Wastewater Treatment Problems in China's Dyeing/Printing Industry           

Dyeing/printing wastewater contains substances that are not biodegradable or nearly not, such as dyes, slurry, additives, oil finishes, acids and bases, fiber impurities as well as inorganic salts. Domestic dyeing/printing enterprises are located mostly in provinces like Zhejiang, Jiangsu and Guangdong. Most of these enterprises treat wastewater inside their own plants, while a few others treat it both inside the plants and through professional sewage treatment plants. At present, common dyeing sewage treatment processes in China generally adopt the physicochemical and biochemical (or flocculation - biochemistry - absorption) technological routine that include activated sludge biological treatment, physical and chemical treatment, membrane treatment and so on.
   As chemical fabrics and dyeing/printing post finishing technologies are developing quickly, a large number of hardly biodegradable substances like PVA (polyvinyl alcohol) slurry and new additives have begun to enter the dyeing/printing wastewater. These pollutants bring the COD removal rate of the original biological treatment system from the previous 70% down to 50% or even lower. Moreover, the domestic dyeing/printing industry features high consumption of resources and energy. Compared to overseas companies, China consumes about three times as much water and discharges 1.2 to 1.8 times the pollutants for every unit of dyeing/printing products. At present, there is almost no dyeing/printing enterprise in China that can achieve zero discharge of wastewater.

Problems   

1. Lack of capital investment    

Domestic dyeing/printing products are generally low grade and yield low profits, so companies are not willing to invest much in wastewater treatment. At present, to process one ton of dyeing/printing wastewater, some treatment plants in China spend only RMB1 000 - 1 200. Part of the dyeing/printing wastewater, after getting treated, still cannot meet China's discharge standards.

2. High energy consumption and low reuse of resources    

In making dyeing/printing products, domestic enterprises' water consumption is usually about 3-5 t/(100 m fabrics), two to three times that of the world's average level. For some enterprises in central and western China this figure can reach about 4.5-5.5 t/(100 m fabrics), with a few even hitting 8.0 t/(100 m fabrics). In addition, China's standard coal consumption for producing every 1 000 meters of dyeing/printing cloth is 1.8 times the international advanced level, and the total energy consumption is 3 times the latter. In recent years, although national and local governments all attach great importance to clean production and wastewater treatment in the dyeing/printing industry, its water and energy consumption has stayed at high level, and the wastewater reuse rate is still less than 10%, the lowest of all industries.

3. Poor treatment technology   

Pollutants in dyeing wastewater are mainly organic materials, but since the BOD (biological oxygen demand)/COD ratio of the wastewater is generally low, these pollutants are biodegradable yet low biodegradable. To raise the COD removal rate, most enterprises in China adopt the method of extending the flocculation and biochemical reaction time, but this method leads to a large wastewater treatment area, long processing time and high construction costs without achieving substantial effects. In places like Shanghai, Shandong and Liaoning provinces, some enterprises change the parallel running of aeration tanks to running them in series. Through this method, although the biochemical treatment efficiency is increased, the improvement in wastewater purification is poor. Therefore, how to improve the COD removal rate is a key problem that urgently needs to be solved in dyeing/printing wastewater treatment.
   The dye uptake rate of home-made dyes is relatively low, so enterprises in China usually put more dyes in the process of dyeing/printing, thus producing almost twice the pollutants of developed countries. With the development of dyeing/printing process technology, the stability of dye structure is also improved, which in turn makes decolorization ever more difficult. In actual wastewater treatment, some enterprises combine many methods, including the common physicochemical method, various chemical engineering methods like absorption by solid absorbents, extraction, stripping, distillation and high-temperature deep oxidation, as well as biochemical method. Although this combined effort can achieve some effects, it is hard to get popularized due to its complexity. The activated carbon absorption method can work effectively. But it also faces several setbacks like regeneration difficulty in China as well as high investment, energy consumption and transport fees. Therefore, decolorization is also a prevalent problem in China's dyeing/printing wastewater treatment at present.
    Moreover, checked by factors like low economic level, lack of environmental protection awareness and strong regional protectionism, China's dyeing/printing industry still lags way behind foreign countries in its management level.

Conclusion   

Traditional dyeing/printing wastewater treatment focuses too much on the end-of-pipe treatment, and has many formidable obstacles like high expenses, ineffective treatment and huge resource waste. In treating wastewater, it should adhere to the "clean production" concept throughout the whole production process and uphold the principle of "resource utilization" in the later part of the treatment. In addition, enhancing internal management is also an effective way to control pollution.