Studies on decolouration, toxicity and the possibility for recycling of acid dye effluents using ozone treatment

Investigations carried out to study the feasibility of application of ozone technology for decolouration, toxicity and towards recycling of acid dye effluents for dyeing of silk fabric are reported in this paper. The dyes Acid Red 88, Acid Red 18, Acid Orange 7, Acid Orange 10 and Acid Red 73 were used in this study. The used dye baths were treated with ozone till complete colour removal and reused. For all the dyes two successive recycling processes were carried out. Characteristics of effluents and ozone treated effluents were assessed in terms of total dissolved solids (TDS) and chemical oxygen demand (COD). Gambusia affinis was used for the bioassay tests to find the LC50 value of the treated effluents. The effect of recycling on quality of dyeing was determined using colour difference (ΔE*) and relative unlevelness index (RUI). This study reveals that treatment of acid dye effluents with ozone does not have an effect on TDS reduction. But it reduces the COD of effluents including the effluents obtained in recycling processes. The toxicity of the ozone treated effluents increases with increasing time. A dye having a greater number of sulphonic acid groups in its structure reveals higher toxicity. Resulting effluents from the application of Acid Red 88 and Acid Red 73 on silk fabric can be recycled twice and those from Acid Red 18 and Acid Orange 10 can be recycled once with acceptable ΔE* values. Acid Orange 7 effluent is found to be unsuitable for recycling because of its higher colour difference values. The levelness of the shade determined on the basis of RUI, produced on the silk fabric in the recycling processes, is either excellent or good in all the cases, which include those producing higher colour difference.