the UWA Profiles and Research Repository - …...a wall (Rahman, Wang, Rahim, Loo, & Miswan, 2014). The installation and maintenance costs vary with configuration, with plant viability

Performance of green walls in Mediterranean climates: A literature reviewJuly 2018

C OldhamA Karima

School of Engineering, The University of Western Australia

© 2018 Oldham C, Karima A

This work is copyright. Apart from any use permitted under the Copyright Act 1968, no part of it may be reproduced by any process without written permission from the publisher. Requests and inquiries concerning reproduction rights should be directed to the publisher.

Publisher: The University of Western Australia

Date of publication: 2018

An appropriate citation for this document is:

Oldham C and Karima A. (2018) Performance of green walls in Mediterranean climates: A literature review. The University of Western Australia. Report to Water Corporation of Western Australia.

Performance of green walls in Mediterranean climates: A literature review 1

Table of ContentsSummary ............................................................................................................................................................................2

Introduction ......................................................................................................................................................................3

Overview of green wall configurations ......................................................................................................................5

Green façades ..........................................................................................................................................................5

Attached green façades ...............................................................................................................................5

Detached green façades...............................................................................................................................5

Living walls ...............................................................................................................................................................5

Bio-walls ...................................................................................................................................................................6

Performance of green walls ..........................................................................................................................................7

Thermal comfort ....................................................................................................................................................7

Biodiversity and amenity ......................................................................................................................................8

Greywater treatment ............................................................................................................................................9

Design considerations ..................................................................................................................................................10

Selection of plant species ...................................................................................................................................10

Selection of growing media ................................................................................................................................10

Installation and maintenance costs ..................................................................................................................11

Irrigation regime ...................................................................................................................................................12

Conclusions .....................................................................................................................................................................13

Key knowledge gaps for green walls under Mediterranean climates ......................................................13

Guiding principles for the Bentley Primary School demonstration project ..........................................13

References .......................................................................................................................................................................14

2 Performance of green walls in Mediterranean climates: A literature review

SummaryGreen walls have been used for millennia for thermal comfort and aesthetics. The traditional green wall design of vines grown over a trellis, has recently been re-engineered into a variety of possible configurations (living walls, green façades and biofaçades), in an attempt to enhance their performance in urban environments, and also to utilise the systems for treatment of domestic greywater.

A key challenge for green walls in the Perth context is their ability to survive Perth’s hot dry summer, and it would be preferable to access research conducted in arid and semi-arid zones on green wall design, performance and viability, costs and maintenance. However, while green wall systems have been used for millennia in drier climates, there is a paucity of research on systems in arid or semi-arid climates.

Like all green infrastructure in urban environments, design and performance objectives will vary depending on needs. For urban residential areas, green walls are expected to provide thermal comfort, and are typically oriented on a building with this in mind. They are also expected to provide amenity and aesthetics. Plant species must be carefully selected for both of these objectives, and in Perth may also be expected to be water-wise.

The objective that green walls also treat domestic greywater requires additional design considerations, including the selection of plants that are not impacted by more plentiful and consistent watering regimes and by higher nutrient supply, the selection of growing media to enhance nutrient uptake, and an irrigation regime that consumes the greywater supply yet optimises saturation regimes in the media.

This literature review provides a succinct summary of recent research on green wall performance, and highlights knowledge gaps that may impact on appropriate design and operation of green walls in Perth urban areas. Importantly, we have also carefully defined terminology used for different green wall configurations.

This literature review provides the setting for the establishment of a green façade demonstration site at the Bentley Primary School, supported by the WA Department of Communities, the CRC Water Sensitive Cities and the University of Western Australia. The project aims to quantify the thermal comfort and greywater treatment performance of green façades, using different wall orientations and plant species.


IntroductionRapidly increasing urbanisation around the world faces, and contributes to, multidimensional challenges including adverse and unpredictable climatic conditions, environmental pollution and pressures on potable water supplies (CRCWSC, 2017). To enhance the sustainable development of urban centers, biophilic design has been recommended (Susorova, 2013). In biophilic cities, green infrastructure or technologies, urban design aspires to mimic and support the natural environment, with specific guidance on the design, planning and management of engineered elements at building, lot, precinct and regional scales. One specific objective of biophilic cities is to integrate local water management with the protection and restoration of urban ecosystems. Examples of urban ecosystems, or green infrastructure, that are increasingly seen in biophilic cities are green walking trails, bio-filtration systems and rain gardens, constructed wetlands, green rooves, vegetated swales and green walls (CRCWSC, 2017).

