costly and destructive, and the land used to expand centralized sewage treatment plants is often limited. In the existing on-site sewage treatment technology, a membrane bioreactor can effectively and efficiently remove organic and microbial pollutants in water without subsequent complex treatment [9]. However, the high operation and maintenance costs caused by membrane fouling have hindered the wide application of membrane bioreactors in domestic wastewater treatment [10]. The removal effect of pollutants in wastewater using an aerobic sequencing batch reactor combined with activated carbon adsorption is good [11], but the high cost of activated carbon, difficulty in regeneration, and repeated utilization are not convenient for long-term sustainable use. A constructed wetland is a natural-based low-cost wastewater treatment scheme. However, in densely populated urban areas, the horizontal space is very limited and the wide application is also low. Incorporating decentralized systems into the urban environment is an innovative strategy to alleviate the capacity bottleneck of existing centralized sewage infrastructures [12]. Therefore, the green wall originally installed on the side of the building for aesthetic and microclimate benefits has become an effective on-site sewage treatment solution worth considering.
Compared with other technologies, a VGS is a more suitable on-site technology for gray water with low energy consumption and cost. The decentralized treatment method represents a sustainable idea that can reduce the load of existing sewage treatment plants while maintaining all the benefits of traditional green walls [13]. The water discharged after in situ treatment can be further used for non-drinking purposes (flushing toilets, garden irrigation, flushing streets, etc.). Studies in the last ten years demonstrated continual attention on the application of VGSs in rainwater and sewage treatment (Figure 1). However, so far, to the best of our knowledge, there is not any comprehensive review on this topic. In order to explore and recommend the best green wall elements and understand and optimize its rainwater wastewater treatment potential, this paper reviews the existing case studies of VGSs in water treatment and reuse and summarizes the types, treatment mechanisms, design parameters, and benefits of this system for rainwater wastewater reuse treatment, so as to create a more flexible and effective VGS in the future.

2. Vertical Greening Systems

2.1. Definition and Classification

Vertical greening refers to the concept of planting green plants on the walls, windows, shading components, and external spaces of buildings [14], which is a method of greening in three-dimensional space corresponding to ground greening. In high-density urban areas, it simulates natural ecology, alleviates the urban heat island phenomenon, enhances carbon absorption and oxygen release capacity, and both beautify the environment and maintain an ecological balance.
The terminologies of VGSs are diverse [15], such as “green facades” [16], “living walls” [17], “biowalls” [18], “vertical gardens” [19], and “green walls” [20]. VGSs are divided into two main systems according to their construction method, growth medium type, and plant root position: green facade (external wall) and green living wall [20]. On this basis, according to the specific design, the module is further divided into sub categories, as follows (Figure 2).
The green facade (external wall), that is, through the climbing of vine plants or the setting of supporting structure in front of the wall to achieve wall greening, does not require a complex living wall system and only depends on the natural winding and the suspension ability of plants, which can be divided into direct or indirect (Figure 3) [21].
The living wall is composed of pre-cultivated plants, supporting the element growth substrate and vertical planting module of the irrigation system and is vertically fixed on the structural wall or frame, which is divided into continuous and modular (Figure 4) [20,22,23]. The continuous living wall is based on a single supporting structure, while the modular living wall is composed of multiple supporting structures to form the whole living wall system, which is divided into a block design and flowerpot (or compartment) design [24].

2.2. Governance Mechanisms

VGSs mainly use the synergistic effect of plants, substrates, and microorganisms to purify rainwater and sewage by simulating the ecological functions of natural wetlands. Specifically, rain sewage is first intercepted by plant leaves and partially evaporated into the air, and the remaining part is purified via filtration, adsorption, precipitation, redox, and other processes of plant roots and substrates [25]. At the same time, the activities of microorganisms in the substrate also promote the decomposition of organic matter and the transformation of nutrients, such as nitrogen and phosphorus.
The substrate plays an important role in the system, providing support for plant growth and providing an adhesive surface for microorganisms in the system [26]. Through the use of physical forces (such van der Waals forces), it may absorb nitrogen from water and adhere it to the matrix’s pores or surface. The mineral components (rich in cations, such as Ca2+, Mg2+, Al3+, and Fe3+) in the substrate and the surface of the organic matter have a negative charge or positive charge, which can adsorb PO43−-P (phosphate) or P (phosphorus) in the water [27]. Plant biomass can improve the removal rate of nitrogen in low-polluted water [28]. Plant root redox-related microorganisms play an important role in nutrient cycling [29]. In addition, substrates can also indirectly promote biological processes, such as nitrification and denitrification, to remove pollutants by providing attached surfaces and nutrients for plants and microorganisms [30]. The main benefit of plants is to provide oxygen through roots and promote microbial activity and the direct absorption of nutrients [31]. In addition, plants can also change the soil porosity to increase the time for pollutants to be exposed to the substrate to promote the removal process [32], but for plants with slow growth and shallow roots, they are mainly removed through substrate adsorption and microbial activity [33]. Microorganisms use their own metabolic functions to decompose and transform pollutants in water and also promote the transfer of oxygen to the root zone for aerobic bacteria to colonize and degrade organic pollutants [34]. As an economical and energy-saving new biological process, iron ammonia oxidation is more suitable for the treatment of domestic sewage, because it has great potential in wastewater denitrification [35].
In summary, water remediation in VGSs involves a wide range of biological and physicochemical processes. Using physical filtration and sedimentation to remove pollution, combined with chemical reaction and adsorption technology, and relying on biological processes, such as microbial degradation and biofilm attachment, water purification is achieved [36]. Figure 5 shows the various processing methods involved in this process.

