Environmental Assessment of a Constructed Wetland with Ornamental Vegetation for Wastewater Treatment: A Sustainable Option for Neighborhoods (The Case of Veracruz, Mexico)

Abstract

The discharge of wastewater into bodies of water and subsoil poses a serious pollution problem. In many neighborhoods or districts, there are often no wastewater treatment systems due to the high costs involved, which may compromise human health. Constructed wetlands (CWs) offer an ecological solution to improve water quality and enable its reuse. They promote the removal of contaminants through physical, chemical, and biological processes. The objective of this study was to evaluate Canna hybrids, Zingiber spectabile, and Alpinia purpurata—ornamental plants not typical of wetlands—regarding their function as phytoremediators and their growth under such conditions. Utilizing CWs with ornamental plants for water treatment in neighborhoods could improve the adoption of this ecotechnology. To this end, eight cells were built: two were controls (without plants), two contained Canna hybrids, two had Zingiber spectabile, and two included Alpinia purpurata, all designed for a hydraulic retention time of three days. Inlet and outlet water samples were collected biweekly for six months. The results showed that the cells with Canna hybrids and Zingiber spectabile removed from 40 to 70% of total nitrogen and phosphorus. In terms of organic matter, measured as COD and TSS, the removals ranged from 55 to 90%. In contrast, cells with Alpinia purpurata demonstrated removal rates of only 30 to 50%, which were statistically lower (p ≤ 0.05), indicating a slower adaptation to wetland conditions. This slower adaptability is directly related to the growth of the species, as Alpinia purpurata also exhibited the lowest growth rates. The study concluded that using CWs with the studied ornamental plants is a viable alternative for treating wastewater and, at the same time, they may add a commercial value to the vegetation. Additionally, they can enhance the aesthetic landscape with colorful flowers that attract birds and insects and the treated water could be utilized to irrigate sports areas or urban planters.

1. Introduction

Water pollution is a significant global issue that arises out of the lack of adequate technologies for its treatment. Conventional systems, including chemical reactors, activated sludge, and biological systems, often entail high construction costs that may exceed 10 million Mexican pesos. In addition, specialized personnel for their operation is required [1,2]. This has led to a substantial deficit in the implementation of wastewater treatment systems, particularly in rural areas (communities with fewer than 2500 inhabitants), where the small and dispersed population poses additional challenges. While large cities require extensive water treatment systems, there is potential to establish localized systems on a neighborhood or district basis to address the issues of wastewater discharge and facilitate the reuse of treated water using environmentally and economically viable technologies.

Constructed wetlands (CWs) serve as a sustainable alternative for wastewater treatment. These systems, or eco-technologies, enhance water quality by mimicking the physical, chemical, and biological processes found in natural wetlands, all within an engineered framework. Key design considerations include hydraulic retention time, filter materials, phytoremediation plants, as well as the volume and type of wastewater to be treated [3,4].

CWs consist of shallow cells or channels and can be classified based on the type of flow as either superficial or subsurface. In superficial systems, water passes through a substrate, typically composed of soil or fine materials. These conditions often support various types of rooted vegetation, including emergent plants, submerged plants, and floating plants [5]. Subsurface systems, on the other hand, are shallow channels or cells filled with rough or porous materials, such as gravel, tezontle, tepezil, sand, or even rough or folded plastic waste. These substrates promote the establishment of microbial communities that aid in the removal of contaminants from the water flowing through the cells. Additionally, the substrate serves as an anchor for the roots of the emergent plants, which also contributes to the absorption or adsorption of contaminants. This capability of plants is referred to as phytoremediation [6]. In these types of wetlands, water can enter from either a horizontal or vertical direction [2,7].

In Mexico, for example, initial studies predominantly began around the 2000s, and 67 studies have explored this alternative, mainly in laboratory or pilot scale conditions. Only 18 CWs operating areas ranging from 31 to 11,600 m², have been reported [8].

