Phytocapping for Municipal Solid Waste Landfills: A Sustainable Approach

Abstract

This paper reviews the historical developments in landfill technology and its drawbacks. It introduces phytocapping, in light of previous research, as a promising, eco-friendly and sustainable technology for municipal solid waste (MSW) landfill covers in order to reduce landfill gas and leachate generation. This paper highlights the challenges to successful phytocapping, such as selection criteria for appropriate plants and growth media, and the importance of new research into overcoming these challenges. It also presents a database of plants used in landfill phytocapping studies worldwide. In addition, the performance, economics, and sustainability of phytocapping technology are evaluated in comparison to ordinary MSW landfill methods.

1. Introduction

Municipal solid waste (MSW) management is one of the major global environmental concerns. The world produced 1.3 billion tonnes of MSW in 2012, and it is projected to exceed 2.2 billion tonnes annually by 2025 [1,2]. Typical MSW that goes to landfills contains biodegradable, non-biodegradable and inert wastes [3,4], and the term “landfill” describes a physical facility that stores MSW below the earth’s surface [5]. Biodegradable waste may undergo aerobic and anaerobic decomposition at different phases, producing greenhouse gases and leachate. The production of leachate can accelerate due to water infiltration through inadequate capping and can be an environmental and health risk if not appropriately managed [6,7]. Proper capping, post-closure construction and maintenance are crucial to reducing water ingress [8,9]. Traditional low-permeable landfill covers, like compacted clay capping, geosynthetic clay liners (GCLs), polyvinyl chloride (PVC) and high-density polyethylene (HDPE), are commonly used as capping [3,10,11,12,13,14,15].

Over time, these traditional landfill covers have deteriorated, primarily due to climatic conditions [16,17]. One innovative way to address this landfill cover deterioration issue is the application of a layer of growth medium and vegetation on top of traditional landfill covers—a practice known as phytocapping. This can help to maintain formal cap integrity [15,18,19,20]. The success of phytocapping depends on several factors, including climatic conditions, the choice of suitable plant species, the survivability and overall growth of plant species, and the development of a cost-effective and adequate growth medium with sufficient thickness, which can be challenging. This paper discusses the development of landfill technology and its impacts on society, health, and the environment. The possibilities, scope, and challenges of landfill phytocapping technology are also explored.

2. Development of Landfill Waste Management Technology to Overcome Environmental Challenges

Waste management in the early 19th century involved the disposal of collected waste far away from populated areas. This waste was often used for land reclamation by depositing it in swamps or wetlands without following any regulations or restrictions [15]. Sanitary landfills were developed between 1920 and 1930 in response to concerns about health threats and environmental hazards [15]. In the 1960s and 1970s, significant contamination of local environments was found due to the unplanned disposal of industrial waste in landfills, leading to further improvements in landfill technology [15,21]. An engineered landfill concept was introduced in around the 1990s, employing a clay liner to prevent water infiltration into buried waste and subsequent leachate movement to the surrounding area [3,15].

2.1. Conventional Landfill Cell and Capping

Modern MSW landfills aim to minimise health threats and environmental hazards with properly designed landfill components. The essential elements of a conventional MSW landfill (see Figure 1) include a base and side liner, leachate collection system and top cover [12]. A landfill liner is a low permeable barrier used at the base and side of a landfill to control leachate migration to underground aquifers and nearby water bodies. Compacted clay liners [22,23], GCLs [11,13,24] and PVC/HDPE liners [11,25] are commonly used as base and side liners as they have significantly less permeability. The leachate collection system accumulates and extracts leachate from landfills via perforated pipes. Leachate reticulation [15,26], leachate disposal [27], leachate treatment [10,15], and bioreactors [26] are the standard technologies implemented for leachate management.

Placing a top cover over a landfill is the most vital post-closure step to isolate landfill waste, mainly water and heat, from the external environment [3] and keep the deposited waste relatively inactive [26]. Sustainable landfill covers are essential to minimise water infiltration, reducing environmental hazards like air, water, and soil pollution due to greenhouse gasses (GHGs) and leachate generation. Compacted clay [13,27,28], GCLs [10,11,12,13], PVC lining [14] and HDPE cover [13,29,30] are the most commonly used technologies for landfill caps. Based on a landfill survey conducted in Australia in 2005 involving 230 landfills, Bateman [10] found that compacted clay capping, geosynthetic clay liners and PVC/HDPE were used in 70%, 20% and 10% of cases, respectively.

Although the success of an MSW landfill cell largely depends on the performance of its landfill capping, these conventional techniques are costly, and little is known about their long-term benefits and sustainability [3,7].

2.2. Role of Water Infiltration in Greenhouse Gas Production during Anaerobic Decomposition of MSW and Its Impact on the Environment

Conventional landfill technology mitigates many environmental consequences associated with the old practice of unplanned dumping of MSW. However, it still poses some ecological concerns, such as generating greenhouse gases, fire and explosions, flora damage, odours, and soil and water contamination [8,15], mainly due to uncontrolled water infiltration and the corresponding decomposition of MSW.

The decomposition of MSW occurs by aerobic and anaerobic degradation processes. The aerobic degradation process occurs in the presence of oxygen, and aerobic bacteria break down organic matter to produce carbon dioxide, water, and heat. Aerobic biodegradation usually ceases after a short while due to oxygen depletion [26], and the principal bioreaction in landfills, i.e., anaerobic degradation, starts [31]. The anaerobic process consists of four phases (see Figure 2), and in the first stage, fermentative bacteria hydrolyse complex organic matter into soluble molecules in the presence of infiltrated water. Next, in Phase 2 and Phase 3, these soluble molecules are converted into organic acids, carbon dioxide (CO₂) and hydrogen with the help of fermentation and acetogenic bacteria. Finally, in Phase 4, methane (CH₄) is formed by methanogenic bacteria, either by breaking down acids mainly into methane and carbon dioxide or by reducing carbon dioxide with hydrogen [26,31].

According to Hartz and Ham [32] and Rees [33], methane production is proportional to moisture content, and an exponential increase in landfill gas production has been reported from a water content of up to 60% (by wet mass). Boeckx and Van Cleemput [34] and Visvanathan et al. [35] have reported that methane oxidation rates in landfill cover soil peaked at between 15% and 20% moisture content and temperatures of from 30 °C to 36 °C.