Vegetation around buildings has been used throughout human history for multiple benefits (Köhler, 2008). Recently termed living architecture, green walls are promoted as supporting water and air purification, shading, thermal comfort and wind reduction, food production, urban biodiversity and amenity (Cameron, Taylor, & Emmett, 2014; Sclar, 2013). Green walls are also reported to reduce energy usage by moderating heating, cooling and ventilation requirements (Susorova, 2013).

Over the millennia, green walls have traditionally been vertically grown climbers or creepers that are rooted in the ground, or in containers at the base of a wall. A more recent design uses a modular system of planters attached to a wall of the building. The plants can be directly attached to the building façade (e.g. ivy) or on a separate structural system (e.g. grapevine on a trellis) close to or attached to a wall (Rahman, Wang, Rahim, Loo, & Miswan, 2014). The installation and maintenance costs vary with configuration, with plant viability and degree of thermal comfort also being highly variable (Safikhani, Abdullah, Ossen, & Baharvand, 2014; Sclar, 2013)

Green walls have also been used as treatment systems for greywater with opportunities for re-use of the (cleaned) water exiting the wall, either within the building e.g. toilet flushing, or outside the building e.g. irrigation of public open space. Greywater is discharged from kitchens, washing machines, basins, baths and showers, and is a less concentrated wastewater stream (compared to blackwater), making it suitable for low level on-site treatment and re-use (Fowdar, Hatt, Breen, Cook, & Deletic, 2017). The constant supply of residential greywater for treatment by green walls, offers the potential for new sustainable non-potable water supplies for urban areas. This would decrease demand for potable water supplies and also decrease energy use at centralised wastewater treatment systems (CRCWSC, 2017).

Many of the developed treatment technologies for greywater are expensive and energy intensive, e.g. sand filters, fixed film reactors, rotating biological reactors, membrane bioreactors, and sequencing batch reactors (Fowdar et al., 2017). Alternatively, wetlands and bio-filtration systems use a nature-based approach to treatment, with lower energy needs. Wetlands have been used across all climate zones, increase biodiversity and amenity, but require more space. Bio-filtration systems can be designed to be of similar size to wetlands, or in high density cities may be minimal in size (Penn, Hadari, & Friedler, 2012). Recently, specialised bio-filtration systems have been developed, based on ancient architectural vegetated façades, and have been used for greywater treatment and thermal comfort; these systems have a number of possible configurations, and in general are termed green walls.

There been considerable research on the performance of green walls, particularly their provision of thermal comfort in tropical climates. There has been far less research on the performance of walls in arid, semi-arid or Mediterranean climates (despite green façades being used there through the ages) or on green walls ability to purify water. This article critically analyses existing research literature on green walls, specifically identifying the climate zones where the research has been conducted, and flagging where research findings are likely useful within the Perth context (i.e. under a Mediterranean climate). Table 1 summarises the literature accessed for this review.


Table 1 Mapping of accessed research articles on green walls across climate zones and research topic.

Climate Tropical Temperate Arid Mediterranean

Thermal comfort 1, 13 2 3

Greywater Treatment 4, 14, 15

Biodiversity 5 16

Acoustic insulation 6

Energy saving 7 7, 9, 10

Air purification 13 8 8

Cost analysis 11, 12

1. Rahman et al., 20142. Cameron et al., 20143. Coma, de Gracia, Chàfer, Pérez, & Cabeza, 20174. Fowdar et al., 20175. Kazemi, Beecham, & Gibbs, 20116. Pérez-Urrestarazu, Fernández-Cañero, Franco-Salas, & Egea, 20157. Feng & Hewage, 2014a8. Feng & Hewage, 2014b9. Coma et al., 201710. Mazzali, Peron, Romagnoni, Pulselli, & Bastianoni, 201311. Perini & Rosasco, 201312. Perini & Rosasco, 201613. Ghazalli, Brack, Bai, & Said, 201814. Prodanovic, Zhang, Hatt, McCarthy, & Deletic, 201815. Prodanovic, Hatt, McCarthy, Zhang, & Deletic, 201716. Madre, Clergeau, Machon, & Vergnes, 2015


Overview of green wall configurationsAcross the research literature, different terminologies have been used to describe different configurations of green walls, leading to considerable confusion. A clarification of the terminologies previously used is provided below, as well as definition of terminologies subsequently used throughout this literature review (summarised in Table 2).

