Study on an integrated constructed wetland system for colour removal of textile dyeing effluent

Abstract

Textile dyeing effluent contains various types of azo-dyes which are not 100% bio degradable. In addition to that, different types of chemicals, surfactants and various types of additives are also present in the effluent. Due to the higher concentration of colour in the dyeing effluent, it is impossible to discharge effluent into the environment. Studies have been carried out to remove COD, BOD and dissolved solids via constructed wetlands. However there is no significant study done on colour removal of textile dyeing effluent via wetlands. Thus, the main aims of this study are to design and test a suitable integrated constructed wetland system to remove colour from the textile dyeing effluent and to investigate the effect of using different plant species. Three integrated pilot units were set with different operating conditions. The first unit consists of two subsurface flow sub-units with the plant Typha latifolia (cattails) and a free water surface unit with Lemnaceae(Duckweed). The second unit contains two subsurface flow units without plants to find the effect of the material used as the bed and a free water surface unit without plants. The third unit consists of two subsurface flow units with the plant Typha latifolia (cattails) and the free water surface unit with Hydrilla verticillata (Hydrilla). Starting from a single day, the resident time was gradually increased up to 4 days where 90.87% colour removal was achieved. Hence industrial wastewater was fed into the process with a flow rate of 8.0 l/day at a retention time of 3 days. The measured parameters were pH, Dissolved Oxygen, Chemical Oxygen Demand (COD), Suspended Solids and Colour. Influent and effluent from each unit were sampled and analysed for colour using the absorpmetric method. The experiment was carried out for a total period of thirty-nine days. The units with plants showed a significant colour reduction compared to the unit without plants. During the start-up phase of the experimental set up a significant amount of colour removal of 60-70% was observed but there were small deviations due to the accumulation of colour in the root system of duckweed. After that, when the system reached a stable status, the percentage colour removal of 95-98% was achieved using the units with plants. Furthermore, the third unit in which the free water surface consists of Hydrilla verticillata (Hydrilla) showed the highest rate of colour removal of 97.8%. Thus, cattails increased the colour removal in the subsurface flow unit. It can be concluded that the colour of the textile dyeing effluent could be removed using the integrated constructed wetland system where the free water surface consists of Hydrilla verticillata (Hydrilla).