Generally, methane production continues for up to 10 to 20 years [36]. Methane is the predominant hydrocarbon in the atmosphere [37], and a single methane molecule possesses a 100-year warming potential. Methane emissions from landfills account for 15% to 20% of greenhouse gas production in the USA [38], and the climate-changing potential of methane is about 40% greater than that of carbon dioxide [39]. Furthermore, methane’s superior infrared activity is roughly 21 to 25 times more harmful than that of carbon dioxide molecules [40,41]. Flammable methane can migrate away from landfill areas by diffusion and advection [8]. It can escape through landfill covers as well as landfill-side soil. The lateral travel distance depends on the soil’s characteristics, which may create gas pockets in surrounding infrastructure and create explosive hazards [42,43,44,45]. Fire and explosions may also occur on site due to air entry and the formation of a mixture of methane and oxygen [46]. Improper gas control measures can lead to landfill gases, generally methane and carbon dioxide, escaping upward and displacing the oxygen from the plant root zone, causing the death of plants [8].

Methane oxidation at the root zones of plants is a process where methane is converted into carbon dioxide and water by methane-oxidising bacteria known as methanotrophs [47,48]. The root zone, rich in oxygen because roots allow aeration in soil, creates an aerobic environment favourable for methanotrophs. These bacteria use methane as a carbon and energy source and produce the enzyme methane monooxygenase, which catalyses methane oxidation. Methane oxidation into carbon dioxide, which has a lower global warming potential than methane, significantly reduces the greenhouse gas emissions from landfills [47,48].

Apart from methane and carbon dioxide, the other harmful gases landfills produce are hydrogen sulphide, ammonia, and volatile organic compounds. Although carbon dioxide is less potent than methane, it is the primary greenhouse gas responsible for climate change and soil acidification. Hydrogen sulphide is highly toxic, leading to health risks to humans and animals, promoting odour pollution and acid rain [49]. Ammonia affects air quality, leading to nutrient pollution, causing eutrophication and harming aquatic life [50]. On the other hand, volatile organic compounds affect air quality by forming ground-level ozone, which harms human health and vegetation [51].

The production and migration of leachate due to excess infiltration contaminates the underlying or surrounding soil and aquifers by depositing organic solvents, nitrate, phosphates, salts and heavy metals such as chromium, manganese, iron, nickel, zinc, cadmium, mercury and lead [52,53,54]. Plant uptake of heavy metals from the contaminated soil and water can not only retard plant growth and productivity but also create a route for heavy metals to enter human and animal food, posing a significant threat to the ecosystem [53]. Leachate contains high concentrations of nitrate and phosphates, creating eutrophic conditions when mixed with surface water and producing massive planktonic algae and cyanobacteria on the water’s surface. It can limit light penetration into the water, adversely impacting fish, other aquatic species and the ecosystem [55]. Groundwater pollution due to leachate has been reported in many countries, including Iran [56], Canada [57], Sweden [58], India [59] and Australia [60].

It is clear from the above discussion that the most significant hazards of landfills are the generation of GHG and leachate, which are directly related to the amount of infiltration water, mainly from landfill capping. Therefore, all these recent hazards can be significantly reduced by designing a sustainable, environmentally friendly, long-lasting landfill capping method to minimise water infiltration into buried waste, and by reducing hydrolysis reactions from the anaerobic process, as this is the most vulnerable and vital element of a landfill.

2.3. Conventional MSW Landfill Capping and Challenges

Landfill caps are permanent top cover layers to separate buried waste from the surrounding environment [61]. Their primary function is to prevent wind transportation of litter and odour and reduce or prevent water percolation [15], thereby controlling greenhouse gas and leachate production [3]. Generally, low hydraulically conductive materials such as compacted clay [28], GCLs [11], PVC [14] and HDPE [30] are used for landfill capping to achieve these challenging goals [30].

Compacted clay capping is generally economical and typically has expected low-saturated hydraulic conductivity ranging from 10⁻⁷ to 10⁻⁹ cms⁻¹ [4]. It is recommended to use soil with specific characteristics, including a fine content of 20–30%, a plasticity index of 7–10% and a gravel fraction of less than 30% to achieve the desired low hydraulic conductivity [62]. The performance of clay caps depends on on-site water balance, and their performance deteriorates with time [63,64,65], primarily due to crack generation. A decrease in moisture content within clay caps due to evapotranspiration leads to soil water tension, which causes tensile stresses leading to cracks [30,66]. In areas with cold climates, they often deteriorate over time due to freezing and thawing effects [66,67]. In addition, compacted clay caps require regular monitoring and maintenance to prevent any damage that can cause water infiltration. However, monitoring and maintaining clay capping is time-consuming, laborious, and expensive.

GCLs typically consist of a bentonite layer between two geotextile layers. GCLs are a preferred choice due to their very low hydraulic conductivity (10⁻¹⁰ cms⁻¹), thickness (≤10 mm) and ease of construction [30,62,68]. Some advantages of using GCLs include rapid installation, requiring less skilled labour, the ability to withstand significant differential settlement, ease of repair, effective gas barrier properties and reduction in overburden stress on compressible MSW layers [68]. However, GCLs can fail in temperate regions due to the freezing/thawing effect [69] and are susceptible to leakages during construction and maintenance [70,71]. Moreover, GCLs exhibit low shear strength, lower leachate attenuation capacity and are prone to deterioration over time [68]. Composite capping systems like PVC and HDPE are costly, become less durable, and lose their plasticisers with age [72].

While commonly used landfill capping technologies offer short-term benefits, their long-term efficiency remains uncertain, and their performance in protecting the environment from the detrimental impact of landfills is questionable [30,62,66,68].

3. Development of Phytocapping Technology

Phytocapping was introduced by the US Department of Energy in 1991 through a project delivered by the Idaho National Engineering and Environmental Laboratory [67,73]. Phytocapping has two significant application areas: mine closure and MWS landfills. In mining sites, phytocapping, called phytoremediation, can help to extract heavy or trace metals. In phytoremediation, hyperaccumulator plants are used, which can accumulate heavy metals from contaminated soil in their bodies without suffering from toxicity distress [74,75]. Phytoremediation has been the subject of numerous studies worldwide, focusing on the extraction of certain trace metals such as arsenic (As), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), mercury (Hg), manganese (Mn), nickel (Ni), lead (Pb), selenium (Se) and zinc (Zn) [76,77,78,79,80,81,82,83,84,85,86,87]. According to Web of Science [88], India, China and the USA are the top countries contributing to the topic area of “phytoremediation”, followed by Pakistan, Australia, Italy, Germany and Canada. A snapshot of the publication numbers and the origins of publications on the topic is shown in Figure 3.

