Introduction

Wastewater management remains a critical challenge in rapidly urbanizing regions, especially in developing countries, where infrastructure development often lags behind population growth1,2. As urbanization accelerates, municipal wastewater treatment systems confront growing demand3. Inadequate treatment frequently results in untreated wastewater discharge into natural water bodies, including Lake Tana, Ethiopia’s largest freshwater lake4. This environmental degradation not only worsens pollution but also threatens public health and disrupts aquatic ecosystems, thereby emphasizing the urgent requirement for sustainable and effective wastewater treatment methods5.

For decades, wastewater treatment has relied on conventional methods such as activated sludge, trickling filters, and anaerobic digesters, which are widely used to remove organic matter and reduce pathogens. However, these methods have considerable limitations, particularly in resource-limited places like Bahir Dar. Specifically, high capital investment, elevated energy consumption, and the need for skilled personnel to operate and maintain these systems often render them inaccessible6. Moreover, their reliance on external energy sources and operational complexities limits long-term viability, particularly in low-income regions7. To address this issue, alternative cost-effective and environmentally friendly treatment approaches have been increasingly investigated. Among these alternatives, phytoremediation has emerged as a promising technique for wastewater treatment. Phytoremediation is a technique that uses diverse plant species for wastewater treatment by absorbing, degrading, and transforming pollutants through rhizosphere-mediated processes involving root and microbial interactions8. It provides energy-efficient and sustainable alternatives that can complement or even replace conventional methods, especially in resource-constrained environments.

Vetiver grass (Vetiveria zizanioides) has gained considerable attention due to its exceptional phytoremediation potential. Vetiver, native to tropical and subtropical regions, is known for its deep and extensive root system, which stabilizes soil, uptakes contaminants, and supports microbial populations that enhance pollutant degradation9. Additionally, its root creates a conducive environment for microbial communities within the rhizosphere, promoting the breakdown of organic matter and facilitating nutrient removal through nitrification, denitrification, and phosphorus adsorption10. Moreover, its high transpiration rate helps to reduce wastewater volume, making it an ideal passive, low-energy treatment option for places with limited resources. The potential of using vetiver for wastewater treatment has been explored in various settings, ranging from pilot-scale constructed wetlands to small-scale pot and channel experiments. Badejo et al. (2018) evaluated vetiver in a vertical-flow constructed wetland treating domestic effluents, observing removal efficiencies of 85% for BOD, 78% for COD, 68% for total nitrogen (TN), and 54% for total phosphorus (TP) over 60 days11. Although phytoremediation using Vetiver grass has been investigated in Ethiopia, no published studies have systematically evaluated the effect of planting density on pollutant removal efficiency under real municipal wastewater conditions in Bahir Dar City. Moreover, density-dependent performance is highly relevant for practical design, yet this factor remains understudied in field-scale applications. In Ethiopia, Angassa et al. (2019) investigated the use of constructed wetlands planted with vetiver in Addis Ababa for municipal wastewater treatment, achieving BOD and COD removal efficiencies exceeding 80%12. The study also demonstrated excellent removal efficacies; however, it utilized constant plant densities and provided limited insight into ideal layouts for variable climates and loading rates. Another study also investigated multiple macrophyte species, including vetiver, in treating secondary effluents, exhibiting COD removal of 82% and TN removal of 59%13. However, these investigations did not consider plant density variability or root biomass development, which influence removal efficiency.

Thus, the current study evaluates phytoremediation using Vetiver grass for the removal of key pollutants, including BOD, COD, nitrogen, and phosphorus from municipal wastewater. Three planting densities, namely 20, 40, and 60 plants/m2, were used to investigate their efficacy for pollutant removal. By comparing three planting densities and supporting associated seasonal and operational constraints, the study provides new, context-specific evidence to inform cost-effective wastewater treatment methods. Hence, the study aims to contribute to the growing research on phytoremediation and provide practical insights for implementing vetivers in urban wastewater management, particularly in resource-limited environments, such as Bahir Dar City.

