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Efficiency study of a halophyte based constructed wetland for the removal of Cd, Ni and Zn from municipal wastewater

Agioutanti Rozalia

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Summary

In recent years, the extensive use of heavy metals in numerous activities lead to the accumulation of significant concentrations of them in Wastewater Treatment Plants (WWTP) or in the aquatic environment. The pollution of the marine environment or the recycled water due to inadequate processing of waste water is a major environmental problem. The aptly processing of waste water, specifically municipal wastewater, with the use of efficient technologies is essential, so that the pollutant load will decrease.Constructed Wetlands (CWs) are among the proven efficient technologies for wastewater treatment. They are mechanical systems, designed and constructed according to the functions of natural wetlands, like the vegetation, the substrate and the microbial activity for water and wastewater treatment. They are low cost, low-energy, easily operated and maintained, able to achieve multiple goals of contaminant removal and have a strong potential for application in developing countries, particularly by small rural communities.The goal of this work is the remediation of secondary treated municipal wastewaters polluted with heavy metals by a halophyte-based constructed wetland with the specific aim to explore the performance of a Juncus acutus L. based CW mesocosm for the removal of Cd, Ni and Zn from secondary treated wastewater. A horizontal subsurface flow (HSF) CW mesocosm (1x0.5x0.5m) of 57 L working volume with Juncus acutus L. plants is operated in the Wastewater Treatment Plant of the city of Chania in Greece. The influent CW wastewater consists of two fluxes: the 1st is wastewater directed from the outlet of the secondary treatment of the WWTP and the 2nd flux is artificially polluted water with Cd, Ni and Zn. The second flux is directed to meet the wastewater flux and together are pumped onto the gravel bed at fixed rates from 2 to2.6L/h; resulting at a 1-1.2-day hydraulic retention time (HRT). The CW water level remained always constant.Samples are collected at the inlet and outlet every 24h and analyzed for pH, EC, ORP, DO, TDS and Cd,Ni and Zn concentrations. Once a week, the samples are analyzed also for total nitrogen, total phosphorus, COD, BOD and TSS.The results of 7 months of continuous operation indicate that the system is able to treat heavy metal polluted wastewater as tertiary treatment. The removal capacities were 67-80% for Cd, 49-59% for Zn and 23-50% for Ni. Moreover, the system seems to achieve also reduction of total N, total P and TSS.

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