The vegetation of CWs typically comprises natural wetland species, which are well-adapted to having their roots consistently saturated with water. Commonly utilized species include those from the genera Typha, Scirpus, and Phragmites [9,10]. In recent years, particularly in Mexico, there has been a notable shift towards the incorporation of terrestrial ornamental plants (species not commonly associated with wetlands) into these systems [8,11]. Some of the species frequently used in recent years include Zantedeschia aethiopica, Heliconia sp., Anthurium sp., and Canna indica, or hybrids [12,13,14]. This trend has enhanced wastewater treatment, aided in the production of commercial ornamental plants, and contributed to the creation of more aesthetically pleasing landscapes that can be adapted for new urban developments, neighborhoods, or districts, resulting in the establishment of vibrant green and floral spaces.

The significance of this sustainable approach is particularly critical given the pollution challenges faced in Mexico, especially in rural areas or neighborhoods where water treatment systems are often overlooked. In these regions, the discharge of untreated wastewater, which contains both organic and inorganic contaminants, not only disrupts ecosystems but also poses serious risks to human health [15]. Therefore, evaluating CWs as a viable strategy for eliminating these contaminants is of utmost importance.

Research carried out in Nautla, Veracruz, Mexico, reported the adaptation of ornamental plants (Colocasia esculenta, Pontederia cordata, Cyperus papyrus, Heliconia psittacorum, among others) in a CW with surface and subsurface systems treating municipal wastewater. This implementation provides improved aesthetics with different flowering stages [16]. On the other hand, in Tapachula, Chiapas, Mexico, a CW for wastewater was tested in a coffee farm using ornamental plants (Heliconia stricta and Heliconia psittacorum), also demonstrating its adaptability [17]. In both cases, the use of ornamental plants in tropical regions was demonstrated. However, there are fewer studies that use CWs to treat domestic wastewater in backyards as flower beds, which could be an option for better acceptance of this ecotechnology using other species with commercial value and analyzing its function as a phytoremediator.

This study aimed at evaluating the removal of contaminants, including chemical oxygen demand (COD), total suspended solids (TSS), total nitrogen (TN), and total phosphorus (TP), from wastewater using CWs planted with ornamental species (Canna hybrids, Zingiber spectabile, Alpinia purpurata). Additionally, it sought to measure the growth of these species during the purification process, a factor that is often overlooked in similar research [6,17]. Conducting this type of study can provide valuable insights into the use of CWs with ornamental plants, fostering the development of multifunctional systems that not only treat water, but also produce flowers and enhance landscapes and ecosystems in both rural and urban settings. Thus, the following hypotheses were developed:

1:

Based on the physical conditions of plants, we expect that the growth and removal of pollutants in CWs with Canna hybrids will be higher than in CWs with Zingiber spectabile or Alpinia purpurata.

2:

The removal of pollutants will be higher in systems with plants than in systems without vegetation (control).

2. Materials and Methods

For the development of the study, a CW was adapted in a communal neighborhood in San José Pastorías, Actopan, Veracruz, Mexico. The system was built in 2015. For the present study, filling and vegetation adjustments were made as described below.

2.1. Design and Operation of Horizontal Flow Constructed Wetlands

Eight wetland cells were established in the backyard of a residence located at coordinates −96°57′08″ N and 19°55′83″ S. Each cell measures 1.5 m in length, 0.23 m in width, and 0.6 m in depth (207 L), and they were filled with porous river rock (average porosity of 50%) sourced from a local river, with water flow occurring 10 cm below the surface (see Figure 1). To prevent clogging, the cells were topped with a 40 cm layer of rock with a particle size of approximately 3–4 cm, followed by a second layer of 10 cm with a particle size of around 12 cm.

Out of the eight wetland cells, two were planted with the locally known Canna hybrids, two with maracas (Zingiber spectabile), another two with plantain or red ginger (Alpinia purpurata), each consisting of a monoculture with six plants per cell, while the remaining two cells served as control units (without vegetation). This design was intended to assess the effect of plants on the phytoremediation process (see Figure 1).

Contaminated influent water for the wetlands was sourced from household wastewater, encompassing laundry, dishwashing, and toilet water, for that reason, phosphorous, nitrogen, and organic matter, measured as COD and TSS, were analyzed, respectively. This water was stored in a 1000 L tank, and its flow rate was adjusted to ensure that each wetland cell achieved a hydraulic retention time (HRT) of three days (a measure of the average time that water remains in the wetland for the treatment process). This HRT has been recommended as optimal in CWs [4,5]. In all the cells, a value of 2.9 L/h was adjusted.