Phytoremediation can be divided into phytoextraction, phytovolatilization, phytodegradation and phytostabilization [3,89]. In phytoextraction, plants remove toxins and translocate them into the roots and shoots. The soil’s toxic organic and inorganic pollutants are converted into less harmful, more stable, and less mobile forms via the plant’s phytotransformation [89]. Plants take organic pollutants and heavy metals from the soil, which is followed by their conversion to a volatile form and release into the atmosphere in phytovolatilization. This method only transfers contaminants from the ground to the atmosphere [90,91]. In phytodegradation, plants degrade organic toxins with the help of enzymes such as dehalogenase and oxygenase, secreted by soil microbes (bacteria, fungi, etc.) and plants [92].

MSW landfill phytocapping comprises two major components: the vegetation and the growth media (see Figure 4). The growth media store nutrients and water for plants. The plants act as bio-pumps, removing water from the soil through transpiration, thereby controlling the downward percolation of water [3,4]. This arrangement helps to maintain moisture balance within the clay caps and prevents excessive water percolation, thereby enhancing the performance of landfill caps [3].

Limited research has been conducted on MSW landfill phytocapping compared to other forms of phytoremediation, accounting for only ~5% of the total studies. The publication trend in phytocapping research is summarised in Figure 5. Germany, Italy, Australia, and the USA are the top MSW landfill phytocapping research countries, followed by Singapore, England, Turkey, Spain, and India. Although these studies reveal the concept, scope, and advantages of phytocapping over other conventional landfill capping systems, there is a notable lack of information on aspects such as plant selection, proper design of growth media, the optimal ratio of growth media amendments with soil and the suitable depth required for successful implementation of phytocapping in different geographical and climatic conditions.

3.1. Mechanisms Involved in MSW Landfill Phytocapping

The mechanisms involved in phytocapping are the functions of the plant species, climatic conditions, nutrients and texture of growth media, and clay cap properties [7]. Local climatic adaptation, tolerance to soil conditions, and changes in transpiration rate with moisture availability are the primary factors to be considered when selecting plant species. In addition, it is essential to ensure that the chosen species can tolerate and survive in the heat produced by methane oxidation and, in certain circumstances, cope with bushfires [3,15]. A mixture of several native plant species may fulfil the requirements, because if some species are adversely affected, others may take over and maintain the functionality [3,7].

The hydraulic properties, nutrients, depth of the growth media and the selection of suitable plant species for particular growth media and climatic conditions are the critical parameters for the sustainability of phytocapping technology [66,93,94]. Irrigation of plant species at the early vegetation stage may be necessary for survival and plant growth in specific climate zones. The amounts and intervals of irrigation can be decided based on the hydrological water balance of the phytocapping system. Another vital aspect to consider is plant root growth. Landfill clay cover thickness usually varies from 0.3 m to 1.0 m in different landfills [3,15]. The root systems of woody plants used for phytocaps may penetrate the buried waste layer in a system with a thin cover and may cause excessive landfill gas emissions and plant death [95]. They may also create water infiltration pathways and increase leachate generation, odour, and methane production. Moreover, root systems can alter the water retention properties of growth media by influencing soil structure [96], porosity [97] and bulk density [98].

3.2. Growth Media

The soil texture mostly preferred for growth media is silty loam to clay loam [3]. The growth medium should have adequate hydraulic conductivity to allow only the necessary quantity of water required to keep the compacted clay caps moist and plastic. In addition, it should provide good nutrient content and water-holding capacity for plant survival and growth [67,99]. A growth medium should be suitable for plant root growth and act as a screen to oxidise methane and restrict methane emissions to the atmosphere [100,101].

Landfill growth medium soil is generally sourced from local sites and sometimes might not contain sufficient nutrients and other physical and chemical properties required for plant survival and growth. In such cases, adding inorganic and organic supplements can improve soil conditions. However, organic by-products are preferable due to their low cost and environmental friendliness [102]. Biosolids and MSW by-products are two significant sources of organic amendments [102]. MSW organic by-products are used in many studies to boost the productivity of methanotrophs, which may enhance methane oxidation rather than accelerate vegetation growth [103,104,105,106,107]. However, few studies have used MSW organic by-products to enhance plant growth. The mixing ratio of organic by-products with soil ranged between 1:3 and 5:1, and has also been reported as 1:2 [108] and 1:3 [106,107,109].

According to Ettala et al. [110], using a high concentration of organic matter in phytocapping enhances plant growth and methane mineralisation performance. Phillips et al. [111] noted that landfill soil supplemented with organic waste can reduce landfill soil erosion and improve aggregate stability. However, no study has been conducted to determine the optimum mixing ratio of municipal organic by-products used in landfill cap soil to facilitate vegetation growth in phytocapping landfills.

The depth of growth media typically depends on soil type and texture, growth rate and size of plant species, and average annual rainfall. However, very few studies have focused on landfill growth media depth. Albright et al. [112] studied the ability of final landfill covers to control percolation into underlying waste. They compared the water infiltration for conventional covers with phytocaps, using drainage lysimeters over a range of climates at eleven field sites in the USA. They found that phytocapping landfill covers reduced the percolation over time with plant growth. However, their study did not provide guidelines on the optimum depth of growth media for particular climates and plant species. Venkatraman [67] studied the plant transpiration, root depth, canopy interception, and infiltration of twenty-one phytocap plant species planted on growth media with depths of 0.7 m and 1.4 m in a landfill site in Queensland, Australia. All plants grew better on 1.4 m thick growth media. The maximum root depth found in this study was 0.6 m, with an average of 0.4 m during the study period. Although the optimum depth of growth media for particular plants and climatic conditions is crucial for phytocapping performance, this area is relatively underexplored.

3.3. Plant Selection for Phytocapping

The appropriate native plant species are essential for an effective and sustainable phytocapping system [113]. Past studies have chosen plants based on their ability to provide corridors or habitats for local fauna [110,114] and their ability to produce bioenergy or plant-based products [99,115,116]. Plant species that grow in low nutrient levels [108], have high transpiration rates [109], are hyperaccumulators of toxic metals [117], have high drought and salinity tolerance and high growth rates [118] are commonly chosen for phytocapping. Rooting depth [119] and the ability to uptake water [120] have also been considered essential factors. Some projects have preferred grasses due to their ability to reduce surface runoff and erosion, extensive, massive root systems [94], high water uptake capacity [121], drought tolerance [122] and low likelihood of uprooting and damaging the integrity of growth media due to wind action. A plant’s overall adaptation characteristics, types and depth of growth media, degree of exposure to the surrounding environment and the local climatic conditions [4,67] are also considered.

