Keywords

9.1 Introduction

A Constructed Floating Wetlands (CFW) is a nature-based system designed to function as a floating wetland, integrating features from constructed wetlands and conventional retention ponds (Headley and Tanner 2012; Yeh et al. 2015). This hybrid system is particularly effective in the restoration of water bodies affected by eutrophication. Unlike traditional wetlands, CFWs employ emergent plants that are strategically placed on a floating mat above the water’s surface. This arrangement facilitates their growth and establishment in deeper waters, thus:

  • Allowing them to uptake nutrients from the water column, similar to the hydroponic mechanism (White and Cousins 2013).

  • Providing a significant surface area for the growth of biofilms (Borne 2014; Tanner and Headley 2011), which effectively removes excessive nutrients from the water and filter sediments (Tanner and Headley 2011; Winston et al. 2013).

  • Releasing oxygen and exudates and creating oxidative microcosm (Masters 2012).

Incorporation of ornamental plants within floating systems enhances not only the visual aesthetics of the site but also the overall ecological well-being (Chen et al. 2009). These plants provide nesting and breeding habitats for fauna and serve as fish habitats, thus promoting biodiversity and supporting ecological equilibrium (Weragoda et al. 2010).

Choosing suitable emergent plant species is crucial for CFWs as it affects the effectiveness of removing pollutants and maintaining the overall health of the ecosystem (Li and Guo 2017). Different plant species have different degrees of capacity in CFWs because of their distinct morphological and physiological characteristics, including nutrient absorption efficiencies, growth rates, and root types, underscoring the importance of selecting the most suitable plants for successful CFW design and implementation (Pavlineri et al. 2017; Wang and Sample 2014). Figure 9.1 shows a schematic diagram of CFW.

Fig. 9.1
A schematic of a constructed floating wetland. The parts are labeled from top to bottom as follows. Emergent vegetation, C F W buoyant structure, biofilm covered roots, storm water flow, particles settling down, and Benthick layer.

Schematic diagram of constructed floating wetland

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9.2 Importance of Plants in CFWs

Emergent plants in CFWs serve as the key component in the purification process. They have distinct abilities to directly absorb excess nutrients/pollutants from the water and act as natural filters. They indirectly contribute to the breakdown and transformation of pollutants by providing a favourable site for biofilms. There are many more factors enhanced by plants. The existing research on emergent vegetation in CFWs is relatively limited compared to constructed wetlands, the importance of these plants should not be underestimated.

In the floating bed system of CFWs, plants have a significant impact on both the chemical and physical aspects of the water. They can alter factors like pH, temperature, and dissolved oxygen, thereby creating a favourable environment for enhancing water quality. Additionally, the presence of plants inhibits the growth of algae by competing for essential nutrients and light, thereby reducing the risk of eutrophication.

Roots play a crucial role in the nutrient uptake and pollutant transformation process. In CFWs, macrophytes are suspended in floating mats, allowing their roots to remain in constant contact with the water. This arrangement enables efficient absorption of dissolved pollutants and provides a large surface area for beneficial biofilm formation. The biofilm, in turn, facilitates the biochemical transformation of contaminants, aiding in their removal from the water column.

The mat of old roots of plants in CFWs create anaerobic/anoxic conditions that promote the growth of denitrifying bacteria, which can convert nitrate into harmless nitrogen gas, effectively reducing nitrate levels. Furthermore, these roots act as traps, capturing fine suspended particulates that would otherwise remain in the water. Moreover, microbes living on the surface of plant roots play a crucial role in removing nitrate through a process called dissimilatory nitrate reduction to ammonium. They can remove up to ten times more nitrate than the plants themselves, contributing significantly to the overall purification process.

The efficient nutrient uptake and assimilation by CFW plants are influenced by various factors, such as the characteristics of the wastewater, plant species, and seasonal variations. Plants allocate resources to below-ground tissues, increase root length, and develop thinner roots to enhance their absorption abilities in response to nutrient availability. Moreover, as plants progress through different stages of their life cycle, nutrients absorbed earlier are remobilized and translocated to different parts of the plant, optimizing nutrient utilization and overall system performance.