Green façadesIn the literature, the term green façade has been used to describe a range of green wall configurations. To reduce confusion, in this review, we will use the term to describe systems where vegetation, typically creepers or climbers, is rooted into the ground or in large planters, and is grown directly up a supporting structure that can be freestanding (detached green façade) or immediately adjacent to the wall (attached green façade) (Fowdar et al., 2017). Such systems have been variously termed bio-façades, green façades or green walls (Sclar, 2013). While green façades have been used for millennia, their optimisation for specific outcomes (e.g. thermal comfort, water purification) has become an emerging green technology, with promising potential for high density urban areas because of their small footprint and multiple benefits. Green façades have recently been designed with dual-mode configurations that treat both greywater and storm water (Fowdar et al., 2017). Water inputs to the system may be parallel, where greywater is input over the dry season and storm water and greywater input over the wet season, or sequential, where only storm water is input during the wet season (CRCWSC, 2017).

Attached green façadesIn this configuration, the vegetation (typically climbers) is rooted in the ground at the base of a building and climbs up a supporting structure attached to a building wall. While this configuration is the cheapest green wall system to install, it may cause damage to the building wall (Sclar, 2013). This green façade configuration has also been termed direct green façades or traditional green façades.

Detached green façadesIn this configuration, the vegetation is rooted in the soil and climbs up a structural support that is separate from the building wall (Sclar, 2013). The distance between the structural support (e.g. a trellis) and the building wall may vary; distances of > 1-2 m have been used for walkways/breezeways, verandahs or sun shades, while distances < 0.5 m have been used for temperature and sound control. The initial establishment cost may be higher than an attached living wall as a free-standing support is needed. These systems have been variously termed indirect green façades, double skin green façades and green curtains.

Living wallsLiving walls have recently been developed and are configured using prefabricated modular or monolithic vertical soil or hydroponic systems to root plants on a vertical plane (Sclar, 2013). The modules are typically made of plastic or metal panels, the plants are pre-cultivated prior to installation and the panels subsequently fixed to either a vertical structural support or directly to a building wall (Azkorra et al., 2015). In the latter case, waterproofing is applied to the supporting building wall (CRCWSC, 2017). Flowerpots and ornamental hanging pots have been used in these systems to create a green curtain (Sclar, 2013, CRCWSC, 2017).

Living walls have been classified according to size of the substrate layer. Extensive living walls use thinner substrate layers while intensive living walls use thicker substrate layers. The extensive systems have been more commonly used because they are cheaper to implement (Sclar, 2013).


Though the living walls can be considered as vertical gardens with appropriate irrigation, drainage and maintenance, long-term plant survival may be challenging. The initial installation and on-going maintenance costs are high (Sclar, 2013), and in the event of plant death, replanting requires considerable resources.

Bio-wallsBio-walls are modular pre-cultivated indoor living walls that have been used to improve the indoor environment (Sclar, 2013) and as an acoustic insulator (Azkorra et al., 2015).

Table 2 Definitions of different green wall configurations used in this literature review.

Category Sub-category Configuration

Green façade Vegetation is rooted at the base of a wall, in the soil or in large planters.

Attached The vegetation is supported structurally by the building wall.

Detached The vegetation grows up a free-standing structural support that is not attached to the building wall.

Living wall Vegetation is rooted in multiple containers that are fastened to the building wall, typically in a pre-fabricated array.

Extensive The substrate layer is thinner.

Intensive The substrate layer is thicker.

Bio-wall Used indoors, typically for air purification or acoustic insulation.


