The Energy-saving And Pollution-reduction For Cotton Dyeing Industry

In China, the cotton printing and dyeing industry has a large consumption of resources and energy, and is one of the industries of emitting large amounts of pollutants. Its water consumption, comprehensive energy consumption, and waste emissions per unit of product, are 2 to 3 times more than those of the developed countries, have badly affected the strategy for sustainable development and the development of circular economy. Therefore, promotion of cleaner production in printing and dyeing enterprise, to achieve energy conservation and emission reduction, is imperative. In this study, a typical cotton knitting dyeing factory considered as a case, its cleaner production study, focusing on water conservation and emission reduction, was carried out and the main research results obtained are as follows:First, audit and research concerning cleaner production were carried out in this factory.①Through accounting of water balance, material balance and energy conversion, dyeing workshop was regarded as the focus of water conservation and waste reduction, here all dyestuffs and auxiliaries are almost used up, which is a major source of various wastewater pollutants, and water consumption account for the total consumption of 98.8%; Finishing workshop is key breakthrough of energy saving, and coal water slurry (CWS) constiture 36% of total energy consumption in this unit, but only 10% of its'heat to be effectively used, about 90% heat loss.②The inventory analysis of three types of products, classified on light, intermediate, dark in color, were performed, by which energy consumption, material consumption, and pollution emissions per unit of product were monitored and analyzed, and among them, water consumption per unit of product are 113 t/t product, 121 t/t product and 134 t/t product, respectively, correspongding to three types of products, the CODCr emission factors are, similarly, 92 kg/t product, 94 kg/t product and 93 kg/t product. These results modified and improved cleaner production evaluation index system, and the final evaluation, based on the material consumption and energy consumption, shows that the level of cleaner production of this factory is in state's level-3.③The improvement opportunities of cleaner production in this factory were discussed, and proposed pre-treatment process with bio-enzymes, cold pad-batch dyeing, cold water countercurrent washing, and many other energy-saving initiatives, and economic and environmental benefits of them were assessed.Secondly, on the basis of cleaner production, in order to minimize the use of fresh water in this factory, water conservation and emission reduction were studied.①Water system in this factory were divided into water supply systems, water use systems, and wastewater treatment and recycling systems. Through three kinds of methods, water-saving technology analysis, water consumption per unit of product analysis and wastewater reuse analysis, factory water-saving potential were analyzed, and the results showed that water-saving potential of three systems were 88,000 m3/a , 263,000 m³/a, and 441,000 m³/a respectively.②Using high temperature sample-dyeing machine, different water quality indexes were proposed, and water reuse quality criteria for reusing to different levels, such as cooling water, return water and process water, and the different fabrics dyeing, were studied, and it is further confirmed that water quality criteria and indicator system by practical application via testing whiteness, color fastness, strength loss, etc, to meet production requirements.③Based on the pollutant concentration limit value coming from above water quality standards, using TDS as single restrictive impurity, water pinch analysis was used to calculate smallest volume of fresh water for the factory, during the process that water is modeled as a single impurity water system. Concentration interval diagram (CID) used to determine the system pinch, at the same time, water pinch with the TDS is 5520.00 mg/L, and the minimum amount of fresh water used is 2200 t/d. Considering the factors of COD_Cr and Color, Nearest Neighbors Algorithm (NNA) was used to adjust to water network optimization, and considering of waste water treatment plant for more removal of TDS, which made pinch concentration reduced to 1200 mg/L (when TDS removal of 80%), and then further optimize the water network, the results indicated that the minimum fresh water consumption turn to the 1700 t/d.④With the results of water pinch analysis, it was opposed that three types of measures water reuse, including direct reuse of waste water, on-site reuse after a simple treatment, and reuse after end-of-pipe treatment. Finally, comprehensive analysis showed that the largest wastewater reuse rate of this company was 75%.