Background and purpose: High concentrations of heavy metals in industrial and domestic wastewater cause adverse effects on the environment and human health. Therefore, removing heavy metals from water resources is considered as a positive step towards sustainable development. Today, application of artificial wetlands to remove pollutants such as heavy metals is increasing due to high costs of construction sites, low operation, maintenance, and energy needs. Materials and methods: This experiential research was carried out in a pilot scale in adjacent of wastewater treatment plant in the campus of Mazandaran University of Medical Sciences. The pilot was composed of four rectangular concrete reactors with 6 × 1.5 × 0.8 m length, width and affective depth. One reactor was used as control. The research was performed in five phases. Different hybrid systems of the reactors (combination of vertical and horizontal with parallel and series flow) were evaluated. The samples were digested using an acid digester and concentrations of lead and cadmium were analyzed using atomic absorption spectrophotometer. The data was analyzed using SPSS. Results: By increasing the hydraulic retention time (HRT) from 2 to 6 days, the mean percentage for removal of studied metals increased to 58 (which was 42 at the beginning). Among the different phases, phase four was more efficient which removed 54% of the heavy metals while the lowest removal percentage was detected in phase one. The average percentage removal of cell output for reed planting and the controls were 65% and 35%, respectively. These differences were statistically significant (P≤ 0.05). Conclusion: The findings of this research showed that combination of DF-UF (down flow-Up flow) reactors was the best condition for the removal of lead and cadmium from subsurface wetlands. The optimal hydraulic retention time was 4 to 6 days and 10 to 20 mm gravel could be more appropriate for the size of gravel in wetland.

Keywords: Constructed wetland, hybrid system, heavy metals, wastewater

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