3.3.1. Advantages of Native Plants over Non-Native Plants

Both native and non-native plants may have the potential to be used for landfill phytocapping. Very little published research is available since most landfill recovery projects are not research-oriented [123]. Venkatraman [67] investigated the growth and survivability of 21 tree species in a landfill in Rockhampton, Queensland, Australia. Of the 21 species planted, 19 survived and grew well. Two species (Populus nigra and Salix matsudana), which are very popular in the USA for phytocapping, did not survive the high summer temperatures in Australia. Cittadino et al. [123] studied the survival, growth, and biomass production of native Pennisetum pupureum grass for about four years in a landfill in Argentina. They planted 240 plants directly in the landfill capping system in three lines of 80 plants at one-metre intervals. They found very good survival rates of 100%, 100%, 87.1% and 82.9%, and dry biomass production of 9.27, 16.96, 13.24 and 18.15 t/ha for the consecutive four years, respectively.

Plant growth in landfills is challenging and often poor due to limited water availability and elevated soil temperatures [124,125]. Native species are advantageous as they adapt to local conditions, are less likely to disturb the ecosystem, and regenerate after natural disasters [4,15]. The other benefit of native plants is that they need less care during the initial stage after plantation, and their survival and growth will be better than non-native plants. Also, they reduce the maintenance costs and overall construction costs of landfill phytocapping. Native plants can also regenerate after natural disasters such as storms and bushfires. Therefore, native perennial species are preferred for landfill phytocapping [126,127].

3.3.2. Studied Plant Species in Phytocapping

Phytocapping is a recent capping approach, and little guideline is available on the appropriate selection of plant species [99,114]. The geographical location and the soil conditions at the phytocapping site are the major factors in plant selection, among other factors [3]. Most researchers in landfill phytocapping studies used native plant species. For instance, Salix and Populus species have been studied successfully in Europe [99,110], as has Populus hybrid in the USA [128], but these not suitable for Australian climatic conditions [67]. Herbaceous species, such as grasses (1 m tall), provide dense cover and effectively reduce water splashing from rainfall, water runoff and wind velocity, which make them suitable for effective phytocapping designs [129,130]. According to Albright, Benson, Gee, Roesler, Abichou, Apiwantragoon, Lyles and Rock [112], herbaceous species were widely used in eleven successful phytocapping trials in the USA. Several vigorous species considered to be bioenergy crops may also have the potential for use in phytocapping [99,131].

The studied phytocapping plant species are summarised in

Table 1. Plants Used for Phytocapping Landfill Technology [132,133,134,135,136].

