1. Introduction
Different landscape-based water treatment design elements or methods are currently being used in landscape design in different parts of the world. These elements were initially employed as stormwater management method aimed at controlling runoff. They are now being used for pollutant removal, stream restoration, and groundwater recharge infiltration [1]. Some of these elements like bioswales and constructed wetland, had been confirmed to be an effective and low-cost methods for removing water pollutants [2]. Based on the geometric property, US EPA (2014) classified landscape-based water treatment elements as point, linear and area.
Point elements are used to capture water at a specific location and may use a combination of detention, infiltration, evaporation, settling, and transformation, to manage flow and removal of pollutants. Structures like constructed wetland, infiltration basin, bioretention, sand filter (surface), rain barrel, cistern, wet pond, and dry pond are categorized as point elements.
Linear elements are narrow linear shaped structures that provide filtration and nutrient uptake to manage flow and removal of pollutants. Example of linear elements includes grassed swale, infiltration trench, vegetated filter strip, sand filters (non-surface), vegetated swales etc.
Area elements are land-based management practices that affect an impervious area, land cover, and pollutant input. Structures like a green roof and porous pavement are included in this category. When selecting appropriate landscape-based elements for a specific site, important factors like the effectiveness of the methods to remove pollutants should be considered [1].
This review's main goal is to evaluate and examine the different components that go into designing landscape-based water treatment systems. It focuses on two features in particular that are frequently used: constructed wetlands and vegetated wetlands, or bio-swales. The purpose of this article is to analyze these components and determine how well they treat water for various uses, such as domestic use and irrigation.
Two major worldwide issues are environmental pollution and water scarcity. The impact of population increase, urbanization, and climate change on water supplies highlights the need for sustainable solutions. Water resource management can be done sustainably with the help of landscape-based water treatment designs. In order to treat and purify water, these designs incorporate natural materials including vegetation, soil, and water bodies. This paper helps spread knowledge of the significance of landscape-based water treatment by going over the components. It emphasizes how important these components are to preserving the availability and quality of water.
It is crucial to comprehend the advantages and potential applications of constructed wetlands and vegetated swales/bio-swales. These components can lower pollution, improve water quality, and offer ecosystem benefits. The review particularly looks into the possibility of using vetiver grass to remove pollution. Vetiver grass is an important part of landscape-based water treatment systems since it has shown the ability to extract pollutants from wastewater.
This review's scope includes the following important areas: The components frequently used in landscape-based water treatment design are reviewed in this research. It specifically concentrates on two widely utilized components: constructed wetlands and vegetated swales/bio-swales. The review also attempts to evaluate and investigate how well these components treat water. It investigates their suitability for treating water for various purposes, including irrigation and domestic usage. The paper also explores the benefits and possibilities associated with these elements and it delves into the literature on determining the potential of plants for pollution removal. Specifically, it examines the capacity of Vetiver grass to remove pollutants from wastewater.
Academic articles from reputable online search libraries, such as Web of Science, ScienceDirect, IEEE, and SCOPUS, published between 2002 and 2023 were used for this investigation. In order to refine the search, a comprehensive selection of reliable online libraries is made first, and pertinent keywords (such as Constructed wetland, Landscape-based water treatment, Vegetated swale/Bio-swale, and Vetiver grass) and abstracts are chosen. A comprehensive screening procedure is carried out, which includes quality assessment, data extraction, and assessments of the title and abstract. The chosen papers mostly address landscape-based water treatment elements.
The authors of [4] examined the use of artificial landscape water features to improve ecological quality in urban settings. The article addresses methods of treatment and approaches to raise the quality of the water in these areas, which frequently use recycled water. The objective is to solve the issues of water scarcity while designing landscape features that are both aesthetically pleasant and sustainable.
In [5] the authors highlight the significance of employing landscape-based approaches to solve water scarcity, with a particular focus on China's increasing urbanization. It emphasizes the necessity of water reclamation, sustainable urban development, and water-based planning at the regional level.