Overall, plants in CFWs are central to the purification process, actively removing pollutants from water through direct absorption, facilitating microbial transformation, competing with algae for nutrients, and creating favourable root environments. Their role in maintaining water quality, reducing nutrient levels, and preventing the growth of algal toxins makes them indispensable components of CFWs.

9.3 General Characteristics of Plants Suitable For CFWs

Emergent plant selection is crucial in the process of CFWs. The choice of plants not only affects pollutant removal but also plays a significant role in maintaining the integrity of the local ecosystem (Wang and Sample 2014). Various plant species possess distinct biological properties that determine their suitability for CFWs, including nutrient absorption efficiencies, growth rates, and root types. When selecting plant species for CFWs, it is important to consider both plant-related and non-plant-related factors (Zhao et al. 2012).

9.3.1 Plant-Related Factors

The plant-related factors to be considered in CFW plant selection are as follows:

  • Native and Non-Invasive Species: Native plants are preferred in CFWs as they generally outperform non-native species. They are well adapted to local conditions and pose minimal risk of becoming invasive if they escape from the treatment system (Bi et al. 2019). Native macrophytes have roots that are specifically adapted to thrive in wetland conditions, ensuring their survival and effectiveness in pollutant removal (McAndrew & Ahn 2017).

  • Herbaceous Perennial Plants: Herbaceous perennials are desirable for CFWs because their aboveground biomass can be harvested at the end of the growing season, preventing the release of nutrients back into the water. These plants exhibit new growth in the following season, allowing them to continue absorbing nutrients from the water (Chen et al. 2009). Harvesting the aboveground biomass helps maintain nutrient balance and prevents excessive nutrient buildup.

  • Terrestrial Plant Species: Unlike free-floating plants, the growth of terrestrial plants on CFWs can be controlled (Wang and Sample 2014). This allows for better management and facilitates the maintenance of desired plant populations within the system.

  • Ability to thrive in a Hydroponic Environment: CFWs operate as hydroponic systems where plants grow in water without soil. Therefore, selecting plant species that can adapt to and thrive in this hydroponic environment is essential for their successful implementation in CFWs.

  • Plants with Aerenchyma: Aerenchyma refers to air spaces present in the roots and rhizomes of plants (Chen et al. 2009). Plants with large aerenchymas exhibit increased buoyancy potential, which helps them stay afloat on the water surface. Additionally, aerenchyma facilitates the movement of oxygen from the aerial parts of the plants to the roots and rhizomes, effectively aerating these submerged plant parts (Wang and Sample 2014).

  • Aesthetic Beauty: Whilst not directly related to the functionality of CFWs, the aesthetic appeal of the selected plant species can enhance the visual quality of the treatment system. Choosing plants with attractive foliage, flowers, or overall appearance can contribute to the overall satisfaction and acceptance of CFWs in various settings. Figure 9.2 shows the plant species suitable for CFWs.

Fig. 9.2
Three photographs. They are of Canna indica L with spear shaped leaves and bright flowers, Typha angustifolia with needle leaves, and Acorus calamus L that grows in damp soil.

Plant species for CFWs: (a) Canna indica L.; (b) Typha angustifolia L.; (c) Acorus calamus L.

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9.3.2 Non-plant-Related Factors

The non-plant-related factors to be considered in CFW plant selection are as follows:

  • Economic: Economic feasibility is an important consideration when implementing CFWs. The total cost of CFWs is influenced by factors such as floating mats, plants, and labour for harvesting and planting (Wang and Sample 2014). Therefore, plant species that exhibit luxuriant growth and produce substantial aboveground biomass are advantageous. Harvested biomass from economically viable plants can be used for various purposes, including composting, soil amendments, anaerobic digestion to produce methane, volatile fatty acid (VFA) production, and animal feed. Furthermore, combining harvested plant biomass with solidified manure can increase the nutrient content, thereby offering additional benefits in agriculture (Sooknah and Wilkie 2004).

