Nước thải công nghiệp thường được thải ra hệ thống thoát nước, thành phố trực thuộc Trung ương ở các nước đang phát triển kể từ những năm 1920. Trước đây, phần lớn nước thải được thải ra vùng nước thủy triều mà không cần điều trị bất kỳ. Ít sự chú ý được trả tiền để màu sắc của nước thải cho đến khi những năm 1980, và thậm chí sau đó, sự phản đối trên cơ sở thẩm mỹ, kể từ khi được biết rằng thuốc nhuộm hiện đại tương đối không độc hại. Vào đầu những năm. | 7 Decolorisation of effluent with ozone and re-use of spent dyebath M. M. HASSAN AgResearch Ltd New Zealand and C. J. H AW K YA R D University of Manchester UK Introduction Industrial effluents have usually been discharged into municipal sewage systems in developed countries since the 1920s. Previously the majority of sewage was discharged to tidal waters without any treatment. Little attention was paid to the colour of wastewater until the 1980s and even then the objections were on aesthetic grounds since it was known that modern dyestuffs are relatively non-toxic. At the beginning of the 1970s only physical treatment methods such as sedimentation and equalisation were applied to maintain the pH total dissolved solids TDS and total suspended solids TSS of the discharged water. There were no obligatory discharge limits for the colour of the effluent at that time. Secondary treatments such as the use of filter beds for biodegradation and more recently the introduction of the activated sludge process aerobic biodegradation have reduced the toxicity of sewage water considerably. As a result much of the water is now discharged to local rivers. However sewage treatment works have often been unable to remove the colour from dyehouse effluent completely especially when reactive dyes are included and this causes the receiving river water to become coloured. As a result there have been complaints by the public who are becoming increasingly aware of environmental issues. Wastewater treatment methods can be classified as shown in Fig. . Treatment of large volumes of effluent is a very costly process and investment in effluent treatment is often considered a waste of money as it makes no contribution to profit for an industrial company. However textile wet processing is now under threat in many countries because of the tightening of discharge limits for effluents by environmental agencies. The viability of many textile dyeing printing and finishing plants is already in