3. Research Status

3.1. System Design and Configuration

3.1.1. Plant Selection

The plant species is one of the key variables in VGS design. It establishes the features of the habitat and the necessary growth matrix, which is a critical system construction, in addition to providing ecological value and shaping the landscape’s aesthetics. The screening of plant species that are appropriate for VGSs and have a major impact on rain-sewage purification is important since not all plant species are suited for rain-sewage irrigation or can live in the presence of chemicals in the irrigation medium. The vertical greening plant species involved today are shown in Table 1.
Some studies have found that there are significant differences in the removal efficiency of organic matter, solids, and nutrients between planting systems and non-planting systems. It was found that the presence and type of plants have a great influence on the removal of phosphorus in the gray water of the living wall [38], and the development of roots and the growth rate of plants also affect the effective removal of TN (total nitrogen) [39], which indicates the importance of the correct selection of plants. Similarly, it was found that the average TN-removal rate of plants with higher performance was more than 88% that of the non-vegetation configuration [33]. More interestingly, phosphorus absorption improved over time and was more reliant on the plant type. Among them, the removal rate of good plants during standard operation was 34–53%, indicating that plant growth was an important TP-removal mechanism in the green wall [33]. Previous studies have shown that the presence of plants also increased the bacterial density and the removal rates of organic matter (90.9–95.9% of the chemical oxygen demand (COD) and 95.2–98.5% of the biochemical oxygen demand (BOD5)), nitrogen (74.3–84% of TN and 76–84% of ammonia nitrogen (NH4+-N)), and phosphorus (77.4–96.9% of PO43−-P), increased by 5.9–24.1% compared with the control bed [40]. Compared with the non-vegetated system, the removal rate of the vegetation system is slightly higher, and the addition of vegetation can improve the overall treatment performance of the system [41]. In recent studies, it was confirmed that plant selection is more critical than substrate or irrigation regime selection [42]. Different from the above, some studies have found that the choice of substrate is a factor that affects the treatment performance more than plant selection [43,44]. Some tests found no significant difference in the effects of adding plants on BOD, COD, and TSS (total suspended solid) in VGSs [45].
Due to the limited choice of climbing plants for gray water treatment and aesthetic considerations, ornamental plants have also begun to appear in VGSs. Hydroponic plants (three lettuce varieties) have also been used for gray water reuse in green wall systems [46]. Considering the winter open-air conditions, the vegetation needs to withstand high and continuous gray water treatment, and choosing cold-tolerant, high-soil-moisture, and fast-root-growth perennial plants has begun [47,48]. Mårtensson et al. tested seven edible, seven evergreen, and one both edible and evergreen species in a living wall system in a continental climate region, finding that the evergreen perennial plant species Chamaecyparis pisifera, Euonymus fortuneii, Euphorbia polychroma, Vinca minor, and Waldsteinia ternata can grow in the green wall [49]. The edible evergreen plant Vaccinium vitis-idea is ideal for living walls in this climate region. Fowdar et al. selected a series of ornamental plants according to the ability to tolerate waterlogging conditions, high nutrient environments, and high salinity, including deciduous tree species and evergreen tree species, which performed well in removing organic matter and suspended solids [38]. After that, a series of studies on the gray water treatment of various ornamental plants (including climbing plants and flowers) confirmed that ornamental plants can successfully adapt to the gray water irrigation system and play an important role in the absorption nitrogen and phosphorus in gray water [26,41,45,50].
In addition, the best plant species used in the system will also depend on many key criteria, one of which is the rate of plant development [51]. In order to cope with drought conditions and withstand harsh conditions, such as inclined rooting and a limited growth space [52], the fleshy water storage of roots, leaves, or stems of the plant group plays an important role. Low-transpiration species with succulent roots, buds, or leaves or shallow main root species that can adapt to a thinner substrate layer can be selected [53]. The plant water requirement is also a very important parameter for greening walls. Prodanovic et al. tested five species and observed differences in water requirements between plant species placed in the same substrate [54]. Chung et al. also found that the total daily water consumption and daily water consumption per unit leaf area of different species of climbing plants were significantly different [55]. The development of roots and their position on the wall also have a great influence on the overall performance of plants [56].
Table 1. Vertical green plant species used for wastewater treatment.
Table 1. Vertical green plant species used for wastewater treatment.
PlantsMain FindingsSubstrate ComponentsReferences
Trachelospermum jasminoides, Lonicera japonica, Callistemon laevisSlightly higher removal rate was obtained in the vegetation systemWashed sand or vermiculite[41]
Calamagrostis epigejos, Geranium pratense, Origanum vulgare, Prunella vulgaris, Scirpus sylvaticusSubstrate types and plants improved microorganisms growth [43]
Einadia nutans, Goodenia varia, Dianella revoluta, Myoporum parvifolium, Dichondra repens, Westringia fruticosaPosition of plants and amount of irrigation significantly affected total dry weight of plantsOrganic sandy loam, native soil, potting mix[42]
Calamagrostis epigejos, Geranium pretense, Origanum vulgare, Prunella vulgaris, Scirpus sylvaticusPGPM inoculation enhanced the growth of C. epigejos in the reed-based substrateReed or sandy loam-based substrates[57]
Mixed collocation of 39 kinds of plantsEupatorium cannabinum, Mentha aquatica, Sedum telephium, Eriophorum vaginatum, Thelypteris palustris, and Lythrum salicaria were suitable for gray water treatmentExpanded clay (4–8 mm), zeolite (1–2.5 mm), perlite (0–6 mm), sand (0.06–2 mm), and crushed expanded clay (0–8 mm)[58]
Andropogon gayanus, Chrysopogon zizanioides, Echinochloa pyramidalis, Pennisetum purpureum, Tripsacum laxumPlants increased bacterial density and the removal rate of organic matter, nitrogen, and phosphorus in VFCW10 cm gravel layer (5/15 mm) covered with geotextile cloth and a 60 cm white lagoon sand layer[40]
