Removal of heavy metals in sub-surface horizontal flow constructed wetland: Approach to energy free treatment

The industrial wastewater discharge is a major contributor to environmental degradation in Pakistan. To make conditions more deplorable, discharges from various industries such as paint, electroplating, battery, etc., are rich in heavy metals concentrations. The effluents from industries are generally used for irrigation or are directly discharged into waterways, ultimately leading to the deterioration of the local ecology and becoming part of the food chain through uptake by vegetation. Various studies are being conducted to figure out the most efficient nature-based solutions. One such technique is to develop constructed wetlands. These are engineered systems employing indigenous wetland plants to remove various pollutants from wastewater. In this study, a sub-surface horizontal flow constructed wetland (SSHW) was used to remove heavy metals lead (Pb), cadmium (Cd), and zinc (Zn). The heavy metal aqueous solution was prepared in the laboratory with a concentration of 30 mg/l for each metal. The removal efficiency was calculated for different detention times from day 1 to day 7. Zn showed maximum removal efficiency of 76.56% on day 6, while lead and cadmium showed maximum removal efficiency of 72.6% and 69.67%, respectively, on day 7. The metal's mobility in a plant, translocation factor, and accumulation coefficient was calculated. The results indicated that Typha angustifolia serves as a bio-indicator for all three metals, and most of the accumulation of metals occurred in the plant's root zone. The results indicated that this technology gives good removal of heavy metals and has a scope for its efficiency. Since it has low operational and maintenance costs, it is comparatively a step toward a green economy.