Dye wastewater treatment by vertical-flow constructed wetlands

Wetlands have long played an important role as natural purification systems. Textile industry processes are among the most environmentally unsustainable industrial processes, because they produce coloured effluents in large quantities polluting water resources. In this study, two different azo dyes (Acid Blue 113 (AB113) and Basic Red 46 (BR46)) have been fed as part of synthetic wastewater recipes to a laboratory-scale vertical-flow construction wetland set-up comprising wetlands with gravel media as controls and wetlands planted with Phragmites australis (Cav.) Trin. ex Steud. (Common Reed) for each dye. Two different concentrations (7 mg/l and 215 mg/l) were used for each dye at two different hydraulic retention times (48 h and 96 h). According to results for the low concentration of BR46, there is no significant (p > 0.05) difference between wetlands (unplanted and planted) in terms of dye removal. The use of plants concerning the short contact time scenario for ammonia-nitrogen (NH₄-N) and a low concentration of AB113 is linked to good removal. In case of low dye concentrations, the presence of plants for the long contact time scenario impacted significantly (p < 0.05) positive on the removal efficiencies of nutrients. For chemical oxygen demand (COD), the removal percentages were 50%, 59% and 67% for the control and for the wetlands with short and long retention times, respectively. All reductions were statistically significant (p < 0.05). For the high concentration of BR46, the removal percentages for this dye and COD were 94% and 82%, and 89% and 74% for the long and short retention times, respectively. For the low concentration of AB113, the percentage corresponding removals for the dye were 71%, 68% and 80%. The COD removals were 4%, 7% and 15% for the control, and the short and long retention times, respectively. Finally, for the high concentration of AB113, the percentage removals for the dye and COD were 71% and 73%, and 50% and 52% for the 48-h and 96-h retention times in this order.