Aquatic toxicity of heavy metal-containing wastewater effluent treated using vertical flow constructed wetlands

The effluent from municipal wastewater can be discharged into the surface water once the chemical parameters meet the standard requirements. However, little attention has been paid to assess the toxicity of the effluent in the receiving water. In this study, vertical flow constructed wetlands (VFCWs) were used to treat the heavy metal-containing wastewater. In addition, the toxicity of the effluent towards Chlorella pyrenoidosa and Daphnia magna was evaluated. The results showed that the VFCWs removed nearly 80% of the total nitrogen, nitrate, and Cd²⁺ from the wastewater during a 60 day operation. Acute toxicity assessments demonstrated that incubating the plankton in 10% (v/v) effluent enhanced their growth, but a further increase in the effluent proportion (50% and 100%) exhibited detrimental effects towards the aquatic organisms. The superoxide dismutase (SOD) and peroxidase (POD) activities in C. pyrenoidosa and D. magna peaked but then decreased with the increasing effluent proportion, indicating that incubation in a higher proportion of the effluent could damage the anti-oxidative capabilities of the organisms, which aligned with the enhanced reactive oxygen species (ROS) levels and malondialdehyde (MDA) contents. After 96 h of incubation, the assimilation of Cd²⁺ was most obvious in all the studied effluent gradients in comparison to Cu²⁺ and Pb²⁺. Additionally, the correlation analysis between the plankton growth and heavy metal removal revealed that Cd²⁺ removal clearly benefited the growth of C. pyrenoidosa and D. magna. These results suggest that additional endeavor should be made on the heavy metal removal, to reduce the potential ecological risk of the final discharge.