2.2. Pollutant Measurements

The analysis of total nitrogen (TN), total phosphorus (TP), chemical oxygen demand (COD), and total suspended solids (TSS) was conducted according to APHA standards [18]. Additionally, ambient temperature and light intensity were measured biweekly using an HTC-1 hydrometer (Uplayteck; Veracruz, Mexico) and a HIELEC-MS8233-2000 luxometer (STEREN; Veracruz, Mexico), respectively, at three different times: 9:00, 14:00, and 17:00 h, to calculate the average of each measurement. For plant growth assessment, the height of individual plants was recorded every 15 days using a measuring tape.

For statistical analysis, a one-way analysis was employed using the SPSS program, version 27 for Windows, to compare the results of contaminant removal across treatments involving different vegetation types and the presence or absence of plants. Prior to this analysis, data were confirmed to be parametric using the Kolmogorov–Smirnov test. When significant effects of the variables were found, the Tukey post hoc and Games–Howell tests were used, assuming equal and different variances, respectively. A p-value of 0.05 was used to reveal the statistical significance of all the analysis.

3. Results and Discussions

3.1. Climate Characteristics in the Study Area

In the area where the wetlands were established, the ambient temperature ranged from 19 to 25 °C, with an average of 22.3 °C (see Figure 2). This temperature, typical for the region, supported the growth of vegetation. It is important to note that the vegetation was initially collected from nearby riparian zones, meaning it was already well-adapted to the local climatic conditions. Additionally, the light intensity in the area fluctuated between 70,000 and 90,000 lux, which is characteristic of tropical regions like the one under investigation. Various reports on the species utilized in the wetlands indicate that these climatic conditions are ideal for their adaptation [7,19].

3.2. Growth of Wetland Vegetation

The height and growth of the plant species included in the study were measured, revealing consistent growth patterns. Notably, none of the plants perished, which can be attributed to their ability to adapt to the favorable climatic conditions described. However, variations in size were observed (see Figure 3). Alpinia purpurata exhibited growth ranging from 10 to 50 cm, while Canna hybrids grew from 12 to 140 cm, and Zingiber spectabile displayed a range from 12 to 210 cm. This growth pattern aligns with the species’ adaptability to hydroponic conditions involving wastewater, and may also indicate their potential phytoremediation capabilities, which will be analyzed further.

In the case of Alpinia purpurata, the comparatively lower growth rate corresponds with findings by Marín-Muñiz et al. [19], who noted that this species is slow to acclimate to wetland environments. They also discovered that, while the aerial portions of the plants can rot, the roots remain viable and can regenerate when replanted. This is a phenomenon that could persist until the next planting cycle. It is worth noting that all three species have ornamental qualities and produce flowers, contributing to the aesthetic appeal of the treatment cells, which function as both floral displays and natural water purifiers. The pH in the system oscillated between 6.8 and 7.2 (data not reported), suitable conditions for the growth of plants and microorganisms [4,5].

3.3. Concentrations and Removal of Pollutants

Throughout the research period (180 days), both the initial and final concentrations, as well as the removal of pollutants, were assessed. The concentrations of nitrogen compounds (TN) ranged from 4 to 20 mg/L (see Figure 4a), which are typical levels found in wastewater. Notably, the highest concentrations (16–18 mg/L) were observed at the system’s entry point and in the control cells, where the absence of vegetation hindered their reduction (see Figure 4b). This highlights a positive removal effect attributed to the presence of plants compared to the control group, demonstrating a successful phytoremediation process. It is important to point out that this effect was primarily observed in species with more vigorous growth, such as *Z. spectabile* (58%) and *C. hybrids* (49%). In contrast, *A. purpurata* exhibited a removal rate of only 20%, which was statistically similar to the control’s 11% (p = 0.850) and lower than that of the other two species (p < 0.05). In a CWs mesocosm study with A. purpurata in Misantla, Veracruz, Mexico, a removal percentage of 70–86% of TN was reported. This difference was possibly related to the period of study (two years of investigation). Thus, it is considered that the time in this study was not optimal for its best development and phytoremediation function [13].