The study [6] presents a Landscape Elements Water Management Strategy (LEWMS) that serves as a framework for investigating source control measures in urban environments. This strategy looks at recurring spatial elements (such parks, gardens, and rooftops) in an effort to improve catchment-level water management.
2. Most widely used Landscape-based water treatment design elements
2.1. Vegetated swale/Bioswale and Grass swale
Vegetated swale (Figure 1) is linear, flow-based element designed to convey runoff with the provision of limited pollutant removal through sedimentation and horizontal filtration by vegetation [7]. Because of its limited capacity to remove dissolved pollutants, a vegetated swale element design is considered as pre-treatment of concentrated flows, before discharge to other landscape-based water treatment systems [8]. A vegetated swale can be used in conjunction with check dams perpendicular to the water flow to increase pollutant removal efficiency [9]. Check dams can be constructed using earth, riprap, gabions, railroad ties, or pressure-treated wood logs. A vegetated swale is cost-effective, aesthetical appealing, improve water quality by filtration, proved wildlife habitat and reduce water volume, flow rate and temperature [10]. Well-designed and properly maintained swale system could be expected to remove 70% of Total Suspended Solid (TSS), 30% of total phosphorus (TP), 25% of total nitrogen (TN), and 50-90% of trace metals [9].
According to Iowa state university (2008), there are three types of vegetated swale i.e., grass swales, dry swale with filter media and wet swales. Grass swale (Figure 2(a)) is a broad and shallow earthen channel vegetated with erosion-resistant and flood-tolerant grasses. It is used as a pre-treatment conveyance for other water quality landscape-based water treatment methods. Dry swale (Figure 2(b)) consists of an open channel that has been modified to enhance its water quality treatment capability by adding a filtering medium consisting of a soil bed with an underdrain system. The wet swale (Figure 2(c)) also consists of a broad open channel capable of temporarily storing the water but lacks an underlying filtering bed. The wet swale has water quality treatment mechanisms like stormwater wetlands, which rely primarily on the settling of suspended solids, adsorption, and uptake of pollutants by vegetative root systems.
Grassed-lined and flat-bottomed channel, grass swale is one of effective element used to improve stormwater quality for many years [11]. Grass swale (Figure 3) is cheap to construct, easy to maintain [12]. Grass swale design element could improve water quality through infiltration, sedimentation (due to the low velocity induced by the vegetation), filtration by the grass blades, and likely through some biological processes [11]. Several field studies have proven the effectiveness of grass swales in removal of water pollutants. A study done in Florida, showed that grass swale have removal rate of 98% of TSS, 64% of organic carbon, 45% - 48% of nitrogen,18% of phosphorus and 50% - 70% of metal [12]. Kabir et al. 2014 (cited in Leroy et al. 2016) explained briefly how grassed swales showed a good potential of reducing TSS (44% - 98%) and metals such as Cu, Zn, Pd and Cd (17% - 99%). Sedimentation along the swale length is the most important process in removing water pollutants. Installation of check dams, increasing swale length and channel roughness and decreasing the slope are reported to have improved treatment efficiency through increasing the retention time of the water promoting sedimentation and infiltration [11].
2.2. Constructed wetland
Constructed wetlands are man-made systems that imitate the structural and functional aspect of natural wetlands for removing water pollutants from wastewater through the use of aquatic plants and microorganisms [14]. Constructed wetlands is considered a holistic approach, integrating wastewater treatment, flood protection and stormwater management [15]. It removes physical, biological and chemical contaminants from wastewater through the use of different process including sedimentation (settling of solids), chemical adsorption (fixation), plants uptake and bacterial degradation. Compared to the conventional water treatment processes, constructed wetlands are effective, environmentally friendly and economical viable alternative of wastewater treatment technology. It has become an increasingly popular option for wastewater treatment because of high removal efficiency, low cost, simplicity in operation and high in potential of water and nutrient reuse [16-18].
