Biological and physical-chemical treatment of textile dyeing wastewater for color and COD removal

Biological treatment, ozonation and chemical precipitation were studied at bench scale to investigate color and COD removal efficiencies in three dye waste effluents of cotton, synthetic and woven fabric processing. The aerobic biodegradability of ozonated and unozonated samples was investigated in fill-and-draw activated sludge systems. An ozone dosage of 0.8 g/L was applied for 30 minutes in a semi-batch reactor with volume of 10 L. Alum, FeCl3 and FeSO4 were studied as coagulants during chemical clarification and anionic polyelectrolyte was used to improve COD removal efficiency. The results indicated that color removal efficiencies of 78 - 100 percent were obtained with ozonation at a dosage of 0.8 g/L. COD removal efficiencies of 59 - 71 percent were obtained with combined treatment of ozonation and chemical clarification, while 62 - 82 percent were achieved with a combination of ozonation and biological treatment. A combination of ozonation and biological treatment is proposed for medium strength wastewater to obtain sufficient color and COD removal. For high strength wastewaters, chemical clarification should be used as a pretreatment before the ozonation and biological treatment to be able to achieve the discharge standards.