  • Social acceptability: Public acceptability is important for the successful implementation of the CFW. The customs, traditions, beliefs, and values of local people have to be considered in the selection of plants.

9.4 Plant Selection Process

The plant selection process is shown in Fig. 9.3. The following three steps can be followed in the plant selection procedure:

Fig. 9.3
A flow chart of the selection of plants for C F W. The selection starts with step 1, systematic view, step 2, preliminary screening, and step 3, weighted scoring. Each step has a different flow which finally ends at scoring system.

A flow chart for selecting plants for CFWs

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Step 1. Systematic review: The systematic literature review on CFWs involves a structured approach to gathering information on objectives, plant species used for previous research, plant selection criteria, and scoring parameters under each criterion. Figure 9.4 shows the major steps of plant selection criteria. The process includes defining the research objective, choosing a relevant search engine, determining keywords, conducting a search, collecting and categorizing relevant studies, extracting data, organizing and analyzing the data, identifying patterns and trends, and synthesizing the findings. By following these steps, a comprehensive and structured analysis of research studies on CFWs can be conducted, highlighting important insights and knowledge gaps in the field.

Fig. 9.4
A diagram explains plant selection criteria. It starts with systematic scoring, and follows to preliminary screening, weighted scoring, field study, and finally verification.

Major steps of plant selection criteria

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Step 2. Preliminary screening: This process involves refining information from a systematic review and follows a hierarchical key with five stages. For example, the preliminary screening process aims to select plant species suitable for Sri Lankan conditions, which is described in Fig. 9.5. Only plant species that fulfil all five requirements will be chosen. The selection criteria are based on ecological considerations and CFW requirements. The criteria include the availability of plants in Sri Lanka to ensure local adaptation, the non-invasive nature of plants to prevent disruption of aquatic ecosystems, preference for terrestrial growth for better control and biomass accumulation, preference for perennial life cycles to enable controlled biomass harvest and efficient nutrient absorption, and adaptation to submerged conditions for effective nutrient absorption and treatment in CFWs. By following this systematic screening process, the selected plant species will meet all five requirements, ensuring their suitability for CFWs in Sri Lankan conditions.

Fig. 9.5
A hierarchical key diagram. It starts with plants found in previous study. If it is available in Sri Lanka, it follows successive steps before ending with terrestrial plants that adapt to submerged conditions.

Hierarchical key

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Step 3. Weighted scoring: In this third stage of plant selection for CFWs, a weighted scoring system is employed to identify the most suitable plant species for a field study (Table 9.1). The selection process considers common CFW objectives, including lake water quality improvement, biodiversity enhancement, aesthetic value, economic value, and social acceptability. To assess the plant species’ suitability, criteria and parameters are identified from the systematic review conducted in the first stage. The assessment involves evaluating nutrient removal efficiency and plant growth for improving water quality, breeding surfaces for benthic organisms and shade provision for fish to enhance biodiversity, flowering and foliage beauty for aesthetic value, income and expenditure for economic considerations, and the preference of the local community for social acceptability. Each criterion is assigned a weight based on its relative importance to the overall objectives. Plant species are then scored on a scale of 1–3, indicating their suitability within the identified criteria, with 1 being the least suitable and 3 being the most suitable (Table 9.2). Based on this weighted scoring system, the selection process has identified specific plant species for CFWs. Figure 9.6 illustrates the typical plant species employed in previous studies around the world. These plants have been utilized in various contexts and environments to enhance the performance of CFWs. It is important to note that whilst these plants offer valuable insights, they may not always be the most suitable choices for specific regional conditions.

Table 9.1 Identified common objectives of CFWs, plant selection criteria and parameters
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Table 9.2 Scaling of parameters and plant selection criteria (Author)
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Fig. 9.6
Three close-up views of the following. A, Canna indica L with bright flowers. B, Cyperus papyrus L located near water and has needle leaves. C, Vetiveria zizanioides L with a dense growth of needle leaves.

Typical plant species for CFWs: (a) Canna indica L., (b) Cyperus papyrus L., and (c) Vetiveria zizanioides L.