Studies examining the role of plants in treatment wetlands have indicated that their presence enhances removal efficiency compared to systems without vegetation, with differences ranging from 20 to 35% [10]. This finding aligns with observations made in this study regarding species exhibiting the highest growth rates. Consequently, some research has even excluded control systems from their analyses [20,21]. Beyond their commercial and aesthetic significance, as noted by Zurita et al. [22] and Lara-Acosta et al. [23], the value of these plants is further highlighted by their adaptation to wetland environments and their contributions to phytoremediation processes. Several of these species are particularly important in various regions of Latin America due to their beneficial functions, thereby enhancing their overall utility [24].

Conversely, the concentrations and removals of TP exhibited a trend like that of TN, albeit with differing values. The inputs for this parameter ranged from 2 to 10 mg/L (see Figure 4c). The cells containing species that demonstrated the highest growth also had the lowest concentrations (2–5 mg/L). Similarly, the observed removals were correlated with the growth and stress of the vegetation, as the controls and the cells with A. purpurata showed no statistically significant differences (p > 0.05). This could potentially change in the year of growth for the species, as previously noted. In the case of the other two species, the removals were comparable (55–68%; p = 0.425) but differed significantly from those of the controls and cells with A. purpurata (p < 0.05) (see Figure 4d). The removals observed are like those reported by studies using typical plants (Carex elata, Juncu effusus and Phragmites australis) of natural wetlands, indicating its utility in bioremediation processes, added to aesthetical benefits [25].

It is also worth mentioning that the final removal concentrations for both TN (25 mg/L) and TP (15 mg/L) do not exceed the maximum permissible limits for discharge into rivers or streams. However, when discharging into lakes or lagoons, Mexican standards dictate that the maximum allowable limits must not exceed 15 mg/L for TN and 5 mg/L for TP [26]. Notably, these standards are met with systems that utilize Z. spectabile and C. hybrids, which are widely recommended for effectively eliminating contaminants in wastewater and ensuring compliance with Mexican Official Standards. The effects observed with ornamental plants are comparable to those seen with traditional CWs with species such as Typha or Phragmites spp. [9,10]. This underscores the importance of selecting species well-adapted to wetland conditions and highlights the added value that these plants provide. Thus, it is crucial to leverage the rich biodiversity of ornamental plants and assess their adaptability in wetland environments.

The concentrations of organic matter, measured as chemical oxygen demand (COD), ranged from 50 to 680 mg/L, which is consistent with the previously reported parameters. The Alpinia species exhibited COD concentrations like the controls without plants, although these values were notably lower than those observed at the inlet. In contrast, two species that have demonstrated significant effectiveness as phytoremediators, Z. spectabile and C. hybrids, recorded lower COD concentrations of 5 to 100 mg/L (see Figure 5a). It is important to highlight that the maximum permissible limit for this parameter is 150 mg/L, a threshold that is frequently exceeded by as much as four times the inlet concentrations. For discharges into lakes, only the biochemical oxygen demand (BOD₅), which should not exceed 30 mg/L, is reported; this metric was included in the overall COD measurement. The observed removal rates ranged from 85 to 91% (as shown in Figure 5b) for cells with the fastest-growing plants, with no statistically significant differences among these groups (p = 0.285). However, the removal rates (30–42%; p < 0.05) for the control samples and those with A. purpurata were statistically different from those of the more effective plant cells. The removals observed were similar to those reported in other studies in tropical regions using different species of ornamental plants. This may support the usefulness of the plants under study for the treatment of domestic wastewater [13,18,20,26].

Regarding total suspended solids (TSS) (see Figure 5c), the concentrations throughout the 180 days of analysis varied between 200 and 1080 mg/L. However, systems with the fastest-growing species produced TSS values ranging only from 15 to 105 mg/L, reflecting the removal efficiencies illustrated in Figure 5d. The highest removal rates were between 58 and 63%, while the controls and those with A. purpurata achieved from 25 to 40% of removal. These data further underscore the critical role of wetlands in eliminating a variety of contaminants. Akratos and Tsihrintzis [27] and Varma et al., [28] have described that TSS removal occurs primarily in systems with an 8-day hydraulic retention time (HRT). In this study, the HRT was three days, achieving removals of up to 63%. This demonstrates the good performance of both the design and the phytoremediation process with the species studied, which translates into an important contribution to this field of research.