Performance of green walls Green walls have been extensively used for environmental sustainability, particularly in urban areas, and are expected to perform a range of functions. For example, they can enhance amenity and aesthetics of the surrounding landscape and increases property value (Gabay, Meir, Schwartz, & Werzberger, 2014). A 20-30 cm layer of vegetated wall can contribute to sound insulation up to 1 dB (Pérez et al., 2016) and also improve air quality (Mazzali et al., 2013), by capturing both particulate and gaseous airborne pollutants (Feng & Hewage, 2014b; Rondeau, Mandon, Malhautier, Poly, & Richaume, 2012); nitrogen dioxide and particulate matter concentrations in the adjacent air can be reduced by up to 40-60% (CRCWSC, 2017). Gases are removed via direct uptake, absorption and adherence, while particulate matter is removed through deposition on plant and foliage surfaces (Ghazalli et al., 2018). The different performance characteristics of a range of green wallconfigurations are detailed below.

Thermal comfort Green walls can reduce adjacent wall and air temperatures (Alexandri & Jones, 2008). It was found in several studies that a green envelope around buildings contributed to lower ambient temperatures (Rahman et al., 2014). An average 7°C temperature difference has been reported after the installation of green walls and rooves, with the extent of thermal difference depending on the climatic conditions (Alexandri & Jones, 2008).

The green wall creates a static air layer between it and the adjacent wall, that acts as a thermal insulation layer and it has the ability to control heat gains and losses (Perini & Rosasco, 2016). The green wall systems can also cool the surroundings through a combined effect of shading of sunlight and evapotranspiration (Feng & Hewage, 2014b; Safikhani et al., 2014; Sunakorn & Yimprayoon, 2011). Feng and Hewage (2014b) found that solar radiation reaching the internal wall can be reduced by 40–80% by reflection or absorption through the foliage, which can reduce indoor and outdoor temperature fluctuation by up to 50%. Sclar (2013) also found that up to 30% of energy could be saved using similar systems. Studies shows that the foliage of green façades exploits sunlight, reflecting 5-30%, using 5-20% for photosynthesis, transforming 10-50% into heat, and using 20-40% for evapotranspiration; only 5-30% was passed through the leaves via phototropism effect (CRCWSC, 2017). The foliage thickness can however increase humidity levels, slow heat dissipation and reduce nighttime cooling (Manso & Castro-Gomes, 2015; Sunakorn & Yimprayoon, 2011). Safikhani et al. (2014) found that under dry Mediterranean continental climates, virginia creeper (Parthenocissus quinquefolia) and under tropical and subtropical climates blue trumpet vine (Thunbergia grandiflora) grown on green façade systems provided strong growth, higher foliage density and full leaf coverage while ivy demonstrated lower foliage density, but more rapid growth in height after its first year.

Sclar (2013) suggested that for improved shading coverage and cooling effects, virginia creeper, blue trumpet vine, and ivy may be preferable, with the magnitude of the shading effect dependent on the density of foliage. Pérez et al. (2017) observed that the degree of shading is also depended on the green wall configuration and the building facade orientation. In the northern hemisphere, east and west facing green walls generally performed better than north or south facing walls. As a comparison under a Mediterranean climate, Manso & Castro-Gomes (2015) found that Clematis was adversely affected by the dry summer conditions.

In a comparison of different designs, Coma et al (2017) showed that living walls decreased energy usage by up to 75%, compared to 26% for green façades. The difference between the two designs was more extreme during the daytime heating period and during the summer season. The cooling effects of green walls systems are mainly due to shading and evapotranspiration. The green façade system acts as a ‘double-skin façade’ and creates an air gap between the building and the vegetation, while the containers and growing medium of the modular panel system in living walls create an additional insulation layer that leads to 20% higher energy savings than the green façade system (Feng & Hewage, 2014a, 2014b).


This artificial cooling effect of the green walls contribute to a decrease in energy demand from air-conditioning (Manso & Castro-Gomes, 2015). Sclar (2013) showed that a 5º C decrease in internal air temperature of a building can save up to 8% in energy demand. The energy saving performance of green walls varies under different climatic conditions and the percentage is higher for warmer climates. Perini and Rosasco (2016) found that more energy is saved under Mediterranean climates (up to 40–60%) compared to temperate climates. Green façades saved 68% energy use under tropical climates, compared to only about 37% in cold temperate climates (Feng & Hewage, 2014b).