Akebia quinate, Gelsemium sempervirens, Jasminum azoricum, Pandorea pandorana, Trachelospermum jasminoides, Vitis “Ganzin Glory”P. pandorana showed the largest biomass regardless of the irrigation method, with the smallest biomass for A. quinata, J. azoricum, and T. jasminoidesScoria-based substrate[55]
Mentha aquatica L., Oenanthe javanica (Blume) DC, a mixture of Lysimachia punctata L. and Lysimachia nummularia L.Vegetation types insignificantly affected the treatment performance of VGSs in different seasonsLightweight inert planting material LECA®[50]
Armeria maritima Willd, Campanula persicifolia L., Dianthus deltoides L., Geranium sanguineum L., Hypericum perforatum L., Knautia arvensis (L.) Coult, Leucanthenum vulgare Lam, Saxifraga granulate L.Plants with well-developed fleshy roots, buds, or leaves had higher ability to cope with drought conditionsMineral wool (Grodan TT®)[59]
Hedera helix, Carex morrowii, Iris germanica, Lonicera nitida, Ranunculus asiaticusRanunculus and Iris had poor moisture resistance. Lonicera, Carex, and Hedera could tolerate high humidity, temperature oscillation, and sunlightCoconut fiber (CF) and perlite (PL)[47]
Pittosporum tobira, Polygala myrtifolia, Hedera helixMore than 90% of BOD, COD, and TSS could be removed10 cm coarse gravel (20–40 mm), 10 cm fine gravel (5–15 mm), and 40 cm washed sand (0.1–2 mm)[45]
Peperomia magnoliiaefolia, Kalanchoe blossfeldiana, Aptenia cordifolia, Carpobrotus edulisPlant species and growth medium types significantly affected plant growth and morphological improvementCocopeat, perlite, cocopeat + perlite (1:1), and cocopeat + perlite + vermicompost (1:1:1)[60]
Carex appressa, Nadina domestica), Antirrhinum majus, Ophiopogon japonicus, Agapanthus praecox, Nephrolepis obliterate, Viola tricolor, Liriope muscari, Patersonia occidentalis, Nasturtium officinale, Myoporum parvifolium, Dianella tasmanica, Phomium tenax, UnvegetatedO. japonicus, P. occidentalis, and N. officinale had poor performance with NOx emission. P removal of VGS systems depended on plant growthMixture of perlite and coco coir (with a ratio 1:2)[33]
Ruellia brittoniana, Alternanthera dentata, Typha domingensis, Acalypha wilkesiana, Koeleria glauca, Portulaca grandifloraSubstrate type posed higher influence on pollutant removal than plant speciesPerlite, coco coir, light-weight expanded clay (LECA), sand, spent coffee grounds (SCG), and date stones[44]
Einadia nutans, Festuca glauca, Goodenia varia, Lomandra filiformis, Scaevola albida ‘mauve cluster’, Pelargonium australeGV, PA, and EN were the most resilient plant species. LF and FG gramineous plants had the lowest survival rateOrganic sandy loam, organic sandy loam mixed with 10% v/v kaolin clay, and bentonite clay (1: 3) or mixed with 10% v/v commercial grade zeolite[61]
Caltha palustris, Carex acutiformis, Carex appressa, Carex elata, Carex riparia, Filipendula ulmaria, Juncus effusus, Juncus inflexus, Lonicera crassifolia, Lythrum salicaria, Mentha aquatica, Nasturtium officinale, Valeriana officinalis, Veronica beccabungaNasturtium officinale with a short life cycle was not suitable for VGSs. Height and irrigation only significantly affected the vigor of Caltha palustris and Mentha aquatica100% Vulkaponic, 75% Vulkaponic + 25% plant-based biochar and 75% Perlite + 25% coco peat[62]
Campanula poscharskyana ‘Stella’, Geranium sanguineum ‘Max Frei’, Sesleria heufleriana, Veronica officinalis ‘Allgrün’Veronica and Sesleria changed root growth due to competitionCoir-based substrate and stone wool-based growing medium[56]
Carex appressa, Canna lilies, Phomium, Boston ivy, non-vegetated, Grape vine, Strelitzia reginaeRoot proliferation, root length density, and specific root length were important for plant selection [39]
Vitis vinifera, Parthenocissus tricuspidate, Pandorea jasminoides, Billardiera scandens, Strelitzia Nicolai, Phormium spp., Canna lilies, Strelitzia reginae, Lonicera japonica, Carex appressa, Phragmites australisPresence and type of vegetation significantly affected the TN-removal efficiency500 mm washed sand, 70 mm gravel, 70 mm coarse sand, 160 mm deep horizontal panels[38]
Lactuca sativa ‘Lobjoits Green Cos’, Lactuca sativa ‘Red Salad Bowl’, Lactuca sativa ‘Australische Gele’Multi-leaf lettuce needed more N for its growth. Different plant varieties exhibited different absorption rates of heavy metals, with relatively low value of “Australische Gele”Perlite[63]
Acinos alpinus, Allium schoenoprasum, Calamintha nepeta, Fragaria vesca, Hyssopus officinalis, Rubus stellarcticus, Thymus vulgaris, Chamaecyparis pisifera, Euonymus fortuneii, Euphorbia polychrome, Ilex crenata, Luzula sylvatica, Vinca minor, Waldsteinia ternate, Vaccinium vitis-ideaVaccinium vitis-idea is ideal for living walls. H. officinalis, T. vulgaris, and L. sylvatica were not suitable for rockwool living walls with low irrigation levelsStandard construction soil (AMA A)[49]
Abelia, Wedelia Portulaca, Alternenthera, Duranta, HemigraphisPotential of VGSs in treating grey water was demonstratedLECA® (lightweight expanded clay aggregate)[64]
Typha, Iris, Carex, Cyperus, Ficus, Spathiphyllum, EpiprenumPlants were symbiotic with rhizosphere microorganisms, achieving excellent water purification abilityExpanded clay[65]
Epipremnum aureumRemoval of turbidity, BOD, and TN was similar in the presence or absence of plant biofilmExpanded recycled glass[66]
Helichrysum thianschanicum specie15 db of weighted noise reduction index was obtained using VGS [67]
Campanula poscharskyana ‘Stella’, Fragaria vesca ‘Småland’, Geranium sanguineum ‘Max Frei’, Sesleria heufleriana, Veronica officinalis ‘Allgrün’Fragaria, Geranium, and Veronica exhibited a stable increase in root growth. Campanula showed an initially slow but finally rapid trend in root growthCoir and 2 of rockwool[48]
Sedum Angelina, Sedum ternatum, Sempervivum tectorum, Ajuga reptansVGS provided an alternative solution for rainwater management by green roof systemsPlanting soil mixed with polymer granules[6]
P. laurocerasus, Jasminum officinale ‘Clotted Cream’, Hedera helix, Stachys byzantine, Fuchsia ‘Lady Boothby’, Lonicera ‘Gold Flame’Fuchsia promoted evapotranspiration cooling, while Jasminum and Lonicera were more prominent in shade cooling [68]
Momordica charantia, Ipomoea tricolor, Canavalia gladiate, Pueraria lobata, Apios american MedikusCoverage is crucial for reducing the wall temperature, which was determined by the development of the vine length [69]
“Amerikanischer brauner” lettuce, “Nores” spinach, “Half tall” leaf cabbage, marigolds (Lord Nelson)More than 95%, 80%, 90%, 30%, and 69% of BOD5, COD, TSS, TN, and TP could be removedLightweight expanded clay aggregates (LECA), NR 2–4 mm Filtralite[46]
Lobelia erinusSalt tolerance was an important factor for selecting plants for vertical gardens and green roofsSoilless culture[70]
Sedum ellecombianum, S. floriferum “Weihenstephaner Gold”, S. kamtschaticum, S. spurium “Dragon’s Blood”, S. reflexum, Allium cernuum, A. senescens subsp. Montanum, Turfgrass mixtureAllium cernuum, A. senscens, and S. ellecombianum showed strong tolerance to high salt concentrations [71]