Materials and methods

Study area and site description

This study was conducted in Bahir Dar City, the capital of the Amhara Region, northwestern Ethiopia, located on the southern shore of Lake Tana at approximately 11 ºN and 37 ºE, with an elevation of about 1840 m above sea level (see Fig. 1). The city experiences a tropical highland climate, characterized by a mean annual temperature and rainfall of approximately 20.1 ℃ and 1280 mm, respectively. The rainfall is unimodal, with the wet season extending from June to September, and the dry season from October to May. Municipal wastewater in the city is generated from domestic sewage and is typically discharged into open drainage channels, ultimately into Lake Tana and the Abay River, without undergoing proper treatment. The site was selected because it represents urban wastewater issues of Ethiopian secondary cities, has direct discharge to the Abay River and Lake Tana, has infrastructure deficiencies, and is easily accessible for pilot-scale phytoremediation experiments.

Fig. 1
Fig. 1
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Map of the study area.

Wastewater sampling and characterization

Untreated municipal wastewater samples were collected from Sebatamit Kebele in Bahir Dar City, Ethiopia, where domestic effluents are discharged directly into open drains flowing toward the Abay River. The samples were obtained using three clean 150 L high-density polyethylene (HDPE) containers. Samples were delivered to the laboratory within one hour and stored at 4 ℃ before analysis. The baseline characterization was performed to determine initial pollutant concentrations. The parameters, including biochemical oxygen demand (BOD5), chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP), were examined using APHA Standard Methods14. The wastewater samples were digested in a Block Digester (HACH) using a catalyst containing sulfuric acid, potassium dichromate, mercury (II) sulfate, and silver sulfate to facilitate the oxidation of organic matter. Following the digestion using a spectrophotometer (HACH DR6000 UV-VIS), the COD was determined calorimetrically. The BOD, on the other hand, was determined as the percentage of biodegradable organic matter using the Winkler method with azide adjustment. The samples were incubated at 20 ℃ for 5 days in 300 mL BOD bottles (BOD-293 RS-232). The initial and final DO concentrations were determined by adding sodium hydroxide, sodium azide (to prevent nitrification), manganese sulphate, sodium iodide, and a starch indicator. The TN and TP, nutrients that lead to eutrophication, were quantified via persulfate digestions (alkaline digestion for TN and acid digestion for TP) followed by spectrophotometric analysis. All samples were tested in triplicate.

Experimental design and pollutant removal assessment

Vetiver grass was obtained from the Amhara Agricultural Research Institute and pre-cultivated in soil beds before transplantation. The samples were contained in 150 L cylindrical HDPE containers simulating small-scale constructed wetlands. Each container was supplied with a perforated plant support to stabilize the plant and permit unrestricted root growth and water circulation (see Fig. 2). Four treatments were examined to assess their impact on pollutant removal: control (0 plants/m2), low (20 plants/m2), medium (40 plants/m2), and high (60 plants/m2). Each density treatment was implemented in a separate 150 L container, resulting in a study design with experimental replicates. The wastewater samples were placed in each container, and the trial was conducted for nine weeks to evaluate the long-term treatment efficiency. The wastewater in the containers allowed for consistent exposure of pollutants to vetiver roots, enabling long-term monitoring of phytoremediation efficacy across planting densities. These densities represent a realistic range from low to high planting intensities used in tropical phytoremediation systems. They are compatible with field-based recommendations for improving root biomass without severe intraspecific competition15,16.

Fig. 2
Fig. 2
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Overall experimental layout for vetiver grass treatment.

Monitoring and data collection

The study was conducted under ambient outdoor conditions. The samples were taken from each container at seven-day intervals for nine consecutive weeks. This routine provided a time-series dataset to monitor pollutant removal trends as the vetiver root matured. Samples were tested for BOD5, COD, TN, and TP using the procedures described in sample characterization. Each of them was measured in triplicate per container to determine the variability within the treatment. The visual observations of plant growth and root health were also conducted throughout the study.

Data analysis

The pollutant removal efficiency (%) for BOD, COD, TN, and TP was calculated for each treatment period using Eq. 1. The mean values and standard deviations were computed from triplicate measurements. Plant densities (0, 20, 40, and 60 plants/m2) were analyzed using one-way ANOVA with a 95% confidence level (α = 0.05) to compare removal efficiency. All statistical analyses were conducted using Origin Pro (2025), and results are reported with corresponding p-values.

$$\:\text{R}\text{e}\text{m}\text{o}\text{v}\text{a}\text{l}\:\text{e}\text{f}\text{f}\text{i}\text{c}\text{i}\text{e}\text{n}\text{c}\text{y}\:\left(\text{\%}\right)=\left(\frac{{C}_{in}-{C}_{out}}{{\text{C}}_{\text{o}\text{u}\text{t}}}\right)\times\:100$$
(1)

Where Cin is the pollutant concentration in the influent (pre-treatment) and Cout is the pollutant concentration in the effluent (post-treatment).

