Sustainability of global small-scale constructed wetlands for multiple pollutant control

The escalating global production of wastewater, projected to increase significantly by 2050, poses a severe threat to environmental sustainability and clean water availability. A substantial portion of this wastewater is discharged untreated, particularly in developing nations, hindering progress towards the UN's Sustainable Development Goal 6.3. Constructed wetlands (CWs) offer a cost-effective and energy-efficient nature-based solution for wastewater treatment, with lower capital, energy, and maintenance costs compared to conventional wastewater treatment plants. However, questions remain regarding the long-term effectiveness and sustainability of CWs in simultaneously removing multiple pollutants. While some studies report synergistic pollutant removal, others indicate antagonistic interactions or temporal variations in removal efficiency. A systematic global assessment of small CWs (<8 hectares) is needed to address these knowledge gaps and determine their viability for sustainable wastewater management.

Existing literature on constructed wetlands (CWs) extensively covers pollutant removal, influencing factors, operational mechanisms, design, management, and maintenance. However, a significant knowledge gap exists regarding the long-term sustainability and effectiveness of CWs in removing multiple pollutants concurrently. Some studies have shown synergistic effects, while others report antagonism or neutral relationships between different pollutant removal processes. The temporal variability of pollutant removal efficiency also highlights the need for a comprehensive understanding of CW sustainability. Existing research often lacks a global perspective and focuses on short-term studies, limiting the understanding of long-term performance and the influence of various factors on pollutant removal.

This study compiled data from 364 global sites utilizing small-scale CWs (<8 hectares) for wastewater treatment. Data were extracted from 196 publications in the Web of Science Core Collections (2000-2022) and screened based on several criteria, including outdoor setting, treatment of real wastewater, engineering application, data availability on key parameters, and area size. The key pollutants analyzed were chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP), and ammonium nitrogen (NH4+-N). Influencing factors considered included wetland operating time, area, hydraulic loading rate (HLR), hydraulic retention time (HRT), and air temperature. Linear mixed-effects models (LMEMs) were used to assess the relationships between pairwise pollutant removals, considering study site as a random effect. The Johnson-Neyman technique was employed to identify temporal thresholds for synergistic associations. Ordinary least squares (OLS) regression was used to analyze the relationships between slope_PRE (slope of pairwise pollutant removals) and duration, identifying temporal trends and thresholds. Further analysis investigated the effects of area, HLR, HRT, and temperature on synergistic relationships using LMEMs and the Johnson-Neyman technique to identify thresholds. Structural equation modeling (SEM) and Random Forest (RF) analyses were conducted to determine the direct and main effects of these factors on synergistic pollutant removal.

Small-scale CWs demonstrated high removal efficiencies for organic matter and nutrients, with a 75th percentile removal efficiency of 68.8–84.0% for TN, TP, COD, and NH4+-N. Bivariate analysis consistently revealed synergistic relationships between pairwise pollutant removals across different CW types and continents. These synergistic relationships were found to be sustainable for 3–12 years, after which they began to erode. The study identified optimal thresholds for maintaining these synergistic effects: area size of 17,587 m² (minimum), HLR of 0.45 m/d (maximum), HRT of 8.2 days (maximum), and temperature of 20.2 °C (minimum). Beyond these thresholds, synergistic relationships weakened or transitioned to antagonistic relationships. SEM analysis revealed that wetland area had the most significant negative direct effect on synergistic pollutant removal, while HRT showed a positive effect. Smaller wetlands were found to favor the maintenance of synergistic relationships compared to larger wetlands.

The findings demonstrate the potential of small-scale CWs as a sustainable and effective solution for managing diverse wastewater pollutants, especially in regions with limited land resources. The synergistic removal of pollutants is a key advantage, increasing the efficiency and cost-effectiveness of the treatment. The identified thresholds provide valuable guidance for designing and managing CWs to maximize their long-term sustainability. The negative impact of large wetland area on synergistic interactions suggests a need to optimize CW size for efficient multi-pollutant removal. The significant influence of HRT and HLR further highlights the importance of properly managing hydraulic conditions within CWs. Future research should focus on extending the long-term monitoring of CWs and investigating management strategies to maintain synergistic interactions beyond the identified thresholds.

This study provides robust evidence for the effectiveness and sustainability of small-scale constructed wetlands in removing multiple pollutants from wastewater. Optimal thresholds for maintaining synergistic pollutant removal were identified, offering valuable design and management guidelines. The findings emphasize the potential of small-scale CWs as a sustainable and efficient solution for global wastewater management, particularly in resource-constrained settings. Future research should prioritize long-term monitoring, further investigation of management strategies, and expansion to a wider range of pollutants.

While the study utilized a large dataset from 364 sites globally, data availability was limited in some regions (Africa and Oceania), potentially affecting the generalizability of findings. The focus on four key pollutants might not fully capture the complexity of wastewater composition. Further research should address these limitations by expanding data collection across regions, incorporating a broader range of pollutants, and exploring the influence of additional factors on CW performance.