Nanofiltration for the possible reuse of water and recovery of sodium chloride salt from textile effluent

During the reactive dyeing of cotton, salts such as sodium chloride (NaCl) are placed in a dyebath to aid the exhaustion of various dyes onto the fabric while bases are added to raise the pH from around neutral to pH 11 to achieve fixation. Afterwards, the used dyebath solution, called dyebath spent liquor, is discharged with almost all the salts and bases added as well as unfixed dyes. Consequently, many raw materials are lost in the waste stream ending up in the environment as pollutants. In this study possibilities of reusing the water and salts of dyebaths were investigated using a nanofiltration membrane. When the NaCl concentration in the spent liquor was increased from 10 to 80 g/L, the NaCl rejection by the membrane was found to decrease initially; however, the NaCl rejection increased over time, which was not expected. The aggregation of dye was also studied and found to decrease in the concentrate when the salt concentration was increased. This information is useful for the textile industry in evaluating the treated water quality for the purpose of reuse.

Nanofiltrating solutions of hydrolysed C.I. Reactive Black 5 and NaCl were studied with the aim to reuse and reclaim water and NaCl from spent liquor. It was found that NaCl rejection should decrease but actually increased after NF when NaCl was concentrated in the stirred cell. Although dye aggregation numbers were expected to increase after NF because the dye and NaCl were concentrated in the cell, they decreased. It was assumed that the 0.03% of dye removal from the stirred cell was not responsible for the decrease in dye aggregation number from 4.05 to 2.70, but instead the 1.35 decrease of dye aggregation number was attributed to the 33.7% NaCl across the membrane. It was calculated that the NaCl in the permeate might be seven times more powerful in promoting dye aggregation than the NaCl in the original solution. It was proposed that the NaCl across the membrane might be NaCl associates. This information is useful for the textile industry in evaluating treated water quality for the purpose of reuse.