Green walls can also reduce the indoor temperature by controlling the surrounding wind speed of a building. Wind can lessen a building’s energy efficiency by about 50%, and a green wall can moderate this effect (Sclar, 2013). Interestingly however, Sunakorn and Yimprayoon (2011) found that under tropical climates, green façades can contribute to an unexpected increase in room air velocity when the outside air velocity > 0.5 m/s. The ability of green façades to buffer wind speed depends on a number of specifications such as foliage thickness and permeability, façade orientation, and the direction and velocity of the wind itself (Sunakorn & Yimprayoon, 2011).

Biodiversity and amenityThe selection of suitable plant species is crucial for green wall viability and the enhancement of system efficiency (Safikhani et al., 2014). Studies have also shown that green vegetation applied at an urban scale can improve the urban environment and local climate by contributing to urban biodiversity (Manso & Castro-Gomes, 2015). To avoid harsh climatic conditions including intense solar heating and wind, insects and birds are attracted to the stable microclimates created by the vegetation (Kazemi et al., 2011).

Chiquet et al. (2013) showed that green walls can increase the number of birds in the urban environment as they exploit the climbing plants for nesting, food and shelter. Birds preferred the tall, upper portions of green walls with perennial plants, as they offered a larger area for shelter and a higher level of security throughout the year.

Kazemi et al. (2011) highlighted that, vegetation-inhabiting invertebrates drive urban biodiversity more than vertebrates or vascular plants, and green walls promote this biodiversity. Interestingly, Madre et al. (2015) found that living walls attract more living organisms than green façades. Virginia creeper (Parthenocissus quinquefolia) in green façades can facilitate more invertebrates as they provide higher foliage density (Safikhani et al., 2014). Flowering plants in planter boxes also attract more invertebrates which also can further increase this biodiversity (Kazemi et al., 2011).

Studies have shown that plants in urban areas or buildings attract people more than gardens and they are considered to be psychologically helpful for reducing stress and lowering obesity (Safikhani et al., 2014). People have used green façades for their aesthetic enhancement for millennia, and recent research has shown that they increase property value (Ghazalli et al., 2018; Manso & Castro-Gomes, 2015).

Building walls may experience structural decay over time, due to exposure to sunlight, air temperature fluctuations, acid rain and air pollution. Both green façades and living walls may provide protection for building walls and delay wall degradation, and reduce the need for continual renovation of the wall (CRCWSC, 2017).


Greywater treatmentWhile considerable research on the benefits of green walls has been conducted, including their provision of thermal comfort (Medl, Stangl, & Florineth, 2017), there has been less research on greywater treatment performance. Greywater is considered a promising water source to irrigate green walls; it is consistently produced throughout the year and has high levels of nutrients to support healthy plant growth (Prodanovic et al., 2017).

Perini & Rosasco (2013) suggested that green façades are preferable to living walls for greywater treatment, with easier installation and plumbing at lower cost. An effective and reliable greywater treatment system using a green façade configuration has been developed in Melbourne, Australia, in which the vegetation growth was sustained and treated greywater was reused for non-potable outdoor applications (Fowdar et al., 2017). The volumes of excess treated water output depended on the volume of greywater input, the climate and plant type.


Design considerations

Selection of plant speciesIn dryer climates, native plant species are often preferred in green wall systems, for their lower irrigation needs and adaptation to local conditions and climate. For living wall systems in dryer climates, drought-tolerant plant species such as light weight succulents could be a useful option, as they require lower maintenance and have reduced irrigation needs (Manso & Castro-Gomes, 2015). However, when selecting species for provision of thermal comfort, plants with higher evapotranspiration rates will likely be preferred (Razzaghmanesh & Razzaghmanesh, 2017). This highlights that the different characteristics of selected species must be carefully matched to the design objectives of the green wall.

For green façade systems, climbing plants or creepers are low cost and have lower maintenance needs, with the plants attaching to vertical structural supports and covering the entire façade (Safikhani et al., 2014). However, the life-cycle of these types of plants needs to be considered during system design, with some species having life spans of only 3-5 years, while achieving maximum heights of 5-25 m (Manso & Castro-Gomes, 2015).

