Increased toxicity of textile effluents by a chlorination process using sodium hypochlorite

Chlorinated textile effluents were tested for their toxicity using different bioassays. These assays were the Microtox(R) assay, daphnia (Daphnia similis) 48-hr survival test, medaka embryo f 4-day and juvenile 96-hr survival tests, and tilapia (Oreochromis mossambicus) juvenile 96-hr survival test. By comparing the results of toxicity tests on water samples collected at the instream prior to the chlorination process and at the outlet of the wastewater treatment facility, we found that wastewater toxicity was obviously increased by chlorination using NaOCl as the oxidant, as evidenced by the different bioassays used. Because no significant difference was observed in water chemistry, such as pH, DO, and conductivity, the induced-toxicity may be partially attributable to residue chlorine or other chlorinated compounds generated by chlorination. Future studies are warranted to identify the cause of the increase in the textile wastewater toxicity.