3.1.2. Planting Substrate

The substrate is the most critical part of the VGS, which plays an important role in regulating water flow, supporting plant growth, promoting biofilm growth, filtering pollutants, and adsorbing nutrients. The substrates used for water treatment in VGSs have various types and different mixing ratios (Table 1, Table 2). VGSs are vertically arranged and directly attached to the building. To avoid a higher load on the supporting structure, the substrate with high porosity and poor water retention capacity will lead to an increase in the vertical change of the water content and an uneven air distribution [48]. The morphology, particle size, porosity, cation exchange capacity, water holding capacity, and specific surface area of the substrate will affect plant growth, hydraulics, and water treatment efficiency.
Single substrates such as coco coi [44,48,56,72,73], rockwool [48,56,72], fyto-foam [72], expanded clay [72], light-weight expanded clay (LECA) [44,46,50,64], growstone [72], perlite [44,63,72,73], vermiculite [41,72,73], filtralite [46], sand [41,44,72], expanded clay [65], mineral wool [59], and expanded recycled glass [66] have been used in different VGSs.
Masi et al. tested the green wall gray water treatment filled with LECA in the early stage, but the treatment effect was general [64]. The removal effect of pollutants in gray water was significantly improved after the development of LECA plus sand and LECA plus coconut fiber. Prodanovic et al. evaluated the pollutant removal performance of hydraulically slow (coco coir, rockwool, and fyto-foam) and hydraulically fast (perlite, vermiculite, growstone, expended clay, and river sand) in the living wall [72]. The results showed that the slow medium had high pollutant-removal performance. The average removal rates of TSS, TN, TP, COD, and E.coli were about 90%, 50%, 30%, 70%, and 80%, respectively, but the fine pore size was easily blocked, and perlite had the best hydraulic and treatment performance in the fast medium, while coco coir was the best in the slow medium. Therefore, they considered the mixing of various proportions of perlite and coco coir (4:1, 3:1, 2:1, 1:1, 1:2, 1:3) and pointed out that the large proportion of coir was conducive to improving gray water treatment, while the increase in perlite increased hydraulic permeability [74]. Similarly, follow-up studies have confirmed that when the ratio of coir:perlite is 3:1, the removal rates of COD, TSS, and turbidity in the lake water are the highest [75]. Pradhan et al. studied the removal rate of pollutants (organic matter, solids, nitrogen, and phosphorus), which was greater than 90% when using high-surface-area and small-diameter media (such as coco coir, spent coffee grounds, and sand) [44]. Subsequently, the effects of spent coffee grounds and date stones on pollution removal and treatment are described, which are comparable to those of coco coir and kerosene, respectively. It goes on to explain that the drainage is further enhanced and that the mixing performance of SCG and date stones is superior to that of a mixture of perlite and coco coir [76]. The performance of five substrates (zeolite, perlite, date seeds, coffee grinds, and coco coir) for the removal of six xenobiotic organic compounds (XOCs) was evaluated. Compared with natural minerals (zeolite and perlite), the use of carbon-containing wastes (jujube seed, coffee grinding, and coco coir) to remove XOC is more effective [73]. Previous studies have revealed that the inoculation of plant growth-promoting microbes (PGPMs) on vegetation roofs and VGSs can enhance plant biomass and increase the rainwater-retention capacity [57,77]. Shu et al. constructed four different treatments (whether PGPM is inoculated or not) and observed that the nutrient and metal content of sandy loam substrate runoff was 50–90% lower than that of reed-based substrate runoff, which could absorb nutrients and retain metals more effectively [43].
Table 2. Different types of substrates for VGSs.
Table 2. Different types of substrates for VGSs.
Substrate ComponentsMain ResultsReferences
Washed sand, vermiculite
(Coarse gravel 10 cm, fine gravel 10 cm, sand washing, or vermiculite 40 cm)		Sand-carrying system exhibited significantly higher removal rates of turbidity and COD than vermiculite-carrying system. Smaller sand achieved longer contact time with gray water, obtaining slightly larger adsorption of P[41]
Eed-based substrate without inoculation (NR), sandy loam substrate without inoculation (NL), reed-based substrate with inoculation, sandy loam substrate with inoculation (IL)Type of substrate was a main affecting factor for leaching concentration and total load of elements. Sandy loam substrate exhibited higher ability to absorb nutrients and retain metals than reed-based substrate[43]
Organic sandy loam, native soil, potting mixSignificant differences existed between plant species in the substrates[42]
Zeolite, perlite, date seeds, coffee grinds, coco coirLow solubility and hydrophobic XOCs were effectively removed by coco coir, while polar hydrophilic XOCs were effectively removed by zeolite[73]
