| Summary: | A global increase in industrialisation has resulted in the rapid growth of textile industries in developing countries, leading to a high rise in the overall discharge of a broad range of pollutants. Amongst these pollutants is dye wastewater, which frequently has mutagenic and carcinogenic effects on humans and animals, depending on the receiving watercourses as a source for drinking water. In contrast to traditional high-rate wastewater treatment units, passive biological treatment technologies, such as constructed wetlands are a sustainable and cost-effective alternative technology to treat large quantities of contaminated water, especially in places where land costs are low. Four small scale experiments were conducted between July 2014 and June 2017 using plastic containers simulating constructed wetland planted with Common Duckweed (<I>Lemna minor</I>) for assessing the system performance, as a polishing stage, for the treatment of synthetic wastewater containing dyes: acid blue 113 (AB113), reactive blue 198 (RB198), basic red 46 (BR46) and direct orange 46 (DO46), with the main focus on removal of the dyes. The novelty of this research was to cover five prominent gaps in the literature, related to the treatment of dye effluents using free-floating plant-based constructed wetland systems under hydroponic conditions. These gaps have not been previously investigated with this system, which include: treatment of four dyes, which have not been treated before using this economic system; evaluate the system performance and the removal mechanism, in detail, for long-term operation as a polishing stage; assess the performance of identical systems for treating the same dye wastewaters under both semi-natural and controlled conditions; studying the effect of pH adjustment during the operation period; and treatment of mixed dyes (real cases) of textile effluents. The overall findings showed that the systems removed BR46 more efficiently than the other dyes studied, and ponds containing <I>L. minor</I> significantly (p<0.05) outperformed algae-dominated and control ponds. The potential of <I>L. minor</I> ponds for the treatment of BR46 was significantly (p<0.05) better under controlled conditions than those under semi-natural conditions. The impact of pH was negligible concerning the treatment of BR46. In addition, the potential for BR46 degradation when it forms only part of a dye mixture is lower, compared to its corresponding removal as an individual dye. Furthermore, only ponds containing <I>L. minor</I> completely eliminated BR46 by removing the aromatic amines after dye decolourisation. Regarding the main water quality parameters, the findings showed that the removal efficiencies of chemical oxygen demand were not significantly different in all design variables. The reduction of nutrients was considerably higher in planted ponds than in controls. Based on <I>L. minor</I> monitoring, all dyes reduced the growth of <I>L. minor</I>, and the synthetic textile wastewater negatively affected the plants’ growth compared with wastewater containing fertiliser. The overall outcomes of this research provide a better understanding of the long-term performance of shallow ponds technology using different al conditions and design variables for the treatment of dye wastewater. Consequently, this will be a tremendous value for wetland ponds engineers to foster the practical development of this system as a low-cost alternative for helping developing countries.
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