Plant Species Name and Habitat	Family Name	Availability	Soil Type	pH	Rainfall Tolerance (mm)	Adap-Tability	Max Height (m)	Canopy Dia (m)	USDA Plant Hardi-ness Zone	Resear-Cher(s)
Acacia harpophylla (T)	Fabaceae	AUS, NSW	LoC	-	300–700	D	25	-	-	Venkatraman [67]
Acacia mangium (T)	Fabaceae	AUS, PNG, ID, MY,	SLC	-	-	D	30	-	10–12
Pongamia pinnata (T)	Fabaceae	IN, ID, AUS, PNG, PH, CN, FIJI	SL	6–9	500–2500	D	15–25	-	10–12
Eucalyptus grandis (T)	Myrtaceae	AUS, NSW, NZ	SLC	-	990–1780	-	50	-	9–11
Eucalyptus raveretiana (T)	Myrtaceae	AUS, NSW	SLC	-	-	-	21–30	-	9–12
Eucalyptus tereticornis (T)	Myrtaceae	AUS, PNG, NZ	SLC	7–10	500–2000	-	20–35	-	9–12
Callistemon viminalis (T)	Myrtaceae	AUS, NZ	LoC	-	-	D	10	-	-
Lophostemon confertus (T)	Myrtaceae	AUS, NZ	LoC	4.5–7	900–1700	C	30	-	9–12
Melaleuca leucadendra (T)	Myrtaceae	AUS, MY, ID, PNG	SLC	5.5–8.5	650–1600	D	30	-	10–12
Melaleuca linariifolia (S)	Myrtaceae	AUS, NZ	SLC	4.5–7	-	-	6–10	-	8–11
Ficus microcarpa (T)	Moraceae	AUS, PNG, NZ, PH	SLC	4.3–8.6	300–2700	C, D	8.5	-	6–10
Ficus racemose (T)	Moraceae	CN, IN, MM, MY, PH TH, VN, ID, PNG, AUS, NZ, USA	SLC	4.5–8.0	-	-	12	-	9–12
Populus nigraitalica (T)	Salicaceae	AUS, NZ, USA	SLC	4.5–8.0	-	-	20–30	-	3–9
Syzigium AUStralis (S)	Salicaceae	AUS, NZ, PNG, USA, ID	SLC	4.5–8.0	-	-	3	-	6–12
Casuarina cunninghamiana (T)	Casuarinaceae	AUS, NZ	SLC	4.5–8.5	500–4000	D	8–20	-	8–11
Casuarina glauca (T)	Casuarinaceae	AUS, NZ	SL	4.5–8.5	-	-	18	-	8–11
Cupaniopsis anacardioides (T)	Sapindaceae	AUS, PNG, NZ	SLC	-	-	-	10	-	-
Dendrocalamus latiflorus (T)	Poaceae	AUS, CN, MM, VN	SLC	-	-	C	10–20	-	10–12
Hibiscus tiliaceus (T)	Malvaceae	AUS, PNG, ID	SL	4–9	900–2500	-	6–8	-	10–12
Eucalyptus camaldulensis (T)	Myrtaceae	AUS, NZ	SL	4.5–7.5	400	D, L, MF	20–40	10–15	8–12	Michael [137]
Eucalyptus melliodora (T)	Myrtaceae	AUS, NZ	SL	4.2–7.5	400	D, L, MF	20–35	20–30	9–11
Eucalyptus cladocalyx (T)	Myrtaceae	AUS	SLC	4–9	350–600	D, L, MF	20–25	12–15	-
Eucalyptus polybractea (T)	Myrtaceae	AUS	SLC	4.5–8.5	300	D, MF	2–8	4–8	-
Eucalyptus viridis (T)	Myrtaceae	AUS, NZ	SLC	4.5–8.5	400	D, MF	4–8	3–5	-
Acacia mearnsii (T)	Fabaceae	AUS, NZ	SLC	4.5–8.5	500	D, L, MF	5–15	5–20	8–11
Acacia pycnantha (T)	Fabaceae	AUS, NZ	SLC	4.5–8.5	350	D, L, MF	4–6	2–6	7–10
Allocasuarina verticillate (T)	Casuarinaceae	AUS, NZ, SAF, GR	SLC	4–9	350	D, HF, S	5–8	4–6	8–11
Callitris gracilis (T)	Cupressaceae	AUS, NZ, SAF	SL	4–9	300	D, HF, S	7–20	3–8	-
Melaleuca lanceolata (T)	Myrtaceae	AUS, USA	SLC	4–8.5	250	D, HF, S	3–8	3–5	-
Themeda triandra (G)	Poaceae	AUS, NZ, PNG, SAF, SRI	SLC	4.5–8.5	100	D, L, MF	0.9–1.0	1.0	-
Microlaena stipoides (G)	Poaceae	AUS, NZ, PNG, USA	SLC	4–6	200	D, HF, S	0.1–0.7	0.2–1.0	8–10
Bothriochloa macra (G)	Poaceae	AUS, NZ	SLC	4.5–7.5	450–500	D, L, MF	0.4–0.8	0.2–0.4	-
Austrodanthonia caespitosa(G)	Poaceae	AUS, NZ	SLC	4.5–7.5	300–450	D, L, MF	0.2–0.8	0.2–0.2	-
Poa labillardierei (G)	Poaceae	AUS, NZ	SLC	4.5–7.5	300–500	D, L, MF	0.5–0.6	0.4–0.5	-
Austrostipa elegantissima (G)	Poaceae	AUS	SLC	7–9	-	D, L, HF	0.5–1.0	1.0–1.0	-
Eucalyptus cladocalyx (T)	Myrtaceae	AUS, NZ	SLC	4.5–8	350–600	D, L, MF	20–25	12–15	-	[127]
Eucalyptus polybractea (T)	Myrtaceae	AUS	SLC	4.5–8	300	D, MF	2–8	4–8	-
Allocasuarina verticillate (T)	Casuarinaceae	AUS, NZ, SAF, GR	SLC	4.5–8.5	350	D, HF, S	5–8	4–6	8–11
Atriplex nummularia (S)	Chenopodiaceae	AUS	SLC	4–8.5	230–650	D, HF, S	2–4	1–3	7–10
Acacia mearnsii (T)	Fabaceae	AUS, NZ, SAF	SL	5.0–7.2	660–2280	D, L, MF	10	-	8–11
Grevillea robusta (T)	Proteaceae	AUS, NZ	SLC	4.5–8	450–550	D, L, MF	8–20	5–14	9–11
Salix reicharDii (T)	Salicaceae	AUS, NZ	SLC	4.5–8	400–500	D, L, MF	8–10	-	-
Cynodon dactylon (G)	Poaceae	AUS, FIJI, NZ, PNG, USA, CI	SLC	4.5–8.5	-	-	0.5	0.3	6–9	[112]
Populus species (T)	Salicaceae	AUS, NZ, USA, SAF, CA	SLC	-	-	-	15–50	-	-
Ampelopsis arborea (G)	Vitaceae	AUS, NZ, PNG, PH, USA, MY	L	4–8	-	C	10	-	6–9	[138]
Crataegus species (S)	Rosaceae	AUS, NZ, PNG, FR, GR, USA, ID, CA	SL		-	-	5–15	-	-
Pinus taeda (T)	Pinaceae	AUS, NZ, USA	SL	4.5–7	-	D	40	-	6–9
Quercus nigra (T)	Fagaceae	USA, NZ, AUS	LC	4–9	-	C	20–30	-	5–9
Quercus species (T)		AUS, CN, CA, FR, GR, IN, ID, ITA, JP, MY, MX	LC	4.5–7.5	-	C	25		-
Rhus copallinum (S)	Anacardiaceae	AUS, NZ, PNG, USA, CN	SLC	4–9	-	D	2	2	4–10
Rubus species (S)	Rosaceae	AUS, CN, FR, GR, ID, NZ, PNG, USA	SLC	4–9	-	D	3	-	5–9
Andropogon virginicus (G)	Poaceae	AUS, NZ	S	4–9	-	D	1.2		5–9
Cenchrus echinatus (G)	Poaceae	AUS, PNG, USA	SLC	4.5–9.5	-	-	0.8–1.2		-
Cynodon dactylon (G)	Poaceae	AUS, FIJI, NZ, PNG, USA, CI	SLC	4.5–8.5	-	-	0.5	0.3	6–9
Cyperus echinatus (G)	Cyperaceae	AUS, CN, FIJI, IN, ID, MX, MM, NZ, PNG, USA	SLC	4.5–8.5	-	-	0.5	0.3	-
Digitaria ciliaris (G)	Poaceae	AUS, NZ, PNG, FIJI, SRI	SLC	4–9	-	-	0.5	1.0	7–10
Eremochloa ophiuroides (G)	Poaceae	CN, USA	SLC	4–9	-	-	0.2	0.5	-
Juncus effuse (S)	Juncaceae	AUS, NZ, PNG, USA, GR, FR, CA	SLC	4.0–8.0	-	D	1.5	0.5	-
Paspalum notatum (G)	Poaceae	AUS, MX, USA	SLC	4.0–8.0	-	D	0.75	0.3	-
Collinsonia canadensis (S)	Lamiaceae	AUS, CA, GR, NZ, PNG, USA	SLC	4–9	-	-	0.8	0.4	4–8
Erechtites hieracifolia (S)	Asteraceae	NZ	SLC	4–8.5	-	-	-	-	-
Eupatorium capillifolium (S)	Asteraceae	AUS, CA, NZ, PH, USA	SLC	4–9	-	-	-	-	3–10
Kummerowia striata (S)	Fabaceae	AUS, USA, CN, JP	SLC	4–9	-	-	0.3	-	-
Lepidium virginicum (S)	Brassicaceae	AUS, NZ, USA	SLC	4–9.5	-	-	0.5	-	-
Lespedeza cuneata (S)	Fabaceae	AUS, USA, CA, CN, JP, PNG	SL	4–9.5	-	-	1.0	-	-