Selection of growing mediaFor green walls aiming to treat greywater, the growing media is a key design element and should be carefully selected to optimise treatment performance and the potential re-use of discharge flows (Table 3). The growing media serves three functions: physical filtering and infiltration of greywater, supporting plant growth, and maintaining microbial communities (CRCWSC, 2017). With the potential for larger volumes of media, green façades are the recommended green wall configuration for greywater treatment.

Fowder et al. (2017) recommended using sand based media for green façades, that provide physical (straining, sedimentation), chemical (adsorption, precipitation) and biological (plant and microbial assimilation, other microbial processes) treatment processes. Green façades and their growth media can be considered biofiltration systems, though with some key design differences from wastewater or storm water treatment systems.

In a study to optimise greywater treatment, Prodanovic et al. (2017) selected media based on its weight, water and nutrient retention capacity, water distribution, porosity, capacity to support plant growth, sustainability, and local availability. They compared several media and found coco coir and perlite as the best for slow and fast draining media, respectively. The organic coco coir is also a more sustainable substrate (López-Rodríguez, Pérez-Esteban, Ruiz-Fernández, & Masaguer, 2016). Slow draining media such as fyto-foam, rockwool and vermiculite, is more prone to clogging, while fast media exhibits shorter water retention times. Slow draining media with higher retention times provide enhanced water treatment through the biological removal of nitrogen via denitrification processes. In contrast, fast draining media with lower retention times can still remove nutrients via physico-chemical processes (Prodanovic et al., 2017).

In another study, Prodanovic et al. (2018) showed that enhanced treatment performance was achieved when slow and fast draining media were mixed. They noted however that the different media was redistributed after greywater dosing, creating heterogeneous pockets in the media and affecting the media’s hydraulic properties.

Media for enhanced nutrient retention are also used in larger biofiltration systems designed to treat storm water in urban areas. Ocampo et al. (2017) recently assessed the nutrient attenuation performance of two biofiltration systems in Perth (a raingarden and a bioretention basin) that used media comprised of locally available Gingin loam lined with clay. The raingarden removed the total nitrogen and total phosphorous loads by 80 – 90%, while the biorention basin showed similar nutrient retention when not intercepted


by groundwater. The raingardens did not exhibit the same attenuation of nitrate (NOx-N) loads, likely due to the lack of a saturated zone in the media, and the short residence of the stormwater. The longer residence times in the bioretention basin led to higher retention of nitrate compared to the raingardens.

Alternative nutrient retention soil amendments (media) were compared recently in a field trial on agricultural land close to Perth (ChemCentre, 2018). All three media tested (AlkaLoam®, IronMan Gypsum® and LaBC®) achieved good long term nutrient retention, however released some nutrients, metals and trace-metals in the first few years after establishment (though all under Australian Drinking Water Guidelines). We note that the cause of the releases was not determined and may be due to mobilisation from the base soils under altered pH conditions promoted by the soil amendments. The early release of nutrient and metals from these soil amendments could be of concern if these media were to be used in green wall systems, where the first years of plant establishment are often the most challenging. We note that native plants were successfully established in an IronMan Gypsum blend media (5% blend by mass).

Care must be taken during system establishment to avoid over-compaction and heterogeneous distribution of the media, which can alter the hydraulic performance of the systems (CRCWSC, 2017).

Table 3 Potential re-use of treated greywater (CRCWSC, 2017)

Water quality parameter after treatment

Potential re-useSingle domestic premises

Multi-dwelling and Commercial premises

Treated

• Biological Oxygen Demand (BOD5): 20 mg/L,

• Suspended Solids (SS): 30 mg/L,

• pH: 6 – 9

• Sub-surface irrigation (100-300 mm below ground level)

• Sub-soil irrigation (>300 mm below ground level)