Results and discussion

Wastewater characterization

The pollutant contents in the untreated municipal wastewater were much higher than the permissible discharge limits (Table 1). The BOD and COD levels were 224.6 ± 8.9 mg/L and 300.7 ± 11.2 mg/L, respectively, exceeding the Ethiopian Environmental Protection Authority (EEPA) limits of 30 mg/L (BOD) and 250 mg/L (COD). The nutrient concentrations were also high, with TN of 56.3 ± 2 mg/L and TP of 22.1 ± 0.5 mg/L, surpassing the discharge limits of 10 and 2 mg/L, respectively. These increased pollutant concentrations indicate a high organic and nutrient load, which can cause oxygen depletion and eutrophication in aquatic environments. High BOD and COD levels indicate a substantial biodegradable organic matter, providing a clear baseline for evaluating treatment performance.

Table 1 Wastewater characterization and discharge limits.
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Effect of planting density on pollutant removal efficiency

The efficiencies of all planting densities for COD, BOD, TN, and TP removal increased progressively over the 9-week study period. The high-planting density treatment (60 plants/m2) exhibited the highest removal rate, followed by the 40, 20, and the control in plants/m2 (see Table 2). The control showed the lowest efficiencies. From the statistical analysis, two-way ANOVA, it is demonstrated that no significant variation in COD (p = 0.857) and BOD (p = 0.665) removal among treatments was observed, indicating that organic matter removal was largely unaffected by plant density. This implies that sedimentation and microbial degradation were the primary drivers of COD and BOD reduction, even without vegetation. In contrast, nutrient removal was significantly influenced by planting density for TN (p = 0.027) and TP (p = 0.0017). Although nitrogen and phosphorus were significantly removed compared to the control, differences among planting densities were not statistically significant. This indicates that the presence of plants improves nutrient removal, although higher densities may not always result in proportionally greater advantages.

Table 2 Overall pollutant removal efficacy by plant density (Week 9).
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Several studies support these findings in removing organic pollutants and nutrients from wastewater. Maharjan et al. (2015) reported the removal efficiencies of 71.03% for BOD5 after one month of treatment17. In a separate investigation, 85% of BOD5, 90% of COD, and 85% of total coliform were removed from kitchen effluent18. Additionally, another study reported over 60% phosphate, 40% COD, 40% nitrate, and 90% antibiotic removal from secondary effluent19. Thus, our results are consistent with prior findings, demonstrating vetiver’s effectiveness for nutrient and organic pollutant removal under varied conditions.

The treatment performance of the low-planting density showed a gradual increase in removal over the treatment time, but at a slower rate, particularly for nitrogen and phosphorus (see Fig. 3). Medium-planting density demonstrated a marked improvement in pollutant removal compared to the low-planting density treatment (see Fig. 4). The high planting density exhibited the most substantial and rapid increases in removal of all pollutants (see Fig. 5). The comparison of the removal efficiencies of each plant density, including the control, over the 9th week, is presented in Table 2. The table clearly shows the superior nutrient removal ability of different density plant treatments, especially emphasizing the significant phosphorus removal at medium and high planting densities.

Fig. 3
Fig. 3
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Municipal wastewater treatment by vetiver grass, with a low-planting density treatment (20 plants/m²).

Fig. 4
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Municipal wastewater treatment by vetiver grass, with medium-planting density treatment (40 plants/m²).

Fig. 5
Fig. 5
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Municipal wastewater treatment by vetiver grass, with high-planting density treatment (60 plants/m2).