Robinia pseudoacacia (T)	Fabaceae	AUS, NZ, USA, GR	SLC	4–9	-	-	25	-	4–9	[139]
Robinia hispida (S)	Fabaceae	AUS, NZ, PNG, USA, SAF, ID	SLC	7–11	-	D	3.5	-	4–8
Lolium multiflorum (G)	Poaceae	AUS, NZ, UK, USA	SLC	4–9	-	-	0.3	-	4–8
Eleusine indica (G)	Poaceae	AUS, NZ, FIJI, PNG, USA	SLC	4–9	-	D	0.5	-	8–11
Populus species (T)	Salicaceae	AUS, NZ, USA, SAF, CA	SLC	4–9	-		15–50	-	4–9	[128]
Myrica rubra (T)	Myrtaceae	CN, JP, NZ	SLC	4.5–7	-	-	10–20	-	9–11	[140]
Schefflera heptaphylla (T)	Araliaceae	AUS, NZ, PNG, PH, ID, FIJI, MY	SLC	4.5–7	-	-	20	-	-
Schima superba (T)	Theaceae	AUS, NZ, PNG, TH, VN, MY, CN	SLC	4.5–7	-	-	30	-	-
Bromus diandrus (G)	Poaceae	AUS, FR, NZ, USA, UK	SLC	4–9	-	D	1.0	-	6–9	[112,141]
Panicum virgatum (G)	Poaceae	AUS, NZ, USA, CA	SLC	4–9	-	D	1.8	0.3	10–12
Cynodon dactylon (G)	Poaceae	AUS, NZ, PNG, USA, FIJI, CI	SLC	-	625–1750	D, L, MF	-	-	6–9
Lolium multiflorum (G)	Poaceae	AUS, NZ, UK, USA	SLC	4–9	-	-	0.3	-	4–8
Populus x canadensis (T)	Salicaceae	AUS, NZ, USA, CA	SLC	4–9	-	-	40	12	4–9
Bromus hordeaceus (G)	Poaceae	AUS, NZ, UK, USA, GR, FR	SLC	7–9	-	-	1.0	-	7–10
Avena barbata (G)	Poaceae	AUS, NZ, USA, ITA, GR, FR	SLC	4–9	-	D	0.8	-	4–8
Bromus madritensis (G)	Poaceae	AUS, NZ, USA, UK, FR	SLC	4–9	-	-	1.2		3–7
Erodium cicutarium (G)	Geraniaceae	AUS, NZ, USA, UK, ESP, GR, FR, ITY	SLC	7–9	-	-	0.6	-	-
Brassica nigra (G)	Brassicaceae	AUS, NZ, USA	SL	4–9	-	-	1.2	0.6	6–9
Centaurea solstitialis (G)	Asteraceae	AUS, NZ, USA, UK	SLC	4–9	-	D	0.6		5–9
Lactuca serriola (G)	Asteraceae	AUS, NZ, USA	SL	4–9			1.5	0.3	6–9
Cirsium vulgare (S)	Asteraceae	AUS, NZ, USA, UK, FR	SLC	4–9	-	-	2	-	-
Sonchus asper (G)	Asteraceae	AUS, NZ, PNG, USA, ID, PL	SLC	4–9	-	-	0.5	-	-
Dichelostemma capitatum (G)	Asparagaceae	AUS, NZ, SAF, ITA, GR, CA, EPA	SL	4–9	-	-	0.6	0.1	4–8
Eschscholzia californica (G)	Papaveraceae	AUS, NZ, USA, GR, UK	SLC	4–9	-	-	0.3	0.2	6–11
Castilleja exserta (G)	Orobanchaceae	AUS, USA	SLC	4–9	-	-	0.45	-	-
Lupinus bicolor (G)	Fabaceae	AUS, NZ, PNG, USA, TH, UK, SAF, MX, ITY, GR, FR, CI, CA	SLC	4–9	-	-	0.1	-	-
Larrea tridentata (S)	Zygophyllaceae	USA, MX	SLC	4–9	-	-	4	-	7–10
Salsola tragus (G)	Chenopodiaceae	AUS, NZ, USA, UK, ESP, FI	SL	4–9	-	-	0.5	-	-
Sorghastrum nutans (S)	Poaceae	USA, AUS, MX, BR	-	-	-	-	2.0	-	-
Schizachyrium scoparium	Poaceae	AUS, NZ, USA, PNG, JP, FR, GR, CA	-	-	-	-		-	-
Andropogon gerardii (S)	Poaceae	AUS, NZ, USA, UK, FIJI, ARG	S	4–9	-	-	2.0	-	4–8
Bouteloua curtipendula (G)	Poaceae	AUS, USA, GR, MX, CA, ARG	-	-	-	-	1.0	-	4–9
Festuca arundinacea (G)	Poaceae	AUS, NZ, USA, UK, ITY, FR, CA, PT	-	5.5–7	-	-	1.2	-	-
Pseudoroegneria spicata (G)	Poaceae	AUS, NZ, USA, UK, SAF, PNG, MX, MY, JP	-	-	-	-		-	-
Elymus trachycaulus (G)	Poaceae	AUS, NZ, USA, UK, GR, CA, ARG	-	-	-	-	0.3–1.5	-	-
Medicago sativa (G)	Fabaceae	AUS, NZ, USA, UK, GR, ITY, FR, CA, CN	SLC	4–9	-	D	1.0		4–11
Melilotus indicus (G)	Fabaceae	AUS, NZ, USA, UK, FR, GR	SLC	4–9	-	-	1.0	0.6	5–9
Pascopyrum smithii (G)	Poaceae	AUS, NZ, PNG, SAF, USA, UK,	-	-	-	-	-	-	-
Poa secunda (G)	Poaceae	AUS, NZ, PNG, USA, UK, SRI, SAF, PH, MX, ITY, ID, GR, FR, CA, CH	-	-	-	-	-	-	-
Festuca ovina (G)	Poaceae	AUS, NZ, USA, UK	SL	4–9	-	D	0.3	-	4–8
Bouteloua gracilis (G)	Poaceae	AUS, USA, MX	SLC	4–9	-	D	0.6	-	7–10
Nassella viridula (G)	Poaceae	AUS, NZ, USA, UK, MX, CA	SLC	4–9	-	D	1.2	-	-
Hesperostipa comata (G)	Poaceae	AUS, PNG, USA, CA, ID, TH	-	-	-		1.0	-	-
Bromus carinatus (G)	Poaceae	AUS, NZ, USA, UK, CA	SLC	4–9	-	D	0.8	-	4–8
Nassella pulchra (G)	Poaceae	AUS, NZ, PNG, USA, UK, SAF, MX, MY, JP, ID, ARG, CA	SLC	4–9	-	-	0.5–1.5	-	-
Lupinus succulentus (G)	Fabaceae	AUS, NZ, PNG, USA, UK, MX, ITY, GR, FR, CA, ARG	SLC	4–9	-	-	1.0	-	-
Leymus triticoides (G)	Poaceae	USA, AUS	SLC	4–9	-	D	1.2	-	-
Elymus lanceolatus (G)	Poaceae	AUS, NZ, USA, UK, PNG, GR, CA, ARG, ID, ITY, SAF, SWE	-	-	-	-	1.3	-	-
Bromus marginatus (G)	Poaceae	AUS, NZ, USA	SLC	4–9	-	-	1–1.5	-	-
Koeleria macrantha (G)	Poaceae	AUS, USA, UK, SAF, CA	SLC	4–9	-	-	0.5	-	-
Yucca filamentosa (G)	Asparagaceae	USA, NZ	SLC	4–9	-	-	1.2	0.6	4–10
Bromus commutatus (G)	Poaceae	NZ, UK, AUS, USA	-	-	-	-	0.4–1.2	-	-
Poa compressa (G)	Poaceae	AUS, USA, UK, NZ	-	-	-	-	0.3–0.4	-	-
Poa pratensis (G)	Poaceae	AUS, NZ, UK, USA	SLC	4–9	-	-	1.0	-	3–9
Achillea millefolium (G)	Asteraceae	AUS, NZ, USA, UK, SWE, CA	SLC	4–9	-	-	0.6	0.6	4–8
Ericameria nauseosa (S)	Asteraceae	USA, CA, NZ	SLC	4–9	-	-	2.0	2.0	7–9
Rosa acicularis (S)	Rosaceae	AUS, NZ, USA, PNG, MX, ITY, IN, GR, CI, CN, CA, BRA	SLC	4–9	-	-	1–3	-	-
Balsamorhiza sagittate (G)	Asteraceae	USA	SLC	4–9	-	-	0.3	-	4–8
Liatris punctata (G)	Asteraceae	AUS, NZ, USA, PNG, PH, SWE, ID, ITA, GR, BRA, ARG	SLC	4–9	-	-	0.6	-	3–7
Linum lewisii (G)	Linaceae	AUS, NZ, USA, UK, ESP, ITA, FR, PH	-	-	-	-	0.8	-	-
Lupinus sericeus (G)	Fabaceae	AUS, NZ, USA, PNG, GR, MX, FR, CA, SWE, ITA, ARG	-	-	-	-	0.5	-	-
Astragalus cicer (G)	Fabaceae	AUS, USA, UK, SWE, ESP, NZ, MX, ITA, IN, GR, FR, CN, AUT, ARG, CA, CL	-	-	-	-	0.6–1.0	-	-
Agropyron cristatum (G)	Poaceae	AUS, NZ, USA	-	-	-	-	0.3–0.5	-	3–9
Ericameria nauseosa (S)	Asteraceae	NZ, USA, CA	SLC	4–9	-	-	2.0	2.0	7–9
Acacia melanoxylon (T)	Fabaceae	AUS, NZ, USA, ARG, FR, GR	SLC	4–9	-	-	30	-	9–11	[20]
Melaleuca salignus (T)	Myrtaceae	AUS	-	-	-	-	15	-	-
Corymbia intermedia (T)	Myrtaceae	AUS, PNG, SRI	-	-	-	-	20–30	-	-
Eucalyptus tereticornis (T)	Myrtaceae	AUS, PNG, NZ, SAF, USA, IN	SLC	4–9	-	-	20–35	-	9–12