Although many scholars generally classify constructed wetlands into three (free surface flow, subsurface flow, and hybrid system), floating treatment wetlands are also being used widely as wastewater treatment recently.
2.2.1. Free surface flow constructed wetland (FSF CW)
Free surface flow constructed wetland is the oldest type of artificial wetland characterized by water flow above the ground and plants rooted in the sediment layer or floated on the water surface [16]. Kadlec and his co-workers (2000) as cited by Zhang et al. (2014) explained that free surface flow constructed wetland is effective in the removal of organic matters through microbial degradation and removal of suspended solids through filtration and sedimentation (Figure 4). It has been reported that free surface flow constructed wetland could attain removal efficiency rate 70% and above of TSS, Chemical Oxygen Demand (COD), Biological Oxygen Demand (BOD) and pathogens.
2.2.2. Subsurface flow constructed wetland (SF CWs)
Subsurface flow constructed wetland is a system designed with horizontal or vertical surface flow through a permeable medium like sand, gravel or crushed rock [18]. In the case of subsurface flow constructed wetland, the water level is designed to remain below the top substrate [19]. Wastewater flows by gravity, horizontally or vertically as indicated in Figure 5, through the bed substrate containing a mixture of microbes and plant roots [20]. There are two basic types of subsurface flow constructed wetland: horizontal subsurface flow (HSF) and vertical subsurface flow (VSF) [16]. According to recent studies, elimination efficiencies of 75.10% for HSF and 89.29% for VSF for BOD, and 66.02% for HSF and 64.41% for VSF for COD, can be attained. Additionally, it was stated to achieve elimination rates of 51.97% for HSF and 50.55% for VSF for TN, 65.96% for HSF and 59.61% for VSF for TP [18].
2.2.3. Hybrid system
It was agreed that it is difficult to treat many characters of wastewaters using a single system, indicating the necessity of introducing hybrid system [18]. Hybrid system is a system comprising of various type of constructed wetlands (FSF, VSF and HSF etc.) combined aimed at achieving relatively higher treatment effect [21]. The VSF constructed wetlands is intended to remove organics and suspended solids with provision of nitrification, whereas, denitrification and further removal of organics and suspended solids occur in HSF constructed wetland [18]. According to Zhang et al (2014), removal efficiency of 93.82% for TSS, 84.06% for BOD5, 85.65% for COD and 54.75% for TP and 66.88% for TN were attained with the use of hybrid system.
2.2.4. Floating treatment wetland (FTW)
Floating treatment wetland (also called artificial floating wetland, floating plant bed system, integrated floating system, integrated ecological floating bed) is soilless planting structure constructed with floating material such as hydroponic plants (floating plants), sediment-rooted emergent wetland plants and related ecological communities of algae and small invertebrates [22]. The upper part of the hydroponic plant grows and remains above the water, develop an extensive root system below the water and perform direct nutrient uptake from the polluted/wastewater as shown in Figure 6 [23]. Previous studies have shown FTW has a better removal efficiency of BOD (79.31%), COD (55.20%), TN (62.45%) and TP (49.58%) when compared to the other constructed wetlands [18].
For selecting a plant to be used in constructed wetland, the plant’s tolerance and removal capacity of pollutants are the most important criteria [24]. Although several plant species have been used in floating treatment wetlands, research in over 100 countries including Australia, China, Vietnam, and Thailand, has demonstrated that Vetiver. zizanioides L. has high tolerance and removal capacity to a wide range of pollutants including salinity, sodicity, aluminum, manganese, arsenic, cadmium, chromium, nickel, copper, mercury, lead, selenium and zinc [25]. In the current research undertaking, emphasis of literature review was placed on studies conducted on pollutant removal capacity of Vetiver. Zizanioides L.