Treated and disinfected

• Biological Oxygen Demand (BOD5): 20 mg/L,

• Suspended Solids (SS): 30 mg/L,

• pH: 6 – 9

• Thermo-tolerant coliforms: 10 cfu /100 mL

Surface irrigation

• Toilet flushing

• Laundry use

• Car washing

• Subsurface irrigation

• Surface irrigation by drip only

Installation and maintenance costsThe cost-benefit analysis of green technology in urban developments has been investigated. Life cycle costs of green infrastructure, covering its design, construction, operation, maintenance and ultimate demolition, may be up to 10% of a building’s total cost (Gabay et al., 2014). As noted above, the installation of green walls can create savings of up to 40% of air conditioning electricity bills. The maintenance costs for green wall systems is dependent on the configuration, plants used, irrigation regime, fertilizer requirements, and any on-going vegetation replacement (Feng & Hewage, 2014b). A study of the comparative costs of green façade and living walls estimated the total installation costs for green façades were around $250 – $500 per m2 while living walls cost around $1,500 per m2, and maintenance costs were up to 12% of the installation costs (CRCWSC, 2017). Perini and Rosasco (2016) demonstrated that green façades supported with plastic mesh were cheaper to install than façades supported with


steel mesh, however the increasing awareness of the lower sustainability of plastic may justify higher establishment costs.

With careful plant selection, green façades can have life spans of 50 years or longer, typically do not require replacement of plants but may require annual pruning (Perini & Rosasco, 2013). In contrast, living walls tend to have life spans of 3.5-10 years, may need periodic plant substitution and panel replacement, and require ongoing maintenance of the more complex irrigation systems (Feng & Hewage, 2014b).

Irrigation regimeFowder et al. (2017) demonstrated that green façades that maintain a saturated zone at the bottom of the biofilters (Figure 1), exhibit improved plants survival during dry periods and enhanced pollutant control, particularly nitrate removal. In contrast, the biofiltration systems monitored by Ocampo et al. (2017) showed excellent nutrient retention (TN and TP) even after dry periods and the assumed drying out of the media. We note that the latter biofiltration systems situated in Perth, were vegetated specifically with water-wise native plants (e.g. Grevillea thelemanniana, Baumea juncea, Melaluca lateritia, Ficinia nodosa, Eucalyptus rudis) that were expected to cope with extended summer hot, dry periods.

The irrigation regimes expected in green walls used for greywater treatment will provide more regular watering than would occur under natural rainfall in Perth. However, the results of Ocampo et al. (2017) are promising as they suggest that the maintenance of saturated media is not critical for high nutrient attenuation.

Figure 1 Green façade technology for greywater treatment (Fowdar et al., 2017)


ConclusionsGreen walls are an emerging green technology in the water treatment sector. The review of research literature outlined above suggests that living walls are likely to be challenged by a Mediterranean climate, particularly the long dry summers. Living walls also have higher installation and maintenance costs. While green façades are expected to perform better under Mediterranean climates, their performance during dry, hot summers is less known. The key knowledge gaps around green façade performance are listed below, and will be used to guide research at the Bentley Primary School demonstration project.

Key knowledge gaps for green walls under Mediterranean climates• Optimal plant species for use in Mediterranean and semi-arid climates. In the Perth context, the plants

must tolerate long, hot summers, as well as summer and winter winds, depending on wall orientation.

• System performance over the establishment period.

• Greywater treatment performance.

• Thermal comfort performance.

• Optimal watering regime for enhance system performance.

• Potential for a treated water source and options for re-use.

Guiding principles for the Bentley Primary School demonstration project• The study will focus on the establishment and performance of detached green façades.

• Performance will be defined (in order of priority) as the systems’ ability to provide a) increased amenity, b) thermal comfort, and c) greywater treatment.

• Performance will be assessed across different wall orientations (facing north, east and west).

• Performance will be assessed across different gaps between the building wall and the façade vegetation (2 m and 0.5 m).

• Locally available media will be used and tested for its performance.

• Locally available deciduous and perennial plant species will be tested.

• The performance of native and non-native plants will be tested.

• The greywater supply will be primarily used for plant survival; any excess treated water for re-use would be an additional benefit.

• The green façade systems will be monitored to assess their water use, thermal boundary layers (i.e. thermal comfort), and nutrient treatment capabilities.

• North facing green façades would use deciduous vegetation to allow shading in summer and access to sunlight in winter.

• The green façades only 0.5 m from building walls will use heat tolerant plant species, to tolerate the summer heat radiating from the walls during the early years of establishment, before significant shading has occurred.

• The green façades 1.5 m from a wall, will use wind tolerant species.


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the UWA Profiles and Research Repository - …...a wall (Rahman, Wang, Rahim, Loo, & Miswan, 2014). The installation and maintenance costs vary with configuration, with plant viability