NB: SLC—sandy loam clay soil; T—tree; G—grass; S—shrub; AUS—Australia; AUT—Austria; MY—Malaysia; MM—Myanmar; TH—Thailand; VN—Vietnam; ID—Indonesia; IN—India; CN—China; CL—Chile; SAF—South Africa; NZ—New Zealand; PH—Philippines; ARG—Argentina; PNG—Papua New Guinea; CA—Canada; JP—Japan; FR—France; GR—Germany; MX—Mexico; PL—Poland; ITA—Italy; PT—Portugal; SWE—Sweden; ESP—Spain; FI—Finland; ARG—Argentina; BRA—Brazil; CI—Cook Islands; SRI—Sri Lanka; D—drought; L—lime; C—cold; MF—moderate frost; HF—heavy frost; S—soil salinity.

. The focus of this table is to demonstrate the habitat, family, availability, climatic adaptability, and physical characteristics of these widely studied species and to classify them according to the United States Department of Agriculture’s (USDA’s) plant hardiness zone. The USDA plant hardiness zone map is the standard for determining which plants are most likely to thrive in a particular USA location, as shown in Figure 6. This map will help to select the appropriate plant species for a specific climatic region.

Table 1 also provides information on the most popular plant habitats and families. It has been found that about 35% of studied plant species were selected from the Poaceae family, followed by Myrtaceae (14%) and Fabaceae (13%), due to their worldwide availability. It has also been observed that grass is more commonly used (making up about 50% of studied species) for landfill phytocapping than tree and shrub species due to its dense colonising properties. The number of native plant species used in different countries for phytocapping studies is shown in the world map in Figure 7.

3.4. Water Balance Performance Study of Landfill Phytocapping

A water balance performance study is integral to designing, implementing, and maintaining phytocapping systems in MSW management. It ensures that the system effectively reduces water infiltration, manages leachate, supports vegetation health, and protects the environment. The USA Environmental Protection Agency (USEPA) evaluated the water balance performance of phytocaps through the Alternative Cover Assessment Program (ACAP), which spanned seven states with varying climates and included 11 sites representing diverse climatological conditions, soils, and vegetation. In Australia, the ACAP was evaluated through laboratory investigations and field trials across five states, covering tropical to semi-arid climates. The potentially available water for infiltration, calculated as the difference between precipitation and evapotranspiration through phytocapping, along with the potentially available water for infiltration of landfill sites in both the USA and Australia (Figure 8), demonstrates the efficacy of phytocapping under different climatic conditions [112,141,142].

3.5. Phytocapping: Challenges and Opportunities

Figure 8a,b represents the potentially available water and infiltration through phytocaps from some landfills in Australia. In some cases, negative potentially available water (Figure 8a) indicates higher evapotranspiration than precipitation, necessitating irrigation for plant survival and growth over time; as plant biomass and canopy size increase, infiltration through phytocaps decreases (Figure 8b). However, fluctuations in these variables are observed due to plant die-off from extreme weather conditions, which reduces water uptake from the growth media and potentially increases infiltration.

Figure 8c,d depict the potentially available water and infiltration through phytocaps in landfills in the USA. Negative values of potentially available water observed in all studied USA landfills indicate poor potential growth and a higher survival risk of phytocapping plants due to insufficient water. Consequently, phytocapping performance fluctuates significantly, as plant health is crucial for the effectiveness of phytocapping technology.

Both Australian and USA-based studies suggest that while infiltration can be reduced, plant vulnerability increases unless soil water storage is augmented through irrigation. Supplemental irrigation to improve soil water storage can prevent possible cracking of compacted clay landfill caps during prolonged dry periods, ensuring the stability and functionality of the phytocapping system.