PGPM inoculation + reed-based substrate, PGPM inoculation + sandy loam substrate, non-inoculation + reed-based substrate, non-inoculation + sandy loam substrateA well-ventilated sandy loam substrate significantly reduced residual water and achieved higher water-use efficiency. Microbial inoculation reduced the runoff of VGS[57]
90% BM + 10% GAC (granular activated carbon), 80% BM + 20% CO (compost), 80% BM + 20% BC (Biochar), 80% BM + 20% PA (polyacrylate), 60% BM + 20% PA + 20% BC
(BM = 80% coconut fiber and 20% perlite)		Biochar was the optimum additive. Composting showed the removal performance, but easy blocking[78]
Different proportions of coconut fiber and perlite (60% CF + 40% PL, 70% CF + 30% PL, 80% CF + 20% PL, 90% CF + 10% PL and 100% CF + 0% PL)Mixture of 80% CF + 20% PL obtained the optimum weight (0.37 ± 0.02 g/cm3) and hydraulic conductivity (0.46 ± 0.013 cm/s)[47]
Cocopeat, perlite, cocopeat + perlite (1:1, v:v), cocopeat + perlite + vermicompost (1:1:1, v:v)Perlite had the lowest water-holding capacity. Single or combined using of cocopeat and perlite increased the amount of fine dust in air[60]
Perlite, coco coir, LECA, sand, spent coffee grounds (SCG), date seeds, perlite-coco coir (1:1 or 1:2), and date seeds-SCG (1:1 or 1:2)Coco coir, sand, and SCG were more effective in removing solids, organic matter, and nutrients than perlite, date seeds, and LECA.[76]
Coir:perlite (3:1, 1:1, 1:3)The highest removal rates of COD, TSS, and turbidity were achieved at coir:perlite of 3:1[75]
Perlite, coco coir, light-weight expanded clay (LECA), sand, spent coffee grounds (SCG), and date stonesMore than 90% of organic matter, solids, N, and P could be removed by coco coir, SCG, and sand, which had a higher surface area and smaller diameter. Medium selection was more important than plant selection for treatment performance[44]
100% Vulkaponic, 75% Vulkaponic + 25% plant-based biochar, and 75% perlite + 25% coco peatVulkaponic was more effective in removing COD, while Perlite + coco peat was more suitable for BOD removal[62]
Organic sandy loam, organic sandy loam + 10% v/v kaolin clay and bentonite clay (1:3), organic sandy loam + 10% v/v commercial grade zeoliteSignificant differences in total dry weight existed among different plants and three soil substrates. Healthier plants were found in clay than in zeolite and loam[61]
Coir-based substrate (Quick-Plug) and stone wool-based growing medium (Grodan TT100)Plant roots in coir-based substrate were longer than stone wool-based substrate[56]
Perlite:coir (4:1, 3:1, 2:1, 1:1, 1:2, 1:3)A large proportion of coir with slow drip irrigation improved the pollutant removal[74]
Hydraulically slow: coco coir, rockwool, fyto-foam; hydraulically fast: growstone (2–4.75 mm), expanded clay (less than 9.5 mm), vermiculite (2–4.75 mm), perlite, river sand (2–4.75 mm)Slow media achieved a higher removal of N and COD through biological processes than fast media, while it relied on physical and chemical processes[72]
LECA, LECA plus sand, LECA plus coconut fibersLECA plus coconut fibers showed a higher removal rate than LECA plus sand configuration[64]
Coir and two of rockwool (Quick-Plug, Grodan PP100, and Grodan TT100)Taller plants with higher root frequency and stronger root growth were found in Quick-Plug[48]
Each substrate has its unique morphology, porosity, cation exchange capacity, water holding capacity, and specific surface area. A single substrate either has clogging problems or has a short retention time. Therefore, a number of studies have mixed the substrate and integrated the advantages of the substrate to form a complementary effect, thereby improving the overall purification efficiency and stability. Bustami et al. selected three substrate combinations (organic sandy loam, organic sandy loam + kaolin clay + bentonite clay, organic sandy loam + commercial grade zeolite) for testing, and observed that plants showed more significant growth advantages and health statuses in clay than in zeolite or loam [61]. Comparing 100% Vulkaponic, 75% Vulkaponic + 25% plant-based biochar, and 75% perlite + 25% coco peat, it was found that Vulkaponic showed a significant effect on COD, and perlite + coco peat mixed treatment promoted the biodegradation process and achieved the best performance in BOD removal [62]. In the open winter environment, the aim is to find a substrate formula that can optimize the balance between hydraulic conductivity and bulk density, analyze the mixing effect of coconut fiber (CF) and perlite (PL) under different ratios, and finally select the 80% CF/20% PL mixture for experiments [47]. Four carbon-based inhibitors (compost, biochar, granular activated carbon, and polyphenols) were tested in different combinations on the basis of BM (80% coconut fiber and 20% blended) [78]. All of the additives showed good removal efficacy for both Escherichia coli (>98%) and BOD5 (>95%), with biochar emerging as the top performer (51% of COD, 47%of NO3−-N, 71%of anionic surfactant).