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The plants introduced to the CFWs, as illustrated in Fig. 9.7, have been assessed and ranked according to their suitability scores. Amongst these plants, Canna indica L. has emerged as the most suitable species, attaining the highest score, whilst Dracaena sanderiana is considered a moderately suitable option due to its moderate score. On the other hand, Vetiveria zizanioides L. has obtained the lowest score, indicating it to be the least suitable choice. Thus, by following this approach, the most suitable plant species can be identified for CFWs based on their total scores, ensuring alignment with the objectives of the study.

Fig. 9.7
Three close-up views of uprooted crops. They are as follows. a, Canna indica L. b, Vetiveria zizanioides L. c, Dracaena sanderiana. They have elongated stems with leaves and fibrous roots.

Plants introduced to the CFWs: (a) Canna indica L., (b) Vetiveria zizanioides L., and (c) Dracaena sanderiana

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Tables 9.3 and 9.4 illustrate our plant evaluation process using Acorus calamus L. and Canna indica L. as examples. These tables demonstrate how we assess plants against specific criteria, such as nutrient removal efficiency and growth, assigning scores based on their performance within these parameters. For instance, Acorus calamus L. achieved a nitrogen removal percentage of 38.4%, resulting in a score of 2, whilst Canna indica L.’s performance earned it a score of 2 with a nitrogen removal percentage of 57.55%. These scores reflect how well each plant aligns with our criteria. The higher the score, the better a plant meets our objectives for CFWs. These tables provide a standardized and transparent framework for evaluating plant suitability, assisting decision makers in selecting the most appropriate plants for their specific CFW projects.

Table 9.3 Weighted scoring for the plant of Acorus calamus L. (Author)
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Table 9.4 Weighted scoring for the plant of Canna Indica L.
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9.5 Problems with Plant Selection Criteria

Some common challenges with the plant selection criteria are as follows:

  • Insufficient Data Set: One potential risk in the plant selection process for CFWs is relying on a limited number of research publications. This may lead to an incomplete understanding of plant characteristics and their suitability for CFWs. To mitigate this error, it is crucial to conduct a comprehensive literature review that encompasses a diverse range of research sources. By considering a broader spectrum of studies, you can gather a more comprehensive dataset and ensure a more accurate selection of plant criteria.

  • Evolving Objectives: Another mistake that can occur is the selection of static or outdated objectives for CFWs. The field of CFWs is constantly evolving, with new research and advancements shaping the understanding of their objectives. It is important to regularly review and update the objectives based on the latest scientific findings and industry practices. This ensures that the plant selection criteria align with the current state of knowledge and address the most relevant goals of CFWs.

  • Incomplete Plant Characteristics: Inadequate consideration of plant characteristics can be a potential problem in the selection criteria. Whilst you have identified some common characteristics, it is important to acknowledge that there may be other crucial factors to consider. These could include plant root structure, nutrient uptake capabilities, pollutant removal efficiency, and adaptability to specific climatic conditions etc. To address this issue, it is necessary to conduct further research and consult with experts to identify and incorporate all essential plant characteristics relevant to the specific conditions of the CFWs.

Addressing these mistakes and problems in the plant selection criteria for CFWs is crucial for ensuring accurate and effective plant choices. Conducting a thorough and diverse literature review, updating objectives based on current research, considering a comprehensive range of plant characteristics, and actively seeking scientific data will enhance the reliability and success of plant selection in CFW systems.

9.6 Conclusion

The selection of suitable emergent plant species for Constructed Floating Wetlands (CFWs) is vital for effective water purification and ecosystem maintenance. Criteria such as native and non-invasive nature, herbaceous perennials, adaptability to a hydroponic environment, and aesthetic appeal guide the plant selection process. A systematic process involving literature review, screening, and weighted scoring helps identify the most suitable plant species. Amongst these, Canna indica L. stands out as a top choice for CFWs, promising effective water quality improvement, biodiversity support, and aesthetic enhancement. These carefully selected plants are key to the success of CFWs in restoring urban water bodies.