3. Pollutant removal capacity of Vetiver Grass
In Chile floating treatment wetland with vetiver grass were used as water treatment method for decontaminating irrigation water (Figure 7). The grass was able to extract a number of contaminants such as heavy metals, pesticides hydrocarbons and radioactive compounds [26]. The results of those study showed that the grass was able to reduce Boron from 13.35 mg/l to 7.1 mg/l and Arsenic from 0.33 mg/l to 0.06 mg/l in five days duration, and reduction of Manganese from 1 mg/l to 0.24 mg/l and Lead from 2 mg/l to 0 mg/l in 15 days duration.
A series of experimental study were conducted in Nigeria to assess the potential of the native African vetiver grass (Chrysopogon nigritana) species in addressing the challenges of wastewater management as shown in Figure 8. Native African vetiver grass species (Chrysopogon nigritana) and South Indian species (Chrysopogon zizanioides) were introduced as a means of controlling and treating effluents from a fertilizer blending company, quarry industry and leachate from a public refuse dumpsite. Vetiver plants were raised hydroponically as described by Truong and Hart (2001) in plastic buckets of 40 cm diameter and 60 cm height. The treated wastewater was collected at 2-, 4-, and 6-days intervals for post-treatment laboratory analysis. The levels of contaminants after treatment were compared with allowable international levels reported (FAO 1985; WHO 1993 and WHO/FAO, 2007). The result showed that both C. zizanioides and C. nigritana were effective in improving the pH and removing contaminants such as BOD and COD, nitrate, phosphate, cyanide, lead, zinc, iron, cobalt, cadmium, Arsenic and manganese [27].
Guam island economy relays mainly on tourism which in turn depends upon healthy marine and water resources. Regular storm water run-off and non-treated or semi-treated wastewater were reported to have caused adverse impacts to the marine water quality, fish habitats, and aesthetics. Due to lack of appropriate infrastructure, the island was badly in need of low-cost water treatment system. Vetiver grass was applied to Umatac-Merizo Drainage and Inarajan sewage wastewater treatment plant-pond to remove the nutrients (i.e., phosphorus and nitrogen) as well as some of the heavy metals from the lagoon before the treated water is released to the percolating field and eventually to the ocean (Figure 9).
The initial condition of the pond was monitored weekly before the application of vetiver grass grown in floating panels with each panel consisting of 20 holes. After the installation, water samples were taken and tested at weekly interval. Despite unsteady conditions like wind, rain, sunshine, cloud cover and duckweeds the results obtained indicated that vetiver grass was effective in removing pollutant including heavy metals [25].
The efficacy of vetiver grass in eliminating heavy metals from contaminated water, including Cu, Fe, Mn, Pb, and Zn is examined in this study [28]. To evaluate the effectiveness of removal, the study investigates various root lengths and plant densities. In terms of metal uptake, vetiver grass shows promise; removal rates are increased by longer roots and higher densities. The results demonstrate how well it can purify water tainted with metals.
This paper [29] is primarily concerned with treating wastewater using vetiver grass in artificial wetlands. Water quality is improved by the efficient removal of contaminants from wastewater by vetiver grass. The plant's significance for environmentally friendly water management and preservation is emphasized in the research.
Vetiver grass has proven to be an effective phytoremediation technique for cleaning up contaminated water [30]. This study investigates how successful it is in eliminating pollutants when compared to other plants. Vetiver system technology is promising for tackling the problems associated with water contamination.
4. Conclusion
Water treatment design features based on the landscape are thought to be a very viable and cost-effective technique to treat wastewater. Various ecological approaches were considered in order to obtain the acceptable effluent quality for wastewater disposal or reuse. Ecological solutions such as vegetated swale, constructed wetland, and hybrid (floating wetland with vetiver grass) emerged as a potential way for wastewater treatment, according to the literature assessment. It was also found that landscape-based water treatment design elements tested in various parts of the world have the ability to treat waste water in terms of water quality parameters such as BOD, COD, TSS, pH, and sodium adsorption ratio.
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