3.1.3. Geometric Design

VGSs also involve system design optimization. VGSs are classified as either continuous or modular, with the modular category further subdivided into the block design and flowerpot design. Both systems have advantages in different aspects, but compared to continuous [48,56], the modular containerized design is more widely used in sewage treatment. Van de Wouw et al. explored the difference between block and flowerpot designs in rainwater-collection efficiency and evaporation loss after two months of detailed tests in an outdoor environment [79]. The study revealed that the block system needed regular irrigation and the evaporation was significant, but the flowerpot design showed higher rainwater-accumulation capacity, higher irrigation efficiency, and a lower water demand. Well-designed flowerpots can significantly optimize the environment by reducing pathogen breeding, enhancing water and air circulation efficiency, and effectively regulating resource competition among plants. Prodanovic et al. systematically evaluated the difference in the pollutant removal efficiency between a flowerpot and block design and found that both showed considerable purification capacity, and especially, the removal efficiency of the top greening layer was the most significant [80]. Although the block design shows stronger drought tolerance, the flowerpot design has advantages in maintenance convenience and odor management. In addition, the study also found that the number of layers significantly affected the removal efficiency of TSS, COD, and TN. The color, pH, and EC increased with the number of stages, and the two-layer green wall (300–450 mm) was a more economical and efficient design choice [80].