Mechanisms of pollutant removal

The phytoremediation of organic pollutants via vetiver wastewater treatment is primarily driven by sedimentation, filtration, and microbial degradation within the rhizosphere. The dense root network traps particulates (Phyto-filtration)20. Additionally, microbial biofilms oxidize and degrade organic matter21. Although no vetiver was used (control), it could remove organic pollutants due to natural sedimentation, microbial activity within the substrate, and abiotic processes such as adsorption and chemical oxidation (see Table 2)22,23. Indeed, nutrient removal was minimal, confirming the key role of plants in nitrogen and phosphorus cycling. Additionally, nitrogen removal occurs via plant uptake and microbial nitrification-denitrification24,25. Ammonium (NH4+) is oxidized to nitrate (NO3−) by nitrifying bacteria (Nitrosomonas, Nitrobacter), followed by denitrification to nitrogen (N2) gas by Pseudomonas in anoxic zones26. Although microbial populations were not quantified in this study, previous works confirmed diverse, active microbial communities in vetiver rhizospheres, supporting nutrient removal27,28. Phosphorus is removed through plant uptake and adsorption to root surfaces and soil particles29. The high-planting density treatment achieved the best nutrient removal due to greater root surface area and microbial habitat30. However, excessively high densities may reduce oxygen availability, potentially limiting microbial respiration31. These findings are consistent with prior research that reported maximum BOD removal at intermediate plant densities due to balanced root oxygenation and microbial interaction32.

Compared to other wetland species, such as Phragmites australis and Typha latifolia, which removed BOD at 71.1%, COD at 71.5%, nitrogen at 64.5%, phosphate at 86.5%) rates, vetiver grass exhibited superior phytoremediation potential, achieving 89.7% of BOD, 80.6% of COD, 60.5% of nitrogen, and 40.8% of phosphate removal due to its deep root, high biomass, and environmental stress tolerance as reported in literature33. In contrast, the Typha–Heliconia polyculture demonstrates a higher removal rate, particularly in the removal of phosphate at 86.5% and nitrogen at 64.5% compared to their removal by vetiver (40.8% and 64.5%), respectively. This can be attributed to complementary root architectures that improve substrate contact and microbial processes such as denitrification and phosphorus adsorption34. Thus, these findings indicate that vetiver is more effective for organic load reduction, whereas polyculture is better suited for nutrient-rich wastewater treatment.

Unlike energy-intensive activated sludge, vetiver-based wetlands offer passive treatment with low maintenance requirements, making them suitable for decentralized wastewater management in resource-limited environments35. Although large-scale implementation of vetiver is promising, it faces many challenges, such as root zone clogging, seasonal performance variation, and potential nutrient saturation of the substrate over time. High planting densities may require substantial land for a municipal-scale system, which could be limiting in urban areas. Long-term success will require maintenance protocols, pilot testing, and cost-benefit analyses tailored to local conditions. Vetiver-based treatment offers lower operational costs and energy demands than conventional wastewater treatment, suitable for decentralized applications in resource-limited settings. Their adoption requires community training, ongoing maintenance, and supportive policies to integrate green infrastructure into wastewater management. Challenges for long-term use include root zone clogging, which can reduce hydraulic conductivity and necessitate maintenance. Seasonal variability, such as wet and dry cycles in Bahir Dar, may affect the performance via changes in oxygen availability, microbial activity, and nutrient loads, while nutrient saturation in substrates may decrease treatment efficiency, require periodic substrate renewal, or necessitate resting phases. Scaling to municipal volumes at 60 plants/m2 may require a significant amount of land, limiting its use in densely populated regions. Pilot studies and cost–benefit analyses are necessary to optimize the design and assess feasibility under local conditions. Future studies should consider including biomass-based planting density data (kg m−2). While the commonly used metric of plants per unit area helps comparison across phytoremediation trials, it may not fully capture differences in plant biomass accumulation. Incorporating biomass-based density measures alongside plant counts would support more scale-up designs. Despite this constraint, the present study contributes important comparative evidence on density-dependent pollutant removal under real municipal wastewater conditions in Bahir Dar, thereby advancing practical knowledge for wastewater management in similar contexts., thereby advancing practical knowledge for wastewater management in similar contexts.

Conclusion

This study effectively investigated the efficacy of vetiver in the treatment of municipal wastewater. The various vetiver planting densities were involved in the treatment to investigate their pollutant removal efficacy over time. According to the results, the medium and high planting densities demonstrated an enhanced removal of pollutants in comparison to low-planting density and a control. Specifically, the high planting density removed nutrients at higher rates, which were not achieved by the other planting density systems. In comparison to other plant species, vetiver is an effective phytoremediation strategy, particularly for organic pollutant removal, but less effective for nutrient removal. Overall, the results emphasized the importance of optimizing planting density to improve treatment efficiency while considering practical constraints such as land availability and maintenance needs. Vetiver is an effective, low-cost, and environmentally sustainable phytoremediation technique for municipal wastewater treatment, especially in resource-limited regions. In connection with this, future research may focus on the removal of heavy metals and pathogens as well as the long-term system performance under variable environmental conditions.