The studies recommended that phytocapping could be an effective and sustainable alternative to conventional landfill capping. However, it is clear from these studies that a critical balance between the design of proper growth media, selection of plant species, and climatic conditions is essential for effective and sustainable landfill phytocapping, which has not yet been adequately explored.

Most landfill phytocapping studies usually included lab-scale experiments under controlled conditions and over shorter periods. The success of landfill phytocapping depends on the appropriate selection of plants, growth medium soil, topography, geology, and climatic conditions. Further, the design of proper growth media with adequate water-holding capacity and nutrient content for plant growth is paramount. According to Warren, et al. [143], root growth in phytocovers is reduced in growth media with bulk densities above 1500 kg/m³ and is effectively prevented in bulk densities above 1700 kg/m³. Jones [144] demonstrated that more than 30% soil fineness reduced up to 80% of root growth and had a more than 1600 kg/m³ density. In addition, soil compaction reduces soil water-holding capacity. On the other hand, less compaction will increase water percolation through the growth medium soil. Therefore, balancing these two parameters for proper design of growth media is essential.

In addition, it is crucial to examine the use of various growth media and their influence on plants’ overall water-holding capacities and rainfall infiltration. Also, the designed growth media must be environmentally friendly, readily available, and economical for phytocapping in giant landfills. Overall, the challenges for landfill phytocapping are defining the characteristics of the appropriate plants according to climate and soil characteristics, designing relevant growth media with the corresponding depth for plant root growth, and implementing the proper irrigation intensity and duration for the survival of phytocap seedlings to enhance phytocapping performance.

As these characteristics vary widely, full-scale, and long-term field and laboratory applications are still required to establish the effectiveness of landfill phytocapping as sustainable landfill cover in different climatic conditions.

4. Economic Benefits and Sustainability of Phytocapping

Clay is used to cap landfills in many places due to its low hydraulic conductivity [27]. It can be an economical solution where the material is locally available. However, its use can be uneconomical in some regions, e.g., central Queensland, where landfills are small, and the availability of clay is limited [67]. According to the BDA [145], phytocaps can be up to 50% less expensive in Queensland than conventional clay caps. Similar benefits can be expected in other parts of Australia [67]. In a USA-based study, Hauser, Weand and Gill [94] reported that phytocaps cost 35% to 72% less than conventional clay caps. However, these cost analyses considered only the cost of construction. If the long-term maintenance costs are factored in, phytocaps are likely to yield even more economic benefits. Apart from the financial benefits, phytocaps have ongoing environmental and social benefits, such as biodiversity conservation, carbon sequestration, and amenity park values and picnic sites. These sites can also be used for commercial purposes, such as for cut flowers, hardwood timber, or biofuel production [4,67].

A simplified comparative analysis is conducted here among conventional clay capping, phytocapping (clay cap with vegetation), and a new approach to phytocapping (clay cap + compost-like output (CLO) growth medium and vegetation). The process flow for the three specified landfills and capping is presented in Figure 9, which shows the waste compositions, energy requirements, greenhouse gas (GHG) emission factors, leachate production, tree spacing considered for phytocapping, costings and energy recovery potentials.

In conventional landfills and landfills with phytocaps, all the collected waste is dumped together, and additional cost is involved in handling GHG emissions and leachate production. Methane and carbon dioxide contribute equally to GHG emissions and make up most of the landfill gases. Non-GHGs, like nitrogen and hydrogen, constituting a smaller fraction of emissions, were not considered for analysis. The quantity of waste sent to landfills remains consistent across these two methods. An assumption was made regarding the proportions of organic and inorganic waste in MSW, without critically examining it, to facilitate a comparative analysis of the benefits. In the landfill scenario involving phytocap and CLO growth media (phytocap+CLO), a portion of organic waste is utilised for compost production, while the remainder is mixed with clay to prepare the CLO. This landfill scenario allows for waste recovery to generate energy and reduces the amount of organic waste deposited in landfills. As a result, greenhouse gas emissions and leachate production are also minimised (Figure 9).

Evaluating the advantages and drawbacks of the landfill scenarios involved assessing the additional costs or potential benefits compared to the base case, which is the conventional clay capping method. Figure 10 depicts this analysis, where the base case’s economic, social, and environmental net benefits are designated as zero (0 = no impact). Thereby, any benefit and its extent compared to the base case is portrayed in the analysis. The social acceptability of all three landfill scenarios was considered similar, and therefore, the social net benefit was also designated as zero (no impact).

Landfills utilising phytocaps offer greater environmental benefits compared to conventional landfills, primarily due to their potential for carbon dioxide sequestration. However, the phytocap+CLO approach provides environmental advantages by recovering waste, which diminishes greenhouse gas emissions and leachate production. In the phytocap+CLO scenario, where 80% of the waste is converted to compost, the resulting decrease in GHG emissions is substantial, surpassing the carbon dioxide sequestration potential of phytocaps alone.

Economically, there is no difference between the base case and the base case with phytocapping, as the land requirements remain unchanged. However, the phytocap+CLO scenario offers significant economic benefits by reducing the land needed for landfill. While energy and processing costs are associated with composting and CLO production, the overall benefits outweigh these costs.

5. Conclusions

Despite our increasing ability and efforts to recycle and reuse materials and resources, landfills remain the final repositories for many types of waste and, if not appropriately maintained, are a source of environmental pollution by generating greenhouse gases and leachate. The best technique to overcome this problem is to prevent contact between landfill waste and the outside environment, especially water, using sustainable, environmentally friendly landfill cells, mainly landfill caps. The development of landfill technology, its sustainability and the comparative benefits of landfill capping are summarised below:

• Engineered landfill technology was developed after the 1970s, and compacted clay caps, GCLs, PVC and HDPE are commonly used in conventional landfill covers to reduce GHG production by minimising water infiltration. However, these approaches are expensive, and their performance is questionable in the long run.
• Phytocapping is a new landfill capping method that consists of a growth medium layer for growing vegetation over a landfill cap. Selecting appropriate plants and designing suitable growth media is challenging, as these depend on multiple climatic and geological variables. A few research studies have been conducted on landfill phytocapping to evaluate the effectiveness of phytocapping and found that phytocapping with CLO growth media is more economical and sustainable than conventional landfill methods.
• However, all this research has been conducted over short time periods and the findings are mainly based on either laboratory or field results. This is not adequate as it does not provide any acceptable design guidelines for growth media; appropriate mixing ratios for growth media amendments to enhance plant growth; initial irrigation requirements; or the mortality and survivability of phytocapping plants in different landfill environments. Therefore, fruitful research must be conducted in field and laboratory conditions to compare the performances of plants and growth media for phytocapping.