It is worth emphasizing that the lower contaminant removal rates detected among the study species could be due to their physiological or morphological traits. For example, it has been reported that Alpinia purpurata requires adaptation periods of 7 to 12 months [29], a longer period than that evaluated in this study. Therefore, it is suggested that future research using this species considers longer analysis times. Similarly, it has been reported that this species grows better under controlled humidity conditions rather than under permanent water exposure [30], as occurs in CWs. These conditions suggest that the species developed lower growth and phytoremediation processes in the study systems. Additionally, the plant tissue is more rigid than that of the Canna, which is associated with lower aerenchymatic structures, a physiological characteristic that facilitates gas and water exchange in flooded environments and allows for better adaptability [31]. In the case of the Canna species, it was reported to be highly tolerant to the excess of humidity, stress, direct light, and easy to propagate, which is why it is easy to adapt and remove in environments such as CWs, that is, with a high presence of nutrients [32]. Similar to Canna, Zingiber species are highly tolerant to waterlogged soils, which support their rapid growth and nutrient absorption [33].

The findings regarding the removal of contaminants suggest that the treated water could be utilized for irrigating crops beyond just tubers or for flower cultivation. This aligns with the work of Manzano et al. [34], who employed the Z. spectabile species to treat aquaculture waters, allowing for their reuse in irrigating corn crops, which yielded better results compared to raw water. Additionally, De Campos and Soto [35] reviewed various applications of water treated by CWs across different countries. Their analysis revealed that in Egypt, some reuses are associated with crops designated for biodiesel production. In Portugal, treated water is employed in public parks, gardens, and sports fields. Meanwhile, in Greece, urban water reuse encompasses unrestricted irrigation for crops, golf courses, cemeteries, and public parks. Overall, this illustrates the extensive utility of the water treated by CWs, highlighting ecotechnology as an effective strategy for addressing environmental challenges.

The reuse of water treated by CWs illustrates how this ecotechnology promotes the circular economy and supports the Sustainable Development Goals (SDGs), specifically targeting the objective to “ensure availability and sustainable management of water and sanitation for all” (SDG 6) [36]. Waly et al. [37] noted that the implementation of CWs has a positive impact on achieving several SDGs, including SDG 2: Zero Hunger, SDG 3: Good Health, SDG 6: Clean Water, SDG 7: Affordable and Clean Energy, and SDG 15: Life on Land. This analysis also emphasizes the importance of considering synergies for integrated environmental governance and enhancing policy coherence in sustainable development management. Additionally, other studies have shown that CWs contribute to the circular economy through water reuse, commercial vegetation production, nutrient recovery, energy generation, utilizing ecosystems as social laboratories, and providing various ecosystem services [38,39,40,41].

4. Conclusions

The wetlands examined, featuring ornamental plants such as Canna hybrids, Zingiber spectabile, and Alpinia purpurata, proved effective in treating wastewater. The results indicated a remarkable reduction in organic matter, measured as Chemical Oxygen Demand (COD), alongside reductions in nitrogen and phosphorus compounds by nearly 90% and 70–72%, respectively. These values fall within the regulatory limits established in Mexico, highlighting the practical nature of these systems, which are simple to install and operate. The first two plant species demonstrated superior adaptability to wastewater conditions, indicating that the first hypothesis was not fulfilled one hundred percent, as the Zingiber species was not considered to have greater growth than Canna, and on account of it demonstrating its adaptability to CW conditions. On the other hand, the second hypothesis was accepted, corroborating that phytoremediation is a vital process in wastewater treatment. This may contribute more knowledge regarding the use of ornamental plants in CWs treating domestic wastewater. Additionally, it is suggested that health authorities collaborate with academic and community sectors to promote the benefits of utilizing flower boxes with wetlands as an ecological treatment strategy, applicable in rural areas, specific urban locations, and neighborhoods. The implementation of environmental education initiatives in partnership could significantly enhance people’s quality of life. Furthermore, the process of treating and reusing wastewater aligns with the principles of a circular economy embedded within the urban water cycle. In summary, the potential benefits regarding costs, and environmental sustainability in the construction of treatment wetlands in real-world scenarios are emphasized.