3.2. Operating Condition

3.2.1. Irrigation Method

As shown in Table 3, irrigation methods include drip irrigation, micro-spraying, automatic irrigation, infiltration irrigation, and manual irrigation. The drip pipe network is often deployed in VGSs, and the water is evenly distributed to the growth substrate in the vertical and horizontal directions by using the gravity and lateral infiltration mechanism to ensure reasonable irrigation [81]. Jin et al. investigated how different drip irrigation frequencies affect the way VGSs treat black water [82]. The analysis indicated that reducing the drip irrigation period greatly improved the effluent DO concentration and COD-removal efficiency, while the TP-removal rate trended downward. The effects of drip irrigation and vertical irrigation (i.e., top-down irrigation) on nutrient removal efficiency in a gray water treatment process were compared in a greenhouse [62]. Drip irrigation was shown to promote plant development; however, the rates at which COD and BOD were removed were marginally lower. Summer water consumption is 3–4 times higher than winter due to plant water absorption and transpiration, while plant water absorption at the top of the multi-level green wall is 4 times more than the bottom layer [54]. Prodanovic et al. discovered that during drying in the summer, the concentration of DON outflow dramatically rose, and the phosphorus type with the most performance drop was FRP [33]. Additionally, the soil was found to have been mostly moist during the winter drying season, and drying had less of an impact on the soil’s ability to remove phosphate and nitrogen.

3.2.2. Hydraulic Loading Rate (HLR)

Fowdar et al. discovered that the TN-removal effectiveness of most plant species was equivalent under 110 mm/d and 55 mm/d HLR conditions, while the TN-removal rates of non-vegetated controls, S. nicolai, and Grapevine were dramatically enhanced at a low HLR [38]. A low HLR in particular strongly encouraged TP removal, with the exception of C. appressa, the non-vegetated control, Grape vine, and S. reginae. Employing 55 mm/d or a lower HLR is recommended for the optimum therapeutic result. However, Prodanovic et al. pointed out that the change in HLR had no significant effect on TN removal, and efficient plants, such as C. appressa and N. oblerata, were stable and efficient in removing NOx under various HLRs [33]. The results showed that C. appressa, N. oblerata, and L. muscari were able to considerably absorb FRP at a low (2 L/d) to standard HLR (4 L/d); however, at a high HLR (8 L/d), their efficiency was restricted. Stefanatou et al. found that the HLR was set to 39 mm/d in the first four months of the device and increased to 204 mm/d in the later stage [41]. After the increase in HLR, the organic matter in the effluent increased, which strengthened denitrification and significantly improved the removal efficiency. However, in the following phases, the Fv/Fm ratios of T. jasminoides and L. japonica declined due to the rise in organic and hydraulic loading rates. Additionally, Iris pseudacorus and Eichornia crassipes were unable to survive increased nutrient concentrations and higher sewage loading rates. Furthermore, Chua et al. demonstrated that the lake water flow rate had a major impact on the effectiveness of the green wall system: at a flow rate of 166.4 L/h, perlite floating increased the medium’s porosity, which caused a sudden increase in the TSS concentration; the pollutant removal effect is limited when the flow rate deviates from 28 L/h [75].

3.2.3. System State

Assessing the long-term impacts of system aging on treatment performance is crucial for treating water pollution in a VGS in order to ensure continuous and efficient pollutant-removal capabilities [84]. Compared with two modular three-layer green walls (P1 2020 set, P2 2018 set), the usage significantly affected the purification efficiency of TSS (P1: 97.4 ± 4.6%, P2: 89.1 ± 13.8%) and MBAS (P1: 92.7 ± 3.8%, P2: 75.3 ± 8.0%). Kotsiaa et al. pointed out that the phosphorus-removal efficiency of sand decreased year by year, and the phosphorus-removal rate of the system decreased from 100% in the first month to 15% in the next year [45]. An enhanced nitrogen cycle and efficient solid suspended solids interception were facilitated by the more flawless and stable construction of plant and microbial communities in the mature system, which also resulted in a significant improvement in the effluent quality of COD, BOD, NH4+-N, and SS [85].

4. Conclusions and Prospects

As a kind of green and sustainable ecosystem, the VGS has been widely used in on-site rainwater and sewage purification and recycling. The selections of plants and their growth substances are the two main and curial factors for determining the water purification efficiencies of VGSs. Different plant species possess different growth and reproduction characteristics, environmental adaptability, pollutions resistance, and water purification performance. The preferred plants for VGSs mainly include fast-growing climbing and ornamental plants, such as Canna lilies, Jasmine, Grape vine, Boston Ivy, Pittosporum tobira, Pelargonium australe, Mentha aquatica, Lythrum salicaria, etc. Plants with strong roots and growth, such as Lonicera japonica, Euonymus fortunei, Hedera helix, Liriope muscari, and Carex appressa, are also favored. In addition, substrates can significantly affect plant growth and the water purification performance of VGSs. Coco coir, LECA, growstone, perlite, and rockwool are commonly used substrates in VGSs. Some novel substates, such as SCG, date seeds, coffee grinds, reed-based substrates, organic sandy loam, biochar, granular activated carbon, and polyacrylate are gradually applied. Furthermore, the combined substances have attracted more attention in recent several years due to their improvement in purification efficiency and stability of VGSs.
The operation of VGSs mainly depends on the irrigation style and hydraulic operation time. The drip irrigation pipe network is often arranged in the VGS, evenly distributing water to substances in vertical and horizontal directions through gravity and lateral infiltration effects. The suitable hydraulic operation time for a VGS is 55 mm/d or less.
Further studies on the combination of VGSs and other water treatment systems are needed. A combination of different plant species and different substances is also one of the future research focuses. Meanwhile, long term high-efficiency and stability should be given attention. In addition, the economic benefits and sustainable development of VGSs still need to be evaluated. Precautions for preventing plants exposed to sewage water from dying or becoming sick should be explored. The large challenges of the present application of VGSs are odor emission and breeding of mosquitoes and insects, as well as the high cost of system installation and plant maintenance. Therefore, it is necessary to develop new and more economical VGS systems in future works.

Author Contributions

Conceptualization, W.W., X.Z. (Xiaolin Zhou), and S.W.; methodology, W.W., X.Z. (Xiaolin Zhou), and S.W.; software, W.W., X.Z. (Xiaolin Zhou), and S.W.; validation, Z.J. and K.B.; formal analysis, W.W., X.Z. (Xiaolin Zhou), and S.W.; investigation, W.W., X.Z. (Xiaolin Zhou), and S.W.; resources, W.W., X.Z. (Xiaolin Zhou), and S.W.; data curation, W.W., X.Z. (Xiaolin Zhou), and S.W.; writing—original draft preparation, W.W., X.Z. (Xiaolin Zhou), and S.W.; writing—review and editing, M.Z., X.Z. (Xiangyong Zheng), and C.F.; visualization, Z.J. and K.B.; supervision, X.Z. (Xiangyong Zheng) and C.F.; project administration, W.W., X.Z. (Xiaolin Zhou), and M.Z.; funding acquisition, X.Z. (Xiangyong Zheng) and C.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key R&D Program of China (No. 2022YFE0106200).

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to funder restrictions.

Acknowledgments

The authors express their sincere gratitude for the work of the editor and the anonymous reviewers.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Publications on the application of VGSs in rainwater and sewage treatment in the last ten years.
Figure 1. Publications on the application of VGSs in rainwater and sewage treatment in the last ten years.
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Figure 2. Depending on how they were built, several types of green walls are categorized. Reproduced with permission from ref [20], Copyright 2015, Elsevier.
Figure 2. Depending on how they were built, several types of green walls are categorized. Reproduced with permission from ref [20], Copyright 2015, Elsevier.
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Figure 3. Green facade and image: (a) direct green facade; (b) indirect greening facade.
Figure 3. Green facade and image: (a) direct green facade; (b) indirect greening facade.
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Figure 4. Living wall sketch and image: (a) continuous living wall; (b) modular living wall-type 1; (c) modular living wall-type 1.
Figure 4. Living wall sketch and image: (a) continuous living wall; (b) modular living wall-type 1; (c) modular living wall-type 1.
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Figure 5. The removal mechanisms of plants, media, and microbes. Reproduced with permission from ref [37], Copyright 2018, Elsevier.
Figure 5. The removal mechanisms of plants, media, and microbes. Reproduced with permission from ref [37], Copyright 2018, Elsevier.
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Table 3. VGSs of different irrigation methods.
Table 3. VGSs of different irrigation methods.
Irrigation MethodIrrigation FrequencyMain ResultsReferences
(1) 133 mL/d (2 min @ 4 L/h); (2) 100 mL/d (3 min @ 2 L/h)There was no significant difference between the two irrigation methods[42]
Slow drip irrigation system(1) Standard (five times per week); (2) dry period (no inflow)—14 d (two weeks)1. Drying reduced the nitrogen- and phosphorus-removal performance of the system, but the plant species with high performance were less affected
2. FRP is the type of phosphorus with the largest decrease in performance after drying		[33]
Irrigation pipeline and lineDrip and top-down irrigationPlant growth benefits from drip irrigation; however, COD and BOD removal rates are low[62]
Rubber tube and drip irrigation sprinkler (0.1 L/min)(1) 5 min/60 min; (2) 5 min/40 min: (3) 5 min/20 minShortening the drip irrigation time significantly enhanced the COD-removal efficiency, but the TP-removal rate showed a decreasing trend[82]
Drip tubes(1) non-irrigation; (2) irrigated 4 times per day (3.30 min each, 2 L/h)An irrigation system is essential to the live wall vegetation’s proper existence[83]
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Wang, W.; Zhou, X.; Wu, S.; Zhao, M.; Jin, Z.; Bei, K.; Zheng, X.; Fan, C. Vertical Green Wall Systems for Rainwater and Sewage Treatment. Sustainability 2024, 16, 7593. https://doi.org/10.3390/su16177593

AMA Style

Wang W, Zhou X, Wu S, Zhao M, Jin Z, Bei K, Zheng X, Fan C. Vertical Green Wall Systems for Rainwater and Sewage Treatment. Sustainability. 2024; 16(17):7593. https://doi.org/10.3390/su16177593

Chicago/Turabian Style

Wang, Wen, Xiaolin Zhou, Suqing Wu, Min Zhao, Zhan Jin, Ke Bei, Xiangyong Zheng, and Chunzhen Fan. 2024. "Vertical Green Wall Systems for Rainwater and Sewage Treatment" Sustainability 16, no. 17: 7593. https://doi.org/10.3390/su16177593

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