Treatment of a textile effluent: application of a combination method involving adsorption and nanofiltration

A combination of adsorption and nanofiltration (NF) was adopted for the treatment of a textile dyehouse effluent containing a mixture of two reactive dyes. The effluent stream was first treated in a batch adsorption process with sawdust as an adsorbent to reduce the dye concentration of the effluent by about 83% for Dye 1 and 93% for Dye 2. The effluent from the adsorption unit was passed through an NF unit for the removal of the remaining small amount of dyes and to recover the associated chemicals (mainly salt) in the effluent stream. The dyes remaining after this step were less than 1 ppm. The percentage removal of COD was greater than 99%, and the salt recovery was on the order of 90%. Equilibrium studies were carried out with synthetic solutions of the dyes (both single component as well as two-component systems) at room temperature. The adsorption rates were studied in detail using varying amounts of the adsorbent. NF of the effluent was performed in a cross-flow system using a 400 molecular weight cut-off membrane. A detailed study was carried out to observe the effect of the process parameters, namely applied pressure and bulk velocity on the process outputs such as dye rejection, COD removal, permeate flux and salt recovery. Finally, direct NF of the effluent (with the original high concentration) was undertaken, and the performance of the process was compared with the combination method. The permeate flux for the proposed combined method was found to be about twice that for the direct NF method. The dye rejection improved significantly compared to adsorption.

A combination method involving adsorption followed by NF was successfully used for the treatment of a textile effluent containing a mixture of two reactive dyes. A waste carbonaceous material (sawdust) was used as the adsorbent. Adsorption equilibrium data fit well to the Langmuir isotherm model, indicating the monomolecular nature of adsorption. For dye mixtures, the bi-solute Langmuir isotherm modified with interaction factors was used. Kinetic data show that with 1 g/l adsorbent dose, about 83% and 92% dye removal is possible (for dye 1 and 2, respectively) from the effluent stream for an adsorbent particle size of 0.044 mm and stirrer speed of 2000 rpm. NF of the effluent (after adsorption) was found to be highly effective for the separation of the dyes. Both the dye removal and the reduction in COD are more than 99% for all the operating conditions. Salt recovery was higher than 88%. The performance of the proposed method was compared with that of direct NF of the same effluent. Although the salt recovery and the COD reduction were comparable in both methods, the dye concentrations in the permeate stream were above the permissible limit with the direct NF method. The major drawback in direct NF is a substantial reduction in